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Different Sensitivities of Earthquake‐Induced Water Level and Hydrogeological Property Variations in Two Aquifer Systems

Determining the factors that control the sensitivity of aquifer responses to earthquakes may provide insight into the interaction between hydrogeological and tectonic processes in the shallow crustal zone. Such response sensitivity varies randomly at the spatial scale and its determining factors have not been evaluated quantitatively and are still under debate. In this paper, we analyze seismically induced water level changes and then infer aquifer permeability changes following multiple earthquakes in two long‐term groundwater‐monitoring wells. While earthquake‐related water level changes were observed in one well, the second was insensitive to seismic events. These different seismic responses provide a unique opportunity to identify factors that control earthquake‐related changes. Wavelet transform, coupled with water level responses to tidal forcing, provides a methodology for comparing pre‐ and post‐seismic responses to periodic behavior. These methods found aquifer hydrogeological properties (e.g., transmissivity, storativity, degree of confinement) to be functions of their local hydrogeologic and tectonic settings. Key factors that may affect hydrogeological responses include the seismic energy arriving around the well, the degree of aquifer confinement, and the well location relative to local faults. Such factors favor seismic shaking and fracture unclogging, thus determining the sensitivity of hydrogeological responses to earthquakes. These findings are useful for designing wells for monitoring earthquakes, understanding earthquake‐inducing mechanisms, evaluating underground waste repositories, and estimating hydrogeological parameters using inversion.