Ground Motion Polarization in the Damage Zone of the Active, Strike-Slip Mattinata Fault, Southern Italy

We have recently observed the occurrence of directional amplification effects in fault zones using both earthquakes and ambient noise records. In several faults we have found that ground motion polarization tends to have a high angle to cleavages produced by the stress related to the kinematics in the fault damage zone. We thus interpret this effect as due to the higher compliance of the fractured rocks of the damage zone in a direction transversal to the cleavage strike. Here we have tested the technique of the wavefield polarization using ambient vibrations recorded across the seismically active Mattinata Fault, in the Gargano Promontory, Italy. This fault has been chosen for the high number of structural investigations led out so far. The Mattinata Fault outcrops for over 40 km and shows an ondulated trajectory that is characterized by a number of significant tectonic-related morphological features compatible with a general left-lateral strike-slip kinematics. These features include a pull-apart basin and a transpressional zone. The main associated cleavage consists of a marked array of disjunctive, spaced pressure-solution surfaces developed within the 200-300 m wide fault damage zone. In order to relate the orientation of cleavage to the ground motion polarization, we measured 20-50 min of ambient noise at about 30 sites chosen in the fault damage zone close to rock outcrops where also structural geological measurements were carried out. Ground motion polarization is assessed both in the frequency and time domain through the individual-station horizontal-to-vertical spectral ratio and covariance-matrix analysis, respectively. Two ambient noise measurements were performed close to permanent broadband stations of the Italian Seismic Telemetric Network. Results are consistent with those inferred on earthquake records at the two permanent stations, confirming that ambient noise yields results consistent with earthquake records as previously observed in other studies. We found that, in spite of the high complexity of results, the observed polarization pattern is not random and that there is a general tendency of polarization to be oriented orthogonally to the outcropping pressure-solution cleavage. In the segments where the Mattinata Fault presents pure strike-slip kinematics, most polarization measurements confirm the orthogonal angle to the fault-related cleavages. Similar results are also found from the polarization measurements in the pull-apart region. In both situations, we locally found sites where a different polarization occurs and it is often associated to a corresponding orthogonal cleavage. More complex seems the comparison in the transpressional zone of the fault. This might be due to the complexity of deformation of transtensional fault regimes that prevents from the development of regionally homogeneous fracture sets required to produce ground motion polarization. A further campaign with a higher spatial density of both ground motion polarization and fracture measurements in the transpressional zone might solve the found puzzle. With the exception of this latest case, the analyses on the Mattinata Fault confirm the existence of a high angle (orthogonal) relation between ground motion polarization and fractures, and the proposed model.