THE VARIATION OF GROUND MOTION POLARIZATION ACROSS THE PERNICANA FAULT, MT.ETNA

Abstract

Across the Pernicana fault on Mt. Etna, Di Giulio et al. (2009) found a significant and persistent variation in the polarization angle when moving from the fault hangingwall to the fault footwall. This effect was recurrently observed on several stations and both on volcanic tremor and ambient noise. In this work we propose an interpretation of this variation, calculating the brittle deformation pattern associated to the fault through the package FRAP3 (Salvini, 2002). The Pernicana fault system represents the Northern boundary of the main flank instability of Mt Etna volcano, from the eastern to the south-western portions of the volcanic edifice, where down-slope movements are produced with significant slip rates. It reaches a length of more than 18 km, and the kinematics is mainly left-lateral even though a transtensive component is locally present due to the flank instability. The western portion of the Pernicana fault system, striking N90° and close to Piano Pernicana area, is characterized by the most intense deformation (Acocella and Neri, 2005). In this area Di Giulio et al. (2009) performed volcanic tremor and noise measurements on a dense grid along and across the fault zone, in order to calculate the local polarization azimuths. The Horizontal-to-vertical spectral ratios (HVSR) showed large directional resonances of horizontal components within the damaged fault zone, resonance everywhere occurring around 1 Hz. Conversely the polarization azimuth varies from N160 at stations installed on the fault hangingwall to N120 at stations lying in the fault footwall. Previous studies (e.g. Pischiutta et al., 2011) successfully related that ground motion horizontal polarization in fault zones can be produced by the brittle deformation fields in the damage zone, with a predominant near-perpendicular relation between fractures and polarization strikes. Given this premise, 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 trantensive 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 found by Di Giulio et al (2009) to the distribution of the fracture systems, which appear 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.