Within-Station Variability in Kappa: Evidence of Directionality Effects

Abstract

One of the most commonly used parameters to describe seismic attenuation is the high-frequency spectral decay parameter Kappa (κr), yet the physics behind it remain little understood. A better understanding of potential factors that lead to large scatter in estimated values of κr constitutes a critical need for ground-motion modeling and seismic hazard assessment at large. Most research efforts to date have focused on studying the site-to-site and model-to-model variability of κ, but the uncertainties in individual κr estimations associated with different events at a selected site (which we refer to as the within-station variability of κr) remain uncharacterized. As a direct corollary, obtaining robust estimates of the site-specific component κ0, and their corresponding interpretation become a challenge. To understand the sources of the variability observed in κr (and κ0) at a single site, we select 10 Japanese Kiban–Kyoshin network (KiK-net) downhole arrays and investigate the systematic contributions from ground-motion directionality. We observe that κr estimated from a single horizontal component is orientation dependent. In addition, the influence of ground-motion directionality is a function of local site conditions. We propose an orientation-independent κr-value, which is not affected either by ground-motion directionality or by the events’ azimuths. In addition, we find that focal depth of events used in κr calculations affects the estimation of the regional attenuation component κR, which, in turn, influences the within-station variability in the κ0 model.

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

“…Although the ground response is coupled in the two horizontal directions, it is common in site response assessments to treat the two horizontal components independently. In addition, Ji et al (2020) recently observed κ values that are directionally dependent, and this finding supports our approach of not discarding κ values that are more than 20% different.…”

A Frequency-Dependent Model for the Shape of the Fourier Amplitude Spectrum of Acceleration at High Frequencies

“…Some of the record-to-record variability that we find for η may be regarded as similar to the within-station variability that Ji et al (2020) and Bora et al (2017) show for κ 0 . They report that the within-station variability of κ 0 can be even larger than the variability of κ 0 between different stations.…”

“…The two horizontal spectra of an earthquake are combined into a single one by taking the root-mean-square (rms). We opted to average the horizontal components to reduce the influence of the azimuthal direction on the κ, η and ζ measurements (Ji et al, 2020).…”

“…Some differences between the results of this study and the other studies might not be related to differences in the method. They could also be attributed to differences in the datasets or the record and spectrum processing (Ji et al, 2020).…”

“…They report that the within-station variability of κ 0 can be even larger than the variability of κ 0 between different stations. Ji et al (2020) attributed this effect to dataset choices, the record, and the spectrum processing. They observed, for example, that the estimated value of κ is strongly orientation-dependent if only a single horizontal component is considered.…”