Crustal fracturing and presence of fluid as revealed by seismic anisotropy: case histories from seismogenic areas in the Apennines (Italy)

An automatic analysis code, Anisomat+, was developed, tested and improved to calculate anisotropic parameters: fast polarization direction and delay time. Anisomat+ was applied on data coming from three zones of the Apennines in Italy. For each area, anisotropic parameters have been interpreted to determine the fracture and stress field taking into account the geological and structural settings. It was recognized that the averages of fast directions are NW-SE–oriented at all sites, in agreement with the orientation of maximum horizontal stress as well as with the strike of the main fault structures. The mean values of normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for L'Aquila region, Alto Tiberina Fault area and Val d'Agri basin, suggesting a 3-4% of crustal anisotropy. Moreover, for each area, the spatial distribution of anisotropic parameters is examined, and for L’Aquila 2009 seismic sequence also temporal distribution is discussed.