Understanding Kappa (κ) for Basins of the Intermountain West: Insights from the 2020 Magna, Utah, Earthquake

The recent 2020 Magna Utah earthquake provides a unique opportunity for researchers and practitioners to evaluate the efficacy of site-specific ground response and ground motion hazards analyses for the variable depth basins of the Wasatch Front and the Intermountain West (IMW). The thick sediments in portions of the valleys of the IMW allow for this event to be utilized to evaluate effectiveness of site-specific ground response and ground motion hazards analyses. In particular, Kappa (κ), the high-frequency spectral decay parameter, is useful in site-specific seismic hazard analysis and site response analysis. Values of κ capture high-frequency attenuation and capture several site effects, including material damping. Ground motion recordings from the 2020 Magna Earthquake are utilized here to study κ distributions in and around the Salt Lake Valley from center of the basin to the rock exposed rim. Average κ from two horizontal components is used here to minimize the effect of near-fault ground motion directionality. Results of this study show that additional site attenuation beyond material damping may be caused by scattering from spatially distributed variability in the profile. Traditional geotechnical damping models in ground response analysis may not be able to thus infer the overall strong ground motion attenuation at a site. However, a solution presents itself for the engineer to understand ground response with more fidelity: use of seismological site instrumentation and its application of site response analysis, including host-to-target adjustments for site-specific seismic hazard analysis.