0 2 4 6 8 1 10 Permeability anisotropy k x /k z k y /k z k x /k y 1x10 -19 1x10 -18 1x10 -17 1x10 -16 0 2 4 6 8 0.4 0.5 0.6 Permeability [m 2 ] k x k y k z Initial compaction with an increase in mean stress Friction coefficient ѥ 0 8 6 4 2 Development of permeability anisotropy of antigorite serpentinite gouge during shear deformations Keishi Okazaki 1※ , Ikuo Katayama 1 , Hiroyuki Noda 2 , Miki Takahashi 3 ※okazakikeishi@hiroshimau.ac.jp 1 Earth and planetary system science, Hiroshima University, Japan, 2 InsGtute for Research on Earth EvoluGon (IFREE), Japan Agency for MarineEarth Science and Technology (JAMSTEC), Japan 3 Geological Survey of Japan, Advanced Industrial Science and Technology, Japan Serpen&nite in subduc&on zone and earthquake, slow earthquake Experimental apparatus :Antigorite serpentinite from Nomo metamorphic rocks, Japan Pressure vessel Control panel “Gas pressure medium high temperature high pressure triaxial deformaGon apparatus” (Wibberlery and Shimamoto, 2003) Max. Pc: 220MPa, Max. Pp: 200MPa, Temp.800℃, Advantages: 1. Accurate measurements of axial load and fluid flow 2. Pore pressure control →DeformaGon experiments under hydrothermal condiGon and with conGnuous permeability measurement during deformaGon are possible. Mineral composiGon: AnGgorite (~98%), Spinel, MagneGte, Diopside and no olivine relict. Crushed and sieved to extract grains less than 100 μm in diameter. Mean grain diameter: 1.51 μm (d50), aspect raGo: 0.74 (measured using Morphologi G3, Malvern Instruments Ltd). Summary : 1. Permeability in three orthogonal direcGons of anGgorite serpenGnite gouge was measured during precut fricGonal experiments. 2. PermeabiliGes in all direcGons decreases by one order of magnitude at iniGal compacGon by increasing mean stress without showing significant anisotropy. 3. At the steady state in terms of shear stress, permeability anisotropies kx/kz and ky/kz stayed at their steady state value as high as nearly one order magnitude. 4. Microstructures of recovered samples suggest that the permeability anisotropy is caused by developments of R, Y and Pshear structures that may prevent fluid flow normal to the fault in serpenGnite gouge. 5. Permeability anisotropies may enhance fluid flow along subducGon plate interface and acGve fault zones. Permeability anisotropy and fluid flow in fault zones Forcing block Forcing block R1 Y P Forcing block Forcing block Epoxy Epoxy a d b c 0 50 100 150 200 Effective normal stress [MPa] 0 20 40 60 80 100 120 Shear stress [MPa] GR606 GR609 GR610 GR611 GR614 GR633 GR623 ~ 0.49 ~ 0.64 H 2 O (wet) - Ar (dry) Normal stress ~ 175 MPa corresponding to 6~7 km depth Increasing Pp Confining pressure 150 Mechanical effect Sample Ar Sample Furnace Internal loadcell Pc generators Pp generator “Effects of fluids on rock deformation“ =one of the largest uncertainGes in the subducGon zone!! ・Absorp7on of water on mineral surface ・SerpenGnizaGon (hydraGon) of ultramafic rock ・Decreasing in rock fricGon and flow stress (Morrow et al., 2000; Giger et al.,2008 ) ・AlteraGon of brilleducGle transiGon zone ・EffecGve pressure low (e.g. Terzaghi, 1923) ・Thermal pressurizaGon (Sibson, 1973) ・Faultvalve behavior (Sibson ,1992) Slow earthquakes (Obara, 2002, etc…): occur in high Vp/Vs raGo (~high fluid pressure = low Pe) zone of subducGon zone ↘ Serpen7nized mantle wedge? Lower plane of the double seismic zone : dehydraGon embrillement of serpenGnite? (Kirby et al., 1996; Peacock, 2001) reacGvates outerrise fault?(Nakajima et al., 2011) DEPSS DEPSS Department of Earth and Planetary Systems Science Hiroshima University, JAPAN _Hiroshima →How is fluid kept along fault zone? →Permeability anisotropy must act an important role keeping fluid pressure along fault zones!! Alumina precut spacer with Pp hole Antigorite gouge sample Polyolefin jacket Porous alumina WC spacer Alumina spacer Hole for pore pressure 20mm k // k k ⊥ ○Riedel shears (R1, Y and P) are developed normal to the plane including the fault normal and slip direcGons. But they are not straight as recognized in a secGon normal to the slip direcGon. ○Len7cular structure is developed in the direcGon normal to the slip direcGon in the fault. →They prevent fluid flow normal to the fault in serpen7nite gouge. PermeabiliGes in all direcGons decreases by one order of magnitude unGl shear stress reaches steadystate (apparent slip ~ 1 mm) without showing significant anisotropy. Ater the shear stress reaches steadystate, anisotropy of permeability becomes remarkable. Structure development Steady state? Permeability anisotropies kx/kz and ky/kz stayed at their steady state value as high as 8 at γ =3. →Fluids are likely to move parallel to the fault surface and might be kept around fault zone with minimal loss. →This value seems to be not enough to maintain excess pore pressure from previous models (Rice, 1992, Katayama et al., 2012). Fault healing(e.g. Tenthorey et al, 2003) and cap rocks (e.g. Peacock et al., 2011; Katayama et al., 2012) potenGally act important roles to increase permeability gap and to maintain excess pore pressure. Moho Oceanic crust Mantle wedge (Peridotite) Serpentinized mantle wedge Megathrust earthquake Slow earthquakes (SSE, LFE, NVT) Oceanic lithosphere (Philippine Sea plate) Oceanic ridge ? ? Outer-rise earthquake ? ? Inland fault Intra-slab earthquake ? Shear Strain (γ) Serpen&nite in subduc&on zone and its poten&al significance in regular and slow earthquakes: 10 1 0 1 10 2 0 0.5 1 1.5 2 2.5 3 3.5 4 Number Density Normalized by log boxcar Starting material Effective Grain Diameter, μm −1 −0.5 0 0.5 1 1.5 2 0 0.2 0.4 0.6 0.8 1 log10(Effective Grain Diameter), μm Aspect Ratio Starting Material 0 2 4 6 8 10 12 1x10 -19 1x10 -18 1x10 -17 10 -16 Permeability [m ] Permeability [m 2 ] b c GR663 (k // ) GR642 (k // ) GR645 (k - ) GR657 (k - ) GR654 (k ) GR655 (k // ) GR664 (k ) Experimental condi7on: Pc = 150MPa, Pp = 100MPa Slip rate = 0.575 μm/s, Pore fluid: water, Temp. = RT 0 1 2 3 4 5 6 Axial displacement [mm] 0 0.2 0.4 0.6 0.8 "$! !&$ 0 1 2 3 4 5 6 Axial shortening [mm] 0.7 0.9 1.1 1.3 1.5 Gouge thickness [mm] Gouge thickness [mm] Friction coefficient Hit point GR663 (k // ) GR642 (k // ) GR645 (k - ) GR657 (k - ) GR654 (k ) GR655 (k // ) GR664 (k ) L z = 1.184 - 0.325 d a 0.210 displacement [mm] GR663 (k // ) GR642 (k // ) GR645 (k - ) GR657 (k - ) GR654 (k ) GR655 (k // ) GR664 (k ) Permeability measurement on An&gorite serpen&nite gouge during shear deforma&on Microstructures of recovered samples L 2 k //,−,⊥ = L i 2 k i = i ∑ L x 2 k x + L y 2 k y + L z 2 k z * k: permeability, L: length of each component 0 1 0 0.5 1 1.5 2 2.5 3 Number Density Normalized Uniform Distribution Starting material Aspect Ratio Shear Strain (γ)