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[SSS11-P16] Estimation of seismic ground motions on the surface ground at MeSO-net stations
Keywords:MeSO-net, soil amplification factor, seismic ground motion, microtremor exploration, S-wave velocity structure
In the near future, it is concerned that a large earthquake will occur beneath the Tokyo metropolitan area and there will be severe damage for a lot of houses, buildings and infrastructures. Meanwhile, there are some earthquake observation networks in the area nowadays. Some of the networks provide us with the indexes of earthquake ground motions in a short time. Therefore, the information enables us to understand situation of earthquake ground motions in the area.
In the past national project of Japan, “Metropolitan Seismic Observation network” (hereinafter, referred to as “MeSO-net”) was constructed for the investigation of tectonic plate structures and the estimation of seismic source fault models in the Tokyo metropolitan area around ten years ago [1]. The MeSO-net has about 300 stations with sensors installed at around 20 meters underground in the area. Also, they are spatially distributed at intervals of 2 to 5 kilometers in the area. This network can provide us with the picture of earthquake ground motions with higher spatial resolution, provided that seismic records are acquired on the ground surface.
In recent years, we have worked a subproject of “acquisition of spatially very-high-resolution earthquake observation data and development of the database by private-public partnerships” as a member in a national project or “Tokyo Metropolitan Resilience Project [2].” As one of the issues, we have tackled estimation of seismic ground motions on the ground surface based on bore-hole seismic records observed at each MeSO-net station. This issue needs soil amplification factors between the ground surface and 20 meters below ground.
The procedure is as follows. First, a seismometer has installed on the ground surface at each MeSO-net station during two to three months and some earthquake observation records have been obtained in addition to the bore-hole records at each MeSO-net station. And then, an observed spectral ratio has been obtained by calculating Fourier spectra of seismic records on the ground surface and those in the bore-hole. Also, calculating JMA seismic intensity scale based on records on the ground surface and those in the borehole, increment of seismic intensity has been obtained by subtraction of them. Secondly, a miniature and centerless array microtremor exploration [3] was performed at each MeSO-net station. This method can be adopted principally for estimation of an S-wave velocity structure in the surface ground. It consists of 4-point miniature array with a radius of 60 centi-meters and 3-point centerless array with around 10 meters on a side. An S-wave velocity structure was estimated based on a disperse curve of phase velocity and an H/V spectral ratio calculated from microtremor data at each MeSO-net station. Then a transfer function was obtained using 1-dimensional multiple reflection theory based on the S-wave velocity structure at each MeSO-net station. Finally, an S-wave velocity structure can be enhanced by modifying a transfer function based on an observed spectral ratio at each MeSO-net station. Also, the existing S-wave velocity structures and other geotechnical information can be enhanced based on S-wave velocity structures at MeSO-net stations.
In this study, we will develop the scheme for estimation of earthquake ground motions at each MeSO-net station based on bore-hole seismic records by means of some soil amplification factors mentioned above. In this presentation, we will report on the results in the analyses of seismic data and microtremor ones at this moment.
References
[1] MEXT (2011): https://www.mext.go.jp/a_menu/kaihatu/jishin/1319360.htm (in Japanese)
[2] NIED (2021): https://forr.bosai.go.jp/e/sub_b.html
[3] I. Cho, S. Senna, and H. Fujiwara (2013), “Miniature array analysis of microtremors,” Geophysics, Vol.78, No.1, pp. KS13-KS23, doi:10.1190/geo2012-0248.1.
In the past national project of Japan, “Metropolitan Seismic Observation network” (hereinafter, referred to as “MeSO-net”) was constructed for the investigation of tectonic plate structures and the estimation of seismic source fault models in the Tokyo metropolitan area around ten years ago [1]. The MeSO-net has about 300 stations with sensors installed at around 20 meters underground in the area. Also, they are spatially distributed at intervals of 2 to 5 kilometers in the area. This network can provide us with the picture of earthquake ground motions with higher spatial resolution, provided that seismic records are acquired on the ground surface.
In recent years, we have worked a subproject of “acquisition of spatially very-high-resolution earthquake observation data and development of the database by private-public partnerships” as a member in a national project or “Tokyo Metropolitan Resilience Project [2].” As one of the issues, we have tackled estimation of seismic ground motions on the ground surface based on bore-hole seismic records observed at each MeSO-net station. This issue needs soil amplification factors between the ground surface and 20 meters below ground.
The procedure is as follows. First, a seismometer has installed on the ground surface at each MeSO-net station during two to three months and some earthquake observation records have been obtained in addition to the bore-hole records at each MeSO-net station. And then, an observed spectral ratio has been obtained by calculating Fourier spectra of seismic records on the ground surface and those in the bore-hole. Also, calculating JMA seismic intensity scale based on records on the ground surface and those in the borehole, increment of seismic intensity has been obtained by subtraction of them. Secondly, a miniature and centerless array microtremor exploration [3] was performed at each MeSO-net station. This method can be adopted principally for estimation of an S-wave velocity structure in the surface ground. It consists of 4-point miniature array with a radius of 60 centi-meters and 3-point centerless array with around 10 meters on a side. An S-wave velocity structure was estimated based on a disperse curve of phase velocity and an H/V spectral ratio calculated from microtremor data at each MeSO-net station. Then a transfer function was obtained using 1-dimensional multiple reflection theory based on the S-wave velocity structure at each MeSO-net station. Finally, an S-wave velocity structure can be enhanced by modifying a transfer function based on an observed spectral ratio at each MeSO-net station. Also, the existing S-wave velocity structures and other geotechnical information can be enhanced based on S-wave velocity structures at MeSO-net stations.
In this study, we will develop the scheme for estimation of earthquake ground motions at each MeSO-net station based on bore-hole seismic records by means of some soil amplification factors mentioned above. In this presentation, we will report on the results in the analyses of seismic data and microtremor ones at this moment.
References
[1] MEXT (2011): https://www.mext.go.jp/a_menu/kaihatu/jishin/1319360.htm (in Japanese)
[2] NIED (2021): https://forr.bosai.go.jp/e/sub_b.html
[3] I. Cho, S. Senna, and H. Fujiwara (2013), “Miniature array analysis of microtremors,” Geophysics, Vol.78, No.1, pp. KS13-KS23, doi:10.1190/geo2012-0248.1.