GEOHISTORICAL DEVELOPMENT OF THE TOCHIYAMA LANDSLIDE IN NORTH-CENTRALJAPAN

The Tochiyama landslide is one of several complex, deep-seated and lar ge-scale landslides occurring in the Hokuriku Province in central Japa n. The landslide is about 2 km long and about 500-1100 m wide; it occu pies an area of approximately 150 ha and has a maximum depth of 60 m. The slide developed on a dip-slope structure, and is divisible into th ree layers in ascending order: older landslide debris and avalanche de posits, younger debris-avalanche deposits, and talus. The landslide co mplex is still active. A triangulation point on the upper part of the landslide shifted downhill by 3.3 m from 1907 to 1983, indicating an a verage rate of 4.3 cm/y. In 1991, the average rate of movement on the sliding surface was also 4.3 cm/y as measured by an automatic system w ith inclinometers installed in borehole No. 1-2. The rate measured for borehole No. 1-3, located 380 m upslope from No. 1-2, was over twice that of No. 1-2 for the same period; it has since accelerated to about 19 cm/y. Thus current movements on the basal sliding surface are inho mogeneous; the head of the slide complex is increasing the horizontal granular pressures on the lower part of the slide block. On the basis of dating of two tephra layers and C-14 dating of carbonized wood inte rcalated within the landslide body, two stages of slide movement have been distinguished. The earlier occurred between about 46,000 to 25,00 0 years ago, and the latter occurred since 1361 A.D. The following seq uence of events is inferred. During the middle Pleistocene, intense te ctonic movements occurred in the Hokuriku Province, and as a consequen ce dip-slopes were developed in the Tochiyama landslide area. Low-angl e fault planes (possibly representing slump features) and fracture zon es then developed within flysch deposits underlying the landslide area , causing a reduction in shear strength. The erosion base level was lo wered during the Wurm glacial age, and due to severe erosion and incis ion of stream valleys, the surface slope angle rapidly increased, and toe resistance decreased. This combination of causes led to the develo pment of a deep-seated primary landslide. As a result of an accumulati on of younger deposits, regional uplift and further local erosion, sta bility of parts of the region decreased and led to landslide activity of a second stage. Reactivated and locally accelerating creep movement s occur today and may forewarn of a stage of reactivated, hazardous ra pid sliding, such as occurred with the adjacent and analogous Maseguch i landslide in 1947.