海嘯沉積物之地球化學特性：花蓮豐濱地區之案例分析

Abstract

近年來諸多海嘯事件吸引世人關注，而地質紀錄保存著不為人知的古海嘯事件。過去的相關研究藉由已知的海嘯事件建立起對海嘯沉積物的初步認知，並與相似自然事件，如颱風、洪水做比較。由於古海嘯紀錄並無文字史料記載，因此前人分析古海嘯的指標，多數為沉積學相關特徵，但受到距海遠近與地形等影響，沉積特徵有時無法明確分辨，因此學界開始將目光轉向地球化學分析，希望能找到海嘯沉積物的化學特徵，進一步突破研究困境。地球化學分析著重於沉積物來源的特徵，因此本研究目標為以地球化學方法研究，結合沉積學分析，針對豐濱地區之海嘯事件層進行研究。 研究目標為豐濱沿海之沖積平原，其表面至 2 公尺深處，由粉砂夾細砂質沉積物所組成，其下為一厚度 30 公分至 1 公尺不等之海灘礫石層，更深處則為厚 6 公尺之稜角狀火成岩巨礫層。在細顆粒沉積層之中，部分區域可見一基質支持、中等圓度、淘選度差之火成岩與砂岩質礫石層，向陸地尖滅，被推測為海嘯事件層。在挖掘之槽溝中選擇五個垂直剖面採集沈積物，分析有機碳、無機碳、氮、硫含量和各種元素的相對含量，並分析粒徑，元素分析結果再佐以主成分分析進行探討。結果發現海嘯事件層有著較高比例的矽與鉻，推測矽源於火成岩風化殘留之石英和鄰近區域的八里灣砂岩層，鉻源自於水璉礫岩中的輝長岩所含的含鉻尖晶石，兩種元素的來源都必須透過海水搬運而來。隨事件層深入陸地，粒徑與上下層位無異，但在元素相對含量上依然保有特徵，因此地球化學分析在研究海嘯事件上有其擅長的地方，佐以沉積學特徵分析可得到良好的辨別事件能力。

High energy events, such as tsunami, although occur episodically, are destructive over a wide range of geographic area, and often cause substantial casualties and/or property damages. With the aid of historical records and precise dating, the target tsunami deposits could be correlated over various spatial and temporal scales. This general concept and methodological framework is often challenged under the circumstance where historical records and typical tsunami sediments are fragmentary or absent. As marine sediments bear a variety of geochemical characteristics distinctive from terrestrial ones, geochemical methods would provide additional evidence to constrain the origin of the possible target sediments but have seldom been applied in Taiwan. In this study, a marine terrace in Fongbin Area was chosen for investigation of geochemical characteristics of sediments that have been previously identified to archive a tsunami event. The terrace is composed of colluvial deposits, within which rounded and poorly sorted gravels with a thickness of 50 cm appear in the upper portion, fade away toward inland, and are interpreted to originate from a tsunami event. Particle size and elemental analyses indicated that the poorly soring tsunami deposits have higher Si and Cr content, which could derive from quartz and Cr-spinel. These two minerals may originate from Paliwan Formation and Suilien Conglomerate and were delivered by a tsunami event. The geochemical difference between tsunami and regular deposits decreases toward inland, but the characteristics still subsists. Apparently, geochemical evidence can give a clue to trace the target layer, even if the grain size and sorting cannot exhibit the feature of tsunami event. It seems that geochemical analyses can provide extra indicators to general features of tsunami deposits, while the difference of sediment source could be fully characterized.