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| | Abstract | | We conducted 2D microtremor array measurements to delineate a deep subsurface S-wave velocity model in Tsukuba City. The survey line was 12 kilo-meter long, and was divided to five sub-linear arrays with overlap. Forty single-component nodal seismograph (McSEIS-AT) and a geophone was used for the measurement of each sub-array, and the measurement time for each sub-array was 45 minutes. The total number of observation points is 200. Before the measurement, a huddle test was performed, and high coherence between any pairs of the geophones was observed in a frequency range of about 0.2 to 10 Hz. Twenty-four phase velocity dispersion curves were obtained at every 400 m by CMP-SPAC method (Hayashi et al, 2015). The frequency range of the valid dispersion curves was about 0.3 to 5 Hz. The phase velocity dispersion curves by linear array were consistent with the dispersion curve obtained by cross-shaped array conducted at several cross-sections on the survey line. In addition to the linear array measurement, we conducted single-station microtremor measurements at 20 points on the survey line using three-component nodal seismograph (McSEIS-AT3C). We observed that the peak period of H/V spectral ratio changed from about 1s to 4s. We conducted multi-station inversion using the phase velocity and the H/V spectral ratio. Genetic algorithm (Yamanaka and Ishida, 1995) was used in the inversion process. The result of the multistation inversion showed that the upper depth of sediments and basements (Vs3.2km/s) were smoother than that of individual inversion results. The depth of the top of the basement varies from about 600 m to 200 m. The results were consistent with PS logging results. |
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