GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Along the valleys and on the slopes, V S is generally < 350 m/s. It is low even within the first 5÷10 m of the substrate, before increasing rapidly with depth. In the Apennines foothills, where coarse deposits of fluvial origin occur, V S30 is usually > 300 m/s. There, the conditions mentioned above for which V S2 /V S1 ≥ 2 commonly occur in the first 30 m of the subsoil, such that V SH has also been calculated. Finally, in the alluvial plains V S is almost always < 250 m/s. The lowest velocities are found where the alluvial sediments are thicker, particularly in the eastern sector of the region (Po delta and north of Rimini). Site effect map One of the possible applications of this database is the calculation of the maximum expected acceleration at a given site (a max ), which is pivotal for the definition of seismic risk mitigation strategies. A ready elaboration is the classification of the territory according to the A-E subsoil categories of the NTC18 procedures. The amplification coefficient S for each soil category is associated with each V S and a max can thus be calculated by: a max = S * a g = (S S * S T ) * a g where a g is the peak acceleration on a rigid and flat ground (see hazard model MPS04, 475 years return period, Meletti et al., 2006), S S is the stratigraphic amplification coefficient (see Tab. 3.2.IV of NTC 2018) and S T the topographic amplification coefficient (assumed equal to 1 since almost all V S measurement sites are in areas not susceptible to topographic amplification). However, Seismic Microzonation studies and local seismic response analysis have shown that local site effects can vary significantly from those obtained by considering the subsoil classes. In particular, along valleys and foothills that often are classified as B soils, a max is generally underestimated, while in areas with very thick unconsolidated soils, categorized as D-type (Po delta and coastal areas north of Rimini), its value is commonly overestimated. A calculation of the local effects at the V S acquisition sites was attempted by applying the level 2 MS criteria (Tento et al., 2014; DGR 476/2021 and DGR 564/2021). To that end, in addition to V S30 or V SH , it is necessary to know the lithostratigraphic succession down to the seismic bedrock. This is relatively well known at the regional scale thanks to numerous MS studies, an extensive geological map coverage and geognostic test results (e.g., RER-ENI, 1998; Martelli et al., 2017; database of geognostic tests 2 and geological cross-sections for the Po Plain area 3 ). Based on this 3 https://ambiente.regione.emilia-romagna.it/it/geologia/cartografia/webgis-banchedati/sezioni-geologiche-prove-geognostiche-pia nura 2 Available also online: https://ambiente.regione.emilia-romagna.it/it/geologia/cartografia/webgis-/banca-dati-prove-geognostiche-regione-emilia-romag na )