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Narao landslide, which occurred on October 6, 1976, is situated at about 22km SW of Nagano city. The scale of the landslide is about 600m long and about 150m wide. The landslide area is near a crossing of Gonda and Takafu formations made up of Miocene sandstone. Many cracks, which are locally oriented in a particular direction, are found on the ground surface. An attempt has been made to relate the crack generation with the characteristics of microtremors observed in the landslide area. For its purpose, an anisotropy of mass was assumed only for the upper part of the layered structure. Amplitude ratios of spectra of microtremors in the directions being parallel and perpendicular to the cracks were caluculated by using the multiple reflection method, and were compared with observations. The results are as follows:1. The particle motion of observed microtremors is polarized in the direction perpendicular to the local strike of cracks on the ground surface.2. The superficial layer in the near-surface structure can be regarded as an anisotropic medium, that is, a transversely isotropic medium with the unique axis perpendicular to the cracks.3. The lower boundary of the anisotropic medium seems to correspond with the sliding plane which appears locally in shallow structure.
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