Reconstruction of a subsoil model for local seismic response evaluation through experimental and numerical methods: The case of the Wellington CBD, New Zealand

Defining a reliable subsoil model is one of the most crucial points in the evaluation of the local seismic response, especially when the study area presents a complex geological setting. Performing a large number of investigations during and subsequently a seismic crisis (following a strong earthquake) is useful for collecting valuable data. By applying a multidisciplinary approach, these data can be used for constraining and test the model, as well as for directly evaluating the effects of strong earthquakes on the environment and the distribution of structural damages in heavily populated areas. In the present study, through a new multidisciplinary experimental–numerical approach, we investigated the role of local site conditions on the damage observed in the Central Business District (CBD) of Wellington (New Zealand) after the 2016 M7.8 Kaikōura event. Numerical 1D/2D site response analyses were carried out to explore the hypothesis of damage exacerbation due to basin effects. Our numerical models were then validated by comparing the corresponding numerical amplification functions with those derived from the application of the standard spectral ratio technique to a large accelerogram dataset. This dataset was obtained by GeoNet stations deployed in the study area, including those operative during the 2016 M7.8 Kaikōura event. Differences in ground motion for near- and far-field events were highlighted. Our results show that the site response in the Wellington CBD was controlled by complex 2D/3D valley effects (mainly edge effects), which were related to the local buried geomorphology. Overall, these findings suggest that the type and location of an earthquake influence the maximum distance at which 2D effects (generated at the basin edge) are still detectable. Comparing our dataset with those from other published studies would be useful for clarifying the factors leading to an exacerbation of the seismic response in alluvial basins. This study has also possible implications for seismic hazard mitigation in cities built on such basins. We conclude that robust numerical 2D models can provide and capture notable characteristics of the ground motion associated with basin effects. These characteristics are fundamental for understanding basin effects and should be considered when formulating building regulations.