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. Plain Language Summary: Permeability of Earth's crust is not a static property. Geochemical, biological, and tectonic processes can change permeability and hence fluid and heat transport through the crust. We show that diurnal and semidiurnal solid Earth tides can be used together to monitor changes in deep fractured aquifers over time and after earthquakes. We find that seismic waves from large, distant earthquakes can change the apparent orientation of fractures. Because stresses from the seismic waves are small, we suggest that oscillatory fluid flows driven by seismic waves unclog blocked fracture connections thus changing the apparent orientation of fractures. Key Points: Water level changes produced by solid Earth tides record the orientation of fractures intersected in deep wellsApparent fracture orientations change after large, teleseismic earthquakesDynamic strains from seismic waves may unclog or clog fractures and change fracture connectivity and the mean orientation sensed by tides [ABSTRACT FROM AUTHOR]