Response of Volumetric Strain and Groundwater Level to Earthquakes

Abstract:

Changes in groundwater level have been widely reported after large earthquakes, but the mechanism for the changes is not well understood. The static elastic strain model, proposed ~40 years ago, has been tested only for its predicted sign, but not for its predicted magnitude of the groundwater-level change. Simultaneous measurements of the coseismic change in groundwater level and volumetric strain at the same location are required for testing the hypothesis, but were previously unavailable. Here we report documented coseismic changes of volumetric strain and groundwater level at three stations in China in response to the 2008 Mw7.9 Wenchuan and the 2011 Mw9.0 Tohoku earthquakes; the data provide an opportunity to test the static strain model in the intermediate distance from the epicenter. We show that the magnitude of the measured volumetric strain is mostly consistent with that required to explain the groundwater-level change, both in sign and in magnitude, but is far greater than that predicted from the static-strain model. The result shows that the documented coseismic change of volumetric strain cannot be the static elastic strain due to the earthquakes. Instead, it is likely to be the poroelastic response of the aquifer to the coseismic change in groundwater level, due perhaps to earthquake-enhanced permeability. The scenario suggests that poroelastic response to coseismic groundwater-level change may be far greater in magnitude than the static elastic strain in the intermediate distance and may thus be significant to consider in characterizing site response to earthquakes.