台灣苗栗近岸海域變形前緣之搜尋與探討

Abstract

台灣位於歐亞板塊與菲律賓海板塊的交界，因為弧陸碰撞，使得台灣西部發育一連串的南北向逆衝斷層。這些逆衝斷層由島中央往西邊發展，越往西邊斷層越年輕、傾角越小、活動力也越弱。台灣經過兩期不同造山時期的碰撞，在北港高區的北界出現了一條在三義與埔里之間以東南－西北走向的三義－埔里地震帶。 我們在苗栗外海做地球物理調查，利用多音束測深蒐集水深資料、側掃聲納觀察地貌、以及淺層震測剖面儀對淺層地層做構造解釋。研究區域水深介於6至40公尺深，在西部與西北部地區的海床上存在著許多更新世晚期即存在的古沙丘。離岸較遠處的區域，因為受海流的影響而出露在海床上。這些沙丘的出現，表示這邊近期並沒有受到較大的構造運動所影響，近年的地震分布也顯示這邊並沒有太多地震發生。 淺層震測剖面儀中，研究區域中共發現了六條張裂形態的正斷層。這些斷層的錯動都出現在頭嵙山層內，表示此區已受到造山運動的影響。其中最大的斷層(F6斷層)和三義－埔里地震帶的軸線延伸方向一致，並具有左移的運動機制。暗示它為沿北港高區生成的剪切帶，此剪切帶往外海延伸，代表台灣造山運動所影響的範圍與變形前緣的位置都已經到達比研究區域更外海位置。

Taiwan is located at the junction of the Eurasian Plate and the Philippine Sea Plate. Because of arc-continent collision occurred in Taiwan Orogeny, resulting in that the western Taiwan has formed series of thrusts aligned in north-south direction. From the Central Mountain Range to the west, the thrust faults are getting younger, and the angle of dip is smaller. The two different collisions which created the Sanyi-Pauli Seismic Zone in Taiwan, and the seismic zone is the product of Taiwan Orogeny. The seismic distributed map in Taiwan in recent years pointed out that Sanyi-Puli Seismic Zone has extended to offshore Miaoli. This evidence also indicated that the stress of Taiwan Orogeny has reached offshore Miaoli. We collected three different sonar data, the multi-beam echo system, the side-scan sonar system, and a Boomer reflection system in offshore Miaoli. The regional topography ranged between 6-40 meters depth. There are many paleo dunes on sea floor in the west of the study area. We will explain those dunes by side-scan images and the seismic profiles. Those sand dunes were created by ocean current or tidal current. The reflection seismic profiles suggest that the old dunes were formed in Late Pleistocene. The ocean current caused several sand dunes shown on outer seabed. These sand dunes on the seabed indicated that there are less earthquakes in this area. In addition, the less earthquakes and the small magnitude of earthquakes occurred in past 15 years in offshore Miaoli. In reflection seismic profiles, we totally found six normal faults. Those normal faults were in Toukoshan Formation, representing that this area has been in Taiwan Orogeny. The largest F6 fault and Sanyi-Puli Seismic Zone had same axis orientation. Additionally, the F6 fault had left moving mechanism, and this feature point out that F6 was generated by Sanyi-Puli seismic zone, indicating that the Taiwan Orogeny has been farer than the study area.