南海トラフにおける間隙水挙動に関する研究の現状と間隙水圧観測の意義

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1 Journal of Geography Current Research Status and Meaning of Fluid Pressure Monitoring at the Nankai Trough Hitoshi MIKADA, Masanori IENAGA, Tada-nori GOTO and Takafumi KASAYA Abstract Experimental monitoring of fluid pressures was initiated in June 2001 at 2 underwater holes drilled during the Ocean Drilling Program ODP Leg 196 to investigate the relationship between deformation and fluid flow processes in the Nankai accretionary prism. ODP Leg 196 visited Sites 1173 drilled on Leg 190 and 808 drilled through the frontal thrust on Leg 131, installing 2 Advanced Circulation Obviation Retrofit Kits ACORKs to monitor fluid pressures along the walls of the drilled holes. Site 808 penetrates the frontal thrust fault and the décollement, while Site 1173 is located about 11 km seaward from the Nankai Trough deformation front. Formation water freshening around the décollement was first observed on ODP Leg 131, and geochemists have been investigating whether or not the freshening was due to the production of deep-sourced dehydration processes. We now know that the role of smectite dehydration and dehydrant quartz-cristobalite phase transition should be estimated quantitatively. One of the important findings of fluid pressure monitoring is that the formation fluid pressures seem to reflect the change in the stress state in and around the accretionary prism. We believe that such fluid pressure monitoring in the accretionary prism and in the sediments on top of the subducting oceanic plate in terms of stress field and dehydrant process of minerals is a key to deepening our knowledge in future investigations of seismogenic processes. Key words ODP, Nankai Trough, ACORK, stress state, fluid circulation, fluid pressure ODP ACORK Depertment of Civil and Earth Resources Engineering, Kyoto University Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology Present Address: SciMarkJ Inc.

2 I Ando, 1975 Moore et al., 2001 Hyndman et al., 1995; Saffer and Bekins, 1998; Moore and Silver, 2002; Bourlange et al., 2003; Henry et al., 2003; Saffer, 2003; Morgan and Ask, 2004; Ge and Screaton, 2005 Moore et al., 2005 Mikada et al., 2002a Bangs et al., 2004; Moore et al., 2005; Ienaga et al., 2006 Kastner et al., 1993; Brown et al., 2001; Bangs et al., 2004; Moore et al., Advanced CORK; CORK Circulation Obviation Retrofit Kit ACORK 13 km Mikada et al., 2002b Hucks et al., 2005 Permeability, Zhang et al., 2000; Bernabe et al., 2003 ACORK K. Becker Davis et al., Savage, 1995; Ozawa et al., 1999

3 1 Ocean Drilling Program Moore et al., 2005 ODP Embayment I-B Trench, I-B Arc, KPR, KSC Fig. 1 Locations of sites drilled during the Ocean Drilling Program Moore et al., Numbers are ODP site numbers. Embayment of the accretionary prism due to the subduction of a seamount is clearly visible in the figure. Two formation pressure-monitoring observatories were installed one at Site 808, at the toe of the accretionary prism, and one at Site 1173, at a sea-ward site of the Nankai Trough. I-B, KPR, and KSC depict Izu-Bonin, Kyushu-Palau Ridge, and Kinan Seamount Chain, respectively. ACORK ACORK II Ando, cm 310 N Ando, 1975;, 1992; Sagiya and Thatcher, 1999 Mazzotti et al., ,500 Okino et al., 1994

4 2 1 Muroto Transect Moore et al PTZ BSR Proto-Thrust Zone Bottom Simulating Reflector Fig. 2 Schematic cross-section along the Moroto transect shown in Fig. 1 Modified from Moore et al., PTZ is the Proto-Thrust Zone; BSR is the Bottom-Simulating Reflector. Major observations pertinent to subduction of the oceanic plate and development of the décollement are shown in the figure. Seismic reflections show a polarity reversal at the décollement layer. As the décollement approaches the updip limit of the seismogenic zone, reflection amplitudes from the décollement decline. Top of oceanic basement, acoustically transparent zone between the top of oceanic basement and the décollement layer, and stepdown of the décollement layer are also depicted in the figure ACORK m 15 Ma Moore et al., Mikada et al., 2002a 40 Mikada et al., 2002a 7 km Mikada et al., 2002a Tsuji et al., 2005 Saffer 2003

5 3 Boulange et al a b mbsf a b Fig. 3 Normal a and fracture b porosities estimated in and around the décollement layer at Site 808 Figure from Bourlange et al., Vertical axis denotes depth in meters below seafloor mbsf. Solid curve in a depicts both density- and resistivity-derived porosities. Resistivity-derived porosity varies between 0.23 and 0.35, while density-derived porosity varies between 0.30 and Discrepancies between resistivity-derived and density-derived porosities are attributed to fracture porosities, which have a strong influence on permeability and on change of permeability due to fluid pressure. Negative values in b are caused by estimation errors. Bourlange et al Stauffer and Bekins 2001 Newberry et al m Thermococcus Kormas et al.,

6 Kormas et al., Logging While Drilling 808 McNeill et al., Ienaga et al., McNeill et al., 2004 III Moore and Silver, m 2 Chloride Kastner et al., 1993; Saffer and Bekins, 1998 Moore and Silver, 2002; 4 Saffer and Bekins m m 2 20 Bourlange et al Saffer and Bekins m Saffer and Bekins 1998 Bourlange et al Jouniaux et al., 1994 Morgan and Ask 2004

7 4 Moore and Silver 2002 Fig. 4 Chloride anomalies observed at Costa Rica, Barbados, and Nankai subduction zones Figure from Moore and Silver, Contrary to chloride anomalies limited to the vicinity of décollement layers at the former two subduction zones, a wide depth zone of anomalous chlorinity was observed at the décollement layer off Muroto. This observation recalled discussions on a relationship between the difference in chlorinity and that of fluid migrations. 2.8 Saffer and Bekins, 1998 IV Hyndman et al. 1995

8 Steurer and Underwood Moore and Saffer Brown et al Henry and Bourlange 2004 Saffer and Bekins Moore et al Moore and Silver 2002 Ge and Screaton km ACORK 5 Hyndman et al Vrolijk,

9 5 808 A 1173 B ACORK Mikada et al. 2002b Fig. 5 Schematic figures for ACORK systems installed at Sites 808 A and 1173 B, off Muroto Modified from Mikada et al., 2002b. Locations of packers and screens for pressure measurements are also shown. They measure fluid pressures on the seafloor through downhole tubing. The locations of the screens are chosen at depths of décollement and stratigraphic equivalent of décollement layers. A strong seismic reflector at Site 808 indicates the location of the décollement layer, which disappears between the two sites. At Site 1173, the diagenetic boundary of a quartz-cristobalite phase transition is a strong seismic reflector. Pressure gauges are installed to detect fluid migration at the locations of proto-thrust fault, décollement layer, diagenetic boundary, and inside the oceanic basement V McNeill et al Ienaga et al. 2006

10 McNeill et al Henry et al Ienaga et al Yamada et al Bangs et al Saffer Stauffer and Bekins 2001 Ienaga et al Davis et al ACORK 11 km 10 6 Davis et al Park et al Out of Sequence Thrust Zone Deep Seismic Reflector 15 Tsuru et al P m MPa VI

11 6 Ienaga, et al A Fig. 6 Schematic stress diagram demonstrating the relationship with observed fracture dips at Sites 808 and 1173 where low and high angle dips were observed, respectively Modified from Ienaga, et al., Friction angle of the sediments was assumed to be degrees. Arrows indicate the direction of fracture and slip to be developed by slanted principal stress. Due to heavy mass loading of the prism, the vertical normal trend of compaction to the sediments is distorted towards the prism. It is qualitatively indicated that thrust type and horizontal fracturing tend to be formed above and in the décollement, respectively. A blow-up image of the décollement layer at the bottom implies that tensile fractures could be developed at the top and at the base of the décollement layer. The area indicated by A denotes a possible zone of tensile field due to spatial changes in principal stress directions. It is necessary to consider possible stress distributions for fluid migrations., Zhang et al., 2000; Bernabe et al., 2003 K. Becker ACORK ACORK

12 7 MT Fig. 7 Schematic structure around seismogenic zone using seismic reflection and magnetotelluric profiles. A decline of seismic reflections is found in the vicinity of the décollement step-down and the updip limit of the seismogenic zone, while a zone of low re-sistivity and microtremors are found near the downdip limit. Both phenomena may be explained by the existence of interstitial fluids. However, in-situ data should be provided for further discussions of fluid circulation on a spatial scale including both accretionary prism and seismogenic zone. ; 2 Obara, VII Moore and Silver 2002 Moore et al., 2005

13 Vrolijk, 1990; Hyndman et al., 1995 Steurer and Underwood, 2003; Moore et al., Logging-While-Drilling In- Situ Mikada et al., 2002a McNeill et al., 2004; Ienaga et al., 2006 Newberry et al., 2004 Kormas et al., Moore et al., 2005 Davis et al., 2006 Clavier et al., 1984 Pore Bourlange et al., 2003 McNeill et al., 2004; Ienaga et al., 2006 ACORK 1, Kasaya et al ACORK

14 VIII 1 MT Keir Becker Pierre Henry J. Casey Moore Earl E. Davis Peter Flemings Greg F. Moore ACORK ACORK Ando, M : Source mechanisms and tectonic significance of historical earthquakes along the Nankai Trough, Japan. Tectonophysics, 27, Bangs, N., Shipley, S., Moore, G., Gulick, S., Kuromoto, S. and Nakamura, Y : Evolution of the decollement from the trench into the seismogenic zone inferred from mapping Nankai trough decollement seismic reflections in 3-D, Muroto transect. Geology, 32, Bernabe, Y., Bruderer-Weng, C. and Maineult, A : Permeability fluctuations in heterogeneous networks with different dimensionality and topology. J. Geophys. Res., 108, 2351, doi: / 2002JB Bourlange, S., Henry, P., Moore, J.C., Mikada, H. and Klaus, A : Fracture porosity in the decollement zone of Nankai accretionary wedge using Logging While Drilling resistivity data. Earth Planet. Sci. Lett., 209, Brown, K.M., Saffer, D.M. and Bekins, B.A : Smectite diagenesis, pore-water freshening, and fluid flow at the toe of the Nankai wedge. Earth Planet. Sci. Lett., 194, Clavier, C., Coates, G. and Dumanoir, J : Theoretical and Experimental Bases for the Dual-Water Model for Interpretation of Shaly Sands, SPE J., April 1984, Davis, E.E., Becker, K., Dziak, R., Cassidy, J., Wang, K. and Lilley, M : Hydrological response to a seafloor spreading episode on the Juan de Fuca ridge, Nature, 430, Davis, E.E., Becker, K., Wang, K., Obara, K., Ito, Y. and Kinoshita, M : A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate, Earth Planet. Sci. Lett., 242, Ge, S. and Screaton, E : Modeling seismically induced deformation and fluid flow in the Nankai subduction zone. Geophys. Res. Lett., 32, L17301, doi: /2005GL Henry, P. and Bourlange, S : Smectite and fluid budget at Nankai ODP sites derived from cation exchange capacity. Earth Planet. Sci. Lett., 219, Henry, P., Jouniaux, L., Screaton, E. J., Hunze, S. and Saffer, D. M : Anisotropy of electrical conductivity record of initial strain at the toe of the Nankai accretionary wedge. J. Geophys. Res., 108, 2407, doi: /2002JB Hucks, A., Flemings, P. B., Becker, K. and Kinoshita,

15 M : Hydrologic Monitoring in the Nankai Accretionary Prism. Eos Trans. Am. Geophys. Union, 86, Fall Meet. Suppl., Abstract T13B Hyndman, R., Wang, K. and Yamano, M : Thermal constraints on the seismogenic protion of the southwestern Japan subduction thrust. J. Geophys. Res., 100, Ienaga, M., McNeill, L. C., Mikada, H., Saito, S., Goldberg, D. and Moore, J. C : Borehole image analysis of the Nankai Accretionary Wedge, ODP Leg 196: Structural and Stress Studies. Tectonophysics, doi: /j.tecto , in press. Jouniaux, L., Lallemant, S. and Pozzi, J.-P : Changes in the permeability, streaming potential and resistivity of a claystone from the Nankai prism under stress. Geophys. Res. Lett., 21, Kasaya, T., Goto, T., Mikada, H., Baba, K., Suyehiro, K. and Utada, H : Resistivity image of the Philippine Sea Plate around the 1944 Tonankai earthquake zone deduced by Marine and Land MT surveys. Earth Planets Space., 57, Kastner, M., Elderfield, H., Jenkins, W.J., Gieskes, J. and Gamo, T : Geochemical and isotopic evidence for fluid flow in the western Nankai subduction zone, Japan. in Hill, I.A., Taira, A., Firth, J. V et al. Eds., Proc. ODP, Sci. Results, 131, College Station TX Ocean Drilling Program, Kormas, K.A., Smith, D.C., Edgcomb, V. and Teske, A : Molecular analysis of deep subsurface microbial communities in Nankai Trough sediments ODP Leg 190, Site 1176A. FEMS Microbiol. Ecol., 45, Mazzotti, S., Le Pichon, X., Henry, P. and Miyazaki, S : Full interseismic locking of the Nankai and Japan-west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS. J. Geophys. Res., 105, McNeill, L.C., Ienaga, M., Tobin, H., Saito, S., Goldberg, D., Moore, J.C. and Mikada, H : Deformation and in situ stress in the Nankai Accretionary Prism from resistivity-at-bit images, ODP Leg 196. Geophys. Res. Lett., 31, L02602, doi: / 2003GL Mikada, H., Becker, K., Moore, J. C., Klaus, A. et al. 2002a : Proc. ODP, Init. Repts., 196 CD-ROM, Ocean Drilling Program, Texas A&M University, College Station, TX. Mikada, H., Becker, K., Moore, J. C. and Klaus, A. 2002b : Leg-196: Deformation and fluid flow processes: Logging while drilling and Advanced CORK in the Nankai Trough accretionary prism. JOIDES J., 28 2, Moore, G.F., Taira, A., Klaus, A. and the Leg-190 Scien-tific Party 2001 : New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochem. Geophys. Geosyst., 2, /2001GC Moore, G. F., Mikada, H., Moore, J. C., Becker, K. and Taira, A : Legs 190/196 synthesis: Deformation and fluid flow processes in the Nankai trough accretionary prism. in Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C. and Klaus, A. Eds., Proc. ODP, Sci. Results., 190/196 Online. synth/synth.html Cited Moore, J.C. and Saffer, D.M : Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: an effect of diagenetic to low-grade metamorphic processes and increasing effective stress. Geology, 29, Moore, J.C. and Silver, E : Fluid flow in accreting and eroding convergent margins, JOIDES J., 28 1, Morgan, J. K. and Ask, M. V. S : Consolidation state and strength of underthrust sediments and evolution of the decollement at the Nankai accretionary margin: Results of uniaxial reconsolidation experiments. J. Geophys. Res., 109, B03102, doi: /2002JB Newberry, C. J., Webster, G., Cragg, B. A., Parkes, R. J., Weightman, A. J. and Fry, J. C : Diversity of prokaryotes and methanogenesis in deep subsurface sediments from the Nankai trough, Ocean Drilling Program Leg 190. Environ. Microbiol., 6, Obara, K : Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan. Science, 296, Okino, K., Shimakawa, Y. and Nagaoka, S : Evolution of the Shikoku Basin. J. Geomagn. Geoelectr., 46, Ozawa, T., Tabei, T. and Miyazaki, S : Interplate coupling along the Nankai trough off southwest Japan derived from GPS measurements. Geophys. Res. Lett., 26, Park, J.-O., Tsuru, T., Takahashi, N., Hori, T., Kodaira, S., Nakanishi, A., Miura, S. and Kaneda, Y : A deep strong reflector in the Nankai accretionary wedge from multichannel seismic data: Implications for underplating and interseismic shear stress release. J. Geophys. Res., 107, 2061, doi: /2001JB Saffer, D. M : Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai. J. Geophys. Res., 108, 2261, doi: /2002JB Saffer, D. M. and Bekins, B. A : Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget. J. Geophys. Res., 103, 30, , 370.

16 Sagiya, T. and W. Thatcher 1999 : Coseismic slip resolution along a plate boundary megathrust: The Nankai Trough, southwest Japan. J. Geophys. Res., 104, : 251p Savage, J. C : Interseismic uplift at the Nankai subduction zone, southwest Japan, J. Geophys. Res., 100, Stauffer, P and Bekins, B. A : Modeling consolidation and dewatering near the toe of the northern Barbados accretionary complex. J. Geophys. Res., 106, Steurer, J.F. and Underwood, M.B : Clay mineralogy of mudstones from the Nankai Trough reference Sites 1173 and 1177 and frontal accretionary prism Site in Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C. and Klaus, A. Eds., Proc. ODP, Sci. Results, 190/196 Online /211.htm Cited Tsuji, T., Matsuoka, T., Yamada, Y., Nakamura, Y., Ashi, J., Tokuyama, H., Kuramoto, S. and Bangs, N. L : Initiation of plate boundary slip in the Nankai Trough off the Muroto peninsula, southwest Japan. Geophys. Res. Lett., 32, L12306, doi: /2004GL Tsuru, T., Park, J.-O., Kido, Y., Ito, A., Kaneda, Y., Yamada, T., Shinohara, M. and Kanazawa, T : Did expanded porous patches guide rupture propagation in 2003 Tokachi-oki earthquake? Geophys. Res. Lett., 32, L20310, doi: /2005GL Vrolijk, P : On the mechanical role of smectite in subduction zones. Geology, 18, Yamada, Y., Baba, K. and Matsuoka, T : Analogue and numerical modelling of accretionary prisms with a decollement in sediments. in Numerical and Analogue Modelling of Crustal-Scale Processes edited by Buiter, S. and Scherurs, G., Geol. Soc. Lond., Spec. Publ., 253, Zhang, D., Zhang, R., Chen, S. and Soll, W. E : Pore scale study of flow in porous media: Scale dependency, REV, and statistical REV. Geophys. Res. Lett., 27,

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untitled World topography Most earthquakes occur in subduction zones Trench-Arc-Backarc system Landward side Seaward side Back- Fore- (Widiyantoro et al., 1999 EPSL) Seismicity cross section across Japan trench