Orthogonal relation between wavefield polarization and fast S wave direction in the Val d’Agri region: An integrating method to investigate rock anisotropy

Wavefield polarization is investigated using 200 seismograms recorded by a network of 20
stations installed on rock outcrops in the Val d’Agri region that hosts the largest oil fields in the
southern Apennines (Italy). Polarization is assessed both in the frequency and time domains
through the individual-station horizontal-to-vertical spectral ratio and covariance-matrix
analysis, respectively. We find that most of the stations show a persistent horizontal polarization
of waveforms, with a NE-SW predominant trend. This direction is orthogonal to the general
trend of Quaternary normal faults in the region and to the maximum horizontal stress related to
the present extensional regime. According to previous studies in other areas, such a directional
effect is interpreted as due to the presence of fault-related fracture fields, polarization being
orthogonal to their predominant direction. A comparison with S wave anisotropy inferred from
shear wave splitting indicates an orthogonal relation between horizontal polarization and fast S
wave direction. This suggests that wavefield polarization and fast velocity direction are effects
of the same cause: The existence of an anisotropic medium represented by fractured rocks
where shear wave velocity is larger in the crack-parallel component and compliance is larger
perpendicularly to the crack strike. The latter is responsible for the observed anisotropic pattern
of amplitudes of horizontal ground motion in the study area.

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