The Chelungpu fault, a NS oriented thrust fault, was activated by the 1999 Chi-Chi earthquake (Mw = 7.6), Taiwan. At its northern end, the fault turned east causing an extra 15 km EW branch, which experienced abnormal 9 m surface rupturing. In this paper, we used shallow reflection seismics to investigate the subsurface structure, attempting to find the relationship between the structure and large ruptures. Over 90 short seismic lines were distributed throughout a 10 km ¿ 20 km area. Approximate 3D subsurface structures can be constructed by carefully compiling all seismic profiles with well data. The revealed structural pattern shows that the Chinshui shale, the key-bed layer on which the Chelungpu fault slips, arises toward the north and becomes quite shallow under the EW branch. This structural variation is believed to be the main factor causing the abnormal rupturing of the Chelungpu fault.