Groundwater Level Changes of Pingding Well due to the 2000 ML6.7 Earthquake in Taiwan
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.
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