Active Normal Faulting in Northern Taiwan: Evidence from LiDAR-derived DTM and Geologic Observations

Complex tectonic environment occurs in northern Taiwan where the Philippine Sea plate is subducting underneath the Eurasian plate. In contrast to other parts of the contractional Taiwan mountain belt, the northern Taiwan area currently undergoes extensional tectonics which may potentially induce destructive normal fault earthquakes around the Taipei metropolitan area. However, mapped active faults in the region are still under debate due to lack of clear evidence; hence, crucial work is needed to determine reliable evidence and pattern of active normal faulting. In order to uncover decisive topographic evidence, we initiated and applied airborne LiDAR topographic mapping in northern Taiwan to virtually remove trees and buildings. Based on recent LiDAR-derived DTM and geologic observations, we discovered consistent evidence for regional extensional deformation in northern Taiwan. We uncovered fracture patterns and branches of normal faults possibly extending from the Shanchiao normal fault into the Tatun volcanic region. We also discovered clear topographic evidence for normal faulting preserved in densely-covered forests within the hard-to-reach mountainous region. The newly acquired LiDAR-derived DTM indicates a series of topographic offsets of a few meters indicating possibly active normal faulting in the region. Furthermore, the discovered topographic offsets seem distributed in a zone of several kilometers suggesting that the effect of extension is distributed rather than localized in one single fault. Several sag ponds or dammed lakes were present, which further support pervasive normal faulting activities. In addition, lake sediment study indicates regional subsidence in the Lanyang Plain area at a rate comparable or higher to that for the Taipei Basin. As indicated by the well-preserved surface ruptures under forests and the long-term subsidence rates, these geologic findings remind us that the northern Taiwan region may well produce destructive normal fault earthquakes with shallow hypocenters although recent GPS and seismicity observations reveal only modest tectonic activities.