| dc.description.abstract | The Longitudinal Valley (LV) is considered as the collisional suture between the Philippine Sea and the Eurasian plates. The northern end of the LV is close to the western end of the Ryukyu Trench. Earthquakes are very frequent in such a complex regime. The Milun Fault is located in the northernmost portion of the LV. However, the offshore portion of the Milun Fault is still enigmatic. Particularly, because of the lack of data the offshore extension of the Milun Fault was rarely addressed. For that, in this study we use new marine seismic reflection, sub-bottom chirp sonar profile, and multi-beam bathymetic data to analyze the seafloor features and tectonic structures of the offshore portion of the Milun Fault. The Milun Fault is a well-known active structure associated with the rupture of the Milun tableland by a M7.3 earthquake in October 1951. From south to north, the geological units include Hualien ridge, the top of Hualien Canyon, and Hsincheng Ridge. Based on the SBP profile and bathymetry, we can observe a creeping linear structure on the sediment and seafloor, and three structure directions and two types structure on the Hualien Ridge. Firstly, we find normal fault for sliding strata in shallow depths. The strike of those normal faults is N30˚E in southern Hualien Ridge and N45˚E in northern Hualien Ridge. Secondly, the Hualen ridge is cut by the strike-slip fault zone in N120˚E. So, the SBP results indicate that the Hualien Ridge is under a clockwise rotation stress. Base on the MCS profile, we could observe the area in the Milun Tableland and Hualien Ridge is a pop-up structure. According to the earthquake focal mechanisms, we could know the region is subject to compressive stress and compression direction is NW-SE. Moreover, the SBP profile and MCS profile indicate that the Milun Fault could extend northeartward to the west side of the Hualien Ridge. However, we need more data to understand whether Milun Fault extends to the Shingcheng Ridge. In summary, the Milun Terraces and Hualien Ridge is a pop-up structure due to the NW-SE regional compressive stress, but we could observe the normal faults and sliding strata due to the extensional stress on the shallow depths. The northern Longitudinal Valley is under a clockwise rotation. The Hualien Ridge is therefore cut by a strike-slip fault zone and is divided into southern Hualien Ridge and northern Hualien Ridge. | en_US |