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Kazushige Obara – One of the best experts on this subject based on the ideXlab platform.

  • Very‐low‐frequency earthquakes indicate a transpressional stress regime in the Nankai Accretionary Prism
    Geophysical Research Letters, 2009
    Co-Authors: Yoshihiro Ito, Youichi Asano, Kazushige Obara
    Abstract:

    [1] We investigated the stress field within the Nankai Accretionary Prism of southwestern Japan, where very-low-frequency (VLF) earthquakes occur associated with thrust faulting. A northwest–southeast azimuth of the maximum horizontal principal stress previously estimated from borehole breakouts in wells drilled in the region by deep-sea drilling vessel Chikyu suggests trench-normal shortening, although strike-slip and normal faulting are also possible within a thrust-dominated tectonic environment. We estimated stress orientations and stress ratios by using stress tenstensor inversion to derive moment tenstensor solutions for VLF earthquakes in three regions along the Nankai Trough: off Kumano, off Muroto, and Hyuga-nada. The stress orientations we obtained indicate that the regions off Kumano and off Muroto are within a transpressional stress regime with trench-normal shortening, whereas the Hyuga-nada region on the westernmost edge of the Nankai Accretionary Prism is in a reverse-faulting regime.

  • very low frequency earthquakes indicate a transpressional stress regime in the nankai Accretionary Prism
    Geophysical Research Letters, 2009
    Co-Authors: Yoshihiro Ito, Youichi Asano, Kazushige Obara
    Abstract:

    [1] We investigated the stress field within the Nankai Accretionary Prism of southwestern Japan, where very-low-frequency (VLF) earthquakes occur associated with thrust faulting. A northwest–southeast azimuth of the maximum horizontal principal stress previously estimated from borehole breakouts in wells drilled in the region by deep-sea drilling vessel Chikyu suggests trench-normal shortening, although strike-slip and normal faulting are also possible within a thrust-dominated tectonic environment. We estimated stress orientations and stress ratios by using stress tenstensor inversion to derive moment tenstensor solutions for VLF earthquakes in three regions along the Nankai Trough: off Kumano, off Muroto, and Hyuga-nada. The stress orientations we obtained indicate that the regions off Kumano and off Muroto are within a transpressional stress regime with trench-normal shortening, whereas the Hyuga-nada region on the westernmost edge of the Nankai Accretionary Prism is in a reverse-faulting regime.

  • a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone Accretionary Prism and incoming philippine sea plate
    Earth and Planetary Science Letters, 2006
    Co-Authors: Earl E Davis, Kazushige Obara, Yoshihiro Ito, Keir Becker, Kelin Wang, Masataka Kinoshita
    Abstract:

    New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai Accretionary Prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the Accretionary Prism toe. The rate of strain (ca. 10− 6 yr− 1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr− 1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer Prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the Prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the Prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the Accretionary Prism toe along the decollement separating the Prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the Prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.

Thomas H. Shipley – One of the best experts on this subject based on the ideXlab platform.

  • sequence stratigraphy structural style and age of deformation of the malaita Accretionary Prism solomon arc ontong java plateau convergent zone
    Tectonophysics, 2004
    Co-Authors: Eric J. Phinney, Paul Mann, Millard F. Coffin, Thomas H. Shipley
    Abstract:

    Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an Accretionary Prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide Accretionary Prism. We name this Prism the “Malaita Accretionary Prism” or “MAP” after Malaita, the largest and best-studied island exposure of the Accretionary Prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.

  • Sequence stratigraphy, structural style, and age of deformation of the Malaita Accretionary Prism (Solomon arc–Ontong Java Plateau convergent zone)
    Tectonophysics, 2004
    Co-Authors: Eric J. Phinney, Paul Mann, Millard F. Coffin, Thomas H. Shipley
    Abstract:

    Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an Accretionary Prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide Accretionary Prism. We name this Prism the “Malaita Accretionary Prism” or “MAP” after Malaita, the largest and best-studied island exposure of the Accretionary Prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.

Eric J. Phinney – One of the best experts on this subject based on the ideXlab platform.

  • sequence stratigraphy structural style and age of deformation of the malaita Accretionary Prism solomon arc ontong java plateau convergent zone
    Tectonophysics, 2004
    Co-Authors: Eric J. Phinney, Paul Mann, Millard F. Coffin, Thomas H. Shipley
    Abstract:

    Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an Accretionary Prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide Accretionary Prism. We name this Prism the “Malaita Accretionary Prism” or “MAP” after Malaita, the largest and best-studied island exposure of the Accretionary Prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.

  • Sequence stratigraphy, structural style, and age of deformation of the Malaita Accretionary Prism (Solomon arc–Ontong Java Plateau convergent zone)
    Tectonophysics, 2004
    Co-Authors: Eric J. Phinney, Paul Mann, Millard F. Coffin, Thomas H. Shipley
    Abstract:

    Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an Accretionary Prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide Accretionary Prism. We name this Prism the “Malaita Accretionary Prism” or “MAP” after Malaita, the largest and best-studied island exposure of the Accretionary Prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.

Yoshihiro Ito – One of the best experts on this subject based on the ideXlab platform.

  • Very‐low‐frequency earthquakes indicate a transpressional stress regime in the Nankai Accretionary Prism
    Geophysical Research Letters, 2009
    Co-Authors: Yoshihiro Ito, Youichi Asano, Kazushige Obara
    Abstract:

    [1] We investigated the stress field within the Nankai Accretionary Prism of southwestern Japan, where very-low-frequency (VLF) earthquakes occur associated with thrust faulting. A northwest–southeast azimuth of the maximum horizontal principal stress previously estimated from borehole breakouts in wells drilled in the region by deep-sea drilling vessel Chikyu suggests trench-normal shortening, although strike-slip and normal faulting are also possible within a thrust-dominated tectonic environment. We estimated stress orientations and stress ratios by using stress tensor inversion to derive moment tensor solutions for VLF earthquakes in three regions along the Nankai Trough: off Kumano, off Muroto, and Hyuga-nada. The stress orientations we obtained indicate that the regions off Kumano and off Muroto are within a transpressional stress regime with trench-normal shortening, whereas the Hyuga-nada region on the westernmost edge of the Nankai Accretionary Prism is in a reverse-faulting regime.

  • very low frequency earthquakes indicate a transpressional stress regime in the nankai Accretionary Prism
    Geophysical Research Letters, 2009
    Co-Authors: Yoshihiro Ito, Youichi Asano, Kazushige Obara
    Abstract:

    [1] We investigated the stress field within the Nankai Accretionary Prism of southwestern Japan, where very-low-frequency (VLF) earthquakes occur associated with thrust faulting. A northwest–southeast azimuth of the maximum horizontal principal stress previously estimated from borehole breakouts in wells drilled in the region by deep-sea drilling vessel Chikyu suggests trench-normal shortening, although strike-slip and normal faulting are also possible within a thrust-dominated tectonic environment. We estimated stress orientations and stress ratios by using stress tensor inversion to derive moment tensor solutions for VLF earthquakes in three regions along the Nankai Trough: off Kumano, off Muroto, and Hyuga-nada. The stress orientations we obtained indicate that the regions off Kumano and off Muroto are within a transpressional stress regime with trench-normal shortening, whereas the Hyuga-nada region on the westernmost edge of the Nankai Accretionary Prism is in a reverse-faulting regime.

  • a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone Accretionary Prism and incoming philippine sea plate
    Earth and Planetary Science Letters, 2006
    Co-Authors: Earl E Davis, Kazushige Obara, Yoshihiro Ito, Keir Becker, Kelin Wang, Masataka Kinoshita
    Abstract:

    New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai Accretionary Prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the Accretionary Prism toe. The rate of strain (ca. 10− 6 yr− 1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr− 1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer Prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the Prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the Prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the Accretionary Prism toe along the decollement separating the Prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the Prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.

Gregory F. Moore – One of the best experts on this subject based on the ideXlab platform.

  • structural architecture and active deformation of the nankai Accretionary Prism japan submersible survey results from the tenryu submarine canyon
    Geological Society of America Bulletin, 2009
    Co-Authors: Kiichiro Kawamura, Gregory F. Moore, Satoshi Hirano, Asuka Yamaguchi, Yujiro Ogawa, Ryo Anma, Shunji Yokoyama, Shunsuke Kawakami, Yildirim Dilek, Tomoyuki Sasaki
    Abstract:

    Two research cruises that deployed submersible surveys were undertaken along the Tenryu Submarine Canyon to directly observe the structural architecture of the eastern Nankai Accretionary Prism off the coast of southern Japan. The surveys have demonstrated that the accreted sediments are strongly deformed turbidite sequences that occur in repeated thrust-anticline structures. From the leading edge of the Prism near the trench toward the arc, the following deformation zones have been identified within the Prism: Frontal Thrust zone, Prism Toe zone, Imbricate Thrust zone, and Tokai Thrust zone (or out-of-sequence thrust or OOST zone). The Frontal Thrust zone is characterized by debris deposits within the hanging wall that have an age of 0–0.43 Ma, as determined from radiolarian biostratigraphy. The Prism Toe zone is characterized by unconsolidated turbidite sequences that are 1.98–3.4 Ma; these sequences are cut by normal and thrust faults. The Imbricate Thrust zone includes consolidated muddy layers and unconsolidated sandy layers that contain numerous fracture cleavages. The OOST zone consists of highly deformed consolidated sediments, ranging in age from 0.18 to 1.03 Ma. From the Prism Toe zone to the Imbricate Thrust zone, the uniaxial compcompressive strength increases gradually from 0.5–3.0 to 1.0–6.0 MPa, while the anisotropy of magnetic susceptibility changes from oblate to prolate shapes, and porosity decreases from 40%–50% to 30%–50%. These data indicate that the eastern Nankai Accretionary Prism appears to have been deformed toward the Imbricate Thrust zone just south of the OOST. Stable isotisotope analyses of calcite veins and calcite cement of the sandstone samples from the Tokai Thrust zone have shown that fluid temperatures for calcite precipitation were 24–63 °C in the OOST zone. The occurrence of highly deformed and consolidated rocks within the Nankai Accretionary Prism likely resulted from tectonic transportation of deeply buried rocks along major out-of-sequence thrust faults, such as the Tokai OOST. We infer therefore that out-of-sequence thrust faults play a major role in transporting deeply buried, deformed rocks in Accretionary Prisms to the shallower depths and even to the seafloor during ongoing subduction.

  • Effective stress and pore pressure in the Nankai Accretionary Prism off the Muroto Peninsula, southwestern Japan
    Journal of Geophysical Research, 2008
    Co-Authors: Takeshi Tsuji, Hidekazu Tokuyama, Patrizia Costa Pisani, Gregory F. Moore
    Abstract:

    [1] We developed a theoretical method for predicting effective stress and pore pressure based on rock physics model. We applied the method to reveal the pore pressure distribution within the Nankai Accretionary Prism off southwestern Japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement. From the crack aspect ratio spectrum estimated from laboratory and well-log data, we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory. By iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3D tomographic inversion, we estimated in situ mean effective stress within the Accretionary Prism. Pore pressure is then the difference between the effective stress and the confining stress. When we calculated pore pressure, we considered compressive state of stress in the Accretionary Prism. Our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement; we determined a normalized pore pressure ratio (λ*) of 0.4-0.7. Abnormal pore pressures develop in the under-thrust sequence as a result of the increase in overburden load because of the thickened overlying Prism and a low permeability barrier across the decollement. Overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward. The increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence, and the pore pressure (mean effective stress) contrast at the decollement becomes smaller landward of the deformation front.

  • legs 190 and 196 synthesis deformation and fluid flow processes in the nankai trough Accretionary Prism
    Proceedings of the Ocean Drilling Program: Scientific Results, 2005
    Co-Authors: Gregory F. Moore, Hitoshi Mikada, Casey J Moore, Keir Becker, Asahiko Taira
    Abstract:

    Ocean Drilling Program Legs 190 and 196 were a two-part program to study deformation and fluid flow in the central Nankai Trough off Shikoku Island. During Leg 190 two reference sites were drilled outboard of the trench (Sites 1173 and 1178), one site into the protothrust zone (Site 1174), two sites into a trench slope basin above a major outof-sequence thrust (Sites 1175, 1176), and one site into an older portion of the Accretionary Prism. During Leg 196, Sites 1173 and 808 (drilled through the frontal thrust during Leg 131) were revisited, employing logging while drilling and installing two Advanced Circulation Obviation Retrofit Kits (ACORKs). Our reference sites defined the stratigraphic framework and physical properties baseline of the accreting/ subducting Shikoku Basin sedimentary section. The proto-thrust and frontal thrust sites documented the dewatering and deformation processes at the toe of the Accretionary Prism. Porosity comparisons between Sites 1173 and 808 suggest that elevated fluid pressures occur beneath the decollement at Site 808. Initial measurements from the ACORK at Site 808 indicate a pressure pulse apparently from the decollement. Negative chloride anomalies at Sites 1174 and 808 could be due to fluid flow from deeper in the Prism, but active smectite dehydration could also be responsible for the anomalies. Resistivity imaging of the frontal thrust shows borehole breakouts with principal stress orientations consistent with core-scale structures and plate convergence 1Moore, G.F., Mikada, H., Moore, J.C., Becker, K., and Taira, A., 2005. Legs 190 and 196 synthesis: deformation and fluid flow processes n 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, 1–26 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Department of Geology and Geophysics, 1680 East-West Road, University of Hawaii, Honolulu HI 96822, USA. gmoore@hawaii.edu 3Department of Civil and Earth Resources Engineering, Kyoto University, Yoshida-Hommachi, Sakyo-ku, Kyoto 606-8501, Japan. 4Earth Sciences Department, University of California, Santa Cruz, Santa Cruz CA 95064, USA. 5Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149, USA. 6Center for Deep Earth Exploration, Japan Agency for Marine-Earth ScieScience and Technology (JAMSTEC), 2-15 Natushima-cho, Yokosuka 237-0061,