Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn Variation of high frequency spectral attenuation (Kappa) in vertical arrays Gülüm Tanırcan ⁎ , S.Ümit Dikmen Department of Earthquake Engineering, Kandilli Observatory and Earthquake Research Institute, Boğaziçi University, Istanbul, Turkey ARTICLE INFO Keywords: High frequency spectral attenuation Kappa (κ) Vertical seismic arrays Istanbul ABSTRACT Near surface attenuation parameter kappa (κ ) of S-waves is calculated from 52 earthquake acceleration re- cordings at surface and base level (V s > 760 m/s, namely engineering bedrock) of three downhole arrays and at two outcrop stations in Istanbul, Turkey. Path dependent (κ R ) and site dependent (κ 0 ) components of κ are calculated from epicentral distance (R epi )- κ correlations. Estimated κ 0 values are for the outcrop stations are in the range of 26–30 ms, while for the surface and base levels of the arrays are 40–53 ms and 18–23 ms, respec- tively. A strong correlation is observed between relative amplification factors of downholes and κ 0 differences between surface-base levels. On the other hand, κ 0 not approaching to zero at base stations suggests that several other factors still contribute to other than path and site effects. Given the earthquake scenario and stochastic simulation approach, 10 ms decrease in κ 0 is found to increase response spectral acceleration up to 0.2 g in 0.1–0.2 s structural periods. 1. Introduction The concept of the high frequency attenuation parameter kappa (κ ) was first introduced by Anderson and Hough [1] observing an ap- proximate linear decay of the acceleration spectrum at frequencies higher than a specific frequency (f e ). Subsequently, they proposed the following mathematical form for the high frequency part of the accel- eration spectrum, A(f) of the waveform containing source and path effects, = − > Af A πκf f f () exp( ) for e 0 (1) where A 0 is the spectral amplitude and f is the frequency. If accel- eration spectrum is available, κ can be readily estimated from the slope of the spectrum over a range of frequencies (f e to f e +Δf) as =− = − +∆ [ ( ) ( ) ] κ λ λ ln A ln A f /π where /Δ f f e e (2) where A is the spectral acceleration at selected frequencies. They also postulated that κ has site dependent, κ 0 and distance dependent, κ R components and a linear relationship with epicentral distance, R epi ex- ists as; = + κ κ κ R . R epi 0 (3) Hence, κ 0 is the intercept of the linear relationship between κ and R epi . Since then, this concept has attracted considerable attention in the seismological community, especially by the researchers working on site amplifications [2], ground motion modeling equations [3–6] and stochastic simulations of ground motion [7]. In this respect, κ char- acterizing the high frequency decay of spectra is a valuable parameter in site studies. Consequently it is used (1) in the source related studies in order to study self-similarity of the source spectrum (2) in the gen- eration of synthetic ground motion using point-source or finite-fault stochastic or hybrid simulation approaches; even in physics based si- mulations using theoretical Green’s functions; (3) in the calibration of ground-motion prediction equations (GMPEs) based on stochastic si- mulations and; (4) in the engineering seismology community in prob- abilistic seismic hazard assessment (PSHA) for critical facilities, where it is common to perform site specific response analyses. Observing the rather empirical nature of this parameter, researchers continued their endeavors to broaden their understanding of the con- cept. Thus, over the years a number of complementary techniques have been proposed to estimate the κ 0 [8–10]. Particular effort was devoted to find a plausible correlation between κ 0 and site shear wave velocities [3,11-12]. In this respect, a range of κ 0 values calculated at the engineering bedrock layer with V s > 760 m/s, underlying layers with lower V s (referred to as the base level of the downhole arrays in this study) and/ or at the outcrop stations can help realistic calculation of site specific strong ground motion simulations. In this study, the parameters of Eq. (3) are investigated utilizing the data compiled from the three seismic downhole arrays and two outcrop stations in Istanbul operated by Kandilli Observatory and Earthquake Research Institute (KOERI). The arrays have different subsoil and topographical conditions, as well as urban fabric around them. The motivation of the study has risen from https://doi.org/10.1016/j.soildyn.2018.06.016 ⁎ Corresponding author. E-mail addresses: birgore@boun.edu.tr (G. Tanırcan), umit.dikmen@boun.edu.tr (S.Ü. Dikmen). Soil Dynamics and Earthquake Engineering 113 (2018) 406–414 0267-7261/ © 2018 Elsevier Ltd. All rights reserved. T