1991 Volume 44 Issue 4 Pages 30-35
After the last explosions of the 1988-1989 Tokachi-lake Volcano Eruptions, the volcano returned to dormant condition. However, the western slope to the summit of the volcano has been covered by the block- and ash-flow deposits which ejected by the 21 times eruptions. The deposits are interbedded with thick snow deposits because that the eruptions occurred during winter time.
On the summit areas of the Tokachi-dake, considerably heavy rainfalls have been recorded on August, 22nd and 23rd, 1989. In particular, the rainfalls on August 23rd triggered debris flows along the upper branch stream of the Furano-gawa. Water running on the surface of the deposits of the summit slope has eroded those flowing along the western slope, accerelated by melting of snow interbeds between the block- and ash-flow deposits.
Consequently, several gullies have been formed on the upper western slope. Debris flows have been generated and deposited a small-scale debris (1, 000m3) on a gentle slope about 7° in degree. Mean diameter of emplaced blocks is about 0.5-0.8m and the maximum diameter is about 1.1m.
The velocity of the gully erosion and debris flow has been measured by the cut down of the cable for seismometers set on the western slope. If the seismometer might have caught the time of commence of the debris mass movement on the summit and that of the generating of the debris flows on the lower slope, the data suggest that the velocity of the debris flow was 2-4m/sec.
Application of the actual data, such as size, deposition volume, slope inclination, to several experimental formula, has revealed that the velocity of the debris flow is about 5m/sec, which is similar to the value estimated by the seismometers.
