Comparison between the seismic amplification values obtained from the Italian second-level microzonation (SM2) abacuses and numerical simulation in Friuli Venezia Giulia region

In Italy, second-level Seismic Microzonation (SM2) provides simplified estimates of seismic amplification through the use of abacuses applied to 1D subsoil models, from which two amplification factors are derived: AFa, representative of short-period amplification, and AFv, representative of long-period amplification. This study evaluates the representativeness of abacus-derived amplification factors by comparing them with those obtained from 1D numerical simulations, focusing on the Friuli Venezia Giulia plain. Data collected from the Italian National Civil Protection repository were used both to apply the abacuses and to perform numerical simulations, allowing a direct comparison between the two approaches. Results show that, in most cases, the abacuses tend to underestimate local site effects compared to numerical simulations, except at sites characterized by shallow seismic bedrock. The highest amplification values are observed at sites with silty soil cover and greater sediment thicknesses. No clear trends emerge between abacus-based and numerically derived amplification factors. The comparison of response spectra further indicates that spectra derived from numerical simulations are generally higher than those obtained from abacuses and from the Italian building code, which tends to underestimate amplification at short periods and overestimate it at longer periods, particularly for soil category E.