The 1999 Chi-Chi earthquake (M_w = 7.6), Taiwan, reactivated the 105 km long, NS trending Chelungpu fault causing extraordinary surface ruptures. The distribution of the ruptures was quite heterogeneous with the largest displacement (up to 12 m) occurring across the Tachia River, 50 km north of the epicenter. In this study, several different scaled seismic lines are constrained and used to predict the major regional fault geometry. The 3D northern lateral ramp of the Chelungpu fault is clearly found, which provides a structural framework to explain the en echelon surface ruptures around the Tachia River.

These data were also applied to identify the strata connected with the site of a 2 km hole (the Taiwan Chelungpu-fault Drilling Project, TCDP). Adequate site conditions are predicted before drilling and rechecked after drilling. A ‘TT-boundary’ is proposed as a structural control of the faulting in the area, which shapes the strata to dip 30° east and 20° south at the drill site. A depth of 1100 m for the Chi-Chi earthquake fault zone was predicted and confirmed. This is a good example, combining different scaled seismic methods to study detailed underground structures, which has the drilling purpose.