Investigation of topographic site effects using 3D waveform modelling: amplification, polarization and torsional motions in the case study of Arquata del Tronto (Italy)

The combined efect of topography and near-surface heterogeneities on the seismic response is hardly predictable and may lead to an aggravation of the ground motion. We apply physics-based numerical simulations of 3D seismic wave propagation to highlight these efects in the case study of Arquata del Tronto, a municipality in the Apennines that includes a historical village on a hill and a hamlet on the fat terrain of an alluvial basin. The two hamlets sufered diferent damage during the 2016 seismic sequence in Central Italy. We analyze the linear visco-elastic seismic response for vertically incident plane waves in terms of spectral amplifcation, polarization and induced torsional motion within the frequency band 1–8 Hz over a 1 km2 square area, with spatial resolution 25 m. To discern the efects of topography from those of the sub-surface structure we iterate the numerical simulations for three diferent versions of the sub-surface model: one homogeneous, one with a surfcial weathering layer and a soil basin and one with a complex internal setting. The numerical results confrm the correlation between topographic curvature and amplifcation and support a correlation between the induced torsional motion and the topographic slope. On the other hand we fnd that polarization does not necessarily imply ground motion amplifcation. In the frequency band above 4 Hz the topographyrelated efects are mainly aggravated by the presence of the weathering layer, even though they do not exceed the soil-related efects in the fat-topography basin. The geological setting below the weathering layer plays a recognizable role in the topography-related site response only for frequencies below 4 Hz