The history of large]scale sediment disasters since the 1930s in Miyazaki, which is a region of high rainfall in Japan, was studied in order to evaluate frequency of large sediment]movement events, and to examine the relationship between magnitude of sediment movement and rainfall. The large sediment]movement events of amount exceeding 100,000 m3 of sediment have occurred seven times in the fifty one years from 1954 to 2005, and thus the frequency of events is calculated once seven years. Those events were triggered by heavy rainfalls greater than 600 mm of a two]day amount or 1,000 mm of a three]day amount. There are two types of the large sediment]movement events: one is a very large landslide of 105 to 106 m3 of collapsed sediment; and the other is a mass of landslides and debris flows of various sizes, amounting to 105 to 106 m3 of transported sediment. Using data of all the landslide disasters in the past forty years from two landslide]prone areas of Miyazaki, the authour examined how landslides increase in number in response to an increase of rainfall. Number of landslides can be regarded as an indicator of magnitude of sediment]movement events. Two types in the relationship between number of landslides and rainfall were found. One is the type that the number of landslides increases rectilinearly in response to an increase of rainfall, and a change of magnitude of sediment movement is continuous from low magnitude to high one. The other is the type that the number of landslides increases remarkably when rainfall exceeds 1,200 mm, and a change of magnitude of sediment movement is discontinuous between low and high magnitudes..

Key wordsFlarge events, magnitude, frequency, landslides, rainfall