Seismic microzonation of a region with complex surficial geology based on different site classification approaches

Mohammad Salsabili,Miroslav Nastev,Alain Rouleau,Ali Saeidi

doi:10.1186/s40677-021-00198-8

Mohammad Salsabili, Miroslav Nastev + Show 2 more

Open Access

https://doi.org/10.1186/s40677-021-00198-8

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Abstract

A seismic microzonation study was conducted to refine the seismic hazard model for the city of Saguenay, Canada. The Quaternary geology underlying Saguenay shows complex glacial and post-glacial stratigraphy with a number of buried valleys filled with fluvioglacial and glaciomarine sediments. High impedance contrast between rock formations and surficial sediments is prone to seismic amplification. To evaluate their applicability, advantages and limitations in capturing the geological specificity of the study area, four site classification methods were applied: the current National Building Code of Canada (NBCC) and Eurocode 8, both mainly based on the average shear-wave velocity for the surficial sediments (VS,avg) and for the top 30 m (VS,30); a method based on the fundamental site period (T0); and a hybrid method based on the combination of VS,30, T0 and VS,avg. The study specifically aimed to evaluate the importance of the site classification parameters on the resulting microzonation maps. VS,30 is capable to present the geological and geotechnical site conditions, however, the results may be further improved by considering Vs,avg in shallow and T0 in thick layers of soil sediments as secondary parameters. The T0 method gives also satisfactory results with T0 showing a better correlation to Vs,30 than to Vs,avg. The versatile hybrid method may be challenging to apply in certain cases with its nine different site categories and parameters.

Highlights

An important aspect of geotechnical earthquake engineering is related to the evaluation of the expected intensity and the dominant period of the seismic ground motion at a given location
Site classification results The regional seismic site classification was conducted based on the National Building Code of Canada (NBCC), Eurocode 8, the fundamental site period (Zhao et al 2006), and the hybrid approach (Pitilakis et al 2018)
Due to the predominantly shallow sediments conditions in the region and the addition of the high VSrock in the VS,30 assessment, site classification based on NBCC ends up in about 90% of the area being classified as rock and dense stiff soil

Summary

Introduction

An important aspect of geotechnical earthquake engineering is related to the evaluation of the expected intensity and the dominant period of the seismic ground motion at a given location. The capacity of VS, as an efficient predictor of the local site amplification has been criticized by several authors (e.g., Castellaro et al 2008; Luzi et al 2011; Braganza and Atkinson 2017; Pitilakis et al 2018) It can be questioned in regions such as Eastern Canada, which are characterised with significant impedance contrasts, where thick surficial sediments with relatively low VS,avg overly stiff bedrock (Braganza et al 2016; Braganza and Atkinson 2017). Under such conditions, the Salsabili et al Geoenviron Disasters (2021) 8:27 impedance contrast contributes to shorten the incoming shear-wave wavelengths and increasing their amplitudes (Hunter and Crow 2012). The amplification can further be increased when the shear waves become trapped in low-velocity valleys filled with fluvioglacial and glaciomarine sediments contributing to a resonance effect at the fundamental period (T0) until their energy is dissipated (Kramer 1996; Hunter and Crow 2012)

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