2026-04-30 九州大学
2011年1月に発生した霧島新燃岳の噴火 (撮影：柚木耕二)。 噴煙を火口から高度8000mまで噴出する3回の大規模な噴火により、約0.03 km3のマグマが放出された。そのマグマは、地下の巨大マグマ貯留域の端部から放出されたが、その量は地下のマグマ総量の0.01% 以下に過ぎないと推定された。
＜関連情報＞
- https://www.kyushu-u.ac.jp/ja/researches/view/1465
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL120131
- https://link.springer.com/article/10.1186/s40623-026-02390-2
自己電位、ヘリウム同位体、広帯域地磁気地電流法による桜島火山の地下におけるマグマと揮発性物質の経路 Magma and Volatile Pathways Beneath Sakurajima Volcano From Self-Potential, Helium Isotopes, and Broadband Magnetotellurics
K. Aizawa, T. Koyama, H. Hase, M. Uyeshima, H. Sumino
Geophysical Research Letters Published: 10 April 2026
DOI:https://doi.org/10.1029/2025GL120131
Abstract
At frequently erupting volcanoes, magma and volatiles ascend through established pathways, but the complete picture of their branching and supply system remains unclear. We integrated self-potential (SP), broadband magnetotelluric (MT), and helium isotope observations to image magma and volatile pathways beneath Sakurajima Volcano. SP surveys delineate zones of groundwater flow and hydrothermal alteration, MT data reveal a columnar conductive zone (C1), and high 3He/4He ratios indicate magmatic volatiles. C1 is offset eastward and inclines northward with depth, coinciding with high 3He/4He locations. Active craters, pressure sources, and deep low-frequency earthquakes align along edge of C1, supporting an edge-ascent model in which magma and volatiles preferentially rise along conductor boundaries while volatiles branched at shallow level are laterally guided by shallow groundwater flow. Interpreting C1 as a large, long-lived magmatic reservoir dominated by crystal mush, we propose that geodetically detected pressure sources represent small, transient magma pockets developing along its edges.
Plain Language Summary
Even at frequently erupting volcanoes like Sakurajima, Japan, the integrated pathways of magma and volcanic gases beneath the surface are not well understood. We combined three types of observations—natural electrical voltage difference on surface (self-potential), underground resistivity (magnetotellurics), and helium gas measurements—to map these pathways. We identified a deep, electrically conductive, column-shaped zone (C1) beneath the volcano that likely serves as a crystal-rich, mushy magma reservoir. Magma and gases tend to rise along the edges of this zone rather than through its center. Near the surface, volcanic gases branch out sideways, guided by groundwater flow. Our results suggest that a long-lived, crystal-rich magma system exists beneath Sakurajima, and that the pressure sources detected by geodetic instruments correspond to relatively small but mobile magma pockets at the edges of this larger system. These findings improve our understanding of how magma and volcanic gases move beneath active volcanoes.
濃密広帯域地磁気地電流観測から推測される霧島火山複合体の地殻貫通マグマ供給システム Trans-crustal magma plumbing system of Kirishima Volcanic Complex as inferred from dense broadband magnetotelluric observations
Koki Aizawa,Dan Muramatsu,Kaori Tsukamoto,Yoshiko Teguri,Takao Koyama,Mitsuru Utsugi,Wataru Kanda,Tasuku Inomata,Hiromichi Shigematsu & Hiroshi Shimizu
Earth, Planets and Space Published:09 March 2026
DOI:https://doi.org/10.1186/s40623-026-02390-2
Abstract
To investigate the magma plumbing system of the Kirishima Volcanic Complex (KVC), we conducted a broadband magnetotelluric (MT) survey over an expanded area of approximately 60 × 60 km, including the region around Shinmoe-dake volcano, which recently experienced magmatic eruptions in 2011, 2018, and 2025. Three-dimensional inversion of the MT data reveals a detailed resistivity structure extending to upper mantle depths. A large low-resistivity zone (C1) is interpreted as a long-lived trans-crustal magmatic system, whereas an adjacent high-resistivity zone (R1) is interpreted as solidified magma associated with past caldera-forming eruptions. The geodetically inferred pressure source associated with the recent eruptions of Shinmoe-dake is situated at the boundary between C1 and R1 and is interpreted as a small, temporary magma pocket. The craters of three active volcanoes (Iwo-yama, Shinmoe-dake, and Ohachi) are located above the edges of C1, and deep low-frequency earthquakes (DLFEs) occur at its eastern edge. Notably, the spatial alignment of the DLFE cluster, pressure source, and Shinmoe-dake crater is approximately linear and follows the margin of C1, suggesting that magma preferentially ascends along the edge of C1. Furthermore, the nearly simultaneous onset of DLFEs, pressure source inflation, and shallow seismicity—with time lags of only a few days—indicates the presence of an established and efficient magma ascent pathway along the edge of C1. The large volume of C1 (> 3000 km3) and its structural characteristics imply a relatively high potential for large-scale eruptions to occur over short timescales in the future. These findings provide a new framework for understanding the long-term magmatic evolution and eruption potential of the KVC.
