池上斷層在萬安沖積扇的構造地形研究

Abstract

本研究使用地形的分析方法，針對萬安沖積扇來討論池上活斷層對萬安沖積扇所產生的變形特性。池上斷層是縱谷中南段的一活動斷層，從大地測量的結果顯示，池上斷層主要是以間震潛移的方式活動，每年的縮短量約有2~3公分。在池上斷層經過萬安沖積扇附近，地表破裂及構造地形現象卻不明顯，因此對池上斷層在萬安沖積扇的延伸情形並不確定。有鑒於此，本研究便採用高精度地形的分析方法，包括製作2公尺數值高程模型（DEM）來分析沖積扇的細部地形，以及在野外實地進行公分級精度的GPS-RTK地形剖面的量測，從觀察池上斷層對萬安沖積扇造成的地表變形，來判斷池上斷層在萬安沖積扇的延伸情形。 結果顯示，從DEM中可以初步先將萬安區域分成3個不同年代的沖積扇，分別為位於北邊地勢最高的沖積扇地形F1；位於南邊的沖積扇地形F2，在形狀上有明顯被截切的情形；位在中間地勢相對較低的萬安溪沖積扇F3。從這三個沖積扇地形上截切及高度的相對關係，推測F1是萬安溪最初在北邊之上游堆積而成的沖積扇，之後由於F1沖積扇不斷增厚使得溪水在從山前出來時受阻，而往沖積扇兩側地勢較低的地方流。而在這之前或是同時，位於南邊的萬安溪支流也堆出了F2的沖積扇，當斷層持續活動，上盤地勢不斷被抬高，萬安溪便在F1和F2之間下切形成沖積扇F3。從細部分析中，可以看到萬安沖積扇F3中有明顯的變形現象，在沖積扇扇端有傾向和沖積扇傾向相反的崖面，此崖面和扇央中高約2公尺的地形崖在F3中形成地塹的平地，有別於一般平滑曲線的沖積扇形貌。至於萬安溪南側的沖積扇F2的形貌，則可以看到上面有幾個土石流的錐體，扇的邊緣有明顯的線崖。綜合將這些位於沖積扇中的線崖和不同沖積扇地形的線崖做比較，首先描繪出池上斷層在萬安溪北側的線形崖面，經由地球物理的地下構造探勘以及野外在萬安堤發現的破裂及變形現象，本研究更進一步確認斷層的延伸至萬安溪堤防上。在萬安溪河道附近，並沒有線形崖的生成，推測是由於萬安溪的河流侵蝕堆積營力大於斷層抬升的速率造成。至於在萬安溪南側的線崖，經參考觀察古地圖台灣堡圖的古河道分布，推測應為河蝕造成，至於斷層位置，可能比線崖的位置更靠近西邊，只是受到新武呂溪河流堆積的影響而不明顯，斷層位置必須依賴精密測量方法或是地物的地下探勘才能更有利的證實。

Based on high resolution geomorphology analyses solving it, including 2 meters resolution digital elevation model (DEM) and GPS - RTK measurement in the field, this study determine the elongate condition of the Chihshang fault in the Wanan area from observing the surface deformation of the Wanan alluvial fans produced by the Chihshang fault. Combining 40M DEM and 2M DEM, we can divide the Wanan alluvial fans into three parts, and they formed at different times. The F1 fan is located in the northern- most part of study area with the highest elevation. The F2 fan is located in the southernmost part of study area. The recent Wanan river alluvial fan which I called F3 is the lowest fan in study area. Based on the relative elevation of the three alluvial fans, I interpreted that the F1 originally deposited further north.Later, the Wanan river flowed along the southern side of the F1 alluvial fan. Before this or at the same time, the southern branches of the Wanan river also deposited the F2 alluvial fan in the southern portion. While the Chihshang falut continuously moved and the hanging-wall gradually raised, the Wanan river cut the F2 then formed F3 alluvial fan. From the DEM analysis, we can find obvious deformations in F3. There is a scarp which faces oppositely to the F3 alluvial fan in the fan head. The other scarp is located in the middle part of the F3 fan. These two scarps form a depression in F3. In the southern portion of the Wanan river, we can observe some alluvial cones in the F2 fan, and there is an obvious scarp in the edge of the F2 fan. In the northern portion of Wanan river, the surface traces of Chihshang fault follows an observed scarp. In the field, we found breaks and fissures of concrete wall on the Wanan river embankment along the surface scarps of the fault,consistent with geophysical surveys. We can’t clearly observe scarp near the Wanan river channel, and we suppose that the erosion rate of Wanan river is more rapid than the fault uplift rate in the southern portion of the Wanan river, we interpreted that the fault scarps were eroded by river. because the edges of terrace show a curve shape. Furthermore, the ancient map also showed that there was an old river channel flowing beside the scarp. The location of surface trace of the fault may be farther west than the geomorphic scarp.