Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes
Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s(2)), slip rates (similar to 1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of fault patches, measured large fault dynamic weakening for slip rates larger than a critical velocity of 0.01-0.1 m/s. The dynamic weakening corresponds to a decrease of the friction coefficient (defined as the ratio of shear stress vs. normal stress) up to 40%-50% after few millimetres of slip (flash weakening), almost independently of rock type. The microstructural evolution of the sliding interfaces with slip may yield hints on the microphysical processes responsible for flash weakening. At the microscopic scale, the frictional strength results from the interaction of micro- to nano-scale surface irregularities (asperities) which deform during fault sliding. During flash weakening, the visco-plastic and brittle work on the asperities results in abrupt frictional heating (flash heating) and grain size reduction associated with mechano-chemical reactions (e.g., decarbonation in CO2-bearing minerals such as calcite and dolomite; dehydration in water-bearing minerals such as clays, serpentine, etc.) and phase transitions (e.g., flash melting in silicate-bearing rocks). However, flash weakening is also associated with grain size reduction down to the nanoscale. Using focused ion beam scanning and transmission electron microscopy, we studied the micro-physical mechanisms associated with flash heating and nanograin formation in carbonate-bearing fault rocks. Experiments were conducted on pre-cut Carrara marble (99.9% calcite) cylinders using a rotary shear apparatus at conditions relevant to seismic rupture propagation. Flash heating and weakening in calcite-bearing rocks is associated with a shock-like stress release due to the migration of fast-moving dislocations and the conversion of their kinetic energy into heat. From a review of the current natural and experimental observations we speculate that this mechanism tested for calcite-bearing rocks, is a general mechanism operating during flash weakening (e.g., also precursory to flash melting in the case of silicate-bearing rocks) for all fault rock types undergoing fast slip acceleration due to the passage of the seismic rupture front.