Groundwater level response to the Wenchuan earthquake of May 2008

Abstract

We have comprehensively analysed the co-seismic response of the groundwater levels of 280 wells in mainland China that were associated with the Wenchuan earthquake (Mw 7.9) that occurred on 12 May 2008. The observed co-seismic responses can be classified as step-like changes in 138 wells, variations in 69 wells and non-responses in 73 wells. After a quantitative analysis of spatial distribution, there was no spatially coherent signal found in the step-like changes (positive values indicate a step-like rise, and negative values indicate a step-like fall), even within 300 km of the epicenter. The amplitude and the phase shift of the M2-wave were compared between the pre- and post-earthquake conditions. The phase was forward and was concentrated at 0.23π~0.4π, regardless of the proximity of wells to the epicenter; however, the change in amplitude randomly increased or decreased. By computing the post-seismic groundwater recession, the characteristic times of the aquifers were τ ∈ (-3.3 × 106, 1.8 × 106). We concluded that by assuming a first-order approximation, i.e. a one-dimensional aquifer, the causal mechanism of the co-seismic response of the groundwater level was that the seismic waves enhanced rock permeability by clearing the facture-filling materials. The shapes of the co-seismic responses were determined by the well-aquifer system and the proximity of the wells to recharge or discharge areas (x/L). The water level rose if the well was closer to the recharge area, the water level decline if the well was closer to the discharge area, and the water level oscillated if the well was farther from the recharge or discharge areas when the seismic wave was transmitted. The water level remained unchanged if the well did not penetrate any confined aquifer.