GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 21 di Asti, Ravenna Fm.); ii) a Pliocene group mainly characterized by clay, silt, and sand (e.g., Argille del Santerno, Portocorsini Fm., and Garibaldi Fm.); iii) a Messinian group consisting of sand, clay, and sandstone with gypsum (e.g., Gessoso-Solfifera and Colombacci Fms.); iv) a Flyschoid group consisting of syn- and post-orogenic terrigeneous sequences (e.g., Marne di Gallare, Marnoso-Arenacea and Cervarola unit); and v) a Meso-Cenozoic calcareous and marly sequence (e.g., Scaglia and Maiolica Fms.) present everywhere below the foredeep basin units. The whole Po Plain area have been historically interested by diffuse seismicity (Chiarabba et al. , 2005; ISIDe Working Group INGV, 2010). In addition the 2012 Emilia seismic sequence bring important information regarding the geometry of northern Apennine fold-and-thrust outer belt (Chiarabba et al. , 2014; Carannante et al ., 2015). The May 20 and 29, 2012 mainshocks- aftershocks seismic sequences concentrated within the Mesozoic–Tertiary carbonates (Govoni et al. , 2014 and reference therein) below the terrigenous sedimentary cover, compatibly with the reactivation of pre-existing normal faults inherited from the previous Mesozoic extensional tectonics (Chiarabba et al., 2014). The structure of the “Sant’Alberto” gas reservoir, located on a blind ramp anticline, has been already highlighted, as an example, in Boccaletti et al. (2004). According to the recent literature concerning the relocation of the regional seismicity (Chiarabba et al. , 2014; Carannante et al. , 2015), this structure, does not appear to have been affected by the 2012 Emilia seismic sequence. Evaluations of ambient seismic noise. The analysis of ambient noise has been performed by computing the power spectral density (PSD) of the seismic signal recorded in some test sites located within the surface projection of DI, and comparing the corresponding probability density functions (PDF) with the standard reference curves NHNM (New High Noise Model) and NLNM (New Long Noise Model) obtained by Peterson (1993). In particular, a seismic station (SPCA) was installed in the inner area of the SM1d well, in order to evaluate the day- night variation of the average level of ambient noise. The recorded signals are transmitted in real time to the INGV data acquisition center of Milan. To quantify the level of noise and its fluctuation, the signal is processed using the PQLX software (http:earthquake.usgs.gov/ research/software/pqls.php). Based on the algorithm developed by McNamara and Buland (2004), the software evaluates in real time the probability density function (PDF) of the power spectral density (PSD) for stationary random seismic data. By comparing the observed PDF of ambient seismic noise with the reference curves of Peterson (1993), it results that, in the frequency band 1-3 Hz, the mean levels of ambient noise observed at SPCA are about 12 dB above the NHNM curve while at higher frequencies, the noise decreases up to 10-20 dB under the curve. Moreover, during the daily recording periods, the signal shows several transient disturbances. This site is thus characterized by elevated average levels of anthropic noise and by a high frequency of transients. The characterization of the seismic noise of the area has been carried out by extending the noisemeasures in four additional sites locatedwithin the surface projection of DI.Ameasurement campaign, performed by means of four temporary stations that recorded simultaneous sequences of 1 hour ambient noise, was carried out. Fig. 3 shows the position of the employed stations. The analysis of results shows that all of the considered sites are affected by elevated levels of ambient seismic noise, with power spectra distributions generally comparable with the NHNM curve of Peterson (1993) in the range of frequencies: 1-20 Hz. Differences up to 10-20 dB are observed in the mean ambient noise levels within the investigated area, depending on the position of the considered site with respect to the anthropic sources of disturbance. Microseismic network design. The analysis of noise presented in the previous section are employed for evaluating the sensibility of the network to be designed. For this purpose, we hypothesize that the areas where noise measurements have been carried out, can be characterized by different levels of noise, depending on the accuracy adopted in the selection of the installation site. We also hypothesize the possible installation of a borehole station at