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.

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; D. Shackelford, Fullerton CA.