Correlation Between Seismicity and Water Level Fluctuations in the Polyphyto Dam, North Greece

The Polyphyto Dam, with a maximum water depth of 95 m and maximum water volume of ~ 1220 × 106 m3, is a medium-sized reservoir established on the Aliakmonas river (North Greece) in 1974. Although the broader area was considered as low seismic risk, a strong and damaging earthquake of magnitude (Mw) 6.6 occurred on May 13, 1995. In the present work, we investigate whether the recent seismicity in the vicinity of the Polyphyto Dam is associated with the water level fluctuations of the filled reservoir. To evaluate possible correlations and the occurrence of reservoir-triggered seismicity, we perform a cross-correlation analysis between the water level fluctuations of the reservoir and the observed regional seismicity rates, on datasets that span more than two decades, particularly for the period 1996–2017. The results of the analysis indicate that increased regional seismicity and moderate-sized events occur in the area at a lag of ~ 1 to 2 months after the water level maxima of the reservoir. This case of delayed seismicity is in accordance with a pore-pressure diffusion mechanism that decreases the effective normal stress along critically stressed fault zones, triggering the observed seismicity. Based on this mechanism, we estimate the hydraulic diffusivity and permeability of the regional fracture network and discuss the associated seismic hazard in the area.