Historical and Futuristic Perspectives on Seismic Site Effects in Turkiye-Izmir-Bayrakli Basin

Abstract

Abstract Due to the unique soil, morphological and subsurface topographical conditions, site effect-induced amplified and prolonged seismic demand traces were observed in historical strong ground motion records from Bayrakli-Izmir-Turkiye. A vivid example of these was documented after the recent October 30, 2020 normal event with Mw 7.0, which took place offshore of Samos Island and Seferihisar. During the event, structural damage and life losses were unexpectedly concentrated in Bayrakli-Izmir, even though the fault rupture was located 70 km away. The presence of strong ground motion stations located on rock and soil sites in Bayrakli enables quantitative assessment of this rather unique response. For this purpose, earthquake records starting from year 2010 till present were compiled and studied to assess intensity- or better to refer as strain- dependent Bayrakli basin-specific, nonlinear amplification factors. 1-D total stress-based equivalent linear site response assessments were performed to better understand and model these effects. Additionally, amplification factors were probabilistically estimated by following the modified analytical procedures. After having calibrated and validated the idealized 1-D models, they were used to predict the responses of the site during future, deterministic, more of design basis like scenario events. The results revealed that the 2475- year design basis spectral amplitudes for a ZE type site could be exceeded in Bayrakli by as much as 20 to 100% in the period range 1.5 to 3.5 seconds. Similarly, the amplification factors by Turkish earthquake code (TEC 2018) are expected to be exceeded under deterministic seismic scenarios on Izmir, Karaburun and Seferihisar faults, with Eurocode (2004) and NEHRP (2020) recommendations to be no exceptions. These clearly address the need to develop Bayrakli-specific seismic zonation and design practices due to its rather unique site effect-governed seismic response.