The strain-rate tensor at a point on the surface of a landslide can be determined by setting up 5 stakes and measuring the rate of change of the distances between them . A suitable arrangement of stakes consists of 4 stakes at the corners of a square with 1 stake at the centre . Eight such patterns of stake arrange ment were used in the Ogawamura-Sodechi Landslide in 1987-1988 . Three independent components of the strain-rate tensor and their errors can be estimated by a least-squares method with 4 governing conditions derived from 8 distance measurements . The principal strain-rates are found to within about±6×10^-4/day. The surface deformation pattern of the landslide, directions of the cracks and seasonnal variations of the landslide movement are well indicated by the strain-rate tensor and its invariants; namely, magnitude of the dilatation rate and maximum shear strain-rate. The previously reported model of the landslide movement is improved by the strain-rate tensor determianation.

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This paper deals with auther's research on details of actual sliding surfaces observed in Collector Wells at Hirayama, Okuooe, Nukedo landslide areas.
Observations on sliding surfaces lead to the recognition that the sliding surface is complex and not a geometrically single one. On the contrary, what we call sliding surface consists of a zonal structure having a thickness of a few centimetres. In this zonal structure there is a fine clay seam containing many cracks and slickensides which are produced by shearing and slipping.
Groundwater seapages are often observed at the position of several centimetres above the principal slip surface and the position of 2 or 6 metres above the sliding surface, whereat no seapages are observed in the fissured shearing zones. The presence of groundwater in the sheared zone, however, is obvious, because it is detectable by means of analyses of drilling records and groundwater logging. The sheared zone is usually detected as a leakage zone by the drilling record analysis and laminar inflow or upward flow in the process of groundwater logging in the borehole. These lead to an important hypothesis that groundwater pressure is transmitted through the fissured shearing zones and works as uplift on the sliding surfaces, though the amount of groundwater is not large.

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Heavy rain lashed Shimane Prefecture in July 1988, triggering slope failures and flood in many places especially in Hamada, Kanagi and Misumi districts. 87 slope failures damaged houses and 4 people were killed by some of them.
These slope failures were investigated and described in this report . These districts have experienced many slope failures caused by heavy rain in July 1983. At that time 765 slope failures occured in Shimane Prefecture and 40% of those were in above districts.
Considering rainfalls required to produce slope failures, each district has almost the same effective rainfall in both cases.

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