Hard‐Rock $κ0$ at KiK‐Net Sites in Japan

Site‐specific seismic hazard studies require the knowledge of the shear‐wave velocity $VS$ and the high‐frequency site attenuation parameter $κ0$ at the reference rock level at depth. The latter one (called $κ0,ref$⁠) is often not available and hard to derive. In this study, we make use of the KiK‐net database in Japan that consists of surface and colocated downhole sensors. We select 175 sites where the bottom sensor is deployed at rock or hard‐rock conditions resulting in a database with many recordings at $VS≥1500m/s$⁠. This allows us to tackle two questions: first, is it possible to derive $κ0,ref$ from surface recordings? Second, does $κ0$ reach an asymptotic level at high $VS$ that could be used as a $κ0,ref$ in site‐specific seismic hazard studies? Our results show that measures of $κ0$ derived from S and coda waves are equivalent. Thus, it is not possible to obtain $κ0,ref$ from surface recordings using coda waves. On the other hand, S‐wave measurements of $κ0$ from surface rock sites are close to $κ0,ref$ if $VS≥760m/s$ or if the sedimentary cover is thin. The values of $κ0$ decrease with increasing $VS$ and reach an asymptotic value. The scatter in the so obtained $κ0,ref$ is high, but it can be reduced by selecting subregions with similar geological conditions. Finally, we observe that borehole and surface $κ0$ are correlated, and that the variability of $κ0,ref$ is only slightly reduced compared to $κ0$ at the surface. Although we cannot exclude any influence of source effects, our findings indicate that $κ0,ref$ has to be considered as a deep site parameter.