A conceptual model for kappa (κ) based on downhole earthquake records

Abstract:

At high frequencies, the acceleration spectral amplitude decreases rapidly; this has been modeled with the spectral decay factor κ (Anderson and Hough, 1984). Its site-specific component, κ₀, an index of site attenuation in the upper crust, is used widely today in ground motion prediction and simulation. Here we use accelerometric data from the EUROSEISTEST valley, a geologically complex and seismically active region in Greece. The strong motion array consists of 14 surface and 6 downhole stations. Site conditions across the stations vary greatly, ranging from soft sediments to hard rock (NEHRP class D to A). We use the results to improve our physical understanding of κ₀ and propose a conceptual model of κ₀ with V_s. This model comprises two new notions. On the one hand, and contrary to existing correlations, we observe that κ₀ stabilizes for high V_s values. This may indicate the existence of regional κ₀ values for hard rock. If so, we propose that borehole measurements (which have almost never been used up to now for κ₀) may be useful in determining these values. On the other hand, we find that material damping in the soil column, as expressed through travel times, does 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 attenuation underneath a site; but starting with a regional value (possibly measured from a downhole station) and adding damping, we might define a lower bound for site-specific κ₀. More precise estimates would necessitate seismological site instrumentation.