"Frequency of explosions at the summit of Minami-dake further increased in December when a total of 75 explosions were recorded with at least one almost every day. A distinguishing characteristic of December volcanic activity was the reciprocal appearance of earthquake swarms and vigorous summit eruptions. Swarms of earthquakes were recorded on 1, 2, 3, 4, 8, 14, 17, 20, 21, 23, 26, 27, and 31 December, in almost every case the days of fewer explosions (except for 2, 3, and 30 December). Even on those 3 days, swarm activity occurred before or after the phase of summit explosive activity. This fact suggests that magma had been repeatedly and effectively transported in the conduit through the month.

"On 3 December at 0103, an explosion was accompanied by an airshock, strong enough to break 188 windowpanes in the city of Kagoshima, ~10 km W of the crater. Another strong shock on 19 December at 0546 broke 85 windowpanes and a car windshield in Ushine city, ~13 km ESE of the crater (table 7). In both cases, the presence of an inversion layer at 1,000 m altitude (3 December) and 1,500 m altitude (19 December) seemed to amplify the airshocks.

Table 7. Major episodes of damage to windows by airshocks, 1976-85. Damage from explosions at Sakura-jima, 1988-90. Car windshields were broken by block or lapilli fall, whereas windowpanes were mainly broken by air shocks. Ashfall damage is not included.

"Lapilli also caused some damage: a windshield on a car was broken at Kurokami (4.5 km E of the crater) on the 5th, and at the volcano's SW foot on the 16th and 19th. An explosion on 19 December at 1810 ejected incandescent rocks and one of them made a 1-m-diameter hole near an inhabited house at Arimura (3 km SSE of the crater).

"In January, the monthly number of explosions declined to 35 and seismic activity also declined. Lapilli from an explosion on 1 January at 1158 broke the windshields of six cars at the SW foot.

"At [KLMO], 474 explosions were recorded in 1985 and cumulative ashfall was 15,908 g/m², the largest since the summit eruptive activity began in 1955."

Geological Summary. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: JMA.