An array measurement of long-period microtremors was done in the western part of the Kanto plain, Japan, in order to extract subsurface structure down to the top of the Earth's crust. The area studied is characterized by the existence of deep sediments with thicknesses of around 2km. The subsurface structure near the array is well-investigated by seismic surveys as well as velocity loggings. We, therefore, can examine the applicability of the array technique of microtremors to the exploration of deep sedimentary layers.
Ten sites with station spacings of 0.3 to 2km were used for the array measurement, and a vertical seismometer with a natural period of 10s was set at each site. From the f-k spectral analysis, the phase velocities of microtremors in a period range from 0.7 to 5s are found to be in normal dispersion, indicating the large contribution of Rayleigh waves to microtremors. We, then, deduced the 1-D subsurface structure by an inversion of phase velocity dispersion curve of the Rayleigh wave. A reasonable structure for the deep sediments could be obtained, when the thicknesses of the sediments with fixed velocities were parametrized in the inversion. The resultant structure is in good agreement with the results of a seismic reflection survey.
From these results, we concluded that the array technique of microtremors can be used for the extrapolation of well-investigated structural data at a site to other sites in a basin. Furthermore, we can estimate S-wave velocities of deep sediments at any site by using the microtremor array technique with P-wave data from seismic refraction survey.
