Density Distribution in Sakurajima Volcano by High Density Gravity Measurement

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We carried out geological and gravity surveys in Sakurajima Volcano in order to reveal the detailed gravity anomaly distribution and estimate a location of a crater which produced the biggest eruption 11,000 years ago.In the geological survey, we investigated a distribution of the Satsuma tephra released from the crater, revealed the detailed stratigraphy, a change and characteristic of the eruptions. We also estimate d that the crater may exists in the western part of Sakurajima from a distribution of the base surge and a direction of trees buried in the base surge.The gravity survey, which was composed of 346 gravity points, showed the detailed gravity anomaly distribution in Sakurajima. In general, the gravity anomaly gradually changed from the positive anomaly (+12mgal) in the southern area to the negative anomaly (-18mgal) in the northern area. It is also revealed that there is an local area with a negative anomaly (-4mgal) in the Taisho lave area in the western part of Sakurajima. This negative anomaly suggested a low density body in the shallow area from results of a simple numerical experiment. Finally we concluded that this negative anomaly would correspond to a location of the crater which produced the Satsuma tephra.