The Three-Dimensional Dynamics of Dipping Faults

Recent two-dimensional dynamic simulations of dip-slip faulting (Niel- sen, 1998; Oglesby et al., 1998, 2000; Shi et al., 1998) have shown that the asym- metric geometry of dip-slip faults that intersect the free surface can have large effects on the dynamics of earthquake rupture. The nonvertical dip angle of such faults leads to larger motion on the footwall than the hanging wall, as well as much larger motion from thrust/reverse faults than from normal faults with the same geometry and stress magnitudes. In the present work we perform full three-dimensional simulations of thrust/reverse, normal, and strike-slip faults, and show that the same effects exist in three dimensions. Strike-slip fault motion is either in between or lower than the motion of both dip-slip faults. Additional three-dimensional effects include strong rake rotation at the free surface. The results confirm the findings of the previous studies and further elucidate the dynamic effects of the free surface on fault rupture, slip, and ground motion. They are also borne out by early analyses of the 1999 Chi- Chi (Taiwan) thrust earthquake, which displayed higher motion on the hanging wall than on the footwall, and a strong oblique component of motion at the surface.

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