Earthquake Nucleation with Thermal Pressurization in Finite-Width Shear Zones: Aging Law and Slip Law

Recent work has suggested that shear heating-induced thermal pressurization may become the dominant fault weakening mechanism during the quasi-static nucleation phase of an earthquake, well before the onset of seismic radiation. For the ``aging'' form of rate- and state-dependent friction, Schmitt & others [AGU, 2007] confirmed this hypothesis using 2D numerical simulations that couple friction on a zero-width fault with shear heating and diffusive transport of heat and pore pressure. Depending on fault zone hydraulic diffusivity, they found that thermal pressurization dominates frictional weakening when slip attains speeds greater than 0.02 to 2 mm/s. In the present work, we explore differences in how thermal pressurization interacts with the ``slip'' (logarithmic) form of rate and state friction. Nucleation with the aging law form of rate-state friction is ``cracklike,'' with the interior of the nucleation zone always slipping at nearly the maximum speed. Maximum slip is at the center, and since thermal pressurization is effectively a slip weakening mechanism, dramatic along-strike localization of the nucleation zone results. With the slip law, however, nucleation is ``quasi-pulselike,'' in that the fastest-slipping portion of the nucleation zone propagates unidirectionally, with velocity decaying behind [for example, Ampuero & Rubin, JGR, 2008]. Consequently, thermal pressurization is diminished since most of the frictional weakening occurs in locations with limited amounts of slip. Despite this, numerical simulations indicate that thermal pressurization dominates frictional weakening at slip speeds in the range of 1-300 mm/s, well before seismic radiation dominates the slip dynamics. Past simulations had a shortcoming in ignoring the finite-width of the actively shearing zone. Such an approximation is valid for frictionally-dominated slow slip speeds, but fails near the time that thermal pressurization prevails. To address that issue, we have developed a code that accounts for the finite-width fault zone. Such a geometry distributes the frictional work over a broader area, thereby diminishing the thermal pressurization effect. Yet we observe that our basic result remains true: thermal pressurization dominates frictional weakening well before radiation damping is important.