| 摘要: | 極端地質事件包括洪氾、山崩、斷層錯移與地震、海嘯、颱風與火山爆發。雖然極端地質事件不常見，但對地質紀錄卻有重要的影響，甚至對人類社會有很大的威脅。台灣西南外海可能發生多種的極端地質事件，包括斷層錯移與地震、海底山崩、海嘯及濁流等。由其台灣南部外海位於馬尼拉隱沒帶北部，此區域的板塊間巨型逆斷層與分歧斷層都是重要的孕震斷層。若上述斷層產生錯移，除了可能產生規模大於8的地震之外，也可能引發海嘯，威脅鄰沿岸城市居民的生命財產以及核電廠的安全。另外，發育於台灣西南部增積岩體的高峽谷，其峽谷源頭與高屏溪出海口相連。在2006屏東外海地震以及2009的莫拉克颱風，沿高屏峽谷皆有海底濁流發育並堆積濁流岩。這些例子顯示濁流岩可以成為極端事件的地質時空紀錄。在台灣南部外海已經有一些海床沉積物岩心可供本子計畫研究。這些岩心包括由法國瑪麗杜凡號所收集的巨型活塞岩心(2010 的MD178、2018 EAGER、2018 HydroSed等航次)，海研一號(OR1-930與1138航次)、海研五號(OR5-0032航次)等岩心，皆可成為本多年期計畫的研究材料。上述岩心站位可分為2類沉積地形。第一類站位位於增積岩體中的獨立盆地，各盆地沒有相連，而且這些盆地沒有來自台灣的濁流輸入。這類站位的岩心將用來研究隱沒帶古地震。另一類站位位於增積岩體區、高屏海底峽谷的中游與下游。這類站位的岩心同時有異重流岩與地震濁流岩出現，研究這批岩心可以了解沉積物由台灣傳輸至南中國海的動力與主控因素。本計畫為多期計畫的第二年，其研究目的分別為：(1) 研究北部馬尼拉隱沒帶的古地震以及對比與比較出現於獨立盆地與高屏峽谷中的濁流岩。(2) 了解台灣沉積物傳輸至南中國海的主控因素以及傳輸動力。(3) 嘗試區分異重流岩以及地震濁流岩。(4) 採集更多南部的獨立盆地長岩心，研究主要隱沒帶孕震斷層的分段與特性。本計畫的研究成果，將可解析出台灣南部外海晚期第四紀極端事件的時空演育，提供地震與海嘯危害評估的基本資料(如提供板塊間巨型逆斷層的再現周期等參數)。台灣沉積物傳輸至南中國海的主控因素以及動力機制可更被了解。 ;Extreme geological events may include floods, landslides, earthquakes, tsunamis, typhoons, and volcanic eruptions. Though extreme events are rare they are of geological importance and may bear significant societal impacts. Offshore southern Taiwan, a variety of geological hazards may be present, including earthquakes, submarine landslides, tsunami and turbidity currents. In particular, the study area in offshore southern Taiwan locates in the northern Manila subduction zone, where plate-interface megathrust and splay fault are capable of generating great (M > 8) earthquakes, which may cause tsunamis with devastating effects on the nuclear facility and populated coastal regions nearby the epicenters. The river-connected Gaoping Canyon, meandering through the accretionary wedge, has witnessed recent major floods (e.g., flooding during 2009 Morakot typhoon) and submarine landslides caused by earthquakes (e.g., 2006 Pingtung earthquake) in the form of turbidity currents and their ensuing deposits. The turbidite layers, therefore, provide with us an opportunity to reconstruct the spatial and temporal variability of the extreme events in the study area. There is an array of core material being collected in offshore southern Taiwan in the past. These cores, including giant piston cores, collected onboard R/V Marion Dufresne (i.e., research cruises of 2010 MD178, 2018 EAGER, and 2018 HydroSed), R/V Ocean Researcher I (i.e., OR1-930, 1138), and R/V Ocean Researcher V (i.e., OR5-0032), are available for this study. The cores are located in two types of depositional settings. One type of cores is located in perched slope basins in the accretionary wedge without connection with canyons. The other type of cores located in the Gaoping Canyon. Turbidites in the perched basins are used to study the paleoseismology while hyperpycnites and seismo-turbidites are both occurring in the Gaoping Canyon and the dynamics and controls on sediment transfer from Taiwan to the South China Sea can be deciphered from these canyon cores.The objectives of this second-year project are to (1) study the paleoseismology in the northern Manila subduction zone and to compare the sedimentology, characteristics, correlation, and timing of turbidites among cores recovered from perched basins and in the Gaoping Canyon, (2) decipher the dynamics and controls on sediment transfer from Taiwan to the South China Sea through the study of cores collected in the Gaoping Canyon, (3) attempt to distinguish hyperpycnites from seismo-turbidites, and (4) collect new cores in other perched basins to decipher possible segmentation of major seismogenic faults.The project results will lead us to better understand the late Quaternary spatial and temporal development of extreme events in the study area. This will provide some essential parameters (return periods for plate-interface megathrusts, for example) for future assessment of seismic and tsunami risks in southern Taiwan and neighboring countries. The major controls on sediment delivery from Taiwan to the South China Sea can also be deciphered. |
