Evaluation of earthquake ground motion and site effects in the Thessaloniki urban area by 3D finite-fault numerical simulations

In this study earthquake ground motion in the Thessaloniki urban area is evaluated using a 3D spectral element numerical approach. The availability of detailed geotechnical/geophysical data from past microzonation studies together with the seismological information regarding the relevant fault sources allowed for the creation of a large-scale 3D numerical model suitable for generating finite-fault physics-based ground shaking scenarios within the city of Thessaloniki up to maximum frequencies of about 1.5 Hz. As a representative case study, the simulation of the historical MW6.5 June 20th 1978 Volvi earthquake is addressed. The results show that reasonable estimates can be obtained in terms both of ground motion time histories at the only available strong motion station and of spatial distribution of peak ground motion parameters, as compared to the regional map of macroseismic intensity and to the predictions of attenuation laws. The numerical model is, then, used to provide insights into 3D site effects occurring in the city of Thessaloniki, by showing the comparison between synthetic and experimental Standard Spectral Ratios (SSR) as well as the effect of non-linear visco-elastic soil behavior and the spatial variability of amplification factors with respect to outcrop bedrock basement. The results of this study demonstrate the potentialities of 3D physics-based numerical modelling for deterministic-based seismic hazard assessment studies in large urban areas characterized by complex geological configurations, such as the Thessaloniki area.