Comparison of 2D and 1D numerical simulations for the evaluation of seismic site effects in sedimentary basins : the case of Norcia basin, Central Italy

Site effects assessment through one-dimensional ground response analyses are the keystone of the current practice for estimating the local site specific amplification. More complex site effects, such as those arising from 2D (or 3D) seismic wave propagation may influence the strong ground motion such that the so called 1D site effects are overshadowed. A recent proposal for accounting for these additional effects is the use of aggravation factors. In particular, the present work deals with the assessment of the period depedant aggravation factors found from 2D ground response analysis performed on two cross sections of the Norcia, Central Italy. Due to the combination of dominant near fault characteristics and amplification of ground motion given by the basin, the October 2016 Central Italy earthquake sequence caused significant life and economic losses in the affected area. In this contribution, the response of the aforementioned basin during the M6.5 October 30 main shock will be considered. The study focuses on the 1D and 2D responses of two distinct cross sections extracted from the idealized 3D depth-velocity model of the basin. Time domain finite element analyses (i.e. QUAD4M, Hudson, et al. 1994) and finite difference analyses (i.e. FLAC2D, Itasca 2016) are performed. In both approaches, soil layers are first assumed linear viscoelastic and, then, material nonlinearity is considered. Initially the response of the cross sections is examined by using low and high frequency wavelets, from which the salient aggravation mechanisms are first identified. Analyses with regard to the October 2016 seismic sequence are conducted by scaling a signal that is recorded by a seismogram located on rock-like outcropping conditions. The final results are presented in terms of maximum aggravation factors and Fourier amplification spectra spatial distributions across the studied profiles.