κ₀ for soil sites: Observations from KiK-net sites and their use in constraining small-strain damping profiles for site response analysis

Abstract

Small-strain damping profiles developed from geotechnical laboratory testing have been observed to be smaller than the damping inferred from the observed site amplification from downhole array recordings. This study investigates the high-frequency spectral decay parameter ( κ0) of earthquake motions from soil sites and evaluates the use of κ0 to constrain the small-strain damping profile for one-dimensional site response analysis. Using data from 51 sites from the Kiban-Kyoshin strong motion network (KiK-net) array in Japan and six sites from California, a relationship was developed between κ0 at the surface and both the 30-m time-averaged shear wave velocity ( V s30) and the depth to the 2.5 km/s shear wave velocity horizon ( Z2.5). This relationship demonstrates that κ0 increases with decreasing V s30 and increasing Z2.5. An approach is developed that uses this relationship to establish a target κ0 from which to constrain the small-strain damping profile used in one-dimensional site response analysis. This approach to develop κ0-consistent damping profiles for site response analysis is demonstrated through a recent site amplification study of Central and Eastern North America for the NGA-East project.

Ergodic site amplification model for central and eastern North America

“…These simulations used velocity profiles with V S 30 = 2000 and 3000 m/s, which were modified from the very hard rock crustal model of Boore and Joyner (1997). The site damping parameter κ 0 was taken as 0.006 s for both profiles, which is generally consistent with a recent data compilation by Xu et al (2020). Figure 7 shows the site amplification at 2000 m/s relative to the 3000 m/s reference as interpreted from these simulations.…”

The effect of soil nonlinearity on high-frequency spectral decay and implications for site response analysis

“…The parameter κ was calculated for each motion using the approach by Anderson and Hough (1984), which uses the slope of the high-frequency portion of the log-linear FAS. The procedures described in Xu et al (2020) were applied to process the acceleration time series for use in this analysis. Specifically, we used the entire acceleration time series, instead of only the shear wave window, because manually picked S-wave arrivals can add uncertainty to the κ calculation (Douglas et al, 2010).…”

“…Most of these studies have investigated κ 0 of earthquake motions at rock or stiff soil sites with the time averaged shear wave velocity of the top 30 m ( V s 30 ) greater than 500 m/s. Xu et al (2020) calculated the κ 0 of surface and downhole earthquake recordings at downhole array sites with V s 30 less than 500 m/s and investigated the site parameters that influence κ 0 at these sites. These previous studies have been focused on small-strain motions, and the resulting values of κ 0 have been used to constrain the small-strain damping used in site response models (e.g.…”

“…These previous studies have been focused on small-strain motions, and the resulting values of κ 0 have been used to constrain the small-strain damping used in site response models (e.g. Cabas et al, 2017; Xu et al 2020).…”

“…Traditionally, the parameter κ (or κ 0 ) has only been used to evaluate small-strain site response analysis (e.g. Cabas et al, 2017; Xu et al, 2020), but in this case, it can be used to control the damping in large-strain site response analysis.…”