Hard-Rock 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 kappa 0 at the reference rock level at depth. The latter one (called kappa 0,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 >= 1500 m/s. This allows us to tackle two questions: first, is it possible to derive kappa 0,ref from surface recordings? Second, does kappa 0 reach an asymptotic level at high VS that could be used as a kappa 0,ref in site-specific seismic hazard studies? Our results show that measures of kappa 0 derived from S and coda waves are equivalent. Thus, it is not possible to obtain kappa 0,ref from surface recordings using coda waves. On the other hand, S-wave measurements of kappa 0 from surface rock sites are close to kappa 0,ref if VS >= 760 m/s or if the sedimentary cover is thin. The values of kappa 0 decrease with increasing VS and reach an asymptotic value. The scatter in the so obtained kappa 0,ref is high, but it can be reduced by selecting subregions with similar geological conditions. Finally, we observe that borehole and surface kappa 0 are correlated, and that the variability of kappa 0,ref is only slightly reduced compared to kappa 0 at the surface. Although we cannot exclude any influence of source effects, our findings indicate that kappa 0,ref has to be considered as a deep site parameter.