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Report on Toya (Japan) — July 1980

Scientific Event Alert Network Bulletin, vol. 5, no. 7 (July 1980)
Managing Editor: David Squires.

Toya (Japan) Cryptodome uplift and seismicity continue

Please cite this report as:

Global Volcanism Program, 1980. Report on Toya (Japan). In: Squires, D (ed.), Scientific Event Alert Network Bulletin, 5:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198007-285030.

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42.544°N, 140.839°E; summit elev. 733 m

All times are local (unless otherwise noted)

Seismic activity (table 3) and cryptodome uplift have continued through May 1980. The rate of cryptodome uplift and outward movement of the N somma wall both averaged about 5 cm/day in November 1979 and 4 cm/day in March 1980. In May, white vapor rose from three vents. Three new parallel faults with a combined throw of about 60 cm passed through the S portions of the active vents.

Table 3. Monthly means of the number of recorded earthquakes per day, July 1979-May 1980. Courtesy of JMA.

Month Mean Daily Earthquakes
Jul 1979 47
Aug 1979 36
Sep 1979 35
Oct 1979 36
Nov 1979 36
Dec 1979 44
Jan 1980 38
Feb 1980 35
Mar 1980 29
Apr 1980 19
May 1980 22

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; D. Shackelford, Fullerton CA.