Evidence of shear-wave anisotropy in the upper crust of central Italy

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Abstract
Shear-wave polarization analysis has been performed on data from twelve earthquakes recorded in central Italy. These data are part of a set of high quality seismograms recorded by a three-component digital network installed in Abruzzo region after a Ms = 5.8 earthquake on 7 May 1984. Analysis performed on 2 to 6 Hz bandpass-filtered seismograms reveals shear-wave splitting. In order to determine the direction for which the time separation between the two split S waves is maximum, the horizontal traces are rotated in the range 10 to 90° with steps of 10°. At each step the cross-correlation between the two shear waves and the time separation is estimated. The azimuth of 50°N yields the highest correlation coefficient and the maximum time separation of about 0.09 sec. This direction and the orthogonal to it represent axes of minimum and maximum shear-wave velocity respectively. If the time delay is distributed over the whole hypocenter-seismic station path, the maximum variation in velocity is at least 1.4 per cent. The theoretical linear polarization of particle motion at the source is verified after correcting the seismograms for the anisotropic propagation effect. A distribution of vertical cracks aligned in the direction 140°N may explain the observed anisotropy. The stress field deduced from the focal mechanism of the Abruzzo earthquake is compatible with this hypothetical crack distribution.