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We investigated the frictional properties of carbonate‐rich gouge layers at a slip rate of 1.3 m/s, under dry and water‐saturated conditions, while monitoring temperature at different locations on one of the gouge‐host rock interfaces. All experiments showed a peak frictional strength of 0.4–0.7, followed by strong slip weakening to steady state values of 0.1–0.3. Experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100°C, contemporaneous with the dynamic slip weakening and consistent with thermodynamic considerations of ongoing vaporization of pore water. Upon pore fluid vaporization, the pore pressure increases, while the temperature is buffered endothermically, such that the pore water moves along the liquid‐vapor transition curve in a pressure‐temperature phase diagram. Pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels, enhancing fluid pressurization while impeding the achievement of high temperatures. Therefore, the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes.

Supplementary Information This file provides supporting information for article "The crucial role of temperature in high-velocity weakening of faults: Experiments on gouge using host blocks with different thermal conductivities" by Yao et al. Topics covered here include: 1. Summary of experiments and supplementary data (Table DR1, Fig. DR1); 2. Details of temperature calculation (Table DR2, Fig. DR2); 3. Supplementary information on microstructures (Fig. DR3); 4. Details of fits to experimental data using a flash heating model updated by Platt et al., (2014) (Fig. DR4).

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[1] Slide-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 t h ranging 0.3-10 5 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(t h ). Healing rates, (increase in μ)/(log (t h )), 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.Citation: Yao, L., T. Shimamoto, S. Ma, R. Han, and K. Mizoguchi (2013), 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,