Table 1 Previous classifications dividing mass movement into two types

Fig. 1 Landslide movement recorded at Yomogihira landslide, Niigata Preecttzre

Fig. 2 Landslide movement recorded at Higashiyama landslide, Ibaraki Prefecture

Table 2 Records on the maximum speed and duration of landslide movement

Fig. 3 (A) Frequency distribution of landslide duration( B) Fregneney distribution of landblide speed

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A Classification of Landslides based on the Frequency Distribution of Their Speed and Duration Keiji MIZIJNO* Many Japanese workers have recognized that landslides can be classified into two types; in type (1) the speed is higher, the duration is shorter and the movement rarely reactivates; in type (2) the speed is lover, the duration is longer and the movement continues intermittently for a long period. The former is named here A-type and the latter, B-type, Previous classifications which svei e based can experience are qualitative and descriptive. Therefore, it is necessary to quantitatively classify these two types. The maximum speed and the duration of movements recorded at 120 landslides on natural slope in Japan were quantitatively investigated on the basis of the existing literature. The movement of the A-type landslide is divided into (1) pre-failure movement, (2) main movement and (3) post-failure movement, as shown in Fig. 1 as an example. The distance and speed of the pro failure and post-failure movements are so small as to be negligible, compared with those of the main movemertt. Therefore, the speed and the duration for landslides of this type are estimated from the data on the main movement. For the B-type landslide, active and inactive periods alternately repeat (Pig. 2). The duration and the maximum speed are estimated from one active period. The histograms of duration and maximum speed constructed using the existing data are shown in Fig. 3. The duration histogram (Fig. 3-A) has a hi-modal distribution, which can be divided into two groups at the bottom of trough of the histogram, i.e., 101.5 ( à32) hours. Dual-peak distribution is found in the speed histogram (Fig. 3-B); a trough between the two peaks is located at 10-2 to 10-1 m/min. Fig. 3-B also shows that (1) landslides with a duration of less than 32 hours have a higher speed than 10-2m/min, and (2) the landslides with a duration of more than 32 hours have a lower speed than 10-1m/min. The above discussion leads to the following classification. A landslide having higher speed than 10-1.5m/rain and shorter duration than 32 hours is named the "rapid-type", and one having lover speed than 10-1.5m/min and longer duration than 32 hours is named the "slow-type". (Key words: landslides, classification, frequency distribution, speed, duration) Geographical Review of Japan 62 A-4 320-331 1989 * Institute of Geoscience, University of Tsukuba.