EARTHQUAKES DRIVEN BY THERMAL PRESSURIZATION OF PORE FLUIDS

Earthquake nucleation requires reduction of frictional strength with slip or slip rate. One school of thought, pioneered by Deiterich and Ruina, has focused on rate-state friction effects at constant effective normal stress. On the other hand, it has been known since work of Sibson and Lachenbruch that shear heating increases pore pressure and, if dilatancy and thermal and fluid diffusion are limited, will cause shear resistance to decrease. In this work we examine the coupled effects of rate-state friction, shear heating, thermal and pore-fluid diffusion. Given permeabilities measured on samples from active fault zones, one can show that at slip speeds appropriate for earthquake nucleation fluid diffusion prevents pore pressures from increasing. Thus, shear heating alone cannot nucleate earthquakes; frictional - weakening as in rate-state friction - is required. However, as slip speeds increase shear heating eventually increases pore pressure faster than can be dissipated by Darcy flow. Ignoring the feedback between thermal weakening and slip speed, Segall and Rice (JGR, in press) estimate that the rate of thermal weakening exceeds the rate of frictional weakening for slip speeds in excess of 0.01 to 10 mm/s, depending on parameters chosen. We report here on numerical calculations, with one-dimensional fluid and heat transport perpendicular to the fault plane, that include all thermal-mechanical couplings. The preliminary results suggest that thermal effects are dominant late in the nucleation process, well before seismic waves are radiated, as well as during fast seismic slip. This would indicate that the earthquake cycle can be divided into a locked and early nucleation phase dominated by rate-state friction and a late-nucleation seismic phase dominated by shear induced thermal pressurization.