Presenter: Lien-Kai Lin
Date: 2016/05/25
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. In such a context, the earthquakes in this junction area are most frequent around the Taiwan area. However, the geological structure of the Milun Fault in northernmost portion of the LV and its offshore area is still enigmatic. Particularly, because of the lack of data the offshore extension of the Milun Fault was seldom addressed. Therefore, in this study we use the 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 that ruptured Milun tableland with 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 could see the two kinds of structure on the Hualien Ridge. For the bathymetry, we could observe the creeping linear structure on the sediment and seafloor on the bathymetry. For the SBP profile, we could find three structure direction and two types structure. First, 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. Second, we find 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 the clockwise rotation stress. On the MCS profile, we could observe the area in the Milun Tableland and Hualien Ridge is the pop-up structure. According to the focal mechanisms, we could know the regional stress is compressive and direction is NW-SE. Moreover, based on the SBP profile and MCS profile, we think the Milun Fault extend to the most western Hualien Ridge in NE direction, but we need more evidence and data to realize whether Milun Fault extent to the Shingcheng Ridge. Summary, the Milun Terraces and Hualien Ridge is pop-up structure formed by NW-SE regional compressive stress, so we could observe the normal faults for sliding strata due to the extensional stress on the shallow depths. The northern Longitudinal Valley is under the clockwise rotation stress, so the Hualien Ridge is cut by strike-slip fault zone and divided into Southern Hualien Ridge and Northern Hualien Ridge.
Reference
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