結合反射震測與折射震測資料探討臺灣南部呂宋隱沒帶的構造特徵

Combining seismic reflection and refraction data to investigate tectonic features of the Luzon subduction complex south of Taiwan

陳憶萍(Yi-Ping Chen)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01293

摘要

毀滅性的地震（地震矩規模 > 8.0）多半是由於巨型逆斷層（megathrust fault）沿著板塊邊界錯動所造成的。而一些被認為從巨型逆斷層分支發育並出露於海床的大型逆斷層，稱為分支斷層（splay fault）。臺灣南部海域的增積岩體明顯被一條主要的分支斷層區分成以褶皺逆衝斷層構造型態為主的下部斜坡區，以及高度變形的上部斜坡區。此分支斷層系統可從臺灣南部海域向北延伸到臺灣西南海域，並可能連接至陸上的旗山斷層。由於分支斷層可能會把原本低角度的巨型逆斷層斷層面抬升為高角度斷層面的分支斷層，一旦這些斷層發生錯動可能會增加垂直方向的位移，促使海嘯的發生。
本研究處理分析TAIGER計畫中於臺灣南部海域所收集之長支距反射震測剖面資料以及海底地震儀所記錄到的折射資料，以期能完整描述呂宋隱沒帶增積岩體中這條分支斷層的型態、分布，以及隱沒帶的構造型態。反射震測資料分析結果顯示：(1) 從海溝軸部開始，向東傾斜的滑脫面深入隱沒帶約20至40公里，然後降階至基盤上；(2) 有一至兩條的分支斷層從不同的分岔點（branching point）向上發育，其斷層面角度約為26度，較一般逆衝斷層角度低。對於地殼深部反射震測剖面無法解析到的部分，則利用折射震測資料建立地殼P波速度構造模式，以探討深部地殼架構。速度構造分析的結果顯示：(1) 研究剖面中隱沒帶莫荷面深度約15至17公里，而地殼厚度約9至11公里。
彙整一系列臺灣南部海域震測剖面所顯示的構造型態，本研究認為分支斷層從北緯18.5度向北延伸到北緯22.5度，其型態變化可能與馬尼拉海溝之走向變化以及隱沒板塊所帶入之沉積物供應量有關。利用地震資料與震測剖面分析結果，本研究提出臺灣南部海域可能的巨型逆斷層帶分布範圍。測線MGL0905-25A剖面中觀察到海底山隱沒構造，但該剖面所辨識分支斷層分岔點地區尚無較大地震記錄，能量可能開始在該處累積，尚未釋放，未來可能有大地震產生。此外，北緯20.7度以北雖未發現海底山隱沒構造，但速度構造剖面顯示增積岩體下方有增生的高速物質，也可能使板塊間耦合程度增加，未來亦可能會發生大地震。

關鍵字

分支斷層 ； 巨型逆斷層帶 ； 呂宋隱沒帶 ； 海底地震儀 ； P波速度構造

並列摘要

Disastrous earthquakes (Mw>8) are mostly caused by megathrusts fault that slip along plate boundaries. Some large thrust faults, called splay faults, have been suggested to emerge from the megathrust fault to the seafloor. A major splay fault has been identified in the accretionary wedge offshore southern Taiwan. This splay fault separates a folds and thrusts dominated lower slope domain from an intensely deformed upper slope domain of the accretionary wedge. This splay fault system extends from offshore southern Taiwan to offshore southwestern Taiwan in a SSE to NNW direction, and may connect to the Chi-Shan fault onshore. As the fault plane angle is raised from a low angle megathrust fault to a high angle splay fault, this splay fault could increase the vertical displacement of the seafloor once the fault is activated, then could enhance tsunami generation.
In this study, we analyze several large-offset multi-channel seismic reflection profile data and refraction data from ocean bottom seismometers across the Luzon subduction zone between 18.5 °N and 21 °N, for the purpose of having a complete description of the splay fault system and tectonic patterns of the Luzon subduction zone south of Taiwan. The results of seismic reflection profile analysis suggest that (1) the eastward dipping decollement steps down to basement at about 20 to 40 km from the trench axis; and (2) there are 1 or 2 splay faults emerging from different branching points, the fault plane angle is about 26 degrees, lower than generally thrust faults. To image the structure of the deeper crust, seismic refraction data are used to build the P-wave velocity model. P-wave velocity model shows that the Moho depth is about 15 to 17 km, and thickness of the crust is about 9 to 11 km. Integrating the results from previous studies and this study, we suggest that the structural variation of the splay fault system from 18.5 °N to 22.5 °N is connected with the trending of the Manila trench and incoming sediment supply. Examining the seismicity and seismic reflection profile data, we suggest the distribution of a possible megathrust zone in the area south of Taiwan. As there is no big earthquake record around the branching point mapped on profile MGL0905-25A at 20.7 °N, where a seamount subduction has been observed, there may be a high potential to produce large earthquakes in the future.

並列關鍵字

Splay fault ； Megathrust zone ； Luzon subduction zone ； Ocean-bottom seismometer ； P-wave velocity model

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