Estimates of kappa in the San Francisco Bay area

Site characterization is a critical component of seismic hazards studies, especially in the development and use of ground motion models (GMMs). One such parameter, kappa (Κ₀), represents local site attenuation and effectively describes regional variations in ground motion [1]. However, estimates of Κ₀ are limited. We estimate the site parameter Κ₀ for 296 broadband and accelerometer stations in the San Francisco Bay area using 225 local events with M>3.5. We first invert for kappa on the high-frequency slope of every record at a station, which represents full attenuation effects [2]. We then perform weighted regression of kappa versus distance to obtain expected kappa at the site (Κ₀). We find that Κ₀ varies extensively in the San Francisco Bay area, with values ranging from 0.004-0.113 s, thus representing the heterogeneous geologic conditions in the region. Κ₀ estimates from this study will be useful to consider in future seismic hazard analyses and ground motion studies in the Bay Area.