Temporal changes in pore fluid pressure during slow earthquake cycle estimated from foliation-parallel extension cracking

Pore fluid pressure (P_f) is of great importance to understand slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine mélange eastern Kyushu, SW Japan. The mélange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: a few tens of microns) may represent episodic tremor and slow slip (Ujiie et al., 2018). The foliation-parallel extension cracks can function as the fluid pathway in the mélange. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel extension vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal σ₁-axis trending NNW–SSE and a sub-vertical σ₃-axis, and the driving pore fluid pressure ratio P* (P* = (P_f – σ₃) / (σ₁ – σ₃)) is ~0.1. When the pore fluid pressure exceeds σ₃, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds σ₃ is the pore fluid overpressure ΔP_f (ΔP_f = P_f – σ₃). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). P_f and ΔP_f in the case of the Makimine mélange are ~280 MPa and 80–160 kPa (assuming depth = 10 km, density = 2800 kg/m³, tensile strength = 1 MPa and Young’s modulus = 7.5–15 GPa). When the pore fluid overpressure is released, the cracks are closed and the reduction of pore fluid pressure is stopped (Otsubo et al., 2020). After the pore fluid overpressure is reduced, the normalized pore pressure ratio λ* (λ* = (P_f – P_h) / (P_l – P_h), P_l: lithostatic pressure; P_h: hydrostatic pressure) is ~1.01 (P_f > P_l). The results indicate that the pore fluid pressure constantly maintains the lithostatic pressure during the extension cracking along the foliation.

References: Gudmundsson (1999) Geophys. Res. Lett., 26, 115–118; Jolly and Sanderson (1997) Jour. Struct. Geol., 19, 887–892; Otsubo et al. (2020) Sci. Rep., 10:12281; Palazzin et al. (2016) Tectonophysics, 687, 28–43; Sato et al. (2013) Tectonophysics, 588, 69–81; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371–5379, https://doi.org/10.1029/2018GL078374.