A source-controlled experiment to investigate the origin of wavefield polarization in fault zones

In the damage zone of the Pernicana Fault on Mt. Etna, Italy, ground motion is strongly polarized in the horizontal plane. Dense measurements of ambient noise provide H/V spectral ratios that exceed a factor of 10 around 1 Hz near the fault trace and decrease away from the fault. Across the fault, polarization azimuth varies approximately from N140° to N170°. In principle, both locally radiated energy due to fault creeping and a site/propagation effect could be invoked to explain observations. Earthquake recordings in the fault zone show a similar behavior suggesting a faultrelated site effect, as the polarization direction is independent of azimuth, distance and depth of different sources. In order to investigate the directional amplification mechanism, we performed a sourcecontrolled experiment using a high-resolution Vibroseis machine (Ivi-MiniVib®). The machine was operating about fifty meters from the fault scarp, where the natural site polarization was oriented N150°. Ground motion was produced through a iron plate vibrating in the horizontal plane with fixed azimuths of motion, parallel and transversal to the observed site polarization. Threecomponent seismological stations were installed along profiles in the two orthogonal directions with a 50-m spacing. When shear excitation is parallel to the site polarization, the seismic signal propagates efficiently maintaining the same horizontal polarization of the source and is well recorded up to distances as large as 300 m. When shear excitation is orthogonal to the site polarization, the ground excitation looses the initial source polarization in less than 50 m. At larger distances, transmitted energy propagates with the natural site polarization independently of the source polarization. These experimental results suggest a shallow origin of the wavefield polarization in fault zones, probably due to scattering and mode conversion related to heterogeneities of the predominantly oriented fractures.