山腳斷層再活動對於台北盆地內地形變化之探討

Abstract

山腳斷層位於台北盆地西側，是一條活動性高且具左向滑移分量的正斷層，斷層長度超過40公里。過去四十萬年來由於山腳斷層的活動，使盆地內第三紀沉積岩的基盤持續下陷，導致盆地西北緣第四紀沉積物最厚可達700公尺以上，然而在台灣北部橫移伸張的應力環境下，山腳斷層仍有再次活動的機會。本研究嘗試建構山腳斷層於地下的三維形貌，並以錯位模擬(dislocation model)的概念模擬山腳斷層的活動，目的在了解山腳斷層發生不同規模之地震時，可能造成的地表同震變形大小與範圍。模擬的結果，隨著模擬的地震規模越大地表下陷範圍與下陷量越大：當山腳斷層發生規模6.5以上的地震時，台北盆地內地形會下陷超過1公尺，當規模達到7時，下陷量超過1公尺的面積達115平方公里，地表下陷最深可超過2.1公尺。另外，我們將模擬的結果與古地震的錯位記錄和台北盆地第三紀沉積岩的基盤深度分佈相比較，結果顯示過去以來山腳斷層的活動在台北盆地西北側或更北方的斷層面上有較大的滑移量，且具有左向滑移的分量。最後我們以河川潰堤與降雨兩者探討地形變化對台北盆地淹水的影響，發現當發生規模7的地震時，可能導致新莊、五股、蘆洲與關渡一帶面積約達40平方公里的區域有被潰堤河水淹沒的危險。我們也以一日降雨450 mm之十年頻率雨型來進行三重與蘆洲區域，於山腳斷層錯動前後地形變動的淹水模擬，當模擬山腳斷層發生規模7.0之地震、最大滑移量存在於關渡一帶地下約9公里之斷層面所造成的地形變動後，結果顯示蘆洲西側之淹水範圍會加大，而三重南側之淹水範圍反而減小，我們認為這是因地震前後區域性低窪地的改變，而改變雨水在地表匯集的區域。

The Shanchiao fault, located in west of the Taipei basin, is a highly active normal fault with minor left-slip component. The fault length is over 40 km. Due to the movement of Shanchiao fault, the depth of Tertiary basement in the Taipei Basin is at least 700 m. We suggest that the Shanchiao fault still has the ability to induce subsidence in the Taipei Basin under present extensional regime of northern Taiwan. In order to discuss the co-seismic ground deformation, we model the behavior of earthquakes along the Shanchiao fault with the dislocation model. In this study, we also construct the three dimensional fault plane of the Shanchiao fault and we suggest that the fault has the larger slip around the Kuandu area. When seismic moment-magnitude reaches 6.5, the footwall of Shanchiao fault could subside over 1 m and when seismic moment-magnitude reaches 7.0, the area that subsides over 1 m could reaches 115 Km2. After comparing the model result with the record of prehistoric earthquakes along the Shanchiao fault and the depth of Tertiary basement, we suppose that the seismism of Shanchiao fault should have left-lateral slip portion and it was more active in the north fault section in the Taipei basin. Finally, we discuss the influences of inundation with the variant landforms in two situation : The break of embankment and heavy rainfall. The predicted area subsides below sea level is about 40 Km2 when a moment-magnitude 7 earthquake occurs along the Shanchiao fault. The region, spreads around Sinjhuang, Wugu, Lujhou and Guandu could be submerged by flood. However, the pattern of inundation that results from heavy rainfall may change because the relatively low region changes after earthquake.