GNGTS 2013 - Atti del 32° Convegno Nazionale

Reference site methods (RSSR and GIT) facilitate the identification of the resonance frequencies, both low-frequency (0.5-1 Hz) and at higher frequencies (1.8 Hz, 2.8 Hz); the adoption while of the single station method (EHV) is not always clearly interpretable, considering both ambient noise that seismic events as shown in Fig. 2, where there emerges a strong discrepancy between EHV and NHV. This difficulty to identify clearly the resonance frequency is not related to the energy of events: the sites OG006, OG007 and OG008 show that the results are practically identical, also considering different datasets and this observation verify the same site response in the case of both strong local earthquakes and regional events. The amplifications of site OG010 considering the borehole ground motion corrected by multiplying the time series by a factor of 2 (according to Bindi et al. , 2013) are in good agreement with those define by Margheriti et al. (2000) obtained multiplying spectral ratios by the coherence between surface and borehole recordings (following Steidl et al. , 1996). The main difference between the two computations is the level of amplification of the fundamental peak at about 0.8 Hz. While our analysis has the same values at all amplified frequency peaks (0.8, 1.8 and 3 Hz), Margheriti et al. (2000) show more amplification at higher values than at the fundamental one. These discrepancies could be related to the largest epicentral distance and magnitude of the 2013 dataset, compared to the local and weak 2000 dataset (Margheriti et al. , 2000), that could not include events with enough energy to excite the low frequency of 0.8 Hz. Comparison observed-predicted. The major events of the Emilia sequence were used to test the prediction of the ground-motion attenuation laws and ground shaking (ShakeMaps). According to the present-day seismic hazard classification, the studied area is characterized by an expected horizontal peak ground acceleration (PGA) between 0.10 g and 0.15 g, with a 10% probability of being exceeded in 50 years. The observed PGA was compared with the values predicted by the ground-motion equations developed by Bindi et al. (2011). Assumptions were the geometric mean of horizontal components, a reverse fault mechanism, and that all the stations belonged to a soft-soil classification (class C, Vs < 360 m/s). From the dataset used, the PGA observed values are in good agreement with the predictions of the attenuation law. For the same dataset, the observed PGA values are compared with the data provided by ShakeMaps (Michelini et al. , 2008). INGV ShakeMaps (http://shakemap.rm.ingv.it/shake/archive ) have been downloaded in grid.xyz file, containing a dense grid of geographic coordinates (latitude and longitude) and the shaking values, expressed in units of percent-g (g%). For each event, the values closer to each OGS stations were extracted. The research was done with a radius of 500m from the station. ShakeMap values obtained were averaged and referred to the station. Fig. 3 summarises the differences between the theoretical PGA of the ShakeMap and the PGA observed at the station. The greatest differences arise at OG003 site for stronger events, where the observed data are higher than the predicted data. At site OG004, on the contrary, the predict values are in large part overestimated, in particular for mid PGA values, while for stronger events the accordance between the two values are good. Such differences observed at those sites may be related to geological peculiarity of the soils, which suffered strong liquefaction phenomena during the main events of May, 2012. In theother sites, theobservedPGAvalues areunderestimatedby the theoretical computations for weak events, while for the higher magnitudes more agreement can be observed. Conclusions. The activities carried out within the project have provided a considerable amount of original seismological data in a poorly instrumented area, recording significant events of both the 2012 Emilia Sequence, and the regional events occurred in 2013. The entire dataset of continuous waveforms has been made available on the OGS web OASIS Database. For the largest events of the Emilia Sequence strong-motion parameters were calculated and published on the OASIS web database. Event time series and metadata (site monographs) are available similarly to the ITACA Database. Since February 2013 the Casaglia station is operating, with a very broad-band borehole sensor and a mid-period surface sensor. 183 GNGTS 2013 S essione 2.2