17:15 〜 19:15
[SGC37-P01] Temporal variations of helium isotopes observed in earthquake swarm regions on the Noto Peninsula, central Japan
Prior to the Noto Peninsula earthquake (Mj 7.6) on January 1, 2024, an intense earthquake swarm had persisted beneath the northeastern peninsula since the end of 2020 (~1,100 days before the mainshock). The preceding swarm occurred deepest beneath the southern region (Cluster S), with activity subsequently expanding to Clusters W, N, and NE , in that order(Amezawa et al., 2023). Earthquake swarms, often interpreted to result from fluids invading the brittle seismogenic zones, have seismicity patterns that are significantly different from an aftershock sequence. Furthermore, the process of aseismic slip, induced by the intrusion of fluids, has been observed to be a trigger for the initiation of earthquakes. Geodetic data analysis reveals that aseismic crustal deformation accompanies swarm activity, suggesting the upwelling of fluid migrating from approximately 16 km depth to the shallow, permeable fault zone(Nishimura et al., 2023). The occurrence of the 2022 Mj 5.4 and 2023 Mj 6.5 earthquakes in proximity to the shallow end of the swarm is plausibly associated with the rising fluids and accelerated aseismic slip during the Noto earthquake swarms. Consequently, the occurrence of fluid supply, small earthquakes, and aseismic slip on a complex network of faults probably triggered the mainshock, which propagated to the west and east sides, resulting in an Mj 7.5 event with a length exceeding 100 km.
The earthquake swarms are thought to have been triggered by various types of fluids, such as meteoric water, hydrothermal fluid, mantle fluid, and metamorphic fluid. Volatiles, such as helium and carbon dioxide, offer insights into geological processes extending from the Earth's deep interior to its surface. In particular, helium isotope ratio shows a clear distinction among mantle, crustal, and atmospheric values, thereby effectively discerning the mantle signals from fluid and gas samples collected from hot spring wells. We have been conducting systematic investigations in the Noto Peninsula to determine the temporal and spatial variations in 3He/4He ratios since 15 years before the onset of the earthquake swarms. The spatial variations in 3He/4He ratios indicate the presence and distribution of pathways for mantle-derived volatiles from the mantle to the Earth's surface. Most samples obtained in and around the peninsula are characterized by 3He/4He ratios lower than the atmospheric value, indicating mantle helium comprises less than 10% of the total helium. Exceptionally, the 3He/4He ratios collected from Notoiida, Suzu-Ukai, Suzu-Nakajima and Suzumitsuke wells near Cluster S in the Noto earthquake swarm are several times higher than the atmospheric value(Umeda et al., 2024). The elevated 3He/4He ratios observed prior to the Mj 7.6 mainshock suggest the influx of mantle fluids, rather than deep fluids produced by simple meteoric water or crustal metamorphism. Furthermore, samples were collected from the aforementioned wells following a period of several months after the occurrence of the mainshock. Slightly higher helium isotope ratios were observed before and after the mainshock. The results suggest that mantle fluids might be ascending from the mantle into the crust over an extended period in the source region of the earthquake swarm at the tip of the Noto Peninsula.
References
Amezawa et al. (2023) Geophysical Research Letters, 50, e2022GL102670; Nishimura et al. (2023) Scientific Reports, 13, 8381.; Umeda et al. (2024) Geophysical Research Letters, 51, e2024GL108581.
The earthquake swarms are thought to have been triggered by various types of fluids, such as meteoric water, hydrothermal fluid, mantle fluid, and metamorphic fluid. Volatiles, such as helium and carbon dioxide, offer insights into geological processes extending from the Earth's deep interior to its surface. In particular, helium isotope ratio shows a clear distinction among mantle, crustal, and atmospheric values, thereby effectively discerning the mantle signals from fluid and gas samples collected from hot spring wells. We have been conducting systematic investigations in the Noto Peninsula to determine the temporal and spatial variations in 3He/4He ratios since 15 years before the onset of the earthquake swarms. The spatial variations in 3He/4He ratios indicate the presence and distribution of pathways for mantle-derived volatiles from the mantle to the Earth's surface. Most samples obtained in and around the peninsula are characterized by 3He/4He ratios lower than the atmospheric value, indicating mantle helium comprises less than 10% of the total helium. Exceptionally, the 3He/4He ratios collected from Notoiida, Suzu-Ukai, Suzu-Nakajima and Suzumitsuke wells near Cluster S in the Noto earthquake swarm are several times higher than the atmospheric value(Umeda et al., 2024). The elevated 3He/4He ratios observed prior to the Mj 7.6 mainshock suggest the influx of mantle fluids, rather than deep fluids produced by simple meteoric water or crustal metamorphism. Furthermore, samples were collected from the aforementioned wells following a period of several months after the occurrence of the mainshock. Slightly higher helium isotope ratios were observed before and after the mainshock. The results suggest that mantle fluids might be ascending from the mantle into the crust over an extended period in the source region of the earthquake swarm at the tip of the Noto Peninsula.
References
Amezawa et al. (2023) Geophysical Research Letters, 50, e2022GL102670; Nishimura et al. (2023) Scientific Reports, 13, 8381.; Umeda et al. (2024) Geophysical Research Letters, 51, e2024GL108581.