Free-Surface-Induced Supershear Transition in 3-D Simulations of Spontaneous Dynamic Rupture on Oblique Faults

摘要

Several observations and simulations suggested that the supershear rupture induced by free surface invariably occurs in strike-slip faults rather than in dip-slip faults. To explore the mechanism of supershear transition induced by free surface on oblique faults, we simulate spontaneous dynamic rupture propagation governed by slip-weakening friction on nonvertical planar faults. We find that oblique thrust faults are more likely to produce free-surface-induced supershear transition than oblique normal faults and significant asymmetry arises between the two opposite directions along fault strike. The supershear rupture may only occur within shallow depth or over the entire seismogenic depth, depending on geometric relations between the slip and rupture propagation vectors. Our results help explain the lack of universal observation of supershear earthquakes. Besides, the asymmetric ground shaking on the oblique faults allows us to predict the possible spatial distribution of the high seismic risk.