High-Frequency Spectral Decay (κ) at Stiff Sites in Northwestern Iran

The high-frequency decay of the Fourier amplitude spectrum of acceleration is commonly characterized by the parameter κ (kappa). In this study, we estimate the total kappa (κr) for nine stiff-soil and rock stations in northwestern Iran using the classical Acceleration Spectrum (AS) method. The dataset consists of 631 three-component recordings at epicentral distances of up to 50 km. We observe no significant increase in κr with distance, indicating that path attenuation is negligible within this range. Therefore, κr values were averaged per station to obtain the site-specific component κ0 for both horizontal and vertical motions. The estimated κ0 values span a relatively wide range despite the similar site conditions: 0.027–0.067 s for the horizontal component and 0.024–0.063 s for the vertical component. We show that azimuthal coverage plays an important role in κr variability, with stations having limited azimuthal sampling exhibiting smaller scatter. On average, κ0h exceeds κ0v, but the ratio varies significantly between stations and does not correlate with stiffness. Comparison with published κ0–VS30 correlations shows that our estimates fall within the broad range of existing data and are more consistent with studies that employed the same classical method rather than broadband approaches. Horizontal-to-vertical spectral ratios (HVSR) reveal that high-frequency amplification patterns likely influence κ estimates, particularly for stations with resonant features within the κ frequency band. Our results highlight the need for improved site characterization in the region to better separate site amplification from site attenuation and to refine κ0 estimates for seismic hazard applications.