講演情報
[S1-02]令和6年能登半島地震に関連する地下水の化学的異常
*鹿児島 渉悟1、佐野 有司2、高畑 直人3、河本 結名1、柴田 智郎4、李 营5、森下 知晃6、平松 良浩6、中島 淳一7 (1. 富山大学、2. 高知大学、3. 東京大学、4. 福岡大学、5. 中国地震局、6. 金沢大学、7. 東京科学大学)
キーワード：
令和6年能登半島地震、物質循環、ヘリウム同位体、水素・酸素同位体
The earthquake swarm at the northeastern Noto Peninsula, including the 1 January 2024 Noto earthquake (M7.6), has continued since December 2020. The behavior of a deep fluid seems to be a driving force of the earthquake swarm (Amezawa et al., 2023; Yoshida et al., 2023; Nishimura et al., 2023). In this area, Umeda et al. (2009; 2024) reported hot spring data with high 3He/4He ratios, indicating high contributions of the mantle-derived component. Earthquakes change underground situations such as structures of groundwater system, which results in changes of groundwater chemistry (e.g., Tsunogai and Wakita, 1995). Also in the northeastern Noto Peninsula, it is expected that groundwater chemistry reflects deep-shallow underground processes.
In this study, groundwater and gas samples at eleven sites were collected continuously from June 2022. In University of Toyama, anion (Cl-, SO42-) concentrations in the water samples were measured using an ion chromatography 883 basic IC plus, and δ18O and δD values of water were determined using an isotope analyzer L2130-i. For the water samples collected in copper tubes and glass containers, dissolved gas components were extracted into gaseous phases by a head space method. Subsequently, In AORI, 4He/20Ne and 3He/4He ratios were measured using a noble gas mass spectrometer Helix-SFT system.
Remarkable changes of chemical data were observed at the site ASY located above the cluster S area which is thought to be on the deep fluid supply source (Amezawa et al., 2023). Between May and October 2023, anion concentrations, δ18O values and δD values were lower than those in 2022. After that, until January 2024, they increased to values close to those observed in 2022. An average of the air-corrected 3He/4He ratios of the samples collected in 2022 was 2.94 Ra (1 Ra: the atmospheric 3He/4He ratio of 1.4×10-6). The air-corrected 3He/4He ratio decreased to 2.33 Ra in July 2023, and increased to 3.18 Ra in January 2024. Temporal variations of anion concentrations, δ18O values and δD values may be related to deformation and permeability change of aquifer rocks. Significant decrease in the 3He/4He ratio observed in 2023 may be due to the addition of radiogenic helium by aquifer rock deformation associated with seismic activity and volumetric strain change (Kagoshima et al., 2025).
In this study, groundwater and gas samples at eleven sites were collected continuously from June 2022. In University of Toyama, anion (Cl-, SO42-) concentrations in the water samples were measured using an ion chromatography 883 basic IC plus, and δ18O and δD values of water were determined using an isotope analyzer L2130-i. For the water samples collected in copper tubes and glass containers, dissolved gas components were extracted into gaseous phases by a head space method. Subsequently, In AORI, 4He/20Ne and 3He/4He ratios were measured using a noble gas mass spectrometer Helix-SFT system.
Remarkable changes of chemical data were observed at the site ASY located above the cluster S area which is thought to be on the deep fluid supply source (Amezawa et al., 2023). Between May and October 2023, anion concentrations, δ18O values and δD values were lower than those in 2022. After that, until January 2024, they increased to values close to those observed in 2022. An average of the air-corrected 3He/4He ratios of the samples collected in 2022 was 2.94 Ra (1 Ra: the atmospheric 3He/4He ratio of 1.4×10-6). The air-corrected 3He/4He ratio decreased to 2.33 Ra in July 2023, and increased to 3.18 Ra in January 2024. Temporal variations of anion concentrations, δ18O values and δD values may be related to deformation and permeability change of aquifer rocks. Significant decrease in the 3He/4He ratio observed in 2023 may be due to the addition of radiogenic helium by aquifer rock deformation associated with seismic activity and volumetric strain change (Kagoshima et al., 2025).