Magma mass increase under Sakurajima Volcano, Japan, inferred from campaign relative gravity and leveling data from 1975 to 1992: an interpretation from volcanic gas studies

Abstract Temporal variations of volume and mass in the magma chambers of Sakurajima Volcano were modeled using leveling and relative gravity data collected around the volcano during the eruptive period from 1975 to 1992, to reveal a physical mechanism for the excessive gravity increase observed at the volcano. The following two deflation sources were estimated from the leveling data: a deeper source of (-4. 0 0. 3) 10⁶ (- 4. 0 ± 0. 3) × 10 6 m ³ 3 /year located at a depth of 7800 ± 400 m beneath Aira Caldera, and a shallower source of (-9. 25^\, + \, 1. 00\, - \, ₀. ₇₅) 10⁵ (- 9. 25 - 0. 75 + 1. 00) × 10 5 m ³ 3 /year located at a depth of 4000 ± 400 m beneath the center of Sakurajima Volcano. These deflation sources cannot fully explain the gravity increase of up to 15. 75 µGal/year observed at the volcano, because a gravity increase of only 3. 42 µGal/year is expected from the two deflation sources. After the effect of the deflation sources was subtracted from the observed gravity change, the residual gravity of up to 12. 32 µGal/year was then modeled by a point mass increase under the volcano. The estimated rate of the mass increase was (4. 35\, - \, ₁. ₀₀^\, + \, 0. 95) 10^10 (4. 35 - 1. 00 + 0. 95) × 10 10 kg/year, and the position of the point mass agreed with that of the shallower magma chamber within its error range. This result suggests that the shallower magma chamber gained mass despite the chamber deflation during the 1975–1992 eruptive period. The mass increase can be quantitatively explained by the accumulation of degassed magma in the shallower chamber; the rate of mass increase was calculated to be 1. 40 to 4. 65 10^10 4. 65 × 10 10 kg/year using the Rhyolite-MELTS software, by considering the magma degassing and resultant accumulation of the denser degassed magma in the shallower chamber. Our modeling results also suggest the importance of gravimetry in addition to crustal deformation observations in quantifying the rate of magma mass supply, because the magma supply to the deeper chamber was calculated to be (+5. 24^\, + \, 1. 20\, - \, ₁. ₂₂) 10^10 (+ 5. 24 - 1. 22 <mml: mo