台灣宜蘭及緬甸西海岸地區之極端事件沉積物辨認與探討

Abstract

宜蘭平原位於台灣東北部，其東南側為琉球海溝。琉球海溝於1771年曾經發生規模超過7.4的地震，產生明和海嘯，明和海嘯於石垣島東南側搬動重量超過700公噸之海嘯石。由於台灣宜蘭海岸地區與石垣島距離僅約為250公里，此海嘯事件可能也曾對宜蘭海岸地區造成過影響。而在緬甸西海岸地區，西臨巽它海溝，於1762年亦曾發生規模至少8.5的大地震。故本研究探討此兩區域的海岸極端沉積事件記錄，並藉由這些紀錄探討古海嘯是否曾對此兩區域之海岸環境造成影響。 藉由前人發表之海嘯沉積物記錄，可以發現一些共通的特性，例如：找到海嘯沉積物的位置都位於沿岸砂脊後方地勢較低處，因平時波浪無法翻越沿岸砂脊，只有在海嘯來時，有可能將淺海與沿岸砂脊上的沉積物，帶至後方地勢較低處堆積。此種事件堆積層最關鍵的特徵為：海相事件沉積物不規則地覆蓋在陸相沉積物（例如土壤或泥）上，侵蝕造成之邊界相當明顯。 本研究利用槽溝挖掘，觀察並記錄兩區域的沉積物組成、分佈以及特徵，推測此兩區域之海岸環境變化，以及是否有受到事件之影響，並搭配碳-14定年、銫-137分析及矽藻分析，試圖了解這些事件之來源及發生年代。在蘭陽溪以北的土圍(TW)槽溝中，發現一砂層覆蓋在泥層上，邊界明顯，根據前人研究及其他槽溝結果，本研究推測為古宜蘭河之地區性氾濫事件所沉積。蘭陽溪以南地區的龍德(LD)槽溝中發現一砂層覆蓋在一有機層上，邊界明顯且不規則，推測可能代表一大水事件，但就目前資料仍無法得知此事件確切來源。在緬甸西海岸卡帕裏海攤附近地區，發現一灰色砂土層覆蓋在富含植物碎屑之有機層上，邊界明顯，根據附近地形以及矽藻分析之結果，本研究推測為沙嘴發育或潟湖被沉積物填滿之過程。 由於本研究之研究區域內沿岸砂脊沉積物與河流沉積物來源及組成相同，因此無法直接利用沉積物組成直接判斷事件來源。若未來須利用類似方法研究極端事件沉積物，則需事先確認可分辨不同來源之沉積物，作為沉積物來源之直接證據。

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.