Variation of Horizontal Ground Motion Polarization across the Pernicana Fault, Mt. Etna

Across the trace of the Pernicana fault on Mt. Etna, a strong polarization of horizontal ground motion was observed in previous studies, at several tens of stations and both on ambient noise and earthquake records. This polarization showed a persistent variation in the polarization angle when moving from the fault hangingwall to the fault footwall. In this work we propose an interpretation of this variation, calculating the brittle deformation pattern associated to the fault through the package FRAP3. Our hypothesis is that ground motion horizontal polarization in fault zones is produced by the brittle deformation fields in the damage zone, with a predominant near-perpendicular relation between fractures and polarization angles. We modeled the fracture field expected for the Pernicana fault system in the Piano Pernicana sector. We assumed a pure left-lateral kinematics in the hanging wall, while in the footwall that is part of the flank instability we added a slight transtensive component to the strike-slip movement. As a result, in the fault hanging wall the synthetic cleavage has a higher probability to develop, with an orientation toward N75 direction. Meanwhile, the extensional fractures appear to be the dominating fracture systems in the fault footwall, with a modeled N40 orientation. As a consequence, we ascribe the variation in polarization azimuth to the distribution of the fracture systems, which appears to be different in the hangingwall and in the footwall. Consistently with previous studies, a near-perpendicular relation between wave polarization and the dominant fracture field is recognized on the Pernicana fault, due to the reduction of rock stiffness caused by the presence of fractures: horizontal vibrations are far more pronounced in the direction perpendicular to fractures.