Hydrologically induced crustal stress changes and their impact on seismicity in Greece

The response of seismic activity to external stress perturbations provides important insights into the physical processes governing earthquake triggering, nucleation, and rupture. Among various perturbations, annual hydrological loading is ubiquitous and offers an opportunity for investigating earthquake triggering processes. However, the physical mechanisms governing seismic responses to such periodic stress variations are not yet fully understood. Here, we explore the hydrologically induced crustal stress changes and their impact on seismicity in Greece by integrating a ~14-year earthquake catalog, GNSS time series, and GRACE-derived hydrological loading. We find that a significant variation in the rate of seismicity in Mainland Greece at annual time scale coincide with hydrological loading. The surface displacements predicted from GRACE-based loading models show good agreement with observed GNSS displacements, confirming that hydrological mass redistribution produces geodetically detectable crustal deformation. Our results demonstrate that hydrological loading produces geodetically observable surface deformation and induces stress perturbations that, although small in amplitude, modulate seismicity rates in Mainland Greece. We further find that historical earthquakes from 424 BC to 1903 (Mw > 5) exhibit a seasonal pattern, with peak seismicity occurring during the May–June period, consistent with the present-day seismicity modulation. The observed correlation among surface deformation, hydrological loading, and seismicity rates indicates that elastic stresses induced by hydrological loading play a key role in modulating seismic activity in Mainland Greece.