The Obasute debris flow deposits, distributed in the Obasute area of Koshoku City, are produced by mass movements from older debris deposits. The older debris deposits form the depositional plane which ranges in altitude from 750 to 850 meters above sea level. Based on the stratigraphical relationship with the Omachi tephras distributed in this area, it is evidented that the age of older debris deposits is estimated as about 100, 000 years before present. The ages of the Obasute debris flow deposits were determined as 13, 500±460 yr. B.P. and 3, 250±260 yr. B.P. by radiocarbon dating. The fact indicates that the intermittent occurrences of twice mass movements are distinguished among the Obasute debris flow deposits.

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In many cases, slide surfaces are observable in testpits, but hardly on the boring cores. This paper describes the results of exploration of slide surfaces in a few testpits in some study areas.
In the testpits, a state of a slide surface is not always visible to the naked eye. But it is possible to collect cohesive soil samples containing a slide surface. Therefore, the position and shape of a slide surface is determined when the constants of shear strength of cohesive soil is measured.
There is broad difference among the constants of shear strength on a slide surface. In application of the method of slices for the stability analysis of a landslide, testpits are necessary as many as possible for determination of those constants. In order to obtain exactly the safety factor of a landslide, we must know correctly the distribution of φ′ along the slide surface, in which constants of shear strength are C′=0 and φ′>0 in the moving landslide.

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The method of mapping for seismic slope stability was introduced by Wilson et al. (1979). They wrote critical acceleration (a_c) a_c=g[(c/γh)+(cos θ tan φ-sin θ)], where slope angle (θ) in the equation can be obtained on slope maps and cohesion (c), friction angle (φ), unit weight of landslide material (γ) and thickness (h) are estimated on engineering geological maps.
This method was applied to Nashimoto area in Izu peninsula. The area consists of three rock types: tuffaceous sandstone, andesite of Tertiary, and basaltic lava of Quaternary Period. The slopes in this area were classified into four ranges (>50°, 50°-40°, 40°-30°, <30°). Slopes steeper than 40° in the region of tuffaceous sandstone and more than 50° in andesite region have high susceptibility. Moderate susceptibility appears at slopes from 40° to 50° in andesite region and more than 50° in basaltic lava region. These estimates almost accord with the acutual data. The improvement of engineering geological maps, the problem of knick points and so on are suggested as mooted points of this method.

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This report shows the several findings of our investigation of the crystalline schist landslide in Shikoku. The observations of extensometers and internal strainmeters make it clear that the movement of the earth's. surface and underground has both expansion and contraction. The movement has an annual variation and a monthly variation. The drainage of four drainpipes is a few liter per minute, but the activity of a landslide declines remarkably.

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