3D numerical modelling for interpreting topographic effects in rocky hills for Seismic Microzonation: The case study of Arquata del Tronto hamlet

The 2016-2017 seismic sequence in the Central Apennines (Italy) demonstrated, once again, the relevant role of the differentiated seismic effects related to different features in complex geological contexts at a short distance (i. e. soft sediment-filled valley, hilly relieves, complex buried geometries). The heavy damages of old settlements located on the top of a rocky hill, such as Arquata del Tronto hill, raised the question on the main role of three-dimensional movements of asymmetrical isolated rocky reliefs in generating disruptions during the seismic shaking. In order to shed light on these effects, the writing authors analysed the seismic response of a digital 3D model of the Arquata del Tronto hill, that was constructed by considering the topography and the underground geological settings of the area. The seismic response was evaluated through 3D numerical simulations of seismic waves propagation with the spectral element method for the period range 0.1-1 s, that covers the typical resonance frequencies of the buildings involved in the studied case. The obtained results explain the recorded strong motion at Arquata site and are consistent with the observed heterogeneous damage distribution. This study evidences the need for accurate 3D models in the evaluation of the seismic response of ridges with topographic and geological asymmetries and demonstrates that the common usage of 2D simplified simulations to approximate the 3D seismic behaviour is not correct. Additionally, the study points out the paramount role that the characterization in engineering - geological terms plays in the numerical 3D simulation of the seismic site response. Thus, in microzoning studies, simplifications related to both dimensional and geo-litho-seismic characterizations should be avoided.