Determination of Subsurface Structure of Izumo Plain, Southwest Japan using Microtremors and Gravity Anomalies

Abstract eng:
Microtremors and gravity anomalies were observed in the Izumo plain, Southwest Japan. Microtremors were recorded by seismic arrays (4 sites) and 3-component single-site observation (400 sites). Gravity data were obtained at about 50 sites. The microtremor data were analyzed by the spatial auto correlation (SPAC) method and the horizontal-to-vertical spectral ratio (H/V). S-wave velocity in quaternary and the distribution of predominant periods were determined by the microtremor array observation and the H/V (horizontal-to-vertical) spectral ratios, respectively. It was found from the result that the average S-wave velocity in quaternary was 230m/s. The predominant periods of H/V spectral ratios indicate thicknesses of quaternary following to the quarter wavelength law. The thickness was determined to be about 10 to 100m with the predominant period of 0.2 to 1.8s. The subsurface structures was determined by followings:1) S-wave velocity structure models obtained at the array observation sites, 2) a 3D bedrock configuration based on H/V and the residual gravity anomaly, 3) 2D density structures determined using the gravity anomaly and the seismic refraction method.

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