Regional study of site effects on the high-frequency spectral-decay parameter

The high-frequency spectral-decay parameter κ has been used in the field of engineering seismology for several decades. The zero-distance κ value (κ₀) is a site parameter in ground motion models and strong motion synthesis. Using surface-borehole pairs of strong motion records from the strong-motion seismograph network KiK-net, we explored the effect of site soil on κ₀, specifically the contribution from ground motion directionality. In the target area (138°E−143°E, 36°N–40°N), and records from earthquakes at 50 pairs of stations were collected. All horizontal orthogonal records were rotated from 0° to 180° with an increment of 10° to capture the difference between the average κ₀ on two horizontal components and the median value of the rotated κ₀ at surface and borehole stations. The same process was applied to Δκ₀, the difference between surface station κ₀ and borehole station κ₀, to measure the contribution by soil column. The nonlinear behavior of soils led to large shear strains, reduced stiffness, and decreased resonance frequency. Thus, the effect of soil nonlinearity on κ is investigated. In the same area, strong ground motion records with peak ground acceleration >100 cm/s² were selected to identify the soil nonlinearity by the moving time window deconvolution method, and the temporal variation of time-average shear wave velocity and κ at the 50 surface stations were assessed.