2-D and 3-D basin site effects in Izmir-Bayrakli during the October 30, 2020 Mw7.0 Samos earthquake

Abstract

During the October 30, 2020 M(w)7.0 Samos Earthquake, the Turkiye-Izmir-Bayrakli district was affected the most due to the geometry and the deep-soft alluvial nature of the basin, although the district is approximately 70 km away from the epicenter. In this study, the seismic response of the Bayrakli basin, and the role of the soil stratigraphy and basin geometry on the recorded amplifications and prolonging of seismic shakings are investigated by using 1-, 2-and 3-D non-linear finite element-based dynamic response analyses. The assessment results are presented in the form of spectral amplification ratios, A(T). The highest A(T) values were estimated at spectral periods T = 0.85 and 1 s. They exceed five in both the east-west and the north-south directions, more pronounced in the latter one. A(T) was decomposed into the product of two independent amplification factors, namely rock to soil amplifications, A(Soil)(T), and 1-D soil column to 3-D soil basin amplifications, A(Basin)(T). A(Basin)(T) values for T = 0.85 and 1 s are estimated as high as 1.4 and 1.6, in the east-west and the north-south directions, respectively. These values suggest that the 3-D geometry of the Bayrakli basin amplifies the spectral accelerations by 40-60% at T = 0.85 and 1 s. The unique combination of a deep-soft alluvial site with 3-D basin geometry, and overlying 7-9 story residential buildings, consistently favors and amplifies the seismic energy in the spectral period range of 0.7-1.0 s. This multi-fold increased seismic demand, combined with poor structural design and construction details, lead to localized structural damage and over 117 life losses.