[S-CG53] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG53-P18] Estimation of pore fluid overpressures for tensile cracking at depth of shallow slow earthquakes

*Makoto Otsubo¹, Kohtaro Ujiie², Jeanne L. Hardebeck³, Hanae Saishu¹, Ayumu Miyakawa¹, Asuka Yamaguchi⁴ (1.Geological Survey of Japan/AIST, 2.University of Tsukuba, 3.U.S. Geological Survey, 4.The University of Tokyo)

Keywords:Slow earthquake, Stress, Crack, Fluid, Subduction

Pore fluid pressure P_f is important for understanding 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 mélanges, eastern Kyushu, SW Japan. In the coastal region of the Makimine mélanges (Late Cretaceous Shimanto accretionary complex of SW Japan; temperature = 300–350°C, Palazzin et al., 2016), the mélange preserves quartz-filled shear, foliation-parallel veins and tension vein arrays. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal σ₁-axis trending NW–SE 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 in the case of the Makimine mélanges are ~280 MPa (assuming depth = 10 km, density = 2750 kg/m³, tensile strength = 5 MPa and Young’s modulus = 7.5–15 GPa). The normalized pore pressure ratio λ* (λ* = (P_f – P_h) / (P_l – P_h), P_l: lithostatic pressure; P_h: hydrostatic pressure) is ~1.03 (P_f > P_l).