Effect of seismogenic depth and background stress on physical limits of earthquake rupture across fault step-overs

Abstract

Earthquakes can rupture geometrically complex fault systems by breaching fault step overs. Quantifying the likelihood of rupture jump across step overs is important to evaluate earthquake hazard and to understand the interactions between dynamic rupture and fault growth processes. Here we investigate the role of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs. Our computational and theoretical study is focused on the canonical case of two parallel strike-slip faults with large aspect ratio, uniform prestress and friction properties. We conduct a systematic set of 3-D dynamic rupture simulations with different seismogenic depth, step over distance, and initial stresses. We find that the maximum step over distance H_c that a rupture can jump depends on seismogenic depth W and strength excess to stress drop ratio S, commonly used to evaluate probable rupture velocity, as H_c ∝ W/S^n, where n = 2 when H_c/W < 0.2 (or S > 1.5) and n = 1 otherwise. The critical nucleation size, largely controlled by frictional properties, has a second-order effect on H_c. Rupture on the secondary fault is mainly triggered by the stopping phase emanated from the rupture end on the primary fault. Asymptotic analysis of the peak amplitude of stopping phases sheds light on the mechanical origin of the relations between H_c, W, and S, and leads to the scaling regime with n = 1 in far field and n = 2 in near field. The results suggest that strike-slip earthquakes on faults with large seismogenic depth or operating at high shear stresses can jump wider step overs than observed so far in continental interplate earthquakes.