Talc is generally considered to be frictionally stable, yet the mechanochemical effect of extensive comminution along the fault gouge remains poorly understood. Here, we report that intact hydrophobic crystalline talc was mechanochemically changed into hydrophilic talc, by comminution using high‐energy ball milling. The weight fraction of water on the intact talc was close to zero, which gradually increased to approximately 0.13 with comminution. Then, we performed thermo‐mechanical‐chemical numerical modeling of thermal pressurization (TP) under seismic slip with parameterization of the water content of hydrophilic talc. In the comminuted hydrophilic talc model, the effective shear stress of the talc‐bearing fault patch converges to near zero, accompanied by pore pressure buildup due to seismic frictional heating and TP. Our results highlight that a fault containing the comminuted talc has the potential to exhibit slip‐weakening frictional behavior and catastrophic fault rupture, beyond the previous thought that the talc is frictionally stable with slip‐strengthening. Plain Language Summary: Through a combined approach of laboratory experiments and numerical modeling, we discovered that comminuted talc makes seismic faults more unstable than previously thought, potentially leading to sudden fault slip and even catastrophic rupture. Unlike intact talc, which is hydrophobic and frictionally stable (i.e., velocity/slip strengthening), talc that has been ground into fine particles becomes hydrophilic and absorbs significant amounts of water. Our laboratory experiments show that as talc is ground, the talc absorbs water, with the weight fraction of water (χ $\chi $) increasing up to 0.13, which affects how the talc behaves during seismic slip. Our thermo‐mechanical simulation shows that highly comminuted and hydrated talc can lead to a rapid reduction in shear stress during fault slip due to thermal pressurization. The water absorbed by the talc can be hydrated by frictional heating, leading to an increase in pore pressure within the fault gouge and ultimately fault weakening. The comminuted talc can simultaneously increase and decrease the pore pressure and temperature elevation, respectively, by endothermic dehydration reaction. Our study provides insight into how faults containing hydrophilic talc through long‐term comminution can seismically slip even with low surface heat flux. Key Points: Comminuted talc exhibits slip‐weakening behavior, contrary to the conventional view of frictional stability with slip‐strengtheningHydrophobic talc turned hydrophilic after ball milling, reaching a water weight fraction of 0.13 with a reduced contact angle (64°–22°)Numerical simulation shows hydrophilic talc leads to strong thermal pressurization and near‐zero shear stress during seismic slip [ABSTRACT FROM AUTHOR]

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