Distribution of coseismic slip for the 1999 ChiChi Taiwan earthquake: New data and implications of varying 3D fault geometry

The 1999 Chi Chi earthquake occurred in central Taiwan and broke the Chelungpu Fault (CLPF) causing a 100-km surface rupture. The CLPF is an extensively geophysically monitored major frontal thrust fault in the western foothills of Taiwan. Previous studies of coseismic slip distributions assumed a variety of fault geometries either refined during the inversion process or held fixed following a priori assumptions. In order to illuminate the relation between the seismic surface deformations and fault structures, we adopt the 3D fault geometry of Yue et al., (2005) inferred from surface geology, seismic lines, bore-hole data and balanced cross-section models. We invert for the coseismic slip distribution using all available geodetic data including GPS, SPOT and SAR (InSAR and pixel tracking) data. Preliminary results suggest that the model predictions agree with geodetic observations on the southern CLPF. However, we find a large discrepancy in the northern CLPF. The surface observations are very sensitive to the distribution of fault slips since the structural model has very shallow fault plane and there were large observed coseismic displacements to the north. The inconsistency suggests that the fault geometry on northern CLPF needs to be modified. In addition, newly available SAR pixel tracking data implies a large fault slip of about 9 m between Wufung and Tsaotun that has not been discussed in previous models. The availability of geodetic imaging data provides us with an unprecedented opportunity to iterate on our models of fault structure and to force consistency between geodetic, geologic, and seismic observations.