**Characteristics of Coseismic Thrust-Related Folding from Paleoseismic Investigation Responsible for the 1999 Chi-Chi Earthquake of Central Taiwan**

Wen-Shan Chen1, Nobuhisa Matsuta2 and Chih-Cheng Yang3

*1Department of Geosciences, National Taiwan University, Choushan Road, Taipei, Taiwan 2Research Center for Seismology, Volcanology and Disaster Mitigation Graduate School of Environmental Studies, Nagoya University D2-2(510), Furo-cho, Chikusa-ku, Nagoya, Zip, 3Taiwan Petroleum Exploration Division, Chinese Petroleum Corporation, Miaoli, Taiwan, 1,3Republic of China 2Japan* 

### **1. Introduction**

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

Wessel, P. & Smith, W. H. F. (1998). New, improved version of the Generic Mapping Tools

The Western Foothills are a west-verging fold-thrust belt related to the recent arc-continent collision of the Asian continental plate with the Philippine Sea plate (Suppe, 1981). The frontal orogenic belt of Western Foothills consists of well constructed imbricated thrust faults, which have recently been locked and rooted into an aseismic decollement (Chen *et al.*, 2001c; Yue *et al*., 2005). Over the past tens years, several great intraplate thrust earthquakes (Mw >7.0) located at the orogenic belt occurred in the world (e.g., the 2008 Wenchuan earthquake and the 1999 Chi-Chi earthquake). On September 21, 1999, central Taiwan was hit by an earthquake of magnitude Mw 7.6. Several buildings and infrastructure were severely damaged, and about 2450 people were killed. The Chi-Chi earthquake was caused by a thrust fault located at the boundary between the Western Foothills and the Taichung piggyback basin; the fault produced a spectacular surficial rupture of 100 km in length (Fig. 1; Central Geological Survey, 1999; Chen *et al*., 2001c). Based on seismic reflection profiles and focal mechanisms of the mainshock, it was inferred that the earthquake occurred on a shallow-dipping (20–30º) thrust ramp of the Chelungpu fault (Chiu, 1971; Kao and Chen, 2000; Wang *et al*., 2002). The earthquake ruptures show complex structural and stratigraphic relationships that can be divided into two segments: the Shihkang and Chelungpu faults. The Shihkang fault occurs within Pliocene shale and is interpreted as a bed-parallel slip fault. The Chelungpu fault of the southern segment thrust is late Pliocene shale over Quaternary fluvial deposits in the footwall.

In this study, several excavations have been carried out along the Chelungpu fault across the fault trace (Fig. 1); the excavations showed complicated structural features within unconsolidated sediments, including a ductile deformation zone at the fault tip. Because offset measurements within the ductile deformation zone are too variable to estimate the slip, in this study, we attempted other methods to estimate a relatively complete 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 deriving the growth history of the fault-tip fold.

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.
