Seismic Activity Immediately before and in the Early Stage of the 1977 Eruption of Usu

The first eruption of Usu Volcano occurred near the summit at 09: 12 a.m. (JST) on August 7, 1977 after 32 years ' dormancy. This eruption was not only preceded by a number of earthquakes, but also followed by remarkable earthquake swarms. During the early eruptive stage four large and two moderate eruptions occurred accompanied by both volcanic tremors and high seismic activity. The data on the earthquakes were obtained from two routine stations, HSS (.&=58 km) and ERM (.& = 167 km), and a temporary station, SSU (.&=2.5 km), and these seismic activities were studied. Thirty-two hours before the first eruption premonitory earthquakes began to occur and for 9 hours before the eruption exponential increase in earthquakes was observed. For 4 to 7 hours before the outbreak, stations near and distant from the volcano also observed premonitory variation from high frequency to low frequency in seismic wave form. A series of the six eruptions occurred through the following five stages: (i) cloud with ash and pumice ascending up high, (ii) seismic activity diminishing in intensity, (iii) successive tremors which lasted to the end of the eruption increasing and decreasing in amplitudes, (iv) ejec-tion of ash and pumice coming to rest, and (v) a calm state with no earthquakes and no tremors dominating for a while. The later the eruption occurs, the slower the decline of seismic activity as well as the growth of tremors becomes. A linear relationship was found between the height of volcanic cloud up to 10 km and the amplitude of tremors. 1