Characteristics of Active Normal Faulting in the Northern Taiwan Mountain Belt: Evidence from LiDAR-derived DTM and Geologic Observations

Complex tectonic environment occurs in the northern Taiwan mountain belt where the Philippine Sea plate is subducting underneath the Eurasian plate and the influence of the Okinawa trough opening progressively reaches the region. In contrast to other parts of Taiwan, the northern Taiwan mountain belt is currently experiencing extensional tectonics which may potentially induce destructive earthquakes with normal faulting mechanism in the Taipei metropolitan area. Nevertheless, mapped active faults in the region are still under debate due to lack of clear evidence; thus, decisive work is needed to ascertain reliable evidence of active normal faulting. 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 fault into the Tatun volcanic region. We also discovered evidence for active normal faulting preserved in densely-covered forests within the northern mountainous region of the Lanyang Plain. The newly acquired LiDAR-derived DTM indicates a series of topographic offsets of a few meters, dependable geomorphic evidence indicating active normal faulting in the region. Furthermore, the discovered topographic offsets seem distributed in a zone of about five kilometers suggesting that the effect of extension is distributed rather than localized in one single fault. Several small sag ponds or dammed lakes were present, which further support the normal faulting activities. Other work on lake sediment indicates regional subsidence in the Lanyang Plain area at a rate comparable or higher to that in the Taipei Basin. The discovered topographic and geologic evidence emphasizes that the northern Taiwan is currently experiencing active tectonic extension and reminds us that the region is capable of producing large earthquakes with shallow hypocenters as indicated by the well-preserved surface ruptures under the forests.