kappa(0): Origin and Usability

Recent studies have focused on the analysis of the nature of kappa(0), as well as its application for improving seismic hazard assessment. It follows that a better understanding of kappa(0) (i.e., what exactly it represents) and of its correct application for improving the estimation of local shaking is mandatory. In this study, by means of the numerical simulation of vertically incident SH waves, we set out to clarify the contribution of the intrinsic attenuation and transmission properties of a media to the different portions of the seismic signal passing through it. Numerical simulations were carried out using models with constant or increasing depth velocities that are perturbed with random vertical impedance contrasts. The analysis allows us to quantify the apparent attenuation related to the transmission part for a given situation and to outline the variability of the results, depending on the chosen window of the signal. We showed that the attenuation due to transmission effects, even in strongly perturbed media, is generally much lower than that due to the intrinsic attenuation that is generally associated with near-surface material. Finally, an example of this method's application to real data that was collected from a borehole sensor array is used. We demonstrate that kappa(0) may be used to approximate the high-frequency trend in the moduli of the transfer function of the medium, due to the contribution of different propagation effects. However, although the model used to describe the wave propagation is oversimplified, when the intrinsic attenuation is strong, Ko could be used to differentiate sites with different subsurface attenuation structures. On the other hand, because kappa(0) might not be able to capture the absolute intrinsic attenuation correctly in cases in which it is not very strong, it should be used with care for the numerical simulation of ground motion.