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 21 September, 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 results indicate that the fault zone is clearly displayed in the profiles with a steep resistivity gradient zone. They also indicate that the rupture 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 Ω-m) about 40-90 meters wide appeared adjacent to the fault zone on the footwall, and a high resistjvity zone (36-100 Ω-m) about 90 meters wide appeared adjacent to the fault zone on the hanging wall. Next to the high resistivity zone on the hanging 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 severely and extensively disturbed on the hanging wall.