Abstract
Hydraulic properties of fault zones are important to understanding the pore pressure development and fault stability. In this work, we examined the relationship between water level changes caused by the 2008 Wenchuan Mw 7.9 earthquake and faults using four wells with the same lithology around the Three Gorges Dam, China. Two of the wells penetrating the fault damage zones recorded sustained water level changes, while the other two wells that are not penetrating any fault damage zones recorded transient water level changes. The phase shift and tidal factor calculated from water level, a proxy of permeability and storage coefficient, revealed that both the permeability and storage coefficient changed in the two wells penetrating the fault damage zones, while the other two wells not penetrating the fault damage zone did not show any change in permeability and storage coefficient. Thus, we tentatively suggest that faults may play an important controlling role on earthquake-induced hydrologic changes because the detrital or clogging components in the fractures may be more easily removed by seismic waves.
Highlights
In the intermediate- and far- field, the amplitude of co-seismic water level changes is positively related to the magnitude of the seismic energy density [3,13,14,15,16]; the sign is repeatedly fixed in a specific well [3,13,17,18] and seems to be controlled by the hydrogeological settings, e.g., water level rise in the recharge area and water level drop in the discharge area [19,20]
In order to further analyze the water level changes induced by the Wenchuan earthquake, we analyzed the four wells’ hydraulic properties, because the hydraulic properties can be changed following an earthquake, which is one plausible explanation for the hydrologic changes induced by earthquakes [5,19,31]
These wells with similar hydrogeological settings showed different changes in the water level, tidal behavior and hydraulic properties following the Wenchuan earthquake (Table 3, Figures 4 and 5), which suggested that the earthquake-induced water level, tidal behavior and hydraulic properties changes in the four wells may not have much to do with the hydrogeological settings
Summary
Previous studies indicated that earthquake-induced water level changes are mainly affected by the seismic input (such as energy) and hydrogeological settings. The sign of co-seismic water level changes is related to the earthquake-induced static strain in the nearfield, i.e., water level rise in the contraction zone and drop in the dilation zone [7,8,9,10,11,12]. In the intermediate- and far- field, the amplitude of co-seismic water level changes is positively related to the magnitude of the seismic energy density [3,13,14,15,16]; the sign is repeatedly fixed in a specific well [3,13,17,18] and seems to be controlled by the hydrogeological settings, e.g., water level rise in the recharge area and water level drop in the discharge area [19,20].
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