Earthquake Energy Balance, Stress Levels, and Heat Production in Rough Fault Simulations

Abstract:

While mature faults like the San Andreas operate at extremely low stress levels, most other faults host earthquakes at ratios of shear to effective normal stress around 0.6. Dynamic weakening of fault strength, from thermal pressurization and other processes, can explain the low stresses on mature faults. But if dynamic weakening is as ubiquitous as suggested by high-velocity friction experiments, then one might expect low-stress operation of all faults. We present a possible reconciliation of these ideas, based on dynamic rupture simulations of ruptures on rough faults with dynamic weakening and off-fault plasticity. Fault geometric complexity introduces an additional resistance to slip, or roughness drag, that acts in addition to frictional resistance. Roughness drag is negligible only for the smoothest faults, which then operate at low stresses like the San Andreas. On other faults, stress levels are ultimately determined by the strength of the off-fault material. We also explore related issues regarding heat production and the earthquake energy balance. Strain energy release is converted to radiated energy and dissipated by both frictional sliding and off-fault plasticity. The latter shifts some fraction of the heat production from the fault to the surrounding medium, reducing local temperature rise on the fault. We examine implications of this for both fault melting and long-term heat flow measurements.