Shear heating-induced thermal pressurization during earthquake nucleation

[1] We model earthquake nucleation (in 2D) on narrow faults with coupled rate-state friction and shear heating-induced thermal pressurization, including diffusive transport of heat and pore pressure. Thermal pressurization increases pore pressure p, decreasing frictional resistance. Observed fault core permeability is generally too low to mitigate thermal pressurization at subseismic slip speeds. Under drained, isothermal conditions, nucleation with the aging law is crack like, with the interior of the slip zone always near maximum slip speed. When thermal pressurization is included, it can dominate weakening at speeds of 0.02–20 mm/s for hydraulic diffusivities chyd from 10−8 to 10−3 m2/s and nominal material parameters well before seismic radiation occurs. Dramatic along-strike localization of slip occurs due to feedback in which the area of maximum slip experiences the greatest weakening, which in turn favors more slip. With the slip law, however, nucleation is pulse like, with slip speed decaying behind the pulse tip. Thermal pressurization is diminished relative to the aging law case since most weakening occurs in locations with limited slip, yet we find that it can overwhelm frictional weakening at slip speeds in the range of 1–100 mm/s for chyd from 10−8 to 3 × 10−5 m2/s. At higher slip speeds, the finite thickness of the shear zone becomes significant, reducing thermal pressurization. Even if not the dominant weakening mechanism, thermal pressurization is likely to be significant at or before the onset of seismic radiation.