The 1999 Chi-Chi earthquake in Taiwan was a relatively unique seismic event which activated the Chelungpu thrust fault with extraordinarily large surface ruptures up to 9.8 m horizontally and 5.6 m vertically. The fault is 80 km long, lying mostly in a north–south direction and with a reverse thrust dipping shallowly to the east. For this paper we used the shallow reflection seismic method (a small-scale method) to map this active fault (a large-scale structure) and provide evidence in support of the thin-skinned thrust model for this earthquake fault. The investigation is comprised of two parts. The first concentrates on the northern portion of the fault where the fault trace was bent 70° to the northeast and was accompanied by abnormally large ground displacements and damage. The seismic sections obtained are of good quality and can be used to explore the detailed faulting mechanism. The second part aims to find the gross features of the 60 × 20-km fault surface using limited shallow seismic measurements (about 50 seismic lines), incorporating the thin-skinned thrust concept. This is a test to examine the feasibility of using a small-scale economical method to study a big structure. The results are quite plausible.