FLASH WEAKENING OF SERPENTINITE AT SEISMIC SLIP RATES
Abstract
Flash heating of highly stressed asperities is one mechanism thought to promote dynamic weakening of fault rocks at seismic slip rates (Rice et al.; 2006, 2009). Previous high velocity friction experiments on serpentinite, an abundant fault rock in the shallow lithosphere, indicate that weakening behavior is characterized by a 1/V decrease in friction above V = 0.1 m/s (Goldsby and Hirth, 2006). Similar results are reported by Hirose et al. (2007) for slip rates up to 1.1 m/s over displacements δ = 5 m. To investigate the processes responsible for weakening, we performed constant velocity and velocity-stepping experiments on antigorite serpentinite, over relatively small displacements (δ = 0.04 m) and high slip rates (0.01 to 0.4 m/s) and then analyzed both the microstructures of the sliding surface and the composition of the fault gouge. Single-velocity experiments reinforce previous results showing the approximately 1/V dependence of friction on velocity above a threshold velocity of Vw = 0.1 m/s. Velocity-stepping experiments show decreasing friction with increasing velocity above Vw, with full weakening occurring in just several millimeters of slip - much smaller displacements than inferred from the higher displacement experiments of Hirose and Bystricky. XRD analyses of comminuted gouge from these samples shows significant formation of talc from the decomposition of antigorite - providing estimates of temperatures at asperity sites (350-500 °C) during slip. These independent constraints on the thermal, microstructural and mechanical evolution of the sliding surface support the flash heating hypotheses for dynamic weakening. The results of these experiments indicate that ruptures propagating at seismic velocities (V = 1 m/s > Vw) into serpentinized regions in the shallow lithosphere may trigger seismicity after limited amounts of displacement. 1-atm Rotary Shear experiments approaching seismic velocities (0.3 m/s) on serpentinite show a decrease in friction from 0.5 to a steady-state weakened friction at 0.2 in serveral mm of slip.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T23C1944K
- Keywords:
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- 8034 STRUCTURAL GEOLOGY / Rheology and friction of fault zones;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8123 TECTONOPHYSICS / Dynamics: seismotectonics