GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.1 283 minimum period of 40 seconds), operating continuously from 2015 to 2017, have been selected. A spatial statistical analysis of the data obtained have been performed, in order to find clusters of seismic stations characterized by similar PSD. Furthermore, a correlation between the noise found at the different stations and several maps (rainfall, wind, coastlines) was carried out to comparing the results and characterizing the type of source. Discriminating the noise level in each station may allow to lead improvements in its performance where necessary (for example changing site or put sensor in the well), in order to reduce the noise level in very noisy stations. This improvement could allow to detect even the smaller energetic seismic events sometimes hidden by high noise values. State of art. Noise is a generic term to describe ambient vibrations of the soil caused by natural or anthropic sources. It is possible to distinguish microseisms and microtremors, corresponding respectively to natural and anthropic sources, low (<1Hz ) and high frequency (>1), respectively. At frequencies higher than 1 Hz, seismic noise variation is correlated to human activities, whereas at lower frequencies variation is associated to natural sources (Gutenberg, 1911, 1958, Asten 1978, Asten and Henstridge 1984, Bonnefoy-Claudet et al., 2006). Especially at frequencies higher than 1 Hz, seismic noise systematically shows daily and weekly variations linked to human activities, whereas at lower frequencies the variation of seismic noise is correlated to natural activities (Kanai and Tanaka., 1961, Bonnefoy-Claudet et al., 2006). Furthermore, the noise depends on the characteristics of the site (geology, topography, etc.) (Seo, 1997), local meteorological conditions (wind, meteorological conditions, oceanic waves) and/or anthropic sources (traffic, power plants, factories, machinery); every station is affected by all these factors. The boundary between these two types of noise (microseisms and microtremors) is close to 1 Hz, but many authors say that it can vary depending on the soil structure and maybe human activity. This limit is still much debated today. Seismic noise is determined by the superposition of the wavefields generated by different sources, each characterized by its own energy content and predominant wave type (P-, S-waves, R- and L-waves, and relative modes). It is possible to observe that literature concerning the composition of the seismic noise wavefield is currently very poor and inconsistent in data; in fact there is no common agreement between authors about the nature of noise. From current knowledge is possible to deduce that the relative proportion of different waves (body/surface waves, Rayleigh/Love, fundamental/higher modes) depend on source properties, characteristics of the investigated area (geological characteristics), but also methodological inaccuracy and/or author interpretation issues. Alarge number of authors presuppose that noise is manly composed by surface waves, with often an important contribution given by Love waves (more 50%) (Arai and Tokimatsu (1998, 2000), Yamamoto (2000), Köhler et al., 2007, Bonnefoy-Claudet , 2004). Methods and Data Analysis. Generally, the noise of a seismic stations is investigated by the Power Spectral Density (PSD), depending by the quality installation and the characteristic noise level of the site. PSD represent a mathematical quantity that defines the spectral frequency content of a signal. The set of analyses of more Power Spectra Density given by the Probability Density Functions (PDFs), describes the frequency content in statistical terms, i.e. represents the highest probability of noise level for a given station. Data processing of signals here implemented, follows the method used by Peterson (1993) and McNamara and Buland, (2004). For this study, the analysis was limited to the frequency band 0.025 to 30 Hz, in accordance with the seismic sensors bandwidth. Four different frequency bands have been identified that are: 0.025-0.12, 0.12-1.2, 1.2-10 and 10-30 Hz. To each of these, a main type source has been associated. The 4 classes here established roughly consistent with bibliographic data. Acommon agreement with several authors was found (Banerji, 1924, 1925; Peterson, 1993; Okada, 2003; McNamara and Buland, 2004; and others) about the origin of seismic noise and its frequency features bands, a: the lower frequency seems to be generated by natural sources (from 0.025 to 10 Hz). Instead, anthropic sources are associated to the highest frequencies bands (10-30 Hz) as assumed by several authors (Groos and Ritter, 2009; Bormann and Wielandt, 2013 and others).