GNGTS 2016 - Atti del 35° Convegno Nazionale

386 GNGTS 2016 S essione 2.2 method allowed us to better quantify the horizontal polarization of the ground motion, giving a clear indication that ambient noise is affected by a significant horizontal polarization at the measurement sites along and across the investigated faults. It is indeed interesting to observe that the recorded ambient noise is polarized in a narrow frequency band (1.0-4.0 Hz), following a roughly east-west trend (e.g. Fig. 3e) that is almost perpendicular to the fault strike. Moreover the TF polarization analysis revealed values of the ellipticity reaching a minimum in a wide frequency band (1.0-4.0 Hz) as well as dip values showing an horizontal trend in the same frequency band (see examples in Figs. 3f and 3g). We stress that in this area geological investigation found clear evidence of the presence of the fault, as fractures on buildings and concrete road walls. It is clear that the maxima of the horizontal polarization occur in the north-east to east-north- east direction, although in some cases the high frequency directionality is more complex. As observed by Burjánek et al. (2010), high-frequency ground motion can indeed be controlled by the vibration of smaller blocks that imply both different resonant frequencies and directions. Burjánek et al . (2012 and reference therein) point out that the ambient noise polarization takes place at about 90 degree angle to the observed fractures which are perpendicular to the sliding direction. In the present study, the polarization angle is parallel to the opening cracks, which appears in contrast to the above mentioned results. A possible explanation of our findings is that there exists a prevailing north-easterly sliding direction of the landslide body, which is strongly affecting the polarization direction especially in the 1-10 Hz frequency range. Conclusions. Ambient noise measurements performed along the southern segment of the Acireale Fault pointed out directional amplifications that are also confirmed by the results of the time–frequency analysis, the largest amplification occurring with high angle to the fault strike. The nature of the observed site effects is highly complex as a number of different mechanisms, such as near-surface structures and slow gravitational deformations, contribute. Consequently, Fig. 1 – Location of the ambient noise measurements, SCT1 is the location of the measurement site shows as example (a); Example of HVSR (b), FFT (c), rotate HVSR (d), polarization polar plot (e), strike polar plot (f) and ellipticity (g) results obtained at SCT1 site, located in the fractured zone.