1994 Volume 47 Issue 3 Pages 273-285
Estimation of the geological structure down to a depth of more than 1, 000m was attempted at two sites KS1 and KS2 in the Ashigara Valley in Odawara City assigned as a test field for the international cooperative research on the Effects of Surface Geology on Seismic Motion (ESG). The technique employed involves the measurement of phase velocities of Rayleigh waves in microtremors.
For the measurement, microtremors were observed at each site by seismic arrays consisting of 7 seismometers deployed within spaces from 100m to 600m in diameter. Applying the frequencywavenumber power spectrum analysis to the observed microtremor data, the phase velocities of the fundamental-mode Rayleigh waves in microtremors were successfully measured in the period range from 0.3s to 0.8s at KS1 and from 0.3s to 1.8s at KS2, respectively.
From these phase velocities, the geological structure was estimated as an S-wave velocity structure to a depth of about 1, 000m at KS1 and to a depth of about 2, 000m at KS2, respectively. Except for the upper structure to a depth of about 20m, the estimated S-wave velocity structure was in good agreement with that determined from the P- and S-wave velocity logging applied to a depth of about 100m at each site. The depth of the basement in the estimated S-wave velocity structures coincides with that in a P-wave velocity structure obtained by a seismic refraction survey, a profile of which runs near KS1 and KS2.
The observation of seismic motion had been made at KS2 on the surface of the ground and a place as deep as 100m in a borehole for the ESG symposium. The averaged spectral ratio of the seismic motions observed on the two places for 16 earthquakes have been obtained.
The period of the first and second peaks in the spectral ratio were calculated for the SH-waves vertically incident on the two places with their estimated velocity structures. The period of the first peak calculated is 1.0s, which coincides with that in the averaged spectral ratio, while the period of the second peak calculated is 0.4s, shorter than that in the averaged spectral ratio by 0.1s. This difference, however, could be well explained by slightly modifying the upper 20m part of the estimated S-wave velocity structure.
The present experiment revealed that the exploration technique using microtremors can be a powerful alternative to conventional seismic methods in estimating geological structures in urban areas.
