1983 年三宅島噴火にともなった地殻変動と重力変化

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Bench marks for precise levels were established along the circling route of Miyakejima Island in 1979 by the Geographical Survey Institute. Since then precise levels were repeated every year and a precise gravity survey was made at the same bench marks once in 1980 before its 1983 eruption. Both the surveys repeated after the 1983 eruption revealed the vertical displacement and the gravity change related to the eruption. During one year preceding the eruption, the island tilted 8.2 seconds of arc towards S 30° W as a whole and a small area ranging about 5 km in width including the eruption sites locally upheaved about 17 cm at the maximum. The gravity change observed at the bench mark of the maximum upheaval is about 65 μgal after the free-air correction. The pressure source responsible for the ground deformation is located at a depth of about 2 km on the rough assumption that the source is a point source. The anomalous mass responsible for the gravity change is determined as an infinite vertical dyke having density contrast 0.3 g/cc to the surroundings, width 7∿8 m and depth 2∿3 km below the earth surface, on the assumption that the dyke plane approximately coincides with the radial fissures which ejected lavas. It is probable that the pressure source and the distribution of the anomalous mass have the different configurations each other. Although gravity and elevation changes associated with the 1983 eruption of Miyakejima were surely detected, more detailed and accurate discussions remain rather difficult because the observation points are distributed only along the circling route, not covering a wide area of the island and the surveys have not been repeated frequently. We should prepare for the future study by intensifying the observation network, and repeating the surveys or adopting continuous observations.
特定非営利活動法人日本火山学会の論文
1984-12-28

1983 年三宅島噴火にともなった地殻変動と重力変化

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