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<p>The high-frequency decay of the Fourier amplitude spectrum of acceleration is commonly characterized by the parameter <i>κ</i> (kappa). In this study, we estimate the total <i>κ</i> (<i>κ</i><sub>r</sub>) for nine stiff-soil and rock stations in northwestern Iran using the classical Acceleration Spectrum (AS) method. The dataset (631 three-component strong-motion recordings out to epicentral distances of 50&#xa0;km) is richer in near-source data than any past study in the region. It thus allows for a more detailed analysis and for minimizing errors and uncertainties stemming from the path, a limitation implicit in most <i>κ</i> studies. Distance dependence due to frequency-dependent anelastic attenuation (<i>Q</i>) is not clearly observed on <i>κ</i><sub>r</sub> for most stations. The estimated site-specific <i>κ</i><sub>0</sub> values span a relatively wide range despite broadly similar site conditions, from 0.027 to 0.067&#xa0;s for the horizontal component and from 0.024 to 0.063&#xa0;s for the vertical component. <i>κ</i><sub>0h</sub> exceeds <i>κ</i><sub>0v</sub>, with <i>κ</i><sub>0h</sub>/<i>κ</i><sub>0v</sub> ratios ranging from 0.9–1.9 and an average of about 1.4. <i>Κ</i><sub>0</sub> also shows no clear correlation with Vs30. Differences between stations may partly reflect uneven azimuthal coverage and source–receiver geometry, influencing the scatter of <i>κ</i><sub>r</sub> values but without clear azimuthal dependence. The event magnitudes span a significant range, from M2.5 to M6.5, and no discernable dependence on magnitude is found, indicating the AS method is appropriately used even for the small events in the dataset. Comparison with published <i>κ</i><sub>0</sub>–Vs30 datasets shows that the estimated values fall within the broad range reported for Iranian sites and are more consistent with studies adopting the AS method than with broadband approaches. Horizontal-to-vertical spectral ratios (HVSR) are used as qualitative indicators of frequency-dependent site amplification, and reveal significant deviations within the <i>κ</i> fitting band (8–35&#xa0;Hz) at several stations. These observations suggest that the measured <i>κ</i> values likely represent a combined effect of site attenuation and amplification, highlighting the need for improved site characterization to better constrain <i>κ</i><sub>0</sub> for seismic hazard applications.</p>

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High-frequency attenuation (κ) at stiff-soil and rock sites in Northwestern Iran

  • 期刊论文

  • 发表日期：

    2026年5月13日

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  • Meghdad Samaei,
  • Olga-Joan Ktenidou

摘要

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 κ (κr) for nine stiff-soil and rock stations in northwestern Iran using the classical Acceleration Spectrum (AS) method. The dataset (631 three-component strong-motion recordings out to epicentral distances of 50 km) is richer in near-source data than any past study in the region. It thus allows for a more detailed analysis and for minimizing errors and uncertainties stemming from the path, a limitation implicit in most κ studies. Distance dependence due to frequency-dependent anelastic attenuation (Q) is not clearly observed on κr for most stations. The estimated site-specific κ0 values span a relatively wide range despite broadly similar site conditions, from 0.027 to 0.067 s for the horizontal component and from 0.024 to 0.063 s for the vertical component. κ0h exceeds κ0v, with κ0h/κ0v ratios ranging from 0.9–1.9 and an average of about 1.4. Κ0 also shows no clear correlation with Vs30. Differences between stations may partly reflect uneven azimuthal coverage and source–receiver geometry, influencing the scatter of κr values but without clear azimuthal dependence. The event magnitudes span a significant range, from M2.5 to M6.5, and no discernable dependence on magnitude is found, indicating the AS method is appropriately used even for the small events in the dataset. Comparison with published κ0–Vs30 datasets shows that the estimated values fall within the broad range reported for Iranian sites and are more consistent with studies adopting the AS method than with broadband approaches. Horizontal-to-vertical spectral ratios (HVSR) are used as qualitative indicators of frequency-dependent site amplification, and reveal significant deviations within the κ fitting band (8–35 Hz) at several stations. These observations suggest that the measured κ values likely represent a combined effect of site attenuation and amplification, highlighting the need for improved site characterization to better constrain κ0 for seismic hazard applications.