1:45 PM - 3:15 PM
[SSS09-P02] Investigation of surface ground amplification factors based on microtremor array observations at K-NET and KiK-net seismic observation stations.
Keywords:strong ground motion, microtrmors, s-wave velocity, surface ground amplification factor
1. Introduction
The National Research Institute for Earth Science and Disaster Resilience (NIED) has developed a shallow and deep integrated ground structure model for the Kanto region for the purpose of broadband strong-motion evaluation, and AVS30 and ground amplification factor based on the developed ground model were released by the Earthquake Headquarters and J-SHIS in March 2021.
The current evaluation of ground amplification factor by surface ground models was conducted by Fujimoto and Midorikawa (2006). In this study, with the aim of verifying the accuracy of the ground amplification factor by AVS30 using a ground model and estimating the ground amplification factor accurately even without PS logging by conducting microtremor array survey, we evaluated the relationship between AVS30 and ground amplification factor based on S-wave velocity structures obtained from microtremor arrays, proposed a new ground The purpose of this study is to propose a new ground amplification factor, and to evaluate the optimal ground amplification factor if microtremor array observations are conducted.
2. Estimation Method of S-wave Velocity Structure and Amplification Ratio
In this study, we used 932 seismic observation sites in K-NET and KiK-net, mainly in the Kanto, Koshinetsu, Tokai, Hokuriku, and South Tohoku regions to estimate the S-wave velocity structures of triangular, L-shaped, and 60 cm radii with no center point. and L-shaped arrays with a radius of 60 cm, each with a center point of no more than 20 m in radius. Microtremor observation equipment is a three-component accelerometer JU410 with 200 Hz sampling for at least 15 minutes. SPAC, ESPAC, and CCA methods were used to analyze phase velocity, and the zero-crossing method (Cho et al., (2021)) was also used to read dispersion curves. The dispersion curves were read at almost all of the observed seismic stations, and good results were obtained that allow AVS30 to be fully evaluated.
AVS30 calculations were performed using a method based on a wavelength of 40 m (C40) based on Nagao and Konno (2002), a method that converts the relationship between frequency and phase velocity into a depth and S-wave velocity structure (e.g. Ballard (1964)), and an inversion analysis using phase velocity and H/V based on Arai and Tokimatsu's (2004, 2005) method. and inversion analysis by H/V, and the S-wave velocity structure of each of the methods based on the Bayesian theory of Cho and Iwata (2019), and the S-wave velocity structure converted by PS detection or the conversion formula between N value and S-wave velocity structure (e.g., Ota and Goto (1978)) and the maximum velocity amplification factor obtained from the AVS30 (e.g., Fujimoto and Midorikawa (2006)) were calculated and compared. We also calculated transfer function functions from the top of the engineering basement equivalent layer (Vs400) and used spectral amplification factors calculated from seismic observation records (velocity response spectrum of surface seismic records/velocity response spectrum in distance attenuation equation, and site amplification characteristics by spectral inversion) to verify the accuracy of S-wave velocity structure. The accuracy of the S-wave velocity structure was verified.
3. Results and Summary
The S-wave velocity structures from the PS logs and microtremor arrays at each seismic station were organized and compared. The following points can be considered from the results of this study.
The S-wave velocity structure is not so large as in the Kanto area. In the lowland areas centering on the Kanto region, there is no significant difference in AVS between the microtopographic segment (PS logging) and the microtremor array, but there is a large difference (variation) in AVS in the mountain basin, plateau, and mountainous microtopographic segment. There are many sites where the difference is more than a factor of 2.5 or more.
The PS logging tends to be larger than the microtremor array results for all AVSs. In the analysis of the microtremor array, the period and amplification characteristics of the spectral amplification factor are relatively well matched because the inversion analysis is performed to match the period characteristics of the phase velocity and H/V. However, the spectral amplification factor from the PS logging result has a worse period characteristic match than the microtremor array observation result. In particular, the agreement at depths shallower than 10 m (AVS10) appears to be poor. This suggests that there may be a problem with the accuracy of S-wave velocities at depths shallower than 10 m in the PS logging layer.
The National Research Institute for Earth Science and Disaster Resilience (NIED) has developed a shallow and deep integrated ground structure model for the Kanto region for the purpose of broadband strong-motion evaluation, and AVS30 and ground amplification factor based on the developed ground model were released by the Earthquake Headquarters and J-SHIS in March 2021.
The current evaluation of ground amplification factor by surface ground models was conducted by Fujimoto and Midorikawa (2006). In this study, with the aim of verifying the accuracy of the ground amplification factor by AVS30 using a ground model and estimating the ground amplification factor accurately even without PS logging by conducting microtremor array survey, we evaluated the relationship between AVS30 and ground amplification factor based on S-wave velocity structures obtained from microtremor arrays, proposed a new ground The purpose of this study is to propose a new ground amplification factor, and to evaluate the optimal ground amplification factor if microtremor array observations are conducted.
2. Estimation Method of S-wave Velocity Structure and Amplification Ratio
In this study, we used 932 seismic observation sites in K-NET and KiK-net, mainly in the Kanto, Koshinetsu, Tokai, Hokuriku, and South Tohoku regions to estimate the S-wave velocity structures of triangular, L-shaped, and 60 cm radii with no center point. and L-shaped arrays with a radius of 60 cm, each with a center point of no more than 20 m in radius. Microtremor observation equipment is a three-component accelerometer JU410 with 200 Hz sampling for at least 15 minutes. SPAC, ESPAC, and CCA methods were used to analyze phase velocity, and the zero-crossing method (Cho et al., (2021)) was also used to read dispersion curves. The dispersion curves were read at almost all of the observed seismic stations, and good results were obtained that allow AVS30 to be fully evaluated.
AVS30 calculations were performed using a method based on a wavelength of 40 m (C40) based on Nagao and Konno (2002), a method that converts the relationship between frequency and phase velocity into a depth and S-wave velocity structure (e.g. Ballard (1964)), and an inversion analysis using phase velocity and H/V based on Arai and Tokimatsu's (2004, 2005) method. and inversion analysis by H/V, and the S-wave velocity structure of each of the methods based on the Bayesian theory of Cho and Iwata (2019), and the S-wave velocity structure converted by PS detection or the conversion formula between N value and S-wave velocity structure (e.g., Ota and Goto (1978)) and the maximum velocity amplification factor obtained from the AVS30 (e.g., Fujimoto and Midorikawa (2006)) were calculated and compared. We also calculated transfer function functions from the top of the engineering basement equivalent layer (Vs400) and used spectral amplification factors calculated from seismic observation records (velocity response spectrum of surface seismic records/velocity response spectrum in distance attenuation equation, and site amplification characteristics by spectral inversion) to verify the accuracy of S-wave velocity structure. The accuracy of the S-wave velocity structure was verified.
3. Results and Summary
The S-wave velocity structures from the PS logs and microtremor arrays at each seismic station were organized and compared. The following points can be considered from the results of this study.
The S-wave velocity structure is not so large as in the Kanto area. In the lowland areas centering on the Kanto region, there is no significant difference in AVS between the microtopographic segment (PS logging) and the microtremor array, but there is a large difference (variation) in AVS in the mountain basin, plateau, and mountainous microtopographic segment. There are many sites where the difference is more than a factor of 2.5 or more.
The PS logging tends to be larger than the microtremor array results for all AVSs. In the analysis of the microtremor array, the period and amplification characteristics of the spectral amplification factor are relatively well matched because the inversion analysis is performed to match the period characteristics of the phase velocity and H/V. However, the spectral amplification factor from the PS logging result has a worse period characteristic match than the microtremor array observation result. In particular, the agreement at depths shallower than 10 m (AVS10) appears to be poor. This suggests that there may be a problem with the accuracy of S-wave velocities at depths shallower than 10 m in the PS logging layer.