2006年恆春地震導致台灣地區地下水位變化之研究

Abstract

2006年12月26日20時26分及34在台灣恆春近海地區先後發生兩次芮氏規模7.0之地震，其震源分別位於墾丁地震站西南方38.4公里及西方33.1公里。檢視全台地下水觀測井，共126口井發生同震地下水位上升或下降變化，水位變化範圍從下降29公分到上升54公分不等，變化量最大的為射寮2號井，也是距離震央最近的一口井。在高頻地震監測井記錄中，可觀察到兩次地震所造成的振盪式水位變化。分析同震水位變化與震源距的關係，發現震源距離與同震上升變化幅度呈現反比關係，而與同震水位下降變化則無明確的相關性。其中115口井同震水位變化皆分布於台灣西南部地區，尤其在嘉南平原及屏東平原。平面空間分布上，嘉南平原東北部多發生同震下降變化，西南部多發生同震上升變化；而屏東平原東部多為同震上升變化，西部多為同震下降變化，同震有升有降變化則分布於過渡區域。剖面空間分布上，淺處主要為同震上升變化，深處主要為同震下降變化，顯示造成靠近山區的淺處同震水位變化受壓縮應變影響，而深處則可能受伸張應變所影響。藉由同震地下水位之空間分布，可反映區域應辦分布或含水層之物理特性。檢視全台118站河川水位測站之同震水位變化後，僅有四站在地震前後出現水位異常變化現象，但是其中原因仍需進一步研究調查。

Two ML7.0 earthquakes occurred at 20:26 and 20:34 local time on December 26, 2006 in the offshore area near Hengchun in southern Taiwan. The coseismic changes of the well water level were observed in 126 monitoring wells. The largest coseismic rise was 54 cm and the largest coseismic fall was 29 cm. The largest coseismic change was observed in the No. 2 well of the Sheliao station, which is the closest to the epicenters of the Hengchun earthquakes. Two oscillatory changes induced by the earthquake doublet can be observed in high-frequency water level records. The magnitude coseismic water level rise increased with decreasing hypocenter distance. But the coseismic fall didn’t show any correlation with hypocentral distance. Coseismic changes were observed in 115 wells in the southeast plain of Taiwan. Most coseismic rises were observed in the shallow wells, while the coseismic fall changes were observed in the deeper wells. This phenomenon indicates that the shallow area is influenced by tectonic compression the deeper area was seems to be dominated by tectonic extension. The spatial distribution of coseismic changes is primarily controlled by the stress redistribution and the physical properties of aquifers. Anomalous changes of water level of streams induced by the earthquake were only observed in 4 stations. Nevertheless, the mechanism of these changes remain to be investigated.