Current Limitations of Near-Surface Attenuation Modeling at High Frequencies

Abstract

Four decades after the introduction of the high frequency spectral decay parameter kappa (κ) to the scientific community, there has been relevant progress on our understanding of how it captures seismic attenuation and its multiple applications in stochastic modeling of ground motions, seismic hazard assessment, and site response analysis. Particularly, κ’s site-specific component, κ₀, has been used to characterize near-surface attenuation, which offers additional information to site response analysis and site-specific seismic hazard assessments beyond that provided by other site parameters such as the average shear wave velocity over the top 30 m of a site (V_s30). The Japanese database KiK-net is rich with a dense array of stations (at the ground surface and at depth) providing thousands of high-quality ground motions. However, the unclear underlying physics of κ and its associated variabilities pose challenges to its further application in site-specific seismic hazard assessments. Thus, the objective of this work is to identify the current limitations and challenges for near-surface attenuation characterizations with κ. The downhole array of KiK-net allows to study the response of soil mediums between surface and borehole sensors directly. The difference between surface and borehole individual κ estimates (what we referred to as Dκ₀), could capture the seismic attenuations contributed from the soil column between those two sensors. Larger borehole κ values (i.e., leading to negative Dκ₀) are the focus of this study because Dκ₀ is expected to be positive. With the assumptions that κ could serve as an attenuation parameter to characterize the dissipations of seismic wave energy, the negative Dκ₀ are lack of physical explanations. The observation of larger κ estimates at depth compared to their counterparts at the ground surface (i.e., negative Dκ₀) could result from multiple reasons. In this work, we specifically study the influences of ground motion directionality and site amplifications on Dκ₀ estimates. A single KiK-net station was selected for this case study to figure out the main challenges that remain in characterizing near-surface attenuation with κ.