| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 105 === The Yilan Plain in northeastern Taiwan is located west of the Ryukyu Trench. In AD 1771, a M7.4 earthquake occurred near the Ryukyu Trench, and generated a tsunami which moves a ~700 tonnes boulder on the southern part of the Ishigaki Island, only 250 km away from the Yilan Plain. Therefore, we suggest that tsunami poses potential threats to the Yilan Plain. Offshore western Myanmar, a ~M8.5 earthquake occurred in 1762 on the nearby Sunda Trench. The 2004 Sumatra Tsunami also reached approximately 3 m high along the western coast of Myanmar according to local residents. These observations also point out the potential for future tsunami hazard along the rapidly developing western Myanmar coast. Therefore, we attempted to search for geological record of previous extreme sedimentary events in these two areas in this study.
We excavated several pits in our two study areas to observe the deposits, collected samples, and used C-14 dating, Cs-137 analyses and diatom analyses to help us understand the age and sedimentary environment of the deposits. In the northern Yilan Plain, we found a 25 cm thick sand layer covering a mud layer with a sharp and horizontal boundary. We infer that this sand layer deposited during a fluvial flooding event. In southern Yilan Plain, we found another sand layer covering a black mud layer with a sharp and irregular boundary. This clearly indicates a high-energy depositional event, but the source of such an event remains unknown. In trenches excavated along the western coast of Myanmar, we found a gray sand layer covering an organic mud layer that has abundant plant debris with a sharp boundary. We infer that the gray sand layer also indicates an event. This event may be a fluvial flooding event or a beach ridge propagation event.
Our results show that extreme sedimentary event deposits are present in both of our study areas. However, in both of our study areas, sediments from beach ridges and sediments from rivers have the same sources. Therefore, we cannot use the composition of event layer sediments to define the sources. Thus, we are still unsure whether some of the event sources are fluvial or marine. More data are needed to further understand the details of these events.
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