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Report on Toya (Japan) — January 1981


Scientific Event Alert Network Bulletin, vol. 6, no. 1 (January 1981)
Managing Editor: Lindsay McClelland.

Toya (Japan) 1980 cryptodome uplift and seismicity summarized

Please cite this report as:

Global Volcanism Program, 1981. Report on Toya (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:1. Smithsonian Institution.



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

All times are local (unless otherwise noted)

Cryptodome uplift and local seismicity continued through 1980. Local seismicity continued an irregular decline through 1980 (figure 19 and table 4). Felt shocks averaged 3/day in 1980, but swarms of 30-40 felt events in a single day occurred about once a month. . . . Careful correlation of seismic records with observed surface deformation and faulting revealed that larger earthquakes occurred simultaneously with measureable fault movements.

Figure (see Caption) Figure 19. Monitoring data from Usu, August 1977-April 1980, showing discharge rate of seismic energy in ergs/day (top) and uplift rate in cm/day of the Usu-Shinzan cryptodome (bottom). Note the increase in February 1978. [Uplift] data are from I. Yokoyama.

Table 4. Monthly number of local seismic events at Usu, 1980-81. Courtesy of JMA.

Month Recorded Events Felt Events
Jan 1980 [1177] 234
Feb 1980 1004 216
Mar 1980 890 162
Apr 1980 582 92
May 1980 [674] 121
Jun 1980 [221] 32
Jul 1980 601 112
Aug 1980 486 82
Sep 1980 620 108
Oct 1980 413 69
Nov 1980 604 106
Dec 1980 [571] 94
Jan 1981 357 63
Feb 1981 289 49
Mar 1981 235 41
Apr 1981 485 92
May 1981 153 35
Jun 1981 151 33
Jul 1981 423 89
Aug 1981 317 64
Sep 1981 244 41
Oct 1981 315 52
Nov 1981 [290] 54
Dec 1981 440 105

The rate of cryptodome uplift decreased through 1980, from 5 cm/day in January to 3-4 cm/day in December [at the peak of] Usu-Shinzan . Northward lateral movement of the N flank continued at a similar rate. As a result, compression of the ground N of the volcano also continued, affecting several towns and villages.

Geological Summary. 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; I. Yokoyama, Hokkaido Univ.