Characterizing Seismogenic Fault Structures of the Lake Pertusillo Reservoir Induced Seismicity (Southern Italy) Using a Relocated Template-Matching Catalog

Abstract

We present a detailed analysis of the small magnitude (ML < 3) Reservoir Induced Seismicity associated with the Pertusillo water reservoir located in the high seismic hazard zone of Val d'Agri (Southern Italy). We apply template-matching detection to a 13-month-long dense passive survey, obtaining a final high-precision double-difference catalog of 5,070 earthquakes (−0.7 < ML < 2.6, MC = 0.2). The new catalog allows precisely tracking the spatiotemporal distribution of the swarm-seismicity and to map the b-value of the Gutenberg-Richter law. We combine seismicity data with available subsurface geophysical data, fostering an improved interpretation of the induced seismicity. We identify four seismicity-clusters showing rapid changes in seismic rate which correlate to severe seasonal oscillations. Seismicity unravels new km-scale faults or better define faults partially-illuminated by template earthquakes. b-value shows a significant spatial variability, with very-high b-value (up to 2) within areas of distributed seismicity and lower (∼1.3) b-value for on-fault seismicity featuring larger magnitude events. Seismicity is confined within a brine-saturated fractured carbonate reservoir, while earthquake distribution and rate are controlled by the fault architecture and rock properties (e.g., inherited fluid pathways, rock fracturing, pore fluid pressure). In particular, most earthquakes reactivate, with extensional kinematics, pre-existing reverse/transpressional faults favorably oriented in the present-day extension. All observations suggest that a poroelastic stress transmission mechanism, due to the seasonal water level oscillation, can explain the Pertusillo lake seismicity. This study confirms the importance of investigating the complex interaction among stress changes caused by human activities, pre-existing faults and local stress field to correctly assess the hazard posed by induced seismicity.