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Marina Pastori, Universita' di Perugia (Italy)
Davide Piccinini, Isituto Nazionale di Geofisica e Vulcanologia (Italy)
Lucia Zaccarelli, Isituto Nazionale di Geofisica e Vulcanologia (Italy)
Luisa Valoroso, Isituto Nazionale di Geofisica e Vulcanologia (Italy)
Francesca Bianco, Isituto Nazionale di Geofisica e Vulcanologia (Italy)
Lucia Margheriti, Isituto Nazionale di Geofisica e Vulcanologia (Italy)
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Shear-wave splitting is measured at 17 seismic stations deployed in the Val d'Agri by INGV which recorded local back-ground seismicity from May 2005 to June 2006. The splitting results suggest the presence of an anisotropic layer above the hypocenters (hypocentral depth between 2.5- 15.5 km). The dominant fast polarisation direction strikes NW-SE parallel to the Apennines orogen and is approximately parallel to the maximum horizontal stress in the region and also parallel to the strike of the main normal faults in the Val d'Agri. The size of the delay times (average value 0.01 s/km) suggests 4.5% shear-wave velocity anisotropy. At stations located at the SW portion of the deployment the delay times are larger on the order of 0.02 s/km indicating the presence of heavily fractured rocks possibly filled with fluids; this area is also locus of most of the microseismicity recorded .
Our results have been obtained by means of a classical manual procedure on about 150 earthquakes, that grounds its calculation on covariance matrix decomposition and cross-correlation method. We are extending our analysis using (semi) automatic procedures. This will allow us to analyse all the dataset of Val d'Agri experiment (about 1000 events). Although the total number of measurements differ significantly, we present the comparison of the results obtained from the manual and automatic procedures.
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