由海底地震儀資料探討台灣東部海域之地震地體構造;Seismotectonics of the eastern Taiwan offshore area from OBS data

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题名:	由海底地震儀資料探討台灣東部海域之地震地體構造;Seismotectonics of the eastern Taiwan offshore area from OBS data
作者:	秦韶晉;Chin,Shao-Jinn
贡献者:	地球科學學系
关键词:	地震地體構造;台灣;碰撞帶;Seismotectonics;Taiwan;Collision zone
日期:	2014-07-25
上传时间:	2014-10-15 14:43:11 (UTC+8)
出版者:	國立中央大學
摘要:	台灣位在歐亞板塊及菲律賓海板塊的碰撞帶上，其東部地區因而形成複雜的地體構造。前人的研究提供了許多地球物理和地質對於構造上的解釋。然而，因為絕大部分的地震測站都位於陸地上的關係，東部海域的海底地震在定位的精確度上無法與陸上的地震相比，在分析時也容易因此導入較大的不確定性。為了瞭解台灣東部的地震活動及構造特徵，本研究利用ATSEE計畫 (Across Taiwan Strait Explosion Experiment) 在2012年7月9日到8月13日之間佈放的8個海底地震儀，配合14個氣象局地震測站的資料作海底地震的定位工作，並利用雙差分地震定位演算法 (HypoDD) 來做地震的重新定位，以得到較精確的地震分布。同時我們也使用SIMUL2000的程式及柔性網格的方式建立台灣東部海域的三維P波速度模型。在25天的資料中，共有979個地震被記錄下來，並有333個地震被重新定位。在23.3⁰N以南，重新定位後的地震呈現明顯向東傾沒的分布型態；23.5⁰N以北的地震皆深於30 km，較南部為深，約略呈向西傾沒的地震分布，但連續性不如南部來得明顯。海底地震往北逐漸變深，顯示菲律賓海板塊向北隱沒的形貌。陸上地震分布的深度沒有明顯的趨勢，並在23.3~ 23.7⁰N之間出現明顯的地震空白帶，可能標示了碰撞到隱沒之間的過渡帶。在空白帶以南，最大主應力的方向與板塊運動的方向平行，顯示這裡的構造主要受到碰撞擠壓的影響。從震源機制解來看，地震空白帶內的應力分布變得較為凌亂，地震幾乎沒有出現在陸地上，但在空白帶以北又重新出現在台灣島的下方。2013年10月31日發生在空白帶內的瑞穗地震為向西傾的逆衝型態，我們推測這可能是菲律賓海板塊的隱沒向南傳遞的證據。 ;Located at the arc-continental collision region between the Eurasian (EU) and Philippine Sea Plate (PSP), Taiwan is characterized by a complex tectonic environment, particularly the eastern part of the island. Based on geodetic, geological and geophysical data, the tectonic structures in the eastern Taiwan have been well studied by several former works. However, the seismotectonic structures in the offshore area of eastern Taiwan remains poorly understood, because most seismic stations are inland and the earthquakes occur offshore cannot be located accurately. For understanding the seismic activities and the structures in the eastern Taiwan, we located the earthquakes from OBS (Ocean Bottom Seismometer) data, which was deployed by ATSEE project (Across Taiwan Strait Explosion Experiment) during Jul. 9th~ Aug. 3rd, 2012, and the data from 14 seismic stations of CWB (Central Weather Bureau) by using the program Antelope and the global velocity model iasp91. For improving the precision of location, we relocated the earthquakes with the Hypo-DD relocation method which could minimize errors owing to the velocity structure without the use of station corrections. Additionally, the SIMUL2000 program and the flexible gridding method were applied to build a 3-D Vp velocity model from the 1-D initial model CWB1D. In the 25- day data, 979 earthquakes were recorded while 333 events were relocated. The features of onshore and offshore seismicity are quite different, especially in the north of 23.3⁰N. With the northward subduction of the PSP, offshore seismicity deepens continuously while there is a seismic gap onshore. Along the collided Luzon arc, the high gravity anomaly (volcanic arc) progressively becomes negative (trench) between 23.3⁰N and 23.7⁰N, and there is a significant difference of the surface velocity field near 23.7⁰N from GPS data, which indicates a stress change in the shallow part. As the PSP subducted, the collision zone deepens northward, resulting in an onshore seismic gap. With the northwestward subduction of the PSP north of the seismic gap, normal and strike-slip earthquakes present at the depth shallower than 35km. Taking together all these observation, we suggests a tectonic transition zone from collision to northwestward subduction at the latitude of 23.3~ 23.7⁰N. Also, the Ruisui earthquake series, located in the seismic gap, may provide an evidence of the southward propagation of the PSP subduction.
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