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

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. Plain Language Summary Flat free surface tends to induce supershear transition provided there exists a long enough propagation distance. Previous studies suggested that this transition invariably occurs in strike‐slip faults rather than in dip‐slip faults. We use numerical simulation to investigate the mechanism of supershear transition induced by free surface on oblique faults, which have comparable strike‐slip and dip‐slip components. We find that oblique thrust faults are more likely to produce supershear transition with the changes in style of faulting from dominated dip slip to dominated strike slip. Moreover, significant asymmetry arises between the two opposite directions along fault strike. The supershear rupture may only occur within shallow depth or over the entire fault 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 distribution of peak ground velocity derived from dynamic modeling indicates that the asymmetric ground shaking on the oblique faults significantly depends on dip angle and slip rake angle, which allows us to predict the possible spatial distribution of the high seismic risk. Key Points Oblique thrust faults are more likely to generate free‐surface‐induced supershear transition than oblique normal ones The asymmetric ground shaking on the oblique faults allows us to predict the possible spatial distribu