6:15 PM - 7:30 PM
[SSS25-P17] Estimation for velocity structure of shallow sediments using microtremor array observation in Furukawa, Japan
In order to clarify the shallow ground structure in the area, the array observation of microtremors was carried out from December 13 to 16, 2014. The three components seismometer was adopted, that is, moving-coil type velocity sensor with the natural frequency of 2 Hz. We set three arrays in the north-western area of Furukawa station, which were the equilateral triangle arrays with radii of 2.9 m to 45 m.
The microtremor data were analyzed by the spatial auto-correlation (SPAC) method (aki, 1957). In the surface layer of ground, the shear-wave velocity is approximately 100 m/s at all the sites and the depth is 10 or 15 m. Also, the depth to the ground basement is around 35 m at all the sites. On the other hand, the differences of the ground properties are found in middle layers. It, however, is difficult to recognize the differences on the horizontal-to-vertical spectral ratios (H/V), because of the similarity of the predominant frequencies.
The estimated S-wave velocity structures suggest that the properties of middle layers affect to the anomaly of damage distribution. This means that the impedance ratio between middle layer and basement plays the important roles to the amplification of the ground motions in this area.
Goto et al. : Very dense seismic array observations in Furukawa district, Japan, Seism. Res. Lett., 83(5), pp.765-774, 2012.
Aki : Space and time spectra of stationary stochastic wave, with special reference to microtremor, Bull. Earthq. Res. Inst., 35, pp.415-456, 1957.