Simulation-based Local Site Amplification Factors for the city of Izmir

Öz

The city of İzmir is exposed to significant seismic hazard from nearby shallow crustal faults and pronounced site effects associated with thick deposits of soft clay, silt, and loose sand, particularly within the Bornova Plain. Because the local site factors in the Turkish Building Earthquake Code (TBEC, 2019) were adopted from National Earthquake Hazard Reduction Program (NEHRP. 2015), which was largely developed using western United States ground-motion data, this study develops simulation-based, region-specific site amplification factors for İzmir. A total of 42,504 one-dimensional site response analyses, comprising 21,252 nonlinear (NL) and 21,252 equivalent-linear (EL) simulations, were performed using 161 generic shear wave velocity (VS) profiles and 132 strong ground motions scaled for six earthquake scenarios. The profiles classified with respect to VS30-dependent (time-averaged VS at top 30 m) site classes were generated by randomizing base-case profiles derived from a newly developed shallow VS model for İzmir. Variability in depth to the VS = 760 m/s reference horizon was explicitly considered. Small-strain damping profiles were adjusted using the high-frequency spectral-decay parameter, κ0, to account for attenuation associated with wave scattering effects. Simulation-based short- and long-period amplification factors, FS and F1, respectively, were compared with TBEC (2019). For ZE (VS30 < 180 m/s) sites, the code factors generally encompass the simulated mean+1 standard deviation(σ) values, consistent with the conservatism embedded in NEHRP Class E factors. For ZC (360 m/s < VS30 < 760 m/s) and ZD sites (180 m/s < VS30 < 360 m/s), simulated FS values show improved agreement with TBEC, whereas F1 values generally remain lower, partly because the site natural periods are shorter than the period range used to derive F1. Region-specific mean and mean+1σ FS and F1 factors are tabulated as alternatives to TBEC (2019) for seismic design applications.

Anahtar Kelimeler

• Site Effects
• Site Amplification
• One-Dimensional Site Response Analysis
• Izmir

İzmir İli için Simülasyon-tabanlı Zemin Büyütme Faktörleri

Öz

İzmir kenti, yakın çevresindeki sığ kabuksal faylardan kaynaklanan önemli deprem tehlikesine ve özellikle Bornova Ovası’ndaki kalın yumuşak kil, silt ve gevşek kum tabakalarının neden olduğu belirgin zemin etkilerine maruz kalmaktadır. Türkiye Bina Deprem Yönetmeliği’ndeki (TBDY, 2019) yerel zemin etki katsayılarının büyük ölçüde Batı ABD verilerine dayalı NEHRP (2015) hükümlerinden uyarlanmış olması nedeniyle, bu çalışmada İzmir’e özgü simülasyon tabanlı zemin büyütme katsayıları geliştirilmiştir. Altı deprem senaryosu için ölçeklendirilmiş 132 kuvvetli yer hareketi ve 161 temsili kayma dalgası hızı (VS) profili kullanılarak toplam 42.504 tek boyutlu zemin tepki analizi gerçekleştirilmiştir. Profiller, İzmir için geliştirilen sığ VS modelinden türetilmiş ve VS=760 m/s referans seviyesine olan derinlik değişkenliği dikkate alınmıştır. Küçük şekil değiştirme sönüm profilleri κ0 parametresi kullanılarak düzenlenmiştir. Simülasyonlardan elde edilen FS ve F1 katsayıları TBDY ile karşılaştırılmıştır. ZE zeminlerinde yönetmelik katsayıları genellikle ortalama+1σ sonuçlarını kapsamakta olup, bu durum NEHRP Zemin Sınıfı E katsayılarının yüksek epistemik belirsizlik nedeniyle muhafazakâr belirlenmesinden kaynaklanmaktadır. ZC ve ZD zeminlerinde FS değerleri TBDY ile daha uyumlu iken, F1 değerleri genellikle daha düşüktür.

Anahtar Kelimeler

• Zemin Etkisi
• Zemin Büyütmesi
• Tek-boyutlu Zemin Tepki Analizi
• İzmir

Kaynakça

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