In order to understand the sediment yield process and the mechanisms of the production of overland flow in devas-tated mountains, in-situ sprinkling experiments were carried out at in the Tanakami region. By comparing the results of these experiments and the results of the same experiments done at a devastated mountain in southern China, the following points are found. 1) In a devastated mountain where the surface soil layer consists of highly permeable soil, the rate of the Horton type surface runoff is small, and the sediment yield by individual transport is small. 2) In a devastated mountain where the surface soil layer consists highly permeable, saturated overland flow occurs. This sug-gested that mass transport is a significant process of sediment yield in these mountains.
Moreover, based on the above information, we re-considered the effects of hillside works for erosion control. with the following results. 3) The increase in soil depth brought about by stepped dam, contribute to the reduction in the saturation of the surface soil layer.

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What change does a seismic ground motion undergo under the influence of complex topological and geological conditions like mountainous areas? In order to make that clear through observations of actual seismic ground motions, we studied characteristics of seismic ground motions at the Shirasu (pyroclastic flow deposit) and granitic areas with aftershocks of the 1997 Kagoshima-ken Hokuseibu Earthquake. Furthermore, we tried to estimate characteristics of seismic ground motion from microtremor at each geological area.
At the top (STP) and foot (SFT) of the Shirasu terrace, about 33 m in difference of altitude and about 500 m away in a straight line, aftershocks were observed simultaneously. Consequently, while a seismic wave was passing through the Shirasu layer, remarkable changes appeared in the frequency characteristics of the horizontal seismic ground motion, i.e. amplification of vibration of 2.5 to 4.5 Hz and damping of vibration of 6 to 10 Hz. Moreover, as a result of the simultaneous observations of the aftershocks at STP and a granitic area (GTM), relatively long-period vibration of 2.5 to 6 Hz was predominating at STP the same as the above-mentioned, and on the other hand short-period vibration of 5.5 to 15 Hz was predominating at GTM.
Microtremors were observed at (1) the top (STP, SGM) of the Shirasu terrace of the Ito pyroclastic flow origin, (2) the granitic area (GTM, GKS) intruded in the Neocene, and (3) Shimanto group (MSB, MOS) of the Mesozoic. Predominant frequencies in spectral ratios were as follows : (1) 2 Hz, (2) 6 to 10 Hz, and (3) 15 to 20 Hz. The amplification characteristics of the surface ground were roughly presumed from the microtremors.
By means of the relation between the predominant frequency of the earthquakes and the nature frequency of the ground, we can probably explain features of the distribution of landslides, e.g. few landslides at the Shirasu area and many ones at the granitic area.

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Hydraulic properties of forest soils, the hydraulic conductivity, K, -pressure head, ψ, relationship and the volumetric water content, θ, -ψ relationship of forest soils, influence the transient process of storm water in the sudace soil layer on the mountain slope and occurrence of hillside landslides. In this study, to estimate influence of forest soils on mountain slope stability, numerical simulation was executed with the vertical infiltration experiment results, hydraulic properties of forest soils, to the Tsuchisawa basin where many hillside landslides occured in 1969. The typical results were as follows:(1)The surface soil layer on the mountain slope have a function of the shock absorber against storm water infiltration to the bed rock. (2) Forest soils have the control function against occurrence of hillside landslides and the water recharge function. Furthermore, regarding satisfactory forest as erosion control facilities, influence of slope shapes on the control function against occurrence of hillside landslides and the water recharge function of forest soils was estimated using the model for predicting hillside landslides, to find criterion for the proper arrangement of forest. It was clarified that it was more effective to arrange satisfactory forest in convex and parallel shapes than concave shapes in order to demand maximum effects of forest. In the future, it is necessary to examine appropriateness of these results obtained by this study and research into a limit of these functions of forest soils.

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Soil tests and fluidization tests for the soils sampled from debris flows at the Gamahara River, at the Harihara River and at the Hachimantai Area were conducted to investigate characteristics of soils and to research the fluidity of the soils. Grain size distribution, saturated water content, liquid limit and plastic limit were measured by the soil tests. A small flume, 1.6m long, 10cm wide and 15 cm deep, with 2 different gradient sections was used for the fluidization tests. The gradient of the upper section of the flume was 45°, and lower section was chaged 10°, 15° and 20°. The soils were settled in the upper section, then the soils slid down in the upper section and flowed into the lower section. The lengths of flow-down in the lower section were measured. We judged fluidization occurred when the flow-down length in the lower section exceeded 110 cm. The water content when the fluidization occurred was measured and de-fined as “fluidity limit”.
Results obtained from the soil tests can be summarized as follows.
The order of A.M.I. of the soils sampled from the slopes where the debris flows generated is Hachimantai Area<Harihara River<Gamahara River. The order of percentage of fine material, saturated water content, liquid limit and plastic limit is same. The A.M.I, of the soils sampled from the slopes where the debris flows generated and from the deposits under the slopes ranged from 0.8 to 0.9, and those of debris flow deposits were about 1.0 in the Harihara River. These data indicate that the water contents of collapsed soils increased on the way of flow-down in the Hari-hara River.
Results obtained from the fluidization tests can be summarized as follows.
The order of the fluidity limit of soils sampled from the deposits of debris flows is Hachimantai Area<Harihara River<Gamahara River like A.M.I, and liquid limit. The fluidity limits of the soils contained coarse material were smaller than those of the soils without coarse material. The fluidity limits were larger than the liquid limits in any soils, and it suggests that liquid limit does not always indicate the threshold of fluidization of collapsed soils which change into debris flows.

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The purpose of this study is to improve the understanding of morphological and geological features of debris flows cuased by earthquakes. Data of 24 slope movements which occurred in Japan after 1900 were collected from number of related literatures and investigated. The ratio of run-out distance of collapsed debris to the height of collapsed part is defined as a run-out ratio. The run-out raio and the degree of disturbance of deposited material were investigated. When the slope movements with run-out ratio more than 7 and high disturbance, and with run-out ratio less than 4 and low disturbance are regarded as debris flows (fluidization occurred) and talus formations (fluidization did not occur) respectively, 8 slope movemets are debris flow type and 16 slope movements are talus formation type out of the 24 slope movements. The results are summarized as follows. 1) The obvious correlation was not observed between volume of collapsed debris and equivalent fiction coefficient, and the equivalent friction coefficients of slope movements at the volume of 10⁴-10⁶m³ order are relatively smaller than the values which are obtained in the past investigations. 2) Regarding the run-out ratio, the strong correlation was observed between the inclination at the downstream of collapsed part and inflow angle to the downstream of collapsed part. When the inclinations at the downstream exceed 0.15 (about 9 degrees), the run-out ratios become more than 7 i.e. debris flows occur. When the inflow angles are less than 70 degrees, most of the collapsed debris change into debris flows. Moreover, the negative correlation was observed between the inflow angle and downstream inclination. 3) Regarding the transverse shape of slopes classfled into ridge, equilibrium and valley, the valley shape slopes are more prone to debris flows compared with the ridge or equilibrium shape slopes. 4) Regarding the geology, the volcanic product area are more prone to debris flows compared with the tertiary or quaternary area.

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