Supershear Rupture, Daughter Cracks, and the Definition of Rupture Velocity

Abstract

Rupture velocity is a fundamental feature of earthquake behavior. How rupture velocity is defined can therefore affect our understanding of earthquake physics. Based on dynamic rupture simulations, we compare the tangent and average rupture velocities calculated via one- and two-dimensional gradients. Two strike-slip scenarios with free-surface-induced supershear ruptures are presented within a homogeneous and a depth-dependent stress regime, respectively. Although both scenarios produce a daughter crack that propagate over the seismogenic depth, in the depth-dependent case the 1D and 2D rupture velocities capture different features: A 1D horizontal gradient measurement implies a supershear rupture, while a 2D gradient measurement reveals sub-Rayleigh rupture propagation everywhere except very close to the free surface. A large area on the fault with the 1D horizontal tangent rupture velocity does not necessarily produce an observable Mach front, which arises the importance of rupture velocity definitions in supershear analysis.