Array measurements of vertical microtremors were done at Koto-ku, Tokyo in the Kanto plain, Japan, to deduce an S-wave velocity profile down to the top of seismic bedrock. The array consists of 13 sites with spacings of 0.2 to 4 km. Rayleigh wave phase velocities at periods from 0.8 to 5 sec were determined from a frequency-wave numeber spectrum analysis of the records. The observed phase velocities were inverted to an S-wave velocity profile by an inversion based on genetic algorithms assuming a four-layer model. The inverted profile has a sediment thickness that is thinner than those deduced by seismic refraction survey and a deep borehole near the array. We included Love wave phase velocity data determined from an earthquake array observation into the inversion of the phase velocity data from microtremor measurements. The resultant new structure is in a better agreement with deep borehole data than that inverted only from microtremor data.
