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

<p>Pore fluid pressure (P<sub>f</sub>) 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 σ<sub>1</sub>-axis trending NNW–SSE and a sub-vertical σ<sub>3</sub>-axis, and the driving pore fluid pressure ratio P* (P* = (P<sub>f</sub> – σ<sub>3</sub>) / (σ<sub>1</sub> – σ<sub>3</sub>)) is ~0.1. When the pore fluid pressure exceeds σ<sub>3</sub>, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds σ<sub>3</sub> is the pore fluid overpressure ΔP<sub>f</sub> (ΔP<sub>f</sub> = P<sub>f</sub> – σ<sub>3</sub>). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). P<sub>f</sub> and ΔP<sub>f</sub> in the case of the Makimine mélange are ~280 MPa and 80–160 kPa (assuming depth = 10 km, density = 2800 kg/m<sup>3</sup>, 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<sub>f</sub> – P<sub>h</sub>) / (P<sub>l</sub> – P<sub>h</sub>), P<sub>l</sub>: lithostatic pressure; P<sub>h</sub>: hydrostatic pressure) is ~1.01 (P<sub>f</sub> > P<sub>l</sub>). The results indicate that the pore fluid pressure constantly maintains the lithostatic pressure during the extension cracking along the foliation.</p><p>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.</p>

Spatial changes in inclusion band spacing as an indicator of temporal changes in slow earthquake recurrence intervals

<p>Repeated slow earthquakes downdip of the seismogenic zones may trigger megathrust earthquakes by transferring stress to the seismogenic zones. Geodetic observations have suggested that the recurrence intervals of slow earthquakes decrease toward a next megathrust earthquake. However, the temporal variation in recurrence intervals of slow earthquakes during megathrust earthquake cycles remains poorly understood due to the limited duration of geodetic and seismological monitoring of slow earthquakes. The quartz-filled, crack-seal shear veins in the subduction mélange deformed near the downdip limit of seismogenic zone in warm-slab environments record the cyclic changes in the inclusion band spacing in the range of 5–65 μm. The two-phase primary fluid inclusions in quartz between inclusion bands show various vapor/liquid ratios regardless of inclusion band spacing, suggesting a common occurrence of fast quartz sealing due to a rapid decrease in quartz solubility associated with a large fluid pressure reduction. A kinetic model of quartz precipitation, considering a large fluid pressure change and inclusion band spacings, indicates that the sealing time during a single crack-seal event cyclically decreased and increased in the range of 0.2–2.7 years, with minimum one cycle duration estimated to be 31–93 years. The ranges of sealing time and one cycle duration may be comparable to the recurrence intervals of slow earthquakes and megathrust earthquakes, respectively. We suggest that the spatial change in the inclusion band spacing is a potential geological indicator of the temporal changes in slow earthquake recurrence intervals, particularly when large fluid pressure reduction occurred by brittle fracturing.</p>