Different coseismic groundwater level changes in two adjacent wells in a fault-intersected aquifer system

Abstract

Previous studies have investigated many cases regarding changes in well water level and attributed differences in the coseismic water level responses to various hydrogeological conditions. However, most of these studies often fail to provide insights into the responses of different portions of one groundwater system because the boreholes in question are usually located in different hydrogeological units with no hydraulic communication. This paper analyzes coseismic and postseismic variation characteristics of the water levels of two adjacent observation wells located near the same fault zone and separated by a distance of approximately 2 km. Well X10 resides in the fault zone and is indirectly connected to the aquifer, whereas Well X11 directly penetrates into the aquifer. Based on the coseismic static strain, seismic energy density and variation in well water level response to earth tides before and after earthquakes, this research investigates the effects of faults on coseismic well water level responses. The results show that under the effects of static compressive stresses, wells X10 and X11 present opposite water level responses due to the presence of the fault. After the earthquake, the water levels in both wells are elevated due to the increased aquifer permeability induced by the earthquake. Nevertheless, because of the fault, the rate of increase of the postseismic water level in Well X10 is obviously lower than that of Well X11. A seismic energy density analysis shows that only three near-field earthquakes with energy densities greater than 10−3 J/m3 can simultaneously trigger the elevation of coseismic water levels in wells X10 and X11. This paper concludes that the oscillation elevation of the coseismic water levels in wells X10 and X11 should be mainly attributed to the increased aquifer permeability. However, the mechanisms underlying this change are quite complex and cannot be explained by a single mechanism, and the increased permeability may be correlated to various factors, such as the coseismic static strain, seismic energy density and borehole condition.