Controls on Pre-seismic Groundwater Pressure Variations: Insights from Poroelastic Sensitivity Analysis and Aquifer Properties

Pre-seismic groundwater anomalies have been reported in diverse tectonic settings, yet their physical origin, observability conditions, and predictive relevance remain debated. Here we develop a quantitative hydro-mechanical framework to identify the conditions under which crustal strain can generate detectable groundwater pressure variations in aquifers. Using poroelastic theory combined with a sensitivity analysis over realistic ranges of aquifer properties and strain magnitudes compiled from the literature, we evaluate the amplitude and detectability of pre-seismic hydraulic head fluctuations in confined and unconfined aquifers. Our results demonstrate that measurable poroelastic signals are expected primarily in well-confined aquifers characterized by high stiffness and Skempton coefficients close to unity, such as fractured carbonates and crystalline rocks. In contrast, unconfined aquifers generally produce responses below instrumental detectability. Comparison with reported observations indicates that many pre-seismic anomalies are consistent with elastic poroelastic coupling, whereas large-amplitude anomalies exceed plausible elastic strain limits and require inelastic deformation processes, including microcracking, damage accumulation, granular rearrangement, and fluid-assisted weakening. In the unified interpretative framework proposed, groundwater systems act as selective filters of crustal deformation. The absence of systematic correlations between groundwater anomalies and earthquake magnitude, depth, or precursor time highlights the dominant role of local aquifer properties rather than source parameters. These findings provide quantitative criteria for site selection, monitoring network design, and physical interpretation of groundwater-based seismic precursors.