以河道地形分析探討臺灣西南部六甲斷層的新構造特性

Abstract

六甲斷層位於臺灣西南部，在地表上呈現明顯的線性並區分出了西部麓山帶與海岸平原，被認為是重要的活斷層。二十世紀以來臺灣西南部發生數次重大災害的地震並同時顯示出斷層的活動，但六甲斷層卻未有明顯的歷史活動記錄，因此六甲斷層的未來活動對人類社會可能存在著潛在威脅，需要去深入了解。另一方面由前人研究得知藉著分析河道地形(例如：河道坡度、河道寬度)可以估算構造的活動性，其基本假設為抬升速率等於河流下切速率，而此方法已有成功估算臺灣中部活動構造的前例，因此本研究將藉由此方法來了解六甲斷層的構造活動性。以取得切穿六甲斷層的河流龜重溪之河道地形資料為主，另輔以龜重溪的階地採樣定年將可以建構出六甲斷層的地下構造模型。 利用RTK-GPS和雷射測距儀分別獲得龜重溪的河流縱剖面及河道寬度數據後進行分析，本研究認為龜重溪受到六甲斷層上盤的褶皺構造影響；另結合龜重溪階地定年的結果可以得到不同河段的下切速率，同時可表示為六甲斷層上盤的抬升速率變化狀況，由六甲斷層向東抬升速率逐漸增加，到牛山背斜軸部為最大值4.6mm/yr，到東側向斜區抬升速率又降了下來，由此推論六甲斷層上盤的抬升機制為褶皺的活動，且其斷層前緣可能並未出露至地表。利用Simple Shear Folding Model建構出的地下構造模型確立了六甲斷層在地下的斷層面角度變化趨勢，並由經驗式得到若未來六甲斷層活動，將造成規模6.5的地震，而地震週期約為140年，因此本研究的成果說明了六甲斷層對人類社會具有潛在的威脅性，是需要受到注意的活動斷層。

The Liuchia fault in southwestern Taiwan has been considered as one of the major active faults in the active Taiwan orogen. It is identified by its clear geomorphic features, and forms a major geologic boundary of Taiwan’s Western Foothills. In the twentieth century, several large earthquakes occurred in southwestern Taiwan and caused significant damages. However, there is no unanimous historical evidence for the activity on the Liuchia fault. Therefore, the Liuchia fault poses large hazard potentials for this populous area. Several previous studies have shown that fluvial channel morphology, such as channel slope and width, is strongly influenced by tectonic activities on the assumption of the rock uplift being balanced by the incision of river channels. Base on these hypotheses, it has been shown previously that the analysis of river channel morphology can successfully estimate the activity of potentially active faults in central Taiwan. As a result, we attempted to obtain information of recent activity of the Liuchia fault by analyzing the channel morphology of the Kueichung River, which flows across the fault. We also attempted to calculate the actual river incision rates from the age of river terraces along the river. Such information would enable us to construct the subsurface geometry of this important active structure. We have obtained a detailed river long profile of the Kueichung River from surveys using RTK-GPS, and the channel width profile from actual field measurements using a Laser Rangefinder. The fluvial channel morphology of the Kueichung River appears to have been affected by active folding in the hanging-wall block of the Liuchia fault. Such active deformation pattern is also evident from river incision rate patterns, calculated from the ages and elevations of river terraces along the channel. From the Liuchia fault toward the east, the uplift rate becomes higher, to a highest of 4.6mm/yr at the axis of the Niushan anticline. The uplift rate then becomes lower toward the Chiendapu syncline. The uplift rate pattern indicates the active folding is the uplift mechanism and the fault tip does not extend to the surface yet. We constructed a model of the subsurface geometry of the Liuchia fault, and obtained the trend of dipping angle for the fault plane underneath. Based on this model, we estimated the activity of the Liuchia fault may cause Mw 6.5 earthquakes in the future, and the recurrence interval is about 140yrs. Our results suggest the Liuchia fault has high hazard potential for this populous area, and we need to pay attention to this structure.