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  4. The distribution and sources of seismo-turbidites in the Southern Okinawa Trough
 
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The distribution and sources of seismo-turbidites in the Southern Okinawa Trough

Date Issued
2007
Date
2007
Author(s)
Ling, Chiao-Yun
DOI
zh-TW
URI
http://ntur.lib.ntu.edu.tw//handle/246246/54794
Abstract
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.
Subjects
沖繩海槽
地震
濁流沈積物
黑潮
蘭陽溪
earthquake
southern Okinawa Trough
seismo-turbidites
Lan-Yang River
Kuroshio
SDGs

[SDGs]SDG11

Type
thesis
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