Quaternary International 115–116 (2004) 189–199 Active structures as deduced from geomorphic features: a case in Hsinchu Area, northwestern Taiwan Yue-Gau Chen a, *, J. Bruce H. Shyu b , Yoko Ota c , Wen-Shan Chen a , Jyr-Ching Hu a , Bor-Wen Tsai d , Yu Wang a a Department of Geosciences, National Taiwan University, No. 1, Sec 4th, Roosevelt Rd, Taipei, Taiwan, ROC b Division of Geological and Planetary Sciences, Caltech, USA c Yokohama National University, Japan d Department of Geography, National Taiwan University, Taipei, Taiwan, ROC Abstract Two previously documented active faults in northwestern Taiwan, the Hsinchu and Hsincheng Faults, are evaluated for their earthquake hazard potential by using a Digital Elevation Model (DEM), aerial photos and field mapping. Both of these thrust faults contain active anticlines in their hanging walls based on folding of young alluvial deposits. Mapping of the Hsinchu Fault suggests the presence of numerous scarps oriented sub-parallel to the fault trace that was previously defined by projection of subsurface geology. We interpret the geometry of the surface scarps to be mainly produced by the thrust component, but in part by a lateral component of fault slip. The scarps associated with the Hsincheng Fault system are less complex and offset a flight of terraces, including the modern flood plain. A back thrust exposed 1–2 km east of the Hsincheng Fault is interpreted as linked with the main thrust. Surface deformation is only apparent south of the Touchien River, and becomes less obvious across another wrench fault system, which is linked to Hsinchu Fault in the west and extends southeastward, parallel to the Touchien River. The wrench fault system mentioned above not only offsets the Hsinchu Fault, but also divides the Hsincheng Fault into segments. The supporting evidence found for this includes pressure ridges and pull-apart sags. Two associated hanging wall anticlines are both recently active based on deformed geomorphic surfaces. Two major NE striking thrusts and one NW wrench fault system are identified, reflecting the compression stress is partitioned in this area, which might be resulted from recently influenced extension tectonics, i.e., Okinawa back-arc rifting. r 2003 Elsevier Ltd and INQUA. All rights reserved. Keywords: Active fault; Thrust fault; Wrench fault; Geomorphic features; Earthquake hazard 1. Introduction Accurate mapping of active structures in Taiwan is critical for assessing their potential for future damaging earthquake. For example, the Chi-Chi earth- quake that struck central Taiwan in 1999 (CGS, 1999; Ma et al., 1999; Kao and Chen, 2000; Chen et al., 2001) led to over 2300 deaths and 10 billions US$ in economic losses. More than 40 previously identified active faults have been mapped on Taiwan (Chang et al., 1998; Lin et al., 2000). These maps usually do not consider important geomorphic evidence of recent displacements, which is particularly important for determining fault slip rates, and credible earthquake scenarios for future events. Geomorphic features, such as fault scarps and active folds, are particularly useful where they deform young sediments (i.e., those o100ka), which can be dated in an effort to constrain the slip rates. This study is aimed at evaluating tectonic strain recorded by young geomorphic features, including fluvial terraces, fault scarps and drainage channel networks. We used the recently available 40m Digital Elevation Model (DEM) of Taiwan and 1:18000 scale aerial photos to map active structures in the Hsinchu area. Furthermore, we compare these local structures to the broader stress field and morphology of the Taiwanese fold-and-thrust belt where it inter- acts with the extensional province defined by the Taipei Basin, and further offshore in the Okinawa Trough (Fig. 1). ARTICLE IN PRESS *Corresponding author. Tel.: +886-2-2369-7648; fax: +886-2-2363- 6095. E-mail address: ygchen@ccms.ntu.edu.tw (Y.-G. Chen). 1040-6182/$-see front matter r 2003 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/S1040-6182(03)00107-1