集集地震地層錯動引致地下水位變化之研究

Study on Groundwater Table Variations Induced by Thrust of the Chi-Chi Earthquake

陳德偉(De-Wei Chen)

指導教授 ： 林美聆

國立臺灣大學/工學院/土木工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02329

摘要

1999年9月 21日凌晨，在南投集集發生了大地震，造成許多生命財產的損失，也造成許多天然災害的發生，而在集集地震發生時，在濁水溪沖積扇的地下水位觀測井，就剛好記錄了地震時所發生的水位變化。
在觀測井的觀測記錄中，在濁水溪沖積扇扇央及扇尾的部分，地下水位大部分為上升的情形。而在靠近車籠埔斷層附近的水位觀測井，則情況相反，大部分為地下水位下降的情形。而觀測井所分佈的位置，則恰好在彰化斷層和車籠埔斷層之間。
所以為了瞭解靠近車籠埔斷層附近的地下水位觀測井，受到地層錯動所引發的行為，本研究利用有限差分軟體 FLAC建立地層錯動模式之模型，來進行研究。在地層錯動的模擬上，採用類似砂箱實驗的方法來進行模擬。首先給予一固定的速度來抬昇基盤，進而控制基盤的抬昇量來進行模擬地層的錯動。關於孔隙水壓的部分，則利用FLAC中的線性擬靜態Biot理論，來模擬孔隙水壓的激發，其理論基礎為體積應變和孔隙壓力有一定的關連性，也就是體積應變的改變同時也會造成孔隙壓力的改變。
進一步將模擬的結果和集集地震發生時，濁水溪沖積扇的地下水觀測網所得到的觀測變化量相比較，進行分析及討論，以瞭解變化的趨勢。本研究的地層錯動模式，對於土層中土壤的變形行為，有良好的模擬。但在關於孔隙水壓的變化，卻沒有辦法正確的進行模擬。模擬出的結果，在壓縮側的土層水壓均遵守Biot理論，造成孔隙水壓上升。但在現地觀測中，觀測井卻是呈現下降的行為。所以本研究對車籠埔斷層和彰化斷層間的水位觀測井，對於孔隙水壓的模擬，沒有辦法模擬得很好。

關鍵字

孔隙水壓 ； 地震 ； 地層錯動

並列摘要

On 1999.9.21 , there is a big earthquake happened at Nantou chi-chi township. The Chi-Chi earthquake caused severe damage to the life and asset, and cause many nature disaster. When the Chi-Chi earthquake happened, the groundwater observational well at Jhuo-Shuei river alluvial fan recorded the groundwater table changes.
Based on of the observation data, at the middle and end of the Jhuo-Shuei river alluvial fan the groundwater table reveal rised. But for the observation wells near the Chelungpu fault, the groundwater table reveal decline. It was found that most of the well with groundwater table decline located between Changhua fault and the Chelungpu fault.
For understanding the behavior of the groundwater observation well near by influenced of the thrust of Chelungpu fault, my research uses the finite difference method FLAC to build the model of the thrusting. For modeling the thrust, my research use the results of the sandbox test to simulate the thrust moving. A constant velocity to lift up the base was given, and the total vertical lift up distance from the base was controlled to model the thrust. My research use the linear quasi-static Biot theory to simulate the pore pressure variations. Biot theory provided relationship between the volume strain change and the pore pressure change.
The modeling result is then compare to the result observed at the Jhuo-Shuei river alluvial fan. The thrust model of my research can simulate the action of the soil layer, but the pore pressure changes, can not be simulated well. According to the result of the observation well, the pore pressure declined. This result is not the same as the simulation conclusion. By the conclusion, my research can’t simulate the pore pressure changes of the wells between the Changhua fault and the Chelungpu fault well.

並列關鍵字

pore pressure ； earthquake ； thrust

參考文獻

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FLAC (2000), Fast Lagrangian Analysis of Continua, Version 4.0.

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