恆春半島西側與南側海底地形特徵初步探討

Preliminary study of marine topographic characteristics around western and southern Hengchun penisula

高溥聲(Pu-Sheng Kao)

指導教授 ： 宋國士

國立臺灣大學/理學院/海洋研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.03115

摘要

本研究位在恆春西台地的西側與南側附近海域，包含南灣、海口外海與西台地西側。此區域內的水下地形資料很少，解析度也不高。所以希望能夠藉由高解析的多音束測深系統、側掃聲納系統和3.5KHz淺層震測系統，對於這個區域的水下地形和底質概況有更多的了解。並從高解析的資料判斷出因為構造所造成的細微地形和底質變化。研究目的主要有兩個：

首先是希望能夠對恆春斷層有更進一步的認識。恆春斷層的陸上斷層線位置ㄧ直沒有明確定位，所以對於其活動型態就沒有辦法確定。因此希望能夠藉由恆春斷層的海域延伸，對活動構造的延伸範圍以及活動型態做進一步的研究。經由南灣量測得到的水下地形顯示，85米等深線處似乎可見恆春斷層之延伸。且在南灣外海谷地的東側谷壁，其走向和本次南灣調查區內的線性地形與陸上推測恆春斷層的走向相符，因此恆春斷層可能於本區或在等深線轉折處向海延伸。側掃聲納系統在南灣近岸的水域並沒有發現明顯的恆春斷層入海蹤跡。另外在西台地北方的海口外海發現了一個類似海底崩移的地形，可是其形態與發生的地點皆和一般的崩移地形不同，台灣北部外海的逆衝斷層尾端也有發現類似的地形，所以表示陸上恆春斷層延伸到海口外海後其運動可能終止，方能形成這樣的地形。所以恆春斷層和潮州斷層可能不相連，或是即使兩者相連，斷層活動也是至此停止。

第二部份是探查造成現今恆春西台地抬升的主要原因。恆春西台地由前人大地測量發現台地東西側皆在抬升，所以表示台地下方應該有另一主要的逆衝斷層發育，這條斷層應該是位在恆春西台地的西岸外海，由地形與西台地西側外海的紅柴峽谷特異性來看，西海岸斷層可能就位在上部紅柴峽谷中。在下部紅柴峽谷的北側發現了一條開口朝向西南方的圓弧形線性地形，由淺層震測剖面判斷此線性地形為一拉張形式的構造線。在下部紅柴峽谷南側的紅柴海脊上也出現一系列開口向西南方的微弧線性地形系列，其性質和北方的圓弧形線性地形極為類似。本研究認為一系列拉張型構造線性地形，形成原因可能為紅柴峽谷內存在西海岸斷層，其斜逆衝至恆春西台地下所引起；或是紅柴峽谷本身為一大背斜構造的軸部張裂，這些線性地形同為此張裂現象造成。

關鍵字

恆春斷層 ； 紅柴峽谷 ； 紅柴海脊 ； 多音束測深系統 ； 側掃聲納系統 ； 3.5KHz震測系統

並列摘要

This research tries to access and understand the sea area around Hengchun west and south tablelands, including Nan-Wan, Hai-Kou and Hengchun west tableland. In the area above, there are few data of its underwater landform, and are in a low resolution to do further analysis. By means of using high definition Multibeam survey system (MBS), Sidescan sonar system (SSS) and 3.5KHz subbottom profiler (SBP), this research tests and gathers more data of this area's underwater landform and bedding status, then evaluates and judges the causes and effects of its millimetric landform, bedding changes and the structure. The major purposes of this research are as follows:

At the first place the research tries to gather more specific information of Hengchun fault, in lack of a clear position of its fault line, researchers were not able to define the ways and forms Hengchun fault performs. By studying the extended sea area of Hengchun fault, to go further into its extension area and the acting forms. Through the data collected from Nan-Wan underwater landform, Hengchun fault and its extension can be observed in 85 meters underwater.

Besides that, the research finds that the east side valley of Nan-Wan outer sea has the same moving direction as the landform of the Nan-Wan research area, and is as the way Hengchun fault shows. Therefore it is likely that Hengchun fault extends into the sea in either this area or in the twister in isobath. The research does not find obvious trace or route Hengchun fault made in close-shore area of Nan-Wan through SSS. The research also finds that there is a likely submarine landslide landform in outer Hai-Kou in the north side of west tableland. However, the location and forming is different from other landslide landforms. There is similar landform as stated above in the end of a thrust fault in outer sea area in north Taiwan. These show that the moving of Hengchun ground fault only extends until outer sea of Hai-Kou to from this kind of landform. Thus the research may suggest that Hengchun fault may not be connected with Chow-Chou fault, or even they are connected, the fault forming also stops in this place.

The second part of this research is trying to clarify and explain the major factors causing the lifting of Hengchun west tableland. Former researchers advise that measurement shows that both east and west sides of the tableland are lifting, which indicating that under the tableland there is one other major thrust fault growing, and it is located in the west coast outer sea in Hengchun west tableland.

From the landform of west tableland and the features of Hongtsai canyon, it is likely that the west coast fault is located in upper Hongtsai canyon. In the north of lower Hongtsai canyon, the research finds an opening towards to the south west landform, from the profile the research suggests the landform as a stretching structure line. The research also finds that in the southern Hongtsai Ridge of lower Hongtsai canyon, there is a series of landform which opens towards to south west. The above landform is considered extremely similar to the one in north. In short, the research would suggest that the lining landform in north and south Hongtsai canyon could be caused by the fauly in west coast thrusting into Hengchun west tableland or the seperation of the fold axis. These lining landforms are both caused by the seperation.

並列關鍵字

Hengchun fault ； Hongtsai canyon ； Hongtsai ridge ； Multibeam ； Sidescan ； 3.5KHz subbottom profiler

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