Analysis of the site-specific and regional components of kappa across crustal domains in Belgium based on the BELSHAKE database

This study investigates the high-frequency attenuation parameter kappa (κ), an important parameter in seismic hazard assessment and ground-motion modeling. Using the BELSHAKE ground-motion database, κ values were first determined from acceleration Fourier amplitude spectra following the classic Anderson and Hough definition. Three fitting methods for estimating site-specific kappa (κ₀) of all stations and the regional kappa gradient (κ_r) in different crustal domains in Belgium were evaluated, the Free Kappa Gradient Method, the Joint Kappa Gradient Method, and the Mixed-Effect Method, and we compared their effectiveness in robustly capturing variations in seismic attenuation. Data from four crustal domains were analyzed, with a filtering process excluding kappa values from induced earthquakes and short-distance, shallow-depth records to enhance the κ-distance relationship and refine κ₀ and κ_r estimation. Our results indicate that the Mixed-Effect Method yields the most robust and reliable estimates. In comparison, the Joint Kappa Gradient Method offers a balance between accuracy and consistency, while the Free Kappa Gradient Method is more sensitive to data availability. This comprehensive analysis advances the estimation of crustal attenuation properties in Belgium, supporting the development of improved seismic models and hazard assessments.