The Narao landslide occurred on October 6, 1976 is one of the typical landslides in tertiary rocks which widely distribute over the northern part of Nagano prefecture. This landslide may be the reactivity of an ancient landslide mass on the weathered base rocks. The sliding mass consists chiefly of weathered sandstone and mudstone with subordinate conglomerate and is particularly thick in the terminal part of the sliding area, where it attains 30-50 meters. The direction and the displacement of the marked point on the sliding mass could be measured by a geodimeter successfully.
The movement of landslide reached to 400-300mm/day at initial stage and decreases to a few mm per day at present. The movement of the sliding mass has been closely related to the changes of the groundwater level.
It is considered that the landslide was influenced by the tectonic movement along the Saikawa fault and a rising of porewater pressure in the base rock was occasion for sliding.

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A precise field sueveying by the combination of the AGA 6BL geodimeter and the WILD T2E theodolite was adopted for the observation of the displacement of mark points set on the surface of the landslide area at Narao in Nagano prefecture. The surveyed data were used to represent the position of the mark point in the three dimensional coordinates. The movement of the landslide (the movement of mark points) can be plotted on the vertical and horizontal projection planes. In this manner, the behavior of the landslide is observed on the respective projections.
On the vertical projection, (1) the landslide behavior at the head and the toe of the area is characterized, and (2) the trace of the movement of the mark point can be used for the examination of the existence of circular sliding surface in the landslide area.
On the horizontal projection, the rate of the change of horizontal distance between two mark pointsapproximately, the strain-can be used for the consideration of the process of the occurrence of cracks on the landslide area. Thus, the precise field surveying in the landslide area may be utilized for the analysis of landslide behavior.

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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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We might conclude that it would be impossible to block the landslides in this village without converting the existing farmland to forestland as much as possible, to minimize bare land liable to erode.
As erosion control practices in this place we suggest the following several engineering works: (1) drainage works such as well works and horizontal pipe works to cut of surface run-off pouring into the landslides, (2) earthwork to arrange the deteriorated land surface, (3) construction of check dams and groundsills on the valley, (4) piling works and retaining wall works to support roads, (5) reforestation program on the farmland.

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