Insights from heterogeneous structures of the 1999 M_w 7.6 Chi-Chi earthquake thrust termination in and near Chushan excavation site, Central Taiwan

We describe and analyze the surface and subsurface deformation of the 1999 Chi-Chi earthquake thrust termination of the Chelungpu fault in and near the Chushan excavation site, central Taiwan. In order to minimize damage to engineering structures within a deformation zone and formulate regulations for earthquake fault zones and fault setbacks, one needs to know the characteristics of the deformation zone and the connection between surface and subsurface deformation. The surface deformation zone of high strain induced by the earthquake faulting ranges from 15 to 70 m in width, and characterized by a 0.5 to 2 m high escarpment is much wider in the hanging wall. Exposures in the trench, 40 m long and 10 m deep, excavated across the earthquake ground rupture, show on the one hand the heterogeneous structure of a steep, monocline-like fold. On the other hand, strikingly different surface deformation profiles on either side of the 14 m wide trench, i.e., semiparabola-like and monoclinal, do not reflect in any obvious way the subsurface structure. The analysis of our detailed mapping results along with well logs suggests that the significant width variation of the surface deformation zone is attributed to secondary faults branching from the dominant fault at several tens of meters depth. The surface profile of the escarpment is controlled by the depth of the relevant fault tip, and the subsurface heterogeneous structure formed during at least four earthquakes.