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

Stuart V. Schmitt, Paul Segall, & Eric M. Dunham

Published November 13, 2015, SCEC Contribution #6005

Earthquake nucleation requires that the shear stress tau locally reaches a fault’s static strength, f sigma_{eff}, the product of the friction coefficient and effective normal stress. Once rupture initiates, shear heating-induced thermal pressurization can sustain rupture at much lower tau/sigma_{eff} 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 tau but with local stress heterogeneities that permit nucleation. We examine end-member cases of either high-tau or low-sigma_{eff} heterogeneities. We find that thermal pressurization can sustain slip at tau/sigma_{eff} values as low as 0.13, compared to static friction of ∼0.7. Background tau (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 tau, sustained slip is pulse-like. Cessation of slip in a pulse tail can result from either diffusive restrengthening of sigma_{eff} or a wave-mediated stopping phase that follows the rupture tip. Slightly larger background tau leads to sustained crack-like rupture. Thermal pressurization is stronger at high-tau heterogeneities, resulting in a lower background tau 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.

Citation
Schmitt, S. V., Segall, P., & Dunham, E. M. (2015). Nucleation and dynamic rupture on weakly stressed faults sustained by thermal pressurization. Journal of Geophysical Research: Solid Earth, 120(11), 7606-7640. doi: 10.1002/2015JB012322.

Related Projects & Working Groups
Fault and Rupture Mechanics