| dc.description.abstract | The Philippine Sea Plate subducts northwestward beneath the Eurasian Plate in northeastern Taiwan, causing the previously formed mountain range collapse due to the reverse subduction polarity, which is also view as post-collisional stage and formed a series of NE-SW trending normal faults in NE Taiwan. Among these structures, the North Ilan Structure (NIS), is located at the junction of the Ilan Plain and the Hseushan Range. According to Taiwan Earthquake Model (TEM) in 2020, the estimated probability of an earthquake with a moment magnitude (Mw) of 6.9 occurring in the NIS area within next 50 years is 13%. However, if the NIS extends to the offshore area, the magnitude could be even greater. This study aims to understand the potential offshore extension of the NIS and analyze its activity. To achieve this, we collected 32 profiles of 24-channel sparker reflection seismic data and 6 sub-bottom profiles based on the possible fault distribution. The results indicate that two major normal faults, A1 and C1, are ruptured to the seafloor in SW part of the survey area, suggesting ongoing fault activity in this region. Fault A1 cross the entire study area and extends southwestward, possibly connecting to the onshore Ilan Fault, on the other hand, fault C1 appears to be a regional normal fault system. Both faults A1 and C1 have separated into multiple subsidiary faults in their SW segments, representing complex reginal forces. In contrast, the NE segments of these faults are controlled by post-arc extension, characterized by tilted block faulting along fault A1. Additionally, the NE segment of fault C1 is connected to a high-angle continental slope, indicating the potential occurrence of submarine landslide, suggesting the southwestward expansion of the south Okinawa Trough. Last, the negative flower structures observed in the SW segments of faults A1 and C1 in the sparker profiles indicate a transtensional stress regime. Overall, two major normal faults A1 and C1 in the offshore area of NE Taiwan appear to be active according to the side scan sonar image and sub-bottom profiles, therefore, monitoring the activity of growth fault C1 is crucial, as its continued activity could lead to submarine landslides and the formation of a new continental slope in NE offshore Taiwan. | en_US |