Groundwater Level Changes in Taiwan Caused by The Wenchuan Earthquake on 12 May 2008

Widespread persistent coseismic groundwater level changes induced by the M _w 7.9 Wenchuan earthquake were recorded in 80 wells in Taiwan, located approximately 2,000?km from the epicenter. The timing and magnitude of these changes, ranging from ?40 to +23?cm, have been examined by statistical evaluation. The coseismic fall dominated in the plain area, but in the mountainous area most changes are coseismic rises. Both the coseismic rise and the coseismic fall were observed in wells of different depths at two monitoring stations. The processes of the coseismic changes can be shown by the high-frequency well water level data. Analysis of the 1?Hz water level data and the broadband seismogram revealed that a temporary water level change, ranging from 0.2 to 2.3?cm, occurred at the arrival of high-energy Love waves and Rayleigh waves. However, a water level change of 0.3–6.4?cm occurred in 7–20?min after the passing of high-energy surface waves. It is doubtful that such a change could be induced directly by ground shaking. The cross-formational flow due to fracturing or unclogging of sediment pores was possibly too slow to cause the coseismic change. Further studies are required to understand the mechanism of coseismic groundwater level changes induced by a distant earthquake.