Fault geometry and the dynamics of the 1999 Chi-Chi (Taiwan) earthquake

The 1999 M 7.6 Chi-Chi (Taiwan) earthquake produced a data set of unparalle led size and quality, particularly in the near-source region where data hav e been previously quite scarce. The large amount of near-source data allows the verification of many predictions of thrust-fault behavior for faults t hat 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 hang ing wall moving more than the footwall (with strongly peaked velocities rig ht 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. Buildi ng on the work of Oglesby and Day (2001), the current work helps to delinea te the effects of fault geometry, nonuniform prestress, and dynamic waves o n the physics of the Chi-Chi earthquake and dip-slip faults in general. In particular, we find that a completely homogeneous prestress pattern still f its the gross features of the near-source ground motion quite well. Additio nally, 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 tha t correctly account for both the effects of fault geometry and dynamic wave s in the rupture and slip processes.