GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.1 305 results in a considerable ground motion amplification at the surface. In deep and wide sedimentary basins, geologic, engineering and seismic bedrocks do not always coincide, and the threshold of 800 m/s indicated in the current seismic codes ( i.e. , engineering bedrock) might not be significant for site amplification if does not also correspond to a seismic impedance contrast. In this sense, Mascandola et al. (2019) identifies the seismic bedrock of the Po Plain - one of the deepest and widest alluvial basin worldwide where several cities and critical facilities are present - in correspondence with a marked increase in the mechanical properties of the subsoil materials, which produces a measurable resonance effect at the surface in the medium-to-long-period range ( i.e., 3.33 -1 s, that is 0.3-1 Hz). This corresponds to a marked seismic impedance contrast where the shear-wave velocity approaches, or exceeds, 800 m/s. In detail, to map the seismic bedrock depth we rely on an extensive collection of both existing and newly acquired ambient vibration measurements, with the aim of defining the soil resonance frequencies and the shear-wave velocity gradients within the soft sediments above seismic bedrock. Based on the collected data, an empirical regression model that relates the thickness of the soil deposits above the seismic bedrock to their resonant frequency is defined and applied to map the seismic bedrock depth in the Po Plain area. The resultant seismic bedrock map is correlated with the main unconformities recognized inside the Quaternary succession (Regione Emilia-Romagna,ENI–AGIP, 1998; Regione Lombardia, Eni Divisione Agip,2002). With the aim of providing a long period soil amplification model for the Po Plain, a regional shear-wave velocity model of soft sediments above seismic bedrock is performed through the interpolation of 54 S-wave velocity profiles selected after a quality check on the available data. The velocity gradients highlight two different zones inside the study area: one at Northwest and another at East-Southeast with higher and lower velocity gradients respectively. To compute the soil amplification functions, the velocity model is discretized into a 0.1° x 0.1° grid (~ 8 km x 11 km). For each grid node, a 1D soil model is defined, and a numerical ground response analysis is carried out to compute site amplification functions. The numerical model is verified at those sites with borehole seismic stations, where recordings of the same earthquake are available both at the surface and bedrock depth. Finally,with the aimof determining the influenceof thedeepgeophysical discontinuities on the long-period hazard up to 3 s, the probabilistic seismic hazard assessment is computed at regional scale with the partially non-ergodic approach ( e.g., Rodriguez-Marek et al. , 2014; Kotha et al. , 2017), and considering the most updated scientific improvements in seismogenetic zonation and ground motion prediction equations. The results will be compared with conventional PSHA estimations that accounts for site effects through the application of frequency-independent soil factors derived from seismic codes (European Committee for Standardization, 2004; Ministero delle Infrastrutture e dei Trasporti, 2018), or by adopting ground motion prediction equations (GMPEs) for specific ground types (Barani and Spallarossa, 2017). References Anderson, J. G., Bodin, P., Brune, J. N., Prince, J., Singh, S. K., Quaas, R., Onate M.: 1986; Strong Ground Motion from the Michoacan, Mexico, Earthquake. Science, 233: 1043-1049. Barani, S., Spallarossa D.: 2017; Soil amplification in probabilistic ground motion hazard analysis, Bull. Earthq. Eng. 15, no. 6, 2525–2545 pp. European Committee for Standardization: 2004; Eurocode 8: design of structures for earthquake resistance. P1: General rules, seismic actions and rules for buildings. Draft 6, Doc CEN/TC250/SC8/N335. Joyner, W. B.:2000; Strong motion from surface waves in deep sedimentary basins, Bulletin of the Seismological Society of America, 90(6B), S95-S112. Kotha, S.R., Bindi, D., Cotton, F.: 2017; From ergodic to region- and site-specific probabilistic seismic hazard assessment: method development and application at European and Middle Eastern sites. Earthq. Spectra 33, 1433-1453 pp. Mascandola, C., Massa M., Barani S., Albarello D., Lovati S., Martelli L., Poggi V.: 2019; Mapping the Seismic Bedrock of the Po Plain (Italy) through Ambient-Vibration Monitoring. Bulletin of the Seismological Society of America, 109(1), 164-177 pp.