Well water temperature responses to earthquakes: single- and double-aquifer models

SUMMARY The response of well water temperature to earthquakes is crucial for understanding subsurface seismic fluid dynamics. However, recent studies have primarily focused on observations at a single depth and have employed single-aquifer models, which may lead to controversies when explaining fluid flow. In this study, we develop single- and double-aquifer models to estimate well-water temperature variations at different depths in response to changes in pore pressure, permeability and aquifer recharge temperature. The results indicate that variations in aquifer pore pressure and permeability result in significant differences in vertical flow velocity and temperature changes at various depths. When the borehole bottom is impermeable, for a single aquifer, temperature variation is maximal above the aquifer and variable at the aquifer depth, but nearly zero below the aquifer; for two aquifers, different pore pressure and permeability changes in each aquifer produce distinct temperature variation patterns, with minimal temperature change below the lower aquifer. If the borehole bottom is permeable, temperature variation becomes obvious below the lower aquifer. When cold or hot water from the aquifers flows into the borehole, significant temperature perturbations remain confined within a few metres of the aquifer within one day. Finally, a field case study investigates the co-seismic water temperature responses at three depths in the Chuan No. 03 well, triggered by the 2008 Mw 7.9 Wenchuan earthquake. The double-aquifer model effectively explains the complex co-seismic temperature fluctuations at different depths. Observation at a single depth risk missing crucial information, and multidepth temperature observation is a promising approach for interpreting and monitoring groundwater responses to earthquakes.