Dynamics of dip-slip faulting: Explorations in two dimensions

Dynamic models of earthquake rupture and slip are a powerful method by whic h to investigate the physics of earthquakes. Owing to both conceptual and c omputational constraints, dynamic earthquake models have largely been limit ed to cases with geometrical symmetry, such as faults in unbounded media or vertical faults. However, there are both observational and theoretical rea sons to believe that nonvertical dip-slip faults behave differently from fa ults with more symmetrical geometries. Previous observations have shown gre ater 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 s how precisely these effects and also elucidate their causes. For typical no nvertical dip-slip faults the breakdown of symmetry with respect to the fre e 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 proce ss can lead to time-dependent normal stress and a feedback between the fric tion/rupture processes and seismic radiation. This interaction leads to thr ust/reverse faults producing much higher fault and ground motion than norma l faults with the same geometry and stress magnitudes. The asymmetric geome try also directly leads to higher motion on the hanging walls of such fault s than on the footwalls. Smulations show that these effects occur for a var iety of dip angles but only for faults that either intersect or closely app roach the free surface. The results emphasize the strong effect that the fr ee surface can have on the dynamics of fault rupture and slip.