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 , 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 . 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 (). 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.
Highlights
The response of groundwater levels to earthquakes is an important topic to help seismologists and GEOMATICS, NATURAL HAZARDS AND RISK hydrogeologists understand earthquake processes and focal mechanisms
There are four possible causal mechanisms that we have summarized from existing studies
The third causal mechanism is the static strain hypothesis, which states that the co-seismic water level changes can be explained by the co-seismic static strain and the change in pore pressure predicted by the poroelastic theory (Wakita 1975; Merifield and Lamar 1984; Roeloffs 1996; Ge and Stover 2000; Jonsson et al 2003; Wang and Manga 2010)
Summary
GEOMATICS, NATURAL HAZARDS AND RISK hydrogeologists understand earthquake processes and focal mechanisms. It can clearly be seen that investigating the causal mechanisms of the co-seismic water level response requires an abundance of observational data and a variety of analytical methods; for example, a single well may respond to many earthquakes (Roeloffs 1998; Brodsky et al 2003; Matsumoto et al 2003; Zhang et al 2015), or many wells may respond to a single earthquake (Wang et al 2001, 2004; Roeloffs et al 2003; Chia et al 2001; Ma and Huang 2017). Near the Sichuan basin, the Qinghai–Tibet Plateau’s east-northward movement meets with strong resistance from the South China Block, causing a high degree of stress accumulation in the Longmenshan thrust formation This caused a sudden dislocation in the Yingxiu–Beichuan fracture, leading to a violent earthquake of Mw 7.9 (Liu et al 2012)
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