Co-Seismic Groundwater Level Changes Induced by the May 12, 2008 Wenchuan Earthquake in the Near Field
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Pure and Applied Geophysics
Co-Seismic Groundwater Level Changes Induced by the May 12, 2008 Wenchuan Earthquake in the Near Field ZHEMING SHI,1 GUANGCAI WANG,1 and CHENGLONG LIU1,2 Abstract—The large scales of co-seismic water level changes in mainland China were observed in response to the tragic 2008 Ms 8.0 Wenchuan earthquake. To better understand the mechanism of these hydrogeological phenomena, groundwater-level data at 17 confined wells, with an epicentral distance of \500 km, were collected. We compare the static strain predicted by dislocation theory with the volumetric strain calculated by the tide effect of the groundwater based on poroelastic theory. The results show that the sign of the co-seismic groundwater level change is consistent with the sign predicted by dislocation theory. Additionally, the magnitude of the strain calculated by the two methods is also concordant in half of the wells. In the rest of the wells, the strains inversed from the groundwater level are one or two orders of magnitude larger than the fault dislocation model. These wells mostly have an epicenter distance larger than 300 km; therefore, the dynamic stress induced by the seismic wave may be responsible for the co-seismic water level changes in these wells. According to these results, we roughly estimate that the effect range of the static stress is approximately 300 km for the Wenchuan earthquake, and the dynamic stresses dominate beyond this epicenter distance. In addition, geological and hydrogeological conditions and other mechanisms may be responsible for these changes. Key words: Groundwater level, earth tides, dislocation theory, dynamic stresses, static stress, Wenchuan earthquake.
1. Introduction It is known that the fluid in the crust is very sensitive to crust strain and solid deformation. The groundwater level in a well from a confined aquifer can change even if the deformation in the crust is small (BODVARSSON, 1970). A confined well-aquifer system can record different frequencies of
1 School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing, 100083, China. E-mail: [email protected] 2 Institute of Geology, China Earthquake Administration, Beijing, 100029, China.
geophysical information, such as seismic waves, ocean tides, earth tides, atmosphere pressure, crust deformation, and rainfall. Therefore, a confined wellaquifer system can be regarded as a broadband seismograph (WANG et al., 1988). Observing the groundwater level is one way to monitor crust deformation and predict earthquakes. Furthermore, many hydrogeological phenomena occur following an earthquake. One of the most interesting phenomena is the change in groundwater level induced by the earthquake. This change in groundwater level may affect the supply of groundwater resources (GOROKHOVICH, 2005; GOROKHOVICH and FLEEGER, 2007; SINGH, 2008; CHEN and WANG, 2009). Seismic waves can affect oil well production (BERESNEV and JOHNSON, 1994). The redistribution of fluid pressure induced by an earthquake may trigger new sei
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