本研究分析地震密度及能量釋放、地表地形剖面，並整合現有之地質與地球物理資料，來探討蘭陽平原地區的新構造活動特性。一般認為，蘭陽平原的形成發育及其地殼變形主要應該是受到鄰近之南沖繩海槽南北伸張應力所造成的影響。然而在近幾年的地震活動顯示，該地區地震主要發生在平原南半部，且其震源機制為帶有少許張裂分量之橫移走向斷層，不似單純的南北張應力的產物。根據蘭陽平原的地形、地質以及地球物理等資料，蘭陽平原的起始沈積發育中心應位於平原北側，呈現東北向之軸線，可與沖繩海槽之張裂中心軸相連接。而根據地質地層的架構及區域地層及構造走向，我們認為橫跨平原中央偏南之走向滑移斷層 (濁水斷層 )應為梨山斷層 (或牛鬥斷層 )北端之延伸。利用 RTK-GPS (Real Time Kinematic-Global Positioning System) 之野外地形實測，我們建構了數條高精度的地形剖面。研究結果發現蘭陽平原北半部有明顯的河階發育，相對地，南半部則無河階地形生成，暗示長期來說北半部有明顯的下陷作用，我們解釋與沖繩海槽的擴張影響有密切關連。另在平原中間南半部，發現濁水斷層的可能地表沿線有約半公尺的地形崖差，雖然可能受到河流侵蝕沈積作用的影響，我們傾向推論是濁水斷層的長期位移累積的現象。此外，利用氣象局地震目錄的資料，統計最近 12年(1994, 2006)蘭陽平原地區淺層的地殼地震活動，可觀察到蘭陽平原地區目前的主要活動構造區域發生在四個地區：龜山島、平原南半部 (濁水斷層帶 )、平原外更南邊中央山脈及近海的地區 (南澳附近)、平原內側西南邊 (三星附近)。其中，濁水斷層帶及南澳附近地區是地震頻率高且釋放能量大；而龜山島地區 (沖繩海槽西端 )的地震帶，雖然地震密度與頻率高，但是釋放能量並不大，而且此地震帶並沒有持續至陸地而連接蘭陽平原起始中心；另外，平原內側之地震帶則是地震密度低但釋放能量大，此上各區域可能分別對應不同的地震構造系統。我們分析 2002-2006年GPS測量資料，顯示蘭陽平原內部的主要地表變形是沿著濁水斷層的左移剪切變形，並帶有明顯的西北 -東南方向的拉張，且斷層南側之塊體位移有明顯順時針旋轉的現象。根據這些觀測分析結果，總結蘭陽平原地區的新構造特性，雖然平原北部外海龜山島地區有高頻率的小地震活動，但是主要還是平原以南的地區為最活躍的構造作用區域；其構造應力來源為板塊斜碰撞在在造山帶邊緣產生的塊體旋轉的作用力、橫向逃脫力，加上沖繩海槽南北向不對稱擴張的作用，雖然沖繩海槽西側延伸在蘭陽平原北部陸地地區的地震作用並不顯著。
Based on reconstruction of high-resolution topographic profiles, seismic data analyses, and combining available geological and geophysical data, we characterize active tectonics in the Ilan Plain area. According to subsurface geological and geophysical data, we interpret that: (1) there exists a spatial coincidence between the Okinawa trough and the initial deposition center of the Ilan Plain in the northern part of the plain. (2) The Chuoshui Fault connects with the northern extension of the Lishan Fault, which represents a major geological boundary between the Hsuehshan and Backbone Ranges, and runs across the middle-south of the plain although overlying by 1-2 km think Quaternay fluvial deposits. We used the technique of Real-Time Kinematic GPS (RTK GPS) to reconstruct high-resolution topographic profiles in the Ilan Plain. We found that upraised river terraces solely occurred in the northern part of the Ilan Plain. This suggests that the northern plain experienced a long-term subsidence, implying that the northern Ilan Plain might be more influenced by the extension of the neighboring Okinawa trough than the southern part. We also observed a series of scarps of about 0.5-m high along the possible surface trace of the blind Choshui Fault, as piece of evidence of an active fault. Based on the analyses of density and energy released from shallow crustal earthquakes between 1994-2006, we located current seismogenic zones in four areas: (1) the Gueishan islet, (2) along the Choshui Fault, (3) farther southeast to the Ilan Plain area including offshore area, and (4) the southwestern tip of the plain. Furthermore, the 2002-2006 GPS measurements showed a strong left-lateral movement and correspond to a clockwise rotation occurred in the southern Ilan Plain area, bounded mainly by the E-W trending Choshui Fault. The present Ilan Plain shows considerable tectonic activities especially in its south part. The tectonic stress may derive from block rotation and lateral extrusion under oblique convergent regime at plate corner as well as the opening of the Okinawa trough.
關聯:
Western Pacific Earth Sciences 8 2008.12[民97.12] 頁17-41
