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

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. Plain Language Summary A rupture traveling faster than the shear wave speed is termed as a supershear rupture, which could trigger the generation of a shock wave in ground motion. To determine whether a rupture is propagating at supershear speed, the direct method is to measure the travel distance of the rupture with a given time interval in numerical simulations. However, other measurements of rupture velocity, such as the 1D horizontal and 2D gradient, are also compared. We find that using the 1D horizontal measure, in some cases a rupture can be classified as a supershear rupture, even though only a small area of the fault is experiencing supershear speed via the 2D gradient calculation, which may not generate a shock wave in ground motion. The results imply that different physical properties may be associated with different metrics of rupture velocity, with implications for determining supershear rupture in nature earthquakes.

Automated summary

Rupture velocity, defined by different gradients, affects the understanding of earthquake physics. In some cases, a rupture may be classified as supershear using a 1D horizontal measure but not via the 2D gradient calculation, highlighting the importance of rupture velocity definitions for supershear analysis.