Takarada S., Hokkaido Branch, Geological Survey of Japan, Kita-8, Nishi-2, Kita-ku, Sapporo 060, Japan. (takarada@gsj.go.jp)
The valley-filling (VF) type Iwasegawa debris avalanche deposit, ca. 0.1 km^3, is distributed along the valleys at the SE foot of Tashirodake Volcano, northern Honshu, Japan. The runout distance (L) was 12.4 km and vertical drop height (H) was 0.72 km. Debris avalanche blocks (DBs) range in size from <35 m proximally and <10 m at the medial area. Debris avalanche matrix (DM) is 30 % -60 % at the proximal area and up to 95 % at the distal area.
The VF type Kaida debris avalanche deposit, 50,000 y.B.P., >0.3 km^3, is distributed along the valleys at E-SE foot of Ontake Volcano, central Japan (L= 46 km and H= 2.5 km). DBs range in size from <25 m proximally and <7 m at the medial area. DM is 25 % - 60 % at the proximal area and up to 95 % at the distal area.
The wide-spreading (WS) type Zenkoji debris avalanche deposit, 7000-8000 y.B.P., ca. 1 km^3, is widely distributed at the SE foot of the Usu Volcano, SW Hokkaido, Japan. The distal material was deposited into the sea as far as 6.5 km from the source. DBs range in size from <300 m proximally and <100 m at the medial area. DM is 0 % - 5 % at the proximal area and up to 20 % at the medial area. A 50 m-wide, 450 m-long, 15 m-high natural levee was observed at the western margin of the avalanche deposit.
These features suggest that the VF type debris avalanches tend to have
smaller DBs and larger amount of DM than the WS type debris avalanche. These
differences may be formed by separation of DBs due to collision against
valley walls and a high production rate of DM due to interaction between
DBs and valley walls in the VF type debris avalanches.