Evidence for ground motion polarization on fault zones of Mount Etna volcano

Abstract

[1] During local and regional earthquakes, an evident amplification of horizontal ground motion is observed at two seismological stations near the Tremestieri fault, on the southeastern flank of Mount Etna volcano. Rotated component spectral ratios show a narrow spectral peak around 4 Hz along a N40°E direction. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal ground motion in the fault zone. The effect is evident during the entire seismogram and independent of source back azimuth as well as distance and depth of earthquakes. The same polarization is observed in ambient noise as well. This consistency allowed us to use microtremors for checking ground motion polarization along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area that can be observed in a broad frequency band. To check whether this ground motion property is recurrent and to understand a possible relationship with fault strike, faulting style, or orientation of fractures, ambient noise was recorded on other mapped faults of the Mount Etna area, the Moscarello, Acicatena, and Pernicana faults. The latter, in particular, is characterized by different strike and faulting style. A systematic tendency of ambient noise to be polarized is found in all of the faults. A picture emerges where normal faults of the eastern flank show an E-W to NE-SW polarization that changes on the Pernicana fault, which develops approximately E-W and is characterized by a prevailing NW-SE to N-S polarization. Directions of polarization were never parallel to the fault strike. Moreover, polarization persists too far away from the fault trace, excluding an effect limited to a narrow low-velocity zone hosted between harder wall rocks. Both these observations rule out an interpretation in terms of faulttrapped waves. The cause of observed polarizations will be the subject of future studies. However, the consistency with recent results of velocity anisotropy in a part of the investigated area suggests a possible role of attenuation anisotropy on horizontal amplitude variations versus azimuth.

Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

“…The azimuth pattern of HVSR complements the rose diagrams giving (1) clear indication of the frequencies where directional effects occur, and (2) the level of horizontal ground motion compared to the vertical one. As found by Rigano et al [2008] at Mount Etna, the directions inferred from the azimuth pattern of HVSR are consistent with those provided by the covariance matrix method.…”

“…Results of conventional covariance matrix analysis are also shown in Figure 6 as rose diagrams. The polarization vector at each station was measured by diagonalizing the covariance matrix using the technique already adopted by Rigano et al [2008]. It moves a time window of 2 s with a 50% partial overlap throughout the entire tremor record of each site (the same used for the computation of HVSR).…”

“…The conventional covariance matrix method is applied to data after band‐pass filtering in the frequency band 0.4–6 Hz. Second, we compute the horizontal‐to‐vertical spectral ratio (HVSR) as a function of the frequency and direction of motion as introduced by Spudich et al [1996] and successively exploited by Cultrera et al [2003] and Rigano et al [2008]. The horizontal plane is divided into a set of directions uniformly spaced at intervals of 10°, from 0° (north) to 180° (south).…”

The role of alternating outcrops of sediments and basaltic lavas on seismic urban scenario: the study case of Catania, Italy

“…In particular, Biondi and Maugeri (2005), Lombardo et al (2004Lombardo et al ( , 2006, Catalano et al (2005), Lombardo and Rigano (2007) evaluated the local seismic response in test sectors of the urban area, whereas preliminary estimates of site response in the whole Catania area were obtained by Giampiccolo et al (2001), Lombardo et al (2001), Priolo et al (2005). Further investigations on the local effects due to fault zones (Rigano et al 2008) as well as natural cavities Lanzo et al 2006;Sgarlato et al 2009) have been recently performed and others are still in progress.…”

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities

“…In Figure 3, some examples of polarization plots obtained by Rigano et al (2008), from microtremor measurements performed along and across the Pernicana and Tremestieri faults, are reported. The authors analyzed both earthquake and ambient noise records and found that in all investigated sites the ground motion polarization azimuths were principally NW-SE and NE-SW oriented, for the Pernicana (Fig.…”