Active structures as deduced from geomorphic features: a case in Hsinchu Area, northwestern Taiwan

摘要

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.