[S13-6] Earthquake source mechanics VI

[S13-6-03] Supershear rupture induced by step over geometry and its effect on near field ground motion

^○Feng Hu¹, Xiaofei Chen^{1, 2} (1.University of Science and Technology of China, China, 2.Southern University of Science and Technology, China)

Based on dynamic rupture simulations on step overs, we show that rupture speeds can jump to supershear speed on the secondary fault of a step over in a 3D full-space after re-nucleation, whose initial shear stress preclude such a supershear transition according to the Burridge-Andrews mechanism, and the rupture speed on the primary fault is sub-Rayleigh. The low normal stress zone and the high shear stress zone beyond the fault step, which radiate from the end of the primary fault if its rupture arrest is sudden, determine the supershear rupture occurrence on the secondary fault. However, a low shear stress zone traveling at the shear wave speed is also radiated, making the rupture speed return to subshear in most cases. Sustained supershear ruptures on the secondary fault are also possible on compressional step overs under certain conditions. Self-arresting phenomenon are observed in the overlapped area on the secondary fault. In a half-space model where supershear rupture is induced by the free surface on the primary fault, the rupture speed on the secondary fault rapidly transits to subshear near the fault step if its width exceeds a critical value. We further calculate near-field ground motions of the half-space cases to understand its high frequency radiation pattern.