The Effect of Depth-Dependent Stress in Controlling Free-Surface-Induced Supershear Rupture on Strike-Slip Faults

摘要

Supershear rupture is crucial in determining ground motion characteristics and its various transition mechanisms is in key connection with rupture dynamics, geometrical complexities and stress heterogeneities. Using 3D dynamic rupture models with numerous depth-dependent stress regimes, we show that depth-dependent stress plays a major role in controlling the occurrence of free-surface-induced (FSI) supershear ruptures. The effect of stress gradient, initial normal stress at the free surface, fault width and critical depth of depth-dependent stress regime are examined. The 2D tangent rupture velocity distributions show that generation of a sustained FSI supershear rupture prefers a larger stress gradient and initial normal stress at the free surface. The truncation of fault width could transit the rupture to a sub-Rayleigh rupture. We propose a dimensionless stress parameter representing a measure of the overall level of depth-dependence in the initial normal stress, and a larger moment magnitude is more likely to trigger a sustained FSI supershear rupture. By comparing the near-field ground motions, three distinct ground motion characteristics are presented using the sustainability of Mach fronts and displacement polarization. This work helps to understand the occurrence of FSI supershear ruptures and its relationship with underground depth-dependent stresses.