Fault-Zone Damage Anisotropy Deciphered from Dynamic Strain Response in a Well-Aquifer System

Although the coseismic water-level response to distant earthquakes is widely recognized, research has predominantly focused on volumetric strain from surface waves, leaving the effect of shear strain largely overlooked. This study investigates the sensitivity of the Beibei well-aquifer system to the coupling effect between the volumetric and shear strain, using high-quality coseismic water-level records from four earthquakes with different azimuths. Our results reveal significant differences among elastic parameters inverted from seismic waves along different azimuths, a discrepancy attributable to crack and damage anisotropy within the fault zone. Notably, the rock damage degree derived from body waves is significantly higher than that from surface waves, indicating more substantial damage along the near-vertical propagation path of body waves. These findings elucidate how fault-zone anisotropy governs hydroseismic responses and offer new insights into the coupled mechanical-hydraulic behavior of aquifer systems under dynamic strain.