The 1999 Chi-Chi earthquake occurred in center Taiwan marks the island’s largest inland seismic event in the twentieth century. It triggered an existing active fault, the Chelungpu fault. The fault was a reverse type dipping 20-50 degrees to the east and extending north-south about 90 km . The observed vertical displacements increased from the south (0.6m) to the north (7m), and the maximum horizontal displacement was 9.6m. Most of fault segments were characterized by pure thrust with minor lateral slip, but the left-lateral component of slip increased in the northern part near Shihkang. The Chelungpu fault developed along the topographic scarp running mosty in the north-south direction. However, it bended toward the northeast near Fengyuan and Shihkang, crossed the Ta-anhsi and the Tachiahsi rivers, finally stopped at the town of Cholan. In geology, the Chelungpu fault was a bed-plane fault sliding along a sedimentary layer, the Chinshui shale. The purpose of this study is to detect the underground structure in the vicinity of the Chelungpu fault by the shallow seismic reflection method. The obtained seismic sections enable us to examine the detailed faulting mechanism along the fault.
