Region- and Site-Specific Measurements of Kappa (κ0) and Associated Variabilities for Iran

Spectral parameter kappa (κ) is widely used to model the decay slope of the Fourier acceleration spectrum at high frequencies. This study presents estimates of κ0, often referred to as the site-related contribution in κ, for the Iranian strong-motion dataset. For our analysis, we use 2798 acceleration time histories from 325 earthquakes (between 1976 and 2020, inclusive) in the magnitude range M 3–7.4 and recorded over 285 stations at distances up to 100 km. The main objective of this study is to estimate κ0 and associated uncertainties at Iranian strong-motion stations. We investigate different sources of uncertainty that contribute to the higher scatter in the station-specific measurements of κ0. According to our results, the choice of frequency range used to approximate linear decay of acceleration spectrum contributes significantly to measurement-related uncertainty. Moreover, our measurements exhibit regional variations in (average) κ0 within Iran; in particular, for Zagros, the average κ0(=0.036) is found to be lower than that for northern Iran (κ0=0.044) and central-eastern Iran (κ0=0.04). In terms of between-station variability, there is a weak negative correlation between κ0 and VS30. The within-station uncertainty (i.e., standard deviation of κ0 values at individual stations) is also found to be decreasing with increasing VS30. The fundamental frequency of the site (f0) also appears to affect both measurement-related uncertainty and between-station variability at stations that exhibit significant 1D soil response behavior.