Earth, Planets and Space (Oct 2023)

Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan

  • Hiroki Miyamachi,
  • Hiroshi Yakiwara,
  • Reiji Kobayashi,
  • Shuichiro Hirano,
  • Takeshi Kubo,
  • Masakazu Souda,
  • Kenyu Sakao,
  • Naohiro Unno,
  • Takeshi Matsushima,
  • Kazunari Uchida,
  • Rintaro Miyamachi,
  • Kenshin Isoda,
  • Yoshiko Teguri,
  • Yoshinosuke Kamiya,
  • Agnis Triahadini,
  • Hiroshi Shimizu,
  • Hiroshi Katao,
  • Takuo Shibutani,
  • Takeshi Tameguri,
  • Yusuke Yamashita,
  • Tsutomu Miura,
  • Jun Nakagawa,
  • Itaru Yoneda,
  • Shinya Kato,
  • Kosei Takishita,
  • Kazuho Nakai,
  • Yuta Maeda,
  • Toshiki Watanabe,
  • Shinichiro Horikawa,
  • Kenjiro Matsushiro,
  • Takashi Okuda,
  • Shuhei Tsuji,
  • Naoki Sogawa,
  • Daima Hasegawa,
  • Kazuo Nakahigashi,
  • Eiji Kurashimo,
  • Tomoaki Yamada,
  • Hideji Abe,
  • Miwako Ando,
  • Shinichi Tanaka,
  • Satoshi Ikezawa,
  • Takaya Iwasaki,
  • Masanao Shinohara,
  • Toshinori Sato,
  • Mare Yamamoto,
  • Ryosuke Azuma,
  • Satoshi Hirahara,
  • Takashi Nakayama,
  • Syuichi Suzuki,
  • Shuhei Otomo,
  • Ryota Hino,
  • Tomoki Tsutsui,
  • Yusuke Inoue,
  • Ryuichi Takei,
  • Yuya Tada,
  • Hiroaki Takahashi,
  • Yoshio Murai,
  • Hiroshi Aoyama,
  • Mako Ohzono,
  • Takahiro Shiina,
  • Masamitsu Takada,
  • Masayoshi Ichiyanagi,
  • Teruhiro Yamaguchi,
  • Natsuki Ono,
  • Kazuma Saito,
  • Chihiro Ito,
  • Yuuki Susukida,
  • Tatsuya Nakagaki,
  • Yasuhisa Tanaka,
  • Yasuhiko Akinaga

DOI
https://doi.org/10.1186/s40623-023-01919-z
Journal volume & issue
Vol. 75, no. 1
pp. 1 – 21

Abstract

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Abstract The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km. Graphical Abstract

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