**2. The 1999 Chi-Chi earthquake ruptures**

126 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

displacement for the thrusting through more detailed observation and discussion of the characteristics of the fault-tip deformation. Here, we also present the results of our paleoseismic studies of the Chelungpu fault to assess the timing, offset, and slip rate of each event. Because active thrust faults commonly show complex geometric patterns of faulting and folding, we present a few examples of excavation that document different styles of surface faulting. We focus on the contribution of the excavation study in providing some information about fault behavior and coseismic fault-tip deformation patterns and in

Fig. 1. Geologic map and location of the excavated sites (A-I). The Chi-Chi earthquake rupture is subdivided into the Shihkang and Chelungpu faults along the frontal Western Foothills. (A) the Fengyuan site, (B) the Wenshan farm site, (C) the Pineapple-field site, (D) the Siangong-temple site, (E) the Tsaotun site, (F) the Wanfung site, (G) the Shijia site, (H) the Mingjian site, and (I) the Chushan site. Average vertical displacement of the Chi-Chi earthquake rupture is calculated from the measured vertical displacements measured at 210

locations along the earthquake rupture.

deriving the growth history of the fault-tip fold.

The Western Foothills in western Taiwan consist of a series of west-verging subparallel thrusts and fault-related fold structures, which have been a zone of active folding and thrusting throughout the late Quaternary (Suppe, 1981). The Chi-Chi earthquake exhibits a fault-bend fold geometry, forming a flat-ramp structure on the frontal upthrown block of the Western Foothills (Yue *et al*., 2005; Lai *et al*., 2006); the concealed Changhua fault of a fault propagation fold that forms the Paukashan anticline in the western side of the earthquake rupture (Fig. 1). The earthquake ruptures have complex structural and stratigraphic characteristics and can be split into two principal segments, namely, the Shihkang and Chelungpu faults. Surface slip was greatest along the northern portion of the Shihkang fault, with a maximum observed vertical slip of 9.5 m. The Shihkang fault, which has a length of about 45 km and is confined within a Pliocene shale, is interpreted as a bedparallel slip fault, based on extrapolation from the Taiwan Core Drilling Project deep borehole. The Chelungpu fault is about 55 km in length and differs from the Shihkang fault in that the Pliocene shale overthrusts a thick sequence of Quaternary deposit. The Shihkang fault consists of a N30–40oW verging oblique thrust with vertical offsets ranging from 3–9.5 m, while the Chelungpu fault is made up of a N70–90oW verging pure thrust with vertical offsets ranging from 0.2–4 m. According to the seismicity, GPS, and SPOT data, the coseismic deformation also defines two different structural domains (Dominguez *et al.*, 2003; Pathier *et al*., 2003; Cattin *et al*., 2004).
