Identification of extreme sedimentary event deposits in Yilan coastal area, Taiwan, and the western coast of Myanmar

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 (the Meiwa Tsunami) with run-up height of more than 30 m on 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 a 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. Many previous studies have pointed out that paleo-tsunami deposits are most likely present in inter-ridge lowlands, and have distinctive characteristics such as a coarse sand layer covering soil layers (or other terrestrial facies sediment layers) with a very sharp boundary. Thus we excavated several trenches in our two study areas to observe and record the deposits, collect samples, and use C-14 dating, Cs-137 analyses and diatom analyses to help us understand the age and sedimentary environment of the deposits. In one trench in 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. Thus we infer that the gray sand is also an event layer. Our preliminary results show that extreme sedimentary event deposits are present in both of our study areas. However, we are still unsure whether some of the event sources are fluvial or marine. More data are needed to further understand the mechanism, influence area and recurrence intervals of these extreme sedimentary events.