南沖繩海槽地震引發濁流沈積物之來源及分布

Abstract

摘要 自從Heezen and Ewing（1952）對於1929年加拿大Grand Banks地震的研究，科學家瞭解到地震會引發濁流的能力，於是濁流沈積物與古地震的研究遂展開。以往對濁流岩之研究多著重在推算地震週期、古地震強度及比較不同成因濁流沈積物間之異同。本研究希望藉由比對近代的地震與其引發濁流沈積物在岩心中的紀錄，進一步瞭解兩者間之相互關係。 南沖繩海槽為菲律賓海板塊隱沒到歐亞大陸板塊下而形成的弧後盆地，其沈積物可能來源為東海陸棚、蘭陽溪及黑潮輸入。由於南沖繩海槽具有高沈積速率且其週邊地區之地震發生頻繁，使本地區成為研究地震與濁流沈積物之關係的絕佳地點。 依據X光攝影及粒徑分析結果，南沖繩海槽沈積物岩心可分成四大類：（1）半濁流沈積（Hemiturbidite）、（2）薄層細粒濁流沈積、（3）厚層細粒濁流沈積及（4）近洋沈積。半濁流沈積之岩心之特徵為其濁流沈積物層厚度較薄，位於南沖繩海槽邊緣陸坡下部；而薄層細粒濁流沈積岩心之濁流沈積物層厚度則較前者為厚，粒徑較粗，位於棉花峽谷下部；至於厚層細粒濁流沈積之岩心，其濁流沈積物層厚度為最厚，粒徑亦最粗，位於海槽最深處，此區為濁流沈積中心。而屬於近洋沈積的岩心，不僅在X光底片上看不出有濁流沈積的構造存在，其粒徑之垂直分布剖面上也無變化。 此外，我們將代表地震事件年代之沈積物進行粒徑分析，進一步探討此區濁流沈積物可能之傳輸途徑。本研究所使用的箱型岩心中，可觀察到2002年ML6.8（24.14oN, 122.19oE, 13.8 km）、1986年ML6.8（23.99oN, 121.83oE, 15.0 km）、1966年ML7.8（24.24oN, 122.67oE, 42 km）、1959年ML7.5（25oN, 122.5oE, 150 km）、1947年ML7.2（24.8oN, 123oE, 110 km）及1922年ML 7.6（24.6oN, 122.2oE, 20 km）等地震所引發之濁流沈積物。粒徑分析結果顯示，南沖繩海槽地區地震引發之濁流沈積物，除1986年地震所引發者之外，皆發源於北坡，再往南傳輸至南沖繩海槽較深處沈積。因此在濁流沈積中心的岩心中，可觀察到厚層之濁流沈積物存在。而1986年地震所引發之濁流沈積物並無法由沈積物粒徑之空間分布辨識出其主要傳輸方向，推究其原因，可能是地震引發之沈積物崩移規模太大，而使得同時有不同方向的濁流形成。 而針對南沖繩海槽沈積物來源與傳輸問題，本研究另藉由沈積物粒徑分布之趨勢分析，以期能有更進一步的瞭解。分析結果顯示，此區沈積物搬運方向主要為由西向東傳輸，顯示蘭陽溪之輸入為主要沈積物來源。搬運方向的強度在南沖繩海槽最西端有一弱區存在，可能是由於黑潮在翻越宜蘭海脊後流向轉彎且流速突降而造成此區沈積物搬運方向較不穩定之故。

Abstract Since the 1929 Grand Banks earthquake, it has been well documented that earthquakes have the potential for triggering turbidity currents. Hence, we are able to use the turbidites, an alternative way to study paleoearthquakes, as records of paleoseismicity. However, the interaction between earthquakes and turbidites is still not well understanding. The Okinawa Trough is an active, incipient, intracontinental backarc basin formed behind the Ryukyu arc-trench system in the western Pacific. Owing to its special geological and hydrological settings, it acts like an efficient receptacle for sediments from Taiwan and the East China Sea shelf. The high sediment flux let us have an opportunity to obtain a high-resolution record of the seismo-turbidites. Such a record might make it possible to identify the spatial and temporal variation of grain size between different earthquake events. All sorts of advantages enable the southern Okinawa Trough becoming an ideal place for turbidite palaeoseismology research. In this study, we utilized the grain size analysis result and X-radiographs of box cores to identify the turbidite layers and their sedimentological features. From these data, we find that the box cores which containing turbidite layers can be divided into three categories. According to their thickness and grain size analysis results, the box cores which belong to the first category are thinner than other two categories and located surrounding outside of the turbidite deposition zone. In the second category, the thickness and median grain size of the turbidite layers are larger than the first category. These cores are sited in the northern part of the turbidite deposition zone. For the last category, which located at the southern part of the turbidite deposition zone, have the thickest turbidite layers and largest median grain size than other categories. The grain size profiles also indicate that there is a fining-upward trend in thick turbidite layers. For the sake of understanding the transport of turbidites generated by the Hualien Earthquake (ML6.8) on March 31, 2002, we analyzed the sediment layers which represent the year of 2002 in our core samples. The spatial distribution of grain size reveals that the earthquake triggered turbidity current may delivered sediments from the north continental slope of southern Okinawa Trough (between the Mien-Hua Canyon and Keelung Sea Valley) and stopped at the deepest part of the southern Okinawa Trough. For the turbidites triggered by the 1966 ML 7.8 (24.24oN, 122.67oE), the 1959 ML 7.5 (25oN 122.5oE) and the 1947 ML 7.2 (24.8oN, 123oE) earthquakes show the same pattern with Hualien Earthquake. However there seems no clear transport direction for the 1986 ML 6.8 (23.99oN, 121.83oE) earthquake. Moreover, only part of our cores recorded the 1922 ML 7.6 (24.6oN, 122.2oE) earthquake, it’s insufficient for us to rebuild the transportation of these sediments. For understanding the transport of sediments in non-earthquake influence years, we also picked some samples from the non-turbidite sections for grain size trend analysis. The result suggested that the Kuroshio is the dominant factor that influences the transportation of sediments in the southern Okinawa Trough, and the Lan-Yang River and eastern Taiwan might be the major source of these sediments.