台中盆地北側斷層構造與同震地表變形研究

Abstract

台灣中部地區於1999年受到集集地震之重創，造成兩千餘位民眾喪生，上千億的財物損失。造成此地震之車籠埔斷層上盤北段於同震時有著近十公尺的位移量，此等變形規模不僅為台灣百年唯一，在世界上亦不多見。由於車籠埔斷層北段有著異常巨大的變形量，因此本研究即針對該地區進行斷層面構造幾何之探討，並建立了一區域尺度之大型三維地質構造模型，以及一公里尺度的小型三維地震斷層面幾何模型，試圖找出此等規模之變形與其斷層構造之關聯，以及在此等級的上盤位移下，伴隨主斷層活動的次生構造幾何形貌與運動方式，最後使用數值方法模擬近地表斷層面上的滑移分布。 西部麓山帶於苗栗的褶皺區塊至台中的覆瓦排列斷層區塊間，於地表上有明顯的分界，大致平行於后里台地以及大安、大甲兩溪，而車籠埔斷層上盤之最大抬升地區的即座落於本轉換帶之南緣，為找出本構造區塊轉換帶與車籠埔斷層上盤抬升之關聯，本研究建立一個橫跨苗栗以及台中地區的三維地質構造模型。此模型使用八條構造平衡剖面建構，而其中一條為本研究自行製作之構造平衡剖面。該剖面之建構是依據中油公司之震測剖面及地表地層位態，配合重定位之震源控制剖面深處而製作完成。 此三維地質構造模型顯示，苗栗與台中構造區塊交界帶似乎受控於一大型向南傾斜之斷坡，橫跨苗栗與台中兩區的構造均受到此斷坡之影響，其中包括出磺坑－關刀山背斜之隱沒、三義斷層上盤大安背斜之生成、以及車籠埔斷層之走向轉折等現象均受到該斷坡之影響。由於車籠埔斷層下盤之大安背斜阻擋車籠埔斷層向北的運動，因此於該背斜處會有較大的同震抬升產生，而此區即為本次集集地震垂直抬升之最大地區。 於集集地震同震最大抬升區之內，本研究使用豐原市的都市計畫樁作為工具，分析該市東側之同震變形。結果顯示該區域內之上盤變形並非單一塊體運動，而是數個塊體以1 m內的位移差共同造成上盤的位移。各塊體依其岩性不同而呈現類似剛體或有10 ~ 20 cm的內部變形之運動模式。本研究結果顯示豐原市東側集集地震地表破裂上盤區域仍有許多次生活動構造，於集集地震時產生大小不一之位移。 本研究使用計算自都計樁之空間位移向量，建構一個公里級尺度的地震斷層構造幾何模型。該模型之形貌顯示公里級尺度之下的斷層傾角可以有相當大的變化，在短短數百公尺之內造成區域性的斷坡與斷坪。而利用鑽井資料測試本研究建構之斷層面幾何模型，可得到僅15 m的深度差異，本研究認為該模型確實可代表集集地震的同震地震斷層形貌。 最後，本研究使用此公里級之地震斷層幾何模型進行數值模擬，以了解斷層上盤於此尺度之下的地表形變模式、材料特性以及斷層面上同震滑移量分布。模擬結果顯示，斷層上盤的地表形變完全受控於其下之斷層形貌，而公里級尺度之下的斷層面上滑移量分布則顯示出量值大小亦受控於斷層面之幾何形貌。以本研斷層面滑移分布為例，滑移最大之位置即位在兩斷坡之交界處，且位在利用地表位移觀察到的一剛性運動塊體之邊界上，此現象顯示斷層面上的滑移量分布與地表觀測到的位移模式有絕對之關聯。 本論文使用數種不同之工具建構台中盆地北側於不同尺度之下的構造形貌，希望經由三維構造模型之呈現，釐清台中盆地北側之構造形貌，以及車籠埔斷層上盤活動模式與本區域之其他構造的關聯。而本論文利用數值方法討論公里級尺度之集集地震同震變形之研究，解析出了以往區域尺度相關研究無法解析之近地表淺層斷層上盤運動模式以及斷層面上滑移量的分布。希望未來有更多地球物理、鑽井以及測量資料檢測本研究建構之模型，並期望在不久的將來有更多研究結果補強本模型之不足，可以更清楚的解析出台中盆地北側的地下地質，特別是可能的發震斷層構造形貌，以期減少本區可能之災害性地震發生時所帶來之傷害。

The northern Taichung basin is a transition zone between two structural domains in central Taiwan: folds-dominated domain in the north and imbricate thrusts- dominated domain in the south. Both of the north and south domains are highly active area. Series of large earthquakes occurred in 1935 in the north domain, and the devastating Mw 7.6 Chi-Chi earthquake occurred in 1999 in the south domain. The main purposes of this study are: to construct the structure geometry in the highly active structural transition area; to reveal detailed deformation pattern in the hanging wall of Chi-Chi earthquake and to construct a kilometer scale geometry model of the earthquake fault; and to simulate the slip distribution on the proposed fault plane. A 3D geological model was established based on seven published balanced cross sections plus one cross section constructed in the first part of the thesis. The constructed cross section is based on the surface geological data and relocated earthquake hypocenters. This 3D geological model indicated the high co-seismic displacements in northern section of Chi-Chi earthquake fault is result from a fault- bend fold in its footwall. The fold is caused by a lateral ramp underneath the Daan and Dacha Rivers. The anticlinal fold is also responsible for the changing direction of the rupture trace in the northern Chi-Chi earthquake fault. The second part is to characterize the deformational behaviors of the hanging wall block due to earthquake fault slip in northern Chi-Chi earthquake fault. In the case of the 1999 Chi-Chi earthquake, hanging wall block of the earthquake fault showed complex deformation pattern at the kilometer scale. Because previous studies mainly characterize the fault at the regional scale, it is of interest and also a challenge to characterize the fault at a smaller scale with a higher resolution. In this study, the geometry of a kilometer-scale patch of the fault plane is reconstructed using the displacement data collected from the densely distributed city planning benchmarks. The study area is approximately 4 km by 8 km in size, and contains as many as 924 benchmarks. Among the benchmarks, 62 have both horizontal and vertical displacement data, and the rest of the benchmarks have the horizontal displacement data. Based on the assumption of constant volume, the earthquake fault geometry is built by using the 62 slip vectors. The derived fault geometry model is rather consistent with the borehole data from the nearby 450 m well. The last part of the thesis applies numerical model to evaluate the fault geometry and to simulate the slip distribution on the fault plane. The fault geometry can be evaluated using the concept of fault parallel flow. The method of fault parallel flow can restore the topography before the earthquake and the deformation controlled by the fault geometry. Comparing pre-earthquake and post-earthquake cross sections of topography shows very high correlation coefficient, above 0.99 in average. Half-space elastic dislocation model is then used to simulate the slip distribution on the fault plane, which is inversed from the displacement data of city planning benchmarks. The result shows significant slip variation at the kilometer scale and is interpreted to be related to the deformation zone. The thesis uses different methods to reveal the spatial distribution and 3D geometry of seismogenic structures in northern Taichung basin at different scale. The results indicate better resolution of the seismogenic can apply to the earthquake hazards mitigation in this area in the future.