By what mechanism(s) do earthquakes propagate ?

Harry GREEN, University of California Riverside, USA | 13 septembre 2012

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The mechanisms by which earthquakes originate seem to be well characterized. At shallow depths (less than about 30-35 km), the consensus is that earthquakes originate when the local stress exceeds static friction. At greater depths, the normal stress inhibits direct failure and an enabling mineral reaction is required : dehydration or decarbonation at depths greater than 50 km and phasetransformation- induced faulting during the exothermic phase transformation from olivine to its highpressure polymorphs above about 350 km. Earthquakes stop at the base of the mantle transition zone (≤700 km) because the mineral reaction to perovskite + ferropericlase cannot initiate failure. How earthquakes propagate after initiation is unclear. At high pressure, frictional sliding processes are prohibited by the high normal stress ; earthquakes apparently propagate by generation of a nanocrystalline gouge that flows by grain-boundary sliding (gbs), a process only weakly dependent on normal stress. Under crustal conditions, recent high-speed friction experiments show a large drop in friction within a second after initiation of sliding. At moderate normal stress the shear heating under such circumstances should initiate devolatilization of any hydrous phases or carbonates present. When this happens, a nanocrystalline gouge is created that potentially could flow by gbs like at high pressure. To test this hypothesis, my colleagues and I are currently performing high-speed friction experiments accompanied by comparison of gouge microstructure with that of high-pressure faulting at the highest resolution of Scanning and Transmission Electron Microscopy.

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