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Mapping water-bottom features involves in the fields of environmental geophysics and engineering structural studies. Standard direct current (DC) resistivity sounding is widely used to map subsurface structures on land, but it is seldom used to map underwater structures, because the deploy ment of underwater electrodes along the water-bottom is expensive and cumbersome. Therefore, a cost-effective and non-destructive survey tech nique is required to map underwater structures. Recently the development of an automatic multi-electrode system for resistivity image profiling (RIP) has been successful in mapping two-di mensional (2-D) or three-dimensional (3-D) geological structures on land faster than standard DC resistivity soundings. In fact, the RIP method provides• higher resolution and greater exploration depth of electrical images. Therefore, it is time to consider whether this advanced RIP technique can be applied to underwater mapping.

We conducted an electric resistivity survey in the Tsaotun area to in vestigate the subsurface structures of the complex deformation and rup ture of Chelungpu fault system induced by the 1999 Chi-Chi earthquake. Four electric resistivity image profiles with pole-pole array across fault zones and topographic lineation were conducted in this survey. The sounding 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 differ ent 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 be tween the Toukoshan Formation and the Chinshui Shale is the characteris tic 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.

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