地震引發水文變化時空分布之研究

Abstract

世界各地發生的強震經常導致明顯的水文變化，本研究依據現地觀測紀錄，探討強震引發的河川流量增減與地下水位升降變化之時空分布，並針對不同含水層於地震期間所記錄到的水位變化，設計簡化的二維模型，使用數值近似方法進行模擬分析，探討斷層位移時水文地質特性對地層孔隙水壓隨時間變化的影響。 1999年M7.6集集地震發生後1至4天引發了台灣中部地區23個水文觀測站的流量出現異常變化，其中22站於震後記錄到流量增加超過60%的現象，推測是山區岩層受到地震震動影響產生裂隙，加速地下水補注臨近河川所致；也可能因為斷層錯動造成地層壓縮，引發孔隙水壓上升所致。距離震央最近位於水里溪上的測站，則於震後記錄到河川流量大幅減少96%，長期觀測資料分析指出，震前該站流量通常大於上游測站流量，但是震後該站流量卻低於上游測站流量，並維持長達8個月之久。震後河川流量減少的原因可能是逆衝斷層錯動時，造成震央地區地殼伸張，降低地層孔隙水壓，引起河川水流下滲河床所致。 台灣南部地區於2016年及2010分別發生M6.4美濃地震及M6.3甲仙地震，全台各有300口及284口觀測井出現同震地下水位變化，其中西南平原地區最多。兩起地震引發了超過80%的同震地下水位上升，間接指出地震發生時大多淺處地層都受到擠壓。在空間分布上，少數複井觀測站在不同深度的井，分別記錄到同震水位上升及同震水位下降的情形，因此地層同震應變可能會隨深度出現明顯變化。本研究利用GPS資料與同震地下水位變化進行比對，發現出現同震地下水位有升有降的觀測站，多位在GPS記錄中位移量相對較大之地區，顯示位移明顯處垂直方向的孔隙水壓變化也較為複雜。 強震引發的地下水位變化，在不同的含水層具有不同的回復特性，自由含水層的同震地下水位變化，震後通常會快速回復，而受壓含水層的同震變化則可持續數月甚至更久。長期的地下水位紀錄分析顯示，觀測井的水文地質特性可能造成同震變化的差異。本研究使用有限元素分析軟體ABAQUS模擬斷層位移時的孔隙水壓變化，分析結果顯示，地層的同震孔隙水壓變化幅度與其壓縮係數密切相關。自由含水層的地下水面與大氣壓力相通，因此同震變化通常會在一天之內迅速回復；而受壓含水層的孔隙水壓消散速率受控於上覆阻水層的水力傳導係數，其同震變化回復速率相當緩慢，可長達一年以上；但是如果地震造成阻水層出現裂隙，將會加速孔隙水壓的消散。因此含水層的水文地質特性，是地震引發的地下水位變化量及其消散速率的重要控制因素。

Earthquakes induced hydrologic changes have been reported worldwide. This study analyzed the spatial and temporal changes of streamflow changes and groundwater-level changes induced by strong earthquakes based on field observations. Simplified two-dimensional models were used to simulate the temporal changes of pore-pressure in different formations due to fault displacement. One to four days after the 1999 MW7.6 Chi-Chi earthquake, changes in streamflow have been observed at 23 stream gauges in central Taiwan. Post-earthquake streamflow increases over 60% were recorded at 22 gauges in four watersheds. The increase in groundwater discharge to the river after the earthquake can be attributed to rock fracturing by seismic shaking as well as pore pressure rise due to compressive strain induced by fault movement. A large decrease in discharge was recorded immediately after the earthquake at the gauge near the earthquake epicenter. Analysis of long-term hydrological data indicated that the post-earthquake discharge at the gauge reduced to a level smaller than that at an upstream gauge for eight months. Such a streamflow decrease might have been caused by a discharge to the streambed due to a co-seismic decrease in pore pressure induced by crustal extension during the rupture of the thrust fault. Co-seismic changes in groundwater level were recorded respectively at 300 and 284 monitoring wells in response to the 2016 M6.4 earthquake and 2010 M6.3 earthquake occurred in southern Taiwan. Most of the changes occurred in the southwest plain of Taiwan. More than 80% of the changes are groundwater-level rise for both earthquakes, indicating co-seismic compression dominant in the shallow crust. Some multiple-well stations recorded co-seismic rise and co-seismic fall at different well depths, revealing co-seismic strain varies significantly in the vertical direction. The multiple-well stations that recorded co-seismic rise and co-seismic fall are located in the area where relatively large displacements were measured by the GPS. This coincidence indicates that the reversal of the polarity of groundwater-level changes in the vertical distribution is related to the local displacement due to the earthquake. Co-seismic groundwater-level changes show different recovery processes in different aquifers. Co-seismic changes usually recover immediately after the earthquake in an unconfined aquifer, while they sustain for months or longer in a confined aquifer. Long-term data analysis indicated that variations in co-seismic groundwater-level changes at different monitoring wells are attributed to hydrogeological characteristics of the well site. The finite element software ABAQUS is used to simulate the pore-pressure changes induced by the displacements due to fault rupture. The calculated co-seismic change in pore pressure is related to the formation compressibility. In the unconfined aquifer, the recovery rate is rapid, usually from a few minutes to a few hours, due to the hydrostatic condition at the water table. In the confined aquifer, the pore pressure dissipation or recovery rate is slow and may last for a year or longer due to the retardation of less permeable confining layer. Fracturing of the confining layer during earthquakes may enhance the dissipation of pore pressure in the confined aquifer. The study results indicated that aquifer characteristics play an important role in determining groundwater-level changes during and after earthquakes.