Effect of shales on the tidal response of water level in wells

Shale is often an important component in groundwater systems and may significantly affect the change of water level in wells during earthquakes. In this work, we examine well water-level data of the 2008 Mw 8.0 Wenchuan earthquake in China and use the tidal response of the water level to characterize some uncommon co-seismic changes. We found that the phase shift to the M2 tide increased in most wells (62.5%); however, it decreased or remained unchanged in the others (37.5%). Those wells with unchanged and decreased phase shift are charactierized by the presence of significant amount of shales, while those wells with positive phase shifts have relatively little shales and thus a component of vertical flow to the water table is likely. Furthermore, for wells with shale in the lithology, there is a strong correlation between shale content and the amplitude response of the water level to tides, with lower amplitude correlated with higher shale content. The results led us to conclude that shales in groundwater systems may have significant effects on the tidal response of water level.