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[SCG40-P38] Fluid pressure from a shallow very-low frequency earthquake is related to the topography
Keywords:fluid pressure, slip tendency, subduction zone, Fault, slow slip, VLFE
Fluid pressure weakens fault strength effectively. Slow earthquakes with low stress drop are believed to occur under high fluid pressure. While previous studies have tried to determine fluid pressure both geologically and geophysically, there are no studies that have used actual slow earthquake data to estimate fluid pressure. Hashimoto et al. (2022) utilized the seismic mechanism of very-low-frequency earthquakes (VLFE) off the Kii Peninsula, deriving the slip tendency (Ts) distribution. This study extends the method to estimate fluid pressure associated with VLFE, investigating the spatiotemporal relationship between fluid pressure and VLFE. Additionally, factors controlling the fluid pressure are discussed.
According to Cox (2010), fluid pressure (Pf) can be determined by the angle between σ1 and the fault plane (θr), the differential stress (Δσ), and lithostatic pressure. Δσ was calculated using the approach of Terakawa et al. (2012). The lithostatic pressure was constrained by depth from CMT solution of VLFE. θr was obtained by applying the regional stress from Hashimoto et al. (2022) to individual VLFE fault planes from CMT solutions. Ts, ranging from 0.1 to 0.6, for each fault plane was also obtained as a depth-independent values.
As a result, fluid pressure ranged from 70 to 170 MPa, corresponding to a fluid pressure ratio of approximately 0.4 to 0.7. Ts values were mainly above 0.3, with a few Ts around 0.1. Temporally, instances with high fluid pressure and low Ts values were observed, but most of values were under conditions close to hydrostatic pressure. The relationship between Ts and fluid pressure is negatively correlated, suggesting that fault planes with low Ts values slipped due to the weakening by high fluid pressure. Furthermore, since Ts is depth-independent, it is presumed that fluid pressure has a minor depth influence.
The spatial distribution of Ts and fluid pressure for VLFE exhibited a northeast-southwest-oriented alignment, which is parallel to the Ts distribution depending on the subduction plate topography. This implies that the topography of the plate boundary controls not only Ts but also fluid pressure.
According to Cox (2010), fluid pressure (Pf) can be determined by the angle between σ1 and the fault plane (θr), the differential stress (Δσ), and lithostatic pressure. Δσ was calculated using the approach of Terakawa et al. (2012). The lithostatic pressure was constrained by depth from CMT solution of VLFE. θr was obtained by applying the regional stress from Hashimoto et al. (2022) to individual VLFE fault planes from CMT solutions. Ts, ranging from 0.1 to 0.6, for each fault plane was also obtained as a depth-independent values.
As a result, fluid pressure ranged from 70 to 170 MPa, corresponding to a fluid pressure ratio of approximately 0.4 to 0.7. Ts values were mainly above 0.3, with a few Ts around 0.1. Temporally, instances with high fluid pressure and low Ts values were observed, but most of values were under conditions close to hydrostatic pressure. The relationship between Ts and fluid pressure is negatively correlated, suggesting that fault planes with low Ts values slipped due to the weakening by high fluid pressure. Furthermore, since Ts is depth-independent, it is presumed that fluid pressure has a minor depth influence.
The spatial distribution of Ts and fluid pressure for VLFE exhibited a northeast-southwest-oriented alignment, which is parallel to the Ts distribution depending on the subduction plate topography. This implies that the topography of the plate boundary controls not only Ts but also fluid pressure.