GNGTS 2013 - Atti del 32° Convegno Nazionale

During this second stage, accelerometric data were continuously recorded at 100 sps for two months up to 9 January 2013 and, such as for the ETNA accelerometer, the NS and EW components of all GeoSIG accelerometers were oriented along the transversal and the longitudinal directions with respect to the main axes of the structure, respectively. Since up to 9 November 2012 we collected data at the second floor of the building, including all the foreshocks and the main event (Tab. 1), in this paper we focalize the attention on the analyses carried out only on data acquired at the second floor of the building during both the stages of acquisition. The hypocentral distances of the analysed foreshocks and aftershocks ranged from 6.6 km to 19.3 km, with PGA between 0.003 and 0.07 g, while the hypocentre of the Ml=5.0 earthquake was at 10.6 km distance from the building with PGA of 0.6 g. Fig. 2 shows the acceleration response spectra along the transversal and the longitudinal directions. Acceleration response spectra were evaluated for all the 6 foreshocks, the main event, and the 7 aftershocks recorded by ETNA-Kinemetrics accelerometer (Tab. 1). It is possible to observe that the results of foreshocks and aftershocks have similar behaviour with fundamental peaks of about 0.2 s, while the response spectra of the main event have fundamental peaks at 0.3 s along the transversal direction and 0.3-0.4 s along the longitudinal direction. Such difference has been confirmed following the temporal evolution of the main frequencies with high accuracy through the S-transform analysis (Stockwell et al. , 1996). Fig. 3 displays the S-transform of the mainshock (Ml = 5.0) evaluated up to 10 Hz along transversal and longitudinal directions and up to 20 Hz along the vertical direction, where it is evident a higher frequency contribution of P-waves. During the main amplitude of S-waves there is a decrease of frequencies down to 2-3 Hz (periods of about 0.3-0.5 s) along the longitudinal direction and frequencies down to 3 Hz (periods of about 0.3 s) along the transversal direction, Fig. 3 – S-transform of the mainshock evaluated up to 10 Hz along the transversal (top left) and longitudinal (top right) directions, and up to 20 Hz along the vertical direction (bottom). 444 GNGTS 2013 S essione 2.3