潮汐與地震引發地下水位變化之數值模擬

Abstract

海洋潮汐週期性漲潮和退潮的荷重變化，會影響濱海地下水井的水位。東石地下水觀測站距離海岸線400公尺，東石3號井和4號井的地下水位，通常會隨著海潮，出現延遲的週期性變化。由於潮汐引發地下水位變化的幅度主要受控於地層彈性係數，而其週期性變化的延遲時間與水力傳導係數相關，但是也受到地層彈性係數的影響，因而可藉由數值模擬的方法及潮汐與地下水位變化的觀測紀錄，來探討含水層特性。地震時斷層錯動產生的應力也會引起地下水位異常的改變，從集集地震時東石3含水層和東石4含水層的同震水位變化，可以瞭解到地震作用下含水層的特性及應力應變狀況。 本研究利用ABAQUS 軟體建立二維變形與孔隙水流耦合模式，模擬潮汐荷重變化對地下水位的影響，並經由水位觀測紀錄加以驗證，可以瞭解地層彈性係數對孔隙水壓的影響，推估含水層特性。模擬的結果顯示，當東石3含水層和東石4含水層的水力傳導係數分別為5×10-3 m/s和5.5×10-3 m/s，楊氏模數分別為4.7×107 Pa和3.0×107 Pa時，對應的地下水位延遲時間和振幅與觀測值接近。從獲得的含水層特性可以推估不同地震位移所對應的同震地下水位變化，將模擬的同震地下水位變化與觀測值對比，求得地震時於東石地區產生的位移大小為模型總長度的0.01%。因此，利用數值模擬的方法配合現地觀測資料可以獲得含水層特性，並探討地震時含水層的應力變化。

Ocean tide may change the groundwater level in the coastal area. The Dongshi groundwater level monitoring station is located about 400 m from the shoreline. The groundwater level of Dongshi-3 and Dongshi-4 wells usually changes with ocean tide, but about 40 minutes behind it. The variations of groundwater level in response to the loading of ocean tide are related to the aquifer properties, such as elastic constants of sediments. The time lag is influenced by both the elastic constants and the hydraulic parameters. On the other hand, the co-seismic groundwater level change in the two wells induced by the Chi-Chi earthquake was attributed to the short-term loading from the lateral direction. In order to understand stress mechanism in Dongshi station, the finite element software ABAQUS was used. We established a two-dimensional simplified model and used tidal method to calculate the Young's modulus of two aquifers where Dongshi-3 and Dongshi-4 are screened. Numerical analysis result revealed the Young’s modulus of Dongshi-3 and Dongshi-4 wells were 4.7×107 and 3.0×107 Pa ,respectively the hydraulic conductivities of Dongshi-3 and Dongshi-4 wells were 5.0×10-3 and 5.5×10-3 m/s. After the aquifer parameters were obtained, the amount of co-seismic groundwater level change induced by Chi-Chi earthquake could be calculated. The simulated co-seismic groundwater level changes were compared with the field observations, and the displacement at Dongshi area was found to be 0.01% of the model. Therefore, numerical analysis is a method that can be used to identify the aquifer parameters and earthquake effects.