It is important to know under ground velocity structure, especially shallow S-wave velocity structure, in urban areas to study on seismic microzoning or to predict strong motion in the high frequency range during a large earthquake. However, it is difficult to carry out a survey such as refraction or reflection because there is no space for a survey. Microtremors exist anytime or anywhere in urban areas. They are thought to construct an ensemble with body and surface waves. If we can detect phase velocities of surface wave from microtremors, we can estimate underground velocity structure from the dispersion relation using an inversion technique. We carried out array observations of short-period micro tremors at eleven sites in Morioka area to detect phase velocities as a function off requency using frequency-wavenumber analysis. Then, we estimated under ground velocity structure which consists of some layers on half-space from the dispersion relation of Raylaigh wave, using a linearized inversion technique. The shallow underground velocity structures were estimated from Rayleigh wave dispersion relations at nine sites. They were consistent with S-wave velocity structures obtained from bore-hole data. It was shown that short-period microtremor array observation was useful for prospecting shallow structure beneath urban areas where there is no space for other geophysical prospecting technique.
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