Three-dimensional earthquake sequence simulations with evolving temperature and pore pressure due to shear heating: Effect of heterogeneous hydraulic diffusivity

A new methodology for three-dimensional (3-D) simulations of earthquake sequences is presented that accounts not only for inertial effects during seismic events but also for shear-induced temperature variations on the fault and the associated evolution of pore fluid pressure. In particular, the methodology allows to capture thermal pressurization (TP) due to frictional heating in a shear zone. One-dimensional (1-D) diffusion of heat and pore fluids in the fault-normal direction is incorporated using a spectral method, which is unconditionally stable, accurate with affordable computational resources, and highly suitable to earthquake sequence calculations that use variable time steps. The approach is used to investigate the effect of heterogeneous hydraulic properties by considering a fault model with two regions of different hydraulic diffusivities and hence different potential for TP. We find that the region of more efficient TP produces larger slip in model-spanning events. The slip deficit in the other region is filled with more frequent smaller events, creating spatiotemporal complexity of large events on the fault. Interestingly, the area of maximum slip in model-spanning events is not associated with the maximum temperature increase because of stronger dynamic weakening in that area. The region of more efficient TP has lower interseismic shear stress, which discourages rupture nucleation there, contrary to what was concluded in prior studies. Seismic events nucleate in the region of less efficient TP where interseismic shear stress is higher. In our model, hypocenters of large events do not occur in areas of large slip or large stress drop.