Yosihiro Sawada provided the following data (table 2) on total summit area elevation changes since activity began in August 1977 [based on results from Hokkaido University]. The uplift rate at Ogari-yama was 5 cm/day in May and 3 cm/day in July.

Table 2. Total elevation change at several features on Usu, as of 31 July 1979. Courtesy of Y. Sawada.

Seismicity continued to decline slowly, from an average of 54 events/day in June to 47/day in July. The actual number of events per day sometimes varied considerably from the mean; in June, daily earthquakes ranged from 5 to 293.

When inspected on 31 July, Gin-numa Crater (formed by the activity of August-October 1978) contained a pool of water. Strong fumaroles on its rim had maximum measured temperatures of 635°C.

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: Y. Sawada, MRI, Tokyo; D. Shackelford, CA.