Groundwater Level Changes Associated with the Wenchuan Earthquake of May 12, 2008

The Mw 7.9 Wenchuan earthquake occurred at 14:28:01 on 12 May 2008 in the Sichuan province of China. When the earthquake occurred, there are 682 monitoring wells recording hourly groundwater level data in Taiwan. Besides, there were 8 monitoring wells sampling at every two minutes, and 4 high frequency earthquake monitoring wells sampling at every second. Persistent anomalous groundwater level changes in response to the distant earthquake were observed at 72 monitoring wells. These wells are 190~2000 km away from the epicenter. Coseismic groundwater level falls were observed in most wells in the west coastal plain. The groundwater level changes at the Magong station, the nearest to the epicenter, revealed both coseismic rise and coseismic fall at different screened depth. Monitoring stations with high sampling rate shows the changing process of well water level. The oscillation of water level in the Liujia well began with the arrival of S wave approximately 450 seconds after the earthquake. The persistent change in the Liujia well began approximately 600 seconds after the earthquake, and a 9-cm persistent coseismic fall was observed several hours later. The spatial distribution of coseismic water level changes is complex. It is inconsistent with the calculated coseismic strains from a simple dislocation model. Seismic shaking cannot explain persistent changes in the rock formation or changes hours after the earthquake as well. Such a distribution is possibly resulting from the interaction between the Philippine Sea plate and the Eurasian plate.