Amplification features and observed damages in İzmir (Turkey) due to 2020 Samos (Aegean Sea) earthquake: identifying basin effects and design requirements

On October 30th 2020, a M7.0 earthquake occurred in the Aegean Sea close to Samos Island (Greece) and the western coast of Turkey that had a strong impact on the city of Izmir, about 70 km away from the epicenter. 116 people were killed, several hundreds were injured and about 4% of the structures sustained damages, including buildings that totally collapsed. Strong amplification of ground motion within Bornova basin (Izmir) where severe damages were observed, has been reported among the detrimental factors that may have contributed to such earthquake losses. This study is devoted to providing evidence that the reason of the observed intensity of ground motion in Bornova basin is mainly related to site effects and in particular to what is referred to as basin effects. The evaluation of observations for basin effects is based on a analysis with empirical techniques in frequency domain as well as with time–frequency analyses to reveal the contribution of different wave types in the response. Empirical findings are correlated with the particular geological structure of the Bornova basin, the latter being enhanced with a representative deep VS model using surface wave inversion on strong motion recordings. Furthermore, empirical findings are validated with 2D numerical analyses of a simplified basin model. Building characteristics and structural damages are also discussed in an attempt to comprehend the relation between them and basin effects. Noting that basin effects are not explicitly included in any modern seismic code, an approach is proposed to introduce the extra amplification due to basin effects in design practice.