Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan
Authors: Miyamachi Hiroki , Yakiwara Hiroshi , Kobayashi Reiji , Hirano Shuichiro , Kubo Takeshi , Souda Masakazu , Sakao Kenyu , Unno Naohiro , Matsushima Takeshi , Uchida Kazunari , Miyamachi Rintaro , Isoda Kenshin , Teguri Yoshiko , Kamiya Yoshinosuke , Triahadini Agnis , Shimizu Hiroshi , Katao Hiroshi , Shibutani Takuo , Tameguri Takeshi , Yamashita Yusuke , Miura Tsutomu , Nakagawa Jun , Yoneda Itaru , Kato Shinya , Takishita Kosei , Nakai Kazuho , Maeda Yuta , Watanabe Toshiki , Horikawa Shinichiro , Matsushiro Kenjiro , Okuda Takashi , Tsuji Shuhei , Sogawa Naoki , Hasegawa Daima , Nakahigashi Kazuo , Kurashimo Eiji , Yamada Tomoaki , Abe Hideji , Ando Miwako , Tanaka Shinichi , Ikezawa Satoshi , Iwasaki Takaya , Shinohara Masanao , Sato Toshinori , Yamamoto Mare , Azuma Ryosuke , Hirahara Satoshi , Nakayama Takashi , Suzuki Syuichi , Otomo Shuhei , Hino Ryota , Tsutsui Tomoki , Inoue Yusuke , Takei Ryuichi , Tada Yuya , Takahashi Hiroaki , Murai Yoshio , Aoyama Hiroshi , Ohzono Mako , Shiina Takahiro , Takada Masamitsu , Ichiyanagi Masayoshi , Yamaguchi Teruhiro , Ono Natsuki , Saito Kazuma , Ito Chihiro , Susukida Yuuki , Nakagaki Tatsuya , Tanaka Yasuhisa , Akinaga Yasuhiko 
Published: October 2023

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