For environmental reasons, it became virtually difficult to investigate the underground structure in urbanized areas using explosive sources. Therefore, an easy and covenient survey method is now required. An array observation of microtremors may be one possible procedure to answer this demand, because it can be made anywhere, even in urban areas and much easier than the other exploration methods. From the above consideration, the observation of short period microtremors were carried out using

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... cross array of seismometers near an urban area in Honjo city, Akita prefecture, on August, in 1987. Their phase velocities in the frequency range 1 Hz to 4 Hz were determined by the frequency-wavenumber spectral analysis (F-K method) of the data of an array. The observed phase velocities were compared with the theoretical dispersion curves computed for a six layered model obtained by PS loggings. As a result, it is shown that the F-K method can be successfully applied to short period microtremors. This result suggests that the utilization of short period microtremors is one of practical methods for estimating a shallow velocity structure in urbanized areas.