Estimation of high-frequency attenuation parameter (kappa) for hard rock stations of Turkish strong motion network

Akgün, Atınç Özgün
Site-specific decay in the Fourier Amplitude Spectrum (FAS) at high frequencies, a.k.a. the zero-distance kappa (κ0), is frequently used in seismic analysis of critical infrastructure; especially for the host-to-target adjustment of the design spectrum and the site response analysis. The zero-distance kappa value for hard rock sites is more crucial but harder to constrain because the amount of strong-motion stations on hardrock sites is limited in the global datasets. The objective of this study is to calculate the zero-distance kappa value for the hard rock strong-motion stations operated by the Disaster and Emergency Presidency of Turkey (AFAD). For this purpose, 6463 recordings from 22 strong-motion stations with measured average shear wave velocities at the first 30 meters (VS30) higher than 740m/s and having at least 100 records have been analyzed. The slope of the decay in the S-wave portion of the FAS (kappa) at high frequencies is determined for a carefully selected and record-specific frequency range. Variation of the kappa with epicentral distance is evaluated to determine the median zero-distance kappa and its uncertainty for each recording station. Estimated median zero-distance kappa values vary between 0.01s to 0.06s and are consistent with the limited amount of previously published data. Only a weak correlation in median zero-distance kappa is observed with increasing VS30, and a rather large scatter in kappa for the same VS30 values is observed. More robust results might be attained by isolating the site amplification effects of weak surficial layers and subcategorization based on available geological and geographical information.
Citation Formats
A. Ö. Akgün, “Estimation of high-frequency attenuation parameter (kappa) for hard rock stations of Turkish strong motion network,” M.S. - Master of Science, Middle East Technical University, 2021.