梅山斷層帶震測三維構造解釋

The Three-Dimensional Seismic Interpretation of Structure of the Meishan Fault Zone

孫偉翔(Wei-Hsiang Sun)

指導教授 ： 石瑞銓

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2015年)

摘要

台灣西部雲嘉地區的歷史地震紀錄中，1906年梅山斷層的危害規模最為凸顯，也是百年以來，雲嘉地區影響最大的一場大地震。該地震規模推估7.1，造成地表13.5公里的破裂，根據當時日本地震學者大森氏調查，梅山斷層屬於右移走向滑移斷層，最大垂直位和水平位移分別為2.4公尺和1.8公尺。梅山斷層經過百年後，未有地震錯動的紀錄，且斷層鄰近區域在近二十年的紀錄顯示低地震活動度的分佈，這意味斷層面可能以鎖住的形式累積應力，使梅山斷層存在高風險的地質敏感區。為了釐清斷層的地下構造，本研究將利用二維震測影像，合成斷層區域的地下三維構造視角，並且再次檢視梅山斷層的型態。

本研究於斷層南側再次施測一條2.5公里的長剖面，並綜合本研究所早期淺層與中油深層震測的剖面，再次提出如下觀點：

1. 梅山斷層帶的寬度應該至少2至3公里寬，有別於過去數百公尺級的看法。由於淺層震測的測線長度相對於公里級的斷層帶，存在不準確性的問題，從震測長剖面的結果皆可發現近地表已發育複雜的破碎帶，倘若將過往淺層震測的尺度與本研究所使用之長剖面比較後，過往的淺層震測顯示的斷層皆為地表破裂分支，應使用長剖面作為主斷層的位置辨別。

2.然而本次研究結果認為，1906年梅山地震的錯動面應為花狀構造的分支，而近地表的分支斷層再往更深部的地殼收束成主斷層。

3. 由三維的構造的小梅背斜軸部連線分析，梅山斷層至今為止，已使小梅背斜右移錯動達2公里的水平位移量，此結果與地表地形分析大致相同，也包括等值線圖的計算結果，這說明鳥嘴層之上的地層具有相同的錯動量，顯示淺層的位移量近乎相同。

關鍵字

小梅背斜 ； 反射震測 ； 梅山斷層

並列摘要

According to the historical records, in 1906, the 7.1 Meishan earthquake occurred in the Yunlin and Chiayi area, was the most prominent earthquake hazard happened in the southwestern Taiwan, in the last century. The Meishan fault and the surface ruptures along with that earthquake were distributed about 13.5 km. The Meishan fault was a right-lateral strike-slip fault, and the largest horizontal and vertical displacement reported by Omori (1907) was about 2.4 m and 1.8m. However, after about 100 years, the seismic activities around the Meishan fault were sort of quiet, especially in the past twenty years. The above phenomena could be considered as a stress accumulation along the fault. In this case, it will be worthwhile of studying the actual location of the Meishan fault is, and its characteristics. In this thesis, I will show the three dimensional interpretation of the subsurface structure around the fault, by using the shallow seismic reflection images those conducted by ourselves and the deeper seismic profiles acquired by CPC in the past. Three dimensional relationships between the Meishan fault, the Chiuchiungkeng fault, and the Hsiaomei anticline will be illustrated as well. The seismic profiles obtained from CPC showed that the near-surface formations were cut by many faults, and the width of fault zone is about 2-3 km. This resulting width of the fault zone is much larger than that obtained from the past study by using the shallow seismic profiles. In our study, we also proposed that the 1906 earthquake rupture was simply part of the Meishan fault zone. Those fault ruptures could merged into a main fault in the deeper strata. The three-dimensional structure of the Hsiaomei anticline showed that the anticlinal axis was horizontally offset by a displacement of about 2 km. The offset is same as that obtained from the geomorphology.

並列關鍵字

Xiaomei anticline ； reflection seismic ； Meishan fault

參考文獻

McCann, W.R., S.P. Nishenko, L.R. Sykes, J. Krause, 1979, Seismic Gaps and Plate Tectonics: Seismic Potential for Major Boundaries. pure and applied geophysics 117, 1082-147.

Omori, F., 1907, Preliminary Note on the Formosa Earthquake of March 17, 1906. Bull. Imp. Earthquake Investigation Committee 1, 53-69.

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