Study On Supershear Rupture And Ground Motion With Free Surface

Supershear rupture does not only provide information about the physical pro-cesses of earthquakes, but it also strongly modifies the nature of seismic radiation and thus the origins of the damaging waves generated by earthquakes. To discover the mechanism of supershear rupture is important to seismic hazard assessment.In this study, we systematically investigated the influence of the parameters of slip-weakening law and the size of nucleation asperity on rupture dynamics of a planar fault in full space and half space using BIEM, particularly the occurrence conditions for sub-Rayleigh (or subshear for dip-slip rupture) and supershear rup-tures. Besides the well-known rupture styles of sub-Rayleigh (subshear for dip-slip rupture) and supershear, we define a new kind of rupture style in this study, that is the Self-Arresting Rupture whose rupture process can be autonomously arrested by itself without any outside interference, for instance the high strength barrier. Based on the vast amount of simulation results, we obtained the rupture phase diagrams for strike-slip and dip-slip ruptures vertically and obliquely embedded in full and half spaces with different buried depths. The rupture phase diagram clearly illustrates the occurrence conditions of three kinds of rupture styles, and the transitions from one to another. In half space, due to the influence of free sur-face, rupture becomes more complicated. For a strike-slip fault with zero buried depth, all ruptures occurred in the parameter range of sub-Rayleigh ruptures in full space case become supershear ruptures. It means that as long as the rupture is able to grow incessantly, it always evolves to a supershear rupture. For dip-slip fault, however, rupture propagates always with subshear speed though slip rate could be almost twice as that in strike-slip. Although the influence of free surface is strong, it is limited to the very shallow ruptures (buried depth<1km). The cause of supershear due to free surface is the stress loading of SV-P transition. Our results are consistent with the observation that supershear rupture is only observed in strike-slip fault, which is up to free surface. Besides, the self-arresting rupture may be the mechanism of small earthquake, especially in which only a small patch is rupture.We compare the characteristics of the strong ground motion between two supershear rupture, i.e., due to high initial energy and free surface. Compared with subshear rupture, supershear rupture can produce unusually large levels of ground motion at larger distances due to the generated Mach wave. There are two S waves in the seismogram, which come from the supershear rupture front and the epicenter. These two S wave appears to be two peak in the supershear due to free surface. Compared with seismogram of the Denali earthquake, its supershear is recognized to be the cause of free surface and propagate at least 10km before it reaches the PS10 station.