GNGTS 2015 - Atti del 34° Convegno Nazionale

Fig. 1 – Web page of ITACA database for AVQ (L’Aquila – V. Aterno – Centro Valle) RAN station. Measurements of ambient noise. In ITACAdatabase, about 400microtremor measurements are available. The major of them was provided by the Department of Civil Protection through 2 campaigns conducted in the period 2011-2014. In order tomake thesemeasurements comparable, all noise-records were processed with the same procedure (Puglia et al., 2011). The main steps of the processing are: i) application of 1st order baseline correction and a 4th order acausal Butterworth band-pass filtering to the entire time series; ii) subdivision of each record in windows and definition of the relevant minimum expected f 0 , following the criteria recommended in the SESAME project (2005); iii) application of 1st order baseline correction and a 5% cosine taper to each window; iv) calculation of the Fourier spectra and smoothing through the Konno and Ohmachi (1998) algorithm, fixing the relevant parameter b to 40; v) computation of the Horizontal –to-Vertical-Spectral Ratio (HVSR) values for each window as the ratio between the vector summation of the Fourier spectra of horizontal components and the spectrum of the vertical component; vi) HVSR curve computation by averaging the single HVSR curves, considering a log-normal distribution of amplitudes to obtain the mean with the relevant 68% confidence interval. Windows exhibiting large transients were generally excluded from the analysis; these windows were visually identified from the velocity time series and from the single window Fourier spectra and spectral ratio. In the analysis of the resonance peaks, the minimum expected f 0 was always set to a value equal to or higher than the lower frequency recordable by the instrument. For example, in the case of Lennartz LE-3D/5s sensor this value is set to 0.2 Hz. For each microtremor measurement, a report (Fig. 2) is generated and published in ITACA on the station monograph under the section “Microtremor H/V spectral ratio”. In this report, GNGTS 2015 S essione 2.2 123