Changes of Hydraulic Transmissivity Orientation Induced by Tele-Seismic Waves

Water level monitoring data from 5 deep wells on the North China Platform are used to study how two large earthquakes impact deep aquifers. After the passage of seismic waves from earthquakes, a subset of these wells shows changes in the phase shifts of water level responses to lunar diurnal (O₁) and semidiurnal (M₂) tides. We fit a model for the tidal responses of hydraulically conductive fractures that intersect the wells to explain the phase and amplitude of responses to both O₁ and M₂ tides. To explain changes after the earthquakes with this model, the apparent orientation of fractures must change. Because the stresses are small, we propose that changes in tidal response occur when the passage of seismic waves modifies the hydraulic connectivity of a network of fractures by unclogging or clogging flow paths, thus changing the apparent orientation of the fractures controlling tidal responses. The seismic energy density that results in changes appears to decrease as the dominant frequency of seismic waves increases.