Variability in Kappa (kappa(r)): Contributions from the Computation Procedure

Kappa (kappa(r)), the high-frequency spectral decay parameter, has been gaining popularity in the engineering seismology field for more than forty years, but it was not until recently that kappa(r) has been used for geotechnical earthquake engineering applications. For instance, kappa(r) has been used in site-specific seismic hazard analysis and site response analysis. Values of kappa(r) capture high-frequency attenuation at short rupture distances. However, the high variability and uncertainty associated with its calculation has hindered its application more broadly in geotechnical engineering analyses. The uncertainties in kappa(r) may stem from either the poor understanding of the physics behind this parameter or the oversimplified computation procedure. Variability due to the computation procedure can be minimized with the implementation of a standardized and robust calculation process. In this work, we evaluate the variability in kappa(r) introduced during the calculation procedure by the selection of the frequency band and discuss potential causes for negative kappa(r) values. We note that the classic kappa(r) model may be an oversimplification of complex wave propagation phenomena, and the divergence between its assumptions and actual site conditions require further investigation.