The 1977 Eruption of Usu Volcanno with a Special Reference to Prediction of Volcanic Activities

Abstract

The activities of Usu Volcano has been continuing since August 1977 though the early eruptive period ended a week after the outbreak of the eruption. The present activities are felt and unfelt earthquakes, crustal deformations in the summit crater and at the foot of the volcano, and occasional steam explosions. Long-period prediction of the outbreak of the 1977 Eruption depended on the case studies of the past eruptions of the volcano. The eruption occurred almost in accordance with the prediction. Short-Period prediction of the outbreak consisted of seismometric, geodetic and geothermic ones. Unfortunately, nevertheless, the eruption broke out only 30 hours after the first swarm of earthquakes before these data would be synthetically analyzed. Predictions of developments of the volcanic activities after the outbreak of the eruption, depend on the geophysical observations of various kinds together with petrological examinations of the ejecta from the new craters. By seismometric observations, the hypocenters and the source mechanisms are determined, and the energy released by volcanic earthquakes is estimated. By geodetic observations, the rates of upheavals in the summit crater and of contractions along the volcano slopes are watched. Both the air-borne and land infrared imageries of the summit crater would give us the important informations concerning the types of the future eruptions, fissure or dome types, and developments of the activities. It is highly desirable that all results of various observations should be synthetically taken into consideration for predictions of the volcanic activities.