GNGTS 2013 - Atti del 32° Convegno Nazionale

The Casaglia surface site, OG010, is a free field station installed at the top of the borehole FERB station. The instrumentation installed is a Lennartz velocimeter with a natural period of 5 seconds. The Casaglia borehole station (FERB) uses a Guralp 360 seconds coupled with an accelerometer CMG-5T (Pesaresi et al. , 2012). All the instrumented sites can be classified as C soil, according the EC8 (soft soils, with Vs in the range 180-360 m/s). Downhole seismic test at the Casaglia site gave the opportunity to constrain the empirical shear waves profiles deduced by empirical methods (e.g. ESAC). The velocity profiles deduced by ESAC method at three sites of the OGS network (Priolo et al. , 2012) are very similar, with a very low value at the surface (120-150 m/s), and increases with depth (215-250 m/s). One site shows slower Vs values, so that it stands on the edge with soil type D (Vs < 180 m/s). This site experienced severe liquefaction phenomena during the main shock of May, 20 2012. The Casaglia site, where direct measurements are available in the first 80 m (Margheriti et al. , 2000), shows the main discontinuity at 15 m, related to the water table, with sharp velocity increase of shear waves (higher than 200 m/s), while down to 80m velocity increases gently with depth. The acquired data were converted from native format to miniSEED format, according to SEED Standard, and archived on OASIS database, where any piece of waveform can be downloaded. The OASIS Web page includes the extracted waveforms of the main events as well as the strong-motion parameters, which will be usable as search keys in the ITACA database. Data from FERB station are available in real time by accessing the portal EIDA (http://eida.rm.ingv.it) . Data. The dataset consists of recordings related to the events of 2012 Emilia sequence; the new data acquired in 2013, referred to the regional events of Lunigiana (mainshock June, 21), Central Adriatic (Ancona sequence following the mainshock of July, 21), some local events with a magnitude greater than 3 (Bondeno, Ml=3.8, April, 5; Bolognese, Ml=3.5, June, 14; Talbignano, Ml=3.5, June, 19) and the event of the Croatia, Ml=4.7, on July, 30, were recorded. For the events of 2012, we extracted the waveforms according to the preliminary OGS Bulletin (July 2012) for magnitude Ml> 3.7. After the bulletin revision (March, 2013), some events have been corrected for the magnitude, which was less than 3.7, but the quality of the records was very good so that they are retained in the list of top events. For the events of 2013, we considered the OGS preliminary bulletin (RTS, July 2013) for local events of low magnitude, and the INGV bulletin (ISIDE, July 2013) for regional events of magnitude greater than 3.0. A total of 46 and 58 events were considered for 2012 and 2013, respectively. Details on selected events can be found in Barnaba et al. (2013). Extracted time series have been visually-inspected by the operator and the events considered are only those whose S-phase recognition is certain at least for two sites. To obtain a homogeneous dataset from the recorded velocity and acceleration time series, the data were processed by removal of means and trends, instrument correction with poles and zeroes to obtain displacement time series, band-pass filtering between 0.2 and 45 Hz, and simple and double differentiation to obtain velocity and acceleration time series. Site response. Site response is estimated considering the conventional spectral ratios both single-station, either by reference site, considering earthquakes and ambient noise. The Casaglia borehole station was taken as a reference site, after correction of the time series to take into account the up-going and down-going wave propagation. Spectral ratios are calculated by three different techniques: the classical reference-site- spectral-ratio (RSSR: Borcherdt, 1970), between the site of interest and the reference site of Casaglia (FERB), after time series correction according to Bindi et al. (2013). The borehole time series are multiplied by the factor of 2, for the free-surface effects, assuming vertical incidence); the generalized inversion technique (GIT: Andrews, 1986), considering asynchronous events; the spectral ratio between the horizontals and the vertical components for each event, for each station (EHV: Lermo and Chávez-García, 1994) . 181 GNGTS 2013 S essione 2.2