Importance of bimaterial interfaces for earthquake dynamics and strong ground motion

P>We perform a numerical investigation of dynamic ruptures on a bimaterial interface in 3-D with regularized slip-weakening friction and a heterogeneous initial shear stress and discuss the resulting strong ground motion. To isolate effects introduced by the material contrast, we perform pairs of simulations with opposite material orientations. We show that for many parameter sets the dynamics of rupture propagation are significantly influenced by the broken symmetry due to the material contrast during rupture propagation. The resulting slip histories of two events with reversed orientations of the material contrast can deviate such that the emanating waves lead to large differences in peak ground motion (peak ground velocity and peak ground acceleration), even when slip-distribution of the individual events are very similar, and therefore their moment magnitudes are basically identical. We also show that the wrinkle-like slip pulse specific to the bimaterial mechanism can be nucleated naturally from an initially crack-like mode of rupture, when the initial stress allows for large propagation distances. Once such a pulse has been nucleated, it travels at a dominant propagation speed close to the generalized Rayleigh velocity. The dynamic weakening of the fault due to the normal stress alteration during slip allows nucleating ruptures to overcome asperities of low initial shear stress in the preferred direction, which is the direction of displacement on the seismically slower side of the fault. In such situations, the orientation of the material contrast determines rupture extent and therefore the size of the earthquake, potentially by orders of magnitude.