The eruptive activity at the summit crater, which was initiated in October 1955, has continued through the period of 1991-1994 as illustrated in Fig. 8, though activity declined for months in 1993 and in 1994. The Kagoshima Meteorological Observatory issued volcanic advisory 22, 10, 6 and 7 times in 1991, 1992, 1993 and 1994, respectively. Slight damages on windowpanes of houses, cars and airplanes due to air-shocks or lapilli were occasionally reported in 1991-1992.

Annual numbers of explosive eruptions in 1991, 1992, 1993 and 1994 were counted as 282, 152, 88 and 126, respectively, by Sakurajima Volcanological Observatory (SVO) of Kyoto University. The total number of explosive eruptions since 1955 as of the end of 1994. The amount of volcanic ash ejected in each year was estimated to be 15.7, 16.8, 3.7 and 2.9 millions tons, respectively by SVO, using the data of the Kagoshima Prefectural Government that has collected volcanic ash regularly at about 60 monitoring posts around the volcano in the distance range of 3 to 55 km from the summit crater since 1978. Both explosive and seismic activities dropped down at the lowest level for one year from May of 1993, which was the first experience since 1972. However, the activity resumed from June of 1994. The volcano has repeated increase and decrease in explosive activity with an overall period of 2-3 years since 1972, as recognized in Fig. 9. Monthly amount of erupted volcanic ash since June of 1978 is illustrated in Fig. 10. The average value in emission rate of volcanic ash during the past 16.5 years is approximately 10 million tons/year and 0.9 million tons/month. The emission rate has varied with periods of 2 to 3 years similarly with explosive activity.

Volcanic earthquakes with rather deep focus (A-type) have originated a few times per month. The focus were located in a vertically elongated zone shallower than 4 km below sea level beneath the summit, as illustrated in Fig. 11. The 8th Joint Observation of Sakurajima Volcano was carried out in 1991-92 by volcano observatories and institutions. No results that suggested significant changes in volcanic activity were obtained.

(Kazuhiro Ishihara)