Supershear transition mechanism induced by step over geometry

Abstract

Few studies have focused on the supershear transition mechanism induced by fault step overs, although seismic observations suggest that rupture speed transitions occur at geometrical complexities on faults. Based on dynamic rupture simulations on fault systems with step overs in a 3‐D full space where the initial stresses preclude a supershear transition on a single buried fault according to the Burridge‐Andrews mechanism, we show that rupture speeds can transit from subshear on the primary fault to supershear on the secondary fault. The low normal stress zone and the high shear stress zone beyond the fault step, which radiate from the end of the primary fault if its rupture arrest is sudden, determine the supershear rupture occurrence on the secondary fault. However, a low shear stress zone traveling at the shear wave speed is also radiated, making the rupture speed return to subshear in most cases. Sustained supershear ruptures are also possible on compressional step overs under certain conditions. Self‐arresting ruptures are observed in the overlap area on the secondary fault. In a half‐space model where supershear rupture is induced by the free surface on the primary fault, the rupture speed on the secondary fault rapidly transits to subshear near the fault step if its width exceeds a critical value.

The Sustainability of Free‐Surface‐Induced Supershear Rupture on Strike‐Slip Faults

“…The most studied supershear transition mechanism is the Burridge-Andrews (BA) mechanism, by which a stress peak traveling at S wave ahead of the sub-Rayleigh crack tip propagating in the direction parallel to slip can trigger a daughter crack forward at supershear speed (Andrews, 1976b;Burridge, 1973). Other mechanisms for the supershear transition also exist, including stress heterogeneities (Day, 1982;Dunham et al, 2003;Liu & Lapusta, 2008), heterogeneous fault geometry (Bruhat et al, 2016;Oglesby & Mai, 2012), fault step overs (Hu et al, 2016;Ryan & Oglesby, 2014), and interactions of the rupture front with the Earth's free surface (Kaneko & Lapusta, 2010;Zhang & Chen, 2006). Heterogeneous fault geometry and stress may lead to unsustained supershear rupture propagation (Bruhat et al, 2016;Day, 1982;Oglesby & Mai, 2012), in which the supershear rupture transitions back to sub-Rayleigh speed.…”

Effect of the time-weakening friction law during the nucleation process

“…Its simplicity in use and accuracy in predicting dynamic rupture behaviors made it widely used in rupture dynamics (Andrews 1985;Madariaga et al 1998;Zhang and Chen 2006a, b;Hu et al 2014Hu et al , 2016bXu et al 2015). As shown in Eq.…”

High Frequency Near‐Field Ground Motion Excited by Strike‐Slip Step Overs

“…An “earthquake bright spot,” which represents a localized area that radiates strong high‐frequency seismic waves (Umeda, ), is detected when both the nucleation and the termination of cracks occur in a localized area (Yamashita & Umeda, ). The rupture speed may change significantly when a rupture jumps across fault discontinuities of strike‐slip step overs (Hu, Xu, et al, ; Kase & Kuge, ; Ryan & Oglesby, ). However, the manner in which a rupture speed transition across a step over affects the characteristics of the high‐frequency near‐field ground motion is still unclear.…”

“…Although many natural earthquakes propagate at sub‐Rayleigh speeds (Heaton, ), multiple numerical and experimental studies, along with observations of natural earthquakes, have proven the existence of supershear ruptures (Andrews, ; Archuleta, ; Bouchon et al, ; Dunham, ; Dunham et al, ; Harris & Day, ; Huang et al, ; Kaneko & Lapusta, ; Liu & Lapusta, ; Shi & Ben‐Zion, ; Xia et al, ; Xu et al, ; Zhang & Chen, ). In addition, step over geometries are one of the causes of supershear transitions on secondary fault segments because of the barrier effects of the geometrical discontinuity (Hu, Xu, et al, ; Kase & Kuge, ; Ryan & Oglesby, ), although unsustained supershear ruptures are often observed on secondary faults (Hu, Xu, et al, ).…”