Changes in groundwater level and tidal response caused by the Wenchuan earthquake, China

Abstract

Earthquakes can trigger various hydrological responses such as changes in groundwater level, tidal response, and aquifer permeability, but the relationship between these changes remains unclear. In this study, the changes in groundwater level and tidal response induced by the 2,008 Ms8.0 Wenchuan earthquake, China, are analyzed for epicenter distances of tens of kilometers to more than 1,000 km. The results show that the water levels in 34 wells changed, including step-like, pulse, oscillation and gradual changes. The water levels of 8 out of 17 wells with tidal responses experienced changes in phase shift based on tidal analysis, and the water levels and phase shifts varied in opposite directions. The changes in phase shift indicate that the earthquake changed the horizontal or vertical permeability of the aquifer, or even the groundwater movement. Several possible mechanisms are proposed to account for these changes: breaching of the aquitard, associated with five of the studied wells in which phase shift increased to different extents but water level decreased; reduction in the fracture aperture, accounting for one well whose phase shift changed from positive to negative but water level increased; clogging/unclogging of the fractures, associated with one well with increasing and then decreasing phase shift; lower aquifer permeability or the occurrence of rapid fluctuations in pore pressure, accounting for another well where phase shift gradually decreased but the water level changed little. Each mechanism may deepen understanding of hydrological responses after earthquakes.