Permanent Hydraulic and Poroelastic Property Changes in a Deep Aquifer Triggered by the Distant Tohoku Earthquake

Abstract While large earthquakes can alter groundwater systems far from the epicenter, their hydromechanical changes remain elusive. We investigate the 2011 Mw 9.0 Tohoku earthquake's impact on a deep well‐aquifer system ∼2,000 km from the epicenter. By employing the groundwater tidal and barometric pressure response methods, we perform tidal response analysis combined with Markov chain Monte Carlo (MCMC) inversion and numerical simulations in the MOOSE. The results indicate that permeability and the vertical leakage coefficient increased by one to two orders of magnitude, whereas the ability of the rock to store water (porosity, specific storage, and bulk compressibility) decreased by 20%–80%. These contrasting changes suggest enhanced hydraulic connectivity accompanied by reduced storage capacity. We interpret them as the result of simultaneous compaction (from volumetric strain) and fracturing (from shear strain) within the deep rock matrix. The study demonstrates that distant megaquakes can cause long‐lasting modifications on deep groundwater systems.