RICEN : Repeated InduCed Earthquakes and Noise at Solfatara, Campi Flegrei, Italy.

Abstract:

RICEN is a series of three experiments whose goal is to study the changes in the elastic properties of the medium at small scales through repeated observations over time through the recognition of changes in seismic records acquired at a dense network. The investigation area is the Solfatara, a volcano in the Campi Flegrei caldera, characterized by an intense hydrothermal shallow activity. RICEN sampled the investigated area with passive and active seismic. For the active part, acquisition was performed by shooting the area with the MiniVIB and recording the ground motion at 240 seismic stations, placed on a regular in front of the Fangaia. Additional two 1D orthogonal profiles were acquired with sensors located at 2m distance. Ambient noise was instead recorded in the area for several days, including also the active part.

Data from the first experiment and from the two profiles were processed and analysed. We obtained 3D P and S wave tomographic models of the subsoil covered by the grid. The P wave velocity model was obtained from a linearized inversion of P wave first arrivals, manually picked on cross-correlated minimum phase traces. The S model was obtained through the inversion of the phase and group velocities dispersion curves, measured on 96 overlapping sub-grids and for each of them assuming a 1D propagation medium. Dispersion curves were also available from ambient noise analysis. Moreover a 2D long profile was analysed to retrieve a tomographic 2D model of the area and the section migrated at depth to retrieve the bottom of the crater in the area. Finally statistical properties of the scattered field, such as the mean free path, were obtained from the analysis of the coherent vs incoherent wavefield and the enhanced backscattering.

Velocity contrasts and the properties of the scattered field indicate a clear separation between gas rich and water rich regions.