Paleoseismic event and active faulting: from ground penetrating radar and high-resolution seismic reflection profiles across the Chihshang Fault, eastern Taiwan

Abstract

Ground penetrating radar (GPR) and high-resolution shallow seismic reflection have been carried out to delineate the subsurface pattern and paleoseismic facies of the active Chihshang Fault, a segment of the Longitudinal Valley Fault, eastern Taiwan. Optimum radar results were obtained along streets in the Chinyuan village. A total of 720m GPR profiles were acquired with 200MHz antenna. Two seismic sections were collected using Mini-Sosie sources, providing a frame to the radar profiles.Three types of structures revealing evidence for a paleoseismic event can be observed in one GPR profile; these are: upward fault termination, colluvial wedge, and sand injection. Other profiles reveal unconformities, reverse-faults, one fault-related fold, and the presence of a colluvial wedge on the downthrown block, all indicating paleoseismic activity.The remarkable feature of the Chihshang Fault at Chinyuan is that the dips of this active fault at surface changes rapidly along strike. Combining the GPR with shallow seismic reflection allows us to reconstruct the three-dimensional fault pattern of the Chihshang Fault and resolve doubts about the abrupt changes of fault dips. These GPR and seismic studies are consistent with the surface observation and reveal the complicated pattern of antithetic subsidiary faults in the near-surface part of the major E-dipping Chihshang thrust fault.