模擬山腳斷層錯動之同震位移模式

Abstract

本研究目的為，藉由彈性錯移理論，評估山腳斷層錯動所可能引起的地表位移模式，及其對台北盆地的影響。 為了建立對近場同震位移的直觀，我們以位移向量對位置向量投影，並繪出位移等值面來建立斷層鄰近三維區域之位移模式。結果顯示，相較於遠場體波的位移幅射模式，近場因為受均質半空間與有限斷層幾何影響，造成部分不對稱的情形；然而在鄰近三維空間中，根據斷層的錯動方式，仍可清楚顯示出相對於斷層幾何中心位移方向的四個等值面分布，其結果相當於遠場P波所造成的影響。這說明在近場我們可根據鄰近三維空間中，相對於斷層中心的位移模式，而推估斷層的錯動形式。 我們依據下列三個主要步驟建立山腳斷層同震位移模式：首先測試選擇不同地下模型對位移模式的影響。接著我們以台北盆地第三紀基盤地形作為邊界條件，希望在長時間尺度下，能建構一個較簡單的斷層幾何模型，以及較平均的斷層錯動方式。最後根據地震規模與斷層破裂面積的經驗關係式，我們可求得一較合理的規模與斷層面平均滑移量，並評估對台北盆地之影響。 結果顯示，在同一個斷層幾何與錯動方式下，選擇均質與一維地下彈性模型之地表垂直位移量差異，在大部分區域小於一維模型結果的30%，而在近斷層附近則小於20%。經由測試不同斷層參數對基盤地形形貌的影響，我們發現基盤地形主要受控於斷層傾角、深度及寬度，因此我們可藉由格點搜尋，得出一組較佳的斷層幾何參數；將求得之斷層參數重新產生基盤地形，並與實際地形比較，結果顯示我們所建構的簡單斷層模型能夠解釋基盤地形大尺度的變化。最後，根據我們所建立的斷層模型，配合地震規模與斷層面積之經驗關係式，當山腳斷層錯動，可能產生的規模大小為5.6的地震，並伴隨0.2公尺的平均錯量；而地表位移結果可能造成淡水河出海口附近，半徑約4公里的範圍低於海平面。

The purpose of this research is to estimate the displacement field due to the Sanchiao fault and it’s effect to the Taipei basin by using the elastic dislocation theory. We constructed the general near-field displacement pattern for a finite fault by position vector projection. Unlike the far-field radiation pattern, our results illustrate interesting asymmetry because of the half-space Green’s function and the finite fault dimension and may help us to estimate the source paramaters. We model the co-seismic displacement through the followimg steps: First, we test the effect of the Earth structure. Then, we construct a simple fault geometry. Previous studies on the Taipei basin indicated that the present day basement structure of the Taipei basin were mainly due to long-term slip of the Sanchiao fault. Thus we assume that the present day basement structure was only due to repeated slip on a regular fault plane. Base on this assumption, we expect to derive an average fault geometry parameters constrained by the structure of the Tertiary basement of the Taipei basin under a long-term time scale. Finally the probable moment magnitude and fault slip can be derived using empirical relation between the magnitude and the fault area. Modeling results suggest that the residual displacement between the homogeneous Earth structure and the layered one are less than 30% of the result of layered model. We test the effect of the fault parameters upon the relative basement structure and conclude that the fault dip, depth and width are the dominating factors controlling the relative structure of the basement. The optimal fault parameters are derived using grid search. The prediction of basement structure with the constructed fault parameters can well present the long-wavelength variation of the basement structure. According to the empirical relation between the magnitude and the fault area, the probable moment magnitude is 5.6 with average dip slip of 0.2 m.