碩士 / 國立臺灣大學 / 海洋研究所 / 101 / Submarine mass movements occur when the seabed could not sustain the stress applied, then the seafloor material will be transported downdip due to gravitational forces and may cause geohazards. In the area off southwest Taiwan, there were large scale submarine cable breakages after the 2006 Pingtung earthquake, the 2009 Morakot typhoon and the 2010 Jiashian earthquake. Investigation results show that both earthquake-induced submarine landslides and the flood-induced hyperpycnal flows have generated turbidity currents that carried considerable amount of sediments from the upper reach of the Gaoping Submarine Canyon (GPSC) to the Manila Trench, and damaged submarine cables lying across the GPSC. Most of the cable broken sites are along the axis of the GPSC, thus canyon morphology could be an important factor controlling transport processes of submarine mass movements and the sediment gravity flows passing through the crossing cables along their ways, and broke many of them. On the other hand, within a minute after the Pingtung Earthquake, submarine cables were broken at two places in the Fangliao Submarine Canyon (FLSC) area, suggesting that submarine mass movements were triggered on sites by the earthquake nearby.
In this study, we use high-resolution bathymetry data (gridded at 100 m interval) and the cable breakage information (including the times and locations of the breakages) to investigate if the canyon morphology controls submarine cable breakages in the GPSC. We also use high-resolution bathymetry data (50-m grid spacing) and chirp sonar profile data to investigate the possible causes of submarine mass movements in the FLSC.
In the GPSC, this study found that both the 2006 Pingtung earthquake and the 2009 Morakot typhoon have produced two sediment gravity flows each, many cable breakages were recorded along gravity flows 2006-1 and 2009-2, however, not all the cables crossing the GPSC were broken, and there is no obvious threshold on local slope values differentiating sites where cables were broken or not. In addition, the local slope values and the gravity flow speeds between two cable breakage sites in the gravity flow 2006-1 and 2009-2 show little correlation. We thus suggest that the local slope values are not the most important factor controlling whether the submarine cables to be broken or not. In the FLSC area, there are linear topographic features parallel to the isobaths at water depths between 200 to 400 m. Distribution map of different types of chirp sonar echo characters have been established. Four types of echo character patterns (flat, mounded, transparent and irregular) and ten sub-types have been identified in this area, and the echo characters of those topographic liner features suggest that they could be formed by the activities of the submarine landslide or turbidity current (type 2-1). Furthermore, the echo types 4-1 and 4-3 show that the pores of the sediment are filled with the liquids or gases, and there are frequent earthquakes which happened in the FLSC. Therefore, the local geological settings should be an important factor controlling the submarine mass movements in the FLSC.
| Identifer | oai:union.ndltd.org:TW/101NTU05279028 |
| Date | January 2013 |
| Creators | Pei-Cheng Hsia, 夏培正 |
| Contributors | Char-Shine Liu, 劉家瑄 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 92 |
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