Strain localization driven by thermal decomposition during seismic shear

Field and laboratory observations show that shear deformation is often extremely localized at seismic slip rates, with a typical deforming zone width on the order of a few tens of mi-crons. This extreme localization can be understood in terms of thermally driven weakening mechanisms. A zone of initially high strain rate will experience more shear heating and thus weaken faster, making it more likely to accommodate subsequent deformation. Fault zones of-ten contain thermally unstable minerals such as clays or carbonates, which devolatilize at the high temperatures attained during seismic slip. In this paper, we investigate how these ther-mal decomposition reactions drive strain localization when coupled to a model for thermal pres-surization of in-situ ground water. Building on Rice et al. [2014], we use a linear stability anal-ysis to predict a localized zone thickness that depends on a combination of hydraulic, fric-tional, and thermochemical properties of the deforming fault rock. Numerical simulations show that the onset of thermal decomposition drives additional strain localization when compared with thermal pressurization alone, and predict localized zone thicknesses of ⇠7 and ⇠13 µm for lizardite and calcite respectively. Finally we show how thermal diffusion and the endother-mic reaction combine to limit the peak temperature of the fault, and that the pore fluid re-leased by the reaction provides additional weakening of ⇠ 2040 % of the initial strength. 1