New Insights On The Val D'Agri Normal Fault Systems (Southern Italy) By Using A Dense Temporary Seismic Network

Three-dimensional Vp and Vp/Vs images of the upper crust in the Val d"Agri area (Southern Apennines seismic belt), down to 12 km depth, were computed by traveltime inversion of P (9269) and S (8726) waves from about 700 low-magnitude earthquakes. Data were recorded by a dense temporary seismic network composed by 36 digital three-component continuously recording stations, deployed over about 1500 km2 with an average receivers spacing of 5 km. The Val d"Agri basin is one of the most seismically active area in the central Mediterranean region. It was struck by a large destructive earthquake in 1857 (Me=7.0) and ongoing seismic activity testifies the high seismic hazard. Despite of the intense hydrocarbon exploration and field surveys, the geometry of the seismogenic sources is still debated. Two contrasting seismotectonic models are present in the literature, claiming for NE or SW dipping basin bounding normal-fault systems. We recorded, in thirteen months, about 2000 low-magnitude earthquakes (0.5<ML<2.7) testifying a high rate of background seismic release (about 3.3e-03 events/day/km2) almost continuous in time. The seismogenic volume is about 12 km thick. Earthquakes are mostly located beneath the south-western margin of the basin. Seismicity concentrates in the shallow crust (3-7 km) within a high-velocity (up to 6.5 km/s) body, clustering at the boundary between high and low Vp/Vs regions. We interpret the body with both Vp and Vp/Vs positive anomalies as the highly fractured and fluid saturated Mesozoic carbonates of the Apulian Platform, the oil reservoir of the region. Our 3D hypocentral locations and focal mechanism solutions strongly support the presence of an active NE-dipping normal-fault system bounding the basin to SW.