5:15 PM - 6:30 PM
[SSS11-P07] Small-aperture Microtremor Array Surveys in the Nara Basin, Japan
Keywords:Nara Basin, microtremor array survey, velocity structure model
The Nara-Bonchi-Toen Fault Zone forms the eastern margin of the Kyoto and Nara basins, Japan. The Nara basin is mostly filled by the lower and lowermost parts of the Plio-Pleistocene Osaka Group. Alluvial and terrace deposits distributed over the Osaka Group inside the basin. Improvement on the velocity structure model from the superficial alluvial deposits to the seismic bedrock is quite important for advancing strong motion prediction for future earthquakes from nearby source faults. As a part of Comprehensive Research Project for the Nara-Bonchi-Toen Fault Zone funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, we have carried out various kind of geophysical surveys to investigate the velocity structure in the Kyoto and Nara basins. Miniature to small aperture microtremor array surveys were carried out at 60 sites in the southern part of the Kyoto prefecture from Aug. 2019 to Mar. 2020 to obtain information on S-wave velocity of shallow subsurface sedimentary layers (Asano et al., 2020). The surveyed area is extended to the whole part of the Nara basin in 2020. The total number of surveyed sites is 140 as of Jan. 29, 2021 (Fig. 1), and the field work is still undergoing to cover the whole area of the Nara basin. We will report our results in the Nara basin in this presentation.
We carried out miniature and small equilateral triangle array observations at each measurement. The circumradius is 0.6 m for a miniature array, and the circumradius for a small array depends on site in a range from 5 to 8 m. A portable integrated microtremor observation system HAKUSAN JU410 was installed at each apex of the equilateral triangle and the center of circumscribed circle. The microtremor was recorded continuously more than 15 minutes at a sampling rate of 200 Hz after amplified by 100 times (200V/G).
The vertical component of the observed microtremors were analyzed to obtain the spatial autocorrelation (SPAC) coefficients (Aki, 1957). The spatial autocorrelation function was calculated in the frequency domain, and the Fourier spectrum was smoothed by the technique of Konno and Ohmachi (1998). Finally, the phase velocity dispersion curve was estimated by the extended SPAC method (Ling and Okada, 1993; Okada, 2003).
The one-dimensional S-wave velocity structure model for each measurement site was estimated assuming the observed phase velocity dispersion curve as the fundamental mode of the Rayleigh wave using the Markov chain Monte Carlo method. The velocity structure model of deep sedimentary layers and the upper crust was given referring to the existing three-dimensional velocity structure models J-SHISV2 (Fujiwara et al., 2012), and the velocity structure model from the lower crust to upper mantle was given referring to JIVSM (Koketsu et al., 2012). The obtained S-wave velocity structure model will be discussed in terms of the spatial distribution of low-velocity layers with referring to surface geology and boring data information.
Acknowledgement: This work was conducted as a part of Comprehensive Research Project for the Nara-Bonchi-Toen Fault Zone of the Ministry of Education, Culture, Sports, Science and Technology, Japan. We appreciate many residents for cooperation of our survey.
We carried out miniature and small equilateral triangle array observations at each measurement. The circumradius is 0.6 m for a miniature array, and the circumradius for a small array depends on site in a range from 5 to 8 m. A portable integrated microtremor observation system HAKUSAN JU410 was installed at each apex of the equilateral triangle and the center of circumscribed circle. The microtremor was recorded continuously more than 15 minutes at a sampling rate of 200 Hz after amplified by 100 times (200V/G).
The vertical component of the observed microtremors were analyzed to obtain the spatial autocorrelation (SPAC) coefficients (Aki, 1957). The spatial autocorrelation function was calculated in the frequency domain, and the Fourier spectrum was smoothed by the technique of Konno and Ohmachi (1998). Finally, the phase velocity dispersion curve was estimated by the extended SPAC method (Ling and Okada, 1993; Okada, 2003).
The one-dimensional S-wave velocity structure model for each measurement site was estimated assuming the observed phase velocity dispersion curve as the fundamental mode of the Rayleigh wave using the Markov chain Monte Carlo method. The velocity structure model of deep sedimentary layers and the upper crust was given referring to the existing three-dimensional velocity structure models J-SHISV2 (Fujiwara et al., 2012), and the velocity structure model from the lower crust to upper mantle was given referring to JIVSM (Koketsu et al., 2012). The obtained S-wave velocity structure model will be discussed in terms of the spatial distribution of low-velocity layers with referring to surface geology and boring data information.
Acknowledgement: This work was conducted as a part of Comprehensive Research Project for the Nara-Bonchi-Toen Fault Zone of the Ministry of Education, Culture, Sports, Science and Technology, Japan. We appreciate many residents for cooperation of our survey.