Fluid Drainage Leads to Thermal Decomposition of Wet Gouge During Experimental Seismic Slip

Abstract Several borehole cores intersecting faults related to coseismic slip display high‐temperature features, including thermal decomposition of fault gouge. We present evidence that these features may be related to fluid drainage of the slip zone during seismic slip. We sheared water‐saturated kaolinite powders under both fluid drained and undrained conditions, expected for seismic slip at shallow crustal depths. Our results show typical dynamic weakening behavior regardless of conditions. Under fluid drained condition, restrengthening accompanied by the thermal decomposition of kaolinite occurs. In addition, thermal decomposition of kaolinite tends to be initiated at high normal stresses (>5 MPa) with short displacement (<5 m). We propose that thermal pressurization acts as a weakening mechanism but ceases because of fluid drainage, triggering kaolinite thermal decomposition. This finding explains seismic‐slip‐related clay anomalies at depth rather than at the surface, as observed in the borehole after the 1999 Mw 7.6 Chi‐Chi earthquake, Taiwan.