For the purpose of grasping characteristics of disaster at mountainous district due to earthquake, the circumstances of disaster were adjusted and analyzed about the Kanto, Niigata, and Izu-Oshima-Kinkai Earthquakes. Further, earthquake accelerations at the stricken districts were presumed and it was tried to grasp the relationship between the accelerations put in a correction according to elevation and the disaster. The results are summarized as follows. (1) Generally, cracks occur by a principal shock and afterward landslides occur by aftershocks or rainfalls. (2) Especially near a fault landslides are easy to occur. (3) Landslides increase rapidly with the increment of slope where an angle of inclination of slope is over 30°. (4) The average depth of landslide is about 1-2m. (5) Acceleration tends to increase in proportion to elevation and area rate of landslide has the same tendency, too. (6) The minimum value of acceleration for occurrence of landslide seems to be about 200gal. (7) A debris flow streams down with passable velocity over 10m/s and oscillation due to earthquake appears to take part in that.
Transportation of debris by debris flows was analyzed through an air-photo survey at a gully entrenching the eastern slope of Mt. Yake in the Japan Alps. It was revealed that the debris movement can be examined in three phases; traction of debris by surface flow at the headwaters, occurrence of debris flows, and conveyance of debris dy the debris flows along the gully. Tractive transport at the headwater was proportional to the depth of effective rainfall, or to toe volume of effective discharge. Hydrological analysis based on the kinematic-wave method revealed that debris flows take place whenever the discharge of the gully exceeds a critical value. Variation of sediment transport along the course of debris flows was characterized by that of the K-value which representst he volume of debris transport divided by the depth of the effective rainfall. The K-value increases downstream reflectihg the entrainment of debris into the debris flows, and becomes more than ten times as large as its initial value. It was shown that the K-value depends on the rainfall which triggers the debris flow at their occurrence point, but on the total rainfall including the antecedent rainfall at the lower reaches.
Early in the morning on 4th July, 1981 a large soilfall broke out on the hillside behind Honkakuji temple, Etajima in Hiroshima Prefecture. Old men, who attended the mass in the hall, were killed and injured. This soilfall is considered the landslide, because it is large scale and the time of occurrence of slope failure is about 6 hours after the peak of rainfall. Then we investigated the cause of this landslide by geological survey, boring, seismic prospecting, electrical prospecting and natural radioactivity survey. According to this investigation, it is considered that underground water, which was due to this disaster, flowed from the fracture zone, to the north and north-east, and acted as the uplift from behind the failure slope.