GNGTS 2015 - Atti del 34° Convegno Nazionale

40 GNGTS 2015 S essione 2.1 The characteristic parameters of each anomaly were obtained by this first analysis (Tab. 3). The considered anomalies are those in which radon values exceed the value of 2-sigma. The Duration is the time interval, expressed in days, months, years, between the start and the end of the anomaly. The Amplitude is the maximum variation, per cent, of the radon value over the 2-sigma value. The Precursor Time is the period between the start time of the anomaly and the occurrence of the linkable earthquake. Distance is the longest distance between the radon monitoring site and the epicentre of the linkable earthquakes. In the other radon time series, an influence of the temperature was detected, but it does not affect the determination of the anomalous periods. For example, the PCA analysis applied to the radon series acquired at Torre Pellice site did not eliminate, in the correct function, the anomalies detected in the original series, but rather made them more evident. Although the results obtained from the analysis of PCA seem to suggest that, although at first sight, temperature and pressure may influence the radon behaviour, there is not an objective correspondence between the meteorological parameters and radon values. As the radon feature, present in most of the data, is a seasonal variation, and considering that the work was also aimed at finding a method, applicable to all the acquired series to clean up them from the influence of meteorological parameters, we tried to identify the seasonal component for all the studied series and to find the most suitable method to eliminate it from them. We chose the ProgramAutoSignal V1.5 (Systat Software, 2004) for Windows. It lets you automate spectral analysis, time-domain analysis, and signal processingwithout programming. To use the application, you have to select the analysis techniques, the algorithm and the options from a menu or toolbar . The software then provides immediate feedback via 2- and 3-D graphs and numeric summaries. Built-in procedures include FFT, autoregressive moving average, complex exponential modelling, minimum-variance methods, eigenanalysis frequency estimation, and wavelets. A library of six Fourier-spectrum methods lets you see a complete picture of the frequency space. A set of 30 data-tapering window functions allows you to solve the leakage problem of standard FFTs. Lomb- Scargle Fourier-domain analysis techniques permit you to handle unevenly sampled data. For theconsidered timeseries, theprogramalsohighlightedan important content at frequencies corresponding to less than one year periods, but only the annual component, attributable to a seasonal variation, was eliminated. It has been avoided removing true anomalies attributable to geodynamic causes. With this method as well, the corrected series shows a variation with respect to original series that does not exceed 2%. Once again, the anomalies detected on the series consisting of the original data without any type of correction are present as well in the corrected series (Fig. 2). Fig. 2 – Daily averages of radon time series acquired at Cazzaso -Friuli-: original data (blue line), data corrected for the seasonal component (red line): The 2-sigma over different periods are marked as reported in legenda.