Report on Toya (Japan) — March 1979
Scientific Event Alert Network Bulletin, vol. 4, no. 3 (March 1979)
Managing Editor: David Squires.
Toya (Japan) Seismicity and cryptodome uplift rate continue to decline
Please cite this report as:
Global Volcanism Program, 1979. Report on Toya (Japan) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 4:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN197903-285030.
42.544°N, 140.839°E; summit elev. 733 m
All times are local (unless otherwise noted)
The average number of earthquakes per day at Usu declined from 76 in January 1979 to 56 in February, and cryptodome uplift rates also decreased, to about 5 cm/day. No explosions have occurred since October.
Geologic Background. Usuzan, one of Hokkaido's most well-known volcanoes, is a small stratovolcano located astride the southern topographic rim of the 110,000-year-old Toya caldera. The center of the 10-km-wide, lake-filled caldera contains Nakajima, a group of forested Pleistocene andesitic lava domes. The summit of the basaltic-to-andesitic edifice of Usu is cut by a somma formed about 20-30,000 years ago when collapse of the volcano produced a debris avalanche that reached the sea. Dacitic domes erupted along two NW-SE-trending lines fill and flank the summit caldera. Three of these domes, O-Usu, Ko-Usu and Showashinzan, along with seven crypto-domes, were erupted during historical time. The 1663 eruption of Usu was one of the largest in Hokkaido during historical time. The war-time growth of Showashinzan from 1943-45 was painstakingly documented by the local postmaster, who created the first detailed record of growth of a lava dome.
Information Contacts: JMA, Tokyo.