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.

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

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    Year   Month   Local Seismic Events
                Recorded     Felt

    1980    Jan     [1177]       234
            Feb      1004        216
            Mar       890        162
            Apr       582         92
            May     [ 674]       121
            Jun     [ 221]        32
            Jul       601        112
            Aug       486         82
            Sep       620        108
            Oct       413         69
            Nov       604        106
            Dec     [ 571]        94

    1981    Jan       357         63
            Feb       289         49
            Mar       235         41
            Apr       485         92
            May       153         35
            Jun       151         33
            Jul       423         89
            Aug       317         64
            Sep       244         41
            Oct       315         52
            Nov     [ 290]        54
            Dec       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.

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; I. Yokoyama, Hokkaido Univ.