Fourier spectra and kappa₀ (κ₀) estimates for rock stations in the NGA-West2 project

In May 2013, the NGA-West 2 project published a flatfile containing metadata for the records in the NGA-West2 database, along with response spectra based on corrected acceleration time histories. These data are important for the creation or updating of current ground motion prediction models. In future, however, it is possible that such models may incorporate additional parameters. Fourier spectra may allow a better fit to the data and κ₀ values (the site-specific high-frequency attenuation factor of [1] (Anderson and Hough, 1984) to better describe high-frequency ground response. This paper describes the project of expanding the NGA-West2 database to include this new information. The process includes the windowing the time series and computation of Fourier amplitude and phase spectra with a view to: 1. creating a complementary Fourier spectrum flatfile for a suite of different time windows, and 2. enriching the metadata with values of κ₀, along with its epistemic uncertainty for the seismic stations. This effort has set as its priority to characterize stations on stiff soil and soft rock (Vs₃₀600 m/s). For these sites the knowledge of κ₀ is particularly critical in view of updating existing ground motion prediction models to better capture high-frequency site response and also of adjusting them to other regions with different κ₀ conditions.