The High-Frequency Decay Parameter (Kappa) in Taiwan

The high-frequency decay parameter kappa (κ) was computed by fitting Fourier amplitude spectra from the seismic network of the Taiwan Strong Motion Instrumentation Program (TSMIP). The κ of shear horizontal waves (SH waves) was calculated for individual recordings, and the relationship between κ values and the epicentral distance (R_epi) of each station was derived for each station. The κ value at R_epi = 0 (denoted as κ₀) can be used as a site parameter. There are totally 26,277 seismograms that have been recorded at 679 TSMIP stations over the period of 1993 through 2014 with local magnitudes of 4.0–7.1 and focal depths less than 30 km. The estimation of κ₀ for Taiwan ranges from 0.0208 to 0.147 s, and the spatial distribution of κ₀ was closely related to geology and velocity. The site-specific κ₀ values from 425 stations were correlated with the averaged shear wave velocity of the top 30 m of strata (V_S30), and the relationship could be described as κ₀ = (0.125 ± 0.005) − (0.011 ± 0.001) × ln(V_S30), and an acceptable linear correlation (R² = 0.57) was performed. The results may be used in the future application of ground motion prediction equations (GPMEs) and serve as simulation parameters. Also, the relationship between κ₀ and the depth to engineering rock (Z_1.0) is not well correlated, whereas an acceptable correlation exists between κ₀ and other site proxies (i.e., elevation and resonant frequency).