Rate/State Frictional Nucleation and Dynamic Rupture on Low-Stress Faults With Thermal Weakening Effects

It is believed that faults sustain large shear stresses during the nucleation phase, but slip at much lower stresses during earthquakes. The nucleation phase is typically characterized by rate- and state-dependent friction, in which slip accelerates after shear stress τ grows to about 0.6 to 0.9 times the effective normal stress σ_eff. During earthquakes, dynamic weakening mechanisms such as shear heating-induced thermal pressurization of pore fluid and flash heating of asperity contacts allow for slip to occur at much lower values of τ. Since 0.6σ_eff is ~100 MPa at seismogenic depths with hydrostatic pore pressure, earthquakes likely nucleate in regions of elevated τ/σ_eff and propagate outward into regions of low τ/σ_eff. Noda & others [JGR, 2009] investigated this issue with numerical simulations in which they included both flash weakening and thermal pressurization. Their ruptures were initiated by a sudden perturbation in τ on a weakly stressed fault. Flash heating quickly dominated the fault's strength, with thermal pressurization contributing a modest amount of additional weakening. Recent work has shown that thermal pressurization may, however, become the dominant weakening mechanism during the quasi-static nucleation phase of an earthquake, well before the onset of seismic radiation [Schmitt & others, JGR, 2011]. We present more physically-motivated simulations of early rupture that model fault weakening mechanisms through nucleation and into the dynamic slip phase. The initial τ/σ_eff heterogeneities remain idealized, but we now consider two types. The first type is a region of elevated τ with uniform σ_eff, while the other type has uniform τ with a low-strength region of diminished σ_eff. We use a quasi-static code to model fault slip from well below steady-state to the early stages of nucleation. Late in nucleation, we switch to a fully dynamic code that otherwise solves the same equations for friction and coupled thermal effects. Our results indicate that there are differences between ruptures on high-stress and low-strength heterogeneities. In rate/state frictional nucleation, slip zones capable of accelerating above steady state have a minimum length dimension L_min. Heterogeneities must be of greater size than a few L_min in order to host instability at low background stress, and since L_min∝σ_eff^-1, nucleation under low strength conditions requires a much larger heterogeneity. Thermal weakening effects are also affected by the heterogeneity type; they are subdued in the low-strength case because of reduced heating with low τ. Detection of precursory slip, early seismic radiation, or thermal anomalies with near-field instruments may be capable of providing observational constraints on our models.