Investigating near-surface structures under the Changhua fault, west-central Taiwan by the reflection seismic method

The Changhua fault is a long fault (80 km), extending along the western border of three gravel terraces in west-central Taiwan. It represents the western-most thrust fault in the deformation front of Taiwan's western foothill area. Photogeologic studies indicate that it could be an active fault, as it has apparent lineation and fault scarps. In this study, we used the shallow reflection seismic method to map the near-surface structures around the fault and to examine its activity level. It is surprising to rind that most of the seismic sections are composed of flat or slant layers and do not show significant traces of fault offsets. Combined with CPC data, we may categorize the fault as a 'blind-thrust' buried at the depth of 3 km. Moreover, this long fault can be divided into northern and southern parts most likely separated by a hidden fault: the Tai-an fault. According to the layer bending attitude, the southern part of the Changhua fault (the Pakua terrace area) could be more active, and may still possess a certain degree of reactivity. By contrast, the northern part is relatively flat and quiet with low potential of activity. Besides detecting the Changhua fault, we also inspected its accompanying back-thrust faults: the Tiehchanshan fault and Hengshan fault, finding that they may not exist.