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

Amoroso O. 1, Ascione A.2, Mazzoli S.2, Virieux J.3, Zollo A.1

1 Dipartimento di Fisica, Università degli Studi di Napoli Federico II
2 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
3ISTerre, Université de Grenoble I, CNRS, Grenoble, France
 
Geophysical Research Letters, 41, 3802–3809, 2014
 
Abstract
Fig 05 nuovaA 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 VP/VS and low VP×VS values, suggesting fluid accumulation within a ~15 kmwide rock volume where intensemicroseismicity is located. The 1980 Irpinia,Ms 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.”
 
 
Legenda

Three-dimensional tomographic model. (a) VP velocity model and micro-earthquake locations projected onto the cross section located in Figure 1a (refer to Figure 1b for the tectonic contacts and geological units). Grey-dashed curves delimit the well-resolved regions of the model according to the checkerboard resolution test (see section 3). Each curve corresponds to a different resolution scale obtained from estimating the resolvability function for each model parametrization used in the multiscale approach. Star shows hypocenter of the Ms 6.9, 1980 Irpinia earthquake. (b) VP/VS ratio for the same depth section as in Figure 3a. (c) VP Å~ VS product for the same depth section as in Figure 3a. (d) Horizontal slice through the P wave tomographic model at depths (Z) of 6 and 8 km. (e) Horizontal slice showing VP/VS ratio at depths (Z) of 6 and 8 km. (f) Horizontal slice showing VP Å~ VS product at depths (Z) of 6 and 8 km.