Dynamics of dip‐slip faulting: Explorations in two dimensions

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Abstract
Dynamic models of earthquake rupture and slip are a powerful method by which to investigate the physics of earthquakes. Owing to both conceptual and computational constraints, dynamic earthquake models have largely been limited to cases with geometrical symmetry, such as faults in unbounded media or vertical faults. However, there are both observational and theoretical reasons to believe that nonvertical dip‐slip faults behave differently from faults with more symmetrical geometries. Previous observations have shown greater ground motion from thrust/reverse faults than normal faults and higher ground motion on hanging walls than on footwalls. In the present work, two‐dimensional dynamic simulations of thrust/reverse and normal earthquakes show precisely these effects and also elucidate their causes. For typical nonvertical dip‐slip faults the breakdown of symmetry with respect to the free surface allows radiated seismic waves to reflect off the free surface and to hit the fault again, altering the stress field on the fault. This process can lead to time‐dependent normal stress and a feedback between the friction/rupture processes and seismic radiation. This interaction leads to thrust/reverse faults producing much higher fault and ground motion than normal faults with the same geometry and stress magnitudes. The asymmetric geometry also directly leads to higher motion on the hanging walls of such faults than on the footwalls. Simulations show that these effects occur for a variety of dip angles but only for faults that either intersect or closely approach the free surface. The results emphasize the strong effect that the free surface can have on the dynamics of fault rupture and slip.