Rapid postseismic strength recovery of Pingxi fault gouge from the Longmenshan fault system: Experiments and implications for the mechanisms of high‐velocity weakening of faults

Abstract

AbstractSlide‐hold‐slide tests were conducted on gray blackish gouge (GBG) and yellowish gouge (YG) from Pingxi fault zone to see how rapidly the strength of Longmenshan fault recovers after the 2008 Wenchuan earthquake. Twenty dry runs were made at a normal stress of 0.8 MPa, at a seismic slip rate of 1.4 m/s, and with hold time th ranging 0.3–105 s. Results exhibit very rapid healing by more than 0.4 in friction coefficient μ in less than 5–10 s, followed by gradual healing in proportion to log(th). Healing rates, (increase in μ)/(log (th)), during rapid and slow healing are 0.188 and 0.015 for GBG and 0.154 and 0.016 for YG, respectively. The average temperature in the outer half of a 5 µm thick slip zone decreases from 260–300°C to 110–170°C in 5–10 s, and hence, temperature drop appears to be correlated with the rapid healing. Previously reported rapid healing at subseismic slip rates (85–90 mm/s) begins to occur in 10–300 s after the stop of sliding, and this cannot be explained by the cooling of gouge. The difference in healing between subseismic slip rates (delayed and rapid healing) and seismic slip rates (immediate and rapid healing) suggests that the dominant weakening mechanism shifts from tribochemical processes at subseismic slip rates to frictional heating at seismic slip rates. Slip‐zone structures are too complex and variable from run to run to reveal microscopic mechanisms for the strength recovery. Rapid healing following seismic slip can be a cause for reduced aftershocks along major coseismic faults.