An Electric Resistivity Study of the Chelungpu Fault in the Taichung Area, Taiwan

We conducted an electric resistivity survey in the Taichung area to investigate the subsurface structures of the Chelungpu fault, where the surface ruptures of the 1999 Chi-Chi earthquake diverged eastwardly from the foot of the hills for most part of the Chelungpu fault to the slope of the hills. Six profiles of resistivity image profiling with pole-pole array were carried out. The data were interpreted by the 2-D inversion method. Based on the formation sequence and the resistivity measured on the outcrops of rocks in and around the study area, the interpretative resistivity layers are correlated to the strata, the resistivity spectra of the 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 thrust faults, several minor faults and a strike-slip fault in the Taichung area. The two major thrust faults are the basal fault in the west and the Chi-Chi fault in the east of the fault system. The basal fault dips 30˚-60 ˚ eastwardly and is the main fault of the Chelungpu fault system, which has a cumulate throw of 2000 m or more. The Chi-Chi fault is a branch of the Chelungpu fault which was developed on the hanging wall after a strike-slip fault had formed in the south of the study area. The Chi-Chi fault dips 40˚-60 ˚ eastwardly and has a cumulate throw of several tens meters. It is very active recently, and may become the dominantly active fault zone in the northern section of the Chelungpu fault system. In the south of the study area, the basal and the Chi-Chi faults are coincident