Refined thresholds for non-linear ground motion and temporal changes of site response associated with medium-size earthquakes

Abstract

S U M M A R YWe systematically analyse non-linear effects and temporal changes of site response associated with medium-size earthquakes, using seismic data recorded by the Japanese Strong Motion Network KIK-Net. We apply a sliding-window spectral ratio technique to surface and borehole strong motion records at six sites, and stack results associated with different earthquakes that produce similar peak ground acceleration (PGA). In some cases we observe a weak coseismic drop in the peak frequency when the PGA is as small as ∼20-30 Gal, and near instantaneous recovery after the passage of the direct S waves. The percentage of drop in the peak frequency starts to increase with increasing PGA values. We also observe a coseismic drop in the peak spectral ratio for two sites. When the PGA is larger than ∼60 Gal to more than 100 Gal, we observe considerably stronger drops of the peak frequencies followed by logarithmic recovery with time. The observed weak reductions of peak frequencies with near instantaneous recovery likely reflect non-linear response with essentially fixed level of damage, while the larger drops followed by logarithmic recovery reflect the generation (and then recovery) of additional rock damage. The results indicate clearly that non-linear site response may occur during mediumsize earthquakes, and that the PGA threshold for in situ non-linear behaviour is lower than the previously thought value of ∼100-200 Gal.Key words: Elasticity and anelasticity; Earthquake ground motions; Site effects; Wave propagation. I N T RO D U C T I O NNon-linear site response is associated with deviations from the linear trend predicted by Hooke's elasticity when the amplitude of ground motion exceeds a certain threshold. This phenomenon is well documented in both laboratory experiments (Ostrovsky & Johnson 2001, and references within) and under in situ conditions (Beresnev & Wen 1996, and references within). Generally, non-linear response has a strong correlation with the level of ground motion (e.g. Hartzell 1998; Su et al. 1998;Johnson et al. 2009) along with rock type and other characteristics of the sites (e.g. Beresnev & Wen 1996;Hartzell 1998;Trifunac et al. 1999;Tsuda et al. 2006). Quantification of soil non-linearity and subsequent consequences for site response are currently key components of estimating seismic hazard, predicting future ground motions (Frankel et al. 2000) and designing geotechnical and structural engineering systems on soils (NEHRP 2003).Comparisons of the site response during strong and weak ground motions provide one of the most typical ways to identify non-linear site behaviour. Several recent studies have also investigated details of the recovery process from non-linear response by computing the temporal evolution of spectral ratios between a target site and a nearby reference site (e. et al. 2009a, b;Rubinstein 2010). These studies generally identify strong reductions of peak frequencies (i.e. resonant frequencies) measured in the spectral ratios during the strong ground motions of a nea...

Temporal changes of site response during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

“…Previous studies have found clear evidences of soil nonlinearity after previous large earthquakes in Japan, including the 1995 M w 6.8 Kobe earthquake, 2000 M w 6.8 Western Tottori earthquake, 2003 M w 8.3 Tokachi-Oki earthquake, 2003 M w 7.0 Miyaki-Oki earthquake, and 2004 M w 6.6 Niigata earthquake (e.g., Pavlenko and Irikura 2002;Sawazaki et al, 2006;Rubinstein et al, 2007;Assimaki et al, 2008;Wu et al, 2009aWu et al, , 2010. The sharp reductions of the peak frequency and peak spectral ratio followed by gradual recovery observed in this study (Fig.…”

Influence of a sedimentary basin infilling description on the 2-D P–SV wave propagation using linear and non-linear constitutive models

“…Beresnev & Wen 1996;Wu et al 2010;Bonilla et al 2011;Rubinstein 2011;Wu & Peng 2011;Régnier et al 2013). However, non-linear soil behaviour is defined and studied in soil mechanics in the shear stressstrain plane.…”

“…Beresnev & Wen 1996;Wu et al 2010;Bonilla et al 2011;Rubinstein 2011;Wu & Peng 2011). We then study the maximum shear strain related to the shear modulus reduction and the damping increase through the curves presented in Fig.…”

Long-term change of site response after the M W 9.0 Tohoku earthquake in Japan

“…The reference value of resonance frequency before the Tohoku earthquake is obtained by averaging all the resonance frequencies measured from the spectral ratio traces with PGA < 100 Gal before the Tohoku main shock. Here we choose the threshold of 100 Gal to avoid the influences of significant nonlinear effects from moderate to large ground motions (Beresnev and Wen, 1996;Wu et al, 2010). Next, we estimate a rough recovery time by the following two steps: we first average the resonance frequencies measured from all the spectral ratios traces with PGA < 100 Gal in the last month of data (07/12/2011-08/12/2011), and then we find the time point when the resonance frequency recover to 95% of the average resonance frequency in the last month.…”