Evidence of low-magnitude continued reservoir-induced seismicity associated with the pertusillo artificial lake (Southern Italy)

The most common observations of reservoir-induced seismicity (RIS) by filling artificial lakes are generally associated with the initial impoundment of the lake or the lake level increase above a previous maximum. Rarely, RIS persists for many years without a decrease in frequency or magnitude; if this occurs, it is called "protracted" or "continued" seismicity. Pertusillo artificial lake in high Agri Valley (southern Italy) is a small reservoir with a water column that fluctuates yearly by about 10-15 m (on average), corresponding to a pressure perturbation at the bottom of the lake of 0.10-0.15 MPa. Although the reservoir was filled in 1963, the microearthquake activity still occurring around the lake allows the categorization of such RIS as continued seismicity. We collected all microearthquakes (ML ≤2.7) recorded near the reservoir by two different seismic networks from January 2005 to June 2012 and analyzed their frequency-magnitude distribution, which showed a b value (b = 1.40) much higher than that expected for the investigated area. Even though the spatial seismicity pattern shows two distinct clusters to the northeast and southwest of the lake, we found that only the southwest seismicity is significantly correlated with the water level. The findings suggest that the physical driving mechanism is the 1D pore fluid pressure diffusion along the northeast-southwest fault zones, connecting the reservoir to the southern termination of the Monti della Maddalena fault system, with an average hydraulic diffusivity of ~7.8 m2/s.