We conducted an electric resistivity survey in the Tsaotun area to investigate the subsurface structures of the complex deformation and rupture of Chelungpu fault system induced by the 1999 Chi- Chi earthquake. Four electric resisitivity image profiles with pole-pole array across fault zones and topographic lineation were conducted in this survey. The somding data were interpreted by the 2-D inversion method.

The results show that the electric resistivity structures interpreted from the resistivity sounding data fit well with ruptures and outcrops appeared on the ground surface. We also evaluated the resistivity ranges for different formations, the top layer thickness and the width of the fault zones.

Resistivity-layer discontinuities found on the profiles correlate to the fault zones of the Chelungpu fault and the back thrust. The boundary between the Toukoshan Formation and the Chinshui Shale is the characteristic contact of the Chelungpu fault. It is a reverse fault including several fracture zones in the Tsaotun area. Some ruptures developed in weaker layers of the Chinshui Shale have been active since the deposition of the Quaternary terrace gravel bed. The main fracture zones have a width of 6-40 m and dip eastward at an angle of 80° - 30° that decrease with depth. The fault diverged into V-shape fractures or more branches near the ground surface in some regions. Not all the fractures of the Chelungpu fault system moved during the Chi-Chi earthquake, but they were active previously.