Seismic imaging of a fluid storage in the actively extending Apennine mountain belt, southern Italy

A picture of the upper crustal structure of the Irpinia active faults system in southern Italy was obtained by combining new geological evidences, lithological properties, and microseismicity distribution. P and S wave velocity models indicate high V-P/V-S and low V(P)xV(S) values, suggesting fluid accumulation within a similar to 15 kmwide rock volume where intensemicroseismicity is located. The 1980 Irpinia, M-s 6.9, earthquake nucleated within the same fault-bounded volume. We suggest that concentration of background seismicity is mainly controlled by high pore fluid pressure. Its increase in fluid-filled cracks around major faults leads to earthquakes' nucleation. Seismic pumping along major faults carries fluids through the conduit system represented by the intensely fractured damage zone. Conversely, the cross-fault barrier behavior of the low-permeability fault core leads to pore fluid pressures building up within the fault-bounded block, thus producing a positive feedback triggering earthquake nucleation within the volume, which behaves as an "earthquake reservoir."