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

Julie Baron,Peter Klin,Ilaria Primofiore,Giovanna Vessia,Giovanna Laurenzano

doi:10.1007/s10518-021-01270-2

Abstract

The combined effect 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 effects 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 flat terrain of an alluvial basin. The two hamlets suffered different 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 amplification, 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 effects of topography from those of the sub-surface structure we iterate the numerical simulations for three different versions of the sub-surface model: one homogeneous, one with a surficial weathering layer and a soil basin and one with a complex internal setting. The numerical results confirm the correlation between topographic curvature and amplification and support a correlation between the induced torsional motion and the topographic slope. On the other hand we find that polarization does not necessarily imply ground motion amplification. In the frequency band above 4 Hz the topography-related effects are mainly aggravated by the presence of the weathering layer, even though they do not exceed the soil-related effects in the flat-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.

Highlights

The estimation of ground motion amplification due to site effects plays a fundamental role in the efforts devoted to seismic-risk mitigation (Amanti et al 2020)
The oversimplification that may cause underestimation of theoretically predicted amplifications typically consists of inadequate two-dimensional (2D) approximation of prominently three-dimensional (3D) topographic features (Paolucci 2002; Luo et al 2020) or in neglecting impedance contrasts in the subsurface structure that possibly concur with topography in forming amplification effects (Graizer 2009; Assimaki and Jeong 2013; Hailemikael et al 2016, to cite a few)
According to Burjánek et al (2014), the observed amplification at sites with comparable topography but with different geology may differ for a factor which is significantly larger than the expected ground motion variability and studies based exclusively on the terrain topography have almost no chance to capture the site effects correctly

Summary

Introduction

The estimation of ground motion amplification due to site effects plays a fundamental role in the efforts devoted to seismic-risk mitigation (Amanti et al 2020). Zhou et al (2020) adopt the back-propagation neural network technique for the derivation of a ground motion amplification model based on topographic geometrical features estimated from the DEM. Since they are based on surface topography alone, these approaches neglect possible interactions between topography and subsurface complexities on wave propagation. According to Burjánek et al (2014), the observed amplification at sites with comparable topography but with different geology may differ for a factor which is significantly larger than the expected ground motion variability and studies based exclusively on the terrain topography have almost no chance to capture the site effects correctly

Objectives

Methods

Results

Conclusion