GNGTS 2015 - Atti del 34° Convegno Nazionale
GNGTS 2015 S essione 1.3 145 In particular, anomalous changes in the emission of Helium, typical geochemical precursor of earthquakes, and methane have been observed during the earthquake of Carlentini of December 13, 1990 (D’Alessandro et al. , 1993). However, some authors assert that the overall heat flux from Etna region has shown to be significantly and strongly controlled by the regional structural framework (Minett and Scott, 1985). Method. A quick estimate of the surface geology effects on seismic motion is provided by the horizontal to vertical noise spectral ratio technique (HVNR). This technique firstly introduced by Nogoshi and Igarashi (1971), was put into practice by Nakamura (1989) and became in recent years widely used since it provides a reliable estimate of the fundamental frequency of soft soil deposits. The good agreement observed between results obtained using earthquake records and ambient noise has pointed out that microtremors are a valid tool to investigate ground motion polarization properties (Rigano et al. , 2008; Di Giulio et al. , 2009; Panzera et al. , 2013; Panzera et al. , 2014). Ambient noise recordings were performed randomly in the area where the main activity of Salinelle is located (yellow points in Fig. 1b) and along two profile Tr#1 and Tr#2 (red and green points in Fig. 1b). A total number of thirty-two recording ����� ���� ������ ����������� ��� sites were used�� ����������� ��� to investigate the main features of the area. ���� ������ �� �� ������� ������ ���� �������� ������� � ���� ������ Time series of 30 minutes length were recorded through a long period velocimeter, using a sampling rate of 256 Hz and processed through the HVNR technique. According to common assumptions ������ ����� ������� (Bard, 1998; Parolai et al ., 2001), the shortest window length of the signal has to be selected in a way that at least 10 cycles of the lowest frequency analyzed are included. ����� ���� ������� �� ��� � ���� ���������� ��� ��� ���� ���������� ���� �� ��� Then, time windows of 100 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 0.05-20.0 Hz and smoothed using a proportional 20% triangular window. ������� ��� ��������� ���� ���� �������� ���������� ��� ����������� Finally the resulting HVNR were computed estimating the logarithmic average of the spectral ratio obtained for each time window, selecting only the most stationary and excluding transients associated to very close sources. ��� ������������ �������� ������ ���� The experimental spectral ratios were also calculated after rotating the horizontal components of motion by steps of 10 degrees starting from 0° (north) to 180° (south) in order to investigate about the possible presence of directional effects. Examples of the results obtained are ������� �� ������ � ����� ������� ����� �� ���������� plotted in figure 2 using contour plots of amplitude, as a function of frequency (x-axis) and direction of motion (y-axis). However, in presence of lateral and vertical heterogeneities or velocity inversion, the HVNR can be “non-informative” due to the occurrence of amplification on the vertical component of motion (Panzera et al. , 2015). Thus in this study we also computed a direct estimate of the polarization angle, for noise data by using the method proposed by Jurkevics (1988). This technique is very efficient in overcoming the bias linked to the denominator behavior that could occur in the HVNR’s technique. Polarization analysis makes full use of the three component vector field to characterize the particle motion and it is based on the evaluation of eigenvectors and eigenvalues of the covariance matrix obtained by three-component seismograms. Signals at each site were band-pass filtered using ��� ����� ���������� ��� ����������� � ������ ������ the whole recordings and considering a moving window of 10 s with 20% overlap, therefore obtaining the strike of maximum polarization for each moving time windows�. The dynamic site properties and, in particular, the shear wave velocity of Salinelle deposits were investigated through non-invasive techniques such as the Multichannel Analysis of Surface Waves (MASW: ����Park et al. , 1999) ��� �������� ������� ��������������� ������ and Extended Spatial AutoCorrelation (ESAC: Okada, 2003)� ��� �������� ��� �� ��������� ���������� ������� �� �� ������� ��� ����� ��� . The combined use of different techniques allowed us to compare and check the obtained results going also all over the limitations of each methodology. A “L” ����� ������������� ��� ���� ��� ��� ���� ������������� ��������� �� ������� �� array configuration was used for the ESAC measurements, recording 20 minutes of noise (blue lines in Fig. 1b). The array was settled using a 26-channel seismograph and 4.5 Hz geophones. The length was 60 m in NE direction and 70 m in NW direction. Time windows of 20 s were considered to calculate dispersion curves of the fundamental mode and the average of the dispersion curves was computed, excluding those not showing a clear dispersion or in which
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