Ground motion polarization on fault zones: observations on Mt. Etna volcano and Hyblean area

Studies about local seismic response nearby fault zones have been recently performed in the Etnean area using earthquakes and ambient noise records. A systematic polarization of the horizontal ground motion was observed during local and regional earthquakes near faults located on the eastern flank of Mt. Etna volcano. The same polarization effect is confirmed by ambient noise measurements. New measurements have been performed in the western flank of the volcano. In this area, two segments of faults, belonging to the Ragalna system, were selected and a total of four transects, perpendicular to the fault strike, having noise measurement points at increasing distance from it, were performed. The results show a stable polarization angle of horizontal motion, in the frequency band 1.0-10.0 Hz, nearby all the investigated faults, with angles of about 60° from the faults strike. Tests were performed to check if the observed directional effects keep the same orientation at increasing distance from the fault lines. Observations point out that directions coming from both rotated spectral ratios and rose diagrams tend to become randomly distributed and/or uniformly scattered as the distance from the investigated faults increases. Investigations performed in a non volcanic area, located in south eastern Sicily (Augusta area), show similar results.