Within‐Station Variability in Kappa: Evidence of Directionality Effects

One of the most commonly used parameters to describe seismic attenuation is the high‐frequency spectral decay parameter Kappa (⁠$κr$⁠), yet the physics behind it remain little understood. A better understanding of potential factors that lead to large scatter in estimated values of $κr$ constitutes a critical need for ground‐motion modeling and seismic hazard assessment at large. Most research efforts to date have focused on studying the site‐to‐site and model‐to‐model variability of $κ$⁠, but the uncertainties in individual $κr$ estimations associated with different events at a selected site (which we refer to as the within‐station variability of $κr$⁠) remain uncharacterized. As a direct corollary, obtaining robust estimates of the site‐specific component $κ0$⁠, and their corresponding interpretation become a challenge. To understand the sources of the variability observed in $κr$ (and $κ0$⁠) at a single site, we select 10 Japanese Kiban–Kyoshin network (KiK‐net) downhole arrays and investigate the systematic contributions from ground‐motion directionality. We observe that $κr$ estimated from a single horizontal component is orientation dependent. In addition, the influence of ground‐motion directionality is a function of local site conditions. We propose an orientation‐independent $κr$‐value, which is not affected either by ground‐motion directionality or by the events’ azimuths. In addition, we find that focal depth of events used in $κr$ calculations affects the estimation of the regional attenuation component $κR$⁠, which, in turn, influences the within‐station variability in the $κ0$ model.