The contribution of scattering to near-surface attenuation

The rapid decrease of the acceleration spectral amplitude at high frequencies has widely been modeled by the spectral decay factor kappa (kappa). Usually, the path-corrected component of kappa, often called kappa(0), is believed to be a local and frequency-independent site characteristic, in turn representing attenuation related to waves propagating vertically through the very shallow layers beneath the study site. Despite the known relevance of kappa(0) in a wide range of seismological applications, most methods for its calculation do not fully consider the influence of the scattering component. To account for the scattering component, we present a summary of statistical observations of the seismic wavefield at sites of the Swiss seismic networks. The intrinsic properties of the wavefield show a clear dependency on the local shallow subsoil conditions with differences in the structural heterogeneity of the shallow subsoil layers producing different scattering regimes. Such deviations from the ballistic behavior (i.e., direct waves that sample only distinct directions) are indicative for local structural heterogeneities and the associated level of scatter. Albeit the attenuation term related to scattering depends nonlinearly on the intrinsic term, the results indicate that the commonly used explanation for the high-frequency decay spectrum might not be appropriate but involving the amount of scattering might allow better constrained estimates of kappa(0).