GNGTS 2015 - Atti del 34° Convegno Nazionale

- ET-1: Flat spectral ratios showing amplitude values not exceeding 2 units; - ET-2: HVSR having fundamental period T ≤ 0.2 s with amplitude exceeding 2 units; - ET-3: HVSR with fundamental period 0.2 < T ≤ 1.0 s with amplitude exceeding 2 units; - ET-4: HVSR showing broad band amplitude exceeding 2 units; The AF of each class were then obtained by averaging the AF of all the stations included in the considered classes (Fig. 2a). ��� �������� ������ ���� � ���� ����� ���� ����� �������� The obtained curves show a good match with those proposed by Luzi et al. (2011) for the whole Italian territory. It has to be specified that the long period seismic site effects were not considered and the predicted amplification functions for the Etna area display higher values compared to Luzi’s functions, as a consequence of the low shear wave velocity assigned to the fractured lavas. Application of the proposed classification on low eastern Etna flank. T�� ���������� he comparison of HVSR and HVNR on the 19 seismic stations all around the Etna volcano allowed us to verify that the noise can replace the earthquakes data in seismic site response estimate. Then in order to evaluate the fundamental period of an investigated site, required for a predominant period classification, it is possible to use the fast and inexpensive HVNR technique. An attempt to use the achieved classification with an extensive noise measurements field, in the eastern flank of Mount Etna, was therefore made. Using a three components velocimeter, 130 noise measurements, georeferenced by means of a GPS instrument, were almost homogeneously spaced along a grid having size of about 1x2 km, extended from north to south of the eastern side on Mt. Etna. Time series of 30 minutes length were recorded using a sampling rate of 128 Hz and processed through the HVNR technique. Time windows of 30 s were considered and the most stationary part of the signal was selected excluding transients associated to very close sources. In this way the Fourier spectra were calculated in the frequency range 0.1-30.0 Hz and smoothed using a proportional 20% triangular window. Following the criteria suggested by the European project Site EffectS assessment using AMbient Excitations (SESAME, 2004), only the spectral ratio peaks having amplitude greater than two units, in the frequency range 0.5-10 Hz, were considered significant. To summarize the obtained results, the HVNR were subdivided into 16 groups showing a similar shape ����� ��� ������� ��������� �������� � ������� �������������� �� using the cluster analysis. Finally, a further classification of the HVNRs was manually performed ����� ��� ������� ����������� ������ �������������� ���� using the adopted predominant period classification (see results in Fig. 2b). Fig. 2 – a) Mean site amplification (red curve) and standard deviation (dashed black curve) for the Etnan area. b) Examples of classification based on the use of HVNR. GNGTS 2015 S essione 2.2 145