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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. Plain Language Summary: Seismic faulting at depth can drive thermochemical reactions within the slip zone, given the high slip velocity and large displacement. Several slip‐zone samples from deep drilling projects following catastrophic earthquakes have exhibited high‐temperature geological characteristics, which were not present in fault zone outcrops hosting surface ruptures. We sheared kaolinite (as an analogue of fault zone materials) under both fluid drained and undrained conditions, simulating conditions expected during seismic slip at borehole depths. Our results show that, regardless of the applied conditions, the materials tend to weaken dramatically during shearing. However, when fluids are allowed to drain from the slip zone, there is a subsequent strengthening accompanied by the thermal decomposition of kaolinite. We suggest that thermal pressurization operates as the weakening mechanism but is ceased due to fluid drainage, resulting in the thermal decomposition of kaolinite gouges. In addition, the thermal decomposition of kaolinite tends to be triggered at large normal stresses. Because kaolinite is a common component in both fault zones and subduction zones at shallow depths, our findings have potential implications for reported thermally driven reactions within slip zones as a potential seismic indicator at shallow crustal depths. Key Points: Thermal pressurization operates as the dynamic weakening mechanism during experimental seismic slip regardless of the ambient conditionsThermal pressurization ceases because of fluid drainage and is followed by kaolinite thermal decompositionThermal decomposition of kaolinite tends to be initiated at large normal stresses in the seismic slip range [ABSTRACT FROM AUTHOR]