遠距地震引發台灣地區同震地下水位變化之研究

Abstract

地震對於地下水之各種影響有許多機制及模型被提出。Cooper等人發現井水的水位對於不同週期的地震表面波有不同程度的振幅反應，進而提出井水位振幅反映出含水層孔隙水壓放大的模型。本研究旨在利用台灣地區現有之地震與水井觀測資料，對遠距強震所產生之振盪式同震水位變化依據其理論進行分析，並與觀測值作比較。地震資料由台灣寬頻地震網提供，水位資料則來自壯圍、花蓮、東和、六甲、那菝和赤山等六口觀測井的秒水位記錄。針對2004年12月至2006年6月期間，規模大於M6.5以上的44次國外強震，本研究將地震波記錄與水位資料進行頻譜分析，估算出水位振幅放大率之觀測值，並嘗試調整導水係數以期理論分析之放大率與觀測值相符。我們發現在觀測井接受到的地震波震動幅度較大，也就是在地震規模大於7.0且震央距離小於5000km，而且震央距離並非太近的事件時，選取最佳的導水係數，結果約為10-4.9∼10-4.5 (1.26×10-5∼3.16×10-5) m2/s之間。依據Cooper理論所計算之水位振幅放大率，與觀測所得之放大率相符程度甚高。此外本研究估計六口觀測井發生可辨識之同震水位變化之臨界體積應變值，進而推測各井對於監測振盪式同震水位變化的敏感度，發現六口觀測井的敏感度可以分為四級：東和井與赤山井最為靈敏(臨界應變> 10-9)，那菝井的敏感度尚佳(臨界應變> 10-8)，花蓮井與壯圍井的敏感度較差(臨界應變> 10-7)，六甲井最不靈敏。此項靈敏度可作為將來持續觀測地震造成地下水位變化之參考。

Numerous mechanisms and models have been proposed to explain groundwater level fluctuations induced by distant earthquakes. Cooper et al. found that the responses of the groundwater level to earthquakes are frequency dependent and derived a solution for the amplification of the aquifer pore pressure due to the harmonic motion of the well water level. This study uses available seismic and monitor well data in Taiwan to analyze coseismic water level changes triggered by teleseismic surface waves. The water level data are obtained from six monitor wells: Hualien, Naba, Liujar, Tungwei, Chishan, Donher with sampling intervals of 1 second. Together with vertical ground velocities recorded at nearby BATS (the Broadband Array in Taiwan for Seismology) stations, the water level amplification is calculated based on the coherence between each seismogram and hydrograph of 44 distant earthquakes with magnitude M≧6.5 during the period from December, 2004 to June, 2006. For the events with M>7.0 and epicentral distances less than 5000 km, the seismically-induced water level fluctuations are clearly visible and the corresponding amplification values generally agree well with those predicted by the Cooper’s theory. The obtained best-fit values of transmissivity for six wells are similar, ranging from 1.26×10-5 to 3.16×10-5 m2/s. Furthermore, the threshold values of volumetric strains for the water level in response to seismic waves are estimated, and the sensibility of each well for monitoring oscillatory water level changes is inferred. The six wells are therefore classified into 4 classes: Donher and Chishan are the most sensitive and the earthquakes with strain > 10-9 are detectable; Naba is the next best (detectable for strain >10-8) ; Hualien and Tungwei are less sensitive (detectable for strain >10-7) and Liujar is imperceptible. These results can be used as references for further studies on monitoring earthquake induced groundwater level changes.