5:15 PM - 6:45 PM
[U15-P04] Spatio-temporal variation of pore fluid pressure using focal mechanism solutions in Noto Peninsula, Japan
Keywords:earthquakes, focal mechanism solutions, fluid, stress, Noto Peninsula
1. Introduction
At around 16:10 on January 1st, 2024, an earthquake with a magnitude of 7.6 occurred at a depth of about 15 km under the Noto Peninsula in Ishikawa Prefecture, Japan. The focal mechanism of the earthquake was a reverse fault type with a compressional axis oriented in the NW-SE, occurring within the upper crust (https://www.jma.go.jp/jma/menu/20240101_noto_jishin.html). Subsequent seismic activity is distributed over approximately 160 km in a NE-SW orientation. The Noto Peninsula had previously experienced a Mw6.7 earthquake on March 25th, 2007, and swarm earthquakes occurred from 2020 to 2023. Discussions on the relationship between earthquakes and fluids in the Noto Peninsula have been ongoing (e.g., Nakajima, 2022; Nishimura et al., 2023). This study estimates the pore fluid pressure in the Noto Peninsula with focal mechanism solutions and discusses the relationship between earthquakes and fluids in the Noto Peninsula.
2.Data and method
We estimate the excess pore fluid pressure on faults during earthquakes with the focal mechanism solutions shallower than 30 km and since 1997 on the F-net catalog (https://www.fnet.bosai.go.jp/top.php) (N=290). The excess pore fluid pressure is obtained based on the method subjected by Terakawa et al. (2010) and which has following four assumptions: (1) The coefficient of friction is 0.6. (2) One of the three principal stresses is σv (overburden pressure). (3).The fault is critical condition to activate under a certain stress (stress which Mohr's stress circle tangent to the yield surface). (4) The excess fluid pressure is the amount of stress from the state of 3rd assumption to the nodal plane (fault plane) of each mechanism solution. Finally, the excess pore fluid pressure ratio (=λ) is calculated to be between 0 and 1, depending on the distance from the yield surface.
3.Results and Discussion
The stress tensor inversion (Hardebeck and Michael, 2006) from the seismic data confirms that the stress field is characterized by a reverse-faulting with NW-SE compression and σ3 trends in the vertical orientation. The average misfit angle obtained from the inversion is low, indicating that it is reasonable to assume a uniform stress field across the studied region. The results of the analysis show that the pore fluid pressure at all faults exceeds the hydrostatic pressure. No clear differences were observed in the frequency distribution of λ across all earthquake activities, the activities in 2007-2008, the swarm earthquake activities between 2020-2023, and the activities after January 1st, 2024. The results showed that earthquakes with a λ value of 0.2 were the most common, while earthquakes with a λ value of 0.8 or higher accounted for less than 4% of the total. Furthermore, temporal variation of λ showed no significant increase before the magnitude 6.7 and 7.6 earthquakes in 2007 and 2024, respectively. The analysis also highlighted a correlation between magnitude and λ, indicating that larger magnitude earthquakes tend to have lower λ values. Regarding large earthquakes studied here, a reduction in fault strength due to fluid involvement is not always necessary, and the accumulation of stress (increase in shear stress) may be the main factor.
At around 16:10 on January 1st, 2024, an earthquake with a magnitude of 7.6 occurred at a depth of about 15 km under the Noto Peninsula in Ishikawa Prefecture, Japan. The focal mechanism of the earthquake was a reverse fault type with a compressional axis oriented in the NW-SE, occurring within the upper crust (https://www.jma.go.jp/jma/menu/20240101_noto_jishin.html). Subsequent seismic activity is distributed over approximately 160 km in a NE-SW orientation. The Noto Peninsula had previously experienced a Mw6.7 earthquake on March 25th, 2007, and swarm earthquakes occurred from 2020 to 2023. Discussions on the relationship between earthquakes and fluids in the Noto Peninsula have been ongoing (e.g., Nakajima, 2022; Nishimura et al., 2023). This study estimates the pore fluid pressure in the Noto Peninsula with focal mechanism solutions and discusses the relationship between earthquakes and fluids in the Noto Peninsula.
2.Data and method
We estimate the excess pore fluid pressure on faults during earthquakes with the focal mechanism solutions shallower than 30 km and since 1997 on the F-net catalog (https://www.fnet.bosai.go.jp/top.php) (N=290). The excess pore fluid pressure is obtained based on the method subjected by Terakawa et al. (2010) and which has following four assumptions: (1) The coefficient of friction is 0.6. (2) One of the three principal stresses is σv (overburden pressure). (3).The fault is critical condition to activate under a certain stress (stress which Mohr's stress circle tangent to the yield surface). (4) The excess fluid pressure is the amount of stress from the state of 3rd assumption to the nodal plane (fault plane) of each mechanism solution. Finally, the excess pore fluid pressure ratio (=λ) is calculated to be between 0 and 1, depending on the distance from the yield surface.
3.Results and Discussion
The stress tensor inversion (Hardebeck and Michael, 2006) from the seismic data confirms that the stress field is characterized by a reverse-faulting with NW-SE compression and σ3 trends in the vertical orientation. The average misfit angle obtained from the inversion is low, indicating that it is reasonable to assume a uniform stress field across the studied region. The results of the analysis show that the pore fluid pressure at all faults exceeds the hydrostatic pressure. No clear differences were observed in the frequency distribution of λ across all earthquake activities, the activities in 2007-2008, the swarm earthquake activities between 2020-2023, and the activities after January 1st, 2024. The results showed that earthquakes with a λ value of 0.2 were the most common, while earthquakes with a λ value of 0.8 or higher accounted for less than 4% of the total. Furthermore, temporal variation of λ showed no significant increase before the magnitude 6.7 and 7.6 earthquakes in 2007 and 2024, respectively. The analysis also highlighted a correlation between magnitude and λ, indicating that larger magnitude earthquakes tend to have lower λ values. Regarding large earthquakes studied here, a reduction in fault strength due to fluid involvement is not always necessary, and the accumulation of stress (increase in shear stress) may be the main factor.