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

Abstract

While prior numerical simulations indicate that the Earth's free surface can induce supershear propagation on strike‐slip faults, copious observations of strike‐slip earthquakes have produced only a few instances of such supershear rupture. Our dynamic rupture simulations with varying initial normal stresses and fault strengths show that free‐surface‐induced supershear rupture on strike‐slip faults may return to sub‐Rayleigh speed as the rupture progresses. Such a phenomenon is defined as unsustained free‐surface‐induced supershear rupture, indicating that a rupture on a strike‐slip fault, even if it encounters the free surface, may not generate supershear rupture that is sustained long enough to be observable. The shape of the rupture front contour on the fault may also lead to a false impression about the rupture velocity, since the daughter crack may also propagate at sub‐Rayleigh speed, with near‐field fault‐perpendicular ground motion. Shallower hypocenter depths increase the likelihood of an unsustained free‐surface‐induced supershear rupture.

The Near‐Fault Ground Motion Characteristics of Sustained and Unsustained Free Surface‐Induced Supershear Rupture on Strike‐Slip Faults

“…The hypocenter depth H is a key factor in controlling the sustainability of free surface‐induced supershear rupture on strike‐slip faults (Hu et al, ). Rupture with a shallower hypocenter depth on a strike‐slip fault is more likely to propagate at sub‐Rayleigh speed at the end of the fault, based on the rupture scenario summation with variety values of S , H , and τ n .…”

“…Rupture with a shallower hypocenter depth on a strike‐slip fault is more likely to propagate at sub‐Rayleigh speed at the end of the fault, based on the rupture scenario summation with variety values of S , H , and τ n . To examine the characteristics of near‐field ground motions for unsustained and sustained free surface‐induced supershear ruptures, three vertical strike‐slip cases with different hypocenter depths of 5, 7.5, and 10 km are compared, recapitulating for reference some results of Hu et al (). The initial normal stress τ n is 120 MPa, corresponding to an S value of 1.9.…”

“…The present work builds on the work of Zhang and Chen (), Kaneko and Lapusta (), and Hu et al (), who explored the mechanism by which a rupture may transition to supershear rupture speeds due to its interaction with the free surface of the Earth. In particular, we continue the work of Hu et al () in examining the cases in which such supershear rupture may be sustained on the fault or may transition back to sub‐Rayleigh speed. We focus in the present work on how the rupture patterns on the fault translate into ground motion, and how such motion differs between unsustained, partially sustained, and sustained supershear rupture cases.…”

“…One possibility is that free surface‐induced supershear rupture may sometimes be ephemeral, similar to supershear rupture induced by heterogeneity in geometry or stress. Recent work by Hu et al () has further examined the mechanism of free‐surface supershear rupture to address the issue of whether such supershear rupture is sustained indefinitely after the transition in speed, or whether it may revert to sub‐Rayleigh speed even in homogenous stress conditions. While reproducing the results of Kaneko and Lapusta () for homogeneous models with similar parameters, they found that for a variety of stress conditions, particularly for S values higher than those explored by Kaneko and Lapusta (), interactions with the free surface can induce supershear rupture near the free surface, but the rupture speed can later return to a sub‐Rayleigh level.…”

On‐Fault Geological Fingerprint of Earthquake Rupture Direction

“…Although supershear rupture propagation is present in our models (e.g., Hu et al, 2019;Kaneko & Lapusta, 2010), it does not become a dominant instability and thus has little effect on slip behavior or temporal changes in slip direction. Cases in which free-surface-induced supershear rupture propagation becomes dominant may influence temporal changes in slip direction and may even lead to observable phase transitions preserved in slickenlines tracks.…”