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Importance of mechanochemical effects on fault slip behavior during earthquakes

Recent analyses of natural faults and laboratory rock friction experiments have revealed that physical transformations and chemical reactions of minerals and fluids occur in faults during earthquakes. However, exactly how these physicochemical processes affect fault weakening and earthquake energetics is not well understood. We present quantitative experimental evidence of mechanochemical changes in a clay mineral (illite) during frictional slip and evaluate the effect of those changes on slip behavior. Friction distorts the crystal structure of illite and dramatically decreases the activation energy (Ea) of the dehydroxylation reaction. Numerical modeling shows that the lower Ea allows the dehydroxylation to occur at relatively low temperatures (200–300°C), which in turn drastically reduces fault strength by pressurization of released water from the mineral. Thus, dynamic thermo‐chemo‐mechanical processes can strongly affect earthquake instability, especially along clay‐rich faults such as those in the shallow part of subduction zone plate boundaries.