Implications of the thermal properties and kinetic parameters of dehydroxylation of mica minerals for fault weakening, frictional heating, and earthquake energetics

Transient frictional heating during earthquake slip induces dehydroxylation of phyllosilicate minerals. As this reaction is endothermic and releases H2O, it affects dynamic fault weakening and the energetics of earthquakes. To quantitatively evaluate this effect, accurate determination of both the kinetic parameters of the reaction and the thermal properties of the minerals is needed. We chose an illite–muscovite sample as representative of the phyllosilicates found in active crustal faults. For this sample, we measured the specific heat capacity and thermal diffusivity, investigated their temperature dependencies, and determined the weight loss and enthalpy of the dehydroxylation reaction to be 5.22 wt.% and 0.2895 kJ g− 1, respectively. We applied Friedman analysis to the weight loss data from heating experiments and found that the dehydroxylation reactions were well fitted by two-step reactions of an n-dimensional nucleation mechanism according to the Avrami–Erofeev equation with n = 0.5 (first step) and two-dimensional diffusion (second step). On the basis of these experimental results, we performed numerical analyses of dynamic fault weakening, which demonstrated that the fluids released by dehydroxylation contribute to pressurization of pore fluids inducing a decrease in effective normal stress on faults, and that the dehydroxylation reaction absorbs heat from the energy released. We also performed a sensitivity analysis on the kinetic function and parameters and the thermal properties, which showed that the contribution of these to fault weakening is considerably smaller than those of frictional coefficient and slip-zone thickness.