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

Kangchen Bai, & Jean-Paul Ampuero

Under Review 2017, SCEC Contribution #7964

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 pre-stress and uniform friction properties. We conduct a systematic set of 3D dynamic rupture simulations in which we vary the 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 $S$, \textbf{commonly used to evaluate probable rupture velocity}, as $H_c \propto W/S^n$, 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 inter-plate earthquakes.

Citation
Bai, K., & Ampuero, J. (2017). Effect of seismogenic depth and background stress on physical limits of earthquake rupture across fault step-overs. Journal of Geophysical Research - Solid Earth, (under review).