Understanding the physics of kappa (κ): insights from a downhole array

At high frequencies, the acceleration spectral amplitude decreases rapidly; this has been modelled with the spectral decay factor κ. Its site component, κ0, is used widely today in ground motion prediction and simulation, and numerous approaches have been proposed to compute it. In this study, we estimate κ for the EUROSEISTEST valley, a geologically complex and seismically active region with a permanent strong motion array consisting of 14 surface and 6 downhole stations. Site conditions range from soft sediments to hard rock. First, we use the classical approach to separate local and regional attenuation and measure κ0. Second, we take advantage of the existing knowledge of the geological profile and material properties to examine the correlation of κ0 with different site characterization parameters. κ0 correlates well with Vs30, as expected, indicating a strong effect from the geological structure in the upper 30 m. But it correlates equally well with the resonant frequency and depth-to-bedrock of the stations, which indicates strong effects from the entire sedimentary column, down to 400 m. Third, we use our results to improve our physical understanding of κ0. We propose a conceptual model of κ0 with Vs, comprising two new notions. On the one hand, and contrary to existing correlations, we observe that κ0 stabilizes for high Vs values. This may indicate the existence of regional values for hard rock κ0. If so, we propose that borehole measurements (almost never used up to now for κ0) may be useful in determining these values. On the other hand, we find that material damping, as expressed through travel times, may not suffice to account for the total κ0 measured at the surface. We propose that, apart from material damping, additional site attenuation may be caused by scattering from small-scale variability in the profile. If this is so, then geotechnical damping measurements may not suffice to infer the overall crustal attenuation under a site; but starting with a regional value (possibly from a borehole) and adding damping, we might define a lower bound for site-specific κ0. More precise estimates would necessitate seismological site instrumentation.