台灣東北地區的地震構造：由碰撞末期轉變為隱沒拉張之構造特性

Seismotectonics of northeastern Taiwan: Structural characteristics of a transitional area from waning collision to subduction and post-collisional extension

黃信樺(Hsin-Hua Huang)

指導教授 ： 吳逸民

國立臺灣大學/理學院/地質科學研究所/碩士(2007年)

https://doi.org/10.6342/NTU.2007.00571

摘要

台灣東北地區（121.0-123.0°E，23.5-25.5°N）受到隱沒帶（琉球弧溝系統）、弧後張裂（沖繩海槽），與弧陸碰撞（呂宋島弧）三者的共同影響，長久以來，一直是一個地震活躍、地質構造複雜，且地表調查不易的區域。在前人的研究中，多半將此區的宜蘭平原與縱谷地區分開討論，認為宜蘭平原受最新期海槽張裂活動的影響正處於伸張環境，地震多為正斷層；縱谷地區則因尚在弧陸碰撞的範圍，斷層機制以帶有右移分量的逆斷層為主，但對宜蘭平原與縱谷地區之間卻少有著墨。
因此，為探討台灣東北地區由碰撞末期轉變為隱沒拉張之構造特性。本研究收集中央氣象局地震觀測網（Central Weather Bureau Seismic Network, CWBSN）1994年1月至2005年12月發生在台灣東北地區，且條件為（1）同時有8個以上的測站清晰收到訊號、（2）芮氏規模（ML）大於3、（3）震源深度小於50公里之地震資料共12030筆，使用三維速度模型與測站修正法重新定位；並結合強地動觀測網（Taiwan Strong Motion Instrumentation Program, TSMIP）在此區密集的P波初動資料，求取高品質的震源機制初動解共169組，用以解析更精細的淺層地震構造。
資料分析結果顯示，斷層型態由北而南呈現正斷層（蘭陽溪以北）、走向斷層（蘭陽溪以南）、逆斷層（縱谷北段）的轉變。在弧陸碰撞轉變為弧後拉張之間，轉換帶是以有系統地走向斷層在運動，且在南澳走向滑移運動密集發生的地方，在重新定位後可區分出兩組不同走向的地震序列：一組是東西向的左移序列；另一組則位於東方外海，走向呈N50°W，有部分右移機制的出現，可能反應了沖繩海槽正向南擴張的現象。此外，在震源空間位置之時序分析中我們也發現在24.3-24.5ºN與24.8-24.9ºN有兩個高地震活動帶，分別對應到宜蘭平原最北的正斷層序列與南澳的走向斷層序列，前者可能受到沖繩海槽淺部張裂性地震活動向西南延伸的影響；後者則意味著此處為一正在活動的地質弱帶。且在南北向剖面中，觀察到一平行班氏帶的地震帶自縱谷北緣向北漸深，可能是海岸山脈已隨菲律賓海板塊隱沒的證據。
綜合上述研究結果，並佐以近年GPS測量與地球物理方面的資料，此走向斷層帶的成因，我們認為台灣東北地區正受到沖繩海槽西南延伸的影響，以宜蘭平原北緣之正斷層向南伸張，造成地層在彎折時發生層間的左移滑動，此一構造行為即沿著南澳一帶的地質弱帶發生，形成一系列的左移斷層系統。而海岸山脈碰撞之效應，因為隨板塊隱沒之故對此區淺部構造之影響已相對不再顯著。

關鍵字

地震構造 ； 地震重定位 ； 震源機制解

並列摘要

Northeastern Taiwan (121.0-123.0°E，23.5-25.5°N) is a tectonically complex area acted by combined effects of slab subduction, backarc opening, and continental-arc collision. According to the previous studies, this area is usually divided into two tectonic entities, the Ilan basin and its sourthern collision zone. The Ilan basin related to Okinawa Trough is a extension environment dominated by normal faults and the collision zone is still a compressive environment because of the arc-continental collision. But the detail structure of the transition area between them has not been studied.
Therefore, this study tried to explore the tectonic characteristics of the transition area from waning collision, to subduction and then post-collisional extension. A total of 12030 earquakes recorded by Central Weather Bureau Seismic Network（CWBSN） from January 1994 to July 2005 were relocated and studied in this study. These events were relocated by using three-dimensional velocity model. By combination the first motion polarities from Taiwan Strong Motion Instrumentation Program（TSMIP）and CWBSN, totally 169 focal mechanism solutions were determined for tectonic analysis.
Base on the results of the relocated seismicity and focal mechanisms, in the study region, the spatial distribution of the focal mechanisms sequentially switches from thrust, strike-slip, and normal fault northward. It is obvious that the transition area between back-arc extension and continental-arc collision is dominated by left lateral strike-slip fault systematically. Furthermore, based on the seismicity profiles, right-lateral focal mechanisms were determined in Nanao earthquake sequence and a concentrated seismicity zone parallel to the subduction zone. It is possible that Coastal Range（CR）has already subducted along with Phillipine Sea Plate and the transition area is affected by Okinawa Trough more than CR. In addition, two active seismicity zones found in latitude 24.3-24.5ºN and 24.8-24.9ºN are corresponding to the normal and left-lateral strike-slip zone, respectively. It might imply that these two zones were located at the tectonic weak zone and thus with high seismicity.
In conclusion, the southward opening of the Okinawa trough is a main driving force in northeastern Taiwan and bending the north Taiwan mountain belt. The left-lateral fault system appears at transition area was caused by the bedding-slip motion for considering the results from relocated seismicity, focal mechanisms and geological settings.

並列關鍵字

seismotectonics ； earthquake relocation ； focal mechanism

參考文獻

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