Nucleation and dynamic rupture on weakly stressed faults sustained by thermal pressurization

Abstract

AbstractEarthquake nucleation requires that the shear stress τ locally reaches a fault's static strength, fσeff, the product of the friction coefficient and effective normal stress. Once rupture initiates, shear heating‐induced thermal pressurization can sustain rupture at much lower τ/σeff ratios, a stress condition believed to be the case during most earthquakes. This requires that earthquakes nucleate at heterogeneities. We model nucleation and dynamic rupture on faults in a 2‐D elastic medium with rate/state friction and thermal pressurization, subjected to globally low τ but with local stress heterogeneities that permit nucleation. We examine end‐member cases of either high‐τ or low‐σeff heterogeneities. We find that thermal pressurization can sustain slip at τ/σeff values as low as 0.13, compared to static friction of ∼0.7. Background τ (and, to lesser extent, heterogeneity width) controls whether ruptures arrest or are sustained, with extremely low values resulting in arrest. For a small range of background τ, sustained slip is pulse‐like. Cessation of slip in a pulse tail can result from either diffusive restrengthening of σeff or a wave‐mediated stopping phase that follows the rupture tip. Slightly larger background τ leads to sustained crack‐like rupture. Thermal pressurization is stronger at high‐τ heterogeneities, resulting in a lower background τ threshold for sustained rupture and potentially larger arresting ruptures. High‐stress events also initiate with higher moment rate, although this may be difficult to observe in nature. For arresting ruptures, stress drops and the dependence of fracture energy on mean slip are both consistent with values inferred for small earthquakes.