2. Activity of Sakurajima Volcano
Sakurajima volcano is a post-caldera volcano of the Aira caldera, located at the southern rim of the caldera. From the analysis of ground deformation data, it is believed that the magma supply system is composed of twin-reservoirs: the main magma reservoir approximately 10 km deep at the center of the caldera and a sub-reservoir a few kilometers under the volcano (Ishihara, 1990).
The eruptive activity at the summit crater of Minami-dake (the south cone of Sakurajima), which was initiated in October 1955, reached its peak in 1960 and declined gradually. In October 1972, the explosive activity resumed and has gradually declined since 1993, as illustrated in Fig. 3. The cumulative number of explosive eruptions is 7209 as of the end of 1998, and the amount of volcanic ash ejected from June, 1978 to December, 1998 is estimated to be 185 millions tons by Sakurajima Volcano Research Center, Kyoto University.
During the period of 1995-1998, dangerous eruptions which threw volcanic blocks near resident area were rare, and few damages were reported, as indicated by a few numbers of volcanic advisories issued by the Kagoshima Meteorological Observatory: 8, 2, 0 and 1 times in 1995, 1996, 1997 and 1998, respectively. The depth of the summit crater had changed with time in the range from 100 to 200 m since 1955. In 1995, it was observed that the crater became significantly deep, as shown in Fig. 2. The estimated depth was 250-300m (Takayama and Ishihara, 1998). This may be why explosive eruptions, which threw large volcanic blocks out of the crater, were rarely observed during the past 4 years.
The largest eruption occurred on August 23-24, 1995. The eruption started at 23:35, August 23 with some precursors: inflation of the summit 4 hours before and an A-type earthquake at 23:16. Volcanic ash, lapilli and pumice were ejected continuously for half a day, and 20 explosive eruptions were successively originated in a few days. Ashfall was observed over the western part of Kyushu, including the area around Mt. Unzen, 150 km northwest of Sakurajima. Total amount of volcanic ash was estimated to be 0.3-0.5 millions tons. However, no serious damages were reported, though the Kyushu expressway, 15 to 20 km from Sakurajima, was closed to delete volcanic ash for one day. It is probably because the eruption started in midnight and no large volcanic blocks reached resident areas (Sakurajima Volcanological Observatory and Earthquake Research Institute, 1995).
The annual number of explosive eruptions during the 4 years did not significantly decrease (Fig. 3): 215, 168, 21 and 79 times in 1995, 1996, 1997 and 1998. However, the amount of ejected volcanic ash has remarkably decreased (Fig. 4): 3.5, 1.8, 0.2 and 1.0 millions tons in 1995, 1996, 1997 and 1998, respectively. The amount of volcanic ash during 1978-1994 was approximately 10 millions ton/year on average. This means that each of recent explosions ejected smaller amount of volcanic ash, compared with explosions during the former period.
Seismic activity at the volcano is illustrated in Fig. 5. B-type earthquakes, which are related to magma intrusion or extrusion through the magma conduit (Iguchi, 1994), have successively decreased, though there is no significant change in activity of A-type earthquakes under the volcano (Fig. 6). The change in activity of B-type earthquakes is roughly correlated with the overall change in ejection rate of volcanic ash.
During one century, the ground of the Aira caldera has indicated inflation during dormant periods and deflation after the two large eruptions in 1914 and 1946. When the summit eruption reached its peak in 1960, inflation stopped, and then started again in 1962, as illustrated in Fig. 7. Inflation of the caldera stopped again in 1975 and then deflation of the caldera had been observed from 1980 until 1989, related to the re-activation of summit eruption from October 1972. By the leveling survey conducted in 1996, it was confirmed that the ground of the caldera was turned into inflation again in 1994, corresponding to the decline of eruptive activity. This suggests that the migration of magma from the main reservoir of the caldera to Sakurajima decreased significantly, probably due to pressure decrease in the main reservoir, and that the storage of magma resumed under the caldera.
The 9th Joint Observation of Sakurajima Volcano was conducted during the period from October 1996 to April 1997 (Ishihara ed., 1998). The characteristics of the recent activity (1994-1996) are summarized in comparison with the highly active period (1974-1993), based on the data obtained by Sakurajima Volcano Research Center, Kyoto University and the Joint Observations conducted 9 times since 1974 (Table 2).
Iguchi, M. (1994) A vertical expansion source model for the mechanisms of earthquakes originated in the magma conduit of an andesitic volcano: Sakurajima, Japan. Bull. Volcanol. Soc. Japan, 39, 49-67.
**Ishihara, K. (ed.) (1998) The 9th Joint Observation of Sakurajima Volcano -October 1996 to April 1997-. Sakurajima Volcano Research Center, DPRI., Kyoto Univ., 132p.
Ishihara, K. (1990) Pressure sources and induced ground deformation associated with explosive eruptions at an andesitic volcano: Sakurajima volcano, Japan. In Ryan, M. P. (ed.), Magma Transport and Storage, John Wily & Sons, pp. 335-356.
**Sakurajima Volcanological Observatory and Earthquake Research Institute (1995) The eruption of August 24, 1995, and recent volcanic activity at Sakurajima. Rep. Coord. Commit. Predict. Volc. Eruption, 63, 88-91.
**Takayama, T. and Ishihara, K. (1998) Aerial photographs of the summit crater of Sakurajima Volcano and the topographic change. Bull. Volcanol. Soc. Japan, 43, 39-42.
(*In Japanese with English abstract, ** In Japanese)