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<p>Intense earthquake swarms had been observed on the Noto Peninsula in north-central Japan since December 2020. We report data obtained by periodical groundwater sampling and helium isotope measurements in the epicentre area starting July 2022 for investigating the cause of this swarm. The data show mantle-derived helium reaching a maximum of 5 R<sub>a</sub> where R<sub>a</sub> is the atmospheric <sup>3</sup>He/<sup>4</sup>He ratio of 1.39 × 10<sup>−6</sup>. This value indicates that the swarm was caused by fluids rising from the mantle. Time-series analysis show an anomalous drop in helium isotopic ratio prior to the 1 January 2024 Noto earthquake (M7.6), which seriously damaged the region. This decrease in helium isotopic ratio is possibly due to the degassing of radiogenic helium via the deformation of the rocks constituting the groundwater aquifer, which is fundamental and valuable data for predicting inland earthquakes in subduction zones. Periodical observation of helium isotopic ratios in deep groundwater is desirable in regions where a large earthquake is expected.</p>

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Helium isotope anomaly in groundwater prior to the 2024 Noto Peninsula earthquake

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年11月26日

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  • Takanori Kagoshima,
  • Yuji Sano,
  • Naoto Takahata,
  • Yume Kawamoto,
  • Tomo Shibata,
  • Ying Li,
  • Tomoaki Morishita,
  • Yoshihiro Hiramatsu,
  • Junichi Nakajima

摘要

Intense earthquake swarms had been observed on the Noto Peninsula in north-central Japan since December 2020. We report data obtained by periodical groundwater sampling and helium isotope measurements in the epicentre area starting July 2022 for investigating the cause of this swarm. The data show mantle-derived helium reaching a maximum of 5 Ra where Ra is the atmospheric 3He/4He ratio of 1.39 × 10−6. This value indicates that the swarm was caused by fluids rising from the mantle. Time-series analysis show an anomalous drop in helium isotopic ratio prior to the 1 January 2024 Noto earthquake (M7.6), which seriously damaged the region. This decrease in helium isotopic ratio is possibly due to the degassing of radiogenic helium via the deformation of the rocks constituting the groundwater aquifer, which is fundamental and valuable data for predicting inland earthquakes in subduction zones. Periodical observation of helium isotopic ratios in deep groundwater is desirable in regions where a large earthquake is expected.