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.