Articles présents dans Web of Knowledge

Résumé: High strain rate loading causes pervasive dynamic microfracturing in crystalline materials, with dynamic pulverization being the extreme end-member. Hydraulic properties (permeability, porosity, and storage capacity) are primarily controlled by fracture damage and will therefore change significantly by intense dynamic fracturing-by how much is currently unknown. Dynamic fracture damage observed in the damage zones of seismic faults is thought to originate from dynamic stresses near the earthquake rupture tip. This implies that during an earthquake, hydraulic properties in the damage zone change early. The immediate effect this has on fluid-driven coseismic slip processes following the rupture, and on postseismic and interseismic fault zone processes, is not yet clear. Here, we present hydraulic properties measured on the full range of dynamic fracture damage up to dynamic pulverization. Dynamic damage was induced in quartz-monzonite samples by performing uniaxial high strain rate (>10(0) s(-1)) experiments in compression using a split-Hopkinson pressure bar. Hydraulic properties were measured on samples subjected to single and successive loadings, the latter to simulate cumulative damage from repeated rupture events. We show that permeability increases by 6 orders of magnitude and porosity by 15% with dissipated energy up to dynamic pulverization, for both single and successive loadings. We present damage zone permeability profiles induced by earthquake rupture and how it evolves with repeated ruptures. We propose that the enhanced hydraulic properties measured for pulverized rock decrease the efficiency of thermal pressurization, when emplaced adjacent to the principal slip zone.

Résumé: This contribution investigates mechanisms controlling subduction development and stabilization over time (coined as 'slabitization'), from a nascent slab to a mature slab viscously coupled to mantle convection, from grain scale to plate tectonics scale. Frozen-in, deep and warm portions of the subduction plate interface with both sides still preserved are found at the base of ophiolites in almost pristine state. Both sides record changes in the mineralogy, structure, fluid content and rheology due to devolatilization of subducting metamorphic rocks. They allow characterizing the evolution, shortly after subduction initiation (similar to 1-10 Ma), of interplate coupling, mantle resistance to slab penetration or incipient mantle wedge metasomatism, as well as transformations occurring at depth in warm or cold subduction zones today. This study combines structural field work, mineralogical and crystallographic data, detailed petrology, thermodynamic modelling and geochemistry from/on both sides of the plate interface, i.e. the base of the mantle wedge (basal ophiolitic peridotites) and crustal fragments from the slab (metamorphic soles). Data collected across the entire Semail ophiolite (Oman, UAE territory) and other similar settings worldwide (e.g., Canada, Turkey, New Caledonia) show a continuous evolution of the subduction plate interface from 1.2-0.9 GPa 900-750 degrees C to 0.7-0.5 GPa 750-600 degrees C, with progressive localization of strain and fluid transfer. Crystallization of neo-formed minerals, enrichment in fluid-mobile elements and their isotopic signature (e.g., for boron) indicate that metasomatism of the mantle base results from interaction with subduction fluids derived from the dehydrating metamorphic sole and slab tip, migrating at velocities similar to 1-10 m/a. Coeval deformation and metamorphic reactions in metabasalts of the downgoing slab reveal the importance of mineral changes (e.g., amphibole content) and deformation modes in controlling fluid delivery, stepwise detachment and accretion of successive slices from the downgoing slab (HTa, HTb and then LT soles) to the mylonitized mantle. This study demonstrates how the interplay between metamorphic reactions, fluid/melt transfer and deformation mechanisms, in particular dissolution-precipitation creep (DPC), controls the mechanical coupling state of the plate interface: (i) suppression of fluid transfer and DPC at depth triggers the onset of viscous coupling. This occurs near similar to 30 km depth during subduction infancy and HTa sole formation; (ii) with increased cooling and fluid availability, strain localization progressively develops downwards and unzips the subduction interface. The downward migration of viscous coupling triggers localized mantle wedge upwelling, potentially leading to short-lived suprasubduction ophiolite or forearc lithosphere formation; (iii) the locus of viscous coupling stabilizes near similar to 80-100 km in mature (and cold) subduction zones, and sets mantle counterflow. This is where and when plates get reattached and slabs become part of the mantle convection system. Recent geochronological data suggest that the duration from subduction nucleation to ophiolite formation is probably slower than suspected (similar to 5-10 Ma), and that another 5-10 Ma may be needed to reach mature subduction and profuse arc magmatism. These results refine our view of the subduction factory and have important implications for how, how much, and which sort of fluid is being fluxed into the mantle wedge at depths where serpentine is no longer stable.

Résumé: With materials of anisotropic electrical conductivity, it is possible to generate a dynamo with a simple velocity field, of the type precluded by Cowling's theorems with isotropic materials. Following a previous study by Ruderman and Ruzmaikin [M. S. Ruderman and A. A. Ruzmaikin, Magnetic field generation in an anisotropically conducting fluid, Geophys. Astrophys. Fluid Dyn. 28, 77 (1984)], who considered the dynamo effect induced by a uniform shear flow, we determine the conditions for the dynamo threshold when a solid plate is sliding over another one, both with anisotropic electrical conductivity. We obtain numerical solutions for a general class of anisotropy and obtain the conditions for the lowest magnetic Reynolds number, using a collocation Chebyshev method. In a particular geometry of anisotropy and wave number, we also derive an analytical solution, where the eigenvectors are just combinations of four exponential functions. An explicit analytical expression is obtained for the critical magnetic Reynolds number. Above the critical magnetic Reynolds number, we have also derived an analytical expression for the growth rate showing that this is a “very fast” dynamo, extrapolating on the “slow” and “fast” terminology introduced by Vainshtein and Zeldovich [S. I. Vainshtein and Y. B. Zeldovich, Reviews of topical problems: Origin of magnetic fields in astrophysics (turbulent “dynamo” mechanisms), Sov. Phys. Usp. 15, 159 (1972)].

Résumé: The structure of liquid silicates is commonly described as a statistical mixture of various atomic entities with relative abundances that can vary with pressure, temperature, and composition. Unfortunately, this view remains largely theoretical due to scarce experimental reports on the silicate melt structure, in particular under pressure. We performed X-ray diffraction of the SiO2 end member to probe the melting curve up to 120 GPa and 7000 K, and the melt structure up to 80 GPa. We confirm the steep increase of the melting curve above 14 GPa when stishovite becomes stable over coesite in subsolidus conditions, with a slope of about 80 K/GPa. Then, around 45 GPa and 5400 K, the melting curve flattens significantly, an effect most likely reflecting the densification of the SiO2 melt structure. The signal of diffuse X-ray scattering is compatible with a change of the Si coordination number from 4 to 6 along the melting curve, in agreement with previous works reporting a similar evolution during the cold compression of SiO2 glass. Because of the limited pressure range (within 10 to 20 GPa) in which the melting curve changes its slope, we speculate a difficult coexistence of tetrahedral SiO4 and octahedral SiO6 units in SiO2 melt at high pressures.

Résumé: Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but arc significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth's history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

Résumé: The elastic properties of buildings change during earthquakes. In particular, fundamental frequencies are observed to shift rapidly during the co-seismic phase and to recover slowly once the strong motion has finished. Although the frequency shift is usually correlated with loading amplitude, the co-seismic frequency variations observed in real structures are not only determined by the absolute amplitude of the strain value. In order to interpret the uncertainties of the prediction of engineering demand parameters for a given intensity measure, we analyze the influence of loading rates (i.e., strain rates) on resonance frequency variations in buildings with different structural states during the loading and unloading phases caused by seismic events. Our observations suggest the existence of a strain rate threshold that activates the nonlinear response of the structure, characterizing the activation of cracking and indicating a strong nexus between the elastic structural response, structural state, and the loading process.

Résumé: Since 2015 six new continuous Global Positioning System (cGPS) and meteorological stations have been installed in Venezuela as part of the Continuously Operating Caribbean GPS Observational Network (COCONet) project (El Baul-CN41, Quebrada Arriba-CN39, Isla de Aves-CN49, Los Roques-CN42, Isla de Margarita-CN44, and Isla La Blanquilla-CN43). Almost simultaneously, in the framework of the VENCREEP project (ANR-France), a survey composed of two geodetic stations (Saucedo-SAU0 and La Pica Arriba de Catuaro-PAC0) and one creepmeter was installed to study the Pilar Fault. Also, new seven cGPS stations of fifteen planned were acquired through the Tsunami project (FONACIT 2013000361), aiming at densifying the current network. The whole set of stations will improve our knowledge about the local and regional geodynamics. We are sure that the results will have implications for seismic hazard studies.

Résumé: Soil erosion is strongly linked to both precipitation patterns and land-use. We examine the effects of erosion and its drivers (i.e. human or/and climate) on soil evolution from the study of lacustrine archives in the northern French Alps. Multi-proxy analyses of the Lake Gers sediment sequence combined with the study of soils and rocks of its catchment allowed to reconstruct its evolution over the past 4.6 kyr. This included C-14 dating, short-lived radionuclides, geochemistry, loss on ignition, grain-size analyses, as well as plant and mammal DNA analyses. A total of 127 instantaneous deposits were identified among the continuous sedimentation in the lake and 93 were interpreted as flood deposits. Erosion was quantified for the whole catchment considering both the continuous sedimentation and the flood deposit thicknesses. The catchment is mainly formed by andesitic sandstones over which Andosols and Podzols can develop. However, low weathered materials from colluviation constitute the main input to the lake. Four main phases of changes in soil weathering were recorded by increases in K2O/TiO2 ratios, associated with an increase in both erosion and flood-frequency. Two phases were associated with climate cooling in the Western Alps, from 2.65 to 2.5 and from 1.45 to 1.3 cal kyr BP. The two others (1.9-1.75 and 0.9-0.4 cal kyr BP) were triggered with deforestation, with cow grazing during the Roman period, and with grazing of cows, sheep and goats during the Medieval period, amplified by the onset of the Little Ice Age. The increase in erosion and flood frequency after 950 cal yr BP indicates substantial damages to soils of the catchment including increase of their erodibility over the last millennia.

Résumé: The Vestgotabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar /39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430-400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the LowerUnit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365-344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgotabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsakkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgotabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

Résumé: Evaporites play a major role on the structuration of collisional orogens especially when they act as decollement units. However, their exact pressure-temperature-deformation (P-T-d) paths are poorly documented. In this study, the first direct P-T-d constraints of the “Nappe des Gypses” formation (western French Alps) have been established. An innovative association of structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geo-chemical analysis shows that the “Nappe des Gypses” formation has recorded the three typical metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different constraints allow the determination of the first P-T path determination for this unit. Metamorphic peak conditions of the “Nappe des Gypses” are at 16.6 +/- 2.3 kbar and 431 degrees C +/- 28 degrees C. This formation was buried at similar conditions than the oceanic units. During the exhumation path, the D1-D2 transition is reached at 350 degrees C +/- 20 degrees C and 6.5 +/- 1.8 kbar and the D2D3 transition is assumed to be at 259 degrees C +/- 24 degrees C and 2.0 +/- 1.0 kbar (Strzerzynski et al., 2012). Peak P-T conditions overlap those of themedian Liguro-Piemontese units but are different fromthose of the Brianconnais units. It implies 1) an active and crucial role of the “Nappe des Gypses” during the exhumation of the Alpine oceanic complex, and 2) confirms the allochthonous and more distal origin of the European Tethyan passive margin of the “Nappe des Gypses” formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 kbar and 200 and 300 degrees C, allows to discuss pore pressure excess and its mechanical consequences on the exhumation process. This process is very likely to amplify the “decollement” effect of the evaporites and allowthe nappe stack formation. This illustrates the role of this formation as a decollement surface. This difference of evolution highlights the major role of the evaporitic formations on the exhumation and structuration of a collisional chain. Suchmethodology could contribute to decipher the role of evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or Zagros. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: The impact of fluorine on phase relations and clinopyroxene-melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the REE and HFSE during crystallisation of alkaline and peralkaline magmas. Clinopyroxene was grown from three H2O-saturated synthetic glasses of tephriphonolite-to-phonolite composition in a rapid quench internally heated pressure vessel at 650-800 circle C and 200 MPa, with Delta log fO(2) fixed to ca. FMQ + 1. The fluorine content in the charges was varied and produced quenched melts from fluorine-free to 1.6 wt.% F. The experiments yield an assemblage of melt, fluid, sodic clinopyroxene, biotite, magnetite, +/- K-feldspar, +/- titanite, +/- fluorite, and +/- hiortdahlite (a Na-Ca-Ti-F sorosilicate). Addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene. Relative to fluorine-free compositions, experiments with 1.6 wt.% fluorine in the melt show a strong decrease in clinopyroxene-melt partition coefficients for the trivalent REE La-Dy and Y with a lesser decrease for the DHREE and DHFSE4+. The diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase, consistent with the formation of REE-F complexes and with modifications to the medium-range structural environment around HFSE4+ An increase in the fluorine content of the melt will thus make the REE and HFSE4+ progressively less compatible and, therefore, available for residual enrichment.

Résumé: In this study focused on France, we explore the uncertainties related to choices made while building a source model for hazard assessment and we quantify the impact on probabilistic hazard estimates. Earthquake recurrence models are initially built from the French Seismic CATalog (FCAT, Manchuel et al. in Bull Earthq Eng, 2018. 10.1007/s10518-017-0236-1). We set up a logic tree that includes two alternative seismogenic source models (ESHM13 and Baize et al. in Bull Soc Geol Fr 184(3):225-259, 2013), two versions of FCAT catalog, two alternative declustering algorithms, and three alternative minimum magnitudes for earthquake recurrence modeling. We calculate the hazard for six cities (i.e. Nantes, Lourdes, Clermont-Ferrand, Briancon, Nice and Strasbourg) that are located in source zones with a minimum amount of data to work with. Results are displayed for the PGA and spectral period 0.2 s, at return periods 475 and 5000 years. Exploration of the logic tree shows that the parameters with the most impact on hazard results are the minimum magnitude used in the recurrence modeling (up to 31%) and the selection of the seismogenic source model (up to 30%). We also use the SHARE European Earthquake Catalog (SHEEC, Woessner et al. in Bull Earthquake Eng, 2015. 10.1007/s10518-015-9795-1) to build earthquake recurrence models and compare hazard values obtained with the FCAT logic tree. Comparisons are limited because of the low number of events available in some sources in SHEEC; however, results show that, depending on the site considered, the earthquake catalog selection can also strongly impact the hazard estimates (up to 50%). The FCAT logic tree is combined with four ground-motion models (Bindi et al. in Bull Earthq Eng 12(1):391-430, 2014; Boore et al. in Earthq Spectra 30(3):1057-1085, 2014; Cauzzi et al. in Bull Earthq Eng 13(6):1587-1612, 2015. 10.1007/s10518-014-9685-y; Drouet and Cotton in Bull Seismol Soc Am 105(4):1883-1902, 2015) to account for the epistemic uncertainty on the prediction of ground-motion. Exploration of the logic tree shows that the contribution of ground-motion model uncertainties can be larger than, equivalent to, or lower than the contribution of the source-model uncertainties to the overall hazard variability. Which component controls overall uncertainty depends on the site, spectral period and return period. Finally, exploring the logic tree provides a distribution for the ratios between hazard levels at 5000 and 475 years return periods, revealing that the ratios only slightly depend on source-model uncertainties, vary strongly from site to site, and can take values between 3 and 5, which is significantly higher than what is commonly assumed in the engineering community.

Résumé: Zoned hydrothermal monazite-(Ce) from Alpine-type fissures and clefts is used to gain new insights into the tectonic history of the Lepontine Dome in the Central Alps and the timing of deformation along the Rhone-Simplon Fault zone on the dome's western end. Hydrothermal monazites-(Ce) (re)crystallization ages directly date deformation that induces changes in physicochemical conditions of the fissure or cleft fluid. A total of 480 secondary ion mass spectrometry (SIMS) spot analyses from 20 individual crystals, including co-type material of the monazite-(Nd) type locality, record ages for the time of similar to 19 to 2.7 Ma, with individual grains recording age ranges of 2 to 7.5 Myr. The combination of these age data with geometric considerations and spatial distribution across the Lepontine region gives a more precise young exhumation history for the area. At the northeastern and southwestern edges of the Lepontine Dome, units underwent hydrothermal monazite-(Ce) growth at 19-12.5 and 16.5-10.5 Ma, respectively, while crystallization of monazite-(Ce) in the eastern Lepontine Dome started later, at 15-10 Ma. Fissure monazite-(Ce) along the western limit of the dome reports younger ages of 13-7 Ma. A younger age group around 8-5 Ma is limited to fissures and clefts associated with the Simplon normal fault and related strike-slip faults such as the Rhone Fault. The data set shows that the monazite-(Ce) age record directly links the fluid-induced interaction between fissure mineral and host rock to the Lepontine Dome's evolution in space and time. A comparison between hydrothermal monazite-(Ce) and thermochronometric data suggest that hydrothermal monazite-(Ce) dating may allow us to identify areas of slow exhumation or cooling rates during ongoing tectonic activity.

Résumé: The Amaga basin between the Western and Central Cordilleras of the Northern Andes of Colombia hosts the Neogene volcanic and volcaniclastic Combia Formation. Deposition of the Combia Formation in relation to Nazca plate subduction and arc volcanism is still a matter of debate. Therefore, the timing, petrography and geochemical characteristics of Combia Formation rocks were studied in the western and eastern parts of the Amaga basin, in order to gain more information on the type of magma generation and volcanic activity that led to the deposition of the Combia Formation. Apatite and zircon fission-track dating largely confirm a 12-6 Ma age for the deposition of the Combia Formation. Petrographic and major element analyses show that mainly trachy-andesite ignimbrites with a calcalkaline composition were deposited in the western Amaga basin, whereas the volcanic rocks of the eastern Amaga basin are lavas flow and fall-out deposits of basaltic andesites of tholeiitic affinity. Trace element and isotopic analyses show that slab dehydration and sediment melting/decarbonation were important in primary magma generation in the mantle wedge, but the primary magma was mixed with lower continental crustal melts (e.g. High-Pb radiogenic), resulting in characteristic isotope signatures in the western and eastern Amaga basin. Then, the hot-zone developed a high Pb-radiogenic, garnet-bearing lower continental crustal (LCC) level as a consequence of the quantity of dehydration of the subducting slab and of changes in the tectonic regime. An extensional pull-apart event (129 Ma), likely facilitated rapid magma ascend to the uppermost crust along a subvertical magma plumbing system throughout the Romeral Fault zone in the eastern Amaga basin, and calcalkaline magmas with adakite-like signature, which may indicate contractile tectonics that allow the formation of middle-to upper-crustal magma chambers with a garnet fractionation at depth and the evolution of silicate melts into the hot zone mainly related to the amount of water (>4 wt %) present.

Résumé: We perform a spectral decomposition of the Fourier amplitude spectra disseminated along with the Engineering Strong Motion (ESM) flat file for Europe and Middle East. We apply a non-parametric inversion schema to isolate source, propagation and site effects, introducing a regionalization for the attenuation model into three domains. The obtained propagation and source components of the model are parametrized in terms of geometrical spreading, quality factor, seismic moment, and corner frequency assuming a omega(2) source model. The non-parametric spectral attenuation values show a faster decay for earthquakes in Italy than in the other regions. Once described in terms of geometrical spreading and frequency-dependent quality factor, slopes and breakpoint locations of the piece-wise linear model for the geometrical spreading show regional variations, confirming that the non-parametric models capture the effects of crustal heterogeneities and differences in the anelastic attenuation. Since they are derived in the framework of a single inversion, the source spectra of the largest events which have occurred in Europe in the last decades can be directly compared and the scaling of the extracted source parameters evaluated. The Brune stress drop varies over about 2 orders of magnitude (the 5th, 50th and 95th percentiles of the increment sigma distribution are 0.76, 2.94, and 13.07 MPa, respectively), with large events having larger stress drops. In particular, the 5th, 50th and 95th percentiles for M > 5.5 are 2.87, 6.02, and 23.5 MPa, respectively whereas, for M < 5.5, the same percentiles are 0.73, 2.84, and 12.43 MPa. If compared to the residual distributions associated to a ground motion prediction equation previously derived using the same Fourier amplitude spectra, the source parameter and the empirical site amplification effects correlate well with the inter-event and inter-station residuals, respectively. Finally, we calibrated both non-parametric and parametric attenuation models for estimating the stress drop from the ratio between Arias intensity and significant duration. The results confirm that computing the Arias stress drop is a suitable approach for complementing the seismic moment with information controlling the source radiation at high frequencies for rapid response applications.

Résumé: X-ray absorption spectroscopy (XAS) is a widely used technique to probe the local environment around specific atomic species. Applied to samples under extreme pressure and temperature conditions, XAS is sensitive to phase transitions, including melting, and allows gathering insights on compositional variations and electronic changes occurring during such transitions. These characteristics can be exploited for studies of prime interest in geophysics and fundamental high-pressure physics. Here, we investigated the melting curve and the eutectic composition of four geophysically relevant iron binary systems: Fe-C, Fe-O, Fe-S and Fe-Si. Our results show that all these systems present the same spectroscopic signatures upon melting, common to those observed for other pure late 3d transition metals. The presented melting criterion seems to be general for late 3d metals bearing systems. Additionally, we demonstrate the suitability of XAS to extract melt compositional information in situ, such as the evolution of the concentration of light elements with increasing temperature. Diagnostics presented in this work can be applied to studies over an even larger pressure range exploiting the upgraded synchrotron machines, and directly transferred to time-resolved extreme condition studies using dynamic compression (ns) or fast laser heating (ms).

Résumé: In tectonically active mountain belts, landslides contribute significantly to erosion. Statistical analysis of regional inventories of earthquake-triggered-landslides after large earthquakes (Mw > 5.5) reveal a complex interaction between seismic shaking, landslide material, and rainfall. However, the contributions of each component have never been quantified due to a lack of in-situ data for active landslides. We exploited a 3-year geodetic and seismic dataset for a slow-moving landslide in Peru affected by local earthquakes and seasonal rainfalls. Here we show that in combination, they cause greater landslide motion than either force alone. We also show the rigidity of the landslide's bulk clearly decreasing during Ml >= 5 earthquakes. The recovery is affected by rainfall and small earthquakes (Ml < 3.6), which prevent the soil from healing, highlighting the importance of the timing between forcings. These new quantitative insights into the mechanics of landslides open new perspectives for the study of the mass balance of earthquakes.

Résumé: This research study has been developed to preserve a remarkable soda straw population from vibrations emitted by nearby rock blasting. The Choranche stalactite cave (Vercors, France) contains thousands of exceptionally long soda straws reaching a few meters in length for 0.5 cm in diameter. These slender speleothems are very vulnerable to vibrations not only because of their structural fragility but also because of their dynamic amplification. We found that soda straws' first natural frequencies lie within the frequency range emitted by nearby rock blasting works (ten to hundreds of Hz). We used in situ blast records, laboratory characterization, and a dynamic 2D finite element code to simulate the load experienced by the soda straw population. We show that induced loads may be increased by a factor 5 due to resonance. Consequently, short soda straws (0.1-1 m) were found more vulnerable to nearby blasting vibrations than longer speleothems (> 1 m), despite greater own weight and inertial forces for the latter. Simulations made on several blast tests yielded an admissible 2.4 mm/s peak particle velocity along the cave. Subsequent blasting works were carried out in compliance with this threshold with no harm done to the soda straw population. We also computed the dynamic response of soda straws exposed to a regional earthquake. In this case, longer speleothems (> 1 m) are primarily affected by resonance because of ground motion lower frequency content. We show that the omission of dynamic resonance or its simplification as made in previous studies may significantly underestimate the induced load in speleothems.

Résumé: A mercury-resistant bacterial strain has been isolated from a rock of the Idrija mercury mine in Slovenia. The rock had 19 g carbon and 2952 mg mercury (Hg) per kg. Mass spectrometry and DNA sequencing showed that the bacterium belongs to the Pseudomonas genus. It is called Pseudomonas idrijaensis. This bacterial strain is sensitive to methylmercury (MeHg) like the reference P. aeruginosa strain PAO1, and is resistant to divalent mercury (Hg(II)) in contrast to PAO1. This difference could be attributed to the presence of the mer operon yet deprived of the merB gene encoding the organomercurial lyase, on the basis of whole genome sequencing. The P. idrijaensis mer operon displays the RTPCADE organization and is contained in the Tn5041 transposon. This transposon identified here occurs in other Gram-negative Hg-resistant strains isolated from mercury ores, aquatic systems and soils, including Pseudomonas strains from 15,000 to 40,000 years old Siberian permafrost. When P. idrijaensis was exposed to mercury chloride, two intracellular Hg species were identified by high energy-resolution XANES spectroscopy, a dithiolate Hg(SR)(2) and a tetrathiolate Hg(SR)(4) complex. P. idrijaensis had a much higher [Hg(SR)2]/ Hg(SR)(4)] molar ratio than bacteria lacking the mer operon when exposed to 4 μg Hg-2(+)/L-resulting in an intracellular accumulation of 4.3 μg Hg/g dw. A higher amount of the Hg(SR)(2) complex provides a chemical signature for the expression of the dicysteinate Mer proteins in response to mercury toxicity. (C) 2020 Elsevier Ltd. All rights reserved.

Résumé: Unveiling the mechanisms of earthquake and volcanic eruption preparation requires improving our ability to monitor the rock mass response to transient stress perturbations at depth. The standard passive monitoring seismic interferometry technique based on coda waves is robust but recovering accurate and properly localized P- and S-wave velocity temporal anomalies at depth is intrinsically limited by the complexity of scattered, diffracted waves. In order to mitigate this limitation, we propose a complementary, novel, passive seismic monitoring approach based on detecting weak temporal changes of velocities of ballistic waves recovered from seismic noise correlations. This new technique requires dense arrays of seismic sensors in order to circumvent the bias linked to the intrinsic high sensitivity of ballistic waves recovered from noise correlations to changes in the noise source properties. In this work we use a dense network of 417 seismometers in the Groningen area of the Netherlands, one of Europe's largest gas fields. Over the course of 1 month our results show a 1.5 per cent apparent velocity increase of the P wave refracted at the basement of the 700-m-thick sedimentary cover. We interpret this unexpected high value of velocity increase for the refracted wave as being induced by a loading effect associated with rainfall activity and possibly canal drainage at surface. We also observe a 0.25 per cent velocity decrease for the direct P-wave travelling in the nearsurface sediments and conclude that it might be partially biased by changes in time in the noise source properties even though it appears to be consistent with complementary results based on ballistic surface waves presented in a companion paper and interpreted as a pore pressure diffusion effect following a strong rainfall episode. The perspective of applying this new technique to detect continuous localized variations of seismic velocity perturbations at a few kilometres depth paves the way for improved in situ earthquake, volcano and producing reservoir monitoring.

Résumé: Despite their generally low volume fraction, Fe\\Ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation. Once nucleated, bubbles respond fast to pressure changes, fostering rapid expansion and explosive behavior. Yet, oxide microlite quantifications are often absent from data of explosive products. We used new, re-analyzed, and published data to build a compilation of oxide number densities (ONDs) and vesicle number densities (VNDs) of trachytic and calc-alkaline products. Four eruptive styles were selected: 1) Vulcanian explosions from Soufriere Hills volcano, Montserrat, Lascar volcano, Chile, and Kilian volcano, France, 2) blasts from Mt. Pelee volcano, Lesser Antilles, Mount St. Helens, USA, and Merapi volcano, Indonesia, 3) a sub-Plinian explosion from Merapi volcano, and 4) lava dome effusions with intermittent collapse from Soufriere Hills and Mt. Pelee volcanoes. Natural samples were separated into two groups according to the dominant texture of the products of each event: 1) vesicular pumice clasts from explosions with a strong vertical component and 2) dense clasts with diktytaxitic textures from dome collapse event and lateral blast. Group 1 samples are either distributed alongside the 1:1 trend be-tween VND and OND that spans from 1015 to 1017 m-3, or have a constant VND of 1016-16.5 m-3 regardless of OND. A large proportion of oxides (55-100%) touch vesicles. A more variable proportion of vesicles (16-72%) are in contact with oxides because of syn-explosive growth and coalescence. Group 2 samples have ONDs in the same broad range as group 1 samples. We also used new and published data to build a compilation of ONDs and VNDs of five series of experimental decompression of rhyolitic and phonolitic melts. In samples with OND approximate to VND, most bubbles are in contact with more than one oxide and 64-88% of the oxides are in contact with bubbles. Such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated. When VND >= OND, nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models, which we used to calculate syn-explosive decompression rates from VNDs at Merapi, Soufriere Hills, and Kilian. These rates and textural evidence suggest that the decompression front accompanying these Vulcanian and sub-Plinian explosions is responsible for syn-explosive bubble nucleation. We calculated the average pre-explosive ascent rates necessary to yield the observed ONDs at Soufriere Hills and Merapi volcanoes. The resulting rates, 0.005-20 m/s, overlap considerably with the range of critical ascent rate inferred for the effusive-explosive transition, supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities. Focusing on the older eruption of Kilian, for which no observation is available, we infer that pre-explosive ascent rates of >7 x 10(-3) m/s were necessary for explosive behavior to occur. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Chromite foundry sands, mixed with binding resins, are employed in the industry to form molds for high demanding casting of metals and steel. As there is no substitute, these sands highly contribute to placing chromium at the top value of the economic importance parameter in the EU classification of critical raw materials. Finding new sources to produce these sands can contribute to lowering its criticality. Chromite foundry sands must meet strict quality parameters, referred to as Cr(2)O(3)content, Fineness Index, SiO(2)content, and Acid Demand. The foundry chromite market is dominated by South Africa production deriving from layered intrusion chromite deposits. Chromite sands from ophiolite chromite deposits, normally used for the metallurgical-grade chromite market, were tested as an alternative starting raw material to produce chromite foundry sands. The study of the silicate impurities assemblage showed that its mineralogy strongly affects the result of the most crucial parameter, the Acid Demand. Ophiolite chromite with serpentine impurities should be depurated to a hardly affordable 0.31% SiO(2)content to meet Acid Demand quality threshold, due to high reactivity of this silicate with the acid environment of the test. Those with olivine impurities require to be depurated to a much more easily affordable 2.11% SiO(2)content. As a result, ophiolite chromite with an olivine dominated silicate assemblage can be used as an alternative source of chromite foundry sands.

Résumé: Magmatic reservoirs located in the upper crust have been shown to result from the repeated intrusions of new magmas, and spend much of the time as a crystal-rich mush. The geometry of the intrusion of new magmas may greatly affect the thermal and compositional evolution of the reservoir. Despite advances in our understanding of the physical processes that may occur in a magmatic reservoir, the resulting architecture of the composite system remains poorly constrained. Here we performed numerical simulations coupling a computational fluid dynamics and a discrete element method in order to illuminate the geometry and emplacement dynamics of a new intrusion into mush and the relevant physical parameters controlling it. Our results show that the geometry of the intrusion is to first order controlled by the density contrast that exists between the melt phases of the intrusion and resident mush rather than the bulk density contrast as is usually assumed. When the intruded melt is denser than the host melt, the intrusion pounds at the base of the mush and emplaced as a horizontal layer. The occurrence of Rayleigh-Taylor instability leading to the rapid ascent of the intruded material through the mush was observed when the intruded melt was lighter than the host one and was also unrelated to the bulk density contrast. In the absence of density contrasts between the two melt phases, the intrusion may fluidize the host crystal network and slowly ascend through the mush. The effect of the viscosity contrast between the intruded and host materials was found to have a lesser importance on the architecture of intrusions in a mush. Analyzing the eruptive sequence of well documented eruptions involving an intrusion as the trigger shows a good agreement with our modeling results, highlighting the importance of specifically considering granular dynamics when evaluating magmas and mush physical processes. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: The Variscan vein-type Panasqueira W-Sn(Cu) deposit, one of the main tungsten deposits in Western Europe, has a long and complicated geological history. The first vein infillings, which consist of the quartz-wolframite association as well as the first generation of topaz, underwent significant deformation. As a consequence, most fluid inclusions of the earliest hydrothermal event are deformed and destroyed. Two preserved fluid inclusion assemblages are, however, found in the topaz overgrowth band and are dense aqueous-carbonic inclusions as well as dense CO2 dominated fluid inclusions. The P-T conditions of fluid trapping are constrained by using the intersection between isochores, as well as graphite-water equilibrium data and yield the following trapping conditions: 500 20 degrees C and 250 +/- 20 MPa. These P-T conditions are incompatible with fluid unmixing. Fluid chemistry results from water-graphite equilibrium, probably in metapelites, at two distinct temperatures: around 450-500 degrees C for the predominant aqueous-carbonic fluid, and higher temperatures of maximal550 degrees C for the CO(2)rich fluid enriched in N-2. These P-T estimates are consistent with deep crustal levels around 8-10 km depth and a high geothermal gradient around c. 60 degrees C/km(-1). The ascending non-magmatic fluids, enriched in volatiles, are essential in the ore genesis. The high thermal gradients may be related either to new magma pulse after the formation of the Panasqueira granite intrusion or to anomalous heat flux produced by the hot fluids ascending from migmatitic levels present at greater depth. This hypothesis necessitates to consider the role of a crustal weakness, which is attested both by the successive intrusions of several granitic magmas at the same place, and the presence of inherited quartz filled structures so-called Seixo-Bravo found only in the Panasqueira area.

Résumé: The process of primary migration, which controls the transfer of hydrocarbons from source to reservoir rocks, necessitates the existence of fluid pathways in low permeability sedimentary formations. Primary migration starts with the maturation of organic matter which produces fluids that increase the effective stress locally. The interactions between local fluid production, microfracturing, stress conditions, and transport remain difficult to apprehend in shale source rocks. Here, we analyze these interactions using a coupled hydro-mechanical numerical model based on the discrete element method. The model is used to simulate the effects of fluid production emanating from kerogen patches contained within a shale rock alternating kerogen-poor and kerogen-rich layers. We identify two microfracturing mechanisms that control fluid migration: (i) propagation of hydraulically driven fractures induced by kerogen maturation in kerogen-rich layers and (ii) compression-induced fracturing in kerogen-poor layers caused by fluid overpressurization of the surrounding kerogen-rich layers. The relative importance of these two mechanisms is discussed considering different elastic properties contrasts between the shale layers, as well as various stress conditions encountered in sedimentary basins, from normal to reverse faulting regimes. The layering in shales causes local stress redistribution that controls the prevalence of each mechanism over the other and the onset of microfracturing during kerogen maturation. The approach is applied to the Draupne formation, a major source rock in the Norwegian continental shelf in the North Sea.

Résumé: Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. Here, we present the first record of an animal fossil preserved in opal formed by weathering with such high-resolution details that even individual cuticle hairs are observed. The fossil consists of the exoskeleton of a nymphal insect belonging to the order Hemiptera and either the family Tettigarctidae or the Cicadidae. This identification is based on anatomical details such as the tibial and femoral morphology of the forelegs. The exoskeleton of the insect was primarily zeolitized during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the continental weathering of the volcanic host rocks. Organic matter is preserved in the form of amorphous carbon. This finding makes opal formed by rocks weathering a new, complementary source of animal fossils, offering new prospects for the search for ancient life in the early history of Earth and possibly other terrestrial planets such as Mars, where weathering-formed opal occurs.

Résumé: An examination of repeating earthquakes in eastern Taiwan revealed previously unrecognized quasiperiodic repetition of aseismic creep along two reverse faults in an active suture zone. Using 202 M-L 2.0 to 4.6 repeating earthquake sequences (RES) during the period from 2000 to 2011, we studied where and how certain faults creep. The RES were found to be highly concentrated in the southern segment of the Longitudinal Valley fault (LVF) and in the northern segment of the Central Range fault (CRF). They are mainly located at a depth of 10-25 km and show strong regional differences in creep behavior. Using the moment release rate of RES and geodetically derived long-term slip rate, we re-estimate the empirical relationship between deep creep and seismic moment for creeping sections in eastern Taiwan. For the 30-km-long LVF, the creep rate increased dramatically from 1.5 to 12.3 cm/year under the influence of the M-L 6.4 Chengkung earthquake of 2003. For the 80-km-long CRF, the high creep rate of 4.3 cm/year appears to have been stable over time and is descriptive of a previously unrecognized deep structure underneath the eastern flank of the Central Range. The quasiperiodic pulsing of the deep slip rate has a predominant interval of 1 year for both segments. After the M-L 6.4 event, the predominant interval for the creeping LVF halved in duration. The time-dependent aseismic slip showed a strong correlation with the creepmeter data, suggesting that the positing of a common mechanism is needed to connect the surface and deep creep variation.

Résumé: Crackling dynamics is characterized by a release of incoming energy through intermittent avalanches. The shape, i.e., the internal temporal structure of these avalanches, gives insightful information about the physical processes involved. It was experimentally shown recently that progressive damage toward compressive failure of quasibrittle materials can be mapped onto the universality class of interface depinning when considering scaling relationships between the global characteristics of the microcracking avalanches. Here we show, for three concrete materials and from a detailed analysis of the acoustic emission waveforms generated by microcracking events, that the shape of these damage avalanches is strongly asymmetric, characterized by a very slow decay. This remarkable asymmetry, at odds with mean-field depinning predictions, could be explained, in these quasibrittle materials, by retardation effects induced by enhanced viscoelastic processes within a fracture process zone generated by the damage avalanche as it progresses. It is associated with clusters of subavalanches, or aftershocks, within the main avalanche.

Résumé: A colonial chronicle written by the indigenous Peruvian author Pachacuti Yamqui Salcamaygua ([1613?]) relates a legend of the sudden appearance of a huge animal – kilometres in length and approximately 4 m in width – and described as the Andean snake-like deity amaru. Pachacuti Yamqui alleged that this fantastic event occurred on the day that the sovereign Pachacuti Inca Yupanqui's eldest son was born around 1440 CE, and was named “Amaru”. We suggest that the underlying event was an earthquake, and that the propagation of the surface rupture across the landscape resembled a sudden appearance of a snake-like being wriggling over the mountains and leaving an undulating surface trace. The concordance between the snake's route and the layout of a major fault complex above Cusco, as well as several ethnographic testimonies, support this hypothesis. Although little is known about pre-1532 CE seismicity, the current tectonic settings of the Cusco area point to seismic awareness of the Incas (ca. 1300-1532 CE). Independent results from architectural and paleoseismological fields in the Cusco area corroborate a significant impact of large earthquakes on local societies. In Peru, without pre-Hispanic written sources, the oral folklore and traditions preserved in Spanish chronicles offer a relevant, but still underexploited resource for identifying paleo-extreme events. Combining multidisciplinary geomorphic observations, archaeological evidence and historical sources, we revisit this legendary episode and its possible implications.

Résumé: Various species of marine mussels have been used, in the last 50 years, as sentinel organisms for monitoring metal contamination along marine coasts. There are two main reasons for this: these mollusks concentrate metals in their soft tissue and they are geographically widespread. In practice, trace metal concentrations in mussel soft tissue reveal (after some correction for biotic effects) the contamination level of their surrounding environment. We present the results of a mercury (Hg) survey in Mytilus spp. collected in the summers of 2016, 2018, and 2019 at 51 stations distributed along the coasts of the Estuary and Gulf of St. Lawrence. Mercury concentrations ranged from 0.063 to 0.507 μg g(-1) (dry weight, dw), with a grand mean of 0.173 +/- 0.076 μg g(-1) dw (+/- 1 standard deviation), and a median of 0.156 μg g(-1) dw for the 504 individuals analyzed. Mercury contents per individual mussel were significantly (p < 0.01) related to shell length and dry tissue weight, with the smaller individuals having the highest Hg concentrations. To take into account these biotic effects, we normalized Hg concentrations of the mussel soft tissue for constant shell length (L) and soft tissue weight (TW) based on the log-log relationships between Hg content and L or TW. The normalized Hg contents of mussels varied from 10.9 to 66.6 ng per virtual individual of 35 mm length and 0.17 g dry weight. A similar normalization procedure applied to 1977-1979 data, yielded a very similar range: 12 to 64 ng. This observation suggests that the Hg bioavailable to marine mussels in the study area did not change over a span of 40 years. Regional Hg distribution patterns indicate a gradual decrease of Hg content in mussels downstream from freshwater discharges to the St. Lawrence Estuary and the Baie des Chaleurs, suggesting that rivers constitute a significant Hg source in these estuarine systems. Atmospheric Hg deposition and concentration in marine waters of the Atlantic Ocean are known to have decreased in the last decades. However, in coastal environments, the response to these changes does not seem to be rapid, probably because of the long residence time of Hg in soils before being exported to coastal areas.

Résumé: The heterogeneous South American geology has coined a wide variety of neotectonic settings where crustal seismogenic sources do occur. This fact has led to different approaches for mapping and inventory neotectonic structures. The South American Risk Assessment project promoted the discussion and update under uniform standards of the available information on neotectonic deformation, for its application in regional Probabilistic Seismic Hazard Assessments. As a result, 1533 hazardous faults have been inventoried onshore South America, 497 of them qualifying to feed the engine model driving probabilistic maps. Main hazardous structures are concentrated throughout the eastern boundary of the Northern Andean Sliver and along the foreland-facing Andean Thrust Front. Space geodesy and seismicity illuminate the seismogenic significance of these deformation belts, although few neotectonic surveys have been conducted to date in the latter region. The characteristics of the main structures or deformation zones are here outlined according to their filiation to neotectonic domains, which are dependant on the geologic, seismotectonic, or morphotectonic settings in Andean and extra-Andean regions. The knowledge accrued on the hazardous faults in South America here compiled, reinforces the fact that some of these structures constitute significant hazard sources for many urban areas and critical facilities and should be incorporated in seismic hazard assessments. However, the available fault data are insufficient in many cases or carry significant epistemic uncertainties for fault source characterization. This contribution aims to summarize the present knowledge on the South American hazardous faults as well as the main challenges for successful fault data incorporation into seismic hazard models.

Résumé: The western limb of the Hellenic Arc defines the boundary of a large intracontinental active extensional domain covering the Aegean and the southwestern part of the Balkan peninsula. Along this boundary a transition from collision in the north to subduction in the south is associated with post-Miocene clockwise rotations of similar to 50 degrees. We present a new GPS velocity field that, with new permanent station velocities in Albania, Bulgaria, Kosovo, Montenegro, and Northern Macedonia, provides insights into previously unresolved aspects on the large-scale dynamics of continental lithosphere and on the relation between long and short-term kinematics. In particular we address (1) the kinematic description of the collision/subduction transition, (2) the relation between long-term finite rotations with geodetically-measured instantaneous rotations, (3) the forces maintaining and resisting the deformation of the extensional domain and (4) the extent of its northern boundary. We use the analogy of rigid elongate inclusions in the velocity field to predict the senses and approximate rates of rotation of crustal blocks in the deforming continental region. Across the collision/subduction transition zone a large rotational pattern in the velocity field is found whose spatial pattern and sense of rotation is compatible with the observed paleomagnetic rotations through the occurrence of fault-bounded block rotations. Geodetic observations show that clockwise vertical axis rotation of the western limb of the Hellenic margin is active today, with distribution and rates that are essentially the same as that defined by the paleomagnetic data for the last 5 Ma. The maximum rates of the rotational component of the velocity field is found along the wide arrangement of fault-bounded blocks in central Greece that accomodate the NE-SW dextral shear transferred from the northern Aegan trough. The northern limit of the rotating margin defines a hinge (Scutari-Pec transverse zone) that has remained stationary throughout the Middle Miocene (relative to the upper plate of the collision/subduction system) with important implications for the distribution of strength of the western margin of the Hellenic Arc. The distribution and style of deformation along the western boundary of the extensional domain is controlled by the relation between gravitational forces, driving the flow of crustal material towards the low-lying adjacent regions sea floor, and heterogeneous resistive forces along the collision/subduction boundary. (c) 2020 Elsevier B.V. All rights reserved.

Résumé: InSAR time series of surface deformation from 16 yr of Envisat (2003-2011) and Sentinel-1 (2014-2019) ESA satellite radar measurements have been constructed to characterise spatial and temporal dynamics of ground deformation over an 80,000 km(2) area in the permafrost of the northeastern Tibetan Plateau. The regional deformation maps encompass various types of periglacial landforms and show that seasonal thaw effects are controlled by the sediment type and local topography. High seasonal ground movements are concentrated on shallow slopes and poor-drainage areas in unconsolidated, frost-susceptible and fine-grained sediments within glacier outwash plains, braided stream plains, alluvial deposits or floodplains. Fast subsidence due to thaw settlement takes place during June/July while frost heave is intense during December/January when two-sided freezing of pore water under pressure causes prolonged ice segregation near the permafrost table. The analysis reveals pervasive subsidence of the ground of up to similar to 2 cm/yr, and increasing by a factor of 2 to 5 from 2003 to today, in high-relief and well-drained areas. The findings suggest that seasonal thaw increasingly affects ice-rich layers at the permafrost table, as well as high-rates of widespread mass movements of non-consolidated sediments, the latter amplified by an increase of effects from frost heave/thaw settlement. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: We describe a late Miocene to early Pliocene axial drainage system in the East Carpathian foreland, which was an important sediment supplier to the Black Sea and the Dacian Basin. Its existence explains the striking progradation of the northwest Black Sea shelf prior to the onset of sediment supply from the continental-scale Danube River in the late Pliocene to Pleistocene. This axial drainage system evolved due to the diachronous along-strike evolution of the Carpathians and their foreland; continental collision, overfilling, slab breakoff, and subsequent exhumation of the foreland occurred earlier in the West Carpathians than in the East Carpathians. After overfilling of the western foreland, excess sediment was transferred along the basin axis, giving rise to a 300-km-wide by 800-km-long, southeast-prograding river-shelf-slope system with a sediment flux of similar to 12 x 103 km3/m.y. Such late-stage axial sedi-ment systems often develop in foreland basins, in particular, where orogenesis is diachronous along strike. Substantial lateral sediment transport thus needs to be taken into account, even though evidence of these axial systems is often eroded following slab breakoff and inversion of their foreland basins.

Résumé: The central part of the Western Andes holds an exceptional concentration of giant paleolandslides involving very large volumes of rock material (v > km(3)). While those gravitational slope failures are interpreted consensually as an erosional response to the geodynamic activity of the Andes (relief formation and tectonic activity), the question of their triggering mechanisms remains enigmatic. To clarify the respective roles of climatic versus seismic forcing on the Andean landslides, new temporal constraints on paleo movements are essential. Here, we focus on one of those giant slope failures, the Aricota giant rockslide that damned the Locumba valley in southern Peru. We conducted fieldwork, high-resolution DEM analysis and cosmogenic nuclide dating to decipher its development history and failure mechanisms. Our results point to the occurrence of two successive events. A giant failure mobilizing a rock volume of ca. 2 km(3) first produced a dam at 17.9 +/- 0.7 ka. Considering its height of ca. 600 m, the Aricota rockslide dam is one of the three largest landslide dams worldwide. At 12.1 +/- 0.2 ka, a second event produced ca. 0.2 km(3) of material, and the rock-avalanche debris spread out over the dam. As the chronology of those two events is pointing to the two main paleoclimatic pluvial periods in this region (Heinrich Stadial 1a and Younger Dryas), we favor the interpretation of a climatic forcing. At a regional scale, the concomitant aggradation of alluvial terraces and fan systems along the nearby valleys highlights higher regional erosion, sediments supply and mass-wasting events during those paleoprecipitation events and strengthens this conclusion. (C) 2019 Elsevier B.V. All rights reserved.

Résumé: The impact of non-linear soil behavior on site response may be described by the non-linear to linear site response ratio RSRNL introduced in Regnier et al. (2013). This ratio most often exhibits a typical shape with an amplitude above one below a site-specific frequency f(NL), and an amplitude below one beyond f(NL). This paper presents an investigation of the correlation between this RSRNL ratio and various parameters used to characterize the site (Site Condition Proxies: SCPs) and the seismic loading level (Ground Motion Intensity Measures: GMIMs). The data used in this analysis come from sites of the Japanese Kiban-Kyoshin (KiK-net) network, for which the nonlinear to linear site-response ratio (RSRNL) is obtained by comparing the surface/down-hole Fourier spectral ratio for strong events and for weak events. The five SCPs are Vs30, the minimum velocity of the soil profile(Vs(m)(in)), an index of the velocity gradient over the top 30 m (B-30), the fundamental frequency f(OHv), as measured from the H/V earthquake ratio, and the corresponding amplitude Amw. The seven GMIMs are PGA, PGV, PGV/V-s30 (peak strain proxy), I-A (Arias Intensity), CAV (Cumulative Absolute Velocity), a(rms), (Root Mean Square Acceleration) and Trifunac-Brady Duration (D-T). The original data set consists of a total of 2927 RSRNL derived from KiK-net recordings at 132 sites. To assign an equal weight to each site, and to avoid any bias linked to sites with many recordings, for each GMIM, this original data set is grouped in 15 different intervals corresponding to fixed fractiles of the statistical distribution of the considered GMIM (every 10% from F10 to F50, and every 5% from F55 to F100). In each group, the average RSRNL Gm for each site is computed. For each of these seven advanced data sets, a neural network approach is used to predict the behavior of RSRNL Gm as a function of the corresponding GMIM, and one or two SCPs. The performance of each model is quantified through the average variance reduction coefficient mu(Rc) in a fixed frequency range. This sensitivity study is performed in the normalized frequency (f/f NL ) domain to identify the best combinations (GMIM, SCPs) providing the largest variance reduction, and then in the absolute frequency domain for the final optimal combination. The optimal combinations [GMIM, two-SCPs] are triplets [PGV/V-s30, V-s30 f(0HV); mu(Rc) = 18.6%], [PGV/V-s30, V-s30 A(0HV); mu(Rc) = 18.16%], [PGV, V-s30 -f(0HV); mu(Rc) = 17.3%] and [PGA, B-30 -A(0HV); mu(Rc) = 17.2%]. The final absolute frequency model with the best triplet makes it possible to predict the non-linear response of a given site knowing its linear, weak-motion response, and two site proxy parameters, for wide ranges of the considered ground motion parameters.

Résumé: Hydrogen gas is seeping from the sedimentary basin of Sao Franciso, Brazil. The seepages of H(2)are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoic basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H-2. Alternatively, ultramafic rocks that may have generated H(2)during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambui group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H(2)accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambui formation to the surface. The fluxes of H(2)escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H(2)production model in the Precambrian continental crust.

Résumé: Knowledge of the elastic properties and equations of state of iron and iron alloys are of fundamental interest in Earth and planetary sciences as they are the main constituents of telluric planetary cores. Here, we present results of X-ray diffraction measurements on a ternary Fe-Ni-Si alloy with 5 wt% Ni and 5 wt% Si, quasi-hydrostatically compressed at ambient temperature up to 56 GPa, and under simultaneous high pressure and high temperature conditions, up to 74 GPa and 1750 K. The established pressure dependence of the c/a axial ratio at ambient temperature and the pressure-volume-temperature (P-V-T) equation of state are compared with previous work and literature studies. Our results show that Ni addition does not affect the compressibility and axial compressibility of Fe-Si alloys at ambient temperature, but we suggest that ternary Fe-Ni-Si alloys might have a reduced thermal expansion in respect to pure Fe and binary Fe-Si alloys. In particular, once the thermal equations of state are considered together with velocity measurements, we conclude that elements other than Si and Ni have to be present in the Earth's inner core to account for both density and seismic velocities.

Résumé: On 7 December 2015, a shallow M-w 7.2 strike-slip earthquake struck the Murghab River Valley in the Central Pamirs of Tajikistan. Seismologically this event was similar to a large seismic event in 1911 whose causative fault has never been identified. We measure the displacement field of the 2015 event from satellite observations using Sentinel-1 radar interferometry, Landsat-8 optical pixel-tracking, and surface rupture mapping from high resolution SPOT-6/7 imagery to characterize the role this earthquake rupture plays in the accommodation of strain on its causative structure, the Sarez-Karakul fault. We present geomorphic mapping and interpretations o f other Quaternary-active reaches of this fault system, which highlight variable rupture history of the different sections. These sections appear to be separated by inherited bedrock structural boundaries. Significantly, the reaches of the fault northeast and southwest of the 2015 rupture exhibit the freshest morphology prior to 2015, indicative of a more recent rupture than elsewhere. Using new high resolution imagery we map fresh scarps at the northern and southern ends of the Sarez-Karakul fault which may represent this 1911 rupture. To test which of these reaches could have been the source of the elusive 1911 event, we compare synthetic seismograms from three plausible fault sources determined from geomorphology, with observed seismic traces from 1911 at early recording stations throughout Europe. We find that the best fitting fault source is in fact southwest of the 2015 rupture, meaning that we have a record of three distinct recent events on the Sarez-Karakul fault system-two of them instrumentally recorded. Our mapping of these separate events reveals a correlation between their boundaries and the active and inherited thrust and suture systems that intersect the northeast striking left-lateral fault, suggesting structural control over the extents of individual ruptures on the active strike-slip fault.

Résumé: Low-frequency earthquakes (LFEs) are detected within tremor, as small, repetitive, impulsive low-frequency (1-8Hz) signals. While the mechanism causing this depletion of the high-frequency content of their signal is still debated, this feature may indicate that the source processes at the origin of LFEs are different from those for regular earthquakes. Key constraints on the LFE-generating physical mechanisms can be obtained by establishing scaling laws between their seismic moment and source durations. Here we apply a simple spectral analysis method to the S waveforms of LFEs from Guerrero, Mexico, to measure their seismic moments and corner frequencies, a proxy to source duration. We find characteristic values of M-0 similar to 3x10(12) N.m (M-w similar to 2.3 ) and f(c)similar to 3.0 Hz with the corner frequency very weakly dependent on the seismic moment. This moment-duration scaling observed for Mexican LFEs is similar to one previously reported in Cascadia and is very different from the established one for regular earthquakes. This suggests that they could be generated by sources of nearly constant size with strongly varying intensities. LFEs do not exhibit the self-similarity characteristic of regular earthquakes, suggesting that the physical mechanisms at their origin could be intrinsically different. Plain Language Summary Low-frequency earthquakes are unusual, small earthquakes that are detected on the deep end of plate boundary faults. They occur during periods of slow slip, when the plates start sliding against each other much slower than during earthquakes, but long enough to release the energy that might otherwise have caused large earthquakes. Thus, they carry precious information about the fault behavior while slow slip is active. In this study, we analyze a selection of low-frequency earthquakes from a slow slip zone of the Mexican subduction, beneath the state of Guerrero. We find that their magnitude is on average higher than in other regions where they have been detected (M(w)1.5 – 3). We also find that whatever the magnitude of these events, they always last about 0.3 s. On the contrary, regular earthquakes are known to last longer when they grow larger, and low-frequency earthquakes in Nankai (Japan) appear to behave similarly. We discuss a physical mechanism explaining such a discrepancy with regular earthquakes and such strong regional variations. The implication of crustal fluids circulating at extremely high pressure in between plates might be one of the key components of this mechanism, as it often is for slow slip processes.

Résumé: Imogolite, a nanotubular aluminosilicate mineral, is commonly found in volcanic soils, where it exerts a control on carbon dynamics. Synthetic imogolites are used for the removal of contaminants from industrial effluents and are considered for a range of other applications including gas adsorption and functionalised heterogeneous catalysts. In spite of their environmental and industrial relevance, the properties of imogolite-water interfaces remain poorly understood. Here, an experimental and computational study is presented in which the structure and energetics of water are characterized on the curved external surface of imogolite and the hydrophilicity of this surface is contrasted with that of gibbsite, its planar counterpart. Atomic force spectroscopy experiments show that in spite of their identical surface structure, imogolite has a lower hygroscopicity than gibbsite. Molecular dynamics simulations provide an explanation for this observation: the curvature of imogolite prevents the formation of in-plane H-bonds along the directions of the nanotube circumference, lowering the enthalpy of adsorption of water molecules. The different arrangement of surface H-bonds and the resulting differences in hydration properties also affects the acidity constants of surface hydroxyl groups. This 'nanotube effect' may be relevant to other nanotubular systems with high curvatures, potentially impacting their wetting properties, their colloidal stability and their affinity towards hydrophobic organic moieties.

Résumé: In late June 2016, the Harmaliere clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-yr-long period of continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 2 x 10(6) m(3). Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic metres. For 4 months, they continuously recorded seismic ambient vibrations and microcarthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (f(B)), the relative variation in Rayleigh wave velocity (dV/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material.

Résumé: The mechanics and seismogenic behavior of fault zones are strongly influenced by their internal structure. In this perspective, the internal structure of the extensional seismically active Vado di Corno Fault Zone (VCFZ, Central Apennines, Italy) was quantified by combining high-resolution structural mapping with 3-D fault network analysis over similar to 2 km along fault strike. The fault zone was exhumed from similar to 2 km depth in carbonate rocks, accommodated 1.5-2 km of extensional throw since Early Pleistocene, and cuts through the Pliocene Omo Morto Thrust Zone (OMTZ) with partial reactivation in extension. The exceptional exposure of the footwall block allowed us to reconstruct in detail the geometry of the OMTZ and quantify the spatial arrangement of master/subsidiary faults and fault zone rocks within the extensional VCFZ. The combination of the structural map and the 3-D fault network with kinematic and topological analyses pointed out the crucial role of the older thrust geometry (i.e., lateral ramps) in controlling the along-strike segmentation and slip distribution of the VCFZ. These observations were discussed in the framework of regional extension through a slip tendency analysis and a simplified mechanical model, which suggest the activation of oblique inherited structures during the lateral propagation of the VCFZ segments. The interaction of the VCFZ with the OMTZ generated along strike and possibly downdip mechanical asperities. Considering the exhumed VCFZ as an analog for the shallow structure of other seismic sources in the Central Apennines, similar settings could play first-order control on the spatio-temporal evolution and rupture heterogeneity of earthquakes in the region.

Résumé: After acquiring 91 km(2) of lidar data from the Zacapu region, West Mexico, we confronted a series of issues that most archaeologists using this technology face. These include the large volume of data available, the limited training of potential “analysts,” the difficult development of a collective mapping tool and protocol, and the reliability of desk-based interpretation of archaeological features. In this article, we present an initiative conducted in 2015 and 2017 as an attempt to answer these methodological and pedagogical issues. We developed a web mapping platform to collectively interpret archaeological features using lidar-derived imagery and to train volunteer students to participate in this desk-based web mapping within a crowdsourcing framework. After evaluating the results of this initiative, we discuss the potential and limitations of this method for both lidar-based research and future training.

Résumé: Surface deformation measured by geodetic data is the sum of single-strain sources deforming at depth. A combination of volume changes from several analytical models (e.g. a point source or dislocation along a plane) can be used to model the different sources. However, solving for the best fit of volume variations, dislocations, position and orientation parameters of all sources is a nonlinear problem, and its solution is generally non-unique. This problem can be converted into a linear one by assimilating the sum of sources to a simplified model formed by three orthogonal planes of dislocations at fixed position and orientation. This strain source model is equivalent to having all neighbouring deformation sources contained in a small size volume. The determination of the strain tensor components can be performed by inverting geodetic data. Because of their high resolution, tiltmeters are well adapted to survey shallow deformation of volcanoes and geological reservoirs. However, they are known to display unknown long-term drift. We propose an approach to jointly estimate the temporal evolution of the strain source and time-dependent instrumental parameters. We verify the approach using synthetic data, giving confidence intervals for each component of the strain tensor. Finally, we link geological information to the internal deformation by interpreting the strain tensor as principal directions of deformation. This approach seems promising for the identification of fracture onset and fault reactivation in geothermal, hydrocarbon exploitations or volcanic systems.

Résumé: Long-period earthquakes and tremors, on a par with volcano-tectonic earthquakes, are one of two main classes of volcano-seismic activity. It is believed that long-period volcanic seismicity is associated with pressure fluctuations in the magmatic and hydrothermal systems beneath volcanoes and can therefore be used as a precursor of the impending eruptions. At the same time, the physical mechanism of the long-period seismicity is still not fully understood. In this work, we have studied the long-period earthquakes that occur at the crust-mantle boundary beneath the Klyuchevskoi volcanic group in Kamchatka in order to establish their recurrence law-the relationship between the magnitude and frequency of occurrence of the events. In the region under study, the earthquakes pertaining to this type are most numerous and characterize the state of the deep magma reservoir located at the crust-mantle boundary. The changes in the seismic regime in this part of the magmatic system can be one of the early precursors of eruptions. For a more thorough characterization of the frequency-magnitude relationship of the discussed events, we compiled a new catalog of the deep long-period earthquakes based on the matched-filter processing of continuous seismograms recorded by the network stations of the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences in 2011-2012. For these earthquakes, we also used a magnitude determination method that provides the estimates close to the moment magnitude scale. The analysis of the obtained catalog containing more than 40 000 events shows that the frequency-magnitude relationships of the earthquakes markedly deviate from the Gutenberg-Richter power-law distribution, probably testifying to the seismicity mechanism and peculiarities of the sources that differ from the common tectonic earthquakes. It is shown that the magnitude distribution of the deep long-period earthquakes is, rather, described by the distributions with characteristic mean values such as the normal or gamma distribution.

Résumé: We present a measurement of K-shell transitions in H-like gold (Au78+) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high-resolution wavelength-dispersive measurement of a K-shell transition in a high-Z H-like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.

Résumé: The 14 July 2013 Vulcanian explosion at Tungurahua occurred after two months of quiescence and was extremely powerful, generating some of the highest infrasound energies recorded worldwide. Here we report on how a combination of geophysical data, textural measurements, and physical and mechanical tests on eruptive products allowed us to determine the processes that led to the pressurization of the conduit and triggering of this large Vulcanian event. Two weeks prior to the 14 July event, daily seismic counts and radial tilt began to steadily increase, indicating the probable intrusion of a new batch of magma into the edifice. The 14 July explosion produced three different juvenile products that were each sampled for this study: airfall; juvenile, vesicular, pyroclastic density current material; and dense plug rocks in the form of ballistic ejecta. Feldspar microlite textures and vesicle size distributions were measured, and used jointly with a two-step recompression model to characterize the spatial distribution of gas in the conduit and the decompression histories of the erupted samples. Model results reveal a vertically stratified conduit with regards to porosity, crystallinity and volatile content prior to the explosion. Overall, our data suggests a complex sequence of events that led eventually to this powerful explosion. 1) Remnants of magma from the previous eruptive phase in May stalled in the shallow conduit, triggering crystallization which, coupled with efficient outgassing, formed a dense (< 2% porosity), highly crystalline plug. This plug had a very low matrix permeability (10(-17) to 10(-18) m(2)) and high tensile strength (9 to 13 MPa), forming an efficient rigid seal and preventing significant outgassing from the conduit. 2) Just below the plug, a high porosity zone (up to 50%) formed, acting as a gas storage zone, with pressurization occurring partly under closed-system degassing. 3) The shallow conduit became pressurized due to the combination of both gas accumulation and the resistance of the plug to magma column extrusion in response to a new influx of magma. 4) On the 14th of July a critical gas over-pressure was reached, overcoming the strength of the dense plug of magma, trigging extreme decompression and evacuation of the top two kilometres of the conduit. 5) The newly intruded magma was not directly involved in the explosion and continued to ascend during the week following the explosion. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: It has been previously shown that the diffraction quality of protein crystals strongly depends on mass transport during their growth. In fact, several studies support the idea that the higher the contribution of the diffusion during mass transport, the better the diffraction quality of the crystals. In this work, we have compared the crystal quality of two model (thaumatin and insulin) and two target (HBII and HBII-III) proteins grown by two different methods to reduce/eliminate convective mass transport: crystal growth in agarose gels and crystal growth in solution under microgravity. In both cases, we used identical counterdiffusion crystallization setups and the same data collection protocols. Additionally, critical parameters such as reactor geometry, stock batches of proteins and other chemicals, temperature, and duration of the experiments were carefully monitored. The diffraction datasets have been analyzed using a principal component analysis (PCA) to determine possible trends in quality indicators. The relevant indicators show that, for the purpose of structural crystallography, there are no obvious differences between crystals grown under reduced convective flow in space and convection-free conditions in agarose gel, indicating that the key factor contributing to crystal quality is the reduced convection environment and not how this reduced convection is achieved. This means that the possible detrimental effect on crystal quality due to the incorporation of gel fibers into the protein crystals is insignificant compared to the positive impact of an optimal convection-free environment provided by gels. Moreover, our results confirm that the counterdiffusion technique optimizes protein crystal quality and validates both environments in order to deliver high quality protein crystals, although other considerations, such as protein/gel interactions, must be considered when defining the optimal crystallization setup.

Résumé: Two very dense seismographic arrays were deployed in a seismically active area in Greece to incorporate the difference in amplitude and phase between two stations located within the dimension of a structure. The spatial variability in seismic ground motion is generally attributed to the wave passage effect, the incoherence effect, and the local site effect. It can cause severe damage on lifeline structures. This article studies the behavior of a reinforced concrete 2D frame structure subjected to differential seismic excitation at the supports. Both linear and nonlinear finite multifiber element models of the seismic behavior of this structure are used. The nonlinear behavior of the structure, under these different cases, displays different damage patterns and maximum displacements. This study allows evaluating the uncertainty that can be propagated through the finite element model, aiming at reducing variability for structural design purposes.

Résumé: The rise and fall of human cultures are strongly modulated by the strong environmental changes taking place during the Holocene. Here, we use the sedimentological and geochemical records of a core taken in the Arabian Sea, west of Kerala, to identify potential factors that may reflect on-land history of local civilizations, in particular the Harappan culture which appeared and collapsed in the Indus Valley during the early and middle Bronze Age. The C-14 record highlights a fourfold increase in sedimentation rate at similar to 5380 cal. yr BP. The short duration of this event (similar to 220 years) suggests a steep regional increase in erosion at the beginning of the Bronze Age. Factor analysis of downcore changes in geochemistry identified two distinct detrital components dominated by silt and clay, respectively, and a component characteristic of chemical erosion. This interpretation is consistent with sediment mineralogy. Comparison with the known climatic record indicates that increased erosion rate at 5380 cal. yr BP around the Arabian Sea is because of the advent of farming. The development of tillage associated with both wheat and barley crops and animal husbandry was favored by trade between Mesopotamia and India. Human activities, therefore, were the trigger of major changes in the sedimentological and geochemical records at sea at the onset of the Bronze Age.

Résumé: The Himalayan syntaxes, characterized by extreme rates of rock exhumation co-located with major trans-orogenic rivers, figure prominently in the debate on tectonic versus erosional forcing of exhumation. Both the mechanism and timing of rapid exhumation of the Namche Barwa massif in the eastern syntaxis remain controversial. It has been argued that coupling between crustal rock advection and surface erosion initiated in the late Miocene (8-10 Ma). Recent studies, in contrast, suggest a Quaternary onset of rapid exhumation linked to a purely tectonic mechanism. We report new multisystem detrital thermochronology data from the most proximal Neogene clastic sediments downstream of Namche Barwa and use a thermo-kinematic model constrained by new and published data to explore its exhumation history. Modeling results show that exhumation accelerated to similar to 4 km/m.y. at ca. 8 Ma and to similar to 9 km/m.y. after ca. 2 Ma. This three-stage history reconciles apparently contradictory evidence for early and late onset of rapid exhumation and suggests efficient coupling between tectonics and erosion since the late Miocene. Quaternary acceleration of exhumation is consistent with river-profile evolution and may be linked to a Quaternary river-capture event.

Résumé: The Abdasht and Soghan ultramafic complexes, Southern Iran, host major actively exploited chromitite bodies, variably enriched in PGE. The Platinum Group Minerals (PGM) – Base Metal Minerals (BMM) assemblage was studied in order to assess PGE remobilization during post-magmatic processes. Studied chromitites have variable textures: massive, banded, nodular and disseminated. Primary silicates are almost completely replaced by serpentine. Ferrian chromitization is present but not widespread. BMM are mostly found in the silicate matrix, while PGM are more common as inclusions within chromites. Primary BMM (pentlandite and bornite) within the silicate matrix are poorly preserved, while primary PGM (laurite) are more abundant. Secondary BMM (heazlewoodite, millerite and Ni-Fe alloys) and PGM (PGE-alloys) are the dominant mineralogical species. During serpentinization the pervasive percolation of low fS(2) fluids induced the desulfurization of the metallic assemblage. Primary sulfides were replaced by S-poor (heazlewoodite) or S-free (Ni-Fe alloys, PGE alloys, PGE-BM alloys) phases. Low fS(2) differentially mobilized PGE. The release of IPGE (fr, Os, Ru) into the fluids follows the order: Ru> > Os>Ir. PPGE (Pt, Pd, Rh) mobility is more difficult to assess, due to their small concentration, but the evidence suggests that Rh is more mobile than Ru, and that Pd and Pt were partially added to the system by fluids. A mass balance calculation on a polyphasic grain, showing differential stages of desulfurization, allowed a semi-quantitative analysis on the desulfurization degree of Abdasht and Soghan complexes, estimated, in the more serpentinized portions, between 75 and 100%. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Monomethylmercury (MMHg) concentrations in aquatic biota from Lake Titicaca are elevated although the mercury (Hg) contamination level of the lake is low. The contribution of sediments to the lake MMHg pool remained however unclear. In this work, seven cores representative of the contrasted sediments and aquatic ecotopes of Lake Titicaca were sliced and analyzed for Hg and redox-sensitive elements (Mn, Fe, N and S) speciation in pore-water (PW) and sediment to document early diagenetic processes responsible for MMHg production and accumulation in PW during organic matter (OM) oxidation. The highest MMHg concentrations (up to 12.2 ng L-1 and 90% of THg) were found in subsurface PWs of the carbonate-rich sediments which cover 75% of the small basin and 20% of the large one. In other sediment facies, the larger content of OM restricted MMHg production and accumulation in PW by sequestering Hg in the solid phase and potentially also by decreasing its bioavailability in the PW. Diagenetically reduced S and Fe played a dual role either favoring or restricting the availability of Hg for biomethylation. The calculation of theoretical diffusive fluxes suggests that Lake Titicaca bottom sediments are a net source of MMHg, accounting for more than one third of the daily MMHg accumulated in the water column of the Lago Menor. We suggest that in the context of rising anthropogenic pressure, the enhancement of eutrophication in high altitude Altiplano lakes may increase these MMHg effluxes into the water column and favor its accumulation in water and biota. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: The Southwestern Alps and the Ligurian margin is a region of moderate seismicity with a high rate of small to moderate events. Identifying the active faults in this very densely populated region is critical to better assess the hazard and mitigate the risk. An accurate 3-D velocity model of the shallow to middle crust is a fundamental step to better locate the seismicity, and hence, the faults from which it originates. We performed ambient noise surface-wave tomography based on all available continuous seismological data from the French and Italian permanent networks (RESIF, INGV, RSNI), and current and past temporary experiments (AlpArray, CASSAT, SISVAR, RISVAL). In addition to these available data, we deployed three more stations to improve the spatial resolution in a region with sparse seismic station coverage. Overall, we used 55 inland seismic stations, 5 oceans bottom seismometers and 2 offshore cabled site/sensors. Data span the 2014-2018 time period. Time series from all available components were cross-correlated to reconstruct both Rayleigh and Love-wave Green's functions. For each station-pair Rayleigh and Love group velocity dispersion curves were semi-automatically picked using a frequency-time analysis. Then we regionalize these group velocities to build 2-D Rayleigh and Love velocity-maps between 1.5 and 9 s period. Using a two-step inversion, we estimate the best 3-D shear wave velocity model. The first step is based on a Neighbourhood Algorithm to recover the best three layers' velocity model at each cell of the model. We then use this three-layer model as a starting model in a perturbational method based on finite elements. At periods up to 5 s, the spatial variation of the velocity is well correlated with the effective geology of the area. Lower velocities are observed in areas where the sedimentary cover is thicker, such as the Var and Paillon valley near Nice, or in the subalpine domain in the northwestern part of the region. Higher velocities are retrieved in areas where massifs are present, such as the Argentera-Mercantour massifs in the northeastern, or the Esterel massif in the southwestern part of the region.

Résumé: Recent increases in industrial activity have resulted in elevated chromium (Cr) pollution in the natural environment, from the degradation of concrete and the leakage of water from landfills or mine waste. The release of toxic chromate ions into the environment could have harmful consequences. Methods for Cr removal are therefore becoming increasingly important to control the release of this highly toxic metal into the environment. This study examines how chromate (CrO42-) can be incorporated into a new, stable solid phase at a dissolving calcite surface. To study the relationship between solutions containing chromate and calcite, we performed two series of time-lapse atomic force microscopy (AFM) experiments to identify calcite dissolution and growth rates in the presence of chromate. In addition, we conducted complementary experiments in a stirred flow-through reactor to determine the amount of Cr removal from solution. All experiments were performed at room temperature and under a constant initial pH in the range 6-12. During the AFM experiments, we observed calcite dissolution via etch pit formation and propagation. In the presence of chromate, nanoparticles of a new phase nucleated and grew at the calcite surface by a coupled dissolution-precipitation process. In experiments with concentrations above 10 ppm Cr or at pH 10 and higher, we observed many nanoparticle precipitates, while precipitates were rarely observed at lower concentrations of Cr. At pH 10.5, the precipitates covered the calcite surface. This partly passivated further calcite dissolution, an observation confirmed by the low amounts of Cr removal measured in the flow-through experiments. Scanning electron microscopy analysis demonstrated that Cr was associated with the precipitates, indicating that Cr could be captured from solution and trapped in solid nanoparticles in the presence of calcite. However, the passivation of the calcite surface by a new precipitate may eventually slow down Cr sequestration.

Résumé: Strain localisation structures, such as shear fractures and compaction bands, are of importance due to their influence on permeability and therefore outgassing, a factor thought to influence eruptive style. In this study, we aim to develop a better understanding of strain localisation in porous volcanic rocks using X-ray tomographic images of samples of porous andesite (porosity = 0.26) acquired before and after deformation in the brittle and ductile regimes. These 3D images have been first analysed to provide 3D images of the porosity structure within the undeformed andesite, which consists of a large, well-connected porosity backbone alongside many smaller pores that are either isolated or connected to the porosity backbone by thin microstructural elements (e.g., microcracks). Following deformation, porosity profiles of the samples show localised dilation (porosity increase) and compaction (porosity reduction) within the samples deformed in the brittle and ductile regimes, respectively. Digital volume correlation (DVC) of the images before and after triaxial deformation was used to quantify the tensor strain fields, and the incremental divergence (volumetric strain) and curl (used as an indicator of shear strain) of the displacement fields were calculated from the DVC. These fields show that strain localisation in the sample deformed in the brittle regime manifested as a similar to 1 mm-wide, dilatational shear fracture oriented at an angle of 40-45 degrees to the maximum principal stress. Pre- and post-deformation permeability measurements show that permeability of the sample deformed in the brittle regime increased from 3.9 x 10(-12) to 4.9 x 10(-12) m(2), which is presumed to be related to the shear fracture. For the sample deformed in the ductile regime, strain localised into similar to 1 mm-thick, undulating compaction bands orientated sub-perpendicular to the maximum principal stress with little evidence of shear. Taken together, our data suggest that these bands formed during large stress drops seen in the mechanical data, within high-porosity zones within the sample, and within the large, well-connected porosity backbone. Pre- and post-deformation permeability measurements indicate that inelastic compaction decreased the permeability of the sample by a factor of similar to 3. The data of this study assist in the understanding of strain localisation in porous volcanic rocks, its influence on permeability (and therefore volcanic outgassing), and highlight an important role for DVC in studying strain localisation in volcanic materials. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: We report in situ structural measurements of shock-compressed single crystal orthoenstatite up to 337 55 GPa on the Hugoniot, obtained by coupling ultrafast X-ray diffraction to laser-driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X-ray diffraction signal at nanosecond timescales at 80 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post-perovskite have been indirectly reported in microsecond-scale plate-impact experiments. Therefore, we interpret the high-pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post-perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments. Plain Language Summary The study of silicate materials at extreme pressures and temperatures provides insight on the evolution of planetary bodies evolution during solar system formation. During their accretion, rocky bodies have undergone several collisions and possibly planetary impacts that have transformed their minerals. The microscopic processes occurring during such events are not fully understood. In this study, we used high-power lasers to generate shock waves into MgSiO3 enstatite crystals, creating conditions comparable to the deepest part of the early Earth mantle and large planetary impacts. During the shock wave transit, within few nanoseconds, we probed the structure of shocked enstatite between 14 3 and 337 55 GPa using intense X-ray pulses from Linac Coherent Light Source X-ray free electron laser facility. We found that, when shocked between 80 +/- 13 GPa and the conditions of equilibrium melting (above 170 GPa), enstatite transforms into a disordered structure instead of forming bridgmanite or post-perovskite-the expected equilibrium phases. This disordered structure is similar to MgSiO3 glass or liquid and is observed up to 337 +/- 55 GPa. This study provides the first direct measurement of shocked enstatite structure and suggests that the observed disordered state is an intermediate phase on the transformation pathway of bridgmanite in natural impacts.

Résumé: Seismic hazard studies provide indicators of seismic motion that are expressed for “free-field,” that is, representative of the ground motion exactly at the free surface, without disturbances due to interactions between soil and buildings or other structures. Most of these studies are based on ground-motion prediction equations, which are, themselves, formulated to predict free-field motion, as they are derived from similarly free data. However, is this really the case? In this study, we use several examples to illustrate how small structures hosting permanent strong-motion stations (often anchored on small concrete slabs) generate soilstructure interaction effects that can amplify the high-frequency part of the earthquake signal (10 Hz) by up to a factor of 2-3 for stations on soils. We also show that the installation depth of a station, even if very shallow (i.e., a few meters), can change the recorded response, mainly by deamplifying the signal in high frequencies (> 10 Hz) by a factor up to 0.3. Such effects imply that there are actual differences between recorded and true free-field signals. Depending on the housing conditions, these effects can have significant impact on response spectra at high frequencies, and on measurements of the kappa parameter. It is, thus, becoming clear that such effects should be taken into account in studies involving high-frequency seismic motion. To do so, scientists need a detailed description of the conditions of installation and housing of seismological and accelerometric stations, which often lacks from the metadata distributed through the various, commonly used web services. Increasing such information and facilitating the access to it would allow the identification of stations that are problematic and of those that are truly close to free-field recording conditions. In a subsequent step, it would be important to quantify the modification curve of the response of stations that experience such effects.

Résumé: We present the geomagnetic field model COV-OBS.x2 that covers the period 1840-2020. It is primarily constrained by observatory series, satellite data, plus older surveys. Over the past two decades, we consider annual differences of 4-monthly means at ground-based stations (since 1996), and virtual observatory series derived from magnetic data of the satellite missions CHAMP (over 2001-2010) and Swarm (since 2013). A priori information is needed to complement the constraints carried by geomagnetic records and solve the ill-posed geomagnetic inverse problem. We use for this purpose temporal cross-covariances associated with auto-regressive stochastic processes of order 2, whose parameters are chosen so as to mimic the temporal power spectral density observed in paleomagnetic and observatory series. We aim this way to obtain as far as possible realistic posterior model uncertainties. These can be used to infer for instance the core dynamics through data assimilation algorithms, or an envelope for short-term magnetic field forecasts. We show that because of the projection onto splines, one needs to inflate the formal model error variances at the most recent epochs, in order to account for unmodeled high frequency core field changes. As a by-product of the core field model, we co-estimate the external magnetospheric dipole evolution on periods longer than 2 years. It is efficiently summarized as the sum of a damped oscillator (of period 10.5 years and decay rate 55 years), plus a short-memory (6 years) damped random walk.

Résumé: Apatite fission-track analysis of middle Bengal fan sediments (IODP expedition 354) and modern Himalayan river sediments shows that most of the detrital apatites are very young compared to their depositional ages, independent of their uranium content. Bengal fan apatites display an average central age lag time as short as 2.26 +/- 1.6 Myr since at least similar to 13 Ma. Such lag times reflect a mean exhumation rate on the order of at least 1-3 km/Myr. The occurrence of detrital apatites with relatively short AFT lag times since at least 13 Ma indicates that there have always been areas of rapid erosional exhumation, supplying detrital apatites to the fluvial system and delivering them to the paleo-Ganges and/or -Brahmaputra plains and finally to the Bengal fan. It also supports that temporary storage of detrital apatites in the floodplains or delta has always been negligible since at least 13 Ma. Comparison of the AFT data of the Bengal fan with those of the Central and Eastern proximal Neogene Himalayan foreland basin shows that both paleo-Ganga and -Brahmaputra catchments provided apatites with similar short lag time to the distal Bengal Fan basin. In the modern drainage system of the Bengal fan, the apatites with young fission-track cooling ages are principally derived from areas where the topography has a sharp relief controlled by threshold hillslope processes and stream power resulting in landslide erosion as a coupled response to tectonic and fluvial forcing. By analogy with the modern erosion processes in the Himalayan range, we suggest that over the past 13 Ma, apatites were mainly derived from areas of sharp relief, where river stream power was high and hill slopes close to the threshold angle. As the exhumation signal is rather consistent since the late Miocene the detrital apatite fission-track data are either not sensitive enough to detect rapid climatically controlled changes in exhumation rates, or overall long-term erosion rates on the orogen scale are not strongly affected by climatic variations such as the variability of the Indian Summer Monsoon. Given the already rapid exhumation rates controlled by tectonics, the impact of climate variability on surface erosion rates cannot be detected with our data, especially in the case of erosion processes dominated by threshold hillslope model. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Reinforced concrete (RC) damage over time can cause severe structural problems (e.g. Morandi Bridge in Italy). Many factors contribute to concrete deterioration (thermal conditions, chemical attacks, shrinkage, creep, carbonation, corrosion, etc). Reinforced concrete deterioration starts at early-age and continues with structure aging. Early-age shrinkage, creep and thermal conditions or initial cracks can have a significant impact on the dynamic behavior of concrete structures. The CEOS.fr National program for instance showed that the natural frequency of a RC beam subjected to early-age restrained shrinkage is highly affected. In order to quantify the impact of early-age drying shrinkage on the dynamic behavior of structures, the GEOMAS Lab at INSA Lyon has conducted a research project combining both numerical modeling and pseudo-dynamic tests on two types of RC portal frames. The first one was kept in endogenous conditions (by covering it using a plastic sheet to stop water exchange with the surrounding environment) during its early age period in a way to limit drying effects leading to cracks, while the second one was kept in non-endogenous conditions (water exchange was possible) similar to construction site conditions, which induced cracks apparition. Both RC portal frames were subjected after their early age period to the same seismic loading. The present paper focuses on the experimental part of the project based on the use of pseudodynamic tests in order to evaluate the behavior of such structures under a seismic loading. The portal frames were instrumented using optical fiber sensors, displacement and load sensors and velocimeters. Image correlation was also used in order to monitor the structures.

Résumé: The withdrawal of a liquid or the translation of a liquid slug in a capillary tube leads to the deposition of a thin film on the inner wall. When particles or contaminants are present in the liquid, they deposit and contaminate the tube if the liquid film is sufficiently thick. In this article, we experimentally investigate the condition under which particles are deposited during the air invasion in a capillary tube initially filled with a dilute suspension. We show that the entrainment of particles in the film is controlled by the ratio of the particle and the tube radii and the capillary number associated with the front velocity. We also develop a model which suggests optimal operating conditions to avoid contamination during the withdrawal of a suspension from a tube. This deposition mechanism can also be leveraged in coating processes by controlling the deposition of particles on the inner walls of channels.

Résumé: We used episodic GNSS measurements to quantify the present-day velocity field in the northwestern Himalaya from the Himalayan foreland to the Karakoram Range. We report a progressive N-S compressional velocity gradient with two noticeable exceptions: in the Salt Range, where important southward velocities are recorded, and in Nanga Parbat, where an asymmetrical E-W velocity gradient is recorded. A review of Quaternary slip along active thrusts both in and out of sequence allows us to propose a 14 mm/yr shortening rate. This constraint, together with a geometrical model of the Main Himalayan Thrust (MHT), allows us to propose estimations of the slip distributions along the active faults. The lower flat of the MHT is characterized by ductile slip, whereas the coupling increases along the crustal ramp and along the upper flat of the MHT. The basal thrust of the Potwar Plateau and Salt Range presents weak coupling, which is interpreted as the existence of a massive salt layer forming an excellent decollement. In the central part of the frontal Salt Range, the velocities suggest the existence of a southward horizontal flux in the massive salt layer. The simulations also suggest that the velocities recorded in Nanga Parbat can be explained by active westward thrusting along the fault that borders the massif to the west. Simulations suggest that the slip along this fault evolves with depth from 5 mm/yr ductile slip near the MHT to no slip along the upper part of the fault.

Résumé: Chalcopyrite (CuFeS2) is the primary ore mineral for copper, but leaching of this mineral under atmospheric conditions is slow due to the formation of surface passivating phases such as elemental sulphur and jarosite. Here, we studied chalcopyrite leaching in a sulphuric acid solution at 75 degrees C and 750 mV (relative to the standard hydrogen electrode), and found that after adding 20 vol% of tetrachloroethylene (TCE) into the leaching solution, elemental sulphur was dissolved from chalcopyrite and surface passivation was removed at the early stage of leaching. The removal of surface sulphur significantly enhanced the leaching rate by approximately 600% compared with TCE-free leaching. However, adding dimethyl sulfoxide (DMSO) did not improve the leaching rate. At the later stage of leaching, the increasing concentrations of Fe3+ from the dissolution of chalcopyrite and K+ possibly from the dissolution of minor amount of gangue minerals resulted in the precipitation of a potassium jarosite layer on the surface of chalcopyrite. The jarosite shell did not passivate TCE-free leaching due to its porous structure. However, in the case of leaching with TCE, elemental sulphur filled the pores, and the jarosite shell became nearly impermeable, resulting in passivation after 80% copper extraction. This study demonstrates a way for effective removal of sulphur passivation at the early stage of chalcopyrite leaching by adding sulphur dissolving solvent such as TCE, but to prevent jarosite formation at the later stage of leaching, it is necessary to keep the concentrations of Fe3+ and K+ at low levels.

Résumé: New estimates of long-term velocities of permanent GPS stations in Southern Mexico reveal that the geologically discernible similar to 650-km long shear zone, which strikes parallel to the Middle America trench, is active. This left-lateral strike-slip, La Venta-Chacalapa (LVC) fault system, is apparently associated with a motion of the Xolapa terrain and at the present time is the northern boundary of a similar to 110-160-km wide forearc sliver with a sinistral motion of 3-6 mm/year with respect to the North America plate. This sliver is the major tectonic feature in the Guerrero and Oaxaca regions, which accommodates most of the oblique component of the convergence between the Cocos and North America plates. Previous studies based purely on the moment tensor coseismic slips exceedingly overestimated the sliver inland extent and allocated its northern margin on or to the north of the Trans-Mexican Volcanic Belt. While the LVC fault system probably slips slowly over geologic scale time and there is not any historic evidence of large earthquakes on the fault so far, its seismic potential could be very high, assuming a feasible order of similar to 10(3)years recurrence cycle. A detailed analysis of long-term position time series of permanent GPS stations in the Guerrero and Oaxaca states, Southern Mexico discards previous models and provides clear evidence of an active LVC fault zone bounding the Xolapa forearc sliver. The southeastward motion of this sliver may have persisted for the last similar to 8-10 Million year and played an important role in the tectonic evolution of the region.

Résumé: Klyuchevskoy and surrounding volcanoes in central Kamchatka form the Northern Group of Volcanoes (NGV), which is an area of particularly diverse and intensive Pleistocene-Holocene volcanism. In this study, we present a new seismic tomographic model of the crust and uppermost mantle beneath NGV based on local earthquake data recorded by several permanent and temporary seismic networks including a large-scale experiment that was conducted in 2015-2016 by an international scientific consortium. Having an unprecedented resolution for this part of Kamchatka, the new model reveals many features associated with the present and past volcanic activity within the NGV. In the upper crust, we found several prominent high-velocity anomalies interpreted as traces of large basaltic shield volcanoes, which were hidden by more recent volcanic structures and sediments. We interpret the mantle structure to reflect asthenospheric flow up through a slab window below the Kamchatka-Aleutian junction that feeds the entire NGV. The interaction of the hot asthenospheric material with fluids released from the slab determines the particular volcanic activity within the NGV. We argue that the eastern branch of the Central Kamchatka Depression, which is associated with a prominent low-velocity anomaly in the uppermost mantle, was formed as a recent rift zone separating the NGV from the Kamchatka Eastern Ranges.

Résumé: The density of liquid iron has been determined up to 116 GPa and 4350 K via static compression experiments following an innovative analysis of diffuse scattering from liquid. The longitudinal sound velocity was also obtained to 45 GPa and 2700 K based on inelastic x-ray scattering measurements. Combining these results with previous shock-wave data, we determine a thermal equation of state for liquid iron. It indicates that Earth's outer core exhibits 7.5%-7.6% density deficit, 3.7%-4.4% velocity excess, and an almost identical adiabatic bulk modulus, with respect to liquid iron.

Résumé: The Mont-Blanc massif, being iconic with its large glaciers and peaks of over 4,000 m, will experience a sharp increase in summer temperatures during the twenty-first century. By 2100, the impact of climate change on the cryosphere and hydrosphere in the Alps is expected to lead to a decrease in annual river discharge. In this work, we modelled the twenty-first century evolution of runoff in the Arve river, downstream of Mont-Blanc's French side. For the first time for this region, we have forced a hydrological model with output from an ice-dynamical glacier model and 16 downscaled climate projections, under RCP4.5 and RCP8.5 scenarios. By 2100, under RCP8.5 (high-emission scenario), the winter discharge of the Arve river remains low but is expected to increase by 80% when compared to the beginning of the century. By contrast, the summer season, currently the most important discharge period, will be marked by a runoff decrease of approximately 40%. These changes are almost similar according to a scenario with a lower warming (RCP4.5) and are mostly driven by glacier retreat. These shifts will have significant downstream impacts on water quantity and quality, affecting hydroelectric generation, agriculture, forestry, tourism and aquatic ecosystems.

Résumé: Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

Résumé: Bioactive metal releases in ocean surface water, such as those by ash falls during volcanic super-eruptions, might have a potentially toxic impact on biocalcifier planktic microorganisms. Nano-XRF imaging with the cutting-edge synchrotron hard X-ray nano-analysis ID16B beamline (ESRF) revealed for the first time a specific Zn- and Mn-rich banding pattern in the test walls of Globorotalia menardii planktic foraminifers extracted from the Young Toba Tuff layer, and thus contemporaneous with Toba's super-eruption, 74,000 years ago. The intra-test correlation of Zn and Mn patterns at the nanoscale with the layered calcareous microarchitecture, indicates that the incorporation of these metals is syngenetic to the wall growth. The preferential Mn and Zn sequestration within the incipient stages of chamber formation suggests a selective incorporation mechanism providing a resilience strategy to metal pollution in the test building of planktic foraminifers.

Résumé: We conducted a microstructural analysis of bedding-parallel faults (BPFs) in Mesozoic clay-rich layers of the Southeast Basin of France. Various microstructures are recognized in thin sections under a petrographic microscope and by cathodoluminescence. The microscale observations are combined with outcrop observations from previous studies to provide insight into the origin of the BPFs and their evolution during successive phases of deformation in a basin that had a polyphase tectonic history. The BPFs slipped while normal faults were formed during the Oligocene extension. Then, another phase of slip occurred later during the basin inversion. These two phases of deformation are expressed by recurrent crack-seal veins, pull-apart veins and stylolites. In addition, calcite veins with an elongate blocky morphology suggest an opening normal to bedding before the reactivation. The BPFs initiated in clay layers that were shallow dipping. Such conditions may appear mechanically unfavourable for an opening normal to bedding or a shearing parallel to bedding. We suggest that the role of rock anisotropy is critical. This study furthermore demonstrates that BPFs can be long-lived brittle structures that may record successive tectonic events.

Résumé: Deep body waves have been reconstructed from seismic noise correlations in recent studies. The authors note their great potential for deep-Earth imaging. In addition to the expected physical seismic phases, some spurious arrivals having no correspondence in earthquake seismograms are observed from the noise correlations. The origins of the noise-derived body waves have not been well understood. Traditionally, the reconstruction of seismic phases from inter-receiver noise correlations is attributed to the interference between waves from noise sources in the stationary-phase regions. The interfering waves emanating from a stationary-phase location have a common ray path from the source to the first receiver. The correlation operator cancels the common path and extracts a signal corresponding to the inter-receiver ray path. In this study, with seismic noise records from two networks at teleseismic distance, we show that noise-derived spurious seismic signals without correspondence in real seismograms can arise from the interference between waves without a common ray path or common slowness. These noise-derived signals cannot be explained by traditional stationary-phase arguments. Numerical experiments reproduce the observed spurious signals. These signals still emerge for uniformly distributed noise sources, and thus are not caused by localized sources. We interpret the presence of the spurious signals with a less restrictive condition of quasi-stationary phase: providing the time delays between interfering waves from spatially distributed noise sources are close enough, the stack of correlation functions over the distributed sources can still be constructive as an effect of finite frequencies, and thereby noise-derived signals emerge from the source averaging.

Résumé: Source locations provide fundamental information on earthquakes and lay the foundation for seismic monitoring at all scales. Seismic source location as a classical inverse problem has experienced significant methodological progress during the past century. Unlike the conventional traveltime-based location methods that mainly utilize kinematic information, a new category of waveform-based methods, including partial waveform stacking, time reverse imaging, wavefront tomography, and full waveform inversion, adapted from migration or stacking techniques in exploration seismology has emerged. Waveform-based methods have shown promising results in characterizing weak seismic events at multiple scales, especially for abundant microearthquakes induced by hydraulic fracturing in unconventional and geothermal reservoirs or foreshock and aftershock activity potentially preceding tectonic earthquakes. This review presents a comprehensive summary of the current status of waveform-based location methods, through elaboration of the methodological principles, categorization, and connections, as well as illustration of the applications to natural and induced/triggered seismicity, ranging from laboratory acoustic emission to field hydraulic fracturing-induced seismicity, regional tectonic, and volcanic earthquakes. Taking into account recent developments in instrumentation and the increasing availability of more powerful computational resources, we highlight recent accomplishments and prevailing challenges of different waveform-based location methods and what they promise to offer in the near future. Plain language summary Earthquakes are a common physical phenomenon involving ground shaking and rupturing of the surface of the Earth. In addition to the well-known catastrophic tectonic earthquakes, similar vibration sources also appear at various scales in engineering fields, such as acoustic emissions resulting from microcracks in building walls and bridges, rock bursts in mines, microseismic events generated by mining and fluid injection/extraction, and microseisms caused by crustal activity. Seismic information provides a powerful tool for geophysical and engineering surveys. The source location describes the spatial and temporal extent of an earthquake and lays the foundation for seismic monitoring. Seismic location methods have made significant progress over the last century. Specifically, a category of new waveform-based location methods has emerged as a counterpart of conventional traveltime-based inversion. These methods directly utilize the notion of a wavefield and, very similar to an optical lens, aim at spatially focusing a source's emitted energy. Waveform-based methods have provided robust and effective source location results at various scales. We summarize the development history and current state of waveform-based location methods and discuss the advantages and challenges through their applications for seismic source location at multiple scales.

Résumé: alpha-MnO2 is a typical tunneled Mn oxide (TMO) that is frequently associated with delta-MnO2 in the environment and exhibits strong adsorption and oxidation activity. The morphology of alpha-MnO2, which is controlled by an oriented attachment (OA) process, is one of the key factors affecting its reactivity. However, the detailed crystal growth process and coupling between morphology and structure of alpha-MnO2 during OA processes remain poorly understood. We propose that the transformation of layer-based delta-MnO2 to tunnel-based alpha-MnO2 occurs via a multistage OA process. In the initial stage, the produced delta-MnO2 nanoflakes are found to spontaneously self-assemble into a nanoribbon with a large number of lattice defects via edge-to-edge OA. The presence of lattice defects promotes the generation of oxygen vacancies, and the Mn(iv) ions in the [MnO6] octahedral layers of delta-MnO2 are reduced to Mn(iii)/Mn(ii). The reduced ions subsequently migrate from the [MnO6] octahedral layers to the interlayers during this process. The hydroxide which acts in coordination with the interlayer Mn(iii)/Mn(ii) and oxygen atoms coordination with adjacent nanoribbons attach to each other driven by hydrogen bonding and form primary nanorods through a face-to-face OA mechanism along the c-axis. Concomitantly, the bonding of [Mn(iii)O-6] octahedra in the interlayer of the nanoribbons with adjacent [MnO6] octahedral layers leads to the fabrication of a new alpha-MnO2 tunnel structure from the original delta-MnO2. These findings provide insights into both the transformation mechanisms of the layer-based to the tunnel-based nanoparticles and methods for the efficient and controlled synthesis of nanomaterials.

Résumé: Numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions. Due to their proximity to continents, they have been frequently labelled “marginal plateaus”, although this term has not been clearly defined or associated with a specific geology or geodynamic process. Until now, these elevations have been interpreted as submerged thinned continental fragments detached from continents, basaltic buildups formed by hotspots, volcanic margins or oceanic plateaus. Many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages. We propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting: “Transform Marginal Plateaus” (TMPs). Based on a compilation of 20 TMPs around the world, we show that most of them have a polyphased history and have undergone at least one major volcanic phase. Our review highlights in particular a hitherto unrecognized close link between hotspots, volcanic activity and transform margins. We also propose that, due to their polyphased history, TMPs may contain several successive basins and overlooked long-lived sedimentary archives. We finally highlight that, because these TMPs were transform plate boundaries perpendicular or oblique to surrounding rifts, many of them were close to last-contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents. Therefore, we discuss broader scientific issues, such as the interest of TMPs in recording and studying the onset and variations of oceanic currents or past biodiversity growth, bio-connectivity and lineage evolution.

Résumé: In this paper, we describe the manifestation of localized states of a diffuse elastic wavefield inside a two- dimensional metamaterial made of a collection of vertical long beams glued to a thin plate. Through mesoscopic physics, we demonstrate that localized states arise due to multiwave interactions at the beam-plate attachment when the beams act as coupled resonators for both compressional and flexural resonances on the metasurface. In practice, when the compressional resonance of the beams clamps the plate on a large frequency band gap, thus preventing wave propagation inside the metamaterial area, the flexural resonance of the same beams permits evanescent waves to be diffused on a narrow frequency band between clusters of neighboring beams. This experiment physically highlights a tight-binding-like coupling in the localized regime for this two-dimensional metamaterial.

Résumé: We present an improved crustal Vs model and Moho depth map using ambient noise wave-equation tomography. The so-called 'ambient noise wave-equation tomography' is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation, which accounts for 3-D and finite-frequency effects. We use cross-correlations of up to 4 yr of continuous vertical-component ambient seismic noise recordings from 304 high-quality broad-band stations in the Alpine region. We use model LSP_Eucrust1.0 obtained from traditional ambient noise tomography as initial model, and we iteratively improve the initial model by minimizing frequency-dependent phase traveltime differences between the observed and synthetic waveforms of Rayleigh waves in the period range 10-50 s. We obtain the final model after 15 iterations with similar to 65 percent total misfit reduction compared to the initial model. At crustal depth, the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies. At Moho and upper-mantle depth, the final model corrects an obvious systematic velocity shift of the initial model. The resulting isovelocity Moho map confirms a Moho step along the external side of the external crystalline massifs of the northwestern Alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern Alps. Ambient noise wave-equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory.

Résumé: Pyrite plays a significant role in governing the mobility of toxic uranium in an anaerobic environment via an oxidation-reduction process occurring at the mineral-water interface, but the factors influencing the reaction kinetics remain poorly understood. In this study, natural pyrites with different impurities (Pb, As, and Si) and different surface pretreatments were used to react with aqueous U(VI) from pH similar to 3.0 to similar to 9.5. Both aqueous and solid results indicated that freshly crushed pyrites, which do have more surface Fe2+/Fe3+ and S2- sites that were generated from breakage of Fe(S)-S bonds during ball milling, exhibited a much stronger reactivity than those treated with acid washing. Besides, U(VI) reduction which involves the possible intermediate U(V) and the formation of hyperstoichiometric UO2+x(s) was found to preferentially occur at Pb- and As-rich spots on the pyrite surface, suggesting that the incorporated impurities could act as reactive sites because of the generation of lattice defects and galena- and arsenopyrite-like local configurations. These reactive surface sites can be removed by acid washing, leaving a pyrite surface nearly inert toward aqueous U(VI). Thus, reactivity of pyrite toward U(VI) is largely governed by its surface compositions, which provides an insight into the chemical behavior of both pyrite and uranium in various environments.

Résumé: The Ridgecrest earthquake pair ruptured a previously unknown orthogonal fault system in the eastern California shear zone. The stronger of the two, an M-w 7.1 earthquake that occurred on 6 July 2019, was preceded by an M-w 6.4 foreshock that occurred 34 hr earlier. In this study, distinct final slip distributions for the two earthquakes are obtained via joint inversion of Interferometric Synthetic Aperture Radar (InSAR), optical imagery, and Global Positioning System (GPS) measurements. Special attention is paid to the merging of dense (e.g., InSAR and optical imagery) and sparse geodetic (e.g., GPS) datasets. In addition, a new approach is introduced for data and model discretization through intermittent model- and data-space reconditioning that stabilizes the inversion, thus ensuring that small changes in the data space do not cause disproportionate large changes to the model space. Although the coseismic slip of the M-w 6.4 earthquake was complex, involving three distinct asperities distributed among an intersecting orthogonal set of faults, the coseismic slip of the M-w 7.1 earthquake was limited to the main northwest-striking fault. In addition to the M-w 7.1 earthquake, that northwest-striking fault plane also hosted one of the M-w 6.4 asperities. Slip on this coplanar foreshock asperity increased the shear stress at the future site of the M-w 7.1 hypocenter, and triggered a vigorous aftershock activity on the main northwest fault that culminated in its rupture. This, in turn, reactivated the coplanar foreshock asperity. In addition to failing twice within 34 hr, we find that the reruptured asperity slipped about six times more during the M-w 7.1 than during the M-w 6.4 earthquake. This repeated failure is indicative of an incomplete stress drop and premature rupture arrest during the M-w 6.4 foreshock, requiring an efficient frictional strengthening and emphasizing the causal link between highly rate-dependent friction, dynamic frictional restrengthening, and partial stress drop that has been observed in numerical studies of frictional sliding.

Résumé: The seismic damage to buried pipelines could be disruptive in terms of safety, life quality, and socioeconomic impacts. In this paper, we investigate the extensive list of available buried pipelines fragilities: we select a series of typical and interesting fragilities in terms of different ground motions, diverse pipeline and soil types typologies and nature (i.e. empirical, analytical); and then we compare the seismic damage results. The aim is to have a palpable overview of the variability of damage results while using different selected buried pipelines fragilities, various ground motion parameters and different hazard scenarios. To achieve this comparative study, the buried pipeline wastewater network of Byblos, Lebanon, is used as a case study. The methodology used to assess damage to the waste-water network encompasses hazard assessment, wastewater network inventory, wastewater damage functions, and model development. The Ergo platform was especially updated by implementing the list of selected fragilities to achieve the damage to wastewater comparison in this article. Results: and recommendations are offered; first for a more adequate usage of those fragilities; second, for improvement of existing and future developed buried pipelines fragility functions; and third, for the Byblos wastewater pipelines network, allowing the establishment of a resilient earthquake preparedness strategy and recovery plan for infrastructure network in Byblos.

Résumé: The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids. Earthquakes may generate faults serving as fluid pathways, but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that in the presence of fluids, densification reactions can occur very fast, on the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic-wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

Résumé: Pyrite (FeS2) from coal, sedimentary rocks, and hydrothermal ore deposits generally contains hazardous selenium (Se) and arsenic (As) that are released in natural waters through oxidative dissolution of the host. Knowing how As and Se are structurally incorporated into pyrite has important implications in controlling or preventing their release because trace metal(loid) substitution accelerates the dissolution of pyrite. Previous extended X-ray absorption fine structure (EXAFS) studies have reported that nominally monovalent arsenic clusters at the sulfur site form As-As pairs at 3.2 angstrom, whereas monovalent Se does not form Se-Se pairs at this distance for unknown reasons. Here, we revisit this question using As and Se K-edge X-ray absorption near-edge structure (XANES) and EXAFS spectroscopy complemented with atomistic calculations. We find that neither As nor Se atoms can be differentiated from a S atom at 3.2-3.3 angstrom with the cluster and dilute model-fits to As- and Se-EXAFS data yielding equivalent least-squares solutions. Thermodynamic calculations of Fe48As3S93 (3.8 wt % As) and Fe48Se3S93 (4.0 wt % Se) structures show that the formation of As-As pairs is energetically favorable and the formation of Se-Se pairs is unfavorable. Thus, the equilibrium distribution of As and Se predicted by calculation agrees with published EXAFS data. However, this agreement is incidental because EXAFS fits are ambiguous with the same EXAFS spectra being fit indifferently with a cluster and a dilute model. Regarding Se, the dilute model-fit is probably correct since Se-Se pairs are precluded thermodynamically. The situation is less clear for As. The lowest energy atomic arrangement of As in Fe48S93As3 is similar to the local structure of As in arsenopyrite (FeAsS), thus supporting the cluster model. However, the energy gain to total energy provided by the formation of As clusters decreases with decreasing As concentration, making them thermodynamically less favorable below 1.0 wt %.

Résumé: The spatial distribution of temporal variations in seismic wavespeed is key to understanding the sources and physical mechanisms of various geophysical processes. The imaging of wavespeed changes requires accurate measurements of traveltime delays with both high lapse-time and frequency resolutions. However, traditional methods for time-shift estimation suffer from their limited resolutions. In this paper we propose a new approach, the wavelet method, to measure the traveltime changes in the time-frequency domain. This method is based on wavelet cross-spectrum analysis, and can provide optimal time frequency joint resolution while being computationally efficient. It can deal not only with coda but also dispersive surface waves even in the presence of cycle skipping. Using synthetic coda, we show that the wavelet method can retrieve traveltime shifts more stably and accurately than traditional methods. An application at Salton Sea Geothermal Field indicates that the wavelet method is less affected by spectral smearing and better discriminates dv/v variations at different frequencies. Furthermore, upon investigations on synthetic coda, we illustrate that the bias on dv/v measurements due to changes in source frequency content is likely to be negligible, either with traditional methods or with the new wavelet method. The wavelet method sheds lights on applications of seismic interferometry that aim to locate changes in space.

Résumé: Quito, the capital city of Ecuador hosting similar to 2 million inhabitants, lies on the hanging wall of a similar to 60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3-5 mm yr(-1) of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (<1 km) locking depth for the central segment, increasing to a few kilometres south and north of it. Including InSAR results in the inversion requires locking to vary both along dip and along strike. 3-D spatially variable locking models show that shallow creep occurs along the central 20-km-long segment. North and south of the central segment, the interseismic coupling is less resolved, and data still allows significant slip deficit to accumulate. Using the interseismic moment deficit buildup resulting from our inversions and the seismicity rate, we estimate recurrence time for magnitude 6.5 + earthquake to be between 200 and 1200 yr. Finally, PS-InSAR time-series identify a 2 cm transient deformation that occurred on a secondary thrust, east of the main Quito fault between 1995 and 1997.

Résumé: Shortening of the continental lithosphere is generally accommodated by the growth of crustal wedges building above megathrusts in the mantle lithosphere. We show that the locus of shortening in the western margin of South America has largely been controlled by the geometry of the slab. Numerical models confirm that horizontal subduction favors compression far from the trench, above the asthenospheric wedge and steeply dipping segment of the subducting slab. As a result, a second crustal wedge grows in the hinterland of the continent, and widens the Andes. In the Bolivian orocline, this wedge corresponds to the Eastern Cordillera, whose growth was triggered by a major episode of horizontal subduction. When the slab returned to a steeper dip angle, shortening and uplift pursued, facilitated by the structural and thermo-chemical alteration of the continental lithosphere. We review the successive episodes of horizontal subduction that have occurred beneath South America at different latitudes and show that they explain the diachronic widening of the Andes. We infer that the present-day segmented physiography of the Andes results from the latitudinally variable, transient interplay between slab dynamics and upper plate tectonics over the Cenozoic. We emphasize that slab flattening, or absence thereof, is a major driving mechanism that sets the width of the Andes, at any latitude.

Résumé: Predicting the proximity of large-scale dynamic failure is a critical concern in the engineering and geophysical sciences. Here we use evolving contractive, dilatational, and shear strain deformation preceding failure in dynamic X-ray tomography experiments to examine which strain components best predict the proximity to failure. We develop machine learning models to predict the proximity to failure using time series of three-dimensional local incremental strain tensor fields acquired in rock deformation experiments under stress conditions of the upper crust. Three-dimensional scans acquired in situ throughout triaxial compression experiments provide a distribution of density contrasts from which we estimate the three-dimensional incremental strain that accumulates between each scan acquisition. Training machine learning models on multiple experiments of six rock types provides suites of feature importance that indicate the predictive power of each feature. Comparing the average importance of groups of features that include information about each strain component quantifies the ability of the contractive, dilatational and shear strain to predict the proximity of macroscopic failure. A total of 24 models of four machine learning algorithms with six rock types indicate that 1) the dilatational strain provides the best predictive power of the strain components, and 2) the intermediate values (25th-75th percentile) of the strain population provide the best predictive power of the statistics of the strain populations. In addition, the success of the predictions of models trained on one rock type and tested on other rock types quantifies the similarities and differences of the precursory strain accumulation process in the six rock types. These similarities suggest the potential existence of a unified theory of brittle rock deformation for a range of rock types. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: We investigate the scaling properties of the mechanical energy budget in accretionary prisms across five orders of magnitude, from the laboratory centimeter-scale to crustal kilometer-scale. We first develop numerical models that match the length scale, fault and material properties, surface topography, and fault geometries observed in scaled dry sand accretionary experiments. As we systematically increase the spatial dimensions of the numerical models by orders of magnitude, we calculate each component of the energy budget both before and after the first thrust fault pair develops. The increase of both the bulk stiffness and slip weakening distance from the laboratory-to crustal-scale produces a scale-invariant partitioning of the energy budget, relative to the total work done on the system. The components scale as power laws with exponents of three. Consequently, accurate laboratory simulations of the energetics of deformation within crustal accretionary wedges require careful scaling of the stiffness and slip weakening distance. Preceding thrust fault development at both the laboratory and crustal scale, the internal work consumes the largest portion of the budget (67-77%) and frictional work consumes the next largest portion (17-27%). Following thrusting, frictional work and internal work consume similar portions of the energy budget (38-50%). The sum of the remaining energy budget components, including gravitational work, seismic work, and the work of fracture propagation, consume <10-15% of the total energy budget preceding and following thrust fault development. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Continued Africa-Eurasia convergence resulted in post-11 Ma surface uplift of the Central Anatolian Plateau (CAP) and the westward escape of the Anatolian microplate. Contemporaneously, a central Anatolian fluvio-lacustrine system developed that covered extensive parts of the rising CAP. Today, the semi-arid CAP interior-except for the Konya closed catchment-drains toward the Black Sea, the Mediterranean Sea, and the Persian Gulf. Lake connectivity and drainage patterns of the fluvio-lacustrine system in the evolving plateau region are, however, largely unknown. Here, we present sedimentological and stable isotopic (delta C-13 and delta O-18) data (N = 665) from 13 well-dated lake sections covering the former fluvio-lacustrine depocenters of the southern CAP. Persistently (>1 m.y.) stable paleoenvironmental and hydrological conditions suggest that a low-relief environment characterized the southern CAP during plateau uplift. Throughout the late Miocene, various open and closed lakes of the southern CAP drained into closed, terminal lakes within the plateau interior. Sedimentation east of the Tuz Golu fault ceased during the early Pliocene (ca. 5.3-3.6 Ma), when the eastern CAP became connected to marine base level as a result of river incision shortly after the switch from regional compression to extension. A final phase of lacustrine carbonate sedimentation characterizes most sampled basins, yet occurred asynchronously over the extent of the CAP. Therefore, the final episode of lacustrine sedimentation is unlikely to have been the result of a climatic event, consistent with the absence of a clear aridification trend in the lacustrine delta O-18 data. Rather, capping carbonates reflect the interplay of surface uplift and transition from inward- to outward-drained plateau regions and concomitant lake reorganization during the formation of the CAP and its margins.

Résumé: Cycling the alpha <-> epsilon transformation in polycrystalline Fe is investigated using in situ x-ray diffraction under quasihydrostatic conditions. The forward alpha -> epsilon transformation starts at 14 +/- 1 GPa and completes at 18 +/- 1 GPa while the reverse epsilon -> alpha transformation starts at 10.5 +/- 0.5 GPa and completes at 6 +/- 1 GPa. The anomalous evolution of c/a ratios of epsilon-Fe measured at the onset of the alpha -> epsilon transition in earlier studies is not observed. Microstructural features are consistent with a Burgers path for the transformation. The forward alpha -> epsilon transformation is sensitive to the average and intergranular stresses, inducing a strong variant selection with the c axes of the new epsilon phase preferentially aligned perpendicular to compression and concentrated into one single orientation. Partial texture memory is observed as the sample returns to the alpha phase but, as transformation cycles go on, irreversible changes occur in the microstructures that are later fully induced by the cycles of phase transformations.

Résumé: Phreatic activity of Merapi started after nearly 2 years of quiescence following the October-November 2010 eruption which was the largest in more than 100 years. A dozen eruptions identified by visual andior seismic observations took place between August 2012 and April 2014. We present in this work the results of a detailed analysis of the April 20, 2014 phreatic eruption. We attempted to reconstruct the eruptive process, which lasted for over 30 min. To this end, we determined the wavefield composition by polarization analysis, located high-frequency earthquakes occurring in the initial part of the eruption process and then determined the seismic source migration of low-frequency part of the tremor-like signal [0.3-3 Hz] over time. Source depth of low-frequency signal was obtained by comparing the slowness vector calculated using 3 stations of the seismic antenna with a slowness vector model obtained by ray tracing in the structure, taking into account the topography and a 1D velocity model obtained by spatial auto-correlation analysis. The results allow to distinguish 3 different phases: 1) High-frequency transients interpreted as the result of a sudden decompression caused by the transition of the volcanic fluid to a gaseous phase that occurred approximately 1.5 km deep. This decompression process in the hydrothermal system generated a migration of the low-frequency seismic source from 900 m to 1800 m above sea level: 2) A second decompression process revealed by high-frequency micro-seismicity and associated to the migration of the low-frequency tremor source which is marked first by a descent phase, followed by a sharp ascent until reaching the surface. The evolution of the back-azimuth during the migration process indicates a slight inclination of the conduit, presumably in the orientation of the dome fracture, in the NW-SE direction. This direction is consistent with the alignment of regional tectonic structures and with the directivity of eruption deposits. 3) The seismic source then remains positioned at the altitude of the dome for over 10 min. This phase probably corresponds to the ash emission process. The average migration speed of the low-frequency seismic source from the starting eruptive process to ash emission is about 5 m/s. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Sustainably maintaining the densely populated upland plains of Madagascar as operationally safe spaces for the food security of the nation and the urban growth of its capital city, Antananarivo, hinges critically on avoiding crop and infrastructure destruction by their through-flowing rivers. The flood regime, however, is also a function of two 'slow' variables hitherto undocumented: tectonic subsidence regime, and floodplain sedimentation rate. From a radiocarbon-dated chronostratigraphy and environmental history of the sediment sequences in three of Madagascar's semi-enclosed upland basins (Antananarivo, Arnbohibary, and Alaotra), we quantify and compare how the precarious equilibrium between the two variables entails differentials in accommodation space for sediment and floodwater. Results show that all these plains have been wetlands for at least 40,000 years, but that the Antananarivo Basin is the most vulnerable because the imbalance between sedimentation and subsidence is the largest. Although the tectonic regime and the endemic forms of gully erosion that occur in the catchments are beyond human control, we advocate that flood mitigation strategies should focus on the natural grassland savanna, which makes up most of the contributing areas to surface runoff in the watersheds. Pastoralists are persistently left out of rural development programmes, yet the rangelands could benefit from the introduction of multi-purpose grasses and legumes known to withstand high stocking rates on poor soils while combining the benefits of nutritiousness, fire and drought resistance, with good runoff-arrest and topsoil-retainment abilities. Future-proofing Madagascar's upland grainbaskets and population centres thus calls for joined-up action on the sediment cascade, focusing on soil and water sequestration through integrated watershed management rather than on hard-defence engineering against overflowing rivers on the plains, which has been the costly but ineffectual approach since the 17th century. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: Combined interpretation of Electrical Resistivity Tomography (ERT) data, with plume and fumarolic gas emissions provides a snapshot of the Whakaari/White Island hydrothermal system 11 months prior to an eruption on December 9,2019. Two and three dimensional inversion of the ERT data images the F0 fumarole as a low resistivity (2-5 Omega m) feature reflecting the two-phase zone surrounding the single phase vapour conduit. Crucially, interpretation of the inversion images is well constrained by a large existing dataset of rock electrical properties, including surface conductivity, porosity and intrinsic formation factor, alongside new measurements of liquid phase electrical conductivity taken from a range of hot-springs on the island. Pore-filling liquids are an order of magnitude more conductive than sea-water with their low to very low pH (3.4 to -03) contributing to their extremely conductive nature. An extensive low-resistivity feature (0.2 Omega m) at similar to 125 m depth is therefore interpreted as a liquid saturated layer exposed to hydrothermal alteration by acid fluids. The intersection of the F0 fumarole trace with this layer is coincident with a previously determined source of deformation, suggesting that heat transport inside the fumarole conduit pressurises surrounding liquid and vapour filled pore space, generating the deformation signal. The timeseries of fumarolic and plume emissions in the year prior to the ERT survey show that the gases transported by the fumarole are a mixture of high temperature magmatic vapour and lower temperature gas equilibrated within the hydrothermal environment. Interpretation of the gas data suggests that the snapshot captured by the ERT image shows the single-phase vapour and two-phase vapourliquid regions adjacent to the conduit were likely at close to their smallest extent, consisting of mostly hydrothermal derived gases. (C) 2020 Elsevier B.V. All rights reserved.

Résumé: This paper presents an observer-based state and parameter estimation for the extended sliding-consolidation model of a landslide. This system is described by a pair of coupled Ordinary Differential Equation (ODE) and Partial Differential Equation (PDE), with a mixed boundary condition for the PDE. The coupling appears both in the ODE and in the Neuman boundary condition of the PDE. The observer consists of a copy of the PDE part of the system and Kalman-like observer for the ODE. It is shown to ensure exponential convergence of the state and parameter estimates by means of Lyapunov tool. Finally, a simulation result of the extended sliding-consolidation model is presented to illustrate the effectiveness of the proposed observer. Copyright (C) 2020 The Authors.

Résumé: Coal-fired power plants are main contributors to atmospheric CO2 emissions. They also produce huge amounts of coal fly ash (CFA) waste, which is typically landfilled, posing significant environmental risks due to its high content of potentially toxic elements (PTE). However, CFA is an alkaline aluminosilicate-rich waste, which offers the possibility of CO2 mineral capture and the production of economically-relevant mineral by-products such as zeolites. Yet, the combined carbonation and zeolite production from CFA resulting in PTE trapping has never been explored. Here we show that under mild hydrothermal conditions (150 degrees C) and depending of various process parameters such as pH and background alkali metal ion in (bi)carbonate solutions, a carbonation efficiency of up to 79 %, with a net CO2 mineral capture of 0.045 g/g CFA can be achieved, even when using a low Ca and Mg (3.72 wt% CaO, 1.74 wt%, MgO) Class F fly ash. Moreover, amorphous zeolitic precursors and different crystalline zeolites (yield up to 60 wt%) are simultaneously obtained, and PTE in CFA are effectively trapped into the newly formed calcite, zeolitic precursors, and zeolite phases. This is the first time that a combined study of carbonation, zeolitization and PTE trapping in newly formed phases has been developed. These results have important implications for carbon capture and storage, as well as for the safe reutilization and disposal of CFA waste.

Résumé: Since the 1970s, it has been shown that calcium phosphate crystals nucleate from one or several early amorphous calcium phosphate phases into several in vivo and in vitro systems. However, the precise chemical composition, structure, and transformation mechanism of these amorphous phases remain controversial. Here, we characterize the reaction mechanism and kinetics of formation of two phosphate crystals, brushite and hydroxyapatite, by using in situ Raman spectroscopy in batch reactors at 25 degrees C. We investigate three pH regimes to control the phosphate speciation in solution and used solutions with or without citric acid, a complexing agent that may stabilize the amorphous phases. As expected, brushite (CaHPO4 center dot 2H(2)O) forms at pH < 9.8. Amorphous calcium phosphate (ACP: CaHPO4 center dot nH(2)O) with a short lifetime (<2 min) and octocalcium phosphate (OCP: Ca-8(HPO4)(2) (PO4)(4)center dot 5H(2)O) are the main transient phases prior to brushite nucleation that occurs after similar to 8 min. At pH > 11, hydroxyapatite (HAp: Ca-10(PO4)(6)(OH)(2)) nucleates after >35 min, depending on the experimental conditions. The reaction mechanism steps for hydroxyapatite are more complex compared to brushite. For hydroxyapatite formation, amorphous calcium phosphate phases with different chemical composition (Ca(HPO4)(1-x)(PO4)((2/3)x)center dot nH(2)O, with x in the range 0.2-1, and with different lifetime may form. Amorphous tricalcium phosphate (ATCP: Ca(PO4)(2/3)center dot n.H2O or Ca-3(PO4)(2)center dot nH(2)O when x = 1) is the most persistent phase which can either transform into OCP and then hydroxyapatite or directly evolve into hydroxyapatite at pH > 12.2. The presence of citric acid retards the transformation kinetics by increasing the nucleation times of brushite and hydroxyapatite but has little effect on the reaction mechanism steps. Finally, this study identifies new reactive pathways that characterize the formation of amorphous calcium phosphate phases and their transformation into brushite microcrystals or hydroxyapatite nanocrystals under abiotic conditions, relevant to a wide range of technological applications and natural environments.

Résumé: The challenge to produce dolomite CaMg(CO3)(2) at low temperature (20-35 degrees C) over laboratory time scales so far has remained unsuccessful, which has led to long-lasting scientific debates in the last two <^> centuries. This mineral exerts a major control on the natural carbon dioxide sequestration into various sedimentary, basaltic, and mantellic rocks. The present study reports on specific abiotic conditions that allow the precipitation of disordered dolomite, high Mg calcite, and high Ca magnesite at room temperature over time scales of hours to days. Here we show that an amorphous calcium magnesium carbonate (ACMC) phase accelerates dolomitization at room temperature. ACMC is initially precipitated by mixing a carbonate (HCO3-/CO32- = 1; pH similar to 10.3 approximate to pK(a2)) alkaline solution with a Mg-Ca ionic solution (Mg molar fraction between 0 and 1). Then, time-resolved in situ Raman spectroscopy monitored the transformation of ACMC into Mg-rich carbonate minerals. The initial Mg molar fraction controlled both the reaction mechanism (e.g., nature of transient crystalline phases) and the kinetics. Nanosized crystallites with short-range order, called disordered dolomite CaMg(CO3)(2), precipitated following a complex reaction pathway. First, nesquehonite (MgCO3 center dot 3H(2)O: nucleation time 2.5 h) and then disordered dolomite (CaMg(CO3)(2): nucleation time 3.2 h) followed by monohydrocalcite (CaCO3 center dot H2O: nucleation time 3.4 h) formed from ACMC transformation. Nesquehonite and monohydrocalcite are transient phases that nourish the slow precipitation of disordered dolomite, which reached a spectral equilibrium after 7 days of reaction. The direct transformation of ACMC into disordered dolomite was also measured. Our experimental results demonstrate that disordered dolomite precipitates at room temperature when an ideal Mg/Ca ratio, high carbonate alkalinity, and high ionic concentration are reached in abiotic systems. This result suggests the possibility of a physicochemical rather than biotic control on the formation of disordered dolomite at low temperature in several geosystems.

Résumé: Although moderate-size earthquakes are poorly studied by lack of near-fault observations, they can provide key information about larger damaging earthquakes. Here we propose a new approach, inspired by double-difference relocation, that uses high-coherency waveforms recorded at neighboring sensors, to study the preparation phase and dynamics of moderate-size earthquakes. We validate this technique by analyzing the 2016,M(w)5.2 Borrego Springs earthquake in Southern California and find consistent rupture velocities of 2 km/s highlighting two main rupture asperities. The analysis of the 2019,Ml5.2 Le Teil earthquake in France reveals slow nucleation at depth that migrates to the surface and propagates northward with a velocity of similar to 2.8 km/s, highlighting two main rupture events also imaged by InSAR. By providing unprecedented resolution in our observation of the rupture dynamics, this approach will be useful in better understanding the preparation phase and rupture of both tectonic and induced earthquakes. Plain Language Summary Small and moderate-size earthquakes are much more numerous yet studied in much less detail than large and damaging earthquakes. This is because to study small earthquakes precisely, one needs seismometers very close to the rupture, which is rarely the case. It is still unclear if small and large earthquakes start the same way, thus studying more systematically small earthquakes could help to answer this question. Here we show that we can use two close-by seismometers, as we would do with 3-D glasses, to observe with a stereoscopic effect, the evolution of intermediate-size earthquakes when they rupture along a fault. We can retrieve the direction of rupture, the speed at which the fault breaks, and the length of the rupture, important parameters that help to characterize the earthquakes. Using this new method, we can sometimes also observe the behavior of the fault before the main rupture which will be useful to better understand how earthquakes start.

Résumé: Field data are needed for a better understanding of sea ice decline in the context of climate change. The rapid technological and methodological advances of the last decade have led to a reconsideration of seismic methods in this matter. In particular, passive seismology has filled an important gap by removing the need to use active sources. We present a seismic experiment where an array of 247 geophones was deployed on sea ice, in the Van Mijen fjord near Sveagruva (Svalbard). The array is a mix of 1C and 3C stations with sampling frequencies of 500 and 1000 Hz. They recorded continuously the ambient seismic field in sea ice between 28 February and 26 March 2019. Data also include active acquisitions on 1 and 26 March with a radar antenna, a shaker unit, impulsive sources, and artificial sources of seismic noise. This data set is of unprecedented quality regarding sea ice seismic monitoring, as it also includes thousands of microseismic events recorded each day. By combining passive seismology approaches with specific array processing methods, we demonstrate that the multimodal dispersion curves of sea ice can be calculated without an active source and then used to infer sea ice properties. We calculated an ice thickness, Young's modulus, and Poisson's ratio with values h=543 cm, E=3.90.15 GPa, and nu=0.340.02 on 1 March, and h=583 cm, E=4.4 +/- 0.15 GPa, and nu=0.32 +/- 0.02 on 5 March. These values are consistent with in situ field measurements and observations.

Résumé: Sandy kaolinite-rich soils, collected in a H-2-emitting circular depression (ca. 500 m in diameter), located in the Sao Francisco basin (Brazil), were exposed to H-2 gas concentrations in the 500-5000 ppm range for up to eight weeks. The samples were found to consume H-2 at a rate of approximately 0.05-0.1 mmol H-2/soil kg/day due to the microbial activity. DNA extraction from these soil samples before and after H-2 exposure, followed by Ribosomal Intergenic Spacer Analysis (RISA) and 16S rRNA gene amplicon sequencing, indicated that (i) the bacterial community is dominated by phyla that have been previously recognized to scavenge atmospheric H-2, and (ii) H-2 exposure leads to a significant modification of the bacterial community distribution. Measured H-2 uptake rates were fitted to the integrated form of the Michaelis-Menten equation and were further implemented in a 1-D reactive transport model. The model simulates gas-soil interactions in a 1-m vertical soil column, assuming homogeneous distribution of H-2-consuming bacteria. The evolution of the H-2 concentration in the unsaturated soil porosity along the column was simulated considering two different scenarios: a deep H-2 source (Case 1) and a biogenic surface source (Case 2). It was shown that, in the case of diffusion-dominated H-2 transport as considered in this study, bacterial activity will control the amplitude of the H-2 flux across the column. Moreover, we determined that bacterial activity can dramatically decrease the H-2 concentration in the soil porosity, by a factor of two compared to the source concentration. According to the simulation, the timeresolved concentration data collected in the Sao Francisco depression [Prinzhofer et al., 2019; International Journal of Hydrogen Energy] are consistent with the combination of a deep (Case 1) and a surficial biogenic (Case 2) H-2 source in this locality.

Résumé: The Terre Adelie Craton displays superimposed strain fields related to the Neoarchean (2.6-2.4 Ga, M1) and Paleo-Mesoproterozoic (1.7-1.5 Ga, M2) metamorphic events. M1 is a regional granulite fades event, constrained by P-T modelling at -0.8-1.0 GPa – 800-850 et, followed by a decompressional retrogression in the upper amphibolite fades at -0.6 GPa – 750 degrees C. M2 Stage 1 P-T peak is constrained at 0.6-0.7 GPa – 670-700 degrees C, followed by a steep P-T path down to 0.3 GPa – 550 degrees C. Retrogression after M2 PT peak occurred in a context of dextral shearing along the Mertz Shear Zone along with thrust motions within the eastern Terre Adelie Craton. In this paper, we present a series of 63 new Ar-40/Ar-39 ages of biotite and amphibole pairs in mafic rocks from a complete traverse of the Terre Adelie Craton. Ar-40/Ar-39 Ar dating constrains M2 amphibolite facies metamorphism at a regional scale between 1700 and 1650 Ma, during stage 1 peak metamorphism. During retrogression, lower amphibolite fades recrystallization mainly occurred along vertical shear zones and mafic dykes between 1650 and 1600 Ma (Stage 2), followed by amphibolite to greenschist facies metamorphism until after 1500 Ma (Stage 3). At the scale of the Mawson continent, this event is related to the growth of an active margin above an oblique subduction zone. The supra-subduction model best explains opening of Dumont D'Urville and Hunter basins at 1.71 Ga followed by their rapid closure and metamorphism at 1.70 Ga. In this context, episodic shear zone reactivation and magmatic dyke emplacement led to a partial reequilibration of the Ar-40/Ar-39 system until <1500 Ma. This latter phase of mafic magmatism largely coincides with a hot spot event at the scale of the Gawler Craton and western Laurentia paleocontinent. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Résumé: Silver nanowires (AgNW) are new nanomaterials designed to be incorporated into transparent conductive films in electronics, microelectrodes, heated surfaces and others. Although in these films, the AgNW are generally protected by a coating material, a risk for release of silver at all stages of the nanoproduct life cycle does exist due to corrodibility of the metal. Since ionic and nanoparticulate Ag represent a toxicological risk for a large number of living cells, there is a need for quantifying the potential Ag release from these product components. We developed an electrochemical method to evaluate possible corrosion activity of silver in AgNW transparent conductive films (TCFs) and concomitant Ag+ release. A polysiloxane polymer was used as protective coating of AgNW TCFs. A consistent correlation is observed between the degree of corrosion and the coatings' characteristics, in particular the thicknesses. A major advantage of the new approach, compared to classical aging studies, is the short experimentation time: 20 min are sufficient for a diagnostic result. The method is an accelerated corrosion and release test. It is environmentally sound methodology with use of very low electric power and with no harmful reagents. A particularly attractive application could be in the field of environmental risk assessment of metals from portable electronics and biosensors.

Résumé: Mining, water-reservoir impoundment, underground gas storage, geothermal energy exploitation and hydrocarbon extraction have the potential to cause rock deformation and earthquakes, which may be hazardous for people, infrastructure and the environment. Restricted access to data constitutes a barrier to assessing and mitigating the associated hazards. Thematic Core Service Anthropogenic Hazards (TCS AH) of the European Plate Observing System (EPOS) provides a novel e-research infrastructure. The core of this infrastructure, the IS-EPOS Platform (tcs.ah-epos.eu) connected to international data storage nodes offers open access to large grouped datasets (here termed episodes), comprising geoscientific and associated data from industrial activity along with a large set of embedded applications for their efficient data processing, analysis and visualization. The novel team-working features of the IS-EPOS Platform facilitate collaborative and interdisciplinary scientific research, public understanding of science, citizen science applications, knowledge dissemination, data-informed policy-making and the teaching of anthropogenic hazards related to georesource exploitation. TCS AH is one of 10 thematic core services forming EPOS, a solid earth science European Research Infrastructure Consortium (ERIC) (www.epos-ip.org.).

Résumé: Depleted oil reservoirs are considered a viable solution to the global challenge of CO2 storage. A key concern is whether the wells can be suitably sealed with cement to hinder the escape of CO2. Under reservoir conditions, CO2 is in its supercritical state, and the high pressures and temperatures involved make real-time microscopic observations of cement degradation experimentally challenging. Here, we present an in situ 3D dynamic X-ray micro computed tomography (mu-CT) study of well cement carbonation at realistic reservoir stress, pore-pressure, and temperature conditions. The high-resolution time-lapse 3D images allow monitoring the progress of reaction fronts in Portland cement, including density changes, sample deformation, and mineral precipitation and dissolution. By switching between flow and nonflow conditions of CO2-saturated water through cement, we were able to delineate regimes dominated by calcium carbonate precipitation and dissolution. For the first time, we demonstrate experimentally the impact of the flow history on CO2 leakage risk for cement plugging. In-situ mu-CT experiments combined with geochemical modeling provide unique insight into the interactions between CO2 and cement, potentially helping in assessing the risks of CO(2 )storage in geological reservoirs.

Résumé: Replacement or deterioration of seismic instruments and the evolution of the installation conditions and sites can alter the seismic signal in very subtle ways; therefore, it is notoriously difficult to monitor the signal quality of permanent seismic stations. We present a simple way to characterize and monitor signal quality, using energy ratios between each pair of the three components, as a complement to existing methods. To calculate stable daily energy ratios over a large frequency range (0.01-5 Hz), we use the daily median energy ratio over all 5 min windows within the day. The method is applied to all GEOSCOPE stations, for continuous BH channel data collected since 2001. We show applications to identify past gain problems (stations ROCAM and CRZF), to provide feedback after field interventions at remote sites (Antarctic station DRV), and to shed light on complex instrument problems (stations ECH and KIP). Our results show that component energy ratios have excellent time resolution and that they are visually simple for identification of problems. They can be used both for ongoing continuous monitoring of the signal quality, or as a tool to identify past problems. The Python code to produce the results in this work and the Python code for daily monitoring used by GEOSCOPE are available (see Data and Resources).

Résumé: The Pulo do Lobo domain is one of the units exposed within the orogenic suture zone between the Ossa-Morena and the South Portuguese zones in the SW Iberian Variscides. This metasedimentary unit has been classically interpreted as a Rheic subduction-related accretionary prism formed during pre-Carboniferous convergence and eventual collision between the South Portuguese Zone (part of Avalonia) and the Ossa-Morena Zone (peri-Gondwanan terrane). Discrete mafic intrusions also occur within the dominant Pulo do Lobo metapelites, related to an intra-orogenic Mississippian transtensional and magmatic event that had a significant thermal input. Three different approaches have been applied to the Devonian-Carboniferous phyllites and slates of the Pulo do Lobo domain in order to study their poorly known low-grade metamorphic evolution. X-ray diffraction (XRD) was used to identify the mineralogy and measure crystallographic parameters (illite “crystallinity” and K-white mica b-cell dimension). Compositional maps of selected samples were obtained from electron probe micro-analysis, which allowed for processing with XMapTools software, and chlorite semiempirical and thermodynamic geothermometry was performed. Thermometry based on Raman spectroscopy of carbonaceous material (RSCM) was used to obtain peak temperatures. The microstructural study shows the existence of two phyllosilicate growth events in the chlorite zone, the main one (M-1) related to the development of a Devonian foliation S-1 and a minor one (M-2) associated with a crenulation cleavage (S-2) developed in middle-upper Carboniferous times. M-1 entered well into epizone (greenschist facies) conditions. M-2 conditions were at lower temperature, reaching the anchizone-epizone boundary. These data accord well with the angular unconformity that separates the Devonian and Carboniferous formations of the Pulo do Lobo domain. The varied results obtained by the different approaches followed, combined with microstructural analysis, provide different snapshots of the metamorphic history. Thus, RSCM temperatures are higher in comparison with the other methods applied, which is interpreted to reflect a faster re-equilibration during the short-lived thermal Mississippian event. Regarding the metamorphic pressure, the data are very homogeneous: very low celadonite content (0 %-10 %) in muscovite (and low values of K-white mica b-cell dimension; 8.995 angstrom mean value), indicating a low pressure-temperature gradient, which is unexpected in a subduction-related accretionary prism. Highlights – A multidisciplinary approach has been applied to study the metamorphism of the Pulo do Lobo metapelites. – Devonian metamorphism entered epizone conditions. – Carboniferous metamorphism reached the anchizone-epizone boundary. – The inferred low-pressure gradient is incompatible with a subduction-related accretionary prism.

Résumé: After several months of eruptive activity, the subaerial cone of Anak Krakatau collapsed on December 22, 2018. The landslide event generated a tsunami that had deadly consequences within the Sunda Strait in Indonesia. Such significant collapse events are common in the geologic record but are a rare phenomenon, in the instrumented record. However, these events can have a potentially large impact on society. We have reconstructed the collapse, along with the activity preceding and following it, by combining information from official reports, remote geophysical observations, and local eyewitness accounts. It appears that the collapse of Anak Krakatau's subaerial cone led to a drastic change in the eruptive style from continuous Strombolian explosions to sustained Surtseyan. Those changes are detectable in the seismo-acoustic measurements, which, when combined with eyewitnesses, allows us to reconstruct the timing and phenomenology of the sequence. Our analysis reveals that intense eruptive activity generated sustained infrasound, unusual but not unique at Anak Krakatau, starting approximately eight hours before the collapse. Within this timeframe, two seismic signals consistent with minor mass movements as well as a momentary quiescence were identified prior to the main collapse. The data presented here indicate that Anak Krakatau failed in one collapse event, producing a tsunami with multiple waves around the volcano, the last one being the largest. Following the collapse, three volcanic plumes could be clearly identified in the satellite data and by eyewitnesses, as well as spectral lines in the seismic data. These lines, observed up to 371 km, suggest a repeating energetic explosive source lasting for seven days. The collapse produced multiple infrasound arrivals observed at regional infrasound stations, but was not recorded on the regional hydroacoustic network. Our analysis of the eruptive sequence demonstrates that a detailed eruption chronology can be reconstructed using remote methods, even in the event of failure or destruction of local monitoring infrastructure. This event also highlights that tsunamigenic flank collapses can occur with little to no warning, and be difficult to interpret in real-time, as a significant amount of non-operational analysis was required after the event, to complete the chronology. (C) 2020 The Author(s). Published by Elsevier B.V.

Résumé: The growing seismic networks and the increasing number of permanent seismic stations can help in improving the physical basis of seismic hazard assessment. For this purpose, the definition of reference site conditions is of great significance. If a reliable estimate of the reference ground motion is known, its modification at any given site can be modelled with respect to that reference site. Since the choice of a well-characterized reference site is not straightforward, mainly due to the high variability in the shallow layers, such choices prove to be affected by large uncertainties. While proxy parameters like the average S-wave velocity over the uppermost 30 m (v(S)(30)) might help in characterizing reference site conditions, such parameters are neither available at all sites nor do they allow concluding that the site is not affected by amplification and attenuation effects. In this study, we identify prospective reference sites across Europe in a harmonized and fully data-driven way. All analysis is based on freely available geological and geophysical data and no on-site measurements or site-specific proxies are required. The study accounts for both the influence of amplification and attenuation in a large frequency range. To address the key conceptual issues, we verify our classification based on machine learning techniques in which the influence of the individual site characterization parameters is investigated. Our study indicates that around 250 sites in Europe over more than 2000 investigated are not affected by local site effects and can de facto be considered as reference sites based on the criteria applied.

Résumé: A milestone of dynamo theory is Cowling's theorem, known in its modern form as the impossibility for an axisymmetric velocity field to generate an axisymmetric magnetic field by dynamo action. Using anisotropic electrical conductivity, we show that an axisymmetric dynamo is in fact possible with a motion as simple as solid-body rotation. On top of that, the instability analysis can be conducted entirely analytically, leading to an explicit expression of the dynamo threshold, which is the only example in dynamo theory.

Résumé: In March/April 2020 the Italian government drastically reduced vehicle traffic and interrupted all non-essential industrial activities over the entire national territory. Italy thus became the first country in the world, with the exception of Hubei, to enact lockdown measures as a consequence of the COVID-19 outbreak and the need to contain it. Italy is also a seismically active area, and as such is monitored by a dense permanent network of seismic stations. We analyse continuous seismic data from many stations in northern and central Italy, and quantify the impact of the lockdown on seismic ambient noise, as a function of time and location. We find that the lockdown reduces ambient noise significantly in the 1-10Hz frequency range; because natural sources of seismic noise are not affected by the lockdown, the seismic signature of anthropic noise can be characterised with unprecedented clarity, by simply comparing the signal recorded before and after the lockdown. Our results correlate well with independent evaluations of the impact of the lockdown (e.g., cell phone displacements), and we submit that ambient-noise seismology is a useful tool to monitor containment measures such as the coronavirus lockdowns.

Résumé: A number of probabilistic seismic hazard assessment (PSHA) maps have been released for Indonesia over the past few decades. This study proposes a method for testing PSHA maps using U.S. Geological Survey ShakeMap catalog considered as historical seismicity for Indonesia. It consists in counting the number of sites on rock soil for which the independent maximum peak ground acceleration (PGA) of the ShakeMap footprints between May 1968 and May 2018 exceeds the thresholds from the PSHA map studied and in comparing this number with the probability of exceedance given in the PSHA map. Although ShakeMap footprints are not as accurate and complete as continuous recorded ground motion, the spatially distributed ShakeMap covers 7,642,261 grid points, with a resolution of 1 km(2), compensating the lack of instrumental data over this period. This data set is large enough for the statistical analysis of independent PGA values on rock sites only. To obtain the subdata set, we develop a new selection process and a new comparison method, considering the uncertainty of ShakeMap estimates. The method is applied to three PSHA maps (Global Seismic Hazard Assessment Program [GSHAP], Global Assessment Report [GAR], and Standar Nasional Indonesia [SNI2017]) for a selection of sites first located in Indonesia and next only in the western part of the country. The results show that SNI2017 provides the best fit with seismicity over the past 50 yr for both sets of rock sites (whole country and western part only). At the opposite, the GAR and GSHAP seismic hazard maps only fit the seismicity observed for the set of rock sites in western Indonesia. This result indicates that this method can only conclude on the spatial scale of the analysis and cannot be extrapolated to any other spatial resolution.

Résumé: Cosmogenic nuclide dating of glacial landforms may lead to ambiguous results for ice retreat histories. The persistence of significant cosmogenic concentrations inherited from previous exposure may increase the apparent exposure ages for polished bedrocks affected by limited erosion under ice and for erratic boulders transported by glaciers and previously exposed in high-altitude rock walls. In contrast, transient burying by moraines, sediments and snow decreases the apparent exposure age. We propose a new sampling strategy, applied to four sites distributed in the Arc and Arve valleys in the Western Alps, to better constrain the factors that can bias exposure ages associated with glacial processes. We used the terrestrial cosmogenic nuclide Be-10 (TCN) to estimate the exposure time from paired sampling of depth profiles in polished bedrock and on overlying erratic boulders. For a given sampling site, the exposure ages for both the polished bedrock and boulder are expected to be the same. However, in six cases out of seven, boulders had significantly higher Be-10 surface concentrations than those of the associated polished surfaces. In present and past glacial processes, the Be-10 distribution with depth for boulders and bedrocks implies the presence of an inheritance concentration of Be-10. Our study suggests that Be-10 concentrations in erratic boulders and in polished bedrocks provide maximum and minimum exposure ages of the glacial retreat, respectively. (c) 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Résumé: Volcanic tremor is key to our understanding of active magmatic systems, but due to its complexity, there is still a debate concerning its origins and how it can be used to characterize eruptive dynamics. In this study we leverage machine learning techniques using 6 years of continuous seismic data from the Piton de la Fournaise volcano (La Reunion island) to describe specific patterns of seismic signals recorded during eruptions. These results unveil what we interpret as signals associated with various eruptive dynamics of the volcano, including the effusion of a large volume of lava during the August-October 2015 eruption as well as the closing of the eruptive vent during the September-November 2018 eruption. The machine learning workflow we describe can easily be applied to other active volcanoes, potentially leading to an enhanced understanding of the temporal and spatial evolution of volcanic eruptions. Plain Language Summary A good understanding of volcanic activity is key to managing volcanic hazards resulting from eruptive activity. Volcanic tremor is a continuous seismic signal often seen during eruptions associated with the flow of magma through the volcano and is thus an extremely useful tool in characterizing the progression and phases of eruptions. In this study we study this signal at the Piton de la Fournaise volcano, on La Reunion island. Using machine learning algorithms, we investigate characteristics of this signal emitted by the volcano during eruptions to reveal the fundamental frequency at which it occurs, as well as changes in eruptive state that occur during some eruptions in our data set. This workflow may be applied to other volcanos to further our understanding of eruptive dynamics.

Résumé: In the continental crust, faults may accommodate deformation through aseismic creep, slow slip events, or seismic slips that produce dynamic damage, or a combination of these endmembers. A variety of parameters controls the occurrence of these mechanical behaviors. In a series of laboratory experiments, we image centimeter-scale faults during sliding under in situ conditions. We perform four experiments of slip on centimeter-scale crystalline rock samples prepared with a saw-cut interface at 45 degrees from the direction to the maximum compressive stress and at stress conditions of 2-3 km depth. We image fault slip and off-fault fracture development using 4D synchrotron X-ray microtomography. Three faults have an initial rough interface, and deformation occurs with increasing differential stress by a combination of slow slip events and off-fault damage, until catastrophic failure and the formation of new faults. Conversely, the pre-cut fault with a smooth initial surface deforms mainly by slow slip, develops numerous striations along its slip plane, and no microfractures are detected in the wall rock. Our experiments reproduce aseismic and seismic faulting behavior, and demonstrate that the roughness of the fault plane is one of the parameters that control the transition between these two behaviors. A fault with a rougher interface may tend to develop more off-fault damage and seismic behavior. For the rough fault experiments, the secondary faulting occurs along a network of faults oriented at high angles from the pre-existing saw-cut plane, a behavior similar to several earthquake sequences that occurred along orthogonal faults.

Résumé: Body waves can be extracted from correlation functions computed from seismic records even at teleseismic distances. Here we use P and PcP waves from the secondary microseism frequency band that are propagating between Europe and the Eastern United States to image the core-mantle boundary (CMB) and D '' structure beneath the North Atlantic. This study presents the first 3-D image of the lower mantle obtained from ocean-generated microseism data. Robustness of our results is evaluated by comparing images produced by propagation in both directions. Our observations reveal complex patterns of lateral and vertical variations of P-wave reflectivity with a particularly strong anomaly extending upward in the lower mantle up to 2600 km deep. We compare these results with synthetic data and associate this anomaly to a V-p velocity increase above the CMB. Our image aims at promoting the study of the lower mantle with microseism noise excitations.

Résumé: The Rhône River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric synthetic-aperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouvière fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates.

Résumé: The Historic Sanctuary of Machu Picchu (Cusco, Peru) is one of the most important archaeological monuments in Peru and worldwide. Machu Picchu is classified as a UNESCO World Heritage site and at risk from climatic change. However, the seismic centennial history of Peru reports large earthquakes generated both along the subduction zone (Mw8) and on active crustal faults along the Andean Cordillera (Mw7). It is therefore important to know if Machu Picchu is located in an area of seismic hazard and then to take measures to mitigate potential seismic hazards. Due to the short historical earthquake catalogue (< 500 years) and the absence of significant recent instrumental seismicity in the site's vicinity (radius of < 30 km), our knowledge about the seismic hazard in Machu Picchu is limited. The earthquakes of 1650 and 1950 affected Cusco city and surrounding areas, but without damage descriptions in Machu Picchu (80 km away) (Silgado Ferro 1978). In this study, we make the first attempt to use the analysis of earthquake archaeological effects (EAEs) and their differentiation from the effects generated by slope movements (creep) to investigate the past occurrence of strong earthquakes at the site. The application of geological structural analysis to the deformations observed in Machu Picchu shows two directions of the mean ground movement: N020 degrees E and N110 degrees E. Two earthquakes that affected Machu Picchu during its construction generated these directions. This kind of data should be used in the future to protect this important archaeological site.

Résumé: We apply interferometry by deconvolution to compute the shear-wave velocity in shallow sediments (0-83.4 m) based on earthquake records from a vertical accelerometric array (ARGOstoli Network [ARGONET]) on Cephalonia Island, Greece. Analysis of the time variation of measured values reveals a cyclical pattern, which correlates negatively to rainfall and a soil moisture proxy. The pattern includes a sharp reduction in velocity at the beginning of rainy seasons and a gradual rise toward dry periods, the overall variation being around 20%-25% within the shallowest depth interval examined (0-5.6 m) and estimated to reach 40% within the top 2 m. The variation itself and its amplitude are verified by surface-wave dispersion analysis, using ambient vibration data. Synthetic standard spectral ratios suggest that this seasonal effect leaves an imprint on soil response, causing differences in the level of high-frequency ground motion between dry and rainy seasons, and this is verified by earthquake records. Furthermore, the near-surface velocity decrease due to soil saturation can be of the same order of magnitude as the nonlinear coseismic variation, masking the physical process of the nonlinear response of the site due to weak-to-strong-motion shaking. Thus, seasonal variations of seismic-wave velocities in shallow sediments may be important for a number of site-effect related topics, such as high-frequency ground-motion variability, soil anisotropy, kappa measurements, nonlinear site response, and so on.

Résumé: Across the extreme south of Patagonia, the Magallanes-Fagnano Fault (MFF) accommodates the left-lateral relative motion between South America and Scotia plates. In this paper, we present an updated view of the geometry of the eastern portion of the MFF outcropping in Tierra del Fuego. We subdivide the MFF in eight segments on the basis of their deformation styles, using field mapping and interpretation of high-resolution imagery. We quantify coseismic ruptures of the strongest recorded 1949, M(w)7.5 earthquake, and determine its eastern termination. We recognize several co-seismic offsets in man-made features showing a sinistral shift up to 6.5 m, greater than previously estimated. Using Be-10 cosmogenic nuclides depth profiles, we date a cumulated offset in post-glacial morphologies and estimate the long-term slip rate of the eastern MFF. We quantify a 6.4 +/- 0.9 mm/a left-lateral fault slip rate, which overlaps geodetic velocity and suggests stable fault behaviour since Pleistocene.

Résumé: The Lesser Antilles arc is a mixed siliciclastic and carbonate active margin made of active volcanic and flat Plio-Quaternary carbonate islands. It was built as a result of a complex tectonic history at the slowly converging boundary between the American plates and the Caribbean plate. The sedimentary processes as a consequence of external forcing (earthquakes, volcanism, hurricanes) were rarely documented in such environment and are poorly understood. We exploited an exceptional dataset of high-resolution marine seafloor data acquired during the last 20 years in the northern part of the Lesser Antilles forearc to document the sediment-transport processes. We achieved a detailed morpho-sedimentary study from multi-beam bathymetry, backscattering, and seismic profiles. Two areas could be characterized: 1) the “Rough Area”, along Barbuda to Guadeloupe carbonated islands, characterized by steep (up to 25 degrees) slopes incised by short canyons, and deep basins controlled by major normal faults; 2) the “Channelized Area”, south of Guadeloupe and bordered by active volcanic islands and carbonate platforms, characterized by gentle slopes incised by long canyons. During sea-level high-stands, the sediment seems exported from the carbonate platform by hurricanes or density cascading but appears to settle at the shelf-edge and canyon heads. During sea-level low-stands, a connection may exist between onshore and offshore systems. However, this sediment supply appears not sufficient to generate canyon formation, likely shaped by regressive processes. Shelf breaks of the carbonate banks, platforms and submarine slopes are affected by sediment failures. Some may be associated to voluminous remobilizations and large mass transport deposits. Large earthquakes are likely the main processes in this area to remobilize sediments toward the deep forearc basins by triggering both slope failures and flushing of the canyon heads.

Résumé: Ambient noise seismology has revolutionized seismic characterization of the Earth's crust from local to global scales. The estimate of Green's function (GF) between two receivers, representing the impulse response of elastic media, can be reconstructed via cross-correlation of the ambient noise seismograms. A homogenized wave field illuminating the propagation medium in all directions is a prerequisite for obtaining an accurate GF. For seismic data recorded on glaciers, this condition imposes strong limitations on GF convergence because of minimal seismic scattering in homogeneous ice and limitations in network coverage. We address this difficulty by investigating three patterns of seismic wave fields: a favorable distribution of icequakes and noise sources recorded on a dense array of 98 sensors on Glacier d'Argentiere (France), a dominant noise source constituted by a moulin within a smaller seismic array on the Greenland Ice Sheet, and crevasse-generated scattering at Gornergletscher (Switzerland). In Glacier d'Argentiere, surface melt routing through englacial channels produces turbulent water flow, creating sustained ambient seismic sources and thus favorable conditions for GF estimates. Analysis of the cross-correlation functions reveals non-equally distributed noise sources outside and within the recording network. The dense sampling of sensors allows for spatial averaging and accurate GF estimates when stacked on lines of receivers. The averaged GFs contain high-frequency ( > 30 Hz) direct and refracted P waves in addition to the fundamental mode of dispersive Rayleigh waves above 1 Hz. From seismic velocity measurements, we invert bed properties and depth profiles and map seismic anisotropy, which is likely introduced by crevassing. In Greenland, we employ an advanced preprocessing scheme which includes matchfield processing and eigenspectral equalization of the cross spectra to remove the moulin source signature and reduce the effect of inhomogeneous wave fields on the GFs. At Gornergletscher, cross-correlations of icequake coda waves show evidence for homogenized incident directions of the scattered wave field. Optimization of coda correlation windows via a Bayesian inversion based on the GF cross coherency and symmetry further promotes the GF estimate convergence. This study presents new processing schemes on suitable array geometries for passive seismic imaging and monitoring of glaciers and ice sheets.

Résumé: The Indus-Yarlung suture of southernmost Tibet marks the initial collisional zone, the ongoing India-Asia collision, and yet more than similar to 30 million years after the onset of collision, a thick detrital sedimentary unit was deposited just north of the suture: the Kailas Formation. The mechanism permitting subsidence of the deep intracontinental Kailas basin in a compressional tectonic regime remains uncertain. We present new apatite (16-11 Ma) and zircon (24-19 Ma) fission track (AFT and ZFT) ages from the Gangdese batholith just north of the Kailas basin. ZFT analysis of modern-river sand from the northern Gangdese magmatic arc indicates an exhumation at 27.3 +/- 1.3 Ma. Thermal modeling indicates that the batholith experienced reheating between 28 and 20 Ma, coeval with deposition in the Kailas basin (between 26 and 21 Ma), followed by overall rapid cooling between 20 and 17 Ma. We interpret this thermal history as a phase of regional Oligocene-Miocene sedimentary burial followed by exhumation. By modeling mantle dynamics in the geodynamic framework of the India-Asia collision, we show that transient dynamic topography over the relative southward folding of the Indian slab is consistent with burial and exhumation of the Gangdese magmatic arc during Oligocene-Miocene time. The northward migration of the Indian continent relative to its own stati onary slab created a wave of dynamic topography that caused subsidence in the overriding plate north of the Himalaya, followed by a phase of surface uplift since similar to 27 Ma of the northern Gangdese magmatic arc. During latest Oligocene-early Miocene time, the dynamic deflection center was in the Kailas area, and it progressively relocated southward to its present position at the Ganges basin.