Using Repeating Microearthquakes to Infer Medium Velocity Variations at Val d’Agri Area (Southern Italy)

We investigated the correlation between water level of Pertusillo artificial lake and both earthquake production and medium velocity changes in the Val d'Agri area (Southern Italy) estimated by Coda Wave Interferometry (CWI) applied to repeating microearthquakes. We analyzed a catalog of 3354 earthquakes occurred between 2019 and 2023. A total of 122 repeaters were identified using a multi-sensor clustering technique. We observed an annual periodicity in both medium velocity variation and lake water level, but the two series are time shifted of about 6 weeks. Adopting a pore pressure diffusion model, we estimated an effective hydraulic diffusivity of 0.1 m2/s. We found two spatial domains that respond differently to pore pressure changes: within 5 km from the lake, positive pore pressure changes increase seismicity production, whereas, in the distance range 5-9 km, opposite pressure changes impact elastic properties and seismic velocities. Moreover, previous tomographic results suggest that these domains have a different lithology that results in a contrast in VP / VS and can promote fluid circulation. These findings suggest that seasonal water level fluctuations at the lake drive seismic activity and velocity changes in the area.