Influence of fault slip rate on shear‐induced permeability

Abstract

We measured permeability in sandstone and granite sheared at slip rates from 10−4 to 1.3 m/s under low‐normal stress at confining pressures up to 120 MPa. As the slip rate increased, the permeability of Berea sandstone decreased by an order of magnitude, whereas that of Indian sandstone and Aji granite increased by 3 orders of magnitude at high slip rates. A fine‐grained gouge layer of thickness developed during slip, and the wear rate was increased abruptly at high slip rates. Microcracks and mesoscale fractures formed at slip rates above 0.13 m/s. Numerical modeling showed that the slip surface temperature increased by several hundred degrees for slip velocities above 0.13 m/s and exceeded the α‐β phase transition temperature of quartz at 1.3 m/s. Both the temperature rise and the temperature gradient at the slip surface were high at fast slip rates. We attributed reduced permeability after slip in porous sandstone to the low‐permeability gouge layer. An abrupt permeability increase in low‐permeability rocks at high slip rates was caused by heat‐induced cracks. An increase in the rate of wear of gouge with increasing slip velocity was caused by frictional heating that reduced the rock strength. The host‐rock permeability that separated reductions and increases in permeability was about 10−16 m2 at 10 MPa effective pressure. Our results suggest that abrupt increases in shear stress during slip in a low‐permeability fault zone caused by thermal cracking, which may decrease the total slip displacement. The abrupt permeability increase at high slip rates in low‐permeability rocks agrees with hydrogeochemical phenomena observed after earthquakes.