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<p>The Beppu and Yufuin geothermal fields are the most famous hot spring areas in Japan, with the highest and second-highest discharge rates, approximately 80,000 and 40,000 L per minute, respectively. These hot springs are located at the foot of three Quaternary active volcanoes: Garan, Tsurumi, and Yufu. To investigate fluid pathways and associated reservoirs supplying the Beppu and Yufuin geothermal fields, we conducted broad-band magnetotelluric (MT) surveys from 2014 to 2025 over an approximately 50&#xa0;km × 50&#xa0;km area surrounding these geothermal zones. A total of 161 survey sites' data were analyzed. The resulting 3-D resistivity structure reveals two conductors in the upper crust. One conductor is located beneath the three volcanoes at depths of 2–10&#xa0;km and extends in an NNE–SSW direction at greater depths. The second conductor is located beneath Beppu Bay at depths of 1–5&#xa0;km. Earthquakes predominantly occurred in resistive zones near these conductors, including the 2007 earthquake swarm, in which hypocenters migrated from a depth of 10&#xa0;km within Beppu Bay to a depth of 3&#xa0;km southwest of Beppu City. We interpret the conductor beneath the volcanoes as a hot zone associated with volcanic fluids, while the conductor beneath Beppu Bay likely represents cold thick sediment. Since eruption sites are positioned around the edge of the conductor beneath the volcanoes, we hypothesize that magmatic fluids preferentially rise along the conductor’s boundaries. Considering the locations of these conductors and the hypocenter migration in 2007, we propose two fluid pathways for the Beppu geothermal fields: one from beneath the three volcanoes and another from beneath Beppu Bay. The primary fluid sources are likely situated to the NNE of the region, based on the locations of deep conductors (&gt; 15&#xa0;km depth). The hypothesis of dual fluid pathways may explain the significant variations in the chemical composition of Beppu’s hot spring waters. Similarly, the Yufuin hot springs may also be fed by two fluid pathways: one originating beneath the three volcanoes and another from the deep portion of a fault. Among the hot springs in the study region, the northern side of the Beppu hot springs is distinctive in that the conductor beneath the adjacent Garan volcano rises to 2&#xa0;km beneath the summit, and the shallow layer is approximately one order of magnitude more conductive than that of other hot springs. This indicates that the contribution of fluids from the shallow part of the volcanoes is dominant on the northern side of the Beppu hot springs.</p> Graphical Abstract <p></p>

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Three-dimensional resistivity structure beneath Beppu and Yufuin geothermal fields imaged by dense broad-band magnetotelluric observations

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年10月16日

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  • Koki Aizawa,
  • Yuto Yamamoto,
  • Asuma Wakabayashi,
  • Dan Muramatsu,
  • Satoru Aniya,
  • Harutou Tanabe,
  • Shiori Fujita,
  • Azusa Shito,
  • Takao Koyama,
  • Ichiro Shiozaki,
  • Masahiro Ichiki

摘要

The Beppu and Yufuin geothermal fields are the most famous hot spring areas in Japan, with the highest and second-highest discharge rates, approximately 80,000 and 40,000 L per minute, respectively. These hot springs are located at the foot of three Quaternary active volcanoes: Garan, Tsurumi, and Yufu. To investigate fluid pathways and associated reservoirs supplying the Beppu and Yufuin geothermal fields, we conducted broad-band magnetotelluric (MT) surveys from 2014 to 2025 over an approximately 50 km × 50 km area surrounding these geothermal zones. A total of 161 survey sites' data were analyzed. The resulting 3-D resistivity structure reveals two conductors in the upper crust. One conductor is located beneath the three volcanoes at depths of 2–10 km and extends in an NNE–SSW direction at greater depths. The second conductor is located beneath Beppu Bay at depths of 1–5 km. Earthquakes predominantly occurred in resistive zones near these conductors, including the 2007 earthquake swarm, in which hypocenters migrated from a depth of 10 km within Beppu Bay to a depth of 3 km southwest of Beppu City. We interpret the conductor beneath the volcanoes as a hot zone associated with volcanic fluids, while the conductor beneath Beppu Bay likely represents cold thick sediment. Since eruption sites are positioned around the edge of the conductor beneath the volcanoes, we hypothesize that magmatic fluids preferentially rise along the conductor’s boundaries. Considering the locations of these conductors and the hypocenter migration in 2007, we propose two fluid pathways for the Beppu geothermal fields: one from beneath the three volcanoes and another from beneath Beppu Bay. The primary fluid sources are likely situated to the NNE of the region, based on the locations of deep conductors (> 15 km depth). The hypothesis of dual fluid pathways may explain the significant variations in the chemical composition of Beppu’s hot spring waters. Similarly, the Yufuin hot springs may also be fed by two fluid pathways: one originating beneath the three volcanoes and another from the deep portion of a fault. Among the hot springs in the study region, the northern side of the Beppu hot springs is distinctive in that the conductor beneath the adjacent Garan volcano rises to 2 km beneath the summit, and the shallow layer is approximately one order of magnitude more conductive than that of other hot springs. This indicates that the contribution of fluids from the shallow part of the volcanoes is dominant on the northern side of the Beppu hot springs.

Graphical Abstract