Variation of high frequency spectral attenuation (Kappa) in vertical arrays

Near surface attenuation parameter kappa (κ) of S-waves is calculated from 52 earthquake acceleration recordings at surface and base level (Vs > 760 m/s, namely engineering bedrock) of three downhole arrays and at two outcrop stations in Istanbul, Turkey. Path dependent (κR) and site dependent (κ0) components of κ are calculated from epicentral distance (Repi)- κ correlations. Estimated κ0 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, respectively. A strong correlation is observed between relative amplification factors of downholes and κ0 differences between surface-base levels. On the other hand, κ0 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 κ0 is found to increase response spectral acceleration up to 0.2 g in 0.1-0.2 s structural periods.