[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-P13] Shallow velocity structures at three KiK-net stations along the Pacific coast of Tohoku region

*Atsushi Wakai¹, Yadab Dhakal¹, Hiroaki Yamanaka² (1.National Research Institute for Earth Science and Disaster Prevention, 2.Tokyo Institute of Technology)

Keywords:S-wave velocity structure, array microtremor exploration, KiK-net

At present, site amplification factors for S-net stations have been gradually evaluated such as Yadab et al. (2022). They are supposed to be evaluated by means of the spectral inversion technique [e.g., Iwata et al., 1986]. The technique requires a constrained condition in the inversion due to the trade-off between the source and site terms. In the present study, we chose three KiK-net stations, which are located along Pacific coast of Tohoku region, as constrained site amplification factors. In this study, we performed array microtremor explorations to estimate shallow basin structures at these KiK-net stations as reference data. The results will be compared in terms of S-wave velocity structure, spectral ratio and disperse curve of phase velocity etc.

Array microtremor measurements were conducted at three KiK-net stations; IBRH14, MYGH11, IWTH27. It consists of 4-point miniature array with a radius of 60cm and 3-point irregular array from around 5m to 10m on a side. These measurements were made quite near the stations. We measured for 15 minutes at each site using JU410 which is an all-in-one microtremor measurement unit manufactured by Hakusan Corp. Sampling frequency was 200Hz.

We evaluated one-dimensional S-wave velocity structures using the analysis method for shallow subsurface structure based on array microtremor data. The method has been proposed and enhanced in recent researches [e.g., Cho et al., 2008]. We made an analysis in the following procedure using a microtremor analysis system [Cho et al., 2016].

1. Auto-analysis and manual reading of disperse curves of phase velocity
2. Extraction of amplification factors such as AVS
3. Auto-analysis of horizontal to vertical spectral ratios (HVSRs)
4. Depth conversion of HVSRs
5. Direct depth conversion of disperse curves of phase velocity [Satoh et al., 1991]
6. Inversion process such as Simple Inversion Method [Pelekis et al., 2011]

This presentation will show comparison between observed data and theoretical one with respect to disperse curve of phase velocity, HVSR and 1-D S-wave velocity structure at the KiK-net stations.

References
Yadab P. Dhakal et al., “Features of horizontal-to-vertical spectral ratios of ambient noises and earthquake motions at the S-net sites,” SSJ Fall Meeting, C000119, 2021
Iwata et al., “Separation of source, propagation and site effects from observed S-waves,” Zisin Second Series, Vol.39, pp.579-593, 1986.
Hakusan Corporation, “Microtremor Measurement Unit JU410,” https://www.hakusan.co.jp/products/keisoku/ju410.html
Cho I. et al., “A new method of microtremor exploration using miniature seismic arrays; quick estimation of average shear velocities of the shallow soil,” BUTSURI-TANSA, Vol. 61, No.6, pp.457-468, 2008 (in Japanese).
Cho I. et al., “Constructing a system to explore shallow velocity structures using a miniature microtremor array – Accumulating and utilizing large microtremor datasets,” Synthesiology, Vol.9, No.2, pp.87-98, 2016.
Satoh T. et al., “Soil profiling by spectral analysis of surface waves,” Proc. 2nd Int. Conf. on Recent Advances in Geotechnical Earthquake Engineering & Soil Dynamics, Vol.2, pp. 1429-1434, 1991.
Pelekis P. C. et al., “An overview of surface wave methods and a reliability study of a simplified inversion technique,” Soil Dyn. Earthquake Eng., Vol.31, No.12, pp. 1654-1668, 2011.

Acknowledgement
This study was supported by JSPS KAKENHI Grant Number JP20K05055.