Offshore extension of the Ilan Fault in northeast Taiwan

The Philippine Sea Plate (PSP) has subducted northwestward beneath the Eurasian Plate (EP) in the northeast Taiwan and has obliquely collided with the EP creating the mountain building. The offshore area of northeastern Taiwan is subject to post-collisional collapse and under extensional regime, forming a series of normal faults. The NE-SW trending Ilan Fault is situated between the Ilan Plain and the Hseushan Range. The Ilan Fault is also call North Ilan Structure (NIS). According to the Taiwan Earthquake Model published by TEC team in 2020, the probability for a Mw 6.9 earthquake happened in NIS in future fifty years is estimated about 13%. However, if the NIS extends to the offshore area, the risk will become greater. This study aims to understand the possible offshore extension of the Ilan Fault and analyze the structural activity. For that, we have collected 24-channel sparker reflection seismic profiles and sub-bottom profilers across the possible fault trace. Based on sparker seismic profiles and sub-bottom profiles, four major normal faults, Fa, Fb, Fc and Fd, trending NE are observed. Faults Fa and Fb are active and have outcropped to the seafloor. A negative flower structure is observed along Fault Fa, which indicates that the regional stress is under transtension. Furthermore, slope failures, slumps and sliding surfaces are found. Overlying on the Last Glacial Maximum (LGM) unconformity, sedimentary layers are tilted in the hanging wall of Fault Fb, indicating Fault Fb is a growth fault. On the other hand, Faults Fc and Fd dip to the NW and SE, respectively. They bound a graben trending NE and only ruptured to the LGM unconformity. It implies that Faults Fc and Fd are no more active.