5:15 PM - 6:45 PM
[U15-P59] On the causality of fluid for the series of earthquakes in Noto peninsula
Keywords:2024 Noto Earthquake, Fluid, Earthquake swarm
In the northeastern Noto Peninsula, an increase in the number of earthquakes has been observed since around 2018, and earthquake swarm has been active since December 2020. Mj6.5 earthquake occurred on May 5, 2023, and Mj7.6 earthquake occurred on January 1, 2024. These events are considered to be caused by the rupture of a southeast-dipping reverse fault extending from the northern Noto Peninsula to the Japan Sea.
Crustal fluid has been considered as main causal factor of this swarm. According to Nishimura et al. (2023), the increase in the volume of the opening of the fault was ~2.92×107 m3 from November 2020 to June 2022. They interpreted that the swarm in this period was caused by the crustal fluid migration from greater depths, and aseismic and a decrease in fault strength due to the rise of crustal fluid caused the swarm. In addition, according to Amezawa et al. (2023), the diffusivity is estimated to be ~9.4×10-2-1.2×10-1 m2/s. Furthermore, according to Kato (2024), and the permeability of the crust was estimated to be ~10-10 m2.
While various seismological studies have indicated the involvement of crustal fluid, in this study, we estimate the amount of fluid that may have been involved in the swarm by using the theory of injection induced earthquakes due to water injection in the field of resource engineering (Mukuhira et al., 2022), and discuss the involvement of crustal fluid with existing interpretations. First, using the seismicity catalog from December 2020 to July 2023 compiled by Yoshida et al. (2023), we estimated the crustal fluid volume involved in the swarm with four approaches based on McGarr's theory, the Seismogenic Index (SI) model, Darcy’s law, and the cubic law. McGarr's theory and the SI model are theories proposed in the field of injection induced seismicity. McGarr's theory describes the relationship between the amount of fluid injected into the crust and the seismic moment to be released. The SI model describes the relationship between the amount of fluid injected into the crust and a-value of the GR law. In this study, the cumulative seismic moments and a-value within a certain period are obtained from seismicity catalog, and we calculated the fluid volume by using McGarr's theory and the SI model. Darcy’s law and the cubic law describe the relationship among the flow area, the pressure difference and the flow rate of fluid. In this study, the flow area was estimated from hypocenters, and the diffusivity and permeability were estimated from the expansion of hypocenters. The pressure difference of crustal fluid was estimated from general values at the depths of swarm. The flow rates were calculated by using Darcy’s law and the cubic law, and we calculated the fluid volume from the results. From the results of the estimation, the fluid volume is limited to the range of 106-108 m3. This is roughly consistent with the fluid volume change estimated by Nishimura et al. (2023) (2.92×107 m3). From this comparison, we discussed the validity of the fluid volume estimated in this study and the applicability of our method used in this study.
We will estimate the fluid volume for the period after July 2023 in the interval where the involvement of fluid in the swarm is still indicated, and discuss the consistency with the geological background and with the amount of dehydration from the plate.
Crustal fluid has been considered as main causal factor of this swarm. According to Nishimura et al. (2023), the increase in the volume of the opening of the fault was ~2.92×107 m3 from November 2020 to June 2022. They interpreted that the swarm in this period was caused by the crustal fluid migration from greater depths, and aseismic and a decrease in fault strength due to the rise of crustal fluid caused the swarm. In addition, according to Amezawa et al. (2023), the diffusivity is estimated to be ~9.4×10-2-1.2×10-1 m2/s. Furthermore, according to Kato (2024), and the permeability of the crust was estimated to be ~10-10 m2.
While various seismological studies have indicated the involvement of crustal fluid, in this study, we estimate the amount of fluid that may have been involved in the swarm by using the theory of injection induced earthquakes due to water injection in the field of resource engineering (Mukuhira et al., 2022), and discuss the involvement of crustal fluid with existing interpretations. First, using the seismicity catalog from December 2020 to July 2023 compiled by Yoshida et al. (2023), we estimated the crustal fluid volume involved in the swarm with four approaches based on McGarr's theory, the Seismogenic Index (SI) model, Darcy’s law, and the cubic law. McGarr's theory and the SI model are theories proposed in the field of injection induced seismicity. McGarr's theory describes the relationship between the amount of fluid injected into the crust and the seismic moment to be released. The SI model describes the relationship between the amount of fluid injected into the crust and a-value of the GR law. In this study, the cumulative seismic moments and a-value within a certain period are obtained from seismicity catalog, and we calculated the fluid volume by using McGarr's theory and the SI model. Darcy’s law and the cubic law describe the relationship among the flow area, the pressure difference and the flow rate of fluid. In this study, the flow area was estimated from hypocenters, and the diffusivity and permeability were estimated from the expansion of hypocenters. The pressure difference of crustal fluid was estimated from general values at the depths of swarm. The flow rates were calculated by using Darcy’s law and the cubic law, and we calculated the fluid volume from the results. From the results of the estimation, the fluid volume is limited to the range of 106-108 m3. This is roughly consistent with the fluid volume change estimated by Nishimura et al. (2023) (2.92×107 m3). From this comparison, we discussed the validity of the fluid volume estimated in this study and the applicability of our method used in this study.
We will estimate the fluid volume for the period after July 2023 in the interval where the involvement of fluid in the swarm is still indicated, and discuss the consistency with the geological background and with the amount of dehydration from the plate.