2000年ML6.7地震引發坪頂觀測井之地下水位異常變化

Abstract

台灣位於環太平洋地震帶上，由於地震發生頻繁，各種地震監測設施密布，而且裝設了600多口的觀測井，記錄地震相關的地下水位反應，因此適合進行地震水文相關的研究。同震地下水位的變化在空間上的分布可以反映地震發生時大地應力的重新分布，本研究針對2000年6月11日發生芮氏規模6.7地震，探討地震相關之地下水位變化。全球衛星定位系統記錄顯示此次地震造成了地表變形，至於水位記錄分析結果指出，地震發生時，全台共有410口觀測井在記錄水位，其中112口記錄到同震水位上升變化，僅有14口出現同震水位下降變化，變化幅度從下降11公分到上升71公分。空間分布上西南平原出現反應的觀測井以上升變化為主，但其中濁水溪沖積扇接近丘陵的井位，以及嘉南平原之部分井位出現下降反應。位於濁水溪沖積扇上、臨近震央的觀測井反應比例最高，共有98口出現同震變化，其中坪頂一號井除了同震上升22公分外，並於震前一小時出現了5公分異常下降的水位變化。井深204公尺的坪頂一號井位於斗六丘陵的北端，井篩開設於半固結的礫石含水層中，比對1997年至2005年之間發生芮氏規模大於5.5的地震，曾出現13次同震水位反應，大多為水位上升的變化，而且這13次地震的震央機乎都在台灣中部及其外海。在所有同震反應中，2000年芮氏規模6.7地震前的水位下降及同震水位上升變化皆具其獨特性，可能反映了地震引發的現地應力改變及地震發生前地殼的應變狀況，而且符合地震發生時應變出現彈性回跳的理論。

Taiwan is located in the Circum-Pacific seismic belt. As earthquakes occurred frequently, various earthquakes monitoring devices have been placed all over this island. More than 600 wells have been installed to monitor earthquake-related groundwater level changes. Co-seismic groundwater level changes may reflect tectonic stress redistribution and crustal strain. This study focuses on the responses of groundwater level recorded by monitoring wells to the 2000 ML6.7 earthquake. When this earthquake occurred, ground deformation was recorded by Global Positioning System data. From the analysis results of water-level record, 410 wells were operational while co-seismic rises were observed in 112 wells and only 14 wells observed co-seismic falls, where the amplitude of co-seismic changes range from 71cm rise to 11cm fall. In spatial distribution, wells located in Southwest plain of Taiwan react as rises, but wells that are near hills and some in Chianan plain react as falls. Monitoring wells located in Choushui river alluvial fan are near to the epicenter and have a high reacting ratio, which 98 wells observed co-seismic water-level changes. One of the well named Pingding No.1 well (PD1) not only reflect 22cm co-seismic water-level change, but also show an abnormal 5cm water-level change one hour before the co-seismic water-level change. The 204m deep well is located at the northern tip of the Douliu Hill, and was installed in an aquifer consisting of semi-consolidated deposits. Further analysis indicated that 13 co-seismic water-level changes were observed in response to earthquakes of magnitude greater than 5.5 during the 1997 to 2005 period; most of the changes are rises. The epicenter of these 13 earthquakes mostly locates around central Taiwan and its outer sea area. Among all the co-seismic responses, the phenomenon due to the 2000 ML6.7 earthquake shows its uniqueness, which might imply the change of tectonic stress induced by earthquake and crustal deformation caused by strain. This phenomenon consists with the poroelastic rebound theory.