台灣位於歐亞板塊與菲律賓海板塊的聚合交界處，活躍的地體構造使台灣地區有相當多的斷層分布。恆春半島位於台灣南端，為弧陸碰撞所產生之增積岩體抬升後的陸域部分，沿岸有大量露出海水面的珊瑚礁體及沙灘，為著名的觀光景點，同時也是墾丁國家公園的所在地。恆春斷層南段位於墾丁國家公園中，為一呈北北西走向，向東傾斜且帶有走向滑移分量的逆斷層。南灣水域的海域環境探勘以及恆春斷層向南部海域的延伸位置與長度，為南灣水域安全性的一個重要評估因素，但是該區域一直缺乏詳細的海底地形及構造調查。本研究使用涵蓋恆春斷層南方海域的多音束水深資料，26條側掃聲納影像、26條淺層底質剖面資料及4條多頻道反射震測等地球物理探勘資料，試圖分析恆春半島南部南灣水域海底地形及構造。 在多音束水深資料中可以看到東、西側有明顯陡坡，坡度10ᵒ 至 20ᵒ，落差40至60公尺，走向與陸上恆春斷層之M、E斷層分別加夾30ᵒ及20ᵒ角。2條側掃聲納影像在東側陡坡的部分發現3個斷層錯動過的地層不整合面，顯示東側陡坡確實為一系列的正斷層所造成。推測由於東側陡坡為陸上墾丁混同層的海域延伸，岩層受到強力剪應力，使得該區多頻道反射震測剖面無法產生較連續的地層訊號，也較難判斷斷層位置。但是將震測剖面與淺層底質剖面相互比較後，可以看出微弱的正斷層徵兆。剖面中的斷層位置與陸上恆春斷層直線距離約5.2公里，但在這之間的6條淺層底質剖面中並沒有發現斷層錯動的訊號。上述資料顯示東側南灣水道為一拉張盆地，由陸上恆春斷層向南部海域延伸後右偏，產生負開花構造拉張陷落而成。 本研究區域南灣水道部分的多音束水深及側掃聲納影像中，發現一系列明顯的沙丘構造。沙丘的分布範圍為水深110至160公尺，呈弧狀條帶分布。沙丘的高度在西側南灣水道約10公尺，東側南灣水道中段約1公尺，東側南灣水道南段約5公尺；分別代表底流流速約每秒95公分、85公分、75公分。底流流速在不同區域的變化，推測是由水道寬度的地形變化所造成。沙丘的緩坡陡坡方向，顯示南灣水道中的底流流向為由西向東南的弧形。由沙丘所推導出的底流流向及流速等資料，與梁等人於2003年所發表之台灣南部洋流速度與方向大致相同。 東側南灣水道北段的淺層底質剖面資料中發現下切谷（incised valley）和退覆沉積層序（offlap），顯示此區域曾經經歷過海水面下降事件。由下切谷中的沉積物厚度約15公尺、所在位置水深約80公尺等資料，推測當時海水面下降至少95公尺，應為末次冰期時所產生。Taiwan is located in a convergent zone between the Eurasian Plate and the Philippine Sea Plate. This context makes the tectonics around Taiwan very active as exhibited by the numerous faults. Hengchun Peninsula in the southern part of Taiwan is part of the uplifted accretionary prism due to the arc-continent collision. The southern part of Hengchun Peninsula is also a very famous tourist spot known for its spectacular natural landscape and numerous watersports. Compared to the importance of the environmental assessment of this area, the bathymetric and structural features information in this area is quite rare. In this study, we use multi-beam echo sounder (MBES), side-scan sonar (SSS), sub-bottom profiler (SBP) and the multi-channel seismic reflection system (MCS) data to analyze the seafloor structures and locate the offshore extension of the Hengchun Fault in the south of the Hengchun Peninsula.In the eastern portion of the study area, a series of high-angle unconformities exhibiting apparent dislocation in the sedimentary layers are observed in the two SBP profiles. These structures are inferred to be portions of a fault zone or shear zone, and maybe related to the Hengchun Fault. The distance between the faults in the SBP profiles and the onland Hengchun Fault is about 5.2 km, but no similar signal is observed in the rest of the 6 SBP profiles in between. According to those previous data, we suggest that the east part of South Bay is a pull-apart basin which cause by the negative flower structure at the southern offshore extension of the Hengchun Fault.There are a series of dune structures in the middle part of the study area which can be observed in the MBES and SSS data. The water depth at the area where dune structures occurred ranges from 100 to 160 m. The height and the length of dunes increase with water depth. The gentle slope of the dunes clearly shows the direction of the bottom current.Incised valley and offlap sequence were found in SBP profiles at the northern section of the east side of South Bay Channel. These seafloor features imply that this area had experienced a sea level drop event before, which might correspond to the last glacial maximum about 10,000 years ago.
