We conducted an electric resistivity survey consisting of six resistivity image profilings and several resistivity measurements on outcrops of strata in the Taichung area to investigate the subsurface structures of the Chelungpu fault. Three magnetotelluric sounding results are added to infer rock formations at depth. Based on the resistivity measurements on outcrops of the strata and the correlations between the interpretative resistivity structures and the rock formations recognized from drilling cores and the outcrops of the strata, the resistivity spectra of rock formations are obtained, and the geological structures are deduced.The results indicate that the Chelungpu fault is a complex fault system consisting of two major thrusts and several minor faults in the Taichung area. The two major thrusts are the main shear zone in the west and the Chi-Chi earthquake rupture in the east of the fault system. They are 300 -800 m apart on the ground surface. The main shear zone dips eastwardly at an angle of 20° ~ 60° and has a cumulate throw of several thousands meters. The Chi-Chi earthquake rupture was developed in the Cholan Formation in shallow depth. It dips 40° ~ 60° eastwardly and has a cumulate throw of several tens of meters indicating faulting movements had occurred many times prior to the Chi-Chi earthquake. The Chi-Chi earthquake rupture may become the dominant active fault zone in the northern segment of the Chelungpu fault system.

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In order to study the electrostratigraphy, the depositional environments and the change of shoreline, a geoelectric survey was carried out on the Tainan coastal plain. The DC resistivity method was used, and the sound ing data were interpreted by the 1-D inversion method.Based on the electric resistivities and the thicknesses of the layers de rived from the sounding data, three electrostratigraphic units are found and specified. They are designated as the T, R, and S fades corresponding to the strata from the surface to 200 meters in depth. The T-facies is char acterized by layers of resistivity ranging from 12 to 400 ohm-m at the top of the layer sequence. This unit is less than 100 meters thick and is widely distributed in the area studied. The S-facies is characterized by a thick layer or layers of resistivity less than 5 ohm-m, and is generally accompanied with thin layers of resistivity less than 12 ohm-m. The S-facies is 50 to 200 meters thick and is mostly overlain by the T -facies in the western and the central parts of the coastal plain, and is absent in the eastern part. The R facies is characterized by a thick layer or layers of resistivity ranging from 12 to 70 ohm-m. It is overlain by the T-facies in the east and by the S-facies in the west. The R-facies intertongues with the S-facies in the north-center and is absent in the south-center. The T-facies was deposited in terrestrial and estuarine environments during the Holocene. The S-facies was depos ited in marine environments in the late Pleistocene to Holocene. The upper part of the R-facies in the east was deposited in terrestrial and estuarine environments in the late Pleistocene. These three units are similar in lithol ogy being mainly composed of layers of clay, mud, silt, and fine sand.A marine transgression occurred when the strata between the depths of 200 and 40 meters were deposited in the area studied except for the Tainan tableland, and was then followed by a marine regression. At the peak pe riod of the marine transgression, the shoreline reached the line connecting Anding, Hsinhua, and the east of Gueizen. Since the beginning of the re-

In order to investigate the subsurface structure of the northern tip of the Chi-Chi earthquake fault, three electric resistivity image profilings were done in the northern Shihgang area where a large surface rupture was formed during the earthquake. The survey was conducted about three weeks after the Chi-Chi earthquake which occurred on 21September,1999 in central Taiwan. The pole-pole electrode configuration with electrode intervals of 6 meters was used for the profilings. Each profile consisted of 32 electrodes and 15 measured layers. The data were interpreted using the 2-D inversion method. The investigation depth was about 80-90 meters. The r�sults indicate that th� fault zone is clearly displayed in the pro files with a steep resistivity gradient zone. They also indicate that the rup ture is a reverse fault with a dip angle of about 60-80 degrees at the depth of 0-80 meters in the northern Shihgang area. The fault zone is about 30 meters wide on the ground surface and is about 10-15 meters wide at the depth of 30-80 meters. The rock sequences are similar on both sides of the fault. They are the Chinshui Shale overlain by layers of sand and gravel. It is inf erred that the fault in the northern Shih gang area is a new branch of the Chelungpu fault. A low resistivity zone (6-13 .Q-m) about 40-90 meters wide appeared adjacent to the fault zone on the footwall, and a high resis tjvity zone (36-100 Q-m) about 90 meters wide appeared adjacent to the fault zone on the hanging wall. Next to the high resistivity zone on the hang ing wall, a low resistivity zone and a high resistivity zone each about 50-100 meters wide appeared one after the other. This low and high resistivity zoning may be correlated to the strain brought on by the seismic stress released in the earthquake, and also implies that the formations were se verely and extensively disturbed on the hanging wall.

Direct current (DC) resistivity soundings with Schlumberger arrays are utilized to map the vertical and horizontal distributions of resistivity in the area between the Pachang-chi and Tsengwen-chi, southern Taiwan. This makes it possible to map the paleo depo-and hydro-environment of the study area. In addition, the transverse resistance computed from the field sound ing data measured near the wells can be related to the transmissivity mea sured directly within the wells. An empirical relation between the tranverse resistance and transmissivity could thus be derived. The hydraulic param eters at the DC sounding locations without any well information could still be estimated from such empirical relationships. Thus, the locations of fresh groundwater zones and the.most promising sites for future drilling could be determined.Results of this study indicate that the DC resistivity method can be used to map the depositional process of the study area, and it can also be applied to predict the hydraulic parameters in locations without available well information in a recent alluvium covered area in southern Taiwan. This is of great benefit to the future management of groundwater in the study area.