Abstract

The 1999 M 7.6 Chi-Chi (Taiwan) earthquake produced a data set of unparalleled size and quality, particularly in the near-source region where data have been previously quite scarce. The large amount of near-source data allows the verification of many predictions of thrust-fault behavior for faults that intersect the surface of the earth. Through rigorous three-dimensional dynamic models of the Chi-Chi earthquake, it can be shown that many aspects of the observed near-source ground motion in this event are direct effects of the asymmetrical dipping fault geometry. These effects include the hanging wall moving more than the footwall (with strongly peaked velocities right at the fault trace) and a transition from predominantly thrust motion in the south of the fault to largely left-lateral motion in the north. Building on the work of Oglesby and Day (2001), the current work helps to delineate the effects of fault geometry, nonuniform prestress, and dynamic waves on the physics of the Chi-Chi earthquake and dip-slip faults in general. In particular, we find that a completely homogeneous prestress pattern still fits the gross features of the near-source ground motion quite well. Additionally, the strike-slip component of motion near the fault trace is seen to be a combination of dynamic and static effects. Finally, dynamic overshoot is seen to be much larger for dip-slip faults than for otherwise identical vertical faults. The results emphasize the necessity of rigorous models that correctly account for both the effects of fault geometry and dynamic waves in the rupture and slip processes.

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