Abstract
On the morning of December 6, 1996, a landslide flowed down into a valley stream and transformed to a debris flow in Otari village, central Japan. The flow washed away those who were engaged in the preventive work near the lower reaches of the stream and killed fourteen persons. It was proven later that a series of ground tremors occurred for 3 minutes, but seemed not to be caused by an earthquake, was recorded at a seismic observatory 5.7 km south-southeast from the landslide area on the same morning. Examination of the waveform showed that the tremors were possibly derived from the downward flow of the debris . The average velocity of the debris runout (of length about 2.8 km) estimated from the duration time of tremors was about 57 km/h . The kinetic analysis of the debris showed maximum velocity reached approximately 100 km/h . The FFT analysis indicated predominant frequencies ranging from 1 to 2 Hz. This range was closer to frequencies of landslides as opposed to high frequencies of debris flows. The seismic energy was equivalent to M=0 .05, in this case. The analysis of past records indicated that there was a relationship between seismic energy triggered by landslide, M and landslide volume and Gamaharasawa's case was also associated with this relationship.
