A research team from Institute of Science and Engineering Faculty of Geosciences and Civil Engineering, Kanazawa University, led by Professor Yoshihiro Hiramatsu and Professor Tomoaki Morishita, in collaboration with colleagues from University of Toyama, the University of Tokyo, and Kochi University, has investigated hot springs located within the epicentral concentration area where deep fluid supply sources are thought to exist directly below, and observed significant chemical and isotopic composition changes after 2022.
An earthquake swarm in the northeastern Noto Peninsula has been ongoing for three years, and it has been suggested that the migration of deep crustal fluids may be the cause of these earthquakes. This research team investigated hot springs in and around the epicentral area where deep fluid sources are thought to exist directly below, and observed significant chemical and isotopic composition changes after 2022. These changes are thought to reflect variations in the mixing rate of deep fluid components, including mantle-derived materials.
The results of this research were presented at the 2024 Congress of the Japan Geoscience Union (May 26-31, 2024). The preliminary manuscript was posted on the website of the JGS on Friday, May 17, 2024 (Japan Standard Time).
We will continue our research to accumulate data after the 2024 Noto Peninsula earthquake and compare them with pre-earthquake data to elucidate the phenomena that occurred underground with seismic activity.
【Key points of this research】
?We regularly surveyed hot springs in the northeastern Noto Peninsula, where the epicenters of the earthquake swarm is concentrated, and measured temporal variations in chemical and isotopic compositions.
?We observed high helium isotope ratios just above the upwelling area of the fluid, indicating contamination by deep fluid components.
?As a result of these investigations, we obtained data that may reflect changes in the mixing rate of deep fluid components, including mantle-derived materials, accompanied by seismic activity and crustal deformation.
Figure 1: Survey sites in the northeastern Noto Peninsula
Cercles indicate the sampling sites, and the cross represents the epicenter of the M7.6 earthquake that occurred on January? ?1, 2024. A cross-sectional schematic diagram along the dashed line is shown in Figure 2. This figure was created by modifying an output from "Geographical Survey Institute Map (Electronic National Land Web) (GSI)" (cf. https://maps.gsi.go.jp/).
Figure 2: Diagram of material circulation in the basement at point ASY
This figure shows the underground material circulation at location ASY estimated from the 3 He/ 4He ratio (*1) data. ? This figure is based on Figure 4 of Nishimura et al. (2023, Scientific Reports 13, 8381).???
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* 1:3He/ 4 He ratio
The stable isotope ratio of helium is expressed as the 3He/4He ratio. Helium has two stable isotopes, with mass number 3 and 4.? 3He is a primordial component that was incorporated into the solid Earth from outer space during the formation of the Earth. 4He consist of alpha particles produced by the radioactive decay of uranium and thorium. The 3He/ 4 He ratio is higher in the mantle compared to the Earth's crust and atmosphere. Therefore, a high 3He/ 4 He ratio in hot spring water and groundwater can be interpreted as indicating a high rate of mixing of materials originating from the deep, such as the mantle.
Click here to see the press release【Japanese only】
Researchers' Information: Yoshihiro Hiramatsu
Tomoaki Morishita
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