Coseismic displacement, bilateral rupture, and structural characteristics at the southern end of the 1999 Chi-Chi earthquake rupture, central Taiwan
[1] The 1999 Chi-Chi earthquake (Mw 7.6) was due to slip along the Chelungpu fault: a major N striking thrust fault in the fold-thrust belt of western Taiwan. The surface rupture extends over 100 km in a N-S trend with NW trending horizontal displacement increasing from 2 to 10 m from south to north. The central segment of the Chelungpu fault is characterized by bedding slip along the Pliocene Chinshui Shale, which has resulted in the development of a monoclinal structure on the hanging wall. At the southern end of the Chi-Chi earthquake rupture, however, the Chelungpu fault connects with the NE striking Tachienshan fault and cuts into Miocene strata with complex structures on the hanging wall. In this study, we use digital cadastral data to calculate coseismic horizontal displacement around the Chushan area. Results show the amount and direction of horizontal displacement to be ca. 1.2 to 2.6 m and ca. 220° to 260°, respectively, on the hanging wall and ca. 1.4 m and 105°, respectively, on the footwall. Coseismic displacement and seismological data pertaining to the hanging wall at the southern end of the Chi-Chi earthquake rupture indicate the existence of three distinct blocks. Horizontal displacement indicates the Chushan block's movement trended SW. This result reflects the Chushiang and Luliao faults being reactivated with different faulting mechanisms and slip azimuths. This study considers a complex coseismic displacement field, resulting from a bilateral rupture process whereby a southward rupture with SW trending movement results in right-lateral strike-slip faulting of the Tachienshan fault and thrusting with a right-lateral component on the NS strike of the Chelungpu fault. The northward rupture is associated with NW trending movement that results in activation of the Luliao and Chushiang faults.