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Sample records for japan trench subduction

  1. Paleoseismology Along the Japan Trench Subduction Zone: Deep-Sea Sediment Records of Earthquakes in Tohoku

    NASA Astrophysics Data System (ADS)

    Kanamatsu, T.; Ikehara, K.; Strasser, M.; Usami, K.; McHugh, C. M.; Fink, H. G.; Nakamura, Y.; Kodaira, S.

    2014-12-01

    After the 2011 off the Pacific coast of Tohoku Earthquake, we have intensively explored earthquake-induced records in the deep-sea basins close to rupture zones of the 2011 and past earthquakes in Tohoku, Our study area focuses on small terminal basins near the trench for the following reason. Mass transport deposits from up-slope area down into the Japan Trench axis floor during the 2011 event were documented (e. g. Oguri et al., 2013). Similarly past earthquake-induced turbidites were expected to have been deposited in a number of small basins in the trench floor and in a lower slope terrace. Here we introduce our recent results and future prospects for paleoseismology in the Japan Trench. We collected cores from areas that are generally composed of diatomaceous fine grain sediment and fine-grained turbidites. Occasionally, wide spread tephra patches/layers are intercalated in the sediment. In the trench floor, two thick turbidite units below (older) than the 2011 event deposits were identified in up to 10m long sedimentary cores. An intercalated tephra within these turbidite units indicates that turbidites were triggered by historical earthquakes in Tohoku (Ikehara et al., in prep). On the other hand, frequent occurrence of thin-bedded turbidites was identified in the lower slope terrace. Several turbidite layers can be correlated over a wide area by tephra-correlation and matching of paleomagnetic secular variations (Usami et al., in this session). Turbidite-stratigraphy from both locations, the trench and lower slope terrace shows a similar pattern, and seems to be tied to the onland tsunami deposit stratigraphy straightforwardly. These observations suggest that deep-sea turbidite records are more reliable proxy for paleoseimic-stratigraphy of the Tohoku area. Encouraged by the results obtained so far, we plan extensive investigations in the area over the next few years to document effectively the spatiotemporal distribution of earthquake records of Tohoku.

  2. Seismic heating signatures in the Japan Trench subduction plate-boundary fault zone: evidence from a preliminary rock magnetic `geothermometer'

    NASA Astrophysics Data System (ADS)

    Yang, Tao; Dekkers, Mark J.; Zhang, Bo

    2016-04-01

    Frictional heating during earthquake rupture reveals important information on earthquake mechanisms and energy dissipation. The amount of annealing varies widely and is, as yet, poorly constrained. Here we use magnetic susceptibility versus temperature measurements during cycling to increasingly elevated temperatures to constrain the maximum temperature a slip zone has experienced. The case study comprises sheared clay cored from the Japan Trench subduction plate-boundary fault zone (décollement), which accommodated the large slip of the 2011 Mw 9.0 Tohoku-oki earthquake. The décollement was cored during the Integrated Ocean Drilling Program (IODP) Expedition 343, the Japan Trench Fast Drilling Project (JFAST). Heating signatures with estimated maximum temperatures ranging from ˜300 to over 500 °C are determined close to the multiple slip surfaces within the décollement. Since it is impossible to tie a specific slip surface to a certain earthquake, thermal evidence for the cumulative effect of several earthquakes is unveiled. This as yet preliminary rock magnetic `geothermometer' would be a useful tool to detect seismic heating along faults that experienced medium temperature rise, a range which is difficult to assess with other approaches.

  3. Structure and lithology of the Japan Trench subduction plate boundary fault

    NASA Astrophysics Data System (ADS)

    Kirkpatrick, James D.; Rowe, Christie D.; Ujiie, Kohtaro; Moore, J. Casey; Regalla, Christine; Remitti, Francesca; Toy, Virginia; Wolfson-Schwehr, Monica; Kameda, Jun; Bose, Santanu; Chester, Frederick M.

    2015-01-01

    The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary 7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary dcollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone 5-15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary dcollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the dcollement suggests that rupture to the trench may be characteristic of this margin.

  4. Heterogeneity of frontal structure of overriding plate controls co-seismic megathrust slip distribution in trench axial zone, Japan Trench and other subduction zones

    NASA Astrophysics Data System (ADS)

    Azuma, R.; Hino, R.; Ito, Y.; Mochizuki, K.; Uehira, K.; Murai, Y.; Sato, T.; Takanami, T.; Shinohara, M.; Kanazawa, T.

    2013-12-01

    The 2011 off the Pacific coast of Tohoku earthquake induced a giant tsunami by a dynamic slip with the overshoot of the frontal part of the overriding plate near the Japan Trench axis. The maximum slip during the earthquake was detected beneath the deformed zone in the trench axial region of the Miyagi-oki region. The variation in coseismic slip rate would be controlled by a crustal structural variation in the plate interface, which results in a variation of a frictional nature. We estimated the P-wave velocity (Vp) structure to investigate the structural variation spatially correlating to the coseismic slip distribution during the 2011 earthquake by performing an airgun-ocean bottom seismometer experiment on the along-trench profile on the deformed zone in the Japan Trench axial region. We detected that the high Vp body in the hanging wall of the plate interface corresponding to the Cretaceous layer, which is more rigid than the sediment in the deformed zone, sticks out towards the trench axis in the Miyagi-oki segment. In the Miyagi-oki segment, the distance from the trench axis to the forefront of the rigid Cretaceous layer is shorter by ~40 km than in the Sanriku-oki. This means that the width of the less rigid sedimentary prism layer is smaller in the Miyagi-oki than that in the Sanriku-oki. We suggest that this along-arc variation of the hanging wall side structure would cause the difference in propagation of the dynamic slip toward the trench axis between the Miyagi-oki and the Sanriku-oki; the slip reached the trench axis in the Miyagi-oki but not in the Sanriku-oki during the 2011 earthquake. In addition, we found that the similar relationship between the hanging wall structure and the slip distribution of megathrust earthquake observed in Tohoku can recognized in the rupture zones of several major megathrust events, the 1952 Kamchatka, the 1964 Alaska, 1960 and 2010 Chile earthquakes. For example, the 1960 Chile earthquake exhibits the largest coseismic slip at the segment where the sticks out and sedimentary prism is the smallest. We suggest that surveying distribution of a backstop interface would supply important information to assess the passible location of peak slip during any future megathrust events.

  5. A Study of Shear-Wave Velocity Structure Over the Seismic-Aseismic Boundary Along the Japan-Trench Subduction Zone -Preliminary Results-

    NASA Astrophysics Data System (ADS)

    Mochizuki, K.; Kasahara, J.; Nakamura, M.; Hino, R.; Nishino, M.; Yamada, T.; Kuwano, A.; Kuno, T.; Sato, T.; Nakamura, Y.; Kanazawa, T.

    2002-12-01

    The Pacific plate subducts beneath the northeast Japanese main island along the Japan Trench, where a number of large earthquakes of the low-dip-angle thrust type occurred in the history. Very detailed earthquake activities along the Japan Trench have been revealed by repeated seafloor seismic observations using ocean bottom seismometers (OBSs) as well as decades-long land observations. It has become clear that the seismic activities along the trench are not uniformly distributed, but that they are heterogeneous among places. Recent studies (e.g., Fujie et al., 2002) show strong correlations between the seismic reflectivity and aseismicity. This fact implies that the strength of seismic coupling between the subducting and overriding plates can be estimated by the intensity of seismic reflection. This result leads to a discussion that the high seismic reflectivity is a manifestation of high fluid content which lowers the strength of the seismic coupling between the two plates. Understanding the shear-wave velocity structure in addition to the compressional-wave velocity structure would greatly help put better constraints on the physical properties, especially on fluid content, along the plate boundaries. We carried out a seismic experiment in 2001 in a region between 38o40'N and 39o00'N on the forearc slope of the Japan Trench. Thirty-nine OBSs were deployed along seven seismic profiles within a 30 km x 50 km region. Airguns were used as artificial seismic sources, and their average total chamber volume was 57 liters. Absolute orientations of OBS horizontal-component seismometers were determined using three component waveforms of the direct water-wave arrivals of airgun shootings (Yoneshima, 2001). Substantial P-to-S conversion at the base of the sediment layer was observed by the most of OBSs. Average apparent velocities of S-wave propagating through the layers below the sediment for each OBS range from 3 km/s to 3.4 km/s, while their corresponding P-wave apparent velocities are from 5 km/s to 6.1 km. Accordingly, the Vp/Vs ratios range from 1.6 to 1.9. The following issues will be investigated. Detailed shear-wave velocity structure including structure around the plate boundary will be obtained by incorporating ray-tracing and/or synthetic wave forms for discussing precise Vp/Vs ratio and shear-wave amplitude variations. Shear-wave amplitudes of the horizontal transverse component are comparable to those of the radial component on some OBSs. This cannot be explained by simple P-to-S conversion at one of the 2-D interfaces within the structure. A study including three-dimensionality will be required.

  6. Constraints on interseismic deformation at Japan trench from VLBI data

    NASA Technical Reports Server (NTRS)

    Argus, Donald F.; Lyzenga, Gregory A.

    1993-01-01

    Space geodetic data from very long baseline interferometry (VLBI) was used to estimate velocity relative to the plate interiors of two sites on the deforming leading edge at the Japan trench. Elastic models of interseismic deformation and results obtained were used to put constraints on the slip rate along the main thrust of the Japan subduction zone. Observed velocities reflect the sum of permanent west-northwest shortening in Honshu, elastic deformation due to locking of the main thrust fault at the Japan trench, and deformation associated with the subducting Phillipine plate. These velocities limit the locked segment of the main thrust at the Japan trench to 27 km vertically and 100 km along the dip. This indicates that the main Pacific plate thrust fault is not strongly coupled and probably does not generate strong earthquakes.

  7. High-resolution seismic imaging in the Japan Trench axis area off Miyagi, northeastern Japan

    NASA Astrophysics Data System (ADS)

    Nakamura, Yasuyuki; Kodaira, Shuichi; Miura, Seiichi; Regalla, Christine; Takahashi, Narumi

    2013-05-01

    obtained high-resolution seismic data reveal the detailed structure of the Japan Trench axis off Miyagi, Japan, in the region of large shallow slip during the 11 March 2011 M9 Tohoku earthquake. Correlation of seismic images with previous drilling results identifies a possible basal chert-rich layer and hemipelagic/pelagic mudstones overlying igneous Pacific crust. Mapping of acoustic basement depicts the subduction of horst-and-graben topography. The basement and basal chert are subducted beneath a seismically chaotic frontal prism, but the majority of overlying hemipelagic mudstones is offscraped and imbricated at the trench axis as a result of plate boundary compression. A possible dcollement is imaged as a seaward dipping reflection at landward part of the trench graben and was likely generated by loading and failure of underthrust sediments. Collectively, these analyses provide a structural framework for understanding sedimentary inputs and the localization of shallow plate boundary slip at the Japan Trench.

  8. Plume head - trench interaction: impact on subduction dynamics

    NASA Astrophysics Data System (ADS)

    Betts, P. G.; Moresi, L. N.; Mason, W. G.; Willis, D.

    2013-12-01

    The geologic record provides numerous examples where plumes and their associated buoyancy swell have disrupted convergent plate margins. These interactions have produced a variety of responses in the overriding plate including transient episodes of arc amagmatism, transient episodes of crustal shortening followed by plume-related magmatism in the overriding plate. The latter observation implies the plume must have transitioned from the subducting plate to the overriding plate. We present several 3D Underworld numerical models of plume heads of variable dimension and buoyancy interacting with a subduction trench. The models indicate that plume heads impact enormously on trench geometry. Arcuate trenches are created as the trench retreats around the edges of the plume head, whereas trench advance occurs in front of the plume resulting in transient crustal shortening in the overriding plate. Stalling of subduction when the plume head impacts the trench causes slab windowing. The size of the slab window is dependent on the size and buoyancy of the plume. The creation of the slab window provides a potential conduit for plume migration to the overriding plate. Alternatively, the plume head may be transferred to the overriding plate as subduction is re-established behind the plume. Models with "strong" slabs, characterized by high yield strengths, display different behavior. Plume-heads are entrained in the slab and are subducted without the development of a slab window.

  9. Oceanic mantle alteration in the trench-outer rise region of the Japan trench

    NASA Astrophysics Data System (ADS)

    Fujie, G.; Takahashi, T.; Kodaira, S.; Obana, K.; Yamada, T.

    2014-12-01

    Dehydration processes and the expulsion of the water from the subducting oceanic plate affect various subduction-zone processes, including arc volcanism and generation of earthquakes and tremor. Since the amount of chemically bound water in the oceanic plate is highest in serpentinized mantle, it is important to reveal the degree of oceanic mantle serpentinization prior to subduction.In 2009 and 2013, to reveal the structural evolution of the incoming oceanic plate prior to subduction, we conducted wide-angle seismic structural surveys in the trench-outer rise region of the Japan trench. We found P-wave velocity (Vp) within the oceanic crust and mantle gradually decreases toward the trench axis, especially beneath the well-developed horst and grabens. Generally, reduction of Vp suggests two possibilities; one is a fracturing (dry mechanism) and the other is fracturing and water penetration (wet mechanism). The Vp/Vs ratio is a key to distinguish these two possibilities. Using P-to-S converted phases, we successfully modeled Vs within the oceanic crust by the travel-time inversion, and we found the Vp/Vs ratio within the oceanic crust gradually increases toward the trench, suggesting the water infiltration into the oceanic crust. However, it is not straightforward to determine Vs within the oceanic mantle because signal-to-noise ratio of S-wave mantle refraction was too poor to pick arrival times. Therefore, we calculated S-wave mantle refractions by assuming wide variety of Vp/Vs ratio within the oceanic mantle and compared with the observed horizontal data. Although S-wave mantle refraction was observed at only limited number of OBSs, we found that Vp/Vs ratio within the oceanic mantle did not change even in the area where mantle Vp become lower. This implies that the mantle Vp reduction in our seismic profile is mainly governed by the "dry mechanism" or mantle hydration might be confined in only topmost mantle.

  10. Oblique convergence and deformation along the Kuril and Japan trenches

    NASA Technical Reports Server (NTRS)

    Demets, Charles

    1992-01-01

    The hypothesis that present-day deformation within the southern Kuril forearc is driven by oblique subduction of the Pacific plate is tested using 397 horizontal slip directions derived from shallow-thrust earthquakes from the Kuril and Japan trenches for the period 1963-1991. A simple two-plate model fits the 397 slip vectors significantly worse than a model that permits strike-slip motion of the southern Kuril forearc relative to the overlying plate. Weighted, mean slip directions along the southern Kuril trench are systematically rotated toward the direction orthogonal to the trench, which implies that the net convergence is partitioned into less oblique subduction and trench-parallel displacement of the southern Kuril forearc. The angular discrepancy between the observed slip direction and the direction predicted by the NUVEL-1 Pacific-North America Euler vector implies that the southern Kuril forearc translates 6-11 mm/yr to the southwest relative to the overlying North American plate. These results are consistent with geologically, geodetically, and seismologically observed convergence at the leading edge of the forearc sliver in southern Kokkaido and with previously inferred extension at the trailing edge of the sliver, which is located at the Bussol Strait at 46 deg N.

  11. Rapid weakening of subducting plates from trench-parallel estimates of flexural rigidity

    NASA Astrophysics Data System (ADS)

    Arredondo, Katrina M.; Billen, Magali I.

    2012-04-01

    The negative buoyancy force of sinking lithosphere (slabs) is the principle driving force for subducting plates, but transmission of this force to the subducting plate depends on the strength of the slab (e.g., Conrad and Hager, 2001). Slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies (i.e., perfectly elastic, perfectly viscous, perfectly plastic, brittle-ductile layered). Because the applicability of these rheologic models cannot be distinguished based on trench-perpendicular plate bending models (Forsyth, 1980), a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100 km of the Kermadec Trench (Billen and Gurnis, 2005). Using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the Tonga and Japan-Izu-Bonin subduction zones. The observed plate weakening provides further evidence for a plate rheology that leads to significant yielding (loss of elastic strength and reduction in effective viscosity) within the bending region of the subducting plate. This rapid weakening within the shallow, low curvature, region of the plate may significantly decrease estimates of energy dissipation related to plate bending, compared to recent calculations assuming high plate strength and constant plate curvature.

  12. Trench curvature and deformation of the subducting lithosphere

    NASA Astrophysics Data System (ADS)

    Schettino, Antonio; Tassi, Luca

    2012-01-01

    The subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation. The downdip component of strain is associated with external forces that are applied to the slab during its sinking, namely the gravitational force and the mantle resistance to penetration. Here, we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone, independently from the long-term physical processes that have determined the actual curvature of the subduction zone. In particular, we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters, such as trench curvature, dip function and subduction velocity. We also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths, and may include large extensional lateral deformation at intermediate depths, whereas a state of lateral mechanical equilibrium can only represent a localized exception. Our formulation overcomes the flaws of the classic 'ping-pong ball' model for the bending of the lithosphere at subduction zones, which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs. A study of the geometry and seismicity of eight modern subduction zones is performed, to assess the validity of the theoretical relationship between trench curvature, slab dip function, and lateral strain rate. The strain pattern within the eight present-day slabs, which is reconstructed through an analysis of Harvard CMT solutions, shows that tectonic plates cannot be considered as flexible-inextensible spherical caps, whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium.

  13. Ocean bottom seismic and tsunami network along the Japan Trench

    NASA Astrophysics Data System (ADS)

    Uehira, K.; Kanazawa, T.; Noguchi, S.; Aoi, S.; Kunugi, T.; Matsumoto, T.; Okada, Y.; Sekiguchi, S.; Shiomi, K.; Shinohara, M.; Yamada, T.

    2012-12-01

    Huge tsunami, which was generated by the 2011 off the Pacific Coast of Tohoku Earthquake of M9 subduction zone earthquake, attacked the coastal areas in the north-eastern Japan and gave severe casualties (about 20,000 people) and property damages in the areas. The present tsunami warning system, based on land seismic observation data, did not work effectively in the case of the M9 earthquake. For example, real tsunami height was higher than that of forecast by this system. It is strongly acknowledged that marine observation data is necessary to make tsunami height estimation more accurately. Therefore, new ocean bottom observation project has started in 2011 that advances the countermeasures against earthquake and tsunami disaster related to subduction zone earthquake and outer rise earthquake around Japan Trench and Kuril Trench. A large scale ocean bottom cabled observation network is scheduled to be deployed around Japan Trench and Kuril Trench by 2015. The network is consisted of 154 ocean bottom observation stations. Ocean bottom fiber optic cables, about 5100 km in total length, connect the stations to land. Observation stations with tsunami meters and seismometers will be placed on the seafloor off Hokkaido, off Tohoku and off Kanto, in a spacing of about 30 km almost in the direction of East-West (perpendicular to the trench axis) and in a spacing of about 50 - 60 km almost in the direction of North-South (parallel to the trench axis). Two or more sets of tsunami meters and seismometers will be installed in one station for redundancy. Two sets of three component servo accelerometers, a set of three component quartz type accelerometers (frequency outputs), a set of three component velocity seismometers will be installed, and two sets of quartz type depth sensors (frequency outputs) will be installed as tsunami meters. Tsunami data and seismometer data will be digitized at sampling frequency of 10 Hz and 100 Hz, respectively, and will be added clock information at land stations. These digitized data will be transmitted to the data centers, JMA (Japan Meteorological Agency), and so on, using IP network.

  14. Flexural bending of the oceanic plates near the Mariana, Japan, and Philippines trenches

    NASA Astrophysics Data System (ADS)

    Tang, M.; Lin, J.; Zhang, F.

    2013-12-01

    We conducted a detailed analysis of flexural bending of oceanic plates near the Mariana, Japan, and Philippines trenches to better understand the similarities and differences among these major subduction systems in the western Pacific Ocean. For each of the systems, we first obtained a 3-D deformation surface of the subducting plate by removing from the seafloor bathymetry the estimated topographic effects of sediment thickness, seamounts, and age-related thermal subsidence. We then calculated theoretical models of plate deformation along a series of trench-perpendicular profiles and inverted for the vertical force (Vo) and bending moment (Mo) at the trench axis, as well as variations in the elastic plate thickness (Te) that can best explain the observed plate deformation. From analysis of profiles across all trenches, we found that Te is reduced significantly from a value seaward of the outer rise (TeMax) to a value near the trench (TeMin), with the transition at distance Xr from the trench axis. Results of analysis reveal that the Mariana trench has the greatest amplitude of flexural bending (i.e., the greatest trench depth) in the range of 1.39 - 5.67 km and an average of 2.91 km, comparing to the Japan trench (range of 1.0 - 4.08 km, average of 2.59 km) and the Philippines trench (range of 0.48 - 4.04 km, average of 2.41 km). In contrast, the Philippines trench has the relatively narrow trench width (Xr range of 36 - 107 km, average of 68 km), in comparison to the Japan trench (Xr range of 47 - 122 km, average of 83 km) and the Mariana trench (Xr range of 60 - 125 km, average of 92 km). The best-fitting models reveal that for the Mariana trench, the effective elastic thickness is reduced significantly from a value seaward of the outer rise (TeMax = 45 - 55 km) to a value trench-ward of the outer rise region (TeMin = 19 - 40 km), with a corresponding reduction in Te in the range of 20 - 60%. In comparison, for the Japan trench, TeMax = 35 - 55 km, TeMin = 14 - 43 km, with Te reduction ranging 20 - 60%; for the Philippines trench, TeMax = 36 - 52 km, TeMin = 14 - 35 km, with Te reduction ranging 23 - 60%. Together these results illustrate that the elastic strength of the oceanic plates is significantly reduced near trenches, most likely due to pervasive trench-parallel normal faulting caused by flexural bending.

  15. Dehydration of incoming sediments at the Japan Trench

    NASA Astrophysics Data System (ADS)

    Shimizu, M.; Kameda, J.; Hamada, Y.; Tanikawa, W.; Kimura, G.

    2014-12-01

    In the 2011 Tohoku-oki earthquake, the seismic fault slip propagated to the trench axis and caused an extremely large tsunami (Ide et al., 2011). Ductile deformation of unconsolidated sediments is commonly prominent in the aseismic shallow parts of the subduction zone. It is unknown how the seismic rupture reached the nearby trench axis. Based on the result of Deep Sea Drilling Project (DSDP) Leg 56 at site 436 (reference, 1977), it is expected that the subducting sediments at the Japan Trench mainly consist of vitric diatomaceous and radiolarian ooze with pelagic clay intervals. Opal and smectite in the pelagic sediments transform respectively into quartz and illite. Kinetic modeling demonstrated that these reactions will progress with active dehydration at 50-60 km horizontally away from the trench axis and with a temperature of 100-120C. This region coincides with the plate-boundary marked by a prominent seismic reflector. It suggests that the main source of highly pressured fluids is the dehydration of pelagic sediments (Kimura et al., 2012). However, detailed dehydration processes are still unclear mainly due to lack of quantitative sediment composition data. Therefore, in this study, we examined whole rock composition including amorphous silica of the core samples recovered at site 436 as well as those from the Japan Trench by the IODP 343 Japan Trench Fast Drilling Project (JFAST). Analysis of amorphous silica at the drilling site of J-FAST documents that dehydration of the sediments is able to contribute to excess pressure at the shallow part of the megathrust if they underthrust as the same composition. At the drilling site of JFAST, a plate-boundary shear zone was identified around 820 mbsf (Chester et al., 2012). Our analysis showed that the shear zone is characterized by extremely high concentration of smectite (~70 wt%).These results suggest that the abundant smectite may have possibly fostered localized rupture and slip during the earthquake, because smectite has low frictional coefficient. In our presentation, we will also show the results of the permeability and porosity measurements for the core samples recovered at site 436.

  16. Rapid Weakening of Subducting Plates From Trench-Parallel Estimates of Flexural Rigidity

    NASA Astrophysics Data System (ADS)

    Arredondo, K.; Billen, M. I.

    2011-12-01

    The percentage of slab pull force transmitted from the slab to the subducting plate depends on the slab strength (e.g., Conrad and Hager, 2001). Slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies (i.e., perfectly elastic, perfectly viscous, perfectly plastic, layered brittle-ductile layered), but applicability of these rheologic models cannot be distinguished based on trench-perpendicular plate bending models alone (Forsyth, 1980). Consequently, a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100 km of the Kermadec trench (Billen and Gurnis, 2005). Using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the Tonga and Japan-Izu-Bonin subduction zones within 100 km of the trench. The observed plate weakening provides further evidence for a plate rheology that leads to significant lithospheric-scale yielding (loss of elastic strength and reduction in effective viscosity) within the bending region of the subducting plate. This rapid weakening within the shallow, low curvature, region of the plate may significantly decrease energy dissipation related to plate bending compared to past calculations that assume constant strength, plate thickness and plate curvature. While a decrease in bending energy dissipation would provide more energy for slab pull, lithospheric plate weakening may decrease transmission to the subducting plate. Additionally, the high degree of lithospheric weakening suggests that plate age has a weaker influence on slab strength and energy dissipation then previously believed, as very old oceanic lithosphere weakens to effective elastic thickness predicted for relatively young plates. Billen, M. I., Gurnis, H.A., 2005. Constraints on subducting plate strength within the Kermadec trench. J. Geophys. Res. 110, B05407, doi:10.1029/2004JB003308. Conrad, C.P., Hager, B.H., 2001. Mantle convection with strong subduction zones. Geophys. J. Int. 144 (2), 271-288. Forsyth, D.W., 1980. Comparison of mechanical models of oceanic lithosphere. J. Geophys. Res. 85, 6364-6368.

  17. Trench migration and overriding plate stress in dynamic subduction models

    NASA Astrophysics Data System (ADS)

    Holt, A. F.; Becker, T. W.; Buffett, B. A.

    2015-04-01

    On Earth, oceanic plates subduct beneath a variety of overriding plate (OP) styles, from relatively thin and negatively buoyant oceanic OPs to thick and neutrally/positively buoyant continental OPs. The inclusion of an OP in numerical models of self-consistent subduction has been shown to reduce the rate that subducting slabs roll back relative to the equivalent single plate models. We use dynamic, 2-D subduction models to investigate how the mechanical properties, namely viscosity, thickness, and density, of the OP modify the slab rollback rate and the state of stress of the OP. In addition, we examine the role of the subducting plate (SP) viscosity. Because OP deformation accommodates the difference between the slab rollback rate and the far-field OP velocity, we find that the temporal variations in the rollback rate results in temporal variations in OP stress. The slabs in our models roll back rapidly until they reach the lower mantle viscosity increase, at which point the rollback velocity decreases. Concurrent with this reduction in rollback rate is a switch from an OP dominated by extensional stresses to a compressional OP. As in single plate models, the viscosity of the SP exerts a strong control on subducting slab kinematics; weaker slabs exhibit elevated sinking velocities and rollback rates. The SP viscosity also exerts a strong control on the OP stress regime. Weak slabs, either due to reduced bulk viscosity or stress-dependent weakening, have compressional OPs, while strong slabs have dominantly extensional OPs. While varying the viscosity of the OP alone does not substantially affect the OP stress state, we find that the OP thickness and buoyancy plays a substantial role in dictating the rate of slab rollback and OP stress state. Models with thick and/or negatively buoyant OPs have reduced rollback rates, and increased slab dip angles, relative to slabs with thin and/or positively buoyant OPs. Such elevated trench rollback for models with positively buoyant OPs induces extensional stresses in the OP, while OPs that are strongly negatively buoyant are under compression. While rollback is driven by the negative buoyancy of the subducting slab in such models of free subduction, we conclude that the physical properties of the OP potentially play a significant role in modulating both rollback rates and OP deformation style on Earth.

  18. Trench Advance By the Subduction of Buoyant Features - Application to the Izu-Bonin-Marianas Arc

    NASA Astrophysics Data System (ADS)

    Goes, S. D. B.; Fourel, L.; Morra, G.

    2014-12-01

    Most subduction trenches retreat, not only today but throughout the Cenozoic. However, a few trenches clearly advance during part of the evolution, including Izu-Bonin Marianas (IBM) and Kermadec. Trench retreat is well understood as a basic consequence of slab pull, but it is debated what causes trench advance. The IBM trench underwent a complex evolution: right after its initiation, it rotated clockwise, leading to very fast retreat in the north and slow retreat in the south. But since 10-15 Ma, IBM trench motions have switched to advance at the southern end, and since 5 Ma also the northern end is advancing. Based on 2-D subduction models, it has been proposed proposed that the change in age of the subducting plate at the IBM trench (from 40-70 m.y. at the initiation of the trench 45 m.y. ago to 100-140 m.y. lithosphere subducting at the trench today) and its effect on plate strength could explain the transition from trench retreat to trench advance, and that the age gradient (younger in the north and older in the south) could explain the rotation of the trench. However, with new 3-D coupled fluid-solid subduction model where we can include such lateral age gradients, we find that this does not yield the observed behaviour. Instead, we propose an alternative mechanism, involving the subduction of the buoyant Caroline Island Ridge at the southern edge of the Mariana trench and show that it can explain both trench motion history and the current morphology of the IBM slab as imaged by seismic tomography.

  19. Outer slope faulting associated with the western Kuril and Japan trenches

    NASA Astrophysics Data System (ADS)

    Kobayashi, Kazuo; Nakanishi, Masao; Tamaki, Kensaku; Ogawa, Yujiro

    1998-08-01

    Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of 3900'N, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of 3900'N. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (<10 km) roughness of plateaus and valleys edged by outward- and inward-facing fault escarpments. Throughout the surveyed areas, escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall, but with no distinct increase below that depth. No significant difference is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, which is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the East Pacific Rise crest. The maximum length and height of escarpments are small in the Kuril Trench compared with those in the Japan Trench, implying a difference in mechanical strength depending on the fault orientation. The crust is weakest along the inherited spreading fabric, second weakest probably along the non-transform offset direction and strongest in directions very oblique to these orientations. Seamounts appear to be more rigid than normal ocean crust, with no particular weak orientations, resulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi-Kashima Seamount.

  20. The Relationships of Upper Plate Ridge-Trench-Trench and Ridge-Trench-Transform Triple Junction Evolution to Arc Lengthening, Subduction Zone initiation and Ophiolitic Forearc Obduction

    NASA Astrophysics Data System (ADS)

    Casey, J.; Dewey, J. F.

    2013-12-01

    The principal enigma of large obducted ophiolite slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which are suggestive of a forearc origin. PT conditions under which boninites and metamorphic soles form and observations of modern forearc systems lead us to the conclusion that ophiolite formation is associated with overriding plate spreading centers that intersect the trench to form ridge-trench-trench of ridge-trench-tranform triple junctions. The spreading centers extend and lengthen the forearc parallel to the trench and by definition are in supra-subduction zone (SSZ) settings. Many ophiolites likewise have complexly-deformed associated mafic-ultramafic assemblages that suggest fracture zone/transform along their frontal edges, which in turn has led to models involving the nucleation of subduction zones on fracture zones or transpressional transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that trench-parallel split arcs). Syn-arc boninites and forearc oceanic spreading centers that involve a stable ridge/trench/trench triple or a ridge-trench-transform triple junction, the ridge being between the two upper plates, are consistent with large slab ophiolite formation in an obduction-ready settting. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite forearc complex is generated by this mechanism. The ophiolite ages along arc-strike; a distinctive diachronous MORB-like to boninitic to arc volcanic stratigraphy develops vertically in the forearc and eruption centers progressively migrate from the forearc back to the main arc massif with time. Dikes in the ophiolite are commonly highly oblique to the trench (as are back-arc magnetic anomalies in modern environments). Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the instantaneously developed subducting and dehydrating slab. We review both modern subduction environments and ancient obducted ophiolite analogues that illustrate this tectonic model for subduction initiation and the creation and rapid divergent-convergent plate tectonic transitions to ophiolitic forearcs.

  1. Seismic Tomography for the Crust and Upper Mantle behind the Japan Trench

    NASA Astrophysics Data System (ADS)

    Wang, Z.

    2014-12-01

    The Pacific plate subducts WNW under the Eurasian plates with a ~30 angle of dip and a rate of ~8 cm/yr from the Japan-Kuril Trench. The Kuril-NE Japan arc of the uppermost mantle, overlying the subducting Pacific slab, is the locus of important processes, including serpentinization of the forearc mantle wedge, repeated genesis of megathrust earthquakes, slab dehydration, arc magmatism and interplate coupling. To improve our knowledge of crustal and upper mantle structures through tomographic imaging, we determined the three-dimensional (3-D) velocity (Vp, Vs) and Vp/Vs structures under the Kuril-NE Japan subduction zone. The Vp, Vs and Vp/Vs models provide compelling evidence for a highly hydrated and serpentinized forearc mantle and the fluids related to low-velocity and high-Vp/Vs anomalies associated with the slab dehydration. Significant slow anomalous Vp and Vs with a high-Vp/Vs ratio are clearly imaged along the volcanic front with an extended depth of ~100 km under the Kuril-NE Japan arc, showing good consistency with the results of previous studies. This is caused mainly by the fluids associated with the extensive dehydration of the subducting Pacific slab. Fluid-related anomalies under the Kuril-NE Japan arc system, attributed to various processes such as slab dehydration and serpentinization of the forearc mantle wedge, are contributed mainly by arc magmatism, interplate coupling and the repeated generation of megathrust earthquakes. The characteristic distribution of high and low Vp/Vs in the forearc continental crust along the trench-parallel direction may reflects the spatial heterogeneity of the amount of the subducted water which related to the difference of the sedimentary unit and seismic activity in the oceanic crust. Our study demonstrates that the directly optimization of Vp/Vs tomographic procedure provides more stable and reliable Vp/Vs image than previous method.

  2. Mechanism for normal faulting in the subducting plate at the Mariana Trench

    NASA Astrophysics Data System (ADS)

    Zhou, Zhiyuan; Lin, Jian; Behn, Mark D.; Olive, Jean-Arthur

    2015-06-01

    We investigate the mechanisms of normal fault initiation and evolution in the subducting Pacific Plate near the Mariana Trench, through bathymetry analysis and geodynamic modeling. We model the subducting plate as an elastoplastic slab subjected to tectonic forcing at the trench, including vertical load, bending moment, and horizontal tensional force. In our simulations, normal faults initiate within the outer rise region and reach maximum throw toward the trench. This result holds over a wide range of tectonic forcing and is consistent with observations of the Challenger Deep region, where multibeam bathymetry data indicate faults initiate near the outer rise at 70-110 km from the trench and reach maximum throw at 10-35 km from the trench. However, models require a horizontal tensional force with magnitude comparable to axial vertical load to jointly explain the observed seafloor bathymetry, location of maximum normal fault throw, and prevalence of normal faults dipping toward the trench.

  3. Mechanism for Normal Faulting in the Subducting Plate at the Mariana Trench

    NASA Astrophysics Data System (ADS)

    Zhou, Z.; Lin, J.; Behn, M. D.; Olive, J. A. L.

    2014-12-01

    We investigate the characteristics of normal faulting between the trench and outer rise in the subducting Pacific plate through analysis of high-resolution multi-beam bathymetry and geophysical data and geodynamic modeling. Analysis of multi-beam bathymetry data reveals significant variations in normal faulting characteristics along the Mariana trench: (1) The vast majority of the observed surface normal faulting scarps are observed to be sub-parallel to the local strike of the Mariana trench axis, indicating that the orientation of normal faults is predominantly controlled by subduction-related stresses rather than by pre-existing abyssal hill fabrics. (2) Trench-parallel normal fault scarps become apparant as the subducting plate approaches the outer rise of the Mariana trench, indicating that normal faulting initiates in this region. (3) Along the Mariana trench, the Challenger Deep region is associated with the greatest trench depth and largest average values of normal fault throw, while regions with seamounts near the trench axis show the smallest average values of fault throw. To explore the mechanisms that control normal faulting in a subducting plate, we perform numerical simulations of elasto-plastic plate subjected to tectonic loading, bending, and horizontal forces from slab pull. Modeling results suggest that bending-induced extensional stresses in the upper plate reaches maximum values near the outer rise, consistent with the onset of normal faulting in this region. However, bending alone does not predict the continued growth of normal faults toward the trench. We hypothesize that this additional fault growth could be related to (1) tectonic stresses induced by steep topographic slopes; and/or (2) slab pulling forces that are originated in the upper mantle due to the negative buoyancy of a subducted slab but are transmitted to the shallower part of the lithospheric plate prior to its subduction.

  4. Along-trench variations in the seismic structure of the incoming Pacific plate at the outer rise of the northern Japan Trench

    NASA Astrophysics Data System (ADS)

    Fujie, Gou; Kodaira, Shuichi; Sato, Takeshi; Takahashi, Tsutomu

    2016-01-01

    To investigate along-trench variations in the seismic structure of the incoming oceanic plate and their effect on water transportation by the oceanic plate, we conducted a wide-angle seismic survey of a trench-parallel transect 270 km long on the outer rise of the northern Japan Trench. The resulting seismic structure models show that the central part of the transect is characterized by rough topography, thick oceanic crust, low seismic velocities, and high Vp/Vs ratios, suggesting pervasive fracturing and high water content (hydration) there. These observations are consistent with the presence of an ancient fracture zone associated with ridge propagation. The trenchward extension of this fracture zone corresponds to an area of low interplate seismicity, low seismic velocities, and high Vp/Vs ratio around the depth of the subduction interface. Our results suggest that this ancient scar on the oceanic plate influences along-trench variations in interplate seismic coupling through its effect on water transportation.

  5. Low Intensity Characteristic of Plate-Boundary S-S Reflections Within a Region of Strong Plate-Boundary P-P Reflections and low Seismicity Along the Japan Trench Subduction Zone

    NASA Astrophysics Data System (ADS)

    Mochizuki, K.; Kasahara, J.; Hino, R.; Nishino, M.; Yamada, T.; Shinohara, M.; Kanazawa, T.

    2003-12-01

    It has been pointed out that the epicenters of the microearthquakes along the forearc slope of the Japan Trench are not uniformly distributed but clustered in seismically active zones that are oriented perpendicular to the trench axis. One of the clear seismic-aseismic boundaries of such seismic clusters can be identified in latitude 39 N. A seismic survey was conducted in 1996 with one profile running across the boundary and parallel to the trench axis, and a P-wave velocity structure model was obtained by travel-time inversion (Fujie, 1999). A strong anti-correlation between the seismicity and the intensity of the plate-boundary P-P reflected waves was found: strong plate-boundary P-P reflected waves were observed in a region where seismicity is quite low, and vice versa (Fujie et al., 2002). They discussed that a thin layer of low P-wave velocities (3~4 km/s) with its thickness up to a few hundred meters along the plate boundary could explain the intensity of the reflections. Results of finite-difference waveform calculations support this estimation (Moghaddam, 2002). Another seismic survey was carried out in 2001 with 7 trench-parallel profiles in the same region as the 1996 survey in order to map and verify the strong anti-correlation. The strong anti-correlation was observed over the seismic-aseismic boundary region, and it was inferred that a thin layer with low P-wave velocities along the plate boundary exists beneath the aseismic zone in the region. Understanding the characteristics of plate-boundary S-S reflections in addition to those of P-P reflections would greatly help put better constraints on the physical properties along the plate boundary. Substantial P-to-S conversion at the base of the sedimentary layers was observed. S-wave velocities, especially those of the sedimentary layers, should be precisely determined in order to have good estimates of the arrival times of the plate-boundary S-S reflected waves. The S-wave velocities of the sedimentary layers were obtained by the tau-p analysis of the OBS horizontal-component waveforms. The Vp/Vs ratios within the sediment layers were estimated to be 5.2 and 2.3 from top to bottom. The Vp/Vs ratios within the deeper structure were assumed to be 1.8 to 1.74 as the Vp varies from 4.5 km/s to 8.0 km/s. The calculated travel times explain well the observed travel times of P-S-converted refraction arrivals. Expected arrival times of the plate-boundary S-S reflected waves were calculated with respect to the obtained Vs structure. Such intense amplitude as observed for the plate-boundary P-P reflections was not identified for the S-S reflections. Irregularity of the interfaces within the structure or differences in the reflectivity coefficients between P-P and S-S reflections at the plate boundary does not explain the opposite appearances of those reflections. Although the strong plate-boundary P-P reflections could be explained by putting a thin low-velocity layer, some additional features for S-S reflections along the plate boundary may be required, such as very low Q values (high attenuation) for the S-wave propagation.

  6. Heat-flow determination in three DSDP boreholes near the Japan trench

    SciTech Connect

    Burch, T.K.; Langseth, M.G.

    1981-10-10

    The first deep borehole determinations of temperature gradients and heat flow of the landward wall of the Japan Trench and forearc were made on IPOD DSDP leg 57. These heat flow values are based on temperature logs corrected to equilibrium, using a detailed model of the drilling disturbance. Heat flow values on a deeply submerged terrace, landward of the trench slope break are 28 and 32 mW m/sup -2/. A measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mW m/sup -2/. The results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench. Our observations indicate only a small increase of heat flow from the trench to the volcanic arc, in agreement with thermal models, which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench. The temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters. Hundreds of conductivity determinations were made using a new technique.

  7. Geophysical evidence of trench-breaching slip along megathrust plate interface in the Japan Trench

    NASA Astrophysics Data System (ADS)

    Kodaira, Shuichi; Nakamura, Yasuyuki; Miura, Seiichi; Fujiwara, Toshiya; Kanamatsu, Toshiya; Ikehara, Ken

    2014-05-01

    Repeated bathymetry and seismic surveys along a profile in the central part of the rupture of the 2011 Tohoku-oki earthquake show that a co-seismic fault reaches the trench axis, forming a deformed sediment mass seaward of the frontal prism above a graben, probably due to large trench-ward movement of the hanging wall block. If the seismic structures we observed in the trench axis represent a structural proxy showing trench-breaching slip, it can be possible by using seismic data, to map an area where co-seismic slip reaches the trench axis. In order to test this hypothesis, we have started a high-resolution seismic imaging project along the entire Japan Trench axis, and the survey has been completed from 38 N to 40 N by the summer of 2013. Based on preliminary results from the survey, we found along the trench axis continuation of key structures which consist of a small-scale fold-and-thrust zone at the trench axis and seismically transparent zone at the landward, except 39.5 N to 40 N where extremely thin incoming sediments are observed due to rough geometry of the top of the igneous crust. Those structures are interpreted to be formed by overprinting "basal friction-driven thrust fault" and "gravity-driven normal fault" that alternatively occurred during an earthquake cycle with slip to the trench. Although we believe that the high-resolution seismic data have a potential to define the spatial distribution of slips to the trench, those data do not yield any information about temporal variations of the slip. In order to examine the temporal variation of slip to the trench, we will therefore integrate the seismic images with geological studies, such as piston-coring. Furthermore, in order to know even longer records of earthquake slips and evidences of seismic fault motions (i.e., high velocity slip) along megathrust interface at the trench axis, we proposed a new ocean drilling project, called JTRACK, which consists of along-and-across trench axis drilling transect in the Japan Trench.

  8. Relation between subduction megathrust earthquakes, sediment thickness at trench, and plate coupling

    NASA Astrophysics Data System (ADS)

    Heuret, A.; Conrad, C. P.; Funiciello, F.; Lallemand, S.

    2011-12-01

    Extreme seismic events (Mw 8.5 and higher) are uniformly characterized by trench-parallel rupture lengths longer than about 250 km, whereas downdip rupture width ranges from less than 70 km (e.g., Central Aleutians) to more than 200 km (e.g., Andaman-Sumatra). The ability of rupture to propagate in the trench-parallel direction thus appears to play a fundamental role in determining the potential magnitude that an earthquake can achieve for a given subduction zone. The rupture length may be influenced by the nature of the plate interface and the normal stresses applied to the plate interface (plate coupling). The nature of the plate interface is potentially modified by sediment subduction. Subduction of a thick section of trench sediment constructs a laterally homogenous layer between upper and lower plates that smoothes subducted sea-floor relief and strength-coupling asperities (Ruff, 1989). Such a homogeneous interface running parallel to the subduction zone tends to favor long trench-parallel propagation of rupture, and thus large earthquake magnitudes. Compressive normal stresses applied along the plate interface may also tune the earthquake magnitude potential (Ruff & Kanamori, 1980). This plate coupling across the subducting interface can be indirectly estimated by Upper Plate Strain analysis, by using the back-arc as a strain sensor from which we can infer the back-arc stress state. Compressive back-arcs indicate that large stresses are transmitted across the plate interface whereas extensional settings indicate weak plate coupling. Here we present the results of a study funded by the European Science Foundation - EURYI project titled "Convergent margin and seismogenesis". Maximal earthquake magnitude, sediment thickness at the trench and Upper Plate Strain are characterized for worldwide subduction zones in order to test how plate coupling and sediment thickness combine to explain the occurrence of mega-events at the subduction interface. Subduction zones are described through an initial set of 505 transects, systematically extracted each 1 of trench, and merged into 62 subduction segments of homogeneous seismogenetic conditions. Maximal earthquake magnitude has been estimated by combining instrumental and historical seismicity. Trench sediment thickness has been constrained for 48 subduction segments; based on a compilation of 165 different seismic-reflection lines (33% of the initial set of transects).

  9. Non-steady-state subduction and trench-parallel flow induced by overriding plate structure

    NASA Astrophysics Data System (ADS)

    Rodrguez-Gonzlez, Juan; Billen, Magali I.; Negredo, Ana M.

    2014-09-01

    The direction of plate tectonic motion and the direction of mantle flow, as inferred from observations of seismic anisotropy measurements, show a good global correlation far from subduction zones. However, this correlation is poor near subduction zones, where below the slab seismic anisotropy is aligned parallel to the trench and above the slab has a complex pattern, which has not been fully explained. Here we present time-dependent three-dimensional (3D) fully-dynamic simulations of subduction to study the effect of overriding plate structure on the evolution of slab geometry and induced mantle flow. We find that along-strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate; the variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones, where both slab dip and overriding plate thermal state are strongly variable and correlated, and thus may be an important mechanism in other subduction zones. The location and strength of trench-parallel flow vary with the time-dependent evolution of the slab, suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non-steady-state behavior of these systems.

  10. Incoming sediments and its deformation observed on high resolution seismic profiles in the northern Japan Trench axis region

    NASA Astrophysics Data System (ADS)

    Nakamura, Y.; Kodaira, S.; Yamashita, M.; Miura, S.; Fujie, G.; Strasser, M.; Ikehara, K.; Kanamatsu, T.; Usami, K.

    2014-12-01

    The Japan Trench axis area has been intensively investigated since the 2011 Tohoku earthquake because the large slip reached to the vicinity of the trench axis. We have conducted three high resolution seismic cruises in the northern part of the Japan Trench axis region. The trench area between 38 - 40 N was covered by 81 E-W seismic lines with 2 - 4 km line interval. A 1200 m-long, 192 channel streamer cable and a cluster gun array with volume of 320 - 380 inch3 were used for these surveys. Post-stack time migrated sections provide detailed image of sediments above the subducting Pacific plate and its deformation by the bending-related normal faults on the outer trench slope, thrust faults and possible slope failures in the trench axis and inner trench slope. The deformation style of the sediments in the trench axis shows variation along the trench strike. To the south of the survey area in 38 - 39 N, the trench axis shows imbricate thrust-and-fold packages, which could be related to the interaction between the frontal prism toe and horst-graben structure. To the north around 40 N, the trench axis is located on a horst, and frontal thrust and imbricate structure are clearly observed on the seismic profiles. Around 39.5 N, the trench inner slope is very steep. It is suggested that slope failures as rotational slumps have occurred in this area. The trench axis is filled by slump deposits and debris with chaotic acoustic characteristics, which is similar with that in the seaward portion of the frontal prism. Seismic profiles on the outer trench slope show the variation on the thickness of the incoming sediments along the trench strike. It is thick, ~ 500 ms, in the northern part of the survey area around 40 N, and it is ~ 250 ms in the southern part around 38 N. The thickness is varied in the area between 38.5 - 39.5 N, and is very thin at 39.5 N. Sediments on the trench outer slope basically conformably cover the igneous basement of the Pacific plate and they were deformed by the bending-related normal faults, however graben fill sediments which onlap the original incoming sediments are also clearly observed on the seismic profiles in the outer trench slope. These graben fill sediments have been deposited in several isolated basins on the outer trench slope. Similar onlap fill sediments are also observed in a few places in the trench axis.

  11. Seismo-turbidites in the Japan Trench inner slope

    NASA Astrophysics Data System (ADS)

    Usami, K.; Ikehara, K.; Kanamatsu, T.; McHugh, C. M.

    2014-12-01

    The Japan Trench is a part of the Pacific-North America plate boundary, and is a seismically active region where the 2011 off the Pacific coast of Tohoku Earthquake occurred. We collected 24 piston cores from small basins of the Japan Trench inner slope (37.5-40 N, 143.5-144.16 E, 4000-6000 m in water depth) during the cruise NT13-19 with R/V Natsushima, to investigate recurrence intervals of earthquakes in this area, based on the analysis of the turbidite record. Many fine-grained turbidites are observed in the cores. High sedimentation rates in this area preserve the turbidites well. Turbidites are more frequently intercalated in the cores recovered from the southern part of the area. On the other hand, several turbidite layers can be correlated widely in the northern part of the area, assisted by tephrostratigraphy and matching of paleomagnetic secular variations. Moreover, some of these turbidites are considered to be correlative to the reported onshore tsunami deposits along the Sanriku-Sendai coast. We will report the occurrence, characteristics and correlation of seismo-turbidites in the Japan Trench inner slope. The evidence suggests that the Japan Trench area is an ideal site for application of turbidite paleoseismology.

  12. The relationship between plate velocity and trench viscosity in Newtonian and power-law subduction calculations

    NASA Technical Reports Server (NTRS)

    King, Scott D.; Hager, Bradford H.

    1990-01-01

    The relationship between oceanic trench viscosity and oceanic plate velocity is studied using a Newtonian rheology by varying the viscosity at the trench. The plate velocity is a function of the trench viscosity for fixed Rayleigh number and plate/slab viscosity. Slab velocities for non-Newtonian rheology calculations are significantly different from slab velocities from Newtonian rheology calculations at the same effective Rayleigh number. Both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate. Non-Newtonian rheology eliminates the need for imposed weak zones and provides a self-consistent fluid dynamical mechanism for subduction in numerical convection models.

  13. Episodic tremor and slip near the Japan Trench prior to the 2011 Tohoku-Oki earthquake

    NASA Astrophysics Data System (ADS)

    Ito, Yoshihiro; Hino, Ryota; Suzuki, Syuichi; Kaneda, Yoshiyuki

    2015-03-01

    Change in the rates of aseismic deformation prior to large earthquakes is a major area of interest in earthquake physics. Here we present evidence that episodic tremor and slip occurred in the shallow subduction zone within the source region of the 2011 Tohoku-Oki earthquake prior to the main shock. Ocean bottom seismometers near the Japan Trench recorded some excitations in amplitude of ambient noise level accompanying both the 2008 and 2011 slow slip events. The observed signals show that low frequencies of 5-8 Hz dominated, suggesting that the excitations were due to small low-frequency tremors accompanying the slow slip events. The largest amplitude tremor was observed just before the 2011 event. The estimated sources of tremors were possibly distributed within the coseismic slip area of the 2011 event, suggesting the shallow plate-boundary thrust near the trench is a general location of slow earthquakes.

  14. Oblique Subduction of the Pacific Plate at the Kuril Trench and Lateral Movement of the Forearc

    NASA Astrophysics Data System (ADS)

    Tanabe, A.; Tabei, T.

    2004-12-01

    Northeast-southwest trending Kuril forearc in the northwest Pacific is formed by oblique subduction of the Pacific plate (PA) at the Kuril Trench. Its northwestern boundary corresponds to a volcanic front and southwestern front collides with the northeast Japan arc in the central Hokkaido. We estimate lateral movement of the forearc in two ways. At first we combine relative plate motion vectors with slip vectors of major interplate earthquakes at the southern Kuril Trench. Plate motion vector is evaluated at each epicenter using the most recent global model REVEL-2000 (Sella et al., 2002), and decomposed into two components: one parallel to the earthquake slip vector that is related to interplate earthquake cycle and margin-parallel residual that may be taken up by the forearc lateral movement. The residuals show that the southern Kuril forearc moves southwestward at a mean rate of 12mm/yr. In more detail, northeastern part of the forearc moves at a faster rate of about 20mm/yr, and the rate decreases to 10mm/yr at the southwestern front. This implies a deceleration due to the collision with the northeast Japan arc. Next we estimate forearc movement using crustal velocities from nationwide continuous GPS array though data are limited only in Hokkaido region. In horizontal velocity field the most dominant is a crustal shortening of the forearc in the direction of plate convergence, which can be modeled as an elastic deformation caused by back slip vectors acted on plate interface. We determine plate interface geometry referring to high-precision unified hypocenters by Japan Meteorological Agency and apply plate motion vectors from REVEL-2000 on the plate interface. Full plate coupling is assumed to a depth of 50km. After subtracting calculated elastic deformations from the observed velocities, residuals are nearly margin-parallel in the forearc and show drastic change at around the northwestern boundary. Mean of the residuals in the forearc is about 10mm/yr. The result is consistent with that of the first estimation.

  15. The Fault Damage Zone of the Shallow Japan Trench Megathrust

    NASA Astrophysics Data System (ADS)

    Keren, T.; Kirkpatrick, J. D.

    2014-12-01

    The Mw 9.0 Tohoku-oki earthquake resulted in an unprecedented coseismic slip of >50 m in the shallow portion of the Japan Trench subduction zone. We present analyses of core recovered during IODP Expedition 343/343T (JFAST) that define structures surrounding the inferred plate boundary dcollement, and use the results to constrain the fault's long-term strength. The plate boundary fault is centered at 821.5 m below the sea floor, with a damage zone extending 15.5 m below and 51.5 m above. The damage zone is defined by shear fractures, subsidiary faults, deformation bands, mode I fractures, breccia zones, and sediment-filled veins. Orientations of mutually crosscutting shear fractures decrease in dip angle nearing the fault in the hanging wall, from 67 at 50 m above the fault down to 25. In the footwall, dips range from 88 at 9.6 m below the fault to 30 at 11 m below. The damage zone characteristics were established using a set of criteria to eliminate drilling-, coring-, and handling-induced damage in core. Core-scale fracture density increases from 21 fractures/m at 51 m above the fault to 247 fractures/m adjacent to the fault in the hanging wall, and from 28 fractures/m at 11 m below the fault to 254 fractures/m adjacent to the fault in the footwall. The fall-off in fracture density is fit by power law functions in the hanging wall and footwall, with decay exponent n values of 0.70 and 1.45, respectively. Microstructures include shear fractures, veins, crystallographic preferred orientation bands, shear zones, and undifferentiated fractures. Microstructure density in the footwall increases from 0.32 fractures/mm 10 m below the fault to 2.04 fractures/mm adjacent to the fault, and is fit by a power law function with n = 1.27. Orientations of shear fractures have attitudes consistent with normal and reverse faults, indicating the stress field underwent significant reorientations multiple times. This is consistent with the inferred stress field changes accompanying great earthquakes measured from borehole breakouts (Lin et al., 2013). Compared to similar offset faults, the dcollement has a steep fall-off in damage, which we interpret to indicate the fault is weak over geologic time; hence small changes in stress magnitudes could result in large stress field rotations.

  16. Perspectives on the Dynamics of Subduction and Trench Rollback: From the Birth of Subduction to Global Plate Motions (Invited)

    NASA Astrophysics Data System (ADS)

    Gurnis, M.; Leng, W.; Alisic, L.; Stadler, G.

    2013-12-01

    We will provide a brief overview of two classes of dynamic models of subduction zones and address issues associated with the forces driving plate tectonics and initiating new subduction zones. A common thread between the models is the origin of the intense back arc spreading and rapid roll back associated with some ocean-ocean subduction zones. We will look at the dynamics driving global plate motions and then look at the time-dependence of trench rollback regionally. Plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries, but observationally constrained high-resolution models of global mantle flow have remained a computational challenge. We capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions, with individual plate margins resolved down to a scale of 1 kilometer. We find that cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high-viscosity slabs, altering the velocity of oceanic plates. Back-arc extension and slab rollback are emergent consequences of slab descent in the upper mantle. We will show that most back arc extension follows subduction initiation and show how this arises in dynamic models.

  17. Effects of mantle and subduction-interface rheologies on slab stagnation and trench rollback

    NASA Astrophysics Data System (ADS)

    ?kov, Hana; Bina, Craig R.

    2013-10-01

    Trench rollback has been a widely discussed phenomenon in recent years, and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the (upper) mantle. Here we concentrate on the effects of rheological description (yield stress, lower-mantle viscosity, viscosity of crust) in controlling the rollback and associated stagnation of slabs in the transition zone (410-660 km depth). We perform numerical simulations of slab evolution in a 2D Cartesian model with strongly nonlinear rheology combining diffusion creep, dislocation creep and a power-law stress limiter. We demonstrate that trench retreat develops in most models considered, regardless of the subducting plate age or prescribed strength. Rollback then mostly produces slabs that are horizontally deflected at the 660-km phase boundary and remain subhorizontal at the bottom of the transition zone. Slab morphologies are in agreement with stagnant, horizontally deflected structures reported in the transition zone by seismic tomography. Furthermore, if the strength of the slab is limited to less than 0.5 GPa, the slab experiences a significant amount of horizontal buckling. The amplitude of the rollback velocity is sensitive to several model parameters. As one might expect, it increases with the age of the subducting plate, thus reflecting its increasingly negative buoyancy. On the other hand, rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates. High friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation. Also, reducing the additional negative buoyancy imparted by the 410-km exothermic phase transition suppresses trench rollback. Interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models, where, for example, buoyancy effects may be counteracted by associated yield-stress weakening.

  18. Quantitative Study of Seismogenic Potential Along Manila Trench: Effects of Scaborough Seamount Chain Subduction

    NASA Astrophysics Data System (ADS)

    Yu, H.; Liu, Y.; Li, D.; Ning, J.; Matsuzawa, T.; Shibazaki, B.; Hsu, Y. J.

    2014-12-01

    Modern seismicity record along the Manila Trench shows only infrequent Mw7 earthquakes, the lack of great earthquakes may indicate the subduction fault is either aseismically slipping or is accumulating strain energy toward rapid release in a megathrust earthquake. We conduct numerical simulations of the plate coupling, earthquake nucleation and dynamic rupture propagation processes along the Manila subduction fault (15-19.5N), taking into consideration the effects of plate geometry (including subducted seamounts), fault strength, rate-state frictional properties and pore pressure variations. Specifically, we use the bathymetry to depict the outline of Manila trench along its strike, 2681 background seismicity (1970/02/13 to 2013/09/06) from Chinese Earthquake Network Center and 540 focal mechanism solutions (1976/01/01 to 2013/01/27) from Global CMT project to constrain the geometry of the subducting Sunda/Eurasian slab. The compilation of seismicity and focal mechanism indicates the plate dipping angle gradually changes from 28 (south of the Scaborough Seamount Chain) to 12 (north of it). This geometric anomaly may due to the subducted part of the seamount chain. Preliminary modeling results using gabbro gouge friction data show that the Scaborough Seamount Chain could be a barrier to earthquake rupture propagation. Only earthquakes larger than Mw7 can overcome the barrier to rupture the entire Manila trench. Smaller earthquakes would cease rupturing when it encounters the seamount chain. Moreover, we propose that Manila trench subduction zone has the potential of rupturing in a Mw8 megathrust earthquake, if the simulation period is long enough for an Mw8 earthquake cycle and dynamic rupture overcomes the subducted Scaborough Seamount Chain. Our model parameters will be further constrained by laboratory rock mechanics experiments conducted on IODP Expedition 349, South China Sea (SCS), drilling samples (work in progress at China Earthquake Administration Key Laboratory of Earthquake Dynamics), and by GPS records from the Luzon Island. Our results improve ability of earthquake and tsunami hazards assessment and mitigation in the populated regions around the SCS, and provide theoretical basis for future ocean bottom seismometer and seafloor geodesy experiments in the SCS.

  19. Seismic constraints on mantle hydration during subduction: Mariana versus Middle America Trench

    NASA Astrophysics Data System (ADS)

    Miller, N. C.; Lizarralde, D.; Wiens, D. A.; Collins, J. A.; Holbrook, W.; Van Avendonk, H. J.

    2013-12-01

    We will present new results from active-source seismic experiments that constrain the amount of seawater entering the upper mantle along bending-induced faults at the outer rise of the Mariana and Middle America Trenches. This seawater may fill cracks in the upper mantle with free water; react strongly with olivine in upper mantle peridotite, filling cracks and fault zones with the hydrous mineral serpentinite; and/or diffuse between fault zones, pervasively serpentinizing the upper mantle. The upper mantle accounts for a large portion of the subducting lithosphere, and a hydrated upper mantle may supply the majority of water fluxing into arcs. Serpentinite is not stable at high temperatures and pressures, and, once subducted, would undergo a reverse reaction, releasing water into the mantle at depth and driving many arc- and global-scale geochemical and geodynamic processes. Hydration of the subducting upper mantle by fluid flow along bending-induced faults should depend on the density and depth of this faulting, as well as on the plate convergence rate and upper mantle temperature, which together control the rate of serpentinization reactions. Thus, a test of the outer-rise hydration hypothesis is to compare observations of the distribution of serpentinization in the upper mantle from subduction zones with different plate ages (temperatures); convergence rates; and angles between the relic abyssal-hill fabric, plate motion direction, and the trench (i.e. subduction obliquity), which control patterns of faulting. We will compare new seismic observations of faulting and serpentinization at the outer rise of the Middle America Trench offshore Nicaragua to observations from the central Mariana Trench. At the Middle America Trench, a moderately aged slab (24 Ma), and thus hot upper mantle, is subducting rapidly (85 mm/yr), both conditions that can limit serpentinization. Offshore Nicaragua, bending reactivates relic abyssal-hill fabric, which is oriented parallel to the trench. Here, measurements of seismic anisotropy and slow absolute wavespeeds suggest that these faults penetrate into the upper-most mantle and supply seawater that serpentinizes the mantle by up to ~13% (~1 wt% water). At the Mariana Trench, the slab is much older (140 Ma), and thus colder, and the convergence rate is slower (41 mm/yr), conditions expected to promote serpentinization. Here, preliminary analysis of new data suggests that upper mantle velocities are also significantly reduced under the outer rise. This velocity reduction is most extreme where bending-induced faulting is most pronounced, consistent with serpentinization via fluid flow along faults, although Cretaceous-age off-axis magmatism and the faults themselves may also affect seismic wavespeed. The goal of in-progress work on these Mariana data is to separate these wavespeed effects, enabling an estimate of serpentinization that can be compared to results from a similar analysis of the Middle America data, advancing our understanding of processes controlling the water input to subduction zones in general.

  20. P wave anisotropic tomography of the Nankai subduction zone in Southwest Japan

    NASA Astrophysics Data System (ADS)

    Wang, Jian; Zhao, Dapeng

    2012-05-01

    The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3-D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using 540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high-velocity (high-V) anomaly which exhibits considerable lateral variations. Significant low-velocity (low-V) anomalies are revealed above and below the PHS slab. The low-V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low-V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench-normal in the mantle wedge under the back-arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench-parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid-ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.

  1. Seismic versus aseismic slip: Probing mechanical properties of the northeast Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Shirzaei, M.; Brgmann, R.; Uchida, N.; Hu, Y.; Pollitz, F.; Matsuzawa, T.

    2014-11-01

    Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku Mw 9.0 earthquake ruptured with >50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17-8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M >8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space.

  2. Three-dimensional Distribution of Azimuthal and Radial Anisotropy in the Japan Subduction

    NASA Astrophysics Data System (ADS)

    Ishise, M.; Kawakatsu, H.; Shiomi, K.

    2014-12-01

    Seismic anisotropy has close relationships with past and present tectonic and dynamic processes. Therefore, detailed description of seismic anisotropy of subduction zones provides important information for our understanding of the subduction system. The most common method of detecting anisotropy is the S-wave splitting measurement. However, conventional S-wave splitting analysis is not an appropriate way to investigate anisotropy in the mantle and slab because the technique has no vertical resolution. Thus, we have improved common traveltime tomography to estimate three-dimensional anisotropic structures of P-wave, assuming that the modeling space is composed of weakly anisotropic medium with a hexagonal symmetry about a horizontal axis (Ishise & Oda, 2005, JGR; Ishise & Oda, 2008, PEPI). Recently, we extended the anisotropic tomography for P-wave radial anisotropy with vertical hexagonal symmetry axis (Ishise & Kawakatsu, 2012 JpGU). In this study, we expand the study area of our previous regional analyses of P-wave azimuthal and radial anisotropic tomography (Ishise & Oda, 2005; Ishise & Kawakatsu, 2012, JpGU; Ishise et al., 2012, SSJ) using Hi-net arrival time data and examine the subduction system around the Japan islands, where two trenches with different strike directions and plate junction are included. Here are some of the remarkable results associated with the PAC slab and mantle structure. (1) N-S-trending fast axis of P-wave anisotropy is dominant in the PAC slab. (2) the mantle wedge shows trench-normal anisotropy across the trench-trench junction. (3) horizontal velocity (PH) tends to be faster than vertical velocity (PV) in the slab. (4) PV tends to be faster than PH in the mantle wedge. The characteristics of the obtained azimuthal and radial anisotropy of the PAC slab and the mantle wedge qualitatively consistent with heterogeneous plate models (e.g., Furumura & Kennet, 2005) and numerical simulations of mantle flow (Morishige & Honda, 2011; 2013). In addition, the azimuthal anisotropy in the PAC slab that we obtained is subparallel to that in the PAC plate before subducting (e.g., Shimamura et al., 1983). Therefore, we suggest that the slab anisotropy is "frozen anisotropy", which is attributed to the episode before subduction, and mantle wedge anisotropy reflects present dynamics.

  3. Subduction of Serpentinized and Weathered Ultramafic Rocks in the Puerto Rico Trench: Preliminary Results

    NASA Astrophysics Data System (ADS)

    Horning, G.; Klein, F.

    2012-12-01

    Exposure of mantle peridotite and its interactions with seawater to form serpentinite are integral parts of seafloor spreading, and play a key role in affecting the rheology, chemistry, and microbial habitability of the oceanic lithosphere at slow- and ultra-slow spreading ridges. Away from the spreading centers, within subduction zones, the formation and dehydration of serpentinized peridotite impacts seismicity, element cycling, and melt generation. Here we present preliminary results of a petrographic and spectroscopic study of altered rocks recovered from the from the north wall of the trench Puerto Rico Trench (PRT). In fact, the PRT represents one of two subduction zones worldwide where slow spreading oceanic lithosphere is presently subducted, and where serpentinized peridotite has been directly evidenced by seafloor sampling {Bowin, 1966}. Thin section petrography, XRF analysis, scanning electron microscopy, and confocal Raman spectroscopy reveal that the peridotite, which in all likelihood originated at the Mid-Atlantic Ridge during the early Cretaceous, was virtually completely serpentinized under static conditions (as it is evidenced by the preserved mesh texture after olivine and bastite after orthopyroxene), and underwent subsequent seafloor weathering. While it is questionable where exactly serpentinization and subsequent seafloor weathering took place, our preliminary results strongly suggest that the material presently subducted in the PRT is not simply composed of serpentine, magnetite, and brucite; it is rather a complex disequilibrium assemblage of minerals including serpentine, brucite, chlorite, talc, magnetite, hematite, goethite, sulfur-rich sulfides and various clay minerals. Furthermore, our results imply that serpentinite and its weathering products influence the loci of dehydration and mineral replacement reactions, as well as the water input and element recycling in subduction zones.

  4. Development of precision acoustic transponders for GPS/Acoustic observation on the deep seafloor near the Japan Trench axis

    NASA Astrophysics Data System (ADS)

    Fujimoto, H.; Kido, M.

    2012-12-01

    The 2011 Tohoku-oki earthquake has let most of researchers recognize the importance of seafloor geodetic observation, especially near the trench axis. Iinuma et al. (2012a) estimated the coseismic slip distribution combining onshore GPS data with the seafloor geodetic data. Their results reveal that a huge (> 50 m) slip occurred in a small area (about 40 km in width and 120 km in length) near the Japan Trench and generated the huge tsunami. After the Tohoku-oki earthquake, seismic coupling near the trench axis has become a key to understand giant earthquakes in subduction zones, and it is GPS/Acoustic (GPS/A) repeated seafloor positioning that can give an observational constraint to it. Observation of postseimic deformation is another and urgent task required in the Japan Trench. Seafloor geodetic observation indicates posteseimic deformation near the Japan Trench axis in the north and south of the huge slip area (Iinuma et al., 2012b). The result is clearly different from that of onshore GPS observation. Postseismic deformation is estimated to be a key observation that can discriminate proper models from several ones that can explain the occurrence of the mega thrust. Tohoku University plans to deploy extensive GPS/A observation array along the Japan Trench in 2012 with a fund from MEXT, Japan, to cope with these requests (Kido et al., in this meeting). Precision acoustic transponders have newly been developed for the array to enable the following three requests: (1) observation on the deep seafloor, (2) compatibility among three institutions in Japan, and (3) observation for ten years. The first is the observation on the deep seafloor near the trench. While the Japan Trench axis is deeper than 7000 m, the existing GPS/A sites along the Japan Trench have remained on the seafloor shallower than 2500 m except the one nearest to the trench, where we observed coseismic slip of 31 m (Kido et al., 2011). We deployed 4 units of the new transponders supplied by Kaiyo Denshi, Ltd., in July this year on the seafloor of water depth around 5570 m. We have confirmed reliable acoustic ranging up to a slant range of 13 km, which is necessary for GPS/A observation on the seafloor of 6000 m water depth. The depth of 6000 m is a limit of cost effective glass-sphere pressure housing. The new transponder can also be adaptable to the GPS/A observation systems of Japan Coast Guard and Nagoya University to increase the chance of observations and to realize mutual comparison of the observed results. The acoustic system of Tohoku University was not so different from that of Nagoya University, but was quite different from that of Japan Coast Guard. Stronger Doppler effect on a longer acoustic signal adopted by the Japan Coast Guard was the most critical problem. We deployed one unit of the new transponder on the seafloor of about 2000 m water depth, and Japan Coast Guard has confirmed precise acoustic ranging with it up to a slant range of about 6000 m by using the acoustic system installed on their survey vessel.

  5. Teleseismic shear wave tomography of the Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Asamori, Koichi; Zhao, Dapeng

    2015-12-01

    We present a high-resolution shear wave tomography of the Japan subduction zone down to a depth of 700 km, which is determined by inverting a large number of high-quality S-wave arrival-time data from local earthquakes and teleseismic events. The subducting Pacific and Philippine Sea (PHS) slabs are revealed clearly as high-velocity (high-V) zones, whereas low-velocity (low-V) anomalies are revealed in the mantle wedge above the two slabs. The PHS slab has subducted aseismically down to a depth of 480 km under the Japan Sea and to a depth of 540 km under the Tsushima Strait. A window is revealed within the aseismic PHS slab, being consistent with P-wave tomography. Prominent low-V and high-Poisson's ratio (σ) anomalies exist below the PHS slab and above the Pacific slab, which reflect hot and wet mantle upwelling caused by the joint effect of deep dehydration of the Pacific slab and convective circulation process in the mantle wedge above the Pacific slab. The hot and wet mantle upwelling has caused the complex geometry and structure of the PHS slab in SW Japan, and contributed to the Quaternary volcanism along the Japan Sea coast. In eastern Japan, low-V zones are revealed at depths of 200-700 km below the Pacific slab, which may reflect hot upwelling from the lower mantle or even the core-mantle boundary.

  6. Electrical image of subduction zone beneath northeastern Japan

    NASA Astrophysics Data System (ADS)

    Ichiki, Masahiro; Ogawa, Yasuo; Kaida, Toshiki; Koyama, Takao; Uyeshima, Makoto; Demachi, Tomotsugu; Hirahara, Satoshi; Honkura, Yoshimori; Kanda, Wataru; Kono, Toshio; Matsushima, Masaki; Nakayama, Takashi; Suzuki, Syuichi; Toh, Hiroaki

    2015-12-01

    We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 ? m, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80-100 km depth and takes an overturned form toward the back arc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent back-arc volcanoes in the system.

  7. Development of GPS/A Seafloor Geodetic Network Along Japan Trench and Onset of Its Operation

    NASA Astrophysics Data System (ADS)

    Kido, M.; Fujimoto, H.; Osada, Y.; Ohta, Y.; Yamamoto, J.; Tadokoro, K.; Okuda, T.; Watanabe, T.; Nagai, S.; Kenji, Y.

    2012-12-01

    The Tohoku-oki earthquake in 2011 revealed that an M9-class giant earthquake could occur even in the old subduction zone and that coseismic slip can reach its frontal wedge, where we considered no significant stress had been accumulated in. One of the leading figure of such finding is in situ seafloor geodetic measurement, such as GPS/A technique for horizontal displacement and pressure gauge for vertical displacement. Japan Coast Guard and Japanese university group had developed several GPS/A sites near the source region of the Tohoku-oki earthquake and detected quite large coseismic movements over 20 m in there. Displacement vectors observed these sites showed systematic variation, i.e., mainly confined in the off-Miyagi area and getting larger near the trench. However, subsequent post-seismic deformation shows inexplicable distribution. In order to elucidate this complex feature, MEXT Japan has decided to construct dense and widely-extended GPS/A network along Japan trench, including deep area (~6000m). We, Tohoku and Nagoya universities, have firstly developed high-powered seafloor transponders with an omnidirectional acoustic unit that works at 6000 m deep ocean and enable acoustic ranging over 13 km slant length. In addition, using high-energy density battery, its lifetime is expected 10 years with normal operation. Secondly, we examined the optimal distribution of GPS/A sites forming a network, taken pre-existing sites into consideration. The new network consists of 20 sites (roughly four transponders at a single site and 86 transponders in total). The distribution is dense near the area of complex post-seismic deformation and extended over 400 km to cover the adjacent area of the source region, in where induced earthquake may be expected. The largest obstacle to draw network plan is seafloor topography. Because a GPS/A site is a seafloor benchmark, its installation must be on flat and locally stable spot. Since a single GPS/A site consists of three or more transponders in an area extending roughly the same dimension of its depth, flat spot is quite limited especially near the trench. The positions of the 20 sites were carefully determined using a high-definition bathymetry map. We already have constructed two sites, one of which is 5500 m depth, and successfully obtained acoustic data. In September, we will install rest of the sites (18 sites) and begin initial campaign survey. The second campaign is planned in November. We will introduce details of the network and report updated result in the talk.

  8. Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3

    USGS Publications Warehouse

    Kobayashi, K.; Cadet, J.-P.; Aubouin, J.; Boulegue, J.; Dubois, J.; von, Huene R.; Jolivet, L.; Kanazawa, T.; Kasahara, J.; Koizumi, K.-i.; Lallemand, S.; Nakamura, Y.; Pautot, G.; Suyehiro, K.; Tani, S.; Tokuyama, H.; Yamazaki, T.

    1987-01-01

    A detailed topographic and geophysical survey of the Daiichi-Kashima Seamount area in the southern Japan Trench, northwestern Pacific margin, clearly defines a high-angle normal fault which splits the seamount into two halves. A fan-shaped zone was investigated along 2-4 km spaced, 100 km long subparallel tracks using narrow multi-beam (Seabeam) echo-sounder with simultaneous measurements of gravity, magnetic total field and single-channel seismic reflection records. Vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported. The northern and southern extensions of the normal fault beyond the flank of the seamount were delineated. Materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount. Canyons observed in the upper landward slope terminate at the mid-slope terrace which has been uplifted since start of subduction of the seamount. Most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern Japan Trench wall. This survey was conducted on R/V "Jean Charcot" as a part of the Kaiko I cruise, Leg 3, in July-August 1984 under the auspices of the French-Japanese scientific cooperative program. ?? 1987.

  9. Subduction trench migration as a constraint on absolute plate motions since 130 Ma

    NASA Astrophysics Data System (ADS)

    Williams, Simon; Flament, Nicolas; Müller, Dietmar; Butterworth, Nathan

    2015-04-01

    The absolute motions of the lithospheric plates relative to the Earth's deep interior are commonly constrained using observations from paleomagnetism and age-progressive seamount trails. In contrast, a reference frame linking surface plate motions to subducted slab remnants mapped from seismic tomography has recently been proposed. Absolute plate motion (APM) models (or "reference frames") derived using different methodologies, different subsets of hotspots, or differing assumptions of hotspot motion, have contrasting implications for parameters that describe the long term state of the plate-mantle system, such as the balance between advance and retreat of subduction zones, plate velocities, and net lithospheric rotation. Previous studies of contemporary plate motions have used subduction zone kinematics as a constraint on the most likely APM model. Here we use a relative plate motion model to compute these values for the last 130 Myr for a range of alternative reference frames, and quantitatively compare the results. We find that hotspot and tomographic slab-remnant reference frames yield similar results for the last 70 Myr. For the 130-70 Ma period, where hotspot reference frames are less well constrained, these models yield a much more dispersed distribution of slab advance and retreat velocities. By contrast, plate motions calculated using the slab-remnant reference frame, or using a reference frame designed to minimise net rotation, yield more consistent subduction zone kinematics for times older than 70 Ma. Introducing the global minimisation of trench migration rates as a key criterion in the construction of APM models forms the foundation of a new method of constraining APMs (and in particular paleolongitude) in deep geological time.

  10. High-resolution seismic profiling in the trench axis area of the Japan Trench and its preliminary correlation with the results of IODP Expedition 343

    NASA Astrophysics Data System (ADS)

    Nakamura, Y.; Cook, B. J.; Conin, M.; Miura, S.; Takahashi, N.; Kodaira, S.; Mori, J. J.; Chester, F. M.; Eguchi, N.; Toczko, S.

    2012-12-01

    IODP Expedition 343 (Japan Trench Fast Drilling Project: JFAST) was conducted in April-May 2012 using D/V Chikyu. For the JFAST project, a high-resolution multi-channel seismic (MCS) survey was carried out to clarify the structure near the drill site. We present the characteristics of the seismic data from the site survey and briefly introduce the preliminary correlation between seismic and drilling results. The high-resolution seismic data were obtained during the KY11-E05 cruise in Oct-Nov 2011. The sounding source was a 320 cubic inch cluster gun array, and the receiver was a 1.2-km, 192-channel streamer cable. The seismic sections clearly show sedimentary strata overlaying the igneous crust in the outer trench slope. The strata and igneous crust are offset by normal faults producing a horst-graben structure. The strata are also deformed in the vicinity of the trench axis by compaction. Comparison with the results from DSDP Site 436 suggested that the Pacific plate has been subducting with a chert-rich layer preserved above the igneous crust layer, which was confirmed by the drilling results. The hanging wall in the lowermost landward trench slope has acoustically chaotic character, and no continuous strong reflectors which could suggest the possible 'splay' faults are not observed. Sonic, and density logging nor vertical seismic profiling (VSP) were made, therefore a synthetic seismogram was calculated using basically only resistivity log data. The time-depth relation was calibrated by cross-correlating the synthetic seismogram with the seismic section for the interval around the strong reflector corresponding to the top of the chert-rich layer. The synthetic seismogram also shows some strong reflections in the shallower part, but they are not well correlated with the seismic profiles. We will continue detailed investigations in the future through integration of core, log and seismic data.

  11. Subduction of Louisville Ridge seamounts: Effects on Tonga-Kermadec Trench and forearc morphology and seismic structure

    NASA Astrophysics Data System (ADS)

    Stratford, W. R.; Peirce, C.; Funnell, M.; Paulatto, M.; Watts, A. B.; Grevemeyer, I.; Bassett, D.; Hunter, J.

    2013-12-01

    Geophysical profiling normal and oblique to the Tonga-Kermadec Trench between 23° and 28° S highlights forearc and trench deformation structures in the vicinity of the subducting Louisville Ridge. A fast southwards migration of the ridge-trench collision zone (~180 km/myr), and the obliquity of the seamount chain to the trench make this an ideal case study for the effects of seamount subduction on lithospheric structure. Wide-angle and multichannel seismic, swath bathymetry and potential field data on four profiles are used to image seafloor and crustal structure. The study area covers three main deformation zones from north to south: post-, current and pre-seamount subduction. Mo'unga Seamount lies in the centre of the trench at the collision zone creating a disparity between the geomorphic and tectonic trench locations and broadening the trench floor. The geomorphic trench, the deepest part of the collision zone, is seaward of the seamount at the base of a graben formed by extensional bending faults on the down-going Pacific Plate. The true plate boundary lies ~16 km west, on the arcward side of Mo'unga Seamount, where a detachment fault separates forearc from Pacific Plate-derived trench fill. The steepness of the detachment fault indicates that the impinging seamount induces arcward rotation of the lower trench slope. Arcward rotation is also observed in the dipping sedimentary layers of the mid-slope basin. As no unconformable overlying sediments are observed, the deformation is inferred to be recent and ongoing. There is a southward decrease in the slope angle of the inner-trench wall and this is reflected in the style of extensional deformation structures in the mid-slope basin. A 30 km wide basin of distributed deformation on the shallow dipping mid-trench slope is observed in the south and a 10 km wide, ~2 km deep, fault-bounded basin on the steeply dipping mid-trench slope is observed in the collision zone and to the north. A greater degree of tectonic collapse of the steep inner-trench slope in the north is indicated by a 15% decrease in arc basement velocities to 4 km below the mid-slope basin floor. These low velocities are attributed to deep fracturing extending into the upper crust and may record the tectonic collapse of the forearc after seamount subduction. P-wave velocity and gravity models of crustal structure also indicate an along-arc increase in crust and plate interface thickness from north to south. Low mantle Pn velocities of 7.8 kms-1 below the forearc are indicative of serpentinisation of the mantle wedge. Transient effects of a north to south progression of enhanced mantle hydration, basal erosion and oversteepening and collapse of the forearc are inferred. This project has enabled the study of mature, active and pre-seamount subduction effects on forearc and trench structure, and highlights the speed at which evidence of these disappear from the seabed geomorphology.

  12. GPS/acoustic seafloor geodetic observation in the subduction zone around Japan (Invited)

    NASA Astrophysics Data System (ADS)

    Sato, M.; Kido, M.; Tadokoro, K.; Fujimoto, H.

    2013-12-01

    GPS/acoustic (GPS/A) seafloor geodetic observation is a precise seafloor positioning technique and has made great progress over the last decade. GPS/A observation determines the positions of acoustic mirror-type transponders installed on the seafloor by combining the two techniques of kinematic GPS and acoustic ranging through a ship or a buoy. The original idea was proposed by Prof. Spiess at the Scripps Institution of Oceanography in 1985 and its protocol and hardware were made through research and development of his group by the mid-1990s. In Japan, three research groups, Japan Coast Guard, Tohoku University and Nagoya University, began to develop the GPS/A observation system in the 1990s, established GPS/A observation sites mainly on the landward slope of the plate boundaries around Japan, such as the Japan Trench and the Nankai trough, and have been carrying out campaign observations since around 2000. The primary purpose of our observation is to detect and monitor the crustal deformation caused by the subduction of the oceanic plate near the plate boundary where large interplate earthquakes have repeatedly occurred. By continuous efforts for over a decade, the positioning precision has achieved a few centimeters and seafloor movements such as intraplate deformation and coseismic displacements have been successfully detected. In particular, regarding the 2011 Tohoku-oki earthquake (M9.0), which occurred off northeastern Japan on March 11, 2011, east-southeastward coseismic displacements of up to 31 m were observed above the focal region, especially close to the epicenter, while those detected by on-land GPS measurements over 100 km away from the epicenter, conducted by the Geospatial Information Authority of Japan, was up to 5.3 m. Coseismic slip models on the plate boundary estimated from not only GPS data but also GPS/A results indicate that a huge slip of more than 50 m generated close to the trench axis, which was much larger than that estimated from GPS data only. This demonstrates the indispensable roles of seafloor geodesy. After the event, Tohoku University and Nagoya University deployed additional GPS/A sites along the Japan Trench in order to monitor postseismic movements offshore spatially, especially close to the trench axis. In addition, Japan Coast Guard deployed additional GPS/A sites along the Nankai Trough, southwestern Japan, where there are growing concerns about the occurrence of a huge earthquake in the future. This expansion will enable us to detect the spatial change of intraplate velocities along the Nankai Trough, which reflects the difference of the degrees of interplate coupling. We have more than 50 GPS/A sites in total and have been carrying out several campaign observations per site per year. Seafloor geodetic data is an important key to understand the mechanism of the occurrence of interplate earthquakes which occur in the sea area.

  13. Two decades of spatiotemporal variations in subduction zone coupling offshore Japan

    NASA Astrophysics Data System (ADS)

    Loveless, John P.; Meade, Brendan J.

    2016-02-01

    Spatial patterns of interplate coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze ∼19 years of geodetic observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, ∼3.75-year epochs each decomposed into best-fit velocity, annual periodic signals, coseismic offsets, and postseismic effects following seven major earthquakes. Nominally interseismic velocities are interpreted in terms of a combination of tectonic block motions and earthquake cycle activity. The duration of the inferred postseismic activity covaries with the linear velocity. To address this trade-off, we assume that the nominally interseismic velocity at each station varies minimally from epoch to epoch. This approach is distinct from prior time-series analysis across the earthquake cycle in that position data are not detrended using preseismic velocity, which inherently assumes that interseismic processes are spatially stable through time, but rather the best-fit velocity at each station may vary between epochs. These velocities reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to the Tokai and Bungo Channel regions, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped during historic great earthquakes. On the Sagami subduction zone south of Tokyo, we also estimate relatively stable coupling through time. On the Japan-Kuril Trench, we image significant coupling variations owing to effects of the 1994 MW = 7.7 Sanriku-oki, 2003 MW = 8.2 Tokachi-oki, and 2011 MW = 9.0 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following the 1994 event until 2011, coseismic-sense slip precedes the Tohoku-oki event, and coupling offshore northern Honshu is reduced after the 2011 earthquake. Despite the occurrence of the 2003 Tokachi-oki earthquake, persistent coupling offshore Hokkaido suggests ongoing seismic hazard, possibly similar to past MW ∼ 9-class earthquakes interpreted from coastal paleoseismic records. This time-dependent analysis of interseismic deformation illuminates rich diversity in the distribution of subduction zone coupling, including spatiotemporal stability in coupling, effective reduction in strongly coupled regions due to aseismic thrust-sense slip events, and broad changes in the distribution of coupling following major earthquakes.

  14. The Effect of Near-Trench Fluid Circulation on Slab Dehydration Depth in the Chile Subduction Zone

    NASA Astrophysics Data System (ADS)

    Spinelli, G. A.; Wada, I.

    2014-12-01

    Fluids released from subducting slabs affect earthquakes, geochemical recycling, melt generation, and mantle wedge flow. The distribution of this fluid release is controlled by the composition/hydration of the slab entering a subduction zone and the pressure-temperature path that the slab follows. We examine the potential for along-strike changes in the thermal state of the south central Chile subduction zone (36 to 45 °S) to affect the distribution of fluid release from the subducting Nazca Plate. Because the age of the plate entering the subduction zone decreases from ~30 Ma at 36 °S to ~1 Ma at 45 °S, a southward warming of the subduction zone has been hypothesized. We model temperatures in the system, then use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the distribution of dehydration-derived fluid release from the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools parts of the system with the youngest subducting lithosphere. For example, relative to simulations with no fluid flow, hydrothermal circulation decreases temperatures in the 45 °S transect by up to 150 °C. Although hydrothermal circulation in the oceanic crust likely ceases by ~50 km landward of the trench, the legacy of its heat redistribution affects slab temperatures and dehydration >100 km farther landward. In the 45 °S transect, using temperatures from a model that includes hydrothermal circulation yields peak slab dehydration centered under the volcanic arc. In contrast, without hydrothermal circulation, peak slab dehydration is predicted at ~70 km seaward of the volcanic arc. For systems with young (<20 Ma) subducting lithosphere, hydrothermal circulation in oceanic crust should be considered in estimating subduction zone temperatures and fluid source distributions.

  15. Hadal disturbance in the Japan Trench induced by the 2011 Tohoku-Oki earthquake.

    PubMed

    Oguri, Kazumasa; Kawamura, Kiichiro; Sakaguchi, Arito; Toyofuku, Takashi; Kasaya, Takafumi; Murayama, Masafumi; Fujikura, Katsunori; Glud, Ronnie N; Kitazato, Hiroshi

    2013-01-01

    In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku-Oki earthquake. Video recordings documented dense nepheloid layers extending ~30-50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated (137)Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained (134)Cs from the Fukushima Dai-ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of (134)Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock. PMID:23715086

  16. Hadal disturbance in the Japan Trench induced by the 2011 Tohoku–Oki Earthquake

    PubMed Central

    Oguri, Kazumasa; Kawamura, Kiichiro; Sakaguchi, Arito; Toyofuku, Takashi; Kasaya, Takafumi; Murayama, Masafumi; Fujikura, Katsunori; Glud, Ronnie N.; Kitazato, Hiroshi

    2013-01-01

    In situ video observations and sediment core samplings were performed at two hadal sites in the Japan Trench on July, 2011, four months after the Tohoku–Oki earthquake. Video recordings documented dense nepheloid layers extending ~30–50 m above the sea bed. At the trench axis, benthic macrofauna was absent and dead organisms along with turbid downslope current were observed. The top 31 cm of sediment in the trench axis revealed three recent depositions events characterized by elevated 137Cs levels and alternating sediment densities. At 4.9 km seaward from the trench axis, little deposition was observed but the surface sediment contained 134Cs from the Fukushima Dai–ichi nuclear disaster. We argue that diatom blooms observed by remote sensing facilitated rapid deposition of 134Cs to hadal environment and the aftershocks induced successive sediment disturbances and maintained dense nepheloid layers in the trench even four months after the mainshock. PMID:23715086

  17. The buoyancy variation of plate coupling from subduction to collision: an example across the northernmost Manila trench

    NASA Astrophysics Data System (ADS)

    Lo, Chung-Liang; Doo, Wen-Bin; Kuo-Chen, Hao; Hsu, Shu-Kun

    2015-04-01

    The Manila trench is the boundary between the South China Sea (SCS) of Eurasian Plate (EU) and Philippine Sea Plate (PSP). The east subducting of SCS is a ceased rifting oceanic crust. To the north, the subduction is obscured and transits to collision extended to the Taiwan orogenesis. The Taiwan Integrated Geodynamics Research (TAIGER) project has implemented several offshore multichannel seismic (MCS) reflection and wide-angle seismic experiments to model the velocity structure of the incipient arc-continental collision. Amongst, along two trench perpendicular transects (MGL0905_23, 25) are associated with ocean bottom seismometer (OBS) deployed in the northern Manila trench. The transect MCS data and tomographic velocity structure provide well constraint on the recognition between the crust and mantle lithosphere that helps to reconstruct synthetic density structure to fit the observation gravity data. The synthetic gravity result along two transects also show that there exists an anomalous high density (~2.97 g/cm3) mass beneath the accretionary prism in the leading edge of overriding plate; however, unfortunately, the MCS and OBS data have no resolution there. Meanwhile, the buoyancies of crust (Hc) and mantle lithosphere (Hm) can be calculated associated with the residual topography based on the isostatic equilibrium. According to the contribution of Hm, the estimation of the plate coupling effect can be approached. Combining two transects data across the northern Manila trench and one profile across the Hengchun Peninsula in southern Taiwan (T29-33, TAICRUST project), a sequence from subduction to collision of plate coupling effect can therefore be evaluated, and also offers the opportunity to examine the lithospheric structure variation in the zone between Taiwan and northernmost Manila trench.

  18. Estimation of seismic velocity in the subducting crust of the Pacific slab beneath Hokkaido, northern Japan by using guided waves

    NASA Astrophysics Data System (ADS)

    Shiina, T.; Nakajima, J.; Toyokuni, G.; Kita, S.; Matsuzawa, T.

    2014-12-01

    A subducting crust contains a large amount of water as a form of hydrous minerals (e.g., Hacker et al., 2003), and the crust plays important roles for water transportation and seismogenesis in subduction zones at intermediate depths (e.g., Kirby et al., 1996; Iwamori, 2007). Therefore, the investigation of seismic structure in the crust is important to understand ongoing physical processes with subduction of oceanic lithosphere. A guided wave which propagates in the subducting crust is recorded in seismograms at Hokkaido, northern Japan (Shiina et al., 2014). Here, we estimated P- and S-wave velocity in the crust with guided waves, and obtained P-wave velocity of 6.6-7.3 km/s and S-wave velocity of 3.6-4.2 km/s at depths of 50-90 km. Moreover, Vp/Vs ratio in the crust is calculated to be 1.80-1.85 in that depth range. The obtained P-wave velocity about 6.6km/s at depths of 50-70 km is consistent with those estimated in Tohoku, northeast Japan (Shiina et al., 2013), and this the P-wave velocity is lower than those expected from models of subducting crustal compositions, such as metamorphosed MORB model (Hacker et al., 2003). In contrast, at greater depths (>80 km), the P-wave velocity marks higher velocity than the case of NE Japan and the velocity is roughly comparable to those of the MORB model. The obtained S-wave velocity distribution also shows characteristics similar to P waves. This regional variation may be caused by a small variation in thermal regime of the Pacific slab beneath the two regions as a result of the normal subduction in Tohoku and oblique subduction in Hokkaido. In addition, the effect of seismic anisotropy in the subducting crust would not be ruled out because rays used in the analysis in Hokkaido propagate mostly in the trench-parallel direction, while those in Tohoku are sufficiently criss-crossed.

  19. Strain Energy Flow and the Accumulation of Large Earthquake Energies in the Island Arc-Trench Dynamics ( NE Japan Arc)

    NASA Astrophysics Data System (ADS)

    Nagumo, S.

    2002-12-01

    A gravitational shear flow of the island-arc rock-mass was newly revealed under the Japan trench inner slope on the artifact-free and distortion-free seismic reflection profiles (Nagumo and Tsuru (2001), Eos. Trans. AGU82(47), Fall.Meet. Suppl. Abstract T12D-0934). Based on such a shear flow phenomena, I envision a picture of the island arc-trench dynamics as below. The hot upper mantle under the volcanic arc generates buoyancy, and uplifts the volcanic arc. The uplifted island arc rock-mass gains excess gravitational potential energy. The surface inclination from the central arc towards the trench axis generates horizontal driving force within the arc mass, and generates shear stresses, shear deformations, and shear strain energies. In the deep part of the crust, such shear stresses exceed the critical value, and the rock-masses are in a state of shear flow. Because of rather hot thermal regime, such a shear flow of rock-masses predominates within the ductile intermediate zone of the crust, which ranges from the basal granitic layer to the upper part of the basaltic layer. The shear flow carries the whole island-arc mass towards trench axis from the central volcanic arc and transports the strain energies, which are involved within the deformed rock-mass, and forms a flow of strain energy. Such a gravitational flow of the rock-mass is a behavior of viscous fluids in a long time scale. When the strain energy flow is obstructed by some mechanism, a part of the strain energy flow is trapped, and stored around the obstacles. The stored energies accumulate with elapse of time, and results in an occurrence of large earthquake. I postulate that the occurrence condition of a large earthquake is such that the average density of the accumulated strain energy within the volume of the self-adjoined domain attains a certain critical level. Then, we can estimate the earthquake radiation energy by the product of the critical strain energy and the volume of the self-adjoined domain. The gravitational potential energy is sufficient enough to generate the large earthquake energies in the forearc region. The energy supply by the subducting oceanic plate is not required. Such a view of strain energy accumulation process may relate to the variety of the asperity, repetition interval of occurrence, and the types of earthquakes, such as main shock type, swarm type and etc. The above picture of the island arc-trench dynamics does not require subduction-accretion tectonics.

  20. On the evolution of Subduction-Transform Edge Propagators (STEPs): application to the Pliny-Strabo 'trenches'

    NASA Astrophysics Data System (ADS)

    Nijholt, Nicolai; Govers, Rob

    2015-04-01

    At the eastern side of the Aegean slab, tomographic evidence shows that a slab edge is present. The subducting African plate needs to tear to permit continued subduction and rollback of the Hellenic trench. This vertical tear is named a Subduction-Transform Edge Propagator (STEP) and is defined as the region of active tearing. At the eastern side of the Hellenic trench, the active STEP is probably located along the Ptolemy 'trench'. The surface expression (deformation zone) of this propagating STEP is observed in bathymetry and seismology, where the Pliny-Strabo 'trenches' are referred to as the STEP fault zone, i.e. the deformation zone in the wake of the active STEP. Due to its immaturity, the plate boundary is a relatively wide zone as strain has not localized yet. A key question is the propagation direction of an active STEP. It is suspected that passive margins may play a critical role in steering a STEP as they represents first order strength contrasts between oceanic and continental lithosphere. Here, we seek to identify the preferred propagation direction for a STEP and also investigate the effect of passive margins on STEP propagation (direction) through mechanical, finite element models. Our model results show that propagation of a STEP along a passive margin-ocean interface is likely for a range of models which show a passive margin-trench orientation within 15 degrees from a perpendicular setup. Passive margins are rarely straight features and with the inclusion of a change in strike of the passive margin ahead of the active STEP, model results show that propagation will occur along the passive margin-ocean interface if this change is less than 25 degrees from a perpendicular setup. Surprisingly, the subduction history and magnitude of the strength(/effective viscosity) contrast across the passive margin are less relevant. The STEP system seems relatively insensitive to small scale details, e.g. so that small, gradual changes in passive margin orientation do not affect STEP propagation. For the eastern side of the Aegean slab, tectonic reconstructions suggest that the passive margin was oriented nearly perpendicular to the STEP fault in the Pliocene. Propagation of the STEP into the oceanic lithosphere of the Eastern Mediterranean basin along the Pliny-Strabo 'trenches' is therefore consistent with our model results, i.e., an expected consequence of the (past) tectonic setting.

  1. Along-Trench Structural Variations, Seamount Subduction, and Inter-Seismic Coupling at the Central Ecuador Convergent Margin

    NASA Astrophysics Data System (ADS)

    Sanclemente, E.; Collot, J. Y.; Ribodetti, A.

    2014-12-01

    The structural interpretation of 2D-Pre-stack Depth Migrated Multichannel Seismic Reflection sections collected during the SISTEUR cruise across the Central Ecuadorian convergent margin was combined with multibeam bathymetry, OBS wide-angle tomographic models, a GPS inversion model, and relocated micro-seismicity to decipher the causes of the along-trench variability of the Inter-Seismic Coupling (ISC). Our study shows that the Central Ecuador margin divides in two contrasting segments, the northern "Manta-Puerto Lpez" and southern "Puerto Lpez-Salinas" segments showing dissimilar long-lived physical properties. The northern segment coincides with a shallow ISC locked zone, and shows a smooth outer-wedge slope scalloped by a gentle, 50 km-wide morphologic re-entrant. No subduction channel is detected across this segment that reveals a large subducted seamount and a 2-4 landward dipping shallow inter-plate contact. In the locked zone, the seamount is in contact with strong (Vp= 5 km/s) oceanic rocks of the margin basement, suggesting that elastic strain can store and trigger a large earthquake. In contrast, the southern margin segment is rather decoupled and shows a highly disrupted outer-wedge seafloor with deep re-entrants and large Mass Transport Deposits. The interplate contact dips landward ~6-7, and is spotted by isolated seamounts separated by a ~0.5-1 km-thick subduction channel that may act as a lubricant favoring inter-plate creeping. In this segment, subducted seamounts collide against low velocity (Vp=3.5 km/s) margin rocks of a thrust sheet complex deformed by normal faults, so that sufficient elastic strain may not accumulate to trigger a large earthquake. Our study supports that ISC variations along the trench are mainly controlled by the thickness of the SC, the roughness of the subducting plate and stiffness variations of margin basement rocks against which subducted seamounts collide.

  2. Seismic structure related to the Philippine Sea plate subduction beneath the southwestern Nansei-Shoto (Ryukyu) Trench

    NASA Astrophysics Data System (ADS)

    Nishizawa, A.; Kaneda, K.; Oikawa, M.; Horiuchi, D.; Fujioka, Y.

    2013-12-01

    The northwestern part of the Philippine Sea plate (PSP) is subducting beneath the Nansei-Shoto (Ryukyu) Trench. Earthquakes larger than M8 have not been recorded instrumentally in the southwestern part of the Ryukyu Trench. However, there was a large tsunami killed around 12,000 people in 1771 and the position of the source area has not been established. The direction of the PSP subduction is almost perpendicular to the Ryukyu Trench to the northeast of 126.5 E and changes to oblique to the southwest. Seismicity varies along the trench axis and lower seismicity seems to correspond to the free-air gravity anomaly high region in the forearc at around 126 E. On the other hand, the Okinawa-Luzon fracture zone extending from southwest to northeast on the PSP subducts beneath Ryukyu Trench to the northeast of 126 E and seems to relate to the high gravity anomaly. We carried out seismic experiments to elucidate these inhomogeneous crustal structures along the southwestern part of the trench and provide valuable information on potential source areas for large earthquakes and/or tsunamis. We conducted five seismic lines across southwestern Ryukyu Trench region. Multichannel reflection seismic (MCS) data using 240 ch. and 3000 m long hydrophone streamer were collected for airgun shots at an interval of 50 m. We also deployed ocean bottom seismographs (OBSs) as a receiver at an average interval of 5 km along each line. A tri-gun cluster with a volume of 1,050 (350 * 3) cubic inches (17.2 liters) was used for the reflection surveys and a non-tuned airgun array with a volume of 6,000 (1500 * 4) cubic inches (98 liters) was shot at an interval of 200 m (90 sec) for the wide-angle seismic survey. Depth conversion from the MCS time section was carried out using the refraction results. In this paper, we present the seismic structure related to the PSP subduction in the forearc region of the Nansei-Shoto island arc. The subducting Okinawa-Luzon fracture zone was able to be clearly imaged beneath two MCS lines to the northeast of 126 E. The fracture zone has several rows of valleys, of which the maximum relative depths are more than 0.5 km. The MCS records show such fracture topography subducts without deformation. P-wave velocity structure beneath the high free-air gravity region in the forearc at 125-127 E shows that materials with a high velocity of around 4 km/s ascend to 1 km below the seafloor at the shallowest region, which indicates that the gravity anomaly high does not reflect the fracture zone directly. There are not clear splay faults in the seaward of the high gravity region, but this might be due to insufficient energy of the airgun source. On the other hand, distinctive signals from the plate boundary and the fault branching from the plate boundary were detected in the oblique subduction area to the southwest of 125 E.

  3. Seismicity detection around the subduting seamount off Ibaraki the Japan Trench using dense OBS array data

    NASA Astrophysics Data System (ADS)

    Nakatani, Y.; Mochizuki, K.; Shinohara, M.; Yamada, T.; Hino, R.; Ito, Y.; Murai, Y.; Sato, T.

    2013-12-01

    A subducting seamount which has a height of about 3 km was revealed off Ibaraki in the Japan Trench by a seismic survey (Mochizuki et al., 2008). Mochizuki et al. (2008) also interpreted that interplate coupling was weak over the seamount because seismicity was low and the slip of the recent large earthquake did not propagate over it. To carry out further investigation, we deployed dense ocean bottom seismometers (OBSs) array around the seamount for about a year. During the observation period, seismicity off Ibaraki was activated due to the occurrence of the 2011 Tohoku earthquake. The southern edge of the mainshock rupture area was considered to be located around off Ibaraki by many source analyses. Moreover, Kubo et al. (2013) proposes the seamount played an important role in the rupture termination of the largest aftershock. Therefore, in this study, we try to understand about spatiotemporal variation of seismicity around the seamount before and after the Mw 9.0 event as a first step to elucidate relationship between the subducting seamount and seismogenic behavior. We used velocity waveforms of 1 Hz long-term OBSs which were densely deployed at station intervals of about 6 km. The sampling rate is 200 Hz and the observation period is from October 16, 2010 to September 19, 2011. Because of the ambient noise and effects of thick seafloor sediments, it is difficult to apply methods which have been used to on-land observational data for detecting seismicity to OBS data and to handle continuous waveforms automatically. We therefore apply back-projection method (e.g., Kiser and Ishii, 2012) to OBS waveform data which estimate energy-release source by stacking waveforms. Among many back-projection methods, we adopt a semblance analysis (e.g., Honda et al., 2008) which can detect feeble waves. First of all, we constructed a 3-D velocity structure model off Ibaraki by compiling the results of marine seismic surveys (e.g., Nakahigashi et al., 2012). Then, we divided a target area into small areas and calculated P-wave traveltimes between each station and all small areas by fast marching method (Rawlinson et al., 2006). After constructing theoretical travel-time tables, we applied a proper frequency filter to the observed waveforms and estimated seismic energy release by projecting semblance values. As the result of applying our method, we could successfully detect magnitude 2-3 earthquakes.

  4. Mass-transport deposits in the northern Ecuador subduction trench: Result of frontal erosion over multiple seismic cycles

    NASA Astrophysics Data System (ADS)

    Ratzov, Gueorgui; Collot, Jean-Yves; Sosson, Marc; Migeon, Sebastien

    2010-07-01

    Investigations of Mass-Transport Deposits (MTDs) and turbidite deposition in the confined North Ecuador subduction trench provide access to paleoseismic information and insights into long-term mechanisms for frontal tectonic erosion at a convergent margin. The studied trench has been the site of four great subduction earthquakes (7.7 ? Mw ? 8.8) during the 20th century. The trench is isolated from major continental sediment input, so that investigated MTDs and turbidites are considered of local origin. Swath bathymetry, seismic reflection and Chirp data, together with sedimentary cores and 14C dating revealed that seven MTDs were emplaced in distinct trench sub-basins since 23 kyr, and 27 turbidites deposited in the southernmost trench sub-basin since 4.9 kyr. Our analysis shows that six MTDs were derived from the margin, while a single one stemmed from the outer trench wall. Temporal correlations between MTDs emplaced within trench sub-basins separated by a structural saddle, indicate that the seven MTDs were emplaced during five main events. Three were triggered locally and tentatively dated 5.8, 1.6 kyr and Recent, whereas four were emplaced in distinct trench sub-basins as a result of two regional events at 22.6 and 15.4 kyr. None of the MTDs occurred during the fast stage of the last sea-level rise ( 13 to 8 kyr). However, dissociation of gas hydrates during the last 8 kyr-stage of slow sea-level rise might have contributed to trigger the three youngest MTDs. The large 1.5-13.5 kyr return time of the MTDs contrasts with that of 189 yr of the turbidites. The later is consistent with the 73 yr return time of two local Mw ? 8.2 earthquakes, implying that turbidites might have been triggered by large earthquakes. The very large MTDs return time is attributed to long-term deformation processes and mechanical weakening of the margin outer wedge, in response to repeated variations in basal friction, pore pressure and margin extensional/contraction strain over multiple earthquake cycles. This process contributes to short-term frontal erosion, the rate of which is estimated to be 8.6 10 - 3 km 3/kyr/km, since at least 15.4 kyr.

  5. GPS/Acoustic Observations Along the Japan Trench for Postseismic Deformation After the 2011 Tohoku-Oki Earthquake

    NASA Astrophysics Data System (ADS)

    Kido, M.; Tomita, F.; Osada, Y.; Fujimoto, H.; Hino, R.; Ohta, Y.; Iinuma, T.; Azuma, R.; Wada, I.

    2014-12-01

    After the 2011 Tohoku-Oki Earthquake, we realized that the interplate coupling can be extended to the trench even for subducting slab of great age. Monitoring the recovering process of the coupling after the earthquake may provide important knowledge on the total perspective of this great earthquake. For this purpose, Japanese government decided to strengthen the monitoring systems by means of seafloor geodesy. Constructing a GPS/Acoustic network along the trench is one of these projects. Before the earthquake, only five (Japan Coast Guard) and three (Tohoku Univ.) benchmarks were working off-Tohoku area, however, in 2012, we have newly installed up to 20 benchmarks along the trench, especially close to the trench. We took place GPS/Acoustic surveys after the installation and have carried out four times of campaign surveys until 2013 (FY). Not all the benchmarks were measured in each campaign, however, three times of campaigns were made for most of the benchmarks ranging 1 or 1.5 years. Because of such a short period of data accumulation and hard conditions on the accuracy due to great depth (>5000 m) than before, the estimated errors in the obtained displacement vectors are typically 5-10 cm for most of the benchmarks, which are not enough to clearly address the postseismic deformation. Nevertheless, some benchmarks, one is at relatively close to the coast and the other is on the incoming Pacific plate off-Miyagi, show significant WNW-ward movements up to 17 cm/yr, which is much larger than interseismic subducting rate. This gives a strong constraint to evaluate the viscoelastic relaxation process after the earthquake. More campaign surveys are planned in the next year to accumulate the data for improvement of the total accuracy in the displacement vectors for all the benchmarks. In addition, we are dedicated in improving the analytic technique to reduce error sources, such as acoustic signal processing and sound speed correction to lessen the uncertainty in each campaign data. In the presentation, we will introduce our measured results above and additional benchmarks applying the analytical improvement.

  6. Variations of short-term slow slip events along the Nankai Trough to the Ryukyu Trench, southwest Japan

    NASA Astrophysics Data System (ADS)

    Nishimura, T.

    2013-12-01

    ETSs (Episodic Tremor and Slips) occur at a depth of 30-40 km on the plate interface of the subducting Philippine Sea plate along the Nankai trough, southwest Japan (e.g., Obara, 2010). Low-frequency tremors along the Nankai Trough suddenly disappeared southwest of the Bungo Channel between Shikoku and Kyushu, which is a junction between the southwest Honshu arc along the Nankai Trough and the Ryukyu arc along the Ryukyu Trench. Any short-term SSEs (slow slip events) have never been reported southwest of the Bungo Chanel, though Mw~7 long-term SSEs repeated in the Bungo Channel every ~6 years (e.g., Ozawa et al., 2013). Here, we apply a SSE detection method using continuous GNSS data (Nishimura et al., 2013) to find out SSEs in a region along the Nankai Trough to the Ryukyu Trench. We also compare the detected SSEs with epicenters of low-frequency tremors, very-low-frequency earthquakes (VLFEs), and regular earthquakes and discuss along-trench variations on short-term SSE characteristics. Daily coordinates of ~800 GEONET GNSS stations in southwest Japan were used to detect displacements caused by short-term SSEs. We divided a whole region into three regions for spatial filtering to reduce a noise of GNSS data. We fitted a step function with a linear trend to spatial-filtered daily coordinates to detect significant displacements in a direction opposite to the relative plate motion between the Philippine Sea plate and southwest Japan. If Akaike's information criterion expressing data fits exceeds a threshold, three-dimensional displacements were inverted to estimate a rectangular fault along the plate interface. We found more than 200 probable short-term SSEs in the region over 16 years. In western Shikoku, we detected 49 Mw ~6.0 ETSs. SSEs with a similar magnitude are found at the same depth range in northeast Kyushu without tremors although the number of detected SSEs is only 10. In the further southwest region, we found several shallow SSEs at a depth of 10-40 km. Some of them accompany regular earthquakes with the maximum magnitude of ~5. A shallow Mw~6.4 SSE was detected east off southern Kyushu around January 27, 2010. VLFEs were coincidentally activated in a shallow adjacent region near the trench. Although a resolving power of SSEs along the Ryukyu arc is limited due to sparse GNSS stations, we found series of repeated SSEs near Kikaijima and Okinawa Islands in a depth of 10-30 km. A cumulative slip of the estimated SSEs over 16 years suggests along-trench variations of SSEs distribution. A band of large (> 10 cm) slip at a depth of 30-40 km stretches from Shikoku to northeast Kyushu which is ~50 km southwest of the western limit of the tremors. However, the cumulative slip in the Bungo channel and northeast Kyushu is one-third of that in western Shikoku. No shallow slip of SSEs was found from Shikoku to northeast Kyushu. Shallow separated patches of SSEs exist in a region southwest of southern Kyushu along the Ryukyu trench. The variation of SSEs may be related with that of large megathrust earthquakes, that is, repeated Mw>8 and few earthquakes along the Nankai Trough and the Ryukyu Trench, respectively.

  7. Fault plane orientations of intermediate-depth and deep-focus earthquakes in the Japan-Kuril-Kamchatka subduction zone

    NASA Astrophysics Data System (ADS)

    Warren, Linda M.; Baluyut, Elena C.; Osburg, Timothy; Lisac, Kristen; Kokkinen, Siiri

    2015-12-01

    In the northwestern Pacific, the Pacific plate subducts to the west at the Japan, Kuril, and Kamchatka trenches. Throughout most of the subduction zone, the subducting slab is planar and dipping at an angle of 30°-60°, with the exception of a fold in the southern Kuril segment. To investigate how the slab deforms in response to the applied forces and which mechanism generates the earthquakes, we analyze the rupture properties of 111 large (MW≥5.7) intermediate-depth and deep-focus earthquakes (60-656 km depth) from 1990 to 2014 in the Japan-Kuril-Kamchatka subduction zone. For each earthquake, we use rupture directivity to estimate rupture direction and rupture speed and to distinguish the fault plane from the auxiliary plane of the focal mechanism. Seventy six percent of the earthquakes with sufficient station coverage are well modeled by unilateral rupture propagation. The estimated rupture speeds range from zero to supershear. The estimated rupture directions allow identification of the fault plane as the more horizontal nodal plane for 30 earthquakes, while an additional 11 earthquakes rupture toward the intersection of the nodal planes, so the fault plane cannot be identified. Combining our newly identified fault planes with previously identified fault planes in the region, we observe that in planar slab segments, most earthquakes slip along a dominant fault orientation. For a steeply dipping slab, this orientation is subhorizontal. In more sharply bent slab segments, such as the Kuril fold, deformation is accommodated along more variable fault orientations, including subvertical faults. The correlation of slab geometry with fault orientation suggests that the local stress field controls fault orientations.

  8. Global Positioning System (GPS) and GPS-Acoustic Observations: Insight into Slip Along the Subduction Zones Around Japan

    NASA Astrophysics Data System (ADS)

    Nishimura, Takuya; Sato, Mariko; Sagiya, Takeshi

    2014-05-01

    The global positioning system (GPS) is one of the most powerful tools available for observation of Earth's surface deformation. In particular, coseismic, postseismic, slow transient, and interseismic deformation have all been observed globally by GPS over the past two decades, especially in subduction zones. Moreover, GPS-acoustic techniques have been developed for practical use in the past decade, allowing observation of offshore deformation immediately above slip regions. Here, we describe the application of GPS and GPS-acoustic observations to the detection of deformation due to plate boundary slip for interplate earthquakes as well as afterslip and slow slip events in subduction zones around Japan, where geodetic data coverage is particularly dense. The data demonstrate temporally variable strain accumulation in the source region of the 2011 Mw 9.0 Tohoku-oki earthquake, and observation of the huge slip of the Tohoku-oki earthquake near the trench using GPS-acoustic methods has considerably advanced our knowledge of stress release and accumulation in this subduction zone.

  9. Multiple-scale hydrothermal circulation in 135 Ma oceanic crust of the Japan Trench outer rise: Numerical models constrained with heat flow observations

    NASA Astrophysics Data System (ADS)

    Ray, Labani; Kawada, Yoshifumi; Hamamoto, Hideki; Yamano, Makoto

    2015-09-01

    Anomalous high heat flow is observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, where the old Pacific Plate (˜135 Ma) is subducting. Individual heat flow values range between 42 and 114 mW m-2, with an average of ˜70 mW m-2. These values are higher than those expected from the seafloor age based on thermal models of the oceanic plate, i.e., ˜50 mW m-2. The heat flow exhibits spatial variations at multiple scales: regional high average heat flow (˜100 km) and smaller-scale heat flow peaks (˜1 km). We found that hydrothermal mining of heat from depth due to gradual thickening of an aquifer in the oceanic crust toward the trench axis can yield elevated heat flow of the spatial scale of ˜100 km. Topographic effects combined with hydrothermal circulation may account for the observed smaller-scale heat flow variations. Hydrothermal circulation in high-permeability faults may result in heat flow peaks of a subkilometer spatial scale. Volcanic intrusions are unlikely to be a major source of heat flow variations at any scale because of limited occurrence of young volcanoes in the study area. Hydrothermal heat transport may work at various scales on outer rises of other subduction zones as well, since fractures and faults have been well developed due to bending of the incoming plate.

  10. Heat flow and bending-related faulting at subduction trenches: Case studies offshore of Nicaragua and Central Chile

    NASA Astrophysics Data System (ADS)

    Grevemeyer, Ingo; Kaul, Norbert; Diaz-Naveas, Juan L.; Villinger, Heinrich W.; Ranero, Cesar R.; Reichert, Christian

    2005-07-01

    Detailed heat flow surveys on the oceanic trench slope offshore Nicaragua and Central Chile indicate heat flow values lower than the expected conductive lithospheric heat loss and lower than the global mean for crust of that age. Both areas are characterised by pervasive normal faults exposing basement in a setting affected by bending-related faulting due to plate subduction. The low heat flow is interpreted to indicate increased hydrothermal circulation by the reactivation and new creation of faults prior to subduction. A previous global approach [1] [Stein C.A., Heat flow and flexure at subduction zones, Geophys. Res. Lett. 30 (2003) doi:10.1029/2003GL018478] failed to detect similar features in the global but sparse data set. Detailed inspection of the global data set suggests that the thickness of the sedimentary blanket on the incoming plate is an important factor controlling the local hydrogeological regime. Areas with a relatively thick sedimentary cover do not show any heat flow anomaly while areas where normal faulting exposes basement suffer from increased hydrothermal activity. Both geochemical data from arc volcanoes and seismological evidence from intra slab events suggest that the flux of water into the deep subduction zone is larger in areas characterised by reactivated hydrothermal circulation. It is reasonable to assume that the larger water flux is caused by serpentinization of the upper mantle, facilitated by bending-related faults cutting into the upper mantle.

  11. Revisit of basal effective friction and pore pressure for Japan trench from topographic point of view

    NASA Astrophysics Data System (ADS)

    Koge, H.; Fujiwara, T.; Kodaira, S.; Sasaki, T.; Kameda, J.; Hamahashi, M.; Hamada, Y.; Kimura, G.

    2013-12-01

    2011 Tohoku-oki earthquake (Mw9.0) produced a fault rupture, extending to the shallow part of the Japan Trench. Based on the bathymetry difference before and after the earthquake, it is demonstrated that the seafloor on outermost landward slope moved ~50 m east-southeastward towards the trench and uplifted ~7 to 10 m. Although the mechanism of the fault rupture is not clear, deformation and frictional properties beneath the forearc are the key to elucidate this important issue. Kimura et al (2012) focused on seismic reflection data along one seismic transect at the Japan Trench (name the transect), and calculated the basal effective friction of the plate boundary by using the critical taper theory. Limited profiles from narrow area, however, never represent general friction property of the plate boundary in the Japan trench. Therefore, several profiles are examined to investigate for the better understanding along-trench variation of the basal frictional properties. Bathymetric and seismic reflection data were taken before and after the Tohoku-oki earthquake to obtain the following angles; slope angle of upper surface, basal dip of the outer wedge. Acuired angles are limited to only the lowest trench slope of the Japan Trench. The limitation enables us to treat the wedge as a more uniform body than would have been achieved with the whole range of seismic cross sections. Applying the critical taper theory to the individual seismic cross sections, force balance among the interior and base of the wedge, fluid pressure ratio, and the basal effective friction of the plate boundary are calculated. Additionally, by using "earth pressure theory", we estimated basal effective friction of ~0.15 under the assumption that branching faults act as back-stops in lower slope areas. References Cubas et al., Geophysical Research Letters: DOI:10.1002/grl.50682 Fujiwara et al., Science 2 December 2011: Vol. 334 no. 6060 p. 1240 Wang and Hu, Journal of Geophysical Research, v.111, p1-16, 2006 Kimura et al., Earth and Planetary Science Letters, v339-340,p32-45, 2012, Kodaira et al., Nature Geoscience, v5, p646-650, 2012 Sasaki Tomoyuki, Japan Geoscience Union abstract, 2003

  12. Origins and evolution of the Gagua Ridge bathymetric feature: A Possible example of failed subduction competition with the Manila trench

    NASA Astrophysics Data System (ADS)

    Eakin, D. H.; Lavier, L. L.; McIntosh, K. D.; Van Avendonk, H. J.

    2013-12-01

    Three E-W tomographic velocity models combined with coincident multi-channel seismic reflection data (MCS) allow us to develop a new geodynamic model to describe the origin and evolution of the Gagua Ridge bathymetric feature in the West Philippine Basin. Data were acquired in the Huatung Basin offshore Taiwan to the southeast during the 2009 TAIGER program. This region is largely dominated by the eastward subduction of the South China Sea along the Manila trench to the southwest, arc-continent collision between the North Luzon volcanic arc and the passive Chinese continental margin to the northwest, and northward subduction of the West Philippine Sea Basin oceanic crust along the Ryukyu trench to the north. While the tectonic framework and development of this complex plate boundary remains enigmatic, we present tomographic evidence showing what appears to be oceanic crust to the east of Gagua Ridge underthrust to the west beneath the oceanic crust of the Huatung Basin to a depth of 15-20 km. This observation is significant as it possibly represents a failed subduction event in the past that may have coincided, competed with, and was subsequently abandoned in favor of subduction along the Manila trench. Underthrusting of this nature is likely the result of oblique compression along structures such as transform fracture zones inherited from the seafloor spreading that are capable of juxtaposing crust of differing age and thermal maturity against one another. One of the keys to obtaining a geodynamic understanding of this process and how it may have evolved in the past is to document the crustal structure of the Gagua Ridge bathymetric feature from north to south. In this area we acquired 3 wide-angle seismic datasets, T1B at 20.6°N, T2 at 21.6°N, and T4A at 22.8°N along with coincident MCS data that bisect Gagua Ridge E-W to constrain the velocity structure at depth and deformation observed near the seafloor. These models illuminate an interesting structural trend beneath Gagua Ridge, with little to no apparent underthrusting in the south along T1B, to more than 15km of apparent underthusting along T2 and T4A to the north. These observations are supported by coincident MCS data that show significant deepening and extensional deformation of the ocean crust on the east flank of Gagua Ridge and uplift and compressional deformation of oceanic crust on the western flank. These observations are also consistent with new geodynamic modeling that indicates the initiation of subduction is likely the result of oblique convergence along relict fracture zones formed as the result of seafloor spreading. The similar geometry and orientation of the Gagua Ridge with respect to the Manila trench combined with the evidence for eastward underthrusting of ocean crust suggest this feature may be an incipient subduction system that competed with, and was abandoned in favor of the Manila subduction zone to the west.

  13. Groundwater, possibly originated from subducted sediments, in Joban and Hamadori areas, southern Tohoku, Japan

    NASA Astrophysics Data System (ADS)

    Togo, Yoko S.; Kazahaya, Kohei; Tosaki, Yuki; Morikawa, Noritoshi; Matsuzaki, Hiroyuki; Takahashi, Masaaki; Sato, Tsutomu

    2014-12-01

    We studied the origin of deep groundwater in the Joban and Hamadori areas in southern Tohoku, Japan, based on δD, δ18O, 129I/I, 36Cl/Cl, and 3H concentrations. Deep groundwater was collected from the basement rocks (Cretaceous granite) and from the margin of the Joban sedimentary basin (latest Cretaceous to Quaternary sedimentary rocks deposited on the basement rocks). We sampled groundwater pumped from depths ranging from 350 to 1,600 m in these areas. A hypothetical end-member of deep groundwater was estimated from the relationship between δ18O and Cl concentrations, and our data reveal a much higher iodine concentration and lower Br and Cl concentrations than found in seawater. The iodine ages inferred from 129I/I are quite uniform and are about 40 Ma and 36Cl/Cl almost reached the secular equilibrium. The relationship between iodine and Cl can be explained by mixing a hypothetical end-member with meteoric water or seawater. Moreover, the I/Cl ratio increases linearly with increasing water temperature. The water temperature was high in Joban, with a maximum of 78°C at a depth of 1,100 m. The geothermal gradient in the Joban basin is 18°C km-1, and the temperature even at a depth of 3 km in the basin was not high enough to supply thermal water to the sampling sites. Thus, sedimentary rocks in the Joban basin are unlikely to be the source of iodine in the deep groundwater. Several active faults such as the Futaba Fault are developed in and around the studied areas. The Iwaki earthquake occurred 1 month after the 2011 Tohoku-oki earthquake, and normal-fault type surface ruptures formed and discharged hot groundwater in Joban. The deep groundwater we studied probably came up through the basement rocks from greater depths. There are no sedimentary rocks younger than Tertiary age beneath the pre-Cretaceous basement rocks, and the subducted sediments in the Japan Trench are a possible source of iodine in the groundwater. The Joban and Hamadori areas may be an ideal window to look into the water circulation in the forearc of the Tohoku subduction zone.

  14. Flat Slab Subduction, Trench Suction, and Craton Destruction: Comparison of the North China, Wyoming, and Brazilian Cratons

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy; Windley, Brian; Wang, Lu; Wang, Zhensheng; Li, Xiaoyong; Zhu, Peimin

    2015-04-01

    We define and test a unifying plate tectonic driving mechanism that explains the characteristics of the destruction of cratonic lithospheric roots. We document and model the relationships between flat slab subduction, trench suction, and craton destruction, using examples from the North China and Wyoming cratons, each of which locally lost approximately 100 km of their lithospheric roots in the Cretaceous and which show spatio-temporal relationships with episodes of flat slab subduction in the mantle transition zone associated with deep mantle hydration, coupled with slab rollback and concomitant influx of fertile mantle material to accommodate the space created by slab rollback. A similar process has more recently operated along the western side of the Brazilian craton where it is thrust beneath the thickened crust of the Andes in an area of trench rollback. The importance of the mutual interaction between these processes for destruction of cratonic roots may be greater than currently perceived. Together with the other processes of subduction erosion and arc subduction, larger amounts of continental lithosphere may have been subducted or otherwise returned to the sub-lithospheric mantle than previously appreciated. When oceanic lithosphere subducts, it hydrates the upper mantle beneath an arc from well-known dehydration reactions. However, some hydrous phases (e.g., Phase A, Phase E, and -γt and β-phase olivine) are stable to much greater depths and dehydrate even when a slab is in the mantle transition zone. It is estimated that 40% of the water subducted in hydrated oceanic crust, mantle, sediments, and subducted continental material reaches the mantle transition zone between 410 and 660 km. For instance lawsonite may contain up to 11% water, and is stable up to 11 GPa or about 300 km and serpentinites can contain up to 13% water and are stable up to 7 GPa, and after conversion to denser hydrous phases such as β-phase olivine they can be stable up to 50 GPa, well past the mantle transition zone. With increasing temperature (i.e., more time in the transition zone for deep flat slabs) these phases decompose to less hydrous wadsleyite and ringwoodite with 2.2-3.3 wt % water, releasing water to the deep mantle, which rises and hydrates the overlying mantle. During flat slab subduction dehydration reactions therefore add water to the overlying mantle wedge. As the subducting slabs roll back, they suck in mantle material to infill the void space created by the slab roll back, and this fertile mantle becomes hydrated. The roll-back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting, generating new magmas that gradually destroy the roots of the overlying craton through melt-peridotite reactions. Calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through this mechanism, warranting a re-evaluation of continental growth rates with time.

  15. Runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope: The tsunamigenesis of the 2011 Tohoku earthquake off the east coast of northern Japan

    NASA Astrophysics Data System (ADS)

    Kimura, Gaku; Hina, Shoko; Hamada, Yohei; Kameda, Jun; Tsuji, Takeshi; Kinoshita, Masataka; Yamaguchi, Asuka

    2012-07-01

    The gigantic 2011, March 11 Mw 9 Tohoku earthquake is examined from the viewpoint of the pre-seismic forearc structure, the seismic reflection properties of a megathrust around the usual up-dip limit of the seismogenic zone, the thermal state of a shallow subduction zone, and the dehydration of underthrust sediments. At the Japan Trench the Pacific Plate is subducting westward beneath northeast Japan at a dip angle of 4.6. The middle and lower slopes of the landward side dip eastward at angles of 2.5 and 8.0, respectively. The forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states, respectively, based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes. The rapid uplift of the forearc that caused the 2011 Tohoku tsunami may have been associated with this internal deformation of the prism. The critical state of the prism indicates that the effective basal friction (?b') of the plate boundary megathrust is <0.03 for the middle prism and >0.08 for the lower prism. The megathrust, especially under the middle slope, is characterized by a prominent reflector with negative polarity. One of multiple possible explanations is that the megathrust hosts highly pressurized fluids. Underthrust sediments in this part of the Japan Trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay. The dehydration kinetics of opal-A to quartz, the clay transformation of smectite-illite, and the thermal structure of the Japan Trench suggest that maximum dehydration of the sediments would take place at 50-60 km horizontally from the deformation front, where the temperature along the megathrust is 100-120 C. The zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity. We hypothesize a possible free slip along this portion of the megathrust during the 2011 Tohoku earthquake, caused by anomalously high fluid pressure resulting from fluid accumulation over centuries.

  16. Subduction of a Late Cretaceous Seamount of the Louisville Ridge at the Tonga Trench: A model of normal and accelerated tectonic erosion

    NASA Astrophysics Data System (ADS)

    Ballance, Peter F.; Scholl, David W.; Vallier, Tracy L.; Stevenson, Andrew J.; Ryan, Holly; Herzer, Richard H.

    1989-10-01

    The hotspot-generated Louisville Ridge is a 4000-km chain of seamounts (typically 2-2.5 km high and 10-40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26S) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2-3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1-2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestrichtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount The principal effects of hotspot-ridge collision with a sediment-starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell, (3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc-derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the 37,000 km of material which fills subducting grabens each million years is arc-derived volcanic and pelagic sediment.

  17. Direct-path acoustic ranging across the Japan Trench axis, Adjacent to the Large Shallow Thrusting in the 2011 Tohoku-Oki earthquake

    NASA Astrophysics Data System (ADS)

    Osada, Y.; Kido, M.; Ito, Y.; Iinuma, T.; Fujimoto, H.; Hino, R.

    2014-12-01

    Seafloor geodetic data, i.e. GPS/acoustic measurement and continuous seafloor pressure monitoring, brought important evidences showing that the 2011 Tohoku-oki earthquake (Mw 9.0) caused huge (> 50 m) coseismic slip near the Japan Trench. The postseismic behavior of the large slipped area is required to clarify to understand why large amount seismic slip could occur there. We started making direct-path acoustic ranging across the trench axis to reveal the convergence rate between the subducting Pacific and overriding continental plates. We expect the change of the baseline length across the trench axis, the plate boundary, reflects the slip rate at the shallow megathrust, which is difficult to estimate only from other geodetic observations largely affected by intraplate deformation caused by the postseismic viscoelastic relaxation process. ?To this end, we developed an ultra-deep seafloor acoustic ranging system. Our previous ranging systems have been designed to measure baseline length ~ 1 km and to be deployed up to 7,000 m water-depth (Osada et al., 2008, 2012). In order to realize the measurement across the Japan Trench, we improved this system to enhance range of acoustic ranging as well as operational depth of instruments. The improved system was designed to allow acoustic ranging up to 3 km and to be durable under the high-pressure equivalent to water depth of 9,000 m. In May 2013, we carried out a test deployment of the new ranging system. The system is composed of three seafloor instruments equipped with precision transponder (PXPs). Two of the PXPs were set on the landward slope of the Japan Trench, where large coseismic slip happened in 2011. Another PXP was deployed on the seaward side of the trench so that the baseline change associated with the slip on the plate boundary fault, if any, can be detected. Continuous records of baseline lengths were successfully obtained for four months. The repeatability of the distance measurements was about 20 mm for each of the two baselines. Although the duration of the observation was not long enough to estimate precise rate of baseline length changes, it is unlikely that the shortening rates of the baseline lengths exceed the rate of plate convergence (~ 8 cm/a). The results do not support occurrence of evident afterslip along the shallow plate boundary fault in 2013.

  18. Major Existence of Very Low Frequency Earthquakes in Background Seismicity Along Subduction Zone of South-western Japan

    NASA Astrophysics Data System (ADS)

    Ishihara, Y.

    2003-12-01

    The condense, high quality and equalized broadband seismic network provided us to recognize the variety of seismic sources. The active volcanoes excite seismic waves with various frequency characteristics. Some cases show the long period seismic waves greater than 10 sec associates with volcanic activities. The tectonic seismic events originated at the close to trench zone are frequently lack of high frequency, greater than 1 Hz, seismic wave component. Meanwhile, the many low frequency earthquakes and tremors whose sources are not explicated are occurred in lower crust and subcrustal region. The subduction zone of Philippine Sea plate in south-western Japan is actively genetic area of low frequency earthquake group. The broadband seismic array of Japan region observed unknown long period ground motions. The seismograms are higher amplitude between 10 and 30 sec period than ground noise level. The earthquake JMA and USGS catalogues don_ft list about these long period seismograms. The arrival order of wave packet means that these events locate subduction zone around Japan. The hypocenters of unknown events are estimated by arrival times of vertical peak amplitude using the assumption that the ground motion dominates Rayleigh wave. The more detailed determination of major events is performed by combined technique for moment tensor inversion and grid search. The moment magnitude of uncatalogued event is greater than 3.5 because of the detection limitation. The largest event is distributed to about 4.5 Mw level and special event is greater than 5.0. The frequency characteristics show that source time is 7 to 20 sec by comparison with synthetic seismograms. We call these uncatalogued events _gvery low frequency earthquake_h. The hypocenters are located to two kinds of zones along the Philippine Sea subducting plate in south-western Japan. The one zone is very close to the trough. The seismicity listed by earthquake catalogues is low level in the zone and hypocenters are distributed in island arc side. The very low frequency earthquakes occur in outer area of seismic generation zone. The other zone is coincided with extended zone generating low frequency seismic events. Very low frequency earthquake occurs intermittently in limited area. Frequently, activity increases for some days and swarm type activity generates occasionally. To evaluate integrated seismicity for anomalous events, same class magnitude, greater than 3.0, events occurred in same and surrounding area are picked up from JMA earthquake catalogue. The analysis shows that seismicity of very low frequency earthquakes is comparative with one of normal earthquakes in background activity. We concluded that very low frequency earthquake is one of factors playing seismically important role.

  19. 3-D Simulation of Steady Plate Subduction with Tectonic Erosion: Current Crustal Uplift and Free-Air Gravity Anomaly in Northeast Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, Chihiro; Sato, Toshinori; Matsu'Ura, Mitsuhiro

    2008-04-01

    Free-air gravity anomaly in plate subduction zones, characterized by island-arc high, trench low and outer-rise gentle high, reflects the cumulative effects of long-term crustal uplift and subsidence. In northeast Japan the island-arc high of observed free-air gravity anomaly takes its maximum about the eastern coastline. On the other hand, the current vertical crustal motion estimated from geological and geomorphological observations shows a gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Such a discrepancy in spatial patterns between the free-air gravity anomaly and current vertical crustal motion can be ascribed to a change in the mode of crustal uplift and subsidence associated with the initiation of tectonic erosion at the North American-Pacific plate interface. We developed a realistic 3-D simulation model of steady plate subduction with tectonic erosion in northeast Japan on the basis of elastic/viscoelastic dislocation theory. Through numerical simulations with this model we found that simple steady plate subduction brings about the crustal uplift characterized by island-arc high with its maximum about the eastern coastline, while steady plate subduction with tectonic erosion, which is represented by the landward retreat of the plate interface, brings about gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Therefore, if we suppose that tectonic erosion started 3 4 million years ago after the long duration of simple steady plate subduction, we can consistently explain both patterns of free-air gravity anomaly and current crustal uplift in northeast Japan.

  20. Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches

    NASA Astrophysics Data System (ADS)

    Lefeldt, M.; Ranero, C. R.; Grevemeyer, I.

    2012-05-01

    Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2-3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.

  1. 3-D simulation for the tectonic evolution around the Kanto Region of Japan using the kinematic plate subduction model

    NASA Astrophysics Data System (ADS)

    Hashima, A.; Sato, T.; Ito, T.; Miyauchi, T.; Kameo, K.; Yamamoto, S.

    2011-12-01

    In the Kanto region of Japan, we can observe one of the most active crustal deformations on the earth. In the southern part of the Boso peninsula to the south, the uplift rate is estimated to be 5 mm/yr from the height of marine terraces. From geological evidence, the Kanto mountains to the west are considered to uplift at 1mm/yr. In contrast, the center part of the Kanto region is stable or subsiding, covered by the Holocene sediments. The depth of the basement reaches 3 km at the deepest. Vertical deformation in the timescale of 1 Myr is being revealed by the analysis of the recent seismic reflection experiments compared with the heights of the dated sediment layers exposed on land. These crustal deformation occurs in a highly complex tectonic setting with four plates interacting with each other: beneath Kanto, situated on the Eurasian and North American plates, the Philippine sea plate subducts and the Pacific plate further descends beneath the North American and Philippine sea plates, forming the unique trench-trench-trench triple junction on the earth. In addition, the Izu-Bonin (Ogasawara) arc on the Philippine sea plate is colliding with the Japan islands due to the buoyancy of the arc crust. At the plate boundaries near the Izu-Bonin arc, large interplate earthquakes occurred at the Sagami trough in 1703 and 1923 (Kanto earthquake) and at the Nankai trough in 1707, 1854 and 1944. To reveal the crustal deformation under these plate-to-plate interactions, we use the kinematic plate subduction model based on the elastic dislocation theory. This model is based on the idea that mechanical interaction between plates can rationally be represented by the increase of the displacement discontinuity (dislocation) across plate interfaces. Given the 3-D geometry of plate interfaces, the distribution of slip rate vectors for simple plate subduction can be obtained directly from relative plate velocities. In collision zones, the plate with arc crust cannot easily descend because of its buoyancy. This can be represented by giving slip-rate deficit. When crustal deformation occurs, it also causes change in geometry of the plate boundary itself. Iterating this effect sequentially backward in time, we can reconstruct the past plate boundary geometry and past crustal deformation fields. Using the above model, we estimate the long-term slip-rate distribution due to plate subduction/collision to explain the crustal deformation in Kanto obtained from geological and geomorphological studies. The basic deformation pattern of the basin-forming movement in the Kanto plain and uplifts in the southern Boso peninsula and in the Kanto and Akaishi mountains cannot be explained by the collision restricted to the Izu peninsula only. It is necessary to assume wider collision extended to the neighboring Sagami and Suruga trough, which is consistent to the width of the arc crust of the Izu-Bonin arc. However, the degree of the collision is relatively small in these areas where large interplate earthquake occurs. The effect of temporal change in geometry of the plate interfaces is not so large in the timescale of 1 Myr.

  2. Reflections from midcrustal rocks within the Mesozoic subduction complex near the eastern Aleutian Trench.

    USGS Publications Warehouse

    Fisher, M.A.; Von Huene, R.; Smith, G.L.

    1987-01-01

    Seismic reflection data show that highly reflective rocks make up the midcrust of the convergent margin adjacent to the eastern Aleutian Trench. These rocks form an arch that strikes obliquely across the strongly expressed NE-SW structural grain of exposed Mesozoic rocks. Deep reflections could be from underplated rocks that have been arched by the imbrication or underplating of strata below the reflective rocks. Speculates that one band of reflections that rises toward but does not reach the surface is from the Eagle River thrust fault, which separates Late Cretaceous melange from deformed turbidite sequences of the same age. -from Authors

  3. Recurrence of postseismic coastal uplift, Kuril subduction zone, Japan

    USGS Publications Warehouse

    Kelsey, H.; Satake, K.; Sawai, Y.; Sherrod, B.; Shimokawa, K.; Shishikura, M.

    2006-01-01

    Coastal stratigraphy of eastern Hokkaido indicates that decimeters of coastal uplitt occurred repeatedly m the late Holocene. Employing radiocarbon dating and tephrochronology, we identify along a 100 km length of the Kuril subduction zone six uplift events since ???2,800 years B.P. Uplift events occur at the same frequency as unusually high tsunamis. Each coastal uplift event, which occurs on average every 500 years, is the product of decade-long post seismic deep slip on the down dip extension of the seismogenic plate boundary following an offshore multi-segment earthquake that generates unusually high tsunamis. Copyright 2006 by the American Geophysical Union.

  4. Anaerobic methanotrophic community of a 5346-m-deep vesicomyid clam colony in the Japan Trench

    PubMed Central

    Felden, J; Ruff, S E; Ertefai, T; Inagaki, F; Hinrichs, K-U; Wenzhöfer, F

    2014-01-01

    Vesicomyidae clams harbor sulfide-oxidizing endosymbionts and are typical members of cold seep communities where active venting of fluids and gases takes place. We investigated the central biogeochemical processes that supported a vesicomyid clam colony as part of a locally restricted seep community in the Japan Trench at 5346 m water depth, one of the deepest seep settings studied to date. An integrated approach of biogeochemical and molecular ecological techniques was used combining in situ and ex situ measurements. In sediment of the clam colony, low sulfate reduction rates (maximum 128 nmol mL−1 day−1) were coupled to the anaerobic oxidation of methane. They were observed over a depth range of 15 cm, caused by active transport of sulfate due to bioturbation of the vesicomyid clams. A distinct separation between the seep and the surrounding seafloor was shown by steep horizontal geochemical gradients and pronounced microbial community shifts. The sediment below the clam colony was dominated by anaerobic methanotrophic archaea (ANME-2c) and sulfate-reducing Desulfobulbaceae (SEEP-SRB-3, SEEP-SRB-4). Aerobic methanotrophic bacteria were not detected in the sediment, and the oxidation of sulfide seemed to be carried out chemolithoautotrophically by Sulfurovum species. Thus, major redox processes were mediated by distinct subgroups of seep-related microorganisms that might have been selected by this specific abyssal seep environment. Fluid flow and microbial activity were low but sufficient to support the clam community over decades and to build up high biomasses. Hence, the clams and their microbial communities adapted successfully to a low-energy regime and may represent widespread chemosynthetic communities in the Japan Trench. In this regard, they contributed to the restricted deep-sea trench biodiversity as well as to the organic carbon availability, also for non-seep organisms, in such oligotrophic benthic environment of the dark deep ocean. PMID:24593671

  5. Anaerobic methanotrophic community of a 5346-m-deep vesicomyid clam colony in the Japan Trench.

    PubMed

    Felden, J; Ruff, S E; Ertefai, T; Inagaki, F; Hinrichs, K-U; Wenzhöfer, F

    2014-05-01

    Vesicomyidae clams harbor sulfide-oxidizing endosymbionts and are typical members of cold seep communities where active venting of fluids and gases takes place. We investigated the central biogeochemical processes that supported a vesicomyid clam colony as part of a locally restricted seep community in the Japan Trench at 5346 m water depth, one of the deepest seep settings studied to date. An integrated approach of biogeochemical and molecular ecological techniques was used combining in situ and ex situ measurements. In sediment of the clam colony, low sulfate reduction rates (maximum 128 nmol mL(-1) day(-1)) were coupled to the anaerobic oxidation of methane. They were observed over a depth range of 15 cm, caused by active transport of sulfate due to bioturbation of the vesicomyid clams. A distinct separation between the seep and the surrounding seafloor was shown by steep horizontal geochemical gradients and pronounced microbial community shifts. The sediment below the clam colony was dominated by anaerobic methanotrophic archaea (ANME-2c) and sulfate-reducing Desulfobulbaceae (SEEP-SRB-3, SEEP-SRB-4). Aerobic methanotrophic bacteria were not detected in the sediment, and the oxidation of sulfide seemed to be carried out chemolithoautotrophically by Sulfurovum species. Thus, major redox processes were mediated by distinct subgroups of seep-related microorganisms that might have been selected by this specific abyssal seep environment. Fluid flow and microbial activity were low but sufficient to support the clam community over decades and to build up high biomasses. Hence, the clams and their microbial communities adapted successfully to a low-energy regime and may represent widespread chemosynthetic communities in the Japan Trench. In this regard, they contributed to the restricted deep-sea trench biodiversity as well as to the organic carbon availability, also for non-seep organisms, in such oligotrophic benthic environment of the dark deep ocean. PMID:24593671

  6. Earthquake generation cycles and tsunami simulations providing possible scenarios for Turkey (Marmara sea) and Japan (Nankai trough and Japan trench)

    NASA Astrophysics Data System (ADS)

    Hori, Takane; Yalciner, Ahmet; Ozel, Nurcan; Kilic, Irfan; Miyazaki, Shin'ichi; Hyodo, Mamoru

    2015-04-01

    In order to obtain comprehensive earthquake and tsunami scenarios for disaster assessment, numerical simulations of earthquake generation cycles and resultant tsunami generations have been performed in Japan. The occurrence of the 2011 Tohoku earthquake has realized us the necessity to consider all the possible scenarios without preconceptions. We have performed large-scale numerical simulations using Earth Simulator and K-computer for earthquake generation cycles along the Nankai trough, southwest Japan, where megathrust earthquakes with some segments have sequentially occurred. We have succeeded to reproduce various rupture pattern seen in historical data and geological evidences (such as tsunami deposit) being consistent with GEONET data during interseismic period. Using the results of such earthquake generation cycle simulations, we performed tsunami generation, propagation and inundation simulation. In Turkey, tsunami simulation methods and tsunami scenario database have been developed. In the research project of SATREPS -Earthquake and tsunami disaster mitigation in the Marmara region and disaster education in Turkey, we are applying such earthquake generation cycle and tsunami simulations to the North Anatolian fault system to obtain possible earthquake scenarios and to improve tsunami scenario data base for Sea of Marmara. For the modeling of the fault system, we will use observation results by the earthquake source modeling group in this project to improve the existing models. The earthquake scenarios will be used also for strong motion predictions by the group of seismic characterization and damage prediction. We will visualize the simulation results for disaster education. Furthermore, we will contribute to improve semi-realtime earthquake analyses and tsunami forecasting. In the presentation, we will show some recent simulation results of earthquake generation cycles and tsunamis for Turkey (Marmara sea) and Japan (Nankai trough and Japan trench). Acknowledgements: Support by Japan-Turkey Joint Research Project by JICA and JST on earthquakes and tsunamis in Marmara Region (SATREPS) is acknowledged.

  7. Variable Holocene deformation above a shallow subduction zone extremely close to the trench

    NASA Astrophysics Data System (ADS)

    Thirumalai, Kaustubh; Taylor, Frederick W.; Shen, Chuan-Chou; Lavier, Luc L.; Frohlich, Cliff; Wallace, Laura M.; Wu, Chung-Che; Sun, Hailong; Papabatu, Alison K.

    2015-06-01

    Histories of vertical crustal motions at convergent margins offer fundamental insights into the relationship between interplate slip and permanent deformation. Moreover, past abrupt motions are proxies for potential tsunamigenic earthquakes and benefit hazard assessment. Well-dated records are required to understand the relationship between past earthquakes and Holocene vertical deformation. Here we measure elevations and 230Th ages of in situ corals raised above the sea level in the western Solomon Islands to build an uplift event history overlying the seismogenic zone, extremely close to the trench (4-40 km). We find marked spatiotemporal heterogeneity in uplift from mid-Holocene to present: some areas accrue more permanent uplift than others. Thus, uplift imposed during the 1 April 2007 Mw 8.1 event may be retained in some locations but removed in others before the next megathrust rupture. This variability suggests significant changes in strain accumulation and the interplate thrust process from one event to the next.

  8. Variable Holocene deformation above a shallow subduction zone extremely close to the trench.

    PubMed

    Thirumalai, Kaustubh; Taylor, Frederick W; Shen, Chuan-Chou; Lavier, Luc L; Frohlich, Cliff; Wallace, Laura M; Wu, Chung-Che; Sun, Hailong; Papabatu, Alison K

    2015-01-01

    Histories of vertical crustal motions at convergent margins offer fundamental insights into the relationship between interplate slip and permanent deformation. Moreover, past abrupt motions are proxies for potential tsunamigenic earthquakes and benefit hazard assessment. Well-dated records are required to understand the relationship between past earthquakes and Holocene vertical deformation. Here we measure elevations and (230)Th ages of in situ corals raised above the sea level in the western Solomon Islands to build an uplift event history overlying the seismogenic zone, extremely close to the trench (4-40?km). We find marked spatiotemporal heterogeneity in uplift from mid-Holocene to present: some areas accrue more permanent uplift than others. Thus, uplift imposed during the 1 April 2007 Mw 8.1 event may be retained in some locations but removed in others before the next megathrust rupture. This variability suggests significant changes in strain accumulation and the interplate thrust process from one event to the next. PMID:26123872

  9. Friction properties of the plate boundary megathrust beneath the frontal wedge near the Japan Trench: an inference from topographic variation

    NASA Astrophysics Data System (ADS)

    Koge, Hiroaki; Fujiwara, Toshiya; Kodaira, Shuichi; Sasaki, Tomoyuki; Kameda, Jun; Kitamura, Yujin; Hamahashi, Mari; Fukuchi, Rina; Yamaguchi, Asuka; Hamada, Yohei; Ashi, Juichiro; Kimura, Gaku

    2014-12-01

    The 2011 Tohoku-Oki earthquake (Mw 9.0) produced a fault rupture that extended to the toe of the Japan Trench. The deformation and frictional properties beneath the forearc are keys that can help to elucidate this unusual event. In the present study, to investigate the frictional properties of the shallow part of the plate boundary, we applied the critically tapered Coulomb wedge theory to the Japan Trench and obtained the effective coefficient of basal friction and Hubbert-Rubey pore fluid pressure ratio (?) of the wedge beneath the lower slope. We extracted the surface slope angle and dcollement dip angle (which are the necessary topographic parameters for applying the critical taper theory) from seismic reflection and refraction survey data at 12 sites in the frontal wedges of the Japan Trench. We found that the angle between the dcollement and back-stop interface generally decreases toward the north. The measured taper angle and inferred effective friction coefficient were remarkably high at three locations. The southernmost area, which had the highest coefficient of basal friction, coincides with the area where the seamount is colliding offshore of Fukushima. The second area with a high effective coefficient of basal friction coincides with the maximum slip location during the 2011 Tohoku-Oki earthquake. The area of the 2011 earthquake rupture was topographically unique from other forearc regions in the Japan Trench. The strain energy accumulation near the trench axis may have proceeded because of the relatively high friction, and later this caused a large slip and collapse of the wedge. The location off Sanriku, where there are neither seamount collisions nor rupture propagation, also has a high coefficient of basal friction. The characteristics of the taper angle, effective coefficient of basal friction, and pore fluid pressure ratio along the Japan Trench presented herein may contribute to the understanding of the relationship between the geometry of the prism and the potential for generating seismo-tsunamigenic slips.

  10. Variable Holocene deformation above a shallow subduction zone extremely close to the trench

    PubMed Central

    Thirumalai, Kaustubh; Taylor, Frederick W.; Shen, Chuan-Chou; Lavier, Luc L.; Frohlich, Cliff; Wallace, Laura M.; Wu, Chung-Che; Sun, Hailong; Papabatu, Alison K.

    2015-01-01

    Histories of vertical crustal motions at convergent margins offer fundamental insights into the relationship between interplate slip and permanent deformation. Moreover, past abrupt motions are proxies for potential tsunamigenic earthquakes and benefit hazard assessment. Well-dated records are required to understand the relationship between past earthquakes and Holocene vertical deformation. Here we measure elevations and 230Th ages of in situ corals raised above the sea level in the western Solomon Islands to build an uplift event history overlying the seismogenic zone, extremely close to the trench (4–40 km). We find marked spatiotemporal heterogeneity in uplift from mid-Holocene to present: some areas accrue more permanent uplift than others. Thus, uplift imposed during the 1 April 2007 Mw 8.1 event may be retained in some locations but removed in others before the next megathrust rupture. This variability suggests significant changes in strain accumulation and the interplate thrust process from one event to the next. PMID:26123872

  11. Geochemical Characteristics of Core Samples from IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST)

    NASA Astrophysics Data System (ADS)

    Ishikawa, T.; Matsuoka, J.; Mori, J. J.; Chester, F. M.; Eguchi, N.; Toczko, S.

    2013-12-01

    The Integrated Ocean Drilling Program (IODP) Expedition 343 drilled three holes through the plate boundary near the Japan Trench to investigate the cause of very large fault slip during the 2011 Tohoku-Oki earthquake. In this paper, we report trace element compositions of the core samples, including plate-boundary fault rocks, recovered from Hole C0019E. The rocks in the C0019E are lithologically subdivided into seven units (Chester et al., Proceedings of the Integrated Ocean Drilling Program, Vol. 343/343T, 2013): Units 1 to 3, wedge sediments of upper plate; Unit 4, plate-boundary fault; Units 5 to 7, sediments of lower plate. In this study, we analyzed the rocks from six lithological units except for Unit 7 (chert) using ICP-MS. The core samples analyzed all show trace element characteristics of typical marine sediments, but there exist some differences between the compositions of upper plate sediments, plate-boundary fault rocks and lower plate sediments. The clay-rich plate-boundary fault rocks (Unit 4) are characterized by elevated concentrations of rare earth elements (REE) and some refractory metals, and are distinct from any other JFAST samples in terms of trace element characteristics. Brown mudstones of lower plate (Unit 5) show trace element characteristics (e.g. REE pattern) roughly similar to those of upper plate sediments (Units 1 to 3), but they are still distinguishable each other. Varicolored sediments of lower plate (Unit 6) show highly varied trace element compositions. Some of them show large negative Ce anomaly, and are distinct from any other JFAST samples. These observations show clear relationship between the lithological units and the trace element characteristics of the JFAST samples. It seems that in Units 1 to 3 and Units 5 to 7 there is no sedimentary rock equivalent to the protolith of the plate-boundary fault rocks, which provides a key for understanding the origin of the plate-boundary fault at the Japan Trench.

  12. First measurement of the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku-Oki earthquake using GPS/acoustic technique

    NASA Astrophysics Data System (ADS)

    Tomita, Fumiaki; Kido, Motoyuki; Osada, Yukihito; Hino, Ryota; Ohta, Yusaku; Iinuma, Takeshi

    2015-10-01

    The subduction rate of an oceanic plate may accelerate after large earthquakes rupture the interplate coupling between the oceanic and overriding continental plates. To better understand postseismic deformation processes in an incoming oceanic plate, we directly measured the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku-Oki earthquake using a GPS/acoustic technique over a period of 2 years (September 2012 to September 2014). The displacement rate was measured to be 18.0 4.5 cm yr-1 (N302.0E) relative to the North American Plate, which is almost twice as fast as the predicted interseismic plate motion. Because the sum of steady plate motion and viscoelastic response to the Tohoku-Oki earthquake roughly accounts for the observed displacement rate, we conclude that viscoelastic relaxation is the primary mechanism responsible for postseismic deformation of the Pacific Plate and that significant subduction acceleration did not occur at least not during the observation period.

  13. The 2000 Nemuro-Hanto-Oki earthquake, off eastern Hokkaido, Japan, and the high intraslab seismic activity in the southwestern Kuril Trench

    USGS Publications Warehouse

    Takahashi, H.; Hirata, K.

    2003-01-01

    The 2000 Nemuro-Hanto-Oki earthquake (Mw6.8) occurred in the southwestern part of the Kuril Trench. The hypocenter was located close to the aftershock region of the great 1994 Kuril earthquake (Mw8.3), named "the 1994 Hokkaido-Toho-Oki earthquake" by the Japan Meteorological Agency, for which the fault plane is still in debate. Analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event. The hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage. They clearly show that the 2000 event was an intraslab event occurring on a shallow-dipping fault plane between 55 and 65 km in depth. The well-focused aftershock distribution of the 2000 event, the relative location of the 1994 event with respect to the 2000 event, and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow-dipping fault plane in the subducting slab. The recent hypocenter distribution around the 1994 aftershock region also supports this result. Large intraslab earthquakes occuring to the southeast of Hokkaido may occur due to a strong coupling on the plate boundary, which generates relatively large stress field within the subducting Pacific plate.

  14. Trench Parallel Bouguer Anomaly (TPBA): A robust measure for statically detecting asperities along the forearc of subduction zones

    NASA Astrophysics Data System (ADS)

    Raeesi, M.

    2009-05-01

    During 1970s some researchers noticed that large earthquakes occur repeatedly at the same locations. These observations led to the asperity hypothesis. At the same times some researchers noticed that there was a relationship between the location of great interplate earthquakes and the submarine structures, basins in particular, over the rupture area in the forearc regions. Despite these observations there was no comprehensive and reliable hypothesis explaining the relationship. There were numerous cons and pros to the various hypotheses given in this regard. In their pioneering study, Song and Simons (2003) approached the problem using gravity data. This was a turning point in seismology. Although their approach was correct, appropriate gravity anomaly had to be used in order to reveal the location and extent of the asperities. Following the method of Song and Simons (2003) but using the Bouguer gravity anomaly that we called "Trench Parallel Bouguer Anomaly", TPBA, we found strong, logical, and convincing relation between the TPBA-derived asperities and the slip distribution as well as earthquake distribution, foreshocks and aftershocks in particular. Various parameters with different levels of importance are known that affect the contact between the subducting and the overriding plates, We found that the TPBA can show which are the important factors. Because the TPBA-derived asperities are based on static physical properties (gravity and elevation), they do not suffer from instabilities due to the trade-offs, as it happens for asperities derived in dynamic studies such as waveform inversion. Comparison of the TPBA-derived asperities with rupture processes of the well-studied great earthquakes, reveals the high level of accuracy of the TPBA. This new measure opens a forensic viewpoint on the rupture process along the subduction zones. The TPBA reveals the reason behind 9+ earthquakes and it explains where and why they occur. The TPBA reveals the areas that can generate tsunami earthquakes. It gives a logical dimension to the foreshock and aftershock distributions. Using the TPBA, we can derive the scenarios for the early 20th century great earthquakes for which limited data is available. We present cases from Aleutian and South America subduction zones. The TPBA explains why there should be no great earthquake in the down-dip of Shumagin, but that there should be a major tsunami earthquake for its up-dip. Our evidences suggest that the process has already started. We give numerous examples for South America, Aleutian-Alaska, and Kurile-Kamchatka subduction zones and we also look at Cascadia. Despite the possible various applications of the new measure, here we draw the attention to its most important application - the detection of critical asperities. Supplied with this new measure, in addition to the available seismological data, seismologists should be able to detect the critical asperities and follow the evolving rupture process. This paves the way for revealing systematically the great interplate earthquakes.

  15. Aftereffects of Subduction-Zone Earthquakes: Potential Tsunami Hazards along the Japan Sea Coast.

    PubMed

    Minoura, Koji; Sugawara, Daisuke; Yamanoi, Tohru; Yamada, Tsutomu

    2015-01-01

    The 2011 Tohoku-Oki Earthquake is a typical subduction-zone earthquake and is the 4th largest earthquake after the beginning of instrumental observation of earthquakes in the 19th century. In fact, the 2011 Tohoku-Oki Earthquake displaced the northeast Japan island arc horizontally and vertically. The displacement largely changed the tectonic situation of the arc from compressive to tensile. The 9th century in Japan was a period of natural hazards caused by frequent large-scale earthquakes. The aseismic tsunamis that inflicted damage on the Japan Sea coast in the 11th century were related to the occurrence of massive earthquakes that represented the final stage of a period of high seismic activity. Anti-compressive tectonics triggered by the subduction-zone earthquakes induced gravitational instability, which resulted in the generation of tsunamis caused by slope failing at the arc-back-arc boundary. The crustal displacement after the 2011 earthquake infers an increased risk of unexpected local tsunami flooding in the Japan Sea coastal areas. PMID:26399180

  16. Thermal conductivities, thermal diffusivities, and volumetric heat capacities of core samples obtained from the Japan Trench Fast Drilling Project (JFAST)

    NASA Astrophysics Data System (ADS)

    Lin, Weiren; Fulton, Patrick M.; Harris, Robert N.; Tadai, Osamu; Matsubayashi, Osamu; Tanikawa, Wataru; Kinoshita, Masataka

    2014-12-01

    We report thermal conductivities, thermal diffusivities, and volumetric heat capacities determined by a transient plane heat source method for four whole-round core samples obtained by the Japan Trench Fast Drilling Project/Integrated Ocean Drilling Program Expedition 343. These thermal properties are necessary for the interpretation of a temperature anomaly detected in the vicinity of the plate boundary fault that ruptured during the 2011 Tohoku-Oki earthquake and other thermal processes observed within the Japan Trench Fast Drilling Project temperature observatory. Results of measured thermal conductivities are consistent with those independently measured using a transient line source method and a divided bar technique. Our measurements indicate no significant anisotropy in either thermal conductivity or thermal diffusivity.

  17. Middle Miocene near trench volcanism in northern Colombia: A record of slab tearing due to the simultaneous subduction of the Caribbean Plate under South and Central America?

    NASA Astrophysics Data System (ADS)

    Lara, M.; Cardona, A.; Monsalve, G.; Yarce, J.; Montes, C.; Valencia, V.; Weber, M.; De La Parra, F.; Espitia, D.; López-Martínez, M.

    2013-08-01

    Field, geochemical, geochronological, biostratigraphical and sedimentary provenance results of basaltic and associated sediments northern Colombia reveal the existence of Middle Miocene (13-14 Ma) mafic volcanism within a continental margin setting usually considered as amagmatic. This basaltic volcanism is characterized by relatively high Al2O3 and Na2O values (>15%), a High-K calc-alkaline affinity, large ion lithophile enrichment and associated Nb, Ta and Ti negative anomalies which resemble High Al basalts formed by low degree of asthenospheric melting at shallow depths mixed with some additional slab input. The presence of pre-Cretaceous detrital zircons, tourmaline and rutile as well as biostratigraphic results suggest that the host sedimentary rocks were deposited in a platform setting within the South American margin. New results of P-wave residuals from northern Colombia reinforce the view of a Caribbean slab subducting under the South American margin. The absence of a mantle wedge, the upper plate setting, and proximity of this magmatism to the trench, together with geodynamic constraints suggest that the subducted Caribbean oceanic plate was fractured and a slab tear was formed within the oceanic plate. Oceanic plate fracturing is related to the splitting of the subducting Caribbean Plate due to simultaneous subduction under the Panama-Choco block and northwestern South America, and the fast overthrusting of the later onto the Caribbean oceanic plate.

  18. Structural Characteristics of the Philippine Sea Plate Subducted beneath the Southwest Japan Arc

    NASA Astrophysics Data System (ADS)

    Iwasaki, T.; Kurashimo, E.; Kato, A.; Sato, H.; Iidaka, T.; Nakayama, Y.; Arai, R.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.; Ito, T.; Ito, K.

    2012-12-01

    The northwestward subduction of Philippine Sea (PHS) plate forms a well-known seismogenic zone beneath the Southwest Japan Arc, along which M8-class megathrust earthquakes repeatedly occur. Since the eastern half of this seismogenic zone is situated beneath highly populated areas including Tokyo, Yokohama and Nagoya cities, the understanding on the generation mechanism of the megathrust event is inevitably important from social point of view as well as geological/geophysical interests. This seismogenic zone is geologically separated by the Izu Collision Zone (ICZ), where the Izu-Bonin Arc (IBA) has collided to the SW Japan Arc since middle Miocene. The region west of the ICZ, beneath which a young oceanic crust is subducted, involves fault areas of the 1946 Nankai (M8.0) and 1944 Tonankai (M7.9) Earthquakes. An area just east of the Tonankai fault is considered to be a source region of the forthcoming Tokai Earthquake. East of the ICZ, the forearc side of the IBA is subducted, where the 1923 Kanto Earthquake of M7.9 occurred. Recent intensive active and passive source seismic experiments along SW Japan have revealed general structural features as well as regional difference which provide an important key in understanding of the generation mechanism of megathrust earthquake. The most prominent and common feature of seismic profiles west of the ICZ is very strong reflection generated from a very thin (200-500 m) low velocity (Vp=3-4 km/s) layer at the top of the plate boundary. Careful ray-tracing analysis indicates that the low velocity layer extends from the lowermost limit of the locked part to the wedge mantle. A seismic profile crossing the 1944 Tonankai Earthquake delineated detailed structural change along the downgoing plate. A thin low velocity layer as mentioned above exists at the top of the plate around the deepest limit of the locked part. But, in the deeper part, reflectors are distributed with a several kilometer thickness extending from the wedge mantle to the subducted oceanic crust/upper mantle. Low frequency earthquakes (LFEs) are concentrated within or beneath this reflective zone. This indicates that dehydrated fluids ascending from the oceanic lithosphere are trapped around the wedge mantle and the top of the subducted plate to generate strong reflectivity. Passive source observation by a dense array in Tokai area provided more direct evidence of the dehydrated fluids within the oceanic lithosphere. The most important characteristic in this region is a significantly low Vp/Vs portion around a subducted ridge 15-20 km landward of the deepest limit of the locked portion, probably formed by high-pressure dehydrated fluids. This portion well corresponds to an area of slow slip which continued from 2000 to 2005. The wedge mantle is also characterized by low Vp/Vs and LFEs, as in the case of the Tonankai area. Several seismic reflection experiments east of the ICZ showed that asperities of the Kanto Earthquake are characterized by less reflective portions of the plate boundary. Although the deeper part of the plate is identified as dipping reflectors, their reflectivity is much weaker than in the cases west of the ICZ. Probably, the dehydration and water supply system in this region is significantly different from that west of the ICZ where the young oceanic plate is subducted.

  19. Source of energy sustaining the Calyptogena populations from deep trenches in subduction zones off Japan

    NASA Astrophysics Data System (ADS)

    Fiala-Mdioni, A.; Boulgue, J.; Ohta, S.; Felbeck, H.; Mariotti, A.

    1993-06-01

    Deep tow camera surveys during the Hakuho Maru cruise (KH-89-1) and Nautile dives during the Kaiko-Nankai cruise (November 1989) demonstrate the presence of dense animal communities at depths around 2000 and 3800 m. The dominant organisms are vesicomyid bivalve molluscs, with two new very large (up to 28 cm long) species of Calyptogena and the two species previously found in the Nankai Trough during the Kaiko cruise (1985), C. laubieri and C. kaikoi. They apparently rely on sulfide-based chemoautotrophy through symbiotic bacteria associated with their gills. Evidence of sulfur-oxidizing metabolism includes ultrastructural features of symbionts, absence of methanol dehydrogenase activity, presence of ATP-sulfurylase and abundant elemental sulfur in the gill. Carbon isotope ratios are close to values obtained in other sulfur-oxidizing symbiont-bearing species (from -35.6 to -38.7% for the 3950 m species and from -36 to -37.4% for the 2050 m species). Nitrogen isotope ratios show highly variable values (from -4 to -9.7% for the 3950 m species and from -0.2 to +4.4% for the 2050 m species). 14C analyses indicates growth based on water CO 2 with limited or no input of fossil carbon.

  20. Anomalously high porosity in subduction inputs to the Nankai Trough (SW Japan) potentially caused by volcanic ash and pumice

    NASA Astrophysics Data System (ADS)

    Huepers, A.; Ikari, M.; Underwood, M.; Kopf, A.

    2013-12-01

    At convergent margins, the sedimentary section seaward of the trench on the subducting oceanic lithosphere provides the source material for accretionary prisms and eventually becomes the host rock of the plate boundary megathrust. The mechanical properties of the sediments seaward of the subduction zone have therefore a first order control on subduction zone forearc mechanics and hydrogeology. At the Nankai Trough (SW Japan) the majority of sediment approaching the subduction zone is clay-rich. Scientific drilling expeditions in the framework of the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP) have revealed an anomalous zone of high porosity in a major lithologic unit known as the Upper Shikoku Basin facies (USB), which is associated with elevated volcanic ash content and high amounts of silica in the interstitial water. The existence of the high porosity zone has previously been associated with advanced silica cementation, driven by the dual diagenetic transition of opal-A to opal-CT, and opal-CT to quartz. However, temperature estimates from recent drilling expeditions offshore the Kii peninsula reveal different in situ temperatures at the proposed diagenetic boundary in the Shikoku Basin. Furthermore, laboratory measurements using core samples from the USB show that cohesive strength is not elevated in the high porosity zone, suggesting that a process other than cementation may be responsible. The USB sediment is characterized by abundant volcanic ash and pumice, therefore the high porosity zone in the USB may be closely linked to the mechanical behavior of this phase. We conducted consolidation tests in the range 0.1 to 8 MPa effective vertical stress on artificial ash-smectite and pumice-smectite mixtures, as well as intact and remolded natural samples from the IODP Sites C0011 and C0012 to investigate the role of the volcanic constituent on porosity loss with progressive burial. Our results show that both remolded and intact natural samples have high porosities of up to ~71 to 75% at a vertical effective stress of 0.1 MPa, which decreases to 39 to 49% at 8 MPa vertical effective stress. The behavior of the remolded samples is in good accordance with compiled in-situ porosity vs. depth profiles from the high porosity zone. This suggests that cementation is not the cause for the anomalously high porosity. The consolidation tests on the artificial samples document that pure ash and pumice samples are highly resistant to consolidation. Between 0.1 to 8 MPa vertical effective stress, the porosity decreases from 51 to 47% for the ash sample and 60% to 46% for the pumice sample. The higher initial porosity in the pumice may be explained by a porous internal grain structure that allows storage of additional water. Mixtures with smectite are characterized by higher compressibility and higher porosity. For a mixture of 80% smectite and 20% pumice the porosity decreases from 65% to 39%, similar to that of the natural samples. Our results suggest that the high porosity zone is caused by the bulk mechanical behavior of pumice in the USB.

  1. Seismic structure along transitions from flat to normal subduction: central Mexico, southern Peru, and southwest Japan

    NASA Astrophysics Data System (ADS)

    Dougherty, Sara L.

    The fine-scale seismic structure of the central Mexico, southern Peru, and southwest Japan subduction zones is studied using intraslab earthquakes recorded by temporary and permanent regional seismic arrays. The morphology of the transition from flat to normal subduction is explored in central Mexico and southern Peru, while in southwest Japan the spatial coincidence of a thin ultra-slow velocity layer (USL) atop the flat slab with locations of slow slip events (SSEs) is explored. This USL is also observed in central Mexico and southern Peru, where its lateral extent is used as one constraint on the nature of the flat-to-normal transitions. In western central Mexico, I find an edge to this USL which is coincident with the western boundary of the projected Orozco Fracture Zone (OFZ) region. Forward modeling of the 2D structure of the subducted Cocos plate using a finite-difference algorithm provides constraints on the velocity and geometry of the slab's seismic structure in this region and confirms the location of the USL edge. I propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, by a process analogous to that which occurred when the Rivera plate separated from the proto-Cocos plate 10 Ma. In eastern central Mexico, observations of a sharp transition in slab dip near the abrupt end of the Trans Mexican Volcanic Belt (TMVB) suggest a possible slab tear located within the subducted South Cocos plate. The eastern lateral extent of the USL is found to be coincident with these features and with the western boundary of a zone of decreased seismicity, indicating a change in structure which I interpret as evidence of a possible tear. Analysis of intraslab seismicity patterns and focal mechanism orientations and faulting types provides further support for a possible tear in the South Cocos slab. This potential tear, together with the tear along the projection of the OFZ to the northwest, indicates a slab rollback mechanism in which separate slab segments move independently, allowing for mantle flow between the segments. In southern Peru, observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest a smooth contortion of the slab. Concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry. The lateral extent of the USL atop the horizontal Nazca slab is found to be coincident with the margin of the projected linear continuation of the subducting Nazca Ridge, implying a causal relationship, but not a slab tear. Waveform modeling of the 2D structure in southern Peru provides constraints on the velocity and geometry of the slab's seismic structure and confirms the absence of any tears in the slab. In southwest Japan, I estimate the location of a possible USL along the Philippine Sea slab surface and find this region of low velocity to be coincident with locations of SSEs that have occurred in this region. I interpret the source of the possible USL in this region as fluids dehydrated from the subducting plate, forming a high pore-fluid pressure layer, which would be expected to decrease the coupling on the plate interface and promote SSEs.

  2. Interplate locking derived from seafloor geodetic measurement at the shallow subduction zone of the northernmost Suruga Trough, Japan

    NASA Astrophysics Data System (ADS)

    Yasuda, K.; Tadokoro, K.; Ikuta, R.; Watanabe, T.; Nagai, S.; Sayanagi, K.

    2013-12-01

    Observation of seafloor crustal deformation is crucial for megathrust earthquake because most of the focal areas are located below seafloor. Seafloor crustal deformation can be observed GPS/Acoustic technique, and this technique has been carried out at subduction margins in Japan, e.g., Japan Trench, Suruga Trough, and Nankai Trough. At the present, the accuracy of seafloor positioning is one to several centimeters for each epoch. Velocity vectors at seafloor site are estimated through repeated observations. Co- and post- seismic slip distribution and interseismic deformation are estimated from results of seafloor geodetic measurement (e.g., Iinuma et al., 2012; Tadokoro et al., 2012). We repeatedly observed seafloor crustal deformations at two sites across the Suruga Trough from 2005 to investigate interplate locking condition at the focal area of the anticipated megathrust, Tokai, earthquake. We observed 12 and 16 times at an east site of the Suruga Trough (SNE) and at an west site of the Suruga Trough (SNW), respectively. We reinstalled seafloor benchmarks at both sites because of run out of batteries in 2012. We calculated and removed the bias between the old and new seafloor benchmarks. Furthermore, we evaluated two type of analysis. One is Fixed triangular configuration Analysis (FTA). When we determine the seafloor benchmark position, we fix the triangular configuration of seafloor units averaging all the measurements to improve trade-off relation between seafloor benchmark position and sound speed structure. Sound speed structure is assumed to be horizontal layered structure. The other one is Fixed Triangle and Gradient structure of sound speed structure (FTGA). We fixed triangular configuration same as FTA. Sound speed structure is assumed to have gradient structure. Comparing FTA with FTGA, the RMS of horizontal position analyzed through FTA is smaller than that through FTGA at SNE site. On the other hand, the RMS of horizontal position analyzed through FTA is larger than that through FTGA at SNW site. We estimated the displacement velocities with relative to the Amurian plate from the result of repeated observation. The estimated displacement velocity vectors at SNE and SNW are 428 mm/y to N94W direction and 4613 mm/y to N77W direction, respectively. The directions are the same as those measured at the on-land GPS stations. The magnitudes of velocity vector indicate significant shortening by approximately 11 mm/y between SNW and on-land GPS stations at the western part of the Suruga Trough. We also calculated the theoretical surface deformation pattern to depict the interplate locking condition. These results show that the plate interface at the shallow zone of the northernmost Suruga trough is strongly locked.

  3. Scattering of teleseismic P-waves by the Japan Trench: A significant effect of reverberation in the seawater column

    NASA Astrophysics Data System (ADS)

    Maeda, Takuto; Furumura, Takashi; Obara, Kazushige

    2014-07-01

    We detected a scattered wave train in data from the high-sensitivity seismograph network in Japan (Hi-net) following the arrival of the near-vertically incident P-wave generated by the 2009 earthquake (Mw 7.8) off the South Island of New Zealand. The scattered wave train represented predominantly vertical ground motion at a period of 20 to 50 s and with an apparent velocity of 3.5 km/s; it propagated cylindrically westward through the Kanto area of central Japan. Array analysis showed that the scattered wave train developed beneath the Pacific Ocean near the Boso triple junction, southeast of the Kanto area. A 3D finite-difference simulation of seismic wave propagation using a high-resolution model incorporating subsurface structure, topography, and bathymetry revealed that the strong scattered waves that were generated along the Japan Trench and propagated normal to the trench axis represented multiple reverberations of seismic waves between the seafloor and the Pacific plate boundary. In addition, strong reverberation of acoustic waves in the seawater column above the Boso triple junction causes elongated scattered waves, which reasonably explains our observations.

  4. Re-examination of large 20th century earthquakes along the southern Japan trench

    NASA Astrophysics Data System (ADS)

    Murotani, S.; Satake, K.

    2012-12-01

    We re-examine hypocenters, focal mechanisms and fault models for large earthquakes off Fukushima and off Boso regions along the southern Japan trench. The 1938 off Fukushima earthquakes, which consist of five earthquakes of Mjma ranging from 6.9 to 7.5, occurred in the southern part of the 2011 Tohoku earthquake source, where many M~6 aftershocks occurred since March 2011. To the south, off Boso region, the 1927 (Mjma 6.9) and 1953 (Mjma 7.4) earthquakes occurred, but the details are not well known. The 1938 off Fukushima (Shioya-oki) earthquakes were the only M>7 earthquakes recorded in the southern part of the 2011 Tohoku earthquake source. The earthquake sequence consists of 5 events as shown in Table. Abe (1977, Tectonophysics) studied these events and estimated the focal mechanisms and seismic moments. However, the slip distributions are not known. We first examined the teleseismic waveforms recorded at Pasadena, De Bilt and Christchurch. Comparison of waveforms from the five earthquakes shows that P wave and following phases from event 4 are the largest. Event 4 at CHR and event 5 at DBN show clear downward initial motions. While the above three stations are located near nodal planes on focal sphere, the comparison indicates the focal mechanism of event 4 and 5 are different each other. After the 2011 Tohoku earthquake, many aftershocks with M~6 occurred in this region with various focal mechanisms, including reverse fault, normal fault and strike-slip fault. We compared the teleseismic waveforms of these aftershocks and the 1938 earthquakes. Few of the teleseismic waveforms from the 2011 aftershocks are comparable with the 1938 events, because the aftershocks were smaller in M and waveforms from many aftershocks overlap. Off the Boso region, the southern neighbor of the 2011 Tohoku earthquake source, several large earthquakes and tsunami earthquakes (e.g., 1677 Empo earthquake) have occurred, but their recurrence is not known. Two earthquakes, one on August 18, 1927 with Mjma 6.9 and Mt 7.4, and the other on November 25 1953 with Mjma 7.4 and Mt 7.8, have different epicenters and the tsunami source areas. While the 1927 epicenter was located to southeast of the 1953 epicenter, the 1927 tsunami source was estimated to the northwest of the 1953 tsunami source (Hatori, 1975). The S-P times at 9 Japanese stations and at University of Tokyo station indicate are larger for the 1927 earthquake than that of the 1953 event, indicating that the epicenter was at far southeast of the tsunami source area.The 1938 off Fukushima earthquakes;

  5. Evolution of a trench-slope basin within the Cascadia subduction margin: the Neogene Humboldt Basin, California

    USGS Publications Warehouse

    McCrory, P.A.

    1995-01-01

    The Neogene Humboldt (Eel River) Basin is located along the north-eastern margin of the Pacific Ocean within the Cascadia subduction zone. This sedimentary basin originated near the base of the accretionary prism in post-Eocene time. Subduction processes since that time have elevated strata in the south-eastern portion of the basin above sea level. High-resolution chronostratigraphic data from the onshore portion of the Humboldt Basin enable correlation of time-equivalent lithofacies across the palaeomargin, reconstruction of slope-basin evolution, and preliminary delineation of climatic and tectonic influence on lithological variation. -from Author

  6. Dual subduction tectonics and plate dynamics of central Japan shown by three-dimensional P-wave anisotropic structure

    NASA Astrophysics Data System (ADS)

    Ishise, Motoko; Miyake, Hiroe; Koketsu, Kazuki

    2015-07-01

    The central Japanese subduction zone is characterized by a complex tectonic setting affected by the dual subduction of oceanic plates and collisions between the island arcs. To better understand of the subduction system, we performed an anisotropic tomography analysis using P-wave arrival times from local earthquakes to determine the three-dimensional structure of P-wave azimuthal anisotropy in the overriding plate and the Pacific and Philippine Sea (PHS) slabs. The principal characteristics of anisotropy in the subducted and subducting plates are (1) in the overriding plate, the distribution pattern of fast direction of crustal anisotropy coincides with that of the strike of geological structure, (2) in the two oceanic plates, fast propagation directions of P-wave were sub-parallel to the directions of seafloor spreading. Additionally, our tomographic images demonstrate that (1) the bottom of the Median Tectonic Line, the longest fault zone in Japan, reaches to the lower crust, and seems to link to the source region of an inter-plate earthquake along the PHS slab, (2) the segmentation of the PHS slab - the Izu Islands arc, the Nishi-Shichito ridge, and the Shikoku basin - due to the formation history, is reflected in the regional variation of anisotropy. The tomographic study further implies that there might be a fragment of the Pacific slab suggested by a previous study beneath the Tokyo metropolitan area. The overall findings strongly indicate that seismic anisotropy analysis provide potentially useful information to understand a subduction zone.

  7. Great Earthquakes With and Without Large Slip to the Trench

    NASA Astrophysics Data System (ADS)

    Mori, J. J.

    2013-12-01

    The 2011 Tohoku-oki earthquake produced a huge amount of slip (40 to 60 meters) on the shallow portion of the subduction zone close to the trench. This large displacement was largely unexpected for this region and caused the very large and damaging tsunami along the northeast coast of Honshu. For other subduction zones around the world, we examine the possibility of large slip to the trench in past large and great earthquakes. Since the trench region is generally far offshore, it is often difficult to resolve the amount of slip from onland geodetic and strong-motion data. We use a variety of observations, including slip distribution models, aftershock locations, local coastal deformation, and tsunami heights to determine which events likely had large amounts of slip close to the trench. Tsunami earthquakes, such as 1992 Nicaragua and 2006 Java likely had large shallow slip. Some typical subduction earthquakes, such as 1968 Tokachi-oki and 2003 Tokachi-oki (located in regions north of the source area of the 2011 Tohoku-oki earthquake) likely did not. We will discuss possible factors that influence the slip distribution on the shallow area of subduction megathrusts. Using results from the Japan Trench Fast Drilling Project (JFAST) which sampled the fault in the region of large slip, we can begin to understand the conditions of very large fault slip. Are there characteristic features in the material properties for faults that have large slip ? Can we determine if these regions have high plate coupling and accumulate stress ?

  8. Reconstruction of ocean plate stratigraphy in the Gwna Group, NW Wales: Implications for the subduction-accretion process of a latest Proterozoic trench-forearc

    NASA Astrophysics Data System (ADS)

    Asanuma, Hisashi; Okada, Yoshihiro; Fujisaki, Wataru; Suzuki, Kazue; Sato, Tomohiko; Sawaki, Yusuke; Sakata, Shuhei; Yamamoto, Shinji; Hirata, Takafumi; Maruyama, Shigenori; Windley, Brian F.

    2015-11-01

    The Gwna Group in Anglesey island and Lleyn peninsula, Wales consists of a latest Proterozoic volcano-sedimentary trench mlange, which has a complicated accretionary structure, and is poorly constrained by isotopic ages. The mlange contains oceanic-trench rocks including pillow basalts, cherts, mudstones and sandstones, which have not previously been interpreted as ocean plate stratigraphy (OPS). We reconstructed imbricated OPS at 5 localities in the coastal Lleyn peninsula. In order to constrain the depositional U-Pb age of the upper clastic sediments, detrital zircons, separated from 9 clastic sediments, were analyzed with a Nu AttoM single-collector inductively-coupled plasma-mass spectrometer. The ages indicate that there are two Gwna Groups (maximum depositional ages of: 1 at 608-601 Ma, and 2 at 564-539 Ma) that were deposited between the late Neoproterozoic and the Middle Cambrian contemporaneously with dated calc-alkaline arc magmatism and regional metamorphism in the Anglesey-Lleyn complex. The age spectra of the detrital zircons show a prominent peak at ca. 650-600 Ma, and several Proterozoic and Archean ages. To account for the older ages, we integrated our new isotopic data with published radiometric and fossil ages, and conclude that the clastic sediments at the top of the OPS were deposited in a trench on the western active margin of Avalonia when it was close to the Amazonian craton, and that the Gwna Group OPS began to be incorporated into an accretionary wedge in an active subduction zone in the latest Proterozoic.

  9. Mass Flux of Continental Material at Cenozoic Subduction Zones--New Global and Trench-sector Calculations Using New Geological and Geophysical Observations

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; von Huene, R.

    2001-12-01

    INTRODUCTION: A decade ago, then available geophysical and geological data implied that more than 65 percent of ocean floor sediment entering most subduction zones (SZ) accompanied the oceanic crust to the mantle (= sediment subduction or SS). The underthrusting slab also eroded the margin's crustal framework and conveyed this material to the mantle (= subduction erosion or SE). Globally, the mass of continental material recycled to the mantle was estimated at 1.3-1.8 km3 / yr (SS. = 0.7 km3 + SE = 0.6-1.1 km3). SEDIMENT SUBDUCTION: New and enhanced seismic reflection data, new drilling observations, and reevaluation of older information stress that the efficacy of SS is higher than earlier assessed. In detail, it appears that 100 percent SS occurs at non-accreting margins (19,000 km), at least 80 percent at accreting margins (16,000 km) where small to moderate size accretionary prisms (width=5-40 km) are forming, and 40-45 percent where larger prisms are accumulating (8,000 km). At Cenozoic SZs (~43,000 km), it is now estimated that the long-term (i.e., >10 Myr) rate of SS is at least 1.0 km3 / yr (solid volume). SUBDUCTION EROSION: New and reassessed seismic, drilling, submersible, coastal mapping and arc-retreat observations suggest a higher long-term rate of SE than formerly estimated at 30 km3 / Myr / km of trench. We now estimate that, except perhaps where large accretionary bodies are forming, the long-term rate of forearc erosion averages at least 40 km3 / Myr (range = 28-62), which corresponds to a global recycling rate of 1.4 km3 / yr. The matching average rate of landward truncation of the submerged forearc is 2.5 km / Myr (range = 1.8-4.2). SUMMARY: The late Cenozoic rate at which continental crust is recycled at SZs is currently estimated at 2.4 km3 / yr (ss=1+ se=1.4) +/- 25 percent, which is basically that now approximated for arc magmatic additions. It can thus be inferred that at Cenozoic SZs rates of crustal addition and recycling have been in general balance. This quasi-stasis may be applicable to the Phanerozoic.

  10. Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics

    NASA Astrophysics Data System (ADS)

    Liu, Xin; Zhao, Dapeng

    2015-04-01

    We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous-Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.

  11. Downdip velocity changes in subducted oceanic crust beneath Northern Japaninsights from guided waves

    NASA Astrophysics Data System (ADS)

    Garth, Tom; Rietbrock, Andreas

    2014-09-01

    Dispersed P-wave arrivals observed in the subduction zone forearc of Northern Japan suggest that low velocity subducted oceanic crustal waveguide persists to depths of at least 220 km. First arrivals from events at 150-220 km depth show that the velocity contrast of the waveguide reduces with depth. High frequency energy (>2 Hz) is retained and delayed by the low velocity crustal waveguide while the lower frequency energy (<0.5 Hz) travels at faster velocities of the surrounding mantle material. The guided wave energy then decouples from the low velocity crustal waveguide due to the bend of the slab and is seen at the surface 1-2 s after the low frequency arrival. Dispersive P-wave arrivals from WBZ earthquakes at 150-220 km depth are directly compared to synthetic waveforms produced by 2-D and 3-D full waveform finite difference simulations. By comparing both the spectrogram and the velocity spectra of the observed and synthetic waveforms we are able to fully constrain the dispersive waveform, and so directly compare the observed and synthetic waveforms. Using this full waveform modelling approach we are able to tightly constrain the velocity structures that cause the observed guided wave dispersion. Resolution tests using 2-D elastic waveform simulations show that the dispersion can be accounted for by a 6-8 km thick low velocity oceanic crust, with a velocity contrast that varies with depth. The velocities inferred for this variable low velocity oceanic crust can be explained by lawsonite bearing assemblages, and suggest that low velocity minerals may persist to greater depth than previously thought. 2-D simulations are benchmarked to 3-D full waveform simulations and show that the structures inferred by the 2-D approximation produce similar dispersion in 3-D. 2-D viscoelastic simulations show that including elevated attenuation in the mantle wedge can improve the fit of the dispersed waveform. Elevated attenuation in the low velocity layers can however be ruled out.

  12. Mantle wedge flow pattern and thermal structure in Northeast Japan: Effects of oblique subduction and 3-D slab geometry

    NASA Astrophysics Data System (ADS)

    Wada, Ikuko; He, Jiangheng; Hasegawa, Akira; Nakajima, Junichi

    2015-09-01

    We develop a 3-D thermal model for the Northeast Japan subduction margin, using a realistic slab geometry for the subducting Pacific plate, and investigate the effects of oblique subduction and 3-D slab geometry on the mantle wedge flow pattern and the thermal structure. In the Tohoku region, the mantle wedge flow pattern is nearly two-dimensional resulting in a thermal structure similar to those obtained by a 2-D model, owing to the simple slab geometry and subduction nearly perpendicular to the margin. However, in Hokkaido, oblique subduction leads to 3-D mantle wedge flow with northerly inflow and west-northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in Tohoku due to lower sinking rate of the slab. Between Hokkaido and Tohoku, the slab has a hinge-like shape due to a relatively sharp change in the dip direction. In this hinge zone, northerly mantle inflow from Hokkaido and westerly mantle inflow from Tohoku converge, discouraging inflow from northwest and resulting in a cooler mantle wedge. The model-predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross-chains. The predicted 3-D thermal structure correlates well with the along-arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down-dip extent of interplate earthquakes.

  13. Small interseismic asperities and widespread aseismic creep on the northern Japan subduction interface

    NASA Astrophysics Data System (ADS)

    Johnson, Kaj M.; Mavrommatis, Andreas; Segall, Paul

    2016-01-01

    The canonical model of fault coupling assumes that slip is partitioned into fixed asperities that display stick-slip behavior and regions that creep stably. We show that this simple asperity model is inconsistent with GPS-derived deformation in northern Japan associated with interseismic coupling on the subduction interface and the transient response to Mw 6.3-7.2 earthquakes during 2003-2011. Comparisons of GPS data with simulations of earthquakes on asperities and associated velocity-strengthening afterslip require that afterslip overlaps areas of the fault that ruptured in previous earthquakes, including the 2011 Mw 9 Tohoku-oki earthquake. Whereas about 55% of the plate interface ruptured in earthquakes during 2003-2011, we infer that only 9% of the plate interface was fully locked between earthquakes. Inferred locked asperities are roughly 25% the size of rupture areas determined by seismic source inversions. These smaller asperities are consistent with interseismic strain accumulation in 2009, although more extensive locking is required a decade earlier in 1998.

  14. The Mariana Trench: A new view based on multibeam echosounding

    NASA Astrophysics Data System (ADS)

    Gardner, J. V.; Armstrong, A. A.

    2011-12-01

    The entire Mariana Trench, from its northern end at Dutton Ridge to the southwestern terminus at the Yap Trench, was mapped in 2010 using a Kongsberg EM122 12-kHz multibeam echosounder. The region ranges in depths from the shoreline at Guam to almost 11,000 m at the Challenger Deep. The northern part of the trench is receiving seamounts and guyots of the Magellan Seamount chain, whereas the southern section is receiving seafloor that carries the Caroline Ridge to the trench. The area immediately seaward of the trench where the Pacific Plate has bent downward toward the subduction zone has been broken by a series of subparallel horst and graben structures generated by extension on the bending upper surface of the Pacific Plate. Four bathymetric "bridges" span across the trench axis and extend from the Pacific Plate to the inner wall of the trench. The bridges stand as much as 2500 m above the trench axis and are composed of Latest Jurassic to Early Cretaceous accreted seamounts and guyots of the Magellan Seamount chain that are in the process of breaking up and being subducted beneath the Philippine Plate. Only two seamounts of the Caroline Ridge are in the vicinity of the trench and they both presently reside on the outer trench wall. The faults of the horsts and grabens have fractured the seamounts and guyots within the trench depression seaward from the axis outward for about 80 km, but within ~5 km of the trench axis the faults have reactivated to compressional thrust faults. The faults tend to parallel the axis of the trench until the immediate vicinity of an accreting seamount or guyot where the faults bend inward toward the trench axis, as has been observed in many other trenches. Most of the accreted seamounts and guyots are not associated with embayments or reentrants on the inner trench wall, as has been documented in the Middle America and Japan Trenches, perhaps because there is not a large accretionary prism that extends seaward of the forearc. The one exception is a large seamount of the Caroline Ridge that has been fractured into several sections, some of which appear to be mostly subducted, that are associated with a 30 km embayment landward from the trench axis. However, there are reentrants along the inner trench wall but without bathymetric expression of an associated subducting seamount or guyot. These reentrants may mark zones where seamounts and guyots have been completely consumed into the trench. There is no evidence from the acoustic backscatter of sediment filling by debris flows and other failure deposits along the entire trench axis, although the inner trench wall has numerous scarps from wall failures. The forearc area has numerous features that resemble diapirs with what appears from the acoustic backscatter to be ponded sediment in bathymetric lows that are surrounded by diapirs. An analysis of the individual soundings within Challenger Deep shows the deepest depth of the Mariana Trench is 10,994 m (2σ ±40 m), based on numerous soundings and sound-speed profiles collected during the cruise in the immediate area. The location of the deepest depth does not coincide exactly with published claims of the deepest depth, although many of the claims are within a few kilometers of the 10,944 m depth.

  15. Identification of subducting plate structure within seismogenic zones and relationships with seismicity

    NASA Astrophysics Data System (ADS)

    Bassett, D.; Watts, A. B.

    2014-12-01

    The roughness of subducting plates is one of the most important parameters controlling the seismogenic behavior of subduction megathrusts, but in most regions the distribution of subducting relief is inferred from structure seaward of trenches. Spectral averaging techniques developed to remove the steep topographic gradients across forearcs are shown to improve resolution of local trench-slope uplift, that may be diagnostic of subducting relief. This interpretation is locally calibrated where the Louisville Ridge subducts at the Tonga trench. From a global extension of these techniques, >200 residual bathymetric anomalies are identified, enabling links between subducting relief, slip behavior and seismicity to be reconsidered. We interpret >150 potential subducting seamounts, 36 of which have height ?1 km and area ?500 km2. These anomalies are similar in wavelength, amplitude and morphology with unsubducted seamounts, are associated with aseismic regions in Tonga and Mariana, and prevented along strike rupture propagation in large recent earthquakes in Java (2006) and Japan (2011). Subducting aseismic ridges in Peru, Ecuador and Costa-Rica are associated with uplift and steepening of the outer-forearc and a local increase in the width and elevation of the volcanic-arc. Associations with complex large earthquakes, higher frequencies of small events, and creep suggest aseismic ridges may also subduct via the development and evolution of an adjacent fracture network. Megathrust complexity is expected to be greatest on subducting ridge flanks. The bathymetric expression of subducting relief is strongest near the trench (<70 km) and above shallow slab depths (<~17 km). Dip-parallel transitions in the surface expression of aseismic ridges may reflect physical transitions in megathrust slip-behavior and/or material properties in the overthrusting wedge. Aseismic ridges and seamounts represent seafloor roughness over different wavelengths and are associated with contrasting modes of isostatic compensation. Consideration of these differences, coupled with improved knowledge of where subducting bathymetric features are likely within seismogenic zones, may reconcile some debated aspects of the mechanics and seismological consequences of bathymetric relief subduction.

  16. Bathymetric patterns of ? and ? diversity of harpacticoid copepods at the genus level around the Ryukyu Trench, and turnover diversity between trenches around Japan

    NASA Astrophysics Data System (ADS)

    Kitahashi, Tomo; Kawamura, Kiichiro; Kojima, Shigeaki; Shimanaga, Motohiro

    2014-04-01

    The diversity of harpacticoid copepods was investigated around the Ryukyu Trench (430-7150 m), which lies below an oligotrophic subtropical ocean. The ? diversity, which is based on the number of genera and Shannon diversity decreased with increasing water depth. The community structure of harpacticoids gradually changed as the water depth increased from the bathyal zone to the hadal zone. Turnover (?) diversity values were equally high between the trench slope, trench floor and abyssal plain. We compared the harpacticoid assemblage obtained from the Ryukyu region with the assemblage from a region around the Kuril Trench (Kitahashi et al., 2013). Turnover diversity values between the two regions (? diversity) were relatively low at shallow depths, but they increased with increasing water depth and reached their maximum between the trench floors and abyssal plains. These findings indicate that the bathymetric patterns of harpacticoid assemblages differ among regions and that these discrepancies reflect differences in environmental conditions, such as primary productivity level.

  17. The Thermal Memory of Smectite and Implications for Seismogenic Heating during the 2011 Tohoku Earthquake in the Japan Trench

    NASA Astrophysics Data System (ADS)

    Schleicher, A.; Boles, A.; Van der Pluijm, B.

    2014-12-01

    The sensitivity of smectite to temperature changes in shallow fault systems can provide information about the heating history of a fault during seismogenic slip or creep. Pelagic-sourced smectite is the most abundant clay mineral that is incorporated into the slip zone drilled during JFAST (Japan Trench Fast Drilling Project) Expedition 343 in the Japan Trench, located at ~820 mbsf. Chemical compositions analyzed by ICP-OES show a significant amount of Fe, and lesser Mg and K in these smectite minerals that are pelagic in origin. In order to investigate the significance of smectite during recently proposed high frictional heat associated with the Tohoku earthquake in March 2011, mineral transformations in JFAST drill core samples are examined during fast (5 minutes) and slow (5 hours) heating sequences. For both series, eight measurement-sets were analyzed, taken at 25° C pre-heating, increasing maximum temperatures in the range 50-220° C, and 25° C post-heating, using a real-time heating stage that is connected to an x-ray diffractometer. We observe that (i) both slow and fast heating causes reduction of water interlayers in smectite between 50-200°C, with a delay of water-release during quick heating at temperatures up to 150° C, (ii) smectite recovers more quickly to the original hydration state after fast heating than slow heating between 50-150°C, and (iii) non-recoverable collapse of all smectite occurs at ~200°C, for fast and slow heating, respectively. Based on these results, we conclude that distributed frictional heating of the Tohoku fault zone cannot have exceed a temperature of ~200° C, because smectitic clays are widely present in these fault rocks. Their occurrence indicates that shear heating at the sampled site of the Tohoku earthquake was relatively low, or restricted to an extremely narrow zone.

  18. Activity of Small Repeating Earthquakes along Izu-Bonin and Ryukyu Trenches

    NASA Astrophysics Data System (ADS)

    Hibino, K.; Matsuzawa, T.; Uchida, N.; Nakamura, W.; Matsushima, T.

    2014-12-01

    There are several subduction systems near the Japanese islands. The 2011 Mw9.0 Tohoku-oki megathrust earthquake occurred at the NE Japan (Tohoku) subduction zone. We have revealed a complementary relation between the slip areas for huge earthquakes and small repeating earthquakes (REs) in Tohoku. Investigations of REs in these subduction zones and the comparison with Tohoku area are important for revealing generation mechanism of megathrust earthquakes. Our target areas are Izu-Bonin and Ryukyu subduction zones, which appear to generate no large interplate earthquake. To investigate coupling of plate boundary in these regions, we estimated spatial distribution of slip rate by using REs. We use seismograms from the High Sensitivity Seismograph Network (Hi-net), Full Range Seismograph Network of Japan (F-net), and permanent seismic stations of Japan Meteorological Agency (JMA), Tohoku University, University of Tokyo, and Kagoshima University from 8 May 2003 (Izu-Bonin) and 14 July 2005 (Ryukyu) to 31 December 2012 to detect REs along the two trenches, by using similarity of seismograms. We mainly follow the procedure adopted in Uchida and Matsuzawa (2013) that studied REs in Tohoku area to compare our results with the REs in Tohoku. We find that the RE distribution along the Ryukyu trench shows two bands parallel to the trench axis. This feature is similar to the pattern in Tohoku where relatively large earthquakes occur between the bands. Along the Izu-Bonin trench, on the other hand, we find much fewer REs than in Tohoku or Ryukyu subduction zones and only one along-trench RE band, which corresponds to the area where the subducting Pacific plate contacts with the crust of the Philippine Sea plate. We also estimate average slip rate and coupling coefficient by using an empirical relationship between seismic moment and slip for REs (Nadeau and Johnson, 1998) and relative plate motion model. As a result, we find interplate slip rate in the deeper band is higher than shallower one along the Ryukyu trench suggesting larger locking along the shallower band. This feature is also similar to the pattern in the NE Japan. Our results indicate that the Ryukyu subduction zone is very similar to the NE Japan subduction zone, while the Izu-Bonin subduction zone appears to be different from the other two zones according to the RE analyses.

  19. Seismic Constraints on the Japan Subduction Zone from Waveform Inversions of SS precursors

    NASA Astrophysics Data System (ADS)

    Dokht, R.; Gu, Y. J.; Sacchi, M. D.

    2014-12-01

    Arrival times of long-period secondary mantle reflections such as SS and PP precursors have made significant contributions to the understanding of the mantle structure and slab dynamics beneath the Pacific Northwest. Due to strong sensitivities to discontinuity depths, the timing information is often correlated with P/S velocity models from high-resolution seismic tomography while stacked reflection amplitudes provide a measure of impedance contrast. A potential pitfall in the interpretations of SS-precursor measurements is velocity-discontinuity depth ambiguity, as the timing of secondary reflections is mainly 'corrected' based on existing smoothed velocity estimates. In this study we quantitatively investigate the amplitude information of a dense SS precursor data set sampling the northwestern Pacific region. We model the full waveforms of SS precursors using the Genetic Algorithm (GA), an effective nonlinear inversion technique, and properly account for the tradeoff between shear wave velocity and discontinuity depth perturbation. The inverted shear velocities clearly show a consistent high-velocity, dipping structure along the Wadati-Benioff zone, likely in connection with the descending Japan slab. The slab appears to stagnate and horizontally deflect within the upper mantle transition zone beneath northeastern China. The integrity of the deflected slab appears to be compromised beneath the Changbai hotspot where a low velocity anomaly interrupts the flat lying high velocity structure and extends upward to, at least, mid MTZ depths. This anomaly does not appear to reach the Changbai hotspot, though its connection with observed low velocities in the lithosphere and asthenosphere may not be ruled out. Our nonlinear waveform inversion results also show a 600-km wide low velocity zone (up to -4% relative to the Preliminary Reference Earth Model (PREM)) atop the 660-km discontinuity on the oceanic side of the subducting Japan slab. The cause of this anomaly remains questionable, though 1) upwelling hot mantle materials in response to slab interaction with the lower mantle and 2) the remnant of the Mesozoic-era superplume in the Pacific ocean may both contribute to its existence.

  20. Invited review paper: Fault creep caused by subduction of rough seafloor relief

    NASA Astrophysics Data System (ADS)

    Wang, Kelin; Bilek, Susan L.

    2014-01-01

    Among the wide range of thermal, petrologic, hydrological, and structural factors that potentially affect subduction earthquakes, the roughness of the subducting seafloor is among the most important. By reviewing seismic and geodetic studies of megathrust locking/creeping state, we find that creeping is the predominant mode of subduction in areas of extremely rugged subducting seafloor such as the Kyushu margin, Manila Trench, northern Hikurangi, and southeastern Costa Rica. In Java and Mariana, megathrust creeping state is not yet constrained by geodetic observations, but the very rugged subducting seafloor and lack of large earthquakes also suggest aseismic creep. Large topographic features on otherwise relatively smooth subducting seafloor such as the Nazca Ridge off Peru, the Investigator Fracture Zone off Sumatra, and the Joban seamount chain in southern Japan Trench also cause creep and often stop the propagation of large ruptures. Similar to all other known giant earthquakes, the Tohoku earthquake of March 2011 occurred in an area of relatively smooth subducting seafloor. The Tohoku event also offers an example of subducting seamounts stopping rupture propagation. Very rugged subducting seafloor not only retards the process of shear localization, but also gives rise to heterogeneous stresses. In this situation, the fault zone creeps because of distributed deformation of fractured rocks, and the creep may take place as transient events of various spatial and temporal scales accompanied with small and medium-size earthquakes. This process cannot be described as stable or unstable friction along a single contact surface. The association of large earthquakes with relatively smooth subducting seafloor and creep with very rugged subducting seafloor calls for further investigation. Seafloor near-trench geodetic monitoring, high-resolution imaging of subduction fault structure, studies of exhumed ancient subduction zones, and laboratory studies of low-temperature creep will greatly improve our understanding of the seismogenic and creep processes and their hazard implications.

  1. Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench

    NASA Astrophysics Data System (ADS)

    Meffre, Sebastian; Falloon, Trevor J.; Crawford, Tony J.; Hoernle, Kaj; Hauff, Folkmar; Duncan, Robert A.; Bloomer, Sherman H.; Wright, Dawn J.

    2012-12-01

    A wide variety of different rock types were dredged from the Tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 Boomerang voyage. 40Ar-39Ar whole rock and U-Pb zircon dating suggest that these fore arc rocks were erupted episodically from the Cretaceous to the Pliocene (102 to 2 Ma). The geochemistry suggests that MOR-type basalts and dolerites were erupted in the Cretaceous, that island arc tholeiites were erupted in the Eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time. The ages generally become younger northward suggesting that fore arc crust was created in the south at around 48-52 Ma and was extended northward between 35 and 28 Ma, between 9 and 15 Ma and continuing to the present-day. The episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the Bonin-Marianas arc. The Bonin-Marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 Ma at the beginning of subduction above a rapidly foundering west-dipping slab. Instead a model where the 52 Ma basalts that are presently in a fore arc position were created in the arc-back arc transition behind the 57-35 Ma Loyalty-Three Kings arc and placed into a fore arc setting after arc reversal following the start of collision with New Caledonia is proposed for the oldest rocks in Tonga. This is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time.

  2. The 3-D distribution of random velocity inhomogeneities in southwestern Japan and the western part of the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Takahashi, Tsutomu; Obana, Koichiro; Yamamoto, Yojiro; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki

    2013-05-01

    waves at high frequencies (>1 Hz) show collapsed and broadened wave trains caused by multiple scattering in the lithosphere. This study analyzed the envelopes of direct S waves in southwestern Japan and on the western side of the Nankai trough and estimated the spatial distribution of random inhomogeneities by assuming a von Krmn type power spectral density function (PSDF). Strongly inhomogeneous media have been mostly imaged at shallow depth (0-20 km depth) in the onshore area of southwestern Japan, and their PSDF is represented as P(m) ? 0.05m-3.7 km3, with m being the spatial wave number, whereas most of the other area shows weak inhomogeneities of which PSDF is P(m) ? 0.005m-4.5 km3. At Hyuga-nada in Nankai trough, there is an anomaly of inhomogeneity of which PSDF is estimated as P(m) ? 0.01m-4.5 km3. This PSDF has the similar spectral gradient with the weakly inhomogeneous media, but has larger power spectral density than other offshore areas. This anomalous region is broadly located in the subducted Kyushu Palau ridge, which was identified by using velocity structures and bathymetry, and it shows no clear correlation with the fault zones of large earthquakes in past decades. These spatial correlations suggest that possible origins of inhomogeneities at Hyuga-nada are ancient volcanic activity in the oceanic plate or deformed structures due to the subduction of the Kyushu Palau ridge.

  3. Multiple major faults at the Japan Trench: Chemostratigraphy of the plate boundary at IODP Exp. 343: JFAST

    NASA Astrophysics Data System (ADS)

    Rabinowitz, Hannah S.; Savage, Heather M.; Plank, Terry; Polissar, Pratigya J.; Kirkpatrick, James D.; Rowe, Christie D.

    2015-08-01

    We determine the trace element stratigraphy of Site C0019, drilled during the Japan Fast Trench Drilling Project (JFAST) International Ocean Discovery Program (IODP) Expedition 343, to illuminate the structure of the plate boundary following the Tohoku-Oki earthquake of 2011. The stratigraphic units at the JFAST site are compared to undeformed Western Pacific sediments from two reference sites (Ocean Drilling Program (ODP) Site 1149 and Deep Sea Drilling Project (DSDP) Site 436). The trace element fingerprints in these reference sedimentary units can be correlated to individual JFAST samples. At the JFAST site, we find that the accretionary wedge and downgoing plate sediments in the core are composed primarily of Holocene to Eocene sediments. There are several age reversals and gaps within the sequence, consistent with multiple faults in the bottom 15 m of the JFAST core. Our results point to several candidate faults that could have slipped during the 2011 Tohoku-Oki earthquake, in addition to the pelagic clay layer that has been proposed as the main décollement fault.

  4. Locomotory activity and feeding strategy of the hadal munnopsid isopod Rectisura cf. herculea (Crustacea: Asellota) in the Japan Trench.

    PubMed

    Jamieson, Alan J; Fujii, Toyonobu; Priede, Imants G

    2012-09-01

    Benthic fauna in the hadal zone (6500-11,000 m) rely on maintaining sufficient locomotory activity to exploit a low, patchy and uniquely distributed food supply while exposed to high pressure, low temperatures and responding to predator-prey interactions. Very little is currently known about the locomotory capabilities of hadal fauna. In situ video footage of the isopod Rectisura cf. herculea (Birstein 1957) (Asellota, Munnopsidae) was obtained from 6945 and 7703 m deep in the Japan Trench (NW Pacific Ocean). Measurements of locomotion revealed routine walking speeds of 0.19 0.04 BL s(-1) (mean s.d.), increasing to 0.33 0.04 BL s(-1) if naturally perturbed by larger organisms. When immediately threatened by the presence of predators (decapod crustaceans), the isopods are capable of eliciting backward escape jumps and burst swimming escape responses of 2.6 1.5 BL s(-1) and 4.63 0.9 BL s(-1), respectively. These data suggest no significant reduction in locomotory capability despite the extreme depths in which they inhabit. These observations also revealed the isopod to be a bait-attending and aggregative species and suggest that it may not be obligatorily selecting infaunal food sources as previously thought. PMID:22875769

  5. Mantle flow and deformation of subducting slab at the junction of Tohoku-Kurile arc

    NASA Astrophysics Data System (ADS)

    Morishige, M.; Honda, S.

    2012-12-01

    Geophysical studies of the plate junction reveal possible evidence of the presence of 3D mantle flow and deformation of subducting slabs. The junction of the Tohoku-Kurile is one of the best studied junctions in the world. The Pacific plate subducts under the North American plate in a direction almost perpendicular to Japan trench, while it subducts obliquely along the Kurile arc. Analysis of seismic anisotropy in this region shows the trench-normal fast polarization direction of S-wave splitting in the back arc even where the oblique subduction occurs. The angle of subduction varies along the strike of the trench, that is, it is smallest near the plate junction and becomes large beneath Kurile arc. There is also an important distinction in the slab behavior. The slab beneath Tohoku stagnates in the transition zone, whereas the slab beneath the Kurile arc penetrates into the lower mantle. In this presentation, we show the dynamic effects of the junction using a numerical model of mantle convection with a realistic curved shape of the trench in spherical geometry. The model is set so that it becomes similar to the geometry of the surface plate boundary in the Tohoku-Kurile arc. In order to enable one-sided subduction, the velocities are imposed both on the surface and in the small 3D boundary region around the trench. We obtain 3D flow in the mantle wedge which is consistent with the observation of seismic anisotropy including the oblique subduction zone. The flow and hence the fast polarization direction in the subslab mantle is almost 2D. We also find that the angle of subduction varies along-strike, which agrees with the observations. This variation can be explained by a torque balance acting on subducting slabs in the case of oblique subduction. This along-arc variation of the angle of subduction partly contributes to the different behavior of slab stagnation in the Tohoku-Kurile arc. Our results show that the shape of the trench is an important factor which considerably affects mantle flow and deformation of subducting slabs. Thus, 3D modeling is necessary to constrain the dynamics of subduction zones near the junction zone and gives us further understanding of subduction processes.

  6. A Paleozoic Japan-type subduction-accretion system in the Beishan orogenic collage, southern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Song, Dongfang; Xiao, Wenjiao; Windley, Brian F.; Han, Chunming; Tian, Zhonghua

    2015-05-01

    Magmatic arcs ascribed to oceanic lithosphere subduction played a dominant role in the construction of the accretionary Central Asian Orogenic Belt (CAOB). The Beishan orogenic collage, situated between the Tianshan Orogen to the west and the Inner Mongolia Orogen to the east, is a key area to understanding the subduction and accretionary processes of the southern CAOB. However, the nature of magmatic arcs in the Beishan and the correlation among different tectonic units along the southern CAOB are highly ambiguous. In order to investigate the subduction-accretion history of the Beishan and put a better spatial and temporal relationship among the tectonic belts along the southern CAOB, we carried out detailed field-based structural geology and LA-ICP-MS zircon U-Pb geochronological as well as geochemical studies along four cross-sections across crucial litho-tectonic units in the central segment of the Beishan, mainly focusing on the metamorphic assemblages and associated plutons and volcanic rocks. The results show that both the plutonic and volcanic rocks have geochemical characteristics similar to those of subduction-related rocks, which favors a volcanic arc setting. Zircons from all the plutonic rocks yield Phanerozoic ages and the plutons have crystallization ages ranging from 464 ± 2 Ma to 398 ± 3 Ma. Two volcanic-sedimentary rocks yield zircons with a wide age range from Phanerozoic to Precambrian with the youngest age peaks at 441 Ma and 446 Ma, estimated to be the time of formation of the volcanic rocks. These new results, combined with published data on ophiolitic mélanges from the central segment of the Beishan, favor a Japan-type subduction-accretion system in the Cambrian to Carboniferous in this part of the Paleo-Asian Ocean. The Xichangjing-Niujuanzi ophiolite probably represents a major suture zone separating different tectonic units across the Beishan orogenic collage, while the Xiaohuangshan-Jijitaizi ophiolitic mélange may represent a Carboniferous back-arc basin formed as a result of slab rollback ascribed to northward subduction of the Niujuanzi oceanic lithosphere. Subduction of this back-arc basin probably took place in the early Carboniferous, generating the widespread arc-related granitoids including adakitic plutons, and overlapping earlier arc assemblages. The Beishan orogenic collage is not the eastern extension of the Chinese Central Tianshan, but it was generated by the same north-dipping subduction system separated by the Xingxingxia transform fault, as revealed by available regional data. This contribution implies that in addition to fore-arc accretion, back-arc accretion ascribed to opening and closure of a back-arc basin may also have been a common process in the construction of the CAOB, resembling that of the Mesozoic-Cenozoic subduction-accretion system in the SW pacific.

  7. Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan

    NASA Astrophysics Data System (ADS)

    Shiina, Takahiro; Nakajima, Junichi; Toyokuni, Genti; Matsuzawa, Toru

    2014-12-01

    At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

  8. On the initiation of subduction

    SciTech Connect

    Mueller, S.; Phillips, R.J. )

    1991-01-10

    Estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation. Three environments proposed as preferential sites of incipient subduction are investigated: passive continental margins, transform faults/fracture zones, and extinct ridges. None of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses. Subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins. The attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates. This is the only tectonic force sufficient to trigger the nucleation of a new subduction zone. The ubiquitous distribution of transform faults and fracture zones, combined with the common proximity of these features to mature subduction complexes, suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere.

  9. Evolution and modulation of a poleward-propagating anticyclonic eddy along the Japan and Kuril-Kamchatka trenches

    NASA Astrophysics Data System (ADS)

    Kaneko, Hitoshi; Itoh, Sachihiko; Kouketsu, Shinya; Okunishi, Takeshi; Hosoda, Shigeki; Suga, Toshio

    2015-06-01

    To investigate the relationships between the movement of an eddy and its interior structure and water properties, four profiling floats were deployed in an anticyclonic eddy in the western North Pacific in 2013 (April-October). Daily float profiles showed rapid changes in temperature and salinity corresponding to strong interactions between eddies north of the subtropical Kuroshio Extension. After the first interaction with a warm-core eddy in April, the isolation of the winter mixed layer from the surface was observed, forming a subsurface remnant layer. Another interaction with a cold fresh eddy at middepths in May resulted in the formation of a multilayer structure. The eddy then moved poleward along the Japan and Kuril-Kamchatka trenches, indicating changes in its propagation pattern coupled to its interior structure. The eddy then moved northward (June-July), stalled (July-August), and moved eastward (August-October). In addition to a general declining trend, the properties of the warm saline core changed over a short time period, coinciding with changes in propagation. A density anomaly at middepths of the eddy changed location during the stalled period; however, denser waters were continuously observed in the southeast part of the eddy during its northward and eastward movement. This unidirectional density anomaly pattern was consistent with the structure of the poleward-propagating eddy, which interacted with the western topographic boundary. Meridional exchanges of heat and material were potentially elevated by the eddy's advection and movement, as well as by water modifications in the eddy associated with exchanges along its perimeter.

  10. New estimates of subducted water from depths of extensional outer rise earthquakes at the Northwestern Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Emry, E. L.; Wiens, D. A.

    2012-12-01

    The presence of water within the subducting slab mantle may have important implications for subduction zone water budgets, intermediate depth earthquakes, and transport of water into Earth's deep mantle. However, the amount of water stored in hydrous slab mantle rocks prior to subduction is not well constrained. Large extensional faults formed as the plate bends at the subduction zone outer rise are thought to be the main pathway by which water can travel into and hydrate the slab mantle; yet for many subduction zones accurate depths of extensional outer rise faulting are also not well known. Therefore, we attempt to identify the maximum observed depth of extensional faulting, and thereby identify the possible depth extent of slab mantle hydration, by accurately locating and determining depths for outer rise and trench axis earthquakes at Northern and Western Pacific subduction zones. For each region, we relocate all earthquakes seaward of the trench axis as well as forearc earthquakes within 60 km landward of the trench axis using ISC arrival times and the hypocentroidal decomposition relative location algorithm. We then model P- and SH- waveforms and their associated depth phases for all earthquakes with Mw 5.0+ since 1990 that exhibit good signal-to-noise ratios and do not have shallow-dipping thrust focal mechanisms, which are indicative of subduction zone plate interface earthquakes. In total, we redetermined epicenters and depths for over 70 earthquakes at the Alaskan, Aleutian, Kamchatka, Kuril, Japan, and Izu-Bonin-Mariana trenches. We find that at most Pacific subduction zones there is evidence for extensional faulting down to 10-15 km within the top of the oceanic plate mantle, and in total, 95% of our analyzed extensional outer rise events occur within the crust or top 15 km of the mantle. However some regions, such as the Bonin and Aleutian Islands, show evidence for extensional faulting as deep as 20 km below the base of the crust. If the mantle of the subducting slab is hydrated down to ~15 km (with ~2-3.5 wt. % water), and assuming published values for the amount of water in the slab crust [1], then we expect that ~10^10 Tg/Myr of water are input into Northwestern Pacific subduction zones. This value for only the Northwestern Pacific subduction zones is then 10 times larger than previous global estimates [1] and indicates a need to reevaluate recent subduction water flux calculations. [1] Van Keken et al (2011), JGR, 116, B01401.

  11. Determination of fault plane orientations from rupture directivity of intermediate and deep earthquakes in the northeastern Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Osburg, Tim

    Earthquakes occur at intermediate (? 70 km) and deep (? 300 km) depths within the Earth. Since their discovery, the cause of intermediate and deep-focus earthquakes has remained a fundamental problem in seismology. While several physical mechanisms have been theorized to explain the occurrence of deep earthquakes, the mechanism may be constrained by determining the fault plane orientation. I used rupture directivity of 52 events in the northeastern Japan subduction zone to determine each event's fault plane orientation. With adequate station coverage, in both azimuth and distance, rupture can be identified along a single nodal plane signifying the fault plane. P-wave duration is shortest in the direction of rupture propagation and greatest in the opposite direction. Measuring the differential rupture duration between seismic stations allows for the determination of rupture direction, rupture velocity relative to the background seismic velocity, and the fault plane for each event. Rupture directivity is determined for a total of 20 events. Results indicate that rupture at intermediate and deep focus depths occur along the subhorizontal nodal plane. While fault plane orientation is apparently uniform in the subducting slab, rupture direction appears scattered. These results suggest multiple mechanisms are responsible for the orientation and azimuthal direction of rupture.

  12. Slow slip and aseismic deformation episodes associated with the subducting Pacific plate offshore Japan, revealed by changes in seismicity (Invited)

    NASA Astrophysics Data System (ADS)

    Marsan, D.; Reverso, T.; Helmstetter, A.; Enescu, B.

    2013-12-01

    Aseismic phenomena, including slow slip, can alter the surrounding seismicity. We here investigate how seismicity can be used in order to reveal episodes of aseismic deformation: transient anomalous increases of seismicity activity are searched for, as signatures of episodic aseismic deformation in a fault zone. An objective method is proposed, that accounts for both earthquake interactions and transient loading. Applying it to the 1990 - 2011 (pre-Tohoku) seismicity of the Japan subduction zone, we find several significant instances of aseismic transients. Small scale and short duration transients are favored updip of the subducting plate.Large scale transients are mostly observed off-shore Ibaraki prefecture, in a partly decoupled zone that extends downdip. The four most intense of such transients have occurred periodically every 5.9 years, and are likely due to slow slip episodes. Other aseismic phenomena, including possible fluid intrusion in the outer-rise, are also detected. Finally, the seismicity in January and February 2011, close to the epicenter of the mega-thrust Tohoku earthquake, is found to be due to aseismic loading, confirming previous studies, although this transient is only one among others, and is not the most intense nor the most significant for the 21 year-long period studied here.

  13. Internal structure of the shallow Japan Trench dcollement: insights into the long-term evolution of the margin and coseismic slip processes

    NASA Astrophysics Data System (ADS)

    Kirkpatrick, J. D.; Ujiie, K.; Mishima, T.; Chester, F. M.; Rowe, C. D.; Regalla, C.; Remitti, F.; Moore, J. C.; Toy, V. G.; Kameda, J.; Bose, S.; Wolfson-Schwehr, M.

    2013-12-01

    The 2011 MW 9.0 Tohoku-oki earthquake ruptured to the Japan Trench, with largest coseismic slip occurring on the shallow part of the dcollement. To better understand the controls on rupture propagation and slip, the structure and composition of the dcollement near the trench were investigated during Integrated Ocean Drilling Project Expedition 343 (the JFAST project). The plate boundary dcollement is localized upon a ?4.86 m thick layer of smectite-rich pelagic clay. Stratigraphic discontinuities at the base of the hangingwall, top of the footwall and surrounding a horse of intra-dcollement mudstone suggest that the fault contains multiple slip surfaces, although most of these were not recovered. The dcollement damage zone is <10 m wide in both the overlying frontal prism and down-going Pacific plate showing that long-term displacement on the plate boundary fault near the Japan Trench is extremely localized and in turn suggesting the fault is weak relative to the bounding sediments. A pervasive composite foliation, or scaly fabric, defined by striated, lustrous surfaces enclosing lenses of less fissile phacoids is distributed throughout the dcollement clay. The asymmetry of phacoids is consistent with top-to-the-trench shear sense. Several narrow, planar discontinuities crosscut the scaly fabric, truncating or disrupting the foliation and in one case juxtaposing domains of the clay with different foliation orientation and intensity, indicating relative displacement. The scaly fabric is indicative of distributed shear across the recovered interval (~1 m), and may represent deformation at interseismic strain rates. The sharp discontinuities within the dcollement clay, however, resulted from slip localization. They are similar to structures produced in friction experiments conducted at coseismic slip rates suggesting they may record earthquake deformation.

  14. Boron and Strontium Isotope Systematics of Interstitial Water from IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST)

    NASA Astrophysics Data System (ADS)

    Ishikawa, T.; Sample, J. C.; Takai, K.; Chester, F.; Mori, J. J.; Eguchi, N.; Toczko, S.

    2012-12-01

    The Integrated Ocean Drilling Program (IODP) Expedition 343 drilled three holes through the plate boundary near the Japan Trench to investigate the cause of very large fault slip during the 2011 Tohoku-Oki earthquake. Twelve interstitial water samples were recovered from the Hole C0019E at the depths predominantly between 689 mbsf and 831 mbsf. In this paper we report boron and strontium isotope compositions of the interstitial water samples. All the interstitial water samples from the Hole C0019E show 87Sr/86Sr ratios lower than that of modern seawater. The 87Sr/86Sr ratios are relatively constant at the depths above 730 mbsf, but clearly decrease with increasing depth below 780 mbsf. The relationship between strontium concentrations and the 87Sr/86Sr ratios indicates that the interstitial water compositions are essentially controlled by three-component mixing, and the waters above 730 mbsf and below 780 mbsf require distinct end-component fluids. This relationship also negates the drilling-induced contamination as a factor controlling the interstitial water composition because neither modern seawater nor drilling mud fluid can be an appropriate end-component. The boron isotope ratios (δ11B values) of the interstitial waters are all higher than that of seawater, and show a minimum around 710 mbsf and a maximum around 820 mbsf. The high δ11B values around 820 mbsf are apparently similar to those observed near the fractured intervals and shear zones in the Ocean Drilling Program (ODP) Site 1150 (Deyhle and Kopf, 2002). Although the origin of the three end-component fluids is a subject of further investigation, two major fault zones inferred at Site C0019, for which the lower one is regarded as a plate boundary fault, seem to be associated with different types of fluids. This conclusion is also supported by significantly different characteristics of some minor and trace element concentrations in the interstitial waters around these two fault zones. Reference: Deyhle and Kopf (2002) Marine Geology, 183, 1-15.

  15. In-Situ Observations of Earthquake-Driven Fluid Pulses within the Japan Trench Plate Boundary Fault Zone

    NASA Astrophysics Data System (ADS)

    Fulton, P. M.; Brodsky, E. E.

    2014-12-01

    Fault valving and transient fluid flow has long been suspected to be an important process in the earthquake cycle, but has not previously been captured by direct measurements during an episode. In particular, earthquakes are thought to drive fluids in fault zones, but again, evidence has been limited to the geologic record. Here we report on the signature of fluid pumping events inside the Tohoku Fault associated with individual earthquakes. As part of the Japan Trench Fast Drilling Project (JFAST), a sub-seafloor temperature observatory was installed across the plate boundary fault zone that ruptured during the 2011 Mw 9.0 Tohoku-oki earthquake. The observatory consisted of 55 autonomous temperature sensing dataloggers extending up to 820 m below sea floor at a water depth of ~7 km. The temporary deployment recorded data from July 2012 through April 2013. In addition to measuring the frictional heat signal from the megathrust earthquake, the high-resolution temperature time series data reveal spatially coherent temperature transients following regional earthquakes. Temperature increases vertically upwards from a fracture zone and decreases downwards, which is consistent with the expected signature of a pulse entering the annulus from the fracture zone. The anomalies are a few hundredths of degree Celsius and occur repeatedly at depths that are independently interpreted to have higher fracture permeability. High-pass filtered data are spatially correlated in areas disturbed by transient fluid advection. Fluid pulses occur in response to over a dozen local earthquakes, including a Mw 5.4 on 14 October 2012, a Mw 5.5 on 11 November 2012, and a doublet of two very local Mw 7.2 intraplate earthquakes on 7 December 2012, along with its associated aftershocks. There does not appear to be a response to large far-field earthquakes such as the 28 October 2012 Mw 7.8 Haida Gwaii or 6 February 2013 Mw 8.0 Santa Cruz Islands earthquakes. These measurements provide the first in situ documentation of seismic pumping at fractured regions of the fault damage zone. Near fault measurements such as these may provide insight into drivers of earthquake occurrence. The redistribution of fluid pressures within fault zones, such as observed here in response to earthquakes, is a potential mechanism that may be involved in earthquake triggering.

  16. Seismic interferometry imaging of subducting Philippine Sea plate and crustal structure in Tokai region, central Japan using natural earthquakes

    NASA Astrophysics Data System (ADS)

    Totani, M.; Watanabe, T.; Yamaoka, K.; Kato, A.; Iidaka, T.; Ikuta, R.; Tsumura, N.; Okubo, M.; Suzuki, S.

    2013-12-01

    Seismic interferometry is an imaging method of subsurface structures using passive seismic records without artificial energy sources. Using natural earthquake records, seismic reflection imaging of deep crustal structures and plate boundaries is expected. We applied seismic interferometry to natural earthquake records, acquired by a wide-aperture linear seismometer array called Tokai Array, to image the P- and S-wave seismic structures in the Tokai region, central Japan. The Tokai Array observation was conducted from April to August 2008 with 81 three-component seismometers spanning about 100km in length. Records of 8 Hi-net stations near the array were also used. At first, we applied auto-correlation analysis to the record of deep earthquakes. Since the auto-correlation analysis assumes one-dimensional wave propagation, we searched deep earthquakes that could be considered as normal incidence using Fresnel zone concept. We selected 13 events of deep earthquakes for the P-wave and 10 events for the S-wave analysis, which occurred along subducting Pacific Sea plate at 200 - 300km in depth. After correcting seismometer response, we applied the band-pass filter from 1.0 to 2.5 Hz and 0.5 to 1.0 Hz for P- and S-wave, respectively, corresponding to the dominant frequency of the seismic records. Then we removed the records showing low S/N ratio. Afterwards, we calculated the auto-correlation to obtain virtual shot record, which is equivalent to zero-offset shot record. We applied a filter that transforms source functions into the simple Ricker wavelet, to remove the effects associated with source functions. After whitening deconvolution to remove multiples, we stacked auto-correlation of every earthquake record, and applied Kirchoff depth migration using the velocity model estimated by seismic tomography method (Kato et al., 2010). The result of auto-correlation analysis shows good agreement with previous researches in the area, such as seismic tomography, receiver function (Kato et al., 2010; Takaoka, et al., 2012). P- and S-wave virtual reflection profiles show reflectors corresponding to continental Moho and subducting Philippine Sea plate interface. Discontinuity of reflectors is found at the interface of geological structure in shallow part. These results indicate auto-correlation analysis has the potential to image plate boundary and crustal structures. In the Tokai Array observation, the seismometers were not evenly located. Creating a virtual shot gather using cross-correlation analysis improves the spatial density of the records. Therefore, the reliability of lateral variation of reflector amplitude may be improved. Application of the cross-correlation analysis is underway. We will discuss the lateral change in the reflection strength along subducting Philippine Sea plate boundary in the presentation. Reference: Kato, A. et al. (2010). Variations of fluid pressure within the subducting oceanic crust and slow earthquakes. Geophys.Res.Lett., 37, L14310, doi: 10.1029/2010GL043723. Takaoka H. et al. (2012). Three-dimensional Attenuation Structure beneath the Tokai Region, Central Japan Derived Using Local Earthquakes Spectra. Zisin 2, 65, 175-187, doi: 10.4294/zisin.65.175 (in Japanese)

  17. Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, Yoshitaka; Yamano, Natsuko

    2014-12-01

    Tectonic mlange zones within ancient accretionary complexes include various styles of strain accommodation along subduction interfaces from shallow to deep. The ductile-brittle transition at shallower portions of the subduction plate boundary was identified in three tectonic mlange zones (Mugi mlange, Yokonami mlange, and Miyama formation) in the Cretaceous Shimanto Belt, an on-land accretionary complex in southwest Japan. The transition is defined by a change in deformation features from extension veins only in sandstone blocks with ductile matrix deformation (possibly by diffusion-precipitation creep) to shear veins (brittle failure) from shallow to deep. Although mlange fabrics represent distributed simple to sub-simple shear deformation, localized shear veins are commonly accompanied by slickenlines and a mirror surface. Pressure-temperature (P-T) conditions for extension veins in sandstone blocks and for shear veins are distinct on the basis of fluid inclusion analysis. For extension veins, P-T conditions are approximately 125 to 220C and 80 to 210 MPa. For shear veins, P-T conditions are approximately 185 to 270C and 110 to 300 MPa. The P-T conditions for shear veins are, on average, higher than those for extension veins. The temperature conditions overlap in the range of approximately 175 to 210C, which suggests that the change from more ductile to brittle processes occurs over a range of depths. The width of the shallow ductile-brittle transition zone can be explained by a heterogeneous lithification state for sandstone and mudstone or high fluid pressure caused by clay dehydration, which is controlled by the temperature conditions.

  18. Global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones

    NASA Astrophysics Data System (ADS)

    Schellart, W. P.; Rawlinson, N.

    2013-12-01

    The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra-Andaman, Japan) and others producing relatively small earthquakes (e.g. Mariana, Scotia). Here we show how such variability might depend on various subduction zone parameters. We present 24 physical parameters that characterize these subduction zones in terms of their geometry, kinematics, geology and dynamics. We have investigated correlations between these parameters and the maximum recorded moment magnitude (MW) for subduction zone segments in the period 1900-June 2012. The investigations were done for one dataset using a geological subduction zone segmentation (44 segments) and for two datasets (rupture zone dataset and epicenter dataset) using a 200 km segmentation (241 segments). All linear correlations for the rupture zone dataset and the epicenter dataset (|R| = 0.00-0.30) and for the geological dataset (|R| = 0.02-0.51) are negligible-low, indicating that even for the highest correlation the best-fit regression line can only explain 26% of the variance. A comparative investigation of the observed ranges of the physical parameters for subduction segments with MW > 8.5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes. For segments with MW > 8.5 distinct (narrow) ranges are observed for several parameters, most notably the trench-normal overriding plate deformation rate (vOPD⊥, i.e. the relative velocity between forearc and stable far-field backarc), trench-normal absolute trench rollback velocity (vT⊥), subduction partitioning ratio (vSP⊥/vS⊥, the fraction of the subduction velocity that is accommodated by subducting plate motion), subduction thrust dip angle (δST), subduction thrust curvature (CST), and trench curvature angle (αT). The results indicate that MW > 8.5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates (-3.0 ⩽ vOPD⊥ ⩽ 2.3 cm/yr), slow trench velocities (-2.9 ⩽ vT⊥ ⩽ 2.8 cm/yr), moderate to high subduction partitioning ratios (vSP⊥/vS⊥ ⩽ 0.3-1.4), low subduction thrust dip angles (δST ⩽ 30°), low subduction thrust curvature (CST ⩽ 2.0 × 10-13 m-2) and low trench curvature angles (-6.3° ⩽ αT ⩽ 9.8°). Epicenters of giant earthquakes with MW > 8.5 only occur at trench segments bordering overriding plates that experience shortening or are neutral (vOPD⊥ ⩽ 0), suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress (deviatoric compression) on the interface (i.e. a normal stress asperity). Notably, for the three largest recorded earthquakes (Chile 1960, Alaska 1964, Sumatra-Andaman 2004) the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress (neutral or deviatoric tension). Stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface. We have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake (MW > 8.5) using the six most indicative subduction zone parameters (vOPD⊥, vT⊥, vSP⊥/vS⊥, δST, CST and αT). We identify a number of subduction zones and segments that rank highly, which implies a capability to generate MW > 8.5 earthquakes. These include Sunda, North Sulawesi, Hikurangi, Nankai-northern Ryukyu, Kamchatka-Kuril-Japan, Aleutians-Alaska, Cascadia, Mexico-Central America, South America, Lesser Antilles, western Hellenic and Makran. Several subduction segments have a low score, most notably Scotia, New Hebrides and Mariana.

  19. Paleostress analysis of a subduction zone megasplay fault - An example from the Nobeoka Thrust, Japan

    NASA Astrophysics Data System (ADS)

    Kawasaki, R.; Hamahashi, M.; Hashimoto, Y.; Otsubo, M.; Yamaguchi, A.; Kitamura, Y.; Kameda, J.; Hamada, Y.; Fukuchi, R.; Kimura, G.

    2014-12-01

    The megasplay faults in subduction zones, branching from plate boundary thrusts, are thought to have a potential to generate earthquakes and accompany tsunamis. Paleo-splay faults exposed on land often preserve clear deformation features of the seismogenic zone and provide information on the fault mechanisms at depth. One of the important information that can be obtained from exhumed faults is paleo-stress field. Here we investigated the Nobeoka Thrust, a fossilized megasplay fault in the Shimanto Belt in Kyushu, which consists of phyllite and sandstone-shale mlanges that have experienced maximum burial temperatures of ~250 -320C, [Kondo et al., 2005, Tectonics 24.6(2005)]. Kondo et al. (2005) described two orientations of slickensides from the outcrop, suggesting the existence of flexural gentle fold in kilometer scale. The paleo-stress fields preserved in the Nobeoka Thrust is likely to represent multiple stages occurring during burial and uplift, enabling the reconstruction of fault motions along the fault. In this study, we analyzed paleo-stress from slip vectors on small faults observed in the drilled cores of the Nobeoka Thrust obtained from scientific drilling performed in 2011. Small faults are expected to be less-reactivated and their population is much larger than that of large faults, providing high statistical reliability. Multiple inverse method [MIM; Yamaji, 2000, Journal of Structural Geology, 22, 441-452] was applied to the small faults. K-means clustering [Otsubo et al. , 2006, Journal of Structural Geology, 28, 991-997] was applied to stress tensors detected by the MIM for estimating optimal solutions. The results reveal stress solution of four directions existing throughout the drilled range. The stress solution is applied to faults distributed among different lithology, and therefore the paleo-stress is thought to have acted on the whole cores. By drawing the stress polygon from the direction of the stress solution and the stress rate, we estimate the stress state of the Nobeoka Thrust and discuss potential insights to the fault stress evolution of megasplay fault in a subduction zone.

  20. Holocene Tsunami Deposits From Large Tsunamis Along the Kuril Subduction Zone, Northeast Japan

    NASA Astrophysics Data System (ADS)

    Nanayama, F.; Furukawa, R.; Satake, K.; Soeda, Y.; Shigeno, K.

    2003-12-01

    Holocene tsunami deposits in eastern Hokkaido between Nemuro and Tokachi show that the Kuril subduction zone repeatedly produced earthquakes and tsunamis larger than those recorded in this region since AD 1804 (Nanayama et al., Nature, 424, 660-663, 2003). Twenty-two postulated tsunami sand layers from the past 9500 years are preserved on lake bottom near Kushiro City, and about ten postulated tsunami sand layers from the past 3000 years are preserved in peat layers on the coastal marsh of Kiritappu. We dated these ten tsunami deposits (named Ts1 to Ts10 from shallower to deeper) in peat layers by radiocarbon and tephrochronology, correlated them with historical earthquakes and tsunamis, and surveyed their spatial distribution to estimate the tsunamisO inland inundation limits. Ts10 and Ts9 are under regional tephra Ta-c2 (ca. 2.5 ka) and represent prehistorical events. Ts8 to Ts5 are between two regional tephra layers Ta-c2 and B-Tm (ca. 9th century). In particular, Ts5 is found just below B-Tm, so it is dated 9th century (Heian era). Ts4 is dated ca 13th century (Kamakura era), while Ts3, found just below Us-b and Ta-b (AD 1667-1663), is dated 17th century (Edo era). Ts2 is dated 19th century (Edo era) and may correspond to the AD 1843 Tempo Tokachi-oki earthquake (Mt 8.0) recorded in a historical document Nikkanki of Kokutai-ji temple at Akkeshi. Ts1 is inferred 20th century and may correspond to the tsunami from the AD 1960 Chilean earthquake (M 9.5) or the AD 1952 Tokachi-oki earthquake (Mt 8.2). Our detailed surveys indicate that Ts3 and Ts4 can be traced more than 3 km from the present coast line in Kirittapu marsh, much longer than the limits (< 1 km) of recent deposits Ts1 and Ts2 or documented inundation of the 19th and 20th century tsunamis. The recurrence intervals of great tsunami inundation are about 400 to 500 years, longer than that of typical interplate earthquakes along the Kuril subduction zone. The longer interval and the apparent large tsunami inundation indicate unusual origin of these tsunamis.

  1. Late Quaternary uplift rate inferred from marine terraces, Muroto Peninsula, southwest Japan: Forearc deformation in an oblique subduction zone

    NASA Astrophysics Data System (ADS)

    Matsu'ura, Tabito

    2015-04-01

    Tectonic uplift rates across the Muroto Peninsula, in the southwest Japan forearc (the overriding plate in the southwest Japan oblique subduction zone), were estimated by mapping the elevations of the inner edges of marine terrace surfaces. The uplift rates inferred from marine terraces M1 and M2, which were correlated by tephrochronology with marine isotope stages (MIS) 5e and 5c, respectively, include some vertical offset by local faults but generally decrease northwestward from 1.2-1.6 m ky- 1 on Cape Muroto to 0.3-0.7 m ky- 1 in the Kochi Plain. The vertical deformation of the Muroto Peninsula since MIS 5e and 5c was interpreted as a combination of regional uplift and folding related to the arc-normal offshore Muroto-Misaki fault. A regional uplift rate of 0.46 m ky- 1 was estimated from terraces on the Muroto Peninsula, and the residual deformation of these terraces was attributed to fault-related folding. A mass-balance calculation yielded a shortening rate of 0.71-0.77 m ky- 1 for the Muroto Peninsula, with the Muroto-Misaki fault accounting for 0.60-0.71 m ky- 1, but these rates may be overestimated by as much as 10% given variations of several meters in the elevation difference between the buried shoreline angles and terrace inner edges in the study area. A thrust fault model with flat (5-10 dip) and ramp (60 dip) components is proposed to explain the shortening rate and uplift rate of the Muroto-Misaki fault since MIS 5e. Bedrock deformation also indicates that the northern extension of this fault corresponds to the older Muroto Flexure.

  2. Modeling the effects of 3-D slab geometry and oblique subduction on subduction zone thermal structure

    NASA Astrophysics Data System (ADS)

    Wada, I.; Wang, K.; He, J.

    2013-12-01

    In this study, we revisit the effects of along-strike variation in slab geometry and oblique subduction on subduction zone thermal structures. Along-strike variations in slab dip cause changes in the descending rate of the slab and generate trench-parallel pressure gradients that drive trench-parallel mantle flow (e.g., Kneller and van Keken, 2007). Oblique subduction also drives trench-parallel mantle flow. In this study, we use a finite element code PGCtherm3D and examine a range of generic subduction geometries and parameters to investigate the effects of the above two factors. This exercise is part of foundational work towards developing detailed 3-D thermal models for NE Japan, Nankai, and Cascadia to better constrain their 3-D thermal structures and to understand the role of temperature in controlling metamorphic, seismogenic, and volcanic processes. The 3-D geometry of the subducting slabs in the forearc and arc regions are well delineated at these three subduction zones. Further, relatively large compilations of surface heat flow data at these subduction zones make them excellent candidates for this study. At NE Japan, a megathrust earthquake occurred on March 11, 2011; at Nankai and Cascadia, there has been a great effort to constrain the scale of the next subduction thrust earthquake for the purpose of disaster prevention. Temperature influences the slip behavior of subduction faults by (1) affecting the rheology of the interface material and (2) controlling dehydration reactions, which can lead to elevated pore fluid pressure. Beyond the depths of subduction thrust earthquakes, the thermal structure is affected strongly by the pattern of mantle wedge flow. This flow is driven by viscous coupling between the subducting slab and the overriding mantle, and it brings in hot flowing mantle into the wedge. The trench-ward (up-dip) extent of the slab-mantle coupling is thus a key factor that controls the thermal structure. Slab-mantle decoupling at shallow depths causes mantle stagnation and a cool condition, which allows serpentinization to occur, whereas coupling at greater depths drives hot flowing mantle, providing the thermal condition required for melt generation in the mantle wedge. The flowing mantle also causes rapid heating of the subducting slab and affects the occurrence of intraslab earthquakes. In the generic model calculations in the study, we also investigate the effect of local fluctuations in the depth of decoupling-coupling transition on the 3-D mantle wedge flow pattern and thermal structure. Kneller, E.A., and P.E. van Keken (2008), Effect of three-dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy, Geochem. Geophys. Geosyst., 9, Q01003, doi:10.1029/2007GC001677.

  3. Using meteoric 10Be to constrain the age and structure of the frontal wedge at the Japan Trench

    NASA Astrophysics Data System (ADS)

    Regalla, C.; Bierman, P. R.; Rood, D.; Motoyama, I.; Fisher, D. M.

    2013-12-01

    We present new meteoric 10Be concentration data from marine sediments recovered during International Ocean Drilling Program (IODP) Exp. 343 that help constrain the age and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku-oki M9 earthquake rupture. Exp. 343 recovered sediments from an ~200 m interval of the frontal wedge at site C0019. Core and log observations identify the plate boundary dcollement at ~820 mbsf, which separates a deformed sedimentary wedge from relatively undeformed underthrust sediments. However, reconstructions of the structural evolution of the wedge are difficult because of similarity in lithology between sediments from the incoming and overriding plate, and the chaotic character of seismic reflectors in the frontal wedge. We utilize the radiogenic decay of 10Be (t1/2 =1.36 Ma) in marine sediments to constrain variations in sediment age with depth in core C0019. Meteoric 10Be was isolated from marine sediments at the University of Vermont using total fusion and 10Be/9Be ratios were measured at the Scottish Universities Environmental Research Centre. Concentrations of meteoric 10Be in core C0019 range from 1.7x107 to 2.1x109 atm/g and are consistent with 10Be concentrations at nearby DSDP sites 436 and 434. We calculate 10Be sediment ages for analyzed samples assuming a range of initial 10Be concentrations from 1.6 to 2.1x109 atm/g. These concentrations are constrained by a 10Be sample co-located with a radiolarian micropaleontology sample at 780 mbsf that yields a Quaternary age, and from previously reported 10Be concentrations for Quaternary sediments in nearby DSDP cores. 10Be and radiolarian micropaleontology samples from similar depths yield consistent ages for late Miocene to Quaternary sediments (R2 = 0.89). Calculated 10Be ages range from 0-10 Ma, with ~50% of analyzed samples yielding ages <2 Ma. Repetition and inversion of high (109 atm/g) and low (107 atm/g) concentration sediments with depth in the core indicate at least three significant stratigraphic inversions within the recovered section between cores 1 and 2 (180 - 650 mbsf) cores 3 and 4 (655-690 mbsf), and cores 15 and 16 (817-819 mbsf). These inversions correspond to emplacement of late Miocene over Quaternary sediments and suggest thrust repetition of wedge sediments. A two-order-of-magnitude decrease in 10Be concentrations (109 to 107 atm/g) occurs across the plate boundary dcollement between cores 16 and 18, with an increase in 10Be age from <1 Ma immediately above the dcollement (819 mbsf) to 8-9 Ma below the dcollement (825 mbsf). Sediments below the dcollement are comparable in age to the basal 100m of the incoming Pacific sediment section at site 436. Increases in 10Be concentration with depth at multiple intervals between 690-815 mbsf in C0019 suggest the potential for small-scale (<10m) stratigraphic disruption and overturned stratigraphic sections. These analyses show that meteoric 10Be in deep marine sediments can be a viable tool to delineate the age and structure of marine forearc sediments and constrain the structural history of frontal prisms.

  4. S-wave anisotropy estimated by seismic interferometry using ambient noise record in the Nankai Trough subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Kimura, T.; Mikada, H.; Araki, E.; Kitada, K.

    2013-12-01

    In the Nankai Trough subduction zone, located beneath the Pacific Ocean off the southeast coast of Japan, interplate earthquakes can be generated repeatedly in association with stress accumulation and release cycle. In this study, we aim to obtain the information of S-wave anisotropy beneath the seafloor, which could be interpreted as a proxy of stress and strain field above the subduction zone. For this purpose, we apply the seismic interferometry technique to ambient noise records acquired by seafloor and subseafloor seismometers deployed above the Nankai Trough subduction zone. In this area, we have twenty seafloor seismometers as a part of DONET (Dense Oceanfloor Network System for Earthquake and Tsunamis) and a borehole seismometer installed in the IODP (Integrated Ocean Drilling Program) C0002G observatory at the bottom of the borehole, 900 m below seafloor. Both observatories were designed and installed to monitor the seismic activity and the process of earthquake generation including the stress accumulation. In this study, we apply the seismic interferometry to ambient noise records observed by these DONET and C0002G seismometers. Seismic interferometry is a method to retrieve the impulse response by the cross-correlation of seismic records simultaneously acquired by the two seismometers. Because the horizontal components are dominated by S-wave energy, we expected that auto- and cross-correlation functions (ACF and CCF), calculated from the horizontal components of each seismometer, would provide us the knowledge of S-wave velocity and anisotropy beneath seafloor, as a proxy of strain and stress field, and fluid migration above the plate boundary. We obtained zero offset 4-C ACF and CCFs comprising V11, V12, V21, and V22, calculated form continuous ambient noise records observed by horizontal components of each seismometer. Vij are ACF and CCFs calculated from ambient noise record observed by i- and j-direction receiver components, and represents impulse response which has i-direction source and j-direction receiver of each seismometer. We used each 1 hour dataset for more than 6 months and obtained Vij as 30 s zero offset impulse responses for each seismometer. In the obtained ACF and CCFs, several coherent events are visible. However, the events in each component are not consistent with that of others. It might result from S-wave splitting affected by anisotropy. S-wave split into two orthogonal directions along anisotropy direction in propagating anisotropy media. We then applied the Alford rotation and the layer stripping method to the obtained 4-C ACF and CCFs to estimate S-wave anisotropy direction and amplitude beneath each seismometer in each layer, shallow sediment and accretionary prism above the plate boundary. Obtained results, including the azimuth and magnitude of anisotropy as functions of depth, show good agreement with S-wave anisotropy directions and principle shear stress directions estimated from two of the other methods, i.e., borehole breakout analysis in the IODP C0009 borehole, and P-S converted wave analysis using airgun OBS data. We expect that our method could make it possible to monitor temporal changes in the azimuth and the magnitude in anisotropy, as a proxy of stress field, using real-time ambient noise records in the subduction zone.

  5. Shear Veins Under High Pore Pressure Condition Along Subduction Interface: Yokonami Mlange, Cretaceous Shimanto Belt, Shikoku, Southwest Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, Y.; Eida, M.

    2013-12-01

    Fluid pressure along subdcution interface is a key parameter to understand the fault strength, wedge geometry and seismogenic behavior. In this study, we focused on shear veins pervasively observed in exhumed accretionary complex, Yokonami mlange, Cretaceous Shiamanto Belt, Southwest Japan to examine paleo-stress, effective friction coefficient, fluid pressure ratio and fluid pressure along subduction interface. Lithology of the Yokonami mlange is mainly sandstones surrounded by foliated black shales with minor components of basalts, cherts, tuffs, and limestones, representing tectonic mlange textures. Shear veins cutting mlange foliations are pervasively observed. Shear veins are composed of quartz and calcite. Slicken lines and slicken steps are always observed on the surfaces of shear veins. Pressure-temperature conditions for shear veins are about 180MPa and about 200 degree C on the basis of fluid inclusion analysis. Since the distribution of shear veins are related to packages of ocean floor stratigraphy, formation of shear vein can be before underplating and after mlange formation along subduction interface. We conducted multiple inversion method using slip data of shear veins to examine paleo-stress. In the result, we obtained maximum shear stress horizontal to foliations with 0.3 of stress ratio that is defined as (sigma2-sigma3)/(sigma1-sigma3). Effective friction coefficient was estimated as about 0.10-0.22 by the lowest value of ratio of normal and shear stresses in the normalized Mohr's circle on each plane of shear vein. If we put friction coefficient under dry condition as 0.7 because shear veins cut lithified mlange through out, fluid pressure ratio is equivalent to 0.68-0.86. This is very high fluid pressure ratio along subduction plate interface. On the basis of this fluid pressure ratio and P-T conditions of shear veins from fluid inclusion analysis, 7-12km of depth and 20-30 degree C of geothermal gradient were estimated. The age of subducting plate in the Yokonami mlange was about 50-60Ma from the difference of depositional ages between chert and black shale. The geothermal gradient estimated from the slab age is consistent with from fluid pressure ratio and fluid inclusion analysis. This suggests that the low effective frictional coefficient and high fluid pressure ratio estimated from geological evidences are reasonable values for the formation of shear veins. Kitajima and Saffer (2012) revealed that very low frequency earthquakes (VLFs) occur in the high pore pressure area. Saito et al. (2013) found the quartz cemented fault rocks represent velocity weakening behavior and suggested that the quartz-rich shear veins in shallow accretionary complex might be a geological evidence of VLFs. Our result in this study can support this idea as a shear veins with high fluid pressure at the time of their formation.

  6. Gravity anomalies, forearc morphology and seismicity in subduction zones

    NASA Astrophysics Data System (ADS)

    Bassett, D.; Watts, A. B.; Das, S.

    2012-12-01

    We apply spectral averaging techniques to isolate and remove the long-wavelength large-amplitude trench-normal topographic and free-air gravity anomaly "high" and "low" associated with subduction zones. The residual grids generated illuminate the short-wavelength structure of the forearc. Systematic analysis of all subduction boundaries on Earth has enabled a classification of these grids with particular emphasis placed on topography and gravity anomalies observed in the region above the shallow seismogenic portion of the plate interface. The isostatic compensation of these anomalies is investigated using 3D calculations of the gravitational admittance and coherence. In the shallow region of the megathrust, typically within 100 km from the trench, isolated residual anomalies with amplitudes of up to 2.5 km and 125 mGal are generally interpreted as accreted/subducting relief in the form of seamounts and other bathymetric features. While most of these anomalies, which have radii < 50km, are correlated with areas of reduced seismicity, several in regions such as Japan and Java appear to have influenced the nucleation and/or propagation of large magnitude earthquakes. Long-wavelength (500 - >1000 km) trench-parallel forearc ridges with residual anomalies of up to 1.5 km and 150 mGal are identified in approximately one-third of the subduction zones analyzed. Despite great length along strike, these ridges are less than 100 km wide and several appear uncompensated. A high proportion of arc-normal structure and the truncation/morphological transition of trench-parallel forearc ridges is explained through the identification and tracking of pre-existing structure on the over-riding and subducting plates into the seismogenic portion of the plate boundary. Spatial correlations between regions with well-defined trench-parallel forearc ridges and the occurrence of large magnitude interplate earthquakes, in addition to the uncompensated state of these ridges, suggest links between the morphology of the forearc and the peak earthquake stress drop on the subduction megathrust. We present our classification of residual bathymetric and gravitational anomalies using examples from Sumatra, Kuril-Kamchatka, Mariana, Peru-Chile and the Tonga-Kermadec margin. We reassess proposed links between trench-parallel residual topography and gravity anomalies and subduction zone seismicity using global earthquake catalogs and a new compilation of published aftershock locations and distributed slip models from over 200 of the largest subduction zone earthquakes. Our results highlight the role of pre-existing structure in both the over-riding and subducting plates in modulating the along- and across-strike segmentation of subduction zones. Understanding the genesis of long-wavelength trench-parallel forearc ridges may provide further insights into links between forearc morphology, the rheology of the overriding and subducting plates and seismicity in subduction zones.

  7. Relationship between 3He/4He ratios and subduction of the Philippine Sea plate beneath southwest Japan

    NASA Astrophysics Data System (ADS)

    Umeda, Koji; Kusano, Tomohiro; Asamori, Koichi; McCrank, Glen F.

    2012-10-01

    Regional and local variations in mantle helium provide insight into the coupling of mantle-crust tectonics, and heat and/or mass transfer from the Earth's interior. In order to further elucidate the geographic distribution of3He/4He ratios in southwest Japan, the data from a total of 924 sites were compiled and synthesized. These include data from 48 additional hot spring and drinking water well sites on the northern Kyushu Island and in the northern Chugoku region. There appears to be good correlation between variations in helium isotope ratios and the geophysical evidence used to determine the configuration of the subducting Philippine Sea plate (PHS). Seismological studies reveal that the leading edge of the aseismic slab does not extend to the northern Chugoku region nor to the Osaka Bay area, where gas samples with significantly elevated 3He/4He ratios occur. This is consistent with a mantle-derived helium in these areas, from melts and/or mantle fluids ascribed to upwelling asthenosphere without being hindered by the descending PHS slab. In contrast, gas samples in the regions where the overriding crust comes into direct contact with the subducting PHS are dominated by radiogenic helium derived from the crust because of the absence of a mantle wedge, the most plausible source of mantle helium. Owing to the abrupt changes in the seismicity and focal mechanisms of intraplate earthquakes, the PHS is considered to have slab tears beneath the Kii Channel and/or the eastern Kii Peninsula oriented in a NW-SE direction. However, the lenear alignment of anomalously high3He/4He ratios does not appear to be NW-SE trending along the assumed slab tears but rather forms an broad, ENE-WSW trending zone between the tears where low-frequency events occur. The emanation of gas with elevated3He/4He ratios in the central peninsula can be explained by the upward mobilization of mantle volatiles derived from the mantle wedge above the PHS and/or transferred from the hydrated slab mantle through the N-S trending fractured medium within the PHS. Accordingly, the helium isotopes observed on the Earth's surface may be efficient geochemical indicators of the configuration of a relatively younger, warm aseismic slab, and be especially useful in seismically inactive areas.

  8. P and S wave tomography of Japan subduction zone from joint inversions of local and teleseismic travel times and surface-wave data

    NASA Astrophysics Data System (ADS)

    Liu, Xin; Zhao, Dapeng

    2016-03-01

    We determined P and S wave velocity tomography of the Japan subduction zone down to a depth of 700 km by conducting joint inversions of a large number of high-quality arrival-time data of local earthquakes and teleseismic events which are newly collected for this study. We also determined 2-D phase-velocity images of fundamental mode Rayleigh waves at periods of 20-150 s beneath Japan and the surrounding oceanic regions using amplitude and phase data of teleseismic Rayleigh waves. A detailed 3-D S-wave tomography of the study region is obtained by jointly inverting S-wave arrival times of local and teleseismic events and the Rayleigh-wave phase-velocity data. Our inversion results reveal the subducting Pacific and Philippine Sea slabs clearly as dipping high-velocity zones from a 1-D starting velocity model. Prominent low-velocity (low-V) anomalies are revealed in the mantle wedge above the slabs and in the mantle below the Pacific slab. The distinct velocity contrasts between the subducting slabs and the surrounding mantle reflect significant lateral variations in temperature as well as water content and/or the degree of partial melting. The low-V anomalies in the mantle wedge are attributed to slab dehydration and corner flows in the mantle wedge. A sheet-like low-V zone is revealed under the Pacific slab beneath NE Japan, which may reflect hot upwelling from the deeper mantle and subduction of a plume-fed asthenosphere as well. Our present results indicate that joint inversions of different seismic data are very effective and important for obtaining robust tomographic images of the crust and mantle.

  9. Seismic attenuation beneath Kanto, Japan: evidence for high attenuation in the serpentinized subducting mantle

    NASA Astrophysics Data System (ADS)

    Nakajima, Junichi

    2014-12-01

    The three-dimensional (3-D) P-wave attenuation ( Q p -1) structure beneath Kanto, Japan, is estimated by using a large number of waveform data. Corner frequencies of earthquakes are initially calculated from spectral ratios of S-coda waves, followed by an inversion to simultaneously determine attenuation terms and frequency-dependent site amplification factors. The attenuation terms are then inverted for estimation of the 3-D Q p -1 structure. The obtained results show that seismic attenuation is highly heterogeneous, and pronounced high-attenuation areas are located in the continental lower crust and mantle of the Philippine Sea slab. Seismic activity is very low in the high-attenuation lower crust, which is most likely attributable to ductile deformation facilitated by fluids supplied from the underlying Philippine Sea slab. The high-attenuation area in the Philippine Sea slab represents the serpentinized mantle, and two M ~ 7 earthquakes are documented to have occurred along the western boundary of this area. Interplate earthquakes on the Pacific slab are absent in areas overlain by the serpentinized Philippine Sea slab, which is likely due to the low viscosity of serpentine promoting continuous ductile deformation rather than brittle failures along the plate boundary.

  10. Multiscale seismic imaging of the Western-Pacific subduction zone

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2011-12-01

    We used multiscale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific subduction zone. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). High-resolution local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by locating suboceanic events precisely with sP depth phases. Strong structural heterogeneity is revealed in the megathrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes including the great 2011 Tohoku-oki earthquake (Mw 9.0). A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. A big mantle wedge (BMW) has formed in the upper mantle above the stagnant slab. Convective circulations in the BMW and deep dehydration of the stagnant slab may have caused the intraplate volcanoes in NE Asia, such as the Changbai and Wudalianchi volcanoes. The active Tengchong volcanism in SW China is caused by a similar process in the BMW above the subducting Burma (or Indian) slab. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a result of very large gravitational instability from phase transitions. Prominent slow anomalies are also revealed in the mantle under the subducting slabs, which may represent either mantle plumes or upwelling flows associated with the deep subduction of the slabs.

  11. Zircon and apatite thermochronology of the Nankai Trough accretionary prism and trench, Japan: Sediment transport in an active and collisional margin setting

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.; Carter, Andrew; Nicholson, Uisdean; Masago, Hideki

    2013-06-01

    The Nankai accretionary complex is the most recent addition to the accretionary complexes of southwest Japan and has preserved a record of sediment flux to the trench during its construction. In this study, we use U-Pb zircon and fission track analysis of both zircons and apatites from sediments taken from the forearc and trench of the Nankai Trough, as well as rivers from southwest Japan to examine the exhumation history of the margin since the Middle Miocene. Modern rivers show a flux dominated by erosion of the Mesozoic-Eocene Shimanto and Sanbagawa accretionary complexes. Only the Fuji River, draining the collision zone between the Izu and Honshu arcs, is unique in showing much faster exhumation. Sediment from the Izu-Honshu collision is not found 350-500 km along the margin offshore Kyushu indicating limited along-strike sediment transport. Sediment deposited since 2 Ma on the midtrench slope offshore the Muroto Peninsula of Shikoku (ODP Site 1176) and on the lower slope trenchward of the Kumano Basin (IODP Sites C0006E and C00007E) shares the dominant source in the Shimanto and Sanbagawa complexes seen in the modern rivers. Prior to 5 Ma, additional sediment was being sourced from further north in more slowly exhumed terrains, ~350 km from the trench axis. Around 9.4 Ma, U-Pb zircon ages of ~1800 Ma indicate enhanced erosion from the North China Craton, exposed in northern Honshu. In the middle Miocene, at ~15.4 Ma, the sediment was being derived from a much wider area including the Yangtze Craton (U-Pb ages ~800 Ma). We suggest that this enhanced catchment may have reflected the influence of the Yangtze River in supplying into the Shikoku Basin prior to rifting of the Okinawa Trough at 10 Ma and migration of the Palau-Kyushu Ridge to form a barrier to transport. The restriction of Nankai Trough provenance to Mesozoic source partly reflects continued uplift of the Shimanto and Sanbagawa complexes since the Middle Miocene.

  12. Detailed structure of the Philippine Sea plate subducting along the Nankai Trough, western Japan, inferred from high-frequency seismic wave analysis

    NASA Astrophysics Data System (ADS)

    Furumura, T.; Padhy, S.; Maeda, T.

    2012-12-01

    A detailed structure of the subducting Philippine Sea plate (PHP) along the Nankai trough in western Japan was studied by analyzing waveforms recorded at dense Hi-net stations in Japan. It is well recognized that the waveforms from intraplate earthquakes dominate in high-frequency (f >1 Hz) signals due to the waveguide effect of the subducting slab (Furumura and Kennett, 2005; 2008). This results in distorted pattern of intensity and peak ground acceleration (PGA) above the hypocenter with a substantial elongation of isoseismic contours correlated with the configuration of the isodepth contours of the subducting PHP beneath western Japan. A detailed analysis of the dense Hi-net waveform data from the intermediate-depth PHP event shows that the high-frequency S-wave signals suddenly disappear as the waves propagate the zone away from the Kii Channel to the boundary of Hyogo and Okayama prefectures and large S-to-P conversion occurs before the arrival of S-wave. Such anomalies do not occur for shallow and deep earthquakes occurring outside the PHP. These observations support the recent debate on the complexities of the configuration of the PHP subducting beneath western Japan such as that shown by Shiomi et al. (2008) based on receiver function images and the PHP-split model beneath the Kii channel shown by Ide et al.(2010) based on the analysis of comprehensive geophysical data. In order to explain the observations associated with sudden lateral change in the PHP structure, we conducted finite difference method (FDM) simulations of seismic wave propagation taking the detailed PHP model into account. It is confirmed that high-frequency guided wave energy decouple from waveguide where the shape of the PHP is suddenly deformed, which results in dramatic attenuation of high-frequency signals associating with large S-to-P conversions developed at sharp plate boundary. The present results also support the recently proposed complicated PHP-split model, however, further analysis is necessary to confirm whether the PHP is split beneath the Kii Channel or not, which is very important to understand the linkage occurrence of the Nankai trough earthquakes and their tectonic and seismic implications.

  13. Application of InSAR to the Detection of Interseismic Deformation of Subduction Zones: A Case Study of Southwest Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, M.

    2013-12-01

    Geodetic data, especially GNSS, is useful for the purpose of the long-term seismic hazard evaluation in subduction zones. However dense GNSS networks are under operation only in limited areas in the world. Furthermore, some GNSS stations are affected by local disturbances. Therefore it is necessary to establish complementary techniques to cover the defects of GNSS observations. Synthetic Aperture Radar may play this role. In order to examine the ability to detect interseismic deformation with a high spatial resolution, we have been conducting SAR interferometry and stacking analysis of PALSAR images in Southwest Japan, where GNSS velocity field is at hand for the ground truth. We are going to report the results for Shikoku and Kyushu as well as technical issues we have found during these processes. We used images of the ascending paths 417 - 420, which were acquired during mid 2006 to 2010, to detect deformation of Shikoku. In total there were more than 20 acquisitions for each path. We stacked images having small artificial changes possibly due to ionospheric disturbances. The variation in stacked interferogram of the path 419 that covers the central part of Shikoku is as large as that simulated from GNSS velocities, but we recognize a different trend in GNSS velocities than the stacking interferogram for the path 417 (eastern Shikoku). Furthermore, fringes in the Chugoku districts are inconsistent with GNSS. We find significant NW-SE trends in the azimuth offsets for the pairs of images acquired on the day of large GNSS-TEC variation. It is worth noting that the wavelength of variation in azimuth offset is much shorter than that seen in GNSS-TEC. Therefore it may be difficult to correct interferograms with GNSS-TEC. On the other hand, it may be suitable for the analysis of interseismic deformation to use descending images of Shikoku, though the number of observation is less than that of ascending. We find less ionospheric disturbances than ascending images. We could obtain consistent result with the GNSS velocity field in eastern Kyushu and Shikoku. On the basis of the above results, we can conclude that it is essential to ensure enough number of observations and careful selection of interferograms to be stacked. A proper correction of ionospheric disturbances is desired for the use of more images. Azimuth offsets may be useful for the evaluation of ionospheric disturbances.

  14. Relationship between temperatures and fault slips on the upper surface of the subducting Philippine Sea plate beneath the Kanto district, central Japan

    NASA Astrophysics Data System (ADS)

    Yoshioka, Shoichi; Takagi, Rumi; Matsumoto, Takumi

    2015-05-01

    To elucidate the relationship between interplate temperatures and generation mechanisms for megathrust earthquakes and slow slip events (SSEs) in the Kanto district, central Japan, we performed numerical simulations on the thermal state. For this purpose, we newly developed a 2-D box-type thermal convection model that is able to handle the subduction of two oceanic plates: the young oceanic Philippine Sea (PHS) plate subducts following subduction of the old oceanic Pacific (PAC) plate beneath it. To constrain temperatures on the upper surface of the PHS plate, we used high-density Hi-net heat flow data on land. We found that low heat flow in the Kanto district was caused mostly by subduction of the cold PHS plate. To explain the heat flow distribution in the Kanto district in more detail, we needed to incorporate frictional heating at the plate interface on the seaward side of the corner of the mantle wedge, and temperature changes due to surface erosion and sedimentation associated with crustal deformation during the Quaternary on land into the models. The most suitable pore pressure ratio to explain the heat flow data was 0.98. The thermally estimated seismogenic zone corresponded well to the fault planes of the 1923 Taisho Kanto earthquake and the western half of the 1707 Genroku Kanto earthquake. The eastern half of the fault plane of the 1707 Genroku Kanto earthquake could be divided into two areas; the northwestern fault plane corresponded to the thermally estimated seismogenic zone, whereas the relationship between the southeastern fault plane and interplate temperatures was ambiguous. The off-Boso SSEs occurred on the plate interface at temperatures lower than approximately 250 °C, and the slipped region passed through the 150 °C isotherm, corresponding to the clay mineral phase transformation from smectite to illite. This might suggest that the SSEs occurred in relation to a dehydration process.

  15. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

    USGS Publications Warehouse

    Von Huene, R.; Scholl, D. W.

    1991-01-01

    At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (???43,500 km) is calculated to be 1.5 km3/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km3/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km3/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km3/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km3/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km3/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km3/yr to a total of 1.1 km3/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km3/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted materia

  16. C sub 1 -C sub 8 hydrocarbons in sediments from Guaymas Basin, Gulf of California: Comparison to Peru margin, Japan Trench and California borderlands

    SciTech Connect

    Whelan, J.K.; Tarafa, M.E. ); Simoneit, B.R.T. )

    1988-01-01

    Surface sea floor sediments, hydrothermal vent samples, and Deep Sea Drilling Project sediments (Hole 481 A) from the Guaymas Basin were examined for C{sub 1}-C{sub 8} hydrocarbons. The proportions of various classes of compounds were examined and compared to those from other geographic areas (Peru upwelling region and Japan Trench) to gain insight into the relative importance of thermal generation, migration and biodegradation. Concentrations of C{sub 2}-C{sub 7} hydrocarbons were about 10-10,000 times higher in geothermally warm Guaymas Basin sediments in comparison to the low concentrations (0.1-10 ppb per compound) typical of geothermally cold sea floor and DSDP diatomaceous sediments. Alkene/alkane ratios of 0.1 or greater were typical of both geothermally cold sediments and also of very hydrocarbon-rich Alvin samples recovered from the sea floor. Because little or no alkene was generally detected in buried sediments exposed to geothermal temperatures greater than 30C, it is suggested that the alkenes are produced by biogenic processes. Normal alkanes predominated over cyclic and branched structures in geothermally cooler (<20{degree}C) sediments, with the proportion of cyclic and branched compounds increasing in hotter sediments. Similarities in compositions of branched and cyclic compounds were observed in some pairs of bitumen-rich Guaymas sea floor samples recovered from different areas, suggesting common mechanisms of light hydrocarbon generation and/or migration. 76 refs.

  17. Identification and genomic analysis of temperate Pseudomonas bacteriophage PstS-1 from the Japan trench at a depth of 7000 m.

    PubMed

    Yoshida, Mitsuhiro; Yoshida-Takashima, Yukari; Nunoura, Takuro; Takai, Ken

    2015-11-01

    Viruses play important roles in aquatic ecosystems, but deep-sea bacteriophages remain largely unexplored. A temperate bacteriophage (termed vB_PstS-1) was identified from the psychrotolerant gammaproteobacterium Pseudomonas stutzeri 1-1-1b, which was isolated from hadopelagic water (depth of 7000 m) of the Japan Trench in the Northwest Pacific Ocean. The genome size of PstS-1 was 48,666 bp; its genome displayed a 59.8% G + C content and a total of 79 coding sequences were identified in its genome. The PstS-1 phage belongs to the family Siphoviridae, but its genomic sequence and organization are distinct from those of any other well-known Siphoviridae phage. The mosaic genomic structure of PstS-1 suggests the occurrence of genetic exchange between distinct temperate phages in deep-sea Pseudomonas populations. The PstS-1 genome also harbors three distinct sequence regions corresponding to spacers within a single clustered regularly interspaced short palindromic repeat (CRISPR) locus in the rhizosphere-associated diazotrophic P. stutzeri A1501 genome. The extension of these spacers to the soil environment and the presence of many homologs of both the hadal deep-sea phage PstS-1 and terrestrial Pseudomonas phages suggest the early co-evolution of temperate phages and their host genus Pseudomonas prior to the divergence of their habitational and physiological adaptation. PMID:26025640

  18. Strength characteristics of Japan Trench borehole samples in the high-slip region of the 2011 Tohoku-Oki earthquake

    NASA Astrophysics Data System (ADS)

    Ikari, Matt J.; Kameda, Jun; Saffer, Demian M.; Kopf, Achim J.

    2015-02-01

    The 2011 Tohoku-Oki earthquake demonstrated that the shallowest reaches of plate boundary subduction megathrusts can host substantial coseismic slip that generates large and destructive tsunamis, contrary to the common assumption that the frictional properties of unconsolidated clay-rich sediments at depths less than ? 5km should inhibit rupture. We report on laboratory shearing experiments at low sliding velocities (< 1mm /s) using borehole samples recovered during IODP Expedition 343 (JFAST), spanning the plate-boundary dcollement within the region of large coseismic slip during the Tohoku earthquake. We show that at sub-seismic slip rates the fault is weak (sliding friction ?s = 0.2- 0.26), in contrast to the much stronger wall rocks (?s > ? 0.5). The fault is weak due to elevated smectite clay content and is frictionally similar to a pelagic clay layer of similar composition. The higher cohesion of intact wall rock samples coupled with their higher amorphous silica content suggests that the wall rock is stronger due to diagenetic cementation and low clay content. Our measurements also show that the strongly developed in-situ fabric in the fault zone does not contribute to its frictional weakness, but does lead to a near-cohesionless fault zone, which may facilitate rupture propagation by reducing shear strength and surface energy at the tip of the rupture front. We suggest that the shallow rupture and large coseismic slip during the 2011 Tohoku earthquake was facilitated by a weak and cohesionless fault combined with strong wall rocks that drive localized deformation within a narrow zone.

  19. Seismicity of the Earth 1900-2007, Japan and Vicinity

    USGS Publications Warehouse

    Rhea, Susan; Tarr, Arthur C.; Hayes, Gavin P.; Villaseñor, Antonio; Benz, Harley

    2010-01-01

    This map shows details of Japan and vicinity not visible in an earlier publication, U.S. Geological Survey Scientific Investigations Map 3064. Japan and its island possessions lie across four major tectonic plates: Pacific plate, North America plate; Eurasia plate; and Philippine Sea plate. The Pacific plate is subducted into the mantle, beneath Hokkaido and northern Honshu, along the eastern margin of the Okhotsk microplate, a proposed subdivision of the North America plate (Bird, 2003). Farther south, the pacific plate is subducted beneath volcanic islands along the eastern margin of the Philippine Sea plate. This 2,200 km-long zone of subduction of the Pacific plate is responsible for the creation of the deep offshore Ogasawara and Japan trenches as well as parallel chains of islands and volcanoes, typical of the Circumpacific island arcs. Similarly, the Philippine Sea plate is itself subducting under the Eurasia plate along a zone, extending from Taiwan to southern Honshu, that comprises the Ryuku Islands and the Nansei-Shonto trench.

  20. Double subduction of the Philippine Sea plate and its implication in large earthquake occurrence in southern Kanto, central Japan

    NASA Astrophysics Data System (ADS)

    Noguchi, S.; Yoshida, A.; Hosono, K.; Kobayashi, A.

    2012-12-01

    Thickness of seismogenic layer beneath the Kanto region exceeds 60 km excepting the Pacific slab that lies underneath. A popular idea to explain the thick seismic zone which has been commonly regarded as the Philippine Sea slab is that it is due to subduction of the forearc of the Izu-Ogasawara volcanic arc (e.g., Seno, 2006). Here, we propose a different idea. If we look at a cross section of the hypocenter distribution in a narrow zone around longitude 139.5E, for example, two seismic planes both of which deepen toward north are recognized. The upper seismic plane is considered to be responsible for the occurrence of the 1923 Kanto earthquake with M7.9 (e.g., Sato et al., 2005). Then, what does the underlying seismic plane indicate? To consider tectonic meaning of the underlying seismic plane, we point out two observational facts. One is that the seismic plane is seen not only beneath the Kanto region and Sagami Bay, but beneath the sea area to the south of the Boso Peninsula. The plane inclines steeply east-northeastward and contacts with the Pacific slab at a depth of about 60 km beneath the southern sea area and about 90 km beneath the Kanto region, i.e., the plane tends to become shallower toward the south. The other is that many earthquakes in the seismic plane show thrust-type mechanism with the P axis in the northwest-southeast direction, i.e., the direction of the subduction of the Philippine Sea plate under the North-American plate. Based on these observations we propose that the underlying seismic plane is subducting beneath the upper part of the Philippine Sea plate, i.e., there occurs double subduction of the Philippine Sea plate in southern Kanto. In making this proposition we stand on the following viewpoints: 1) A seismic plane exhibits existence of a rigid slab. 2) A steep inclination of the slab does not necessarily show subduction in that direction at the present time, though it may be a trace of subduction in the past time. 3) A slab is likely to subduct toward the direction that is least resisted. The double subduction hypothesis leads us to reconsideration of the mode of strain accumulation and large earthquake occurrence in southern Kanto. We think that the 1703 Genroku earthquake with M8.2 might have been caused by a rebounding of the overlaying part along the surface of the underthrusting seismic slab which begins its subduction in the sea area to the south of the Boso Peninsula across the Sagami Trough. The problem of apparently longer interval of interplate large earthquakes in southern Kanto compared to that along the Nankai Trough may be dissolved by taking account of the idea of double subduction. We discuss some other tectonic problems related to the hypothesis as well.

  1. Frictional behaviour of exhumed subduction zone sediments from the Shimanto Belt, Japan, at in-situ P-T conditions and implications for megathrust seismogenesis

    NASA Astrophysics Data System (ADS)

    den Hartog, Sabine; Niemeijer, Andre; Saffer, Demian; Marone, Chris

    2014-05-01

    Seismogenesis on subduction zone megathrusts is generally thought to be limited to a region between the ~100-150C isotherms, at ~5-15 km depth, and the ~350C isotherm, typically at ~40 km depth. This zone is bounded at its up-dip and down-dip limits by aseismic zones. However, in recent years it has been discovered that very low frequency earthquakes (VLFE) and non-destructive Slow Slip Events (SSEs) or slow earthquakes nucleate in these presumed aseismic regions. Slip on megathrusts is likely to localize in the weak subducted sediments along the plate interface, which implies that the fault material is derived at least in part from these sediments. Therefore, understanding the depth distribution of seismicity and SSEs on megathrusts requires knowledge of the frictional behaviour of metapelites. We investigated such behaviour by performing shear experiments on natural megathrust fault gouges, derived from exhumed subduction zone sediments and faults exposed in the Shimanto Belt on Shikoku Island, Japan. These gouges correspond to peak paleo-temperatures of 105C to 280C, representing different stages in the diagenetic and metamorphic evolution of the subducted sediments, covering the shallow aseismic zone as well as the seismogenic zone. The composition of all gouges was dominated by illite/muscovite, with smaller amounts of quartz, feldspar and chlorite. We sheared these gouges at low displacement rates (0.1-100 micron/s) to address the nucleation of megathrust earthquakes and SSEs, using either a double-direct (biaxial) shear machine or a rotary shear machine. The double-direct shear experiments were performed at room temperature, 5% relative humidity and 50 MPa normal stress. The rotary shear experiments, in turn, were conducted at the sample-specific, approximate peak in-situ P-T conditions, i.e. the P-T conditions corresponding to the maximum burial depth of these samples. At room temperature, samples from different peak paleo-temperatures showed similar frictional behaviour, with near-neutral velocity dependence, i.e. stable or aseismic behaviour. When deformed at their approximate in-situ peak P-T conditions, on the other hand, the samples showed a progressive transition from strong velocity-strengthening (stable) behaviour at 105C (notably at 10-100 micron/s), to velocity-weakening (unstable) behaviour at 280C. The results at elevated P-T conditions match previous results on simulated illite-quartz analogue fault gouges and imply a broad transition in the slip stability of subduction megathrusts from stable (velocity-strengthening), to unstable (velocity-weakening) with increasing depth, in agreement with seismological observations.

  2. Texture development in naturally compacted and experimentally deformed silty clay sediments from the Nankai Trench and Forearc, Japan

    NASA Astrophysics Data System (ADS)

    Schumann, Kai; Stipp, Michael; Leiss, Bernd; Behrmann, Jan H.

    2014-12-01

    The petrophysical properties of fine-grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations (CPOs). In this contribution we show that Rietveld-based synchrotron texture analysis is a new and valuable tool to quantify textures of water-saturated fine-grained phyllosilicate-rich sediments, and assess the effects of compaction and tectonic deformation. We studied the CPO of compositionally almost homogeneous silty clay drillcore samples from the Nankai Accretionary Prism slope and the incoming Philippine Sea plate, offshore SW Japan. Basal planes of phyllosilicates show bedding-parallel alignment increasing with drillhole depth, thus reflecting progressive burial and compaction. In some samples calcite and albite display a CPO due to crystallographically controlled non-isometric grain shapes, or nannofossil tests. Consolidated-undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the CPOs of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis. There is at least a qualitative relation between CPO intensity and strain magnitude. Scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening. This indicates that the microfabrics are sensitive strain gauges, and contribute to anisotropic physical properties along with the CPO.

  3. The reasons why the M9 earthquake in the northeastern Japan subduction zone could not be anticipated and why it really occurred

    NASA Astrophysics Data System (ADS)

    Matsuzawa, T.; Iio, Y.

    2011-12-01

    The M9 Tohoku earthquake on 11 March 2011 had a great impact on the seismologists all over the world. This is because the northeastern Japan subduction zone was one of the most investigated subduction zones and the interplate coupling there was thought to be too weak to generate M9 earthquakes. The bases of the judgment of weak coupling are as follows: (1) The portion of the Pacific plate subducting beneath the subduction zone is older than 100 my, which is older than most of the other ocean floors in the world. Note that although some researchers have casted doubt on the relationship between the M9 potential and plate convergence rate and back-arc spreading proposed by Ruff and Kanamori (1980) after the 2004 M9 Sumatra-Andaman earthquake (e.g., McCaffrey, 2007, 2008; Stein and Okal, 2007), the dependency on the age of the oceanic plate had not been rejected. (2) Around 100 year geodetic survey shows dilatational areal strain is dominant in Tohoku (northeastern Honshu, Japan) (Hashimoto, 1990; Ishikawa and Hashimoto, 1999), indicating all the 'locked' areas on the plate boundary might be loosened by M7 earthquakes occurring with repeating intervals of several tens of years. (3) Although the analyses of GPS (e.g., Suwa et al., 2006) and small repeating earthquake data (Uchida and Matsuzawa, 2011) indicate a large 'locked' area off southern Tohoku, the data in the late 2000s show large portions of the locked area seemed to be released by large earthquakes of M6-7 and their afterslip. (4) The activity of moderate-sized earthquakes there is the highest in Japan. (5) Large interplate earthquakes with M6 or larger are usually followed by large afterslip whose scalar moment is sometimes as large as that of the seismic slip of the main shock. Moreover, Hasegawa et al. (2011) shows that the stress on the plate boundary was not large according to the stress rotation after the M9 earthquake. All of these observations indicate that the plate boundary was not strongly locked over 100 years. Then why did the M9 earthquake really occur there? The reason is still under the debate. One of the probable explanations is that the plate boundary had been weakly coupled but the slip of the M9 earthquake was exceptionally large releasing total stress on the boundary. The Pacific plate descending beneath Tohoku is old and cold but the inclination of the plate is less than around 30 degrees and interplate earthquakes can occur as deep as 60 km because the plate is very cold. The shallow subduction angle and deep sesimogenic limit causesd the seismogenic plate boundary as wide as more than 200 km, which was large enough to accumulate slip deficit of more than 20m without large stress increase (Iio et al., 2011). Most of the aftershocks occurring in the hanging plate are of normal fault type (Asano et al., 2011) indicating the seismic slip of the M9 earthquake was overshot (Ide et al., 2011), which might be caused by thermal pressurization of pore fluid (Mitsui and Iio, 2011).

  4. Salty Trench

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This image taken by the panoramic camera on the Mars Exploration Rover Spirit shows a trench dug by the rover on its way toward the 'Columbia Hills.' Measurements taken of the soil contained in the trench by Spirit's alpha particle X-ray spectrometer showed the presence of sulfur and magnesium. Concentrations of those two elements varied in parallel at different locations in the trench, suggesting that they may be paired as a magnesium-sulfate salt. One possible explanation for these findings is that water percolated through underground material and dissolved out minerals, then as the water evaporated near the surface, it left concentrated salts behind.

  5. Trench mouth

    MedlinePLUS

    ... common among soldiers. The mouth normally contains a balance of different bacteria. Trench mouth occurs when there is an overgrowth of normal mouth bacteria. The gums become infected and develop ...

  6. Overriding Plate Controls on Subduction Zone Evolution

    NASA Astrophysics Data System (ADS)

    Sharples, W. K.; Jadamec, M. A.; Moresi, L. N.; Capitanio, F. A.

    2014-12-01

    Seismic data, rock deformation experiments, and geochemical studies indicate variability in the thickness, buoyancy, and strength of the lithosphere at plate boundaries. However, geodynamic models of subduction commonly either omit an overriding plate or do not investigate role of the variation in overriding plate properties on the subduction evolution. We present time-dependent numerical models of subduction that vary the overriding plate thickness, strength, and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology. We examine the emergence of (a) asymmetric versus symmetric subduction, (b) trench retreat versus advance, (c) subduction zone geometry, (d) slab stagnation versus penetration into the lower mantle, and (e) flat slab subduction. The majority of the models result in sustained asymmetric subduction. The models demonstrate that trench retreat is correlated with a thin overriding plate, whereas, trench advance is correlated with a thick and/or strong overriding plate. Slab dip, measured at a depth below the plate boundary interface, has a negative correlation with an increase in overriding plate thickness. Overriding plate thickness exerts a first order control over slab penetration into the lower mantle, with penetration most commonly occurring in models with a thick overriding plate. Periods of flat slab subduction occur with thick, strong overriding plates producing strong plate boundary interface coupling. The results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction, as well as providing an explanation for the variation of slab geometry observed in subduction zones on Earth.

  7. Characterizing Mega-Earthquake Related Tsunami on Subduction Zones without Large Historical Events

    NASA Astrophysics Data System (ADS)

    Williams, C. R.; Lee, R.; Astill, S.; Farahani, R.; Wilson, P. S.; Mohammed, F.

    2014-12-01

    Due to recent large tsunami events (e.g., Chile 2010 and Japan 2011), the insurance industry is very aware of the importance of managing its exposure to tsunami risk. There are currently few tools available to help establish policies for managing and pricing tsunami risk globally. As a starting point and to help address this issue, Risk Management Solutions Inc. (RMS) is developing a global suite of tsunami inundation footprints. This dataset will include both representations of historical events as well as a series of M9 scenarios on subductions zones that have not historical generated mega earthquakes. The latter set is included to address concerns about the completeness of the historical record for mega earthquakes. This concern stems from the fact that the Tohoku Japan earthquake was considerably larger than had been observed in the historical record. Characterizing the source and rupture pattern for the subduction zones without historical events is a poorly constrained process. In many case, the subduction zones can be segmented based on changes in the characteristics of the subducting slab or major ridge systems. For this project, the unit sources from the NOAA propagation database are utilized to leverage the basin wide modeling included in this dataset. The length of the rupture is characterized based on subduction zone segmentation and the slip per unit source can be determined based on the event magnitude (i.e., M9) and moment balancing. As these events have not occurred historically, there is little to constrain the slip distribution. Sensitivity tests on the potential rupture pattern have been undertaken comparing uniform slip to higher shallow slip and tapered slip models. Subduction zones examined include the Makran Trench, the Lesser Antilles and the Hikurangi Trench. The ultimate goal is to create a series of tsunami footprints to help insurers understand their exposures at risk to tsunami inundation around the world.

  8. Marine electromagnetics: A new tool for mapping fluids at subduction zones

    NASA Astrophysics Data System (ADS)

    Key, K.; Naif, S.; Constable, S.; Evans, R. L.

    2013-12-01

    The recent adoption of marine electromagnetic (EM) methods by the hydrocarbon exploration industry has driven technological innovations in acquisition hardware and modeling software that have created new opportunities for studying plate boundary structure at subduction zones. Because the bulk electrical resistivity measured by EM surveys is strongly dependent on crustal porosity and hence fluid content, EM data can provide valuable constraints on crustal hydration in the incoming oceanic plate, fluids released through sediment compaction and dehydration reactions occurring after the plate is subducted, and fluids escaping through the overlying forearc crust. Since water also plays an important role in regulating subduction earthquake processes and frictional behavior along the plate boundary, EM data have the potential to reveal new insights on the causes of large subduction zone earthquakes and their potential for generating tsunamis. As a demonstration of this novel technique, we present new results from the first controlled-source EM survey of a subduction zone, carried out at the Middle America Trench offshore Nicaragua in 2010. During this survey 50 seafloor EM receivers were deployed along a 280 km profile extending from the abyssal plain, across the trench and onto the forearc. Controlled-source EM signals were broadcast to the receivers by deep-towing a low-frequency electric dipole transmitter close to the seafloor along the entire survey profile, generating diffusive EM waves that traveled through the crust and uppermost mantle. Non-linear two-dimensional inversion of the data reveals a significant decrease in crustal resistivity with the onset of bending faults at the trench outer rise and images a continuous zone of low resistivity porous sediments being carried down with the subducting plate to at least 10 km down dip from the trench. Further landward at about 25 km from the trench, a sub-vertical low-resistivity zone extending from the plate boundary into the overlying forearc crust is consistent with the fluid release expected from the smectite-illite transformation and occurs directly beneath the location of known seafloor fluid seeps. Potential future surveys at other margins such as Cascadia, Alaska, New Zealand and Japan and integrated interpretation with other geophysical, geochemical and geological studies offers the chance for greatly enhancing our understanding of subduction processes.

  9. A Detailed 3D Seismic Velocity Structure of the Subducting Pacific Slab Beneath Hokkaido, Tohoku and Kanto, Japan, by Double-Difference Tomography

    NASA Astrophysics Data System (ADS)

    Tsuji, Y.; Nakajima, J.; Kita, S.; Okada, T.; Matsuzawa, T.; Hasegawa, A.

    2007-12-01

    Three-dimensional heterogeneous structure beneath northeastern (NE) Japan has been investigated by previous studies and an inclined seismic low-velocity zone is imaged in the mantle wedge sub-parallel to the down-dip direction of the subducting slab (Zhao et al., 1992, Nakajima et al., 2001). However, the heterogeneous structure within the slab has not been well studied even though it is very important to understand the whole process of water transportation from the slab to the surface. Here we show a detailed 3D seismic velocity structure within the subducted Pacific slab around Japan and propose a water-transportation path from the slab to the mantle wedge. In this study, we estimated 3D velocity structure within the Pacific slab by the double-difference tomography (Zhang and Thurber, 2003). We divided the study area, from Hokkaido to Kanto, into 6 areas due to the limitation of memory and computation time. In each area, arrival-time data of 7,500-17,000 events recorded at 70-170 stations were used in the analysis. The total number of absolute travel-time data was about 140,000-312,000 for P wave and 123,000-268,000 for S wave, and differential data were about 736,000-1,920,000 for P wave and 644,000-1,488,000 for S wave. Horizontal and vertical grid separations are 10-25 km and 6.5 km, respectively. RMS residuals of travel times for P wave decreased from 0.23s to 0.09s and for S wave from 0.35s to 0.13s. The obtained results are as follows: (1) a remarkable low-Vs zone exists in the uppermost part of the subducting slab, (2) it extends down to a depth of about 80 km, (3) the termination of this low-Vs zone almost corresponds to the "seismic belt" recently detected in the upper plane of the double seismic zone (Kita et al.,2006; Hasegawa et al., 2007), (4) at depths deeper than 80 km, a low-Vs and high-Vp/Vs zone is apparently distributed in the mantle wedge, immediately above the slab crust. We consider that these features reflect water-transportation processes from the slab to the mantle wedge. A low- Vs zone in the uppermost part of the subducting slab corresponds to the hydrous oceanic crust since its absolute velocity is about 4.0 km/s, comparable to that expected for the oceanic crust (Hacker et al., 2003). Dehydration reactions occur in the oceanic crust as temperature and pressure increase, and a relatively large amount of water is released at depths of about 80-100 km. The water generated by dehydration reactions could migrate upward and react peridotite at the base of the mantle wedge, forming a thin-serpentine layer there. Then, the layer is dragged by the subducting slab to deeper depths (e.g. Iwamori, 1998). Such water-transportation processes from the slab to the mantle wedge are partly constrained by a recent receiver function analysis (Kawakatsu and Watada, 2007). We further found an along-arc variation of the termination depth of the low-velocity oceanic crust, suggesting the along-arc variation in the amount of fluids released from the slab.

  10. Generation of adakites in a cold subduction zone due to double subducting plates

    NASA Astrophysics Data System (ADS)

    Nakamura, H.; Iwamori, H.

    2012-12-01

    Adakites have been found in various tectonic settings, since the first report for the distinct lavas as a product of slab melting in Adak Island by Kay (1978). In this study, we present geochemical data for an 'adakite' and 'adakitic rock' suite in central Japan with a cold subduction environment due to the two overlapping subudcting plates, the Pacific Plate and the Philippine Sea Plate. Based on the major, trace and isotopic compositions of the rocks, elemental transport from initial slab inventory at the trench to the volcanic rocks as a final product is quantitatively analyzed, considering the thermal structure, slab dehydration, elemental mobility, slab-fluid migration and melting of fluid-added mantle. The analysis demonstrates a large compositional impact of slab-fluid in the arc magma generation in central Japan. The melting conditions have been also estimated inversely by optimizing the predicted magma composition to the observed composition of volcanic rock, with the two parameters: the degree of melting and the proportion of spinel- and garnet-lherzolites involved in melting. Consequently, a low degree of melting of dominantly garnet-lherzolite with a high fluid flux from the two overlapping slabs beneath the region has been argued to be responsible for the compositional characteristics, including the adakitic signatures, of the studied rocks. These results imply that the geochemical approach may provide useful constraints on the P-T condition of melting in the mantle wedge and the thermal structure in subduction zones, being complementary to the geophysical approach. We have also applied this geochemical approach to the adjacent NE Japan where the Pacific plate subducts, which revealed the thermal regime in the mantle beneath the arc-arc transition.

  11. An approximately 9-yr-period variation in seismicity and crustal deformation near the Japan Trench and a consideration of its origin

    NASA Astrophysics Data System (ADS)

    Tanaka, Yoshiyuki

    2014-02-01

    It is well known that the statistical probability of earthquake occurrence changes over the course of a day due to periodic variations in the tidal stress acting on faults. However, periodicity on a decadal scale has been studied by relatively few researchers. It has been reported that an approximately 10-yr periodicity is observed globally for the seismicity of M-8-class large earthquakes. However, the mechanism underlying this periodicity has not yet been revealed. In this study, the decadal-scale periodicity of earthquakes along the Japan Trench is investigated. A new finding is presented that in northeast Japan, the probability of the occurrence of historical earthquakes with an M ≥ 6 that have occurred during the past 1000 yr has increased approximately every 9 yr. Periodicity becomes even more apparent for large earthquakes with an M > 7.5 and approximately half the recorded events intensively occurred within two successive years on a cycle of approximately 9 yr. This implies the presence of a periodic stress disturbance at an appreciably regular interval. The past strain and tilt observations conducted in Japan during the 1950s through the 1970s indicate that, nationwide, gradual compression repeated every 8-10 yr in the direction of relative plate motion. These compression periods are in accordance with the periods of higher seismic activity discussed above. As a first step in investigating the origin of earthquake periodicity, periods associated with lunar motion are considered. It is shown that long-term motion primarily governed by the period of the lunar perigee is synchronized with the cyclic variation in seismicity and crustal deformation described above. Decadal changes in tidal stress, as calculated using an ordinary theory of solid Earth tides, are too small to cause periodic variations in seismicity. Therefore, the conditions by which tidal stress is sufficiently amplified to trigger an earthquake are investigated. The results show that, if one assumes that a tidal force acts on a spherically asymmetric block-like upper mantle beneath the Pacific Plate, the computed phase and amplitude can explain the observations. Otherwise, it is difficult to consider direct tidal force alone as the main source of periodic variations in seismicity. Other possibilities should be considered, such as unknown interactions between the plate boundaries and the ocean/atmosphere with a period of approximately 9 yr or a resonance between the period of the tidal force and a recurrence period of slow slip events in the transition zone on the plate boundary. Apart from understanding the origin, the important fact confirmed in this study is that in some areas, the occurrence of large earthquakes, if considered as a group, appears to be strongly governed by a periodic stress disturbance rather than by completely random processes. Elucidating the wide-range approximately 9-yr mode helps us narrow a range in occurrence time in a probabilistic mid-term prediction of large interplate earthquakes.

  12. Japan.

    ERIC Educational Resources Information Center

    Geiger, Rita; And Others

    The document offers practical and motivating techniques for studying Japan. Dedicated to promoting global awareness, separate sections discuss Japan's geography, history, culture, education, government, economics, energy, transportation, and communication. Each section presents a topical overview; suggested classroom activities; and easily

  13. Japan.

    ERIC Educational Resources Information Center

    Geiger, Rita; And Others

    The document offers practical and motivating techniques for studying Japan. Dedicated to promoting global awareness, separate sections discuss Japan's geography, history, culture, education, government, economics, energy, transportation, and communication. Each section presents a topical overview; suggested classroom activities; and easily…

  14. Buoyant subduction on Venus: Implications for subduction around coronae

    NASA Technical Reports Server (NTRS)

    Burt, J. D.; Head, J. W.

    1993-01-01

    Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

  15. Buoyant subduction on Venus: Implications for subduction around coronae

    NASA Astrophysics Data System (ADS)

    Burt, J. D.; Head, J. W.

    1993-03-01

    Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

  16. Evidence for retrograde lithospheric subduction on venus.

    PubMed

    Sandwell, D T; Schubert, G

    1992-08-01

    Annular moats and outer rises around large Venus coronae such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On Earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronae while compensating back-arc extension is occurring in the expanding coronae interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of eastern Aphrodite Terra. PMID:17736463

  17. Evidence for retrograde lithospheric subduction on Venus

    NASA Technical Reports Server (NTRS)

    Sandwell, David T.; Schubert, Gerald

    1992-01-01

    Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.

  18. Evidence for retrograde lithospheric subduction on Venus

    NASA Astrophysics Data System (ADS)

    Sandwell, David T.; Schubert, Gerald

    1992-08-01

    Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.

  19. Widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough off the Kii Peninsula, Japan

    NASA Astrophysics Data System (ADS)

    Tsuji, Takeshi; Kodaira, Shuichi; Ashi, Juichiro; Park, Jin-Oh

    2013-07-01

    We identified widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough, southeast of the Kii Peninsula, Japan, on the basis of 2D and 3D seismic reflection data. The seafloor seaward of the trough axis is deformed by displacement on these intraoceanic reverse faults, producing topographic highs (part of Kashinosaki Knoll). Because the thrust faults extend to the Moho and offset the Moho reflection, they may be related to serpentinization of the mantle due to seawater invasion. These faults are seismically active, given that their geometries are consistent with the focal mechanisms of intraplate earthquakes and microearthquakes. The thrust faults appear to extend landward to a high-density dome within the accretionary prism off the Kii Peninsula. Because the dome and the associated thick accretionary prism are expected to generate high friction at the plate interface due to their large vertical load, the intraoceanic thrusts are likely to have grown with ongoing subduction. Furthermore, because the geometry of the fault system we identified off the Kii Peninsula has characteristics similar to faults at Zenisu Ridge east of our study area, the thrusts observed in the study area may be considered to be the westward continuation of those at Zenisu Ridge. Since the Euler rotation pole of relative motion between the Philippine Sea plate and Zenisu Ridge is consistent with the high-density dome off the Kii Peninsula, we interpret the high-density dome as well as Kashinosaki Knoll as a westward termination of the Zenisu compression zone.

  20. Contrasting slip zone mineralogy of major thrusts in ancient subduction complexes: examples from the Pasagshak Point Thrust in Alaska and the Nobeoka Thrust in Japan

    NASA Astrophysics Data System (ADS)

    Yamaguchi, A.; Fukuchi, R.; Fujimoto, K.; Ishikawa, T.; Kato, Y.; Nozaki, T.; Meneghini, F.; Rowe, C. D.; Moore, C. A.; Tsutsumi, A.; Kimura, G.

    2014-12-01

    Two well-studied Cretaceous-Tertiary accretionary complexes, the Kodiak complex in Alaska and the Shimanto complex in Japan, were formed by subduction of a relatively young oceanic plate, and have similar lithologies characterized by thick terrigenous sediments with rare pelagic sediments. However, the occurrences of fault rock types and fluid-rock interaction patterns along major thrust zone differ significantly, instead of similar background temperatures (~250C). In this presentation we compare two representative fault zones showing contrasting mineralogy and water-rock interaction patterns. Ultrafine-grained black fault rocks (BFRs) comprise the principal slip zone of the Pasagshak Point Thrust of the Kodiak accretionary complex. The geochemistry of the BFRs is characterized by Li and Sr enrichment, Rb and Cs depletion, and a low 87Sr/86Sr ratio. These geochemical signatures are explained by fluid-rock interactions at >350C, which result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Sr concentrations and low 87Sr/86Sr ratios. In contrast to the Pasagshak Point Thrust, the fault core of the Nobeoka Thrust in the Shimanto accretionary complex is mineralogically characterized by breakdown of plagioclase and enrichment in clay and carbonate minerals. Values of illite crystallinity expressed as a full width at half maximum of the illite (001) peak in clay-fraction XRD increase within fault zones, showing the absence of significant temperature rise. Temperatures of fault plane during fluid-rock interaction may affect the difference in mineralogical characters of the two fault zones. Further mineralogical and geochemical investigations are necessary to explore the nature of fluids and its role in faulting along seismogenic subduction plate boundaries.

  1. Decadal-Scale Decoupling of the Japan Trench Prior to the 2011 Tohoku-Oki Earthquake from Geodetic and Repeating-Earthquake Observations

    NASA Astrophysics Data System (ADS)

    Mavrommatis, A. P.; Segall, P.; Johnson, K. M.; Uchida, N.

    2014-12-01

    We report geodetic evidence for decadal-scale decoupling of the Japan Trench prior to the 2011 M9 Tohoku earthquake, with supporting evidence from repeating earthquakes. We build on the results of Mavrommatis et al. (2014, GRL), where we investigated GPS time series for 15 years before the M9 event, correcting for the copious postseismic deformation following numerous M ~ 6.5+ earthquakes between 2003 and 2011. We modeled interseismic deformation as linear trends plus constant accelerations and corrected for coseismic and postseismic effects of M 6.3+ quakes during the period 1996 to 2011. We find spatially coherent and statistically significant accelerations throughout northern Honshu. While the accelerations in northern Tohoku can be explained by decaying postseismic deformation following the 1994 M7.7 Sanriku quake, the accelerations in south-central Tohoku cannot be explained simply by postseismic effects. Time series in south-central Tohoku reveal an approximately continuous transient spanning the period 1996 to 2011, without any obvious onset. The transient is consistent with a decrease in the average shear strain rate by ~1/3 in 15 years, and can be explained by increasing average slip rate and/or updip migration of the locked-to-creeping transition depth. MCMC inversions of instantaneous velocities in 1996 and 2011 imply a shallowing of the transition depth by 13.5 +/- 9.5 km and an increase in the average slip rate in the depth range from 22 km to the transition depth by 48.5 +/- 10.5 mm/yr. To test these predictions and to provide additional constraints on the changes in slip rate, we use independent observations of repeating earthquakes during the same period. We find that several sequences of repeaters offshore south-central Tohoku exhibit decreasing recurrence intervals, possibly implying increasing slip rate, while sequences offshore Sanriku exhibit increasing recurrence intervals, consistent with decaying afterslip from the 1994 M7.7 Sanriku event. This along-strike dichotomy is consistent with an inversion of the GPS accelerations and suggests that some change in slip rate in depths shallower than ~60 km is required by the data. Further analysis of the repeating earthquakes will provide additional constraints to discriminate between such models.

  2. Japan.

    PubMed

    1989-02-01

    Japan consists of 3900 islands and lies off the east coast of Asia. Even though Japan is one of the most densely populated nations in the world, its growth rate has stabilized at .5%. 94% of all children go to senior high school and almost 90% finish. Responsibility for the sick, aged, and infirmed is changing from the family and private sector to government. Japan was founded in 600 BC and its 1st capital was in Nara (710-1867). The Portuguese, the 1st Westerners to make contact with Japan in 1542, opened trade which lasted until the mid 17th century. US Navy Commodore Matthew Perry forced Japan to reopen in 1854. Following wars with China and Russia in the late 1800s and early 1900s respectively, Japan took part in World Wars I and II. In between these wars Japan invaded Manchuria and China. The US dropped an atomic bomb on Hiroshima and Nagasaki and the Japanese surrendered in September, 1945 ending World War II (WWII). Following, WWII, the Allied Powers guided Japan's establishment as a nonthreatening nation and a democratic parliamentary government (a constitutional monarchy) with a limited defense force. Japan remains one of the most politically stable of all postwar democracies. The Liberal Democratic Party's Noboru Takeshita became prime minister in 1987. Japan has limited natural resources and only 19% of the land is arable. Japanese ingenuity and skill combine to produce one of the highest per hectare crop yields in the world. Japan is a major economic power, and its and the US economies are becoming more interdependent. Its exports, making up only 13% of the gross national product, mainly go to Canada and the US. Many in the US are concerned, however, with the trade deficit with Japan and are seeking ways to make trade more equitable. Japan wishes to maintain good relations with its Asian neighbors and other nations. The US and Japan enjoy a strong, productive relationship. PMID:12178004

  3. Average slip rate at the transition zone on the plate interface in the Nankai subduction zone, Japan, estimated from short-term SSE catalog

    NASA Astrophysics Data System (ADS)

    Itaba, S.; Kimura, T.

    2013-12-01

    Short-term slow slip events (S-SSEs) in the Nankai subduction zone, Japan, have been monitored by borehole strainmeters and borehole accelerometers (tiltmeters) mainly. The scale of the S-SSE in this region is small (Mw5-6), and therefore there were two problems in S-SSE identification and estimation of the fault model. (1) There were few observatories that can detect crustal deformation associated with S-SSEs. Therefore, reliability of the estimated fault model was low. (2) The signal associated with the S-SSE is relatively small. Therefore, it was difficult to detect the S-SSE only from strainmeter and tiltmeter. The former problem has become resolvable to some extent by integrating the data of borehole strainmeter, tiltmeter and groundwater (pore pressure) of the National Institute of Advanced Industrial Science and Technology, tiltmeter of the National Research Institute for Earthquake Science and Disaster Prevention and borehole strainmeter of the Japan Meteorological Agency. For the latter, by using horizontal redundant component of a multi-component strainmeter, which consists generally of four horizontal extensometers, it has become possible to extract tectonic deformation efficiently and detect a S-SSE using only strainmeter data. Using the integrated data and newly developed technique, we started to make a catalog of S-SSE in the Nankai subduction zone. For example, in central Mie Prefecture, we detect and estimate fault model of eight S-SSEs from January 2010 to September 2012. According to our estimates, the average slip rate of S-SSE is 2.7 cm/yr. Ishida et al. [2013] estimated the slip rate as 2.6-3.0 cm/yr from deep low-frequency tremors, and this value is consistent with our estimation. Furthermore, the slip deficit rate in this region evaluated by the analysis of GPS data from 2001 to 2004 is 1.0 - 2.6 cm/yr [Kobayashi et al., 2006], and the convergence rate of the Philippine Sea plate in this region is estimated as 5.0 - 7.0 cm/yr. The difference between the slip deficit rate and the convergence rate is 2.4-6.0 cm/yr, and it is comparable to the average slip rate of S-SSE. Consequently, slow earthquakes such as S-SSEs and tremor that we can detect, roughly cover 50-100 % of quasi-static slip in this region.

  4. Japan.

    PubMed

    1987-02-01

    Japan is composed of 4 main islands and more than 3900 smaller islands and has 317.7 persons/square kilometer. This makes it one of the most densely populated nations in the world. Religion is an important force in the life of the Japanese and most consider themselves Buddhists. Schooling is free through junior high but 90% of Japanese students complete high school. In fact, Japan enjoys one of the highest literacy rates in the world. There are over 178 newspapers and 3500 magazines published in Japan and the number of new book titles issued each year is greater than that in the US. Since WW1, Japan expanded its influence in Asia and its holdings in the Pacific. However, as a direct result of WW2, Japan lost all of its overseas possessions and was able to retain only its own islands. Since 1952, Japan has been ruled by conservative governments which cooperate closely with the West. Great economic growth has come since the post-treaty period. Japan as a constitutional monarchy operates within the framework of a constitution which became effective in May 1947. Executive power is vested in a cabinet which includes the prime minister and the ministers of state. Japan is one of the most politically stable of the postwar democracies and the Liberal Democratic Party is representative of Japanese moderate conservatism. The economy of Japan is strong and growing. With few resources, there is only 19% of Japanese land suitable for cultivation. Its exports earn only about 19% of the country's gross national product. More than 59 million workers comprise Japan's labor force, 40% of whom are women. Japan and the US are strongly linked trading partners and after Canada, Japan is the largest trading partner of the US. Foreign policy since 1952 has fostered close cooperation with the West and Japan is vitally interested in good relations with its neighbors. Relations with the Soviet Union are not close although Japan is attempting to improve the situation. US policy is based on the following 3 principles: 1) the US views Japan as an equal trade partner, 2) that the relationship is global in scope, and 3) that Japan has become increasingly assertive in world matters and plays a greater international role. The combined efforts of the US and Japan will be utilized to promote world peace. PMID:12177912

  5. A Hybrid Tsunami Risk Model for Japan

    NASA Astrophysics Data System (ADS)

    Haseemkunju, A. V.; Smith, D. F.; Khater, M.; Khemici, O.; Betov, B.; Scott, J.

    2014-12-01

    Around the margins of the Pacific Ocean, denser oceanic plates slipping under continental plates cause subduction earthquakes generating large tsunami waves. The subducting Pacific and Philippine Sea plates create damaging interplate earthquakes followed by huge tsunami waves. It was a rupture of the Japan Trench subduction zone (JTSZ) and the resultant M9.0 Tohoku-Oki earthquake that caused the unprecedented tsunami along the Pacific coast of Japan on March 11, 2011. EQECAT's Japan Earthquake model is a fully probabilistic model which includes a seismo-tectonic model describing the geometries, magnitudes, and frequencies of all potential earthquake events; a ground motion model; and a tsunami model. Within the much larger set of all modeled earthquake events, fault rupture parameters for about 24000 stochastic and 25 historical tsunamigenic earthquake events are defined to simulate tsunami footprints using the numerical tsunami model COMCOT. A hybrid approach using COMCOT simulated tsunami waves is used to generate inundation footprints, including the impact of tides and flood defenses. Modeled tsunami waves of major historical events are validated against observed data. Modeled tsunami flood depths on 30 m grids together with tsunami vulnerability and financial models are then used to estimate insured loss in Japan from the 2011 tsunami. The primary direct report of damage from the 2011 tsunami is in terms of the number of buildings damaged by municipality in the tsunami affected area. Modeled loss in Japan from the 2011 tsunami is proportional to the number of buildings damaged. A 1000-year return period map of tsunami waves shows high hazard along the west coast of southern Honshu, on the Pacific coast of Shikoku, and on the east coast of Kyushu, primarily associated with major earthquake events on the Nankai Trough subduction zone (NTSZ). The highest tsunami hazard of more than 20m is seen on the Sanriku coast in northern Honshu, associated with the JTSZ.

  6. Vp structure and interplate reflectivity around the source area of the 1952 Tokachi-oki earthquake, the south Kuril Trench, Japan, deduced by an airgun-ocean bottom seismometer experiment

    NASA Astrophysics Data System (ADS)

    Azuma, R.; Murai, Y.; Katsumata, K.; Nishimura, Y.; Yamada, T.; Mochizuki, K.; Shinohara, M.

    2012-12-01

    The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M > 8 recur along the trench. Coseismic slip up to ~7 m took place during the 1952 Tokachi-oki earthquake in a seismic gap between the source areas of the 1973 Nemuro-oki and 2003 Tokachi-oki earthquakes has been. The seismic gap has also slipped incidental to neighboring asperities. The difference in coseismic slip pattern on the plate interface generally appears as a spatial difference in the seismic structure, such as a reflectivity of the plate interface. We estimated the crustal structure to investigate the variation of reflectivity of the plate interface suggesting the intensity of the interplate coupling by performing an airgun-ocean bottom seismometer experiment on the along-trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi-oki source area including the seismic gap, whereas it was not in the 1973 Nemuro-oki source area. The strong reflectivity of the plate interface in such a seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the maximum slip patch of the source area of the 1952 earthquake is likely different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi-oki and 1973 Nemuro-oki earthquakes. Our results suggest that the 1952 Tokachi-oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake in addition to that of a typical underthrust earthquake.

  7. Subduction Initiation in Eastern Indonesia

    NASA Astrophysics Data System (ADS)

    Hall, R.

    2014-12-01

    Subduction is often reported to be difficult to initiate, yet in the West Pacific and Eastern Indonesia there are many young subduction zones. Few theoretical or modelling studies consider such settings in which subduction commonly began close to boundaries between ocean crust and thickened crust of arc or continental origin. In Eastern Indonesia there are subduction zones at different stages of development. Some young examples such as the Banda Arc developed by propagation of an existing trench into a new area by tearing, probably along an ocean-continent boundary. This 'solves' the problem since the older subducted slab provides the driving force to drag down unsubducted ocean lithosphere. However, similar explanations cannot account for other subduction zones, such as North Sulawesi, nearby examples in which the subducted slab is not yet at 100 km depth, or troughs where subduction appears to be beginning. These examples show that subduction initiated at a point, such as a corner in an ocean basin, where there were very great differences in elevation between land and adjacent ocean floor. Depression of ocean crust by flow of arc/continent crust is associated with granitic magmatism and detachments within the upper crust. Once the oceanic corner reaches depths of c.100 km, eclogite formation may lead to slab pull that causes the new subduction zone to grow in both directions along strike; arc magmatism may or may not begin. The close relationship between subduction and extension in Eastern Indonesia links dramatic elevation of land, exhumation of deep crust, and spectacular subsidence of basins imaged by oil exploration seismic and multibeam data. Exhumed granites and high-grade metamorphic rocks at elevations up to 3 km, separated by Neogene alluvial sediments from carbonate reefs now at depths of 2 kilometres, imply vertical movements of several kilometres in a few million years. These observations raise the question of whether subduction is driving extension or vice versa.

  8. Generation of adakites in a cold subduction zone due to double subducting plates

    NASA Astrophysics Data System (ADS)

    Nakamura, Hitomi; Iwamori, Hikaru

    2013-06-01

    Adakites have been found in various tectonic settings, since the first report for the distinct lavas as a product of slab melting in Adak Island by Kay (J Volcanol Geotherm Res 4:117-132, 1978). In this study, we present geochemical data for an `adakite' and `adakitic rock' suite in central Japan with a cold subduction environment due to the two overlapping subducting plates: the Pacific plate and the Philippine sea plate. Based on the major, trace and isotopic compositions of the rocks, elemental transport from initial slab inventory at the trench to the volcanic rocks as a final product is quantitatively analyzed, considering the thermal structure, slab dehydration, elemental mobility, slab-fluid migration and melting of fluid-added mantle. The analysis demonstrates a large compositional impact of slab-fluid in the arc magma generation in central Japan. The melting conditions have been also estimated inversely by optimizing the predicted magma composition to the observed composition of volcanic rock, with the two parameters: the degree of melting and the proportion of spinel and garnet lherzolites involved in melting. Consequently, a moderately low degree of near-solidus melting of dominantly garnet lherzolite with a high fluid flux from the two overlapping slabs beneath the region has been argued to be responsible for the compositional characteristics, including the adakitic signatures, of the studied rocks. These results imply that the geochemical approach may provide useful constraints on the P- T condition of melting in the mantle wedge and the thermal structure in subduction zones, being complementary to the geophysical approach.

  9. Middle Miocene swift migration of the TTT triple junction and rapid crustal growth in southwest Japan: A review

    NASA Astrophysics Data System (ADS)

    Kimura, Gaku; Hashimoto, Yoshitaka; Kitamura, Yujin; Yamaguchi, Asuka; Koge, Hiroaki

    2014-07-01

    We review recent progress in geological and geophysical investigation in SW Japan, Nankai Trough, and Philippine Sea Plate (PSP), and propose a hypothesis for the Miocene tectonics in SW Japan driven by middle Miocene swift migration of the TTT (trench-trench-trench) triple junction. The new hypothesis is based on the new interpretations as follows. Near-trench magmatism in the outer zone of SW Japan is ascribed to a collision of proto-Izu arc in addition to the previous model of an oceanic ridge of the Shikoku Basin and hot PSP subduction. The indentation structures at Capes Ashizuri, Muroto in Shikoku, and Shiono on the Kii Peninsula were previously explained by "kink folding" due to recent E-W compression. We alternatively suppose the collision of the active arc or topographic peaks such as seamounts inferred from geological and experimental observations. The main crustal component in SW Japan is suggested to be of igneous plutonic rocks rather than the previous interpretation of Cretaceous to Tertiary accretionary complexes. This is typically illustrated in the outer zone to the north of Capes Ashizuri, Muroto, and Shiono from geophysical observation of gravity anomalies, velocity and resistivity, together with geological estimations of caldera age and the size of its root pluton. Episodic crustal growth due to intrusion of igneous rock and subduction of the PSP may have stopped after approximately 12 Ma and restarted at approximately 6 Ma. Our emphasis for this gap is a cessation and resurgence of subduction rather than the previous interpretation, i.e., decreasing of subduction rate.

  10. Crustal structure and configuration of the subducting Philippine Sea plate beneath the Pacific coast industrial zone in Japan inferred from receiver function analysis

    NASA Astrophysics Data System (ADS)

    Igarashi, T.; Iidaka, T.; Sakai, S.; Hirata, N.

    2012-12-01

    We apply receiver function (RF) analyses to estimate the crustal structure and configuration of the subducting Philippine Sea (PHS) plate beneath the Pacific coast industrial zone stretching from Tokyo to Fukuoka in Japan. Destructive earthquakes often occurred at the plate interface of the PHS plate, and seismic activities increase after the 2011 Tohoku earthquake (Mw9.0) around the Tokyo metropolitan area. Investigation on the crustal structure is the key to understanding the stress concentration and strain accumulation process, and information on configuration of the subducting plate is important to mitigate future earthquake disasters. In this study, we searched for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones by using a grid search method. Synthetic RFs were calculated from many assumed one-dimensional velocity structures that consist of four layers with positive velocity steps. Observed receiver functions were stacked without considering back azimuth or epicentral distance. We further constructed the vertical cross-sections of depth-converted RF images transformed the lapse time of time series to depth by using the estimated structure models. Telemetric seismographic network data covered on the Japanese Islands including the Metropolitan Seismic Observation network, which constructed under the Special Project for Earthquake Disaster Mitigation in the Tokyo Metropolitan area and maintained by Special Project for Reducing Vulnerability for Urban Mega Earthquake Disasters, are used. We selected events with magnitudes greater or equal to 5.0 and epicentral distance between 30 and 90 degrees based on USGS catalogues. As a result, we clarify spatial distributions of the crustal S-wave velocities. Estimated average one-dimensional S-wave velocity structure is approximately equal to the JMA2011 structural model although the velocity from the ground surface to 5 km in depth is slow. In particular, the Kanto plain and Boso peninsula are covered in thick sediment layers. The velocity perturbations in the crust are consistent with existing tomography models. There are low-velocity zones in the upper crust to the crust-mantle boundary corresponding to volcanoes. In contrast, non-volcanic mountain foothills are relatively high-velocity zones. We also elucidated the configuration of PHS plate to a depth of about 60 km. The PHS plate subducts to the northwest and the direction coincides with plate motion. The northeastern margin of PHS plate is estimated from the plate thickness, which gradually decreases to the northeast after contact with the underlying Pacific plate beneath the Tokyo metropolitan area. Asperities of some large earthquakes seem to be corresponded to the high-velocity area in the PHS slab. On the other hand, non-volcanic low-frequency earthquakes located in the plate interface are characterized by relatively low-velocity areas. They may indicate the serpentinized mantle wedge which reflects dehydration of the subducting oceanic crust.

  11. Trench Visualization

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This image shows oblique views of NASA's Phoenix Mars Lander's trench visualized using the NASA Ames Viz software package that allows interactive movement around terrain and measurement of features. The Surface Stereo Imager images are used to create a digital elevation model of the terrain. The trench is 1.5 inches deep. The top image was taken on the seventh Martian day of the mission, or Sol 7 (June 1, 2008). The bottom image was taken on the ninth Martian day of the mission, or Sol 9 (June 3, 2008).

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  12. Japan.

    ERIC Educational Resources Information Center

    Jones, Savannah C.

    Materials for a secondary level, interdisciplinary social studies course on Japan are divided into introductory information, 14 classroom units, and study and evaluation materials. Introductory material includes lists of objectives and skills, an outline of Japanese history, and an explanation of Japan's name and flag. The units cover the

  13. Japan.

    ERIC Educational Resources Information Center

    Jones, Savannah C.

    Materials for a secondary level, interdisciplinary social studies course on Japan are divided into introductory information, 14 classroom units, and study and evaluation materials. Introductory material includes lists of objectives and skills, an outline of Japanese history, and an explanation of Japan's name and flag. The units cover the…

  14. Phoenix Trenches

    NASA Technical Reports Server (NTRS)

    2008-01-01

    [figure removed for brevity, see original site] Annotated Version

    [figure removed for brevity, see original site] Left-eye view of a stereo pair [figure removed for brevity, see original site] Right-eye view of a stereo pair

    This image is a stereo, panoramic view of various trenches dug by NASA's Phoenix Mars Lander. The images that make up this panorama were taken by Phoenix's Surface Stereo Imager at about 4 p.m., local solar time at the landing site, on the 131st, Martian day, or sol, of the mission (Oct. 7, 2008).

    In figure 1, the trenches are labeled in orange and other features are labeled in blue. Figures 2 and 3 are the left- and right-eye members of a stereo pair.

    For scale, the 'Pet Donkey' trench just to the right of center is approximately 38 centimeters (15 inches) long and 31 to 34 centimeters (12 to 13 inches) wide. In addition, the rock in front of it, 'Headless,' is about 11.5 by 8.5 centimeters (4.5 by 3.3 inches), and about 5 centimeters (2 inches) tall.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  15. Changes in seismicity and stress loading on subduction faults in the Kanto region, Japan, 2011-2014

    NASA Astrophysics Data System (ADS)

    Gardonio, Blandine; Marsan, David; Lengliné, Olivier; Enescu, Bogdan; Bouchon, Michel; Got, Jean-Luc

    2015-04-01

    Seismic activity has increased in the Kanto region, Japan, following the 2011 M9.0 Tohoku earthquake. We here reassess this increase up to June 2014, to show that normal, Omori-like relaxation characterizes the activity on crustal faults as well as on the Philippine Sea plate, but not on the deeper Pacific plate. There repeating earthquakes display a twofold rate of occurrence (still ongoing in June 2014) as compared to the pre-Tohoku rate, suggesting enhanced creep. We compute the Coulomb stress changes on the upper locked portion of the Philippine Sea plate, which last ruptured in 1923. We find that this fault was little affected by either the coseismic, the postseismic, the accelerated creep, or the 2011 Boso silent slip event.

  16. Flexural Mechanics of Subduction

    NASA Astrophysics Data System (ADS)

    Ribe, N. M.; Li, Z. H.

    2014-12-01

    Deformation of subducting lithosphere is controlled by a balance of four forces: the negative buoyancy of the slab, its internal viscous resistance to stretching and bending, and the drag of the ambient mantle. To elucidate the complex dynamics of this system, we study a model in which a 3-D sheet of viscous fluid with thickness hh and viscosity ??1gammaeta_1 subducts in an ambient fluid with viscosity ?1eta_1 and depth DD. Numerical solutions for the sheet's evolution are obtained using a boundary element method, starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally. The dynamics of the sheet are controlled by its dimensionless `stiffness' S??(h/?b)3Sequiv gamma (h/ell_b)^3, where the `bending length' ?bell_b is the sum of the lengths of the slab and of the flexural bulge. The slab's sinking speed is controlled by its own viscosity if S?1Sgg 1, and by that of the ambient fluid if S?1Sleq 1. Time-dependent solutions with passive tracers demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides. A systematic investigation of the slab's interaction with the bottom boundary as a function of ?2/?1eta_2/eta_1 and D/hD/h delineates a rich regime diagram of subduction modes (trench retreating, slab folding, trench advancing, etc.) that agrees well with laboratory observations. The solutions show that mode selection is controlled by the dip of the slab's leading edge at the time when it first encounters the bottom boundary. We will discuss several geophysical applications of the model, including seismic evidence for slab folding, the radius-of-curvature constraint on the slab/mantle viscosity ratio ?2/?1eta_2/eta_1, and the distribution of seismic anisotropy around subducting slabs.

  17. Frictional properties of incoming pelagic sediments at the Japan Trench: implications for large slip at a shallow plate boundary during the 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Sawai, Michiyo; Hirose, Takehiro; Kameda, Jun

    2014-12-01

    The 2011 Tohoku earthquake (Mw 9.0) produced a very large slip on the shallow part of a megathrust fault that resulted in destructive tsunamis. Although multiple causes of such large slip at shallow depths are to be expected, the frictional property of sediments around the fault, particularly at coseismic slip velocities, may significantly contribute to large slip along such faults. We have thus investigated the frictional properties of incoming pelagic sediments that will subduct along the plate boundary fault at the Tohoku subduction zone, in order to understand the rupture processes that can cause large slip in the shallow parts of subduction zones. Our experimental results on clayey sediment at the base of the sedimentary section on the Pacific Plate yield a low friction coefficient of <0.2 over a wide range of slip velocities (0.25 mm/s to 1.3 m/s), and extremely low fracture energy during slip weakening, as compared with previous experiments of disaggregated sediments under coseismic slip conditions. Integrated Ocean Drilling Program (IODP) Expedition 343 confirmed that the clay-rich sediment investigated here is identical to those in the plate boundary fault zone, which ruptured and generated the Tohoku earthquake. The present results suggest that smectite-rich pelagic sediment not only accommodates cumulative plate motion over interseismic periods but also energetically facilitates the propagation of earthquake rupture towards the shallow part of the Tohoku subduction zone.

  18. Aeromagnetic legacy of early Paleozoic subduction along the Pacific margin of Gondwana

    USGS Publications Warehouse

    Finn, C.; Moore, D.; Damaske, D.; Mackey, T.

    1999-01-01

    Comparison of the aeromagnetic signatures and geology of southeastern Australia and northern Victoria Land, Antarctica, with similar data from ancient subduction zones in California and Japan, provides a framework for reinterpretation of the plate tectonic setting of the Pacific margin of early Paleozoic Gondwana. In our model, the plutons in the Glenelg (south-eastern Australia) and Wilson (northern Victoria Land) zones formed the roots of continental-margin magmatic arcs. Eastward shifting of arc magmatism resulted in the Stavely (south-eastern Australia) and Bowers (northern Victoria Land) volcanic eruptions onto oceanic forearc crust. The turbidites in the Stawell (southeastern Australia) and Robertson Bay (northern Victoria Land zones) shed from the Glenelg and Wilson zones, respectively, were deposited along the trench and onto the subducting oceanic plate. The margin was subsequently truncated by thrust faults and uplifted during the Delamerian and Ross orogenies, leading to the present-day aeromagnetic signatures.

  19. Geophysics. Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface.

    PubMed

    Yamashita, Y; Yakiwara, H; Asano, Y; Shimizu, H; Uchida, K; Hirano, S; Umakoshi, K; Miyamachi, H; Nakamoto, M; Fukui, M; Kamizono, M; Kanehara, H; Yamada, T; Shinohara, M; Obara, K

    2015-05-01

    Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events. PMID:25954006

  20. Trench connection.

    PubMed

    Jamieson, Alan J; Fujii, Toyonobu

    2011-10-23

    'Trench Connection' was the first international symposium focusing primarily on the hadal zone (depths greater than 6000 m). It was held at the University of Tokyo's Atmosphere and Ocean Research Institute in November 2010. The symposium was successful in having attracted an international collective of scientists and engineers to discuss the latest developments in the exploration and understanding of the deepest environments on Earth. The symposium sessions were categorized into three themes: (i) new deep-submergence technology; (ii) trench ecology and evolution; and (iii) the physical environment. Recent technological developments have overcome the challenges of accessing the extreme depths, which have in turn prompted an international renewed interest in researching physical and biological aspects of the hadal ecosystems. This bringing together of international participants from different disciplines led to healthy discussions throughout the symposium, providing potential opportunities and realizations of where the future of unravelling hadal ecology lies. Hadal science is still at relatively rudimentary levels compared with those of shallower marine environments; however, it became apparent at the symposium that it is now an ever-expanding scientific field. PMID:21450723

  1. Amplitude Comparison of Teleseismic P-Wave Phases from the Japan Subduction Zone and the South Sandwich Islands recorded at Uturuncu Volcano, Bolivia

    NASA Astrophysics Data System (ADS)

    Farrell, A. K.; McNutt, S. R.; West, M. E.

    2012-12-01

    Uturuncu volcano (22 15' S, 67 12'W) has been shown to be inflating at a rate of 1-1.5 cm per year by a satellite geodetic survey from May 1996-present. This inflation is centered just southwest of the volcano's summit and at a depth of 15-17 km. This may be caused by the injection of magma into the system. Seismic studies performed as part of the multi-university PLUTONS project can help constrain the location and nature of this inflation. By looking at how teleseismic peak-to-peak waveform amplitudes (velocities in nm/s) vary across the network, we can begin to pinpoint the size and location of attenuating zones beneath the edifice. Analysis of 5 P-wave phases from 4 earthquakes with origins in the Japan subduction zone (NW of the network, ~155 distant) shows a consistent 'shadow zone' of decreased amplitudes in a 13.6 by 33.3 km zone to the SE of the summit. Observations from two teleseismic events originating in the South Sandwich Islands (~45 distant) show similar effects although the geometry differs with respect to individual stations. We expect this trend to hold true for events originating to the NE and SW of the volcano, which would indicate a zone of decreased amplitude in the same region SE of the summit. The attenuation of P-waves that would otherwise be of uniform amplitude could be the result of some ray paths traveling through a shallow, low-velocity and highly attenuating zone of either magma/mush, highly fractured rock, or some other cause. This attenuating zone may be located at or near the center of the inflation zone, and physical processes associated with it could well be closely related to the observed inflation.

  2. Origin of back-arc basins and effects of western Pacific subduction systems on eastern China geology

    NASA Astrophysics Data System (ADS)

    Niu, Y.

    2013-12-01

    Assuming that subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere [1], and recognizing that subduction initiation within normal oceanic lithosphere is unlikely [1], we can assert that passive continental margins that are locations of the largest compositional buoyancy contrast within the lithosphere are the loci of future subduction zones [1]. We hypothesize that western Pacific back-arc basins were developed as and evolved from rifting at passive continental margins in response to initiation and continuation of subduction zones. This hypothesis can be tested by demonstrating that intra-oceanic island arcs must have basement of continental origin. The geology of the Islands of Japan supports this. The highly depleted forearc peridotites (sub-continental lithosphere material) from Tonga and Mariana offer independent lines of evidence for the hypothesis [1]. The origin and evolution of the Okinawa Trough (back-arc basin) and Ryukyu Arc/Trench systems represents the modern example of subduction initiation and back-arc basin formation along a (Chinese) continental margin. The observation why back-arc basins exit behind some subduction zones (e.g., western Pacific) but not others (e.g., in South America) depends on how the overlying plate responds to subduction, slab-rollback and trench retreat. In the western Pacific, trench retreat towards east results in the development of extension in the upper Eurasian plate and formation of back-arc basins. In the case of South America, where no back-arc basins form because trench retreat related extension is focused at the 'weakest' South Mid-Atlantic Ridge. It is thus conceptually correct that the South Atlantic is equivalent to a huge 'back-arc basin' although its origin may be different. Given the negative Clayperon slope of the Perovskite-ringwoodite phase transition at the 660 km mantle seismic discontinuity (660-D), slab penetration across the 660-D is difficult and trench retreat in the western Pacific readily result in the horizontal stagnation of the Pacific plate in the transition zone beneath eastern Asian continent [2]. Dehydration of this slab supplies water, which rises and results in 'basal hydration weakening' of the eastern China lithosphere and its thinning by converting it into weak material of asthenospheric property [3]. We note the proposal that multiple subduction zones with more water (i.e., subduction of the South China Block beneath the North China Craton, NCC; subduction of the Siberian/Mongolian block beneath the NCC) all contribute to the lithosphere thinning beneath the NCC [4]. However, 'South China-NCC' and 'Siberian/Mongolian-NCC' represent two collisional tectonics involving no trench retreat, causing no transition-zone slab stagnation, supplying no water, and thus contributing little to lithosphere thinning beneath the NCC. Furthermore, lithosphere thinning happened to the entire eastern China, not just limited to the NCC, emphasizing the effects of the western Pacific subduction system on eastern China geology. References: [1] Niu et al., 2003, Journal of Petrology, 44, 851-866. [2] Krason & van der Hilst, R., 2000, Geophysical Monograph, 121, 277-288. [3] Niu, 2005, Geological Journal of China Universities, 11, 9-46. [4] Windley et al., 2010, American Journal of Science, 310, 1250-1293.

  3. Geodetic imaging of plate motions, slip rates, and partitioning of deformation in Japan

    NASA Astrophysics Data System (ADS)

    Loveless, John P.; Meade, Brendan J.

    2010-02-01

    Interseismic deformation in Japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces. We use spherical linear block theory constrained by geodetic observations from densely spaced Global Positioning System (GPS) stations to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Japan-Kuril, Sagami, and Nankai subduction zones. The reference model developed in this paper consists of 20 blocks, produces a mean residual velocity magnitude of 1.84 mm/yr at 950 stations, and accounts for 96% of the observed interseismic deformation signal. We estimate fault slip rates in excess of 15 mm/yr along the Niigata-Kobe Tectonic Zone and Itoigawa-Shizuoka Tectonic Line through central Japan, confirming their hypothesized roles as major tectonic boundaries. Oblique convergence across the Nankai Trough is partitioned, with 3/4 of the 30 mm/yr of trench-parallel motion accommodated by strike-slip motion on the subduction interface and the remaining 1/4 accommodated by right-lateral slip on the Median Tectonic Line. In contrast, our models suggest negligible slip partitioning in eastern Hokkaido, where oblique slip on the Japan-Kuril subduction interface accommodates all of the trench-parallel component of relative plate motion. Inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes.

  4. What really causes flat slab subduction?

    NASA Astrophysics Data System (ADS)

    Manea, V. C.; Perez-Gussinye, M.; Manea, M.

    2014-12-01

    How flat slab geometries are generated has been long debated. It has been suggested thattrenchward motion of thick cratons in some areas of South America and Cenozoic NorthAmerica progressively closed the asthenospheric wedge and induced flat subduction. Here wedevelop time-dependent numerical experiments to explore how trenchward motion of thickcratons may result in flat subduction. We find that as the craton approaches the trench andthe wedge closes, two opposite phenomena control slab geometry: the suction between oceanand continent increases, favoring slab flattening, while the mantle confined within the closingwedge dynamically pushes the slab backward and steepens it. When the slab retreats, as inthe Peru and Chile flat slabs, the wedge closure rate and dynamic push are small and suctionforces generate, in some cases, flat subduction. We model the past 30 m.y. of subduction in theChilean flat slab area and demonstrate that trenchward motion of thick lithosphere, 200-300km, currently ~700-800 km away from the Peru-Chile Trench, reproduces a slab geometrythat fits the stress pattern, seismicity distribution, and temporal and spatial evolution ofdeformation and volcanism in the region. We also suggest that varying trench kinematics mayexplain some differing slab geometries along South America. When the trench is stationaryor advances, the mantle flow within the closing wedge strongly pushes the slab backward andsteepens it, possibly explaining the absence of flat subduction in the Bolivian orocline.

  5. Insight into complex rupturing of the immature bending normal fault in the outer slope of the Japan Trench from aftershocks of the 2005 Sanriku earthquake (Mw = 7.0) located by ocean bottom seismometry

    NASA Astrophysics Data System (ADS)

    Hino, Ryota; Azuma, Ryosuke; Ito, Yoshihiro; Yamamoto, Yojiro; Suzuki, Kensuke; Tsushima, Hiroaki; Suzuki, Syuichi; Miyashita, Makoto; Tomori, Toshihiro; Arizono, Mitsuharu; Tange, Go

    2009-07-01

    The distribution of aftershocks of a large (Mw = 7.0) normal faulting earthquake beneath the outer slope of the Japan Trench in 2005, measured in 2007 using ocean bottom seismographs, indicates that the earthquake was involved with a set of conjugate normal faults. Although the faults reach to the upper mantle, the estimated Vp and Vp/Vs show no remarkable changes that can be related to extensive hydration in the crust or uppermost mantle. The absence of horst-graben topographic structure in the rupture area suggests that immaturity of the bending fault system is responsible for the relatively unhydrated lithosphere. Several earthquakes below the aftershock zone may belong to the lower plane seismicity of the shallow double seismic zone. Because no earthquakes were recorded in the area for more than 80 years before the 2005 event, shallow extensional and deep compressive earthquakes may be activated concurrently in the focal area of this earthquake.

  6. A critical assessment of viscous models of trench topography and corner flow

    NASA Technical Reports Server (NTRS)

    Zhang, J.; Hager, B. H.; Raefsky, A.

    1984-01-01

    Stresses for Newtonian viscous flow in a simple geometry (e.g., corner flow, bending flow) are obtained in order to study the effect of imposed velocity boundary conditions. Stress for a delta function velocity boundary condition decays as 1/R(2); for a step function velocity, stress goes as 1/R; for a discontinuity in curvature, the stress singularity is logarithmic. For corner flow, which has a discontinuity of velocity at a certain point, the corresponding stress has a 1/R singularity. However, for a more realistic circular-slab model, the stress singularity becomes logarithmic. Thus the stress distribution is very sensitive to the boundary conditions, and in evaluating the applicability of viscous models of trench topography it is essential to use realistic geometries. Topography and seismicity data from northern Hoshu, Japan, were used to construct a finite element model, with flow assumed tangent to the top of the grid, for both Newtonian and non-Newtonian flow (power law 3 rheology). Normal stresses at the top of the grid are compared to the observed trench topography and gravity anomalies. There is poor agreement. Purely viscous models of subducting slables with specified velocity boundary conditions do not predict normal stress patterns compatible with observed topography and gravity. Elasticity and plasticity appear to be important for the subduction process.

  7. A critical assessment of viscous models of trench topography and corner flow

    NASA Technical Reports Server (NTRS)

    Zhang, J.; Hager, B. H.; Raefsky, A.

    1985-01-01

    Stresses for Newtonian viscous flow in a simple geometry (e.g., corner flow, bending flow) are obtained in order to study the effect of imposed velocity boundary conditions. Stress for a delta function velocity boundary condition decays as 1/R(2); for a step function velocity, stress goes as 1/R; for a discontinuity in curvature, the stress singularity is logarithmic. For corner flow, which has a discontinuity of velocity at a certain point, the corresponding stress has a 1/R singularity. However, for a more realistic circular-slab model, the stress singularity becomes logarithmic. Thus the stress distribution is very sensitive to the boundary conditions, and in evaluating the applicability of viscous models of trench topography it is essential to use realistic geometries. Topography and seismicity data from northern Hoshu, Japan, were used to construct a finite element model, with flow assumed tangent to the top of the grid, for both Newtonian and non-Newtonian flow (power law 3 rheology). Normal stresses at the top of the grid are compared to the observed trench topography and gravity anomalies. There is poor agreement. Purely viscous models of subducting slables with specified velocity boundary conditions do not predict normal stress patterns compatible with observed topography and gravity. Elasticity and plasticity appear to be important for the subduction process.

  8. Finite element modeling on stress field of subduction zones and island arcs during megathrust earthquake cycles

    NASA Astrophysics Data System (ADS)

    Muto, J.; Shibazaki, B.; Iidaka, T.; Ohzono, M.

    2013-12-01

    A subduction zone earthquake cycle includes a great earthquake and subsequent strain accumulation in to the next earthquake. Such cycles in viscoelastic earth perturbs crustal stresses. The observations of shear-wave splitting during crustal earthquakes in the forearc of the NE Japan have revealed the presence of almost NS polarization azimuths, while the volcanic front to backarc show the EW polarization azimuths. This indicates that the stress field in the forearc crust is not horizontal EW compression during the interseismic period. In order to clarify how crustal stress fields are perturbed during earthquake cycles, we have conducted a finite element model on subduction zones earthquake cycles in the NE Japan. We developed a two-dimensional finite element model oriented perpendicular to the Japan Trench extending 1000 km to the west and 600 km to the east of the Trench and 800 km depth. The model also transects an area of large coseismic slip of the 2011 Tohoku Oki earthquake with the slip magnitude exceeding 60 m. The subsurface crustal and mantle wedge structures, and subducting slab geometry were developed based on an offshore seismic reflection survey and high-precision seismic tomography of the crust, mantle wedge structures, and subducting slab in this region. Deformation along plate boundary is the kinematically assigned using the split node method. For a subduction plate boundary, a shallow portion is assumed to be locked and from a certain depth downdip, the boundary is assumed to slip at the full plate convergence rate of 80 mm/yr during interseismic period. At the coseismic step, the amount of slip corresponding to slip deficit during the interseismic period is achieved along the shallow portion. From preliminary results for cycles up to 10 earthquakes, the horizontal stress was oscillated through the cycles: horizontal EW compression during interseismic periods and sudden extension by coseismic deformations. The horizontal stress in the shallower portion of the forearc side just prior to an earthquake gradually becomes extension regime with cycles. The portion of this extension regime roughly corresponds to the region with NS polarization azimuths of the shear wave splitting of crustal earthquakes in the NE Japan. This indicates that the formation of extensional stress regime in the forearc during intersesimic period might be originated from the buckling of the island arc lithosphere and relaxation of compressive stress during the intersesimic period.

  9. When Boundary Layers Collide: Plumes v. Subduction Zones

    NASA Astrophysics Data System (ADS)

    Moresi, L. N.; Betts, P. G.; Miller, M. S.; Willis, D.; O'Driscoll, L.

    2014-12-01

    Many subduction zones retreat while hotspots remain sufficiently stable in the mantle to provide an approximate reference frame. As a consequence, the mantle can be thought of as an unusual convecting system which self-organises to promote frequent collisions of downgoing material with upwellings. We present three 3D numerical models of subduction where buoyant material from a plume head and an associated ocean-island chain or plateau produce flat slab subduction and deformation of the over-riding plate. We observe transient instabilities of the convergent margin including: contorted trench geometry; trench migration parallel with the plate margin; folding of the subducting slab and orocline development at the convergent margin; and transfer of the plateau to the overriding plate. The presence of plume material beneath the oceanic plateau causes flat subduction above the plume, resulting in a "bowed" shaped subducting slab. In the absence of a plateau at the surface, the slab can remain uncoupled from the over-riding plate during very shallow subduction and hence there is very little shortening at the surface or advance of the plate boundary. In plateau-only models, plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction re-establishes directly behind the trailing edge of the plateau. The plateau shortens during accretion and some plateau material subducts. In a plateau-plus-plume model, accretion is associated with rapid trench advance as the flat slab drives the plateau into the margin. This indentation stops once a new convergent boundary forms close to the original trench location. A slab window formed beneath the accreted plateau allows plume material to flow from beneath the subducting plate to the underside of the overriding plate. In all of these models the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate. The models provide a dynamic context for plateau and plume accretion in accretionary orogenic systems.

  10. New Constraints on Mantle Flow in Subduction Systems

    NASA Astrophysics Data System (ADS)

    Fouch, M. J.; Lassak, T. M.; Roth, J. B.; Smith, C. M.; Hall, C. E.; Kaminski, E.; Anglin, D. K.

    2005-12-01

    From a compositional, dynamical, and structural standpoint, subduction zones are arguably Earth's most complex tectonic setting. Significant uncertainty remains regarding the extent of coupling between lithospheric plates and surrounding mantle, as well as the extent to which the mantle wedge is hydrated in subduction systems. A key observation used to address these issues is measurements of seismic anisotropy, which while generally straightforward to observe via shear wave splitting, are less straightforward to interpret particularly in regions of compositional and structural complexity. To this end, we have surveyed several Pacific subduction zones to provide new constraints on subduction zone seismic anisotropy. We have also performed numerical modeling to estimate the effects of hydration of the mantle wedge on observations of shear wave splitting in subduction systems. From an observational standpoint, we have obtained over 400 new shear wave splitting measurements from shear phases that sample the Japan, Izu-Bonin, and Cascadia subduction systems. Across the Japan and Izu-Bonin regions, fast polarization directions range from convergence-parallel to trench-parallel and suggest complex mantle deformation. We infer that the total strength of anisotropy is 1-2% and is generated primarily from the mantle wedge and the subducting lithospheric slab with a component of subslab anisotropy generated by diverted mantle flow beneath the Philippine Sea Plate. Conversely, fast polarization directions for southern Cascadia exhibit less variation and are generally close to convergence-parallel across the region. In this area, anisotropy appears to be stronger, on the order of 3-4%, and is located primarily in the mantle wedge and overriding plate. The observed shear wave splitting variations, combined with relative delay time measurements that will be presented separately in this session, are likely due a combination of mantle flow variations and the existence of a serpentinized and/or hydrous olivine wedge. From a modeling standpoint, we have examined the effects of high stress and hydration states to predict shear wave splitting as a result of mantle silicate lattice-preferred orientation (LPO) development in mantle flow models using a theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization on textural development. We utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone. Results demonstrate that, using the combined observations of variations in fast polarization directions and splitting times, it is possible to resolve a shift from anhydrous to hydrous mantle in subduction zone settings when sampling of the mantle wedge is very good. The implications of these results suggest that new seismic experiments in subduction systems are required to fully evaluate potential competing effects between changes in mantle flow and hydration state in subduction zone mantle wedges.

  11. Hot spot and trench volcano separations

    NASA Technical Reports Server (NTRS)

    Lingenfelter, R. E.; Schubert, G.

    1974-01-01

    It is suggested that the distribution of separations between trench volcanos located along subduction zones reflects the depth of partial melting, and that the separation distribution for hot spot volcanoes near spreading centers provides a measure of the depth of mantle convection cells. It is further proposed that the lateral dimensions of mantle convection cells are also represented by the hot-spot separations (rather than by ridge-trench distances) and that a break in the distribution of hot spot separations at 3000 km is evidence for both whole mantle convection and a deep thermal plume origin of hot spots.

  12. Interplate coupling along the central Ryukyu Trench inferred from GPS/acoustic seafloor geodetic observation

    NASA Astrophysics Data System (ADS)

    Nakamura, M.; Tadokoro, K.; Okuda, T.; Ando, M.; Watanabe, T.; Sugimoto, S.; Miyata, K.; Matsumoto, T.; Furukawa, M.

    2010-12-01

    The Ryukyu trench is a major convergent plate boundary where the Philippine Sea plate is subducting at a rate of about 8 cm/yr. Large earthquakes have not been reported along the Ryukyu subduction for the last 300 years. Because the rate of release of seismic moment in the Ryukyu Trench over the last 80 years is 5% in consideration of the plate convergence rate, interseismic coupling in the trench is assumed to be weak.The GPS measurements by Japan Geographical Survey Institute also show the southward motion (2.5 cm/yr) of Ryukyu arc relative to the Amurian plate, which is due to extensional rifting of Okinawa Trough. Backslip by the interplate coupling between the subducting Philippine Sea plate and the overriding Eurasian plate cannot have been detected in the GPS network along the Ryukyu Islands. We have started the GPS/acoustic seafloor observation to detect the inter-plate coupling in the central Ryukyu trench. For this measurement, we used a system capable of performing two main tasks: precise acoustic ranging between a ship station (observation vessel) and seafloor transponders, and kinematic GPS positioning of observation vessels. The seafloor reference point was set at about 33 km landward from the axis of the Ryukyu trench (southeast of Okinawa Island). A set of three acoustic transponders has been installed on the seafloor, at a depth of about 2900m. The transponders are placed to form a triangular. Five campaign observations were carried out for the period from January 2008 to November 2009. Each epoch consists of three observation days. The coordinates of the seafloor benchmark were calculated using the least-squares technique (Ikuta et al., 2008); this technique minimizes the square sum of acoustic travel-time residuals. The RMS of travel time residuals for each campaign analysis is about 70 micro-seconds. The result shows that the benchmark moved to northwest direction for two years at a rate of 4 cm/yr relative to the Amurian plate. Then we estimated the length and width of interplate coupling area using observed movement of the benchmark. The movements of the GPS stations on the Ryukyu Islands and the benchmark are described as the combination of the block rotation of the Ryukyu arc (Nakamura, 2004) and the displacement by the backslip in the coupled area. The results show that the estimated width of the interplate coupling area is 40-50 km from the Ryukyu trench. The results also show that the length of the coupled area is over 40 km. Since the calculated displacements are not sensitive to the change in the length of the coupled area, the accurate length is uncertain. These suggest that the interplate coupling occurs up-dip of the seismogenic zone in the Ryukyu subduction zone. The tsunami earthquake (M8.1) occurred near the south Ryukyu Trench in 1771. This suggests the interplate coupling near the Trench would be the cause of the tsunami earthquakes.

  13. Scattering beneath Western Pacific subduction zones: evidence for oceanic crust in the mid-mantle

    NASA Astrophysics Data System (ADS)

    Bentham, H. L. M.; Rost, S.

    2014-06-01

    Small-scale heterogeneities in the mantle can give important insight into the dynamics and composition of the Earth's interior. Here, we analyse seismic energy found as precursors to PP, which is scattered off small-scale heterogeneities related to subduction zones in the upper and mid-mantle. We use data from shallow earthquakes (less than 100 km depth) in the epicentral distance range of 90-110 and use array methods to study a 100 s window prior to the PP arrival. Our analysis focuses on energy arriving off the great circle path between source and receiver. We select coherent arrivals automatically, based on a semblance weighted beampower spectrum, maximizing the selection of weak amplitude arrivals. Assuming single P-to-P scattering and using the directivity information from array processing, we locate the scattering origin by ray tracing through a 1-D velocity model. Using data from the small-aperture Eielson Array (ILAR) in Alaska, we are able to image structure related to heterogeneities in western Pacific subduction zones. We find evidence for 300 small-scale heterogeneities in the region around the present-day Japan, Izu-Bonin, Mariana and West Philippine subduction zones. Most of the detected heterogeneities are located in the crust and upper mantle, but 6 per cent of scatterers are located deeper than 600 km. Scatterers in the transition zone correlate well with edges of fast features in tomographic images and subducted slab contours derived from slab seismicity. We locate deeper scatterers beneath the Izu-Bonin/Mariana subduction zones, which outline a steeply dipping pseudo-planar feature to 1480 km depth, and beneath the ancient (84-144 Ma) Indonesian subduction trench down to 1880 km depth. We image the remnants of subducted crustal material, likely the underside reflection of the subducted Moho. The presence of deep scatterers related to past and present subduction provides evidence that the subducted crust does descend into the lower mantle at least for these steeply dipping subduction zones. Applying the same technique to other source-receiver paths will increase our knowledge of the small-scale structure of the mantle and will provide further constraints on geodynamic models.

  14. A Crustal Structure Study of the Southern Ryukyu Subduction Zone by Using the Aftershock Data

    NASA Astrophysics Data System (ADS)

    Cho, Y.; Lin, J.; Lee, C.

    2011-12-01

    The region along the Ryukyu subduction zone is known as a tsunami disaster zone. The biggest tsunami (85 m) of Japan history was recorded in the Ishigaki Island, Ryukyu, in 1771. The paleo-tsunami events show that it has a frequency of about 150 years. This thread makes the Ryukyu subduction zone as a concerned field for the earthquake studies. However, due to the long distance from the east coast of Taiwan, this is an area out of the effective earthquake detection zone from the Central Weather Bureau network. A main shock of M = 6.9 occurred near the Ishigaki Island in 2009 August 17. After this event, we quickly deployed the OBS and found many aftershocks with the magnitude greater than 5.0. The main shock was 240 km, NE direction from the Hualien city, Taiwan. If a tsunami occurred, it took only less than 15 minutes to arrive the coast. From the recorded data, we picked the P- and S-wave using the 1-D module (iasp91). There were 1500 recorded events during those time range, and most of the earthquakes were located around the Nanao Basin. Based on this, we study the southern Ryukyu subduction zone structure by using the results from focal mechanism solution. From the earthquake relocation it shows that two main groups of aftershocks. They tend in northwest - southeast with a left-lateral strike-slip fault. The left-lateral strike-slip fault is the main structures that link with the splay faults at the southern Ryukyu Trench. The stability and extension of the splay faults are one of the major concerns for the occurrence of mega earthquake. More than 500-km long of the splay fault, such as that in the Indonesia, Chile and Japan subduction zones, has attacked by mega earthquakes in the recent years. The second group of those aftershocks was located in the Gagua Ridge near the Ryukyu Trench. This group may represent the ridge structure relate to the Taitung canyon fault. The front of Ryukyu Trench was being as a locked subduction zone where it is easily to accumulate the earthquake stress. Because of these two earthquake groups are out of range of Taiwan Central Weather Bureau network and lack of information, it is worthwhile to focus our attentions on it.

  15. Strong Quake Strikes Japan

    NASA Astrophysics Data System (ADS)

    Showstack, Randy

    2011-03-01

    As Eos was about to go to press, a powerful earthquake with a preliminary estimated magnitude of 8.9 shook the northeast coast of Japan on 11 March at 05:46:23 UTC. It is the largest known earthquake along the Japan Trench subduction zone since 869 A.D. or earlier, Brian Atwater, geologist with the U.S. Geological Survey (USGS), told Eos. The quake's magnitude would place it fifth in terms of any earthquake magnitude worldwide since at least 1900, according to information from the USGS Earthquake Hazards Program. The amount of energy released in the quake—which occurred 130 kilometers east of Sendai, Honshu, at a depth of 24.4 kilometers—was equivalent to the energy from 30 earthquakes the size of the 1906 quake in San Francisco, Calif., according to David Applegate, USGS senior science advisor for earthquake and geologic hazards. He said the economic losses from the shaking are estimated to be in the tens of billions of dollars.

  16. Geodynamics of trench advance: Insights from a Philippine-Sea-style geometry

    NASA Astrophysics Data System (ADS)

    ?kov, Hana; Bina, Craig R.

    2015-11-01

    For terrestrial parameter sets, trench retreat is found to be nearly ubiquitous and trench advance quite rare, largely due to rheological and ridge-push effects. Recently updated analyses of global plate motions indicate that significant trench advance is also rare on Earth, being largely restricted to the Marianas-Izu-Bonin arc. Thus, we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the Philippine Sea region. In this subduction system, a slab-pull force from distal subduction is transmitted to the overriding plate at the Pacific trench. Our 2D modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries. We observe persistent trench advance punctuated by two episodes of back-arc extension. Characteristic features of the model, such as time interval between extensional episodes, high back-arc heat flow, and stress state of Philippine plate correspond to processes recorded in the region.

  17. Subduction initiation: spontaneous and induced

    NASA Astrophysics Data System (ADS)

    2004-10-01

    The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new subduction zones along the western edge of the Pacific plate during the Eocene. Development of self-sustaining subduction in the case of SNSZ is signaled by the beginning of down-dip slab motion, causing chilling of the forearc mantle and retreat of the magmatic arc to a position that is 100 200 km from the trench. INSZ may affect only part of a plate margin, but SNSZ affects the entire margin in the new direction of convergence. INSZ and SNSZ can be distinguished by the record left on the upper plates: INSZ begins with strong compression and uplift, whereas SNSZ begins with rifting and seafloor spreading. Understanding conditions leading to SNSZ and how hinged subsidence of lithosphere changes to true subduction promise to be exciting and fruitful areas of future research.

  18. Subduction initiation: spontaneous and induced

    NASA Astrophysics Data System (ADS)

    Stern, Robert J.

    2004-10-01

    The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo-Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new subduction zones along the western edge of the Pacific plate during the Eocene. Development of self-sustaining subduction in the case of SNSZ is signaled by the beginning of down-dip slab motion, causing chilling of the forearc mantle and retreat of the magmatic arc to a position that is 100-200 km from the trench. INSZ may affect only part of a plate margin, but SNSZ affects the entire margin in the new direction of convergence. INSZ and SNSZ can be distinguished by the record left on the upper plates: INSZ begins with strong compression and uplift, whereas SNSZ begins with rifting and seafloor spreading. Understanding conditions leading to SNSZ and how hinged subsidence of lithosphere changes to true subduction promise to be exciting and fruitful areas of future research.

  19. Island-Arc Collision Dominates Japan's Sediment Flux to the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Codilean, A. T.; Korup, O.; Hayakawa, Y. S.; Matsushi, Y.; Saito, H.; Matsuzaki, H.

    2013-12-01

    Quantifying volumes and rates of delivery of terrestrial sediment to subduction zones is indispensable for refining estimates of the thickness of trench fills that may eventually control the location and timing of submarine landslides and tsunami-generating mega-earthquakes. Despite these motivating insights, knowledge about the rates of erosion and sediment export from the Japanese islands to their Pacific subduction zones has somewhat stagnated despite the increasing availability of highly resolved data on surface deformation, climate, geology, and topography. Traditionally, natural erosion rates across the island arc have been estimated from catchment topographic predictors of reservoir sedimentation rates that were recorded over several years to decades. We correct for a systematic bias in these predictions, and present new estimates of decadal to millennial-scale erosion rates of the Japanese terrestrial inner forearc, drawing on several unprecedented inventories of mass wasting, reservoir sedimentation, and concentrations of cosmogenic 10Be in river sands. Our data reveal that catchments draining Japan's eastern seaboard have distinctly different tectonic, lithological, topographic, and climatic characteristics, underscored by a marked asymmetric pattern of erosion rates along and across the island arc. Erosion rates are highest in the Japanese Alps that mark the collision of two subduction zones, where high topographic relief, hillslope and bedrock-channel steepness foster rapid denudation by mass wasting. Comparable, if slightly lower, rates characterize southwest Japan, most likely due to higher typhoon-driven rainfall totals and variability rather than the similarly high relief and contemporary uplift rates that are linked to subduction earthquake cycles, and outpace long-term Quaternary uplift. In contrast, our estimated erosion and flux rates are lowest in the inner forearc catchments that feed sediment into the Japan Trench. We conclude that collisional mountain-building of the Japanese Alps causes the highest erosion rates anywhere on the island arc despite similar uplift and precipitation controls in southwest Japan. We infer that, prior to extensive river damming and reservoir construction, the gross of Japan's total sediment export to the Pacific Ocean entered the accretionary margin of the Nankai Trough as opposed to the comparatively sediment-starved Japan Trench. Although this pattern mimics the long-term mass balance of incoming sediment to these subduction zones, future work will be needed to constrain the relative contribution of terrestrial sediment input on 103-yr timescales.

  20. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase I). Final technical report

    SciTech Connect

    Hilde, T.W.C.

    1984-08-01

    The objective was to determine the fate of sediments at convergent lithospheric plate boundaries. The study focuses on the structures of the Circum-Pacific trenches and shallow portions of the associated subduction zones. Sediment distribution and the nature of sediment deformation was defined through the various stages of plate convergence to determine if the sediments are subducted or accreted. The controlling factors for sediment subduction and/or accretion were determined. 50 figs. (ACR)

  1. Evidence for retrograde lithospheric subduction on Venus

    NASA Technical Reports Server (NTRS)

    Sandwell, David T.; Schubert, Gerald

    1992-01-01

    Though there is no plate tectonics per se on Venus, recent Magellan radar images and topographic profiles of the planet suggest the occurrence of the plate tectonic processes of lithospheric subduction and back-arc spreading. The perimeters of several large coronae (e.g., Latona, Artemis, and Eithinoha) resemble Earth subduction zones in both their planform and topographic profile. The planform of arcuate structures in Eastern Aphrodite were compared with subduction zones of the East Indies. The venusian structures have radii of curvature that are similar to those of terrestrial subduction zones. Moreover, the topography of the venusian ridge/trench structures is highly asymmetric with a ridge on the concave side and a trough on the convex side; Earth subduction zones generally display the same asymmetry.

  2. Studying Near-Trench Characteristics of the 2011 Tohoku-Oki Megathrust Rupture Using Differential Multi-Beam Bathymetry before and after the Earthquake

    NASA Astrophysics Data System (ADS)

    Sun, T.; Fujiwara, T.; Kodaira, S.; Wang, K.; He, J.

    2014-12-01

    Large coseismic motion (up to ~ 31 m) of seafloor GPS sites during the 2011 M 9 Tohoku earthquake suggests large rupture at shallow depths of the megathrust. However, compilation of all published rupture models, constrained by the near-field seafloor geodetic observation and also various other datasets, shows large uncertainties in the slip of the most near-trench (within ~ 50 km from the trench) part of the megathrust. Repeated multi-beam bathymetry surveys that cover the trench axis, carried out by Japan Agency for Marine-Earth Science and Technology, for the first time recorded coseismic deformation in a megathrust earthquake at the trench. In previous studies of the differential bathymetry (DB) before and after the earthquake to determine coseismic fault slip, only the rigid-body translation component of the upper plate deformation was considered. In this work, we construct Synthetic Differential Bathymetry (SDB) using an elastic deformation model and make comparisons with the observed DB. We use a 3-D elastic Finite Element model with actual fault geometry of the Japan trench subduction zone and allowing the rupture to breach the trench. The SDB can well predict short-wavelength variations in the observed DB. Our tests using different coseismic slip models show that the internal elastic deformation of the hanging wall plays an important role in generating DB. Comparing the SDB with the observed DB suggests that the largest slip is located within ~ 50 km from the trench. The SDB proves to be the most effective tool to evaluate the performance of different rupture models in predicting near-trench slip. Our SDB work will further explore the updip slip variation. The SDB may help to constrain the slip gradient in the updip direction and may help to determine whether the large shallow slip in the Tohoku earthquake plateaued at the trench or before reaching the trench. Resolving these issues will provide some of the key tests for various competing models that were proposed to explain the large shallow rupture in this event.

  3. Development of seismic anisotropy during subduction-induced mantle flow

    NASA Astrophysics Data System (ADS)

    Faccenda, Manuele; Capitanio, Fabio Antonio

    2013-04-01

    Subduction zones are convergent margins where the rigid lithosphere sinks into the Earth's mantle inducing complex 3D flow patterns. Seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals is commonly used to study flow in the mantle and its relations with plate motions. We present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3-D numerical models of Earth-like subduction. This computational strategy accounts for the non-steady-state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered. In the subduction models a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction, and is independent of the subduction rate. The subslab upper mantle is characterized by two domains with different fabrics: at shallow depth the mantle entrained with the subducting slab develops trench-perpendicular directed anisotropy due to simple shear deformation, while in the deeper mantle slab rollback induces pure shear deformation causing trench-parallel extension and fast seismic directions. Subducting plate advance favours the development of the fabric in the entrained mantle domain, while slab retreat increases the trench-parallel anisotropy in the deeper upper mantle. In the deeper domain the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the centre. As such, strong trench-parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates. The synthetic SKS splitting patterns calculated in the fore-arc are controlled by the magnitude of the anisotropy in the upper domain, with trench-perpendicular fast azimuths in the centre of large plates and trench-parallel toward the plate edges. Instead, above relatively narrow, retreating slabs (≤ 600 km and low subduction partitioning ratio (SPR)), azimuths are trench-parallel due to the strong anisotropy in the lower subslab domain. In all models the anisotropy in the back-arc and on the sides of the subducting plate is, respectively, trench-perpendicular and sub-parallel to the return flow at depth. Results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally

  4. Mmax Inferred from the Back Slip Rate Distributions along the Japan Islands

    NASA Astrophysics Data System (ADS)

    Koketsu, K.; Yokota, Y.; Higuchi, S.

    2012-12-01

    The devastating Tohoku earthquake of magnitude (M) 9.0 occurred on 11 March 2011 UTC along the Japan Trench, where the Pacific plate is subducting beneath the Tohoku district. Koketsu, Yokota, Kato, and Kato (2012) recovered annual rates of back slip, which is the drag of the overriding plate by interplate coupling, using GPS data in northeastern Japan before the Tohoku earthquake. They then recovered coseismic slips through an inversion of GPS data during the earthquake. The distributions of recovered coseismic slips and back slip rates bear a close resemblance to each other. They also calculated the recurrence period of such a megathrust event to be about 400 years using the coseismic moment releases and moment accumulation rate. They confirmed these relations by conducting seismic cycle simulations.The national program of seismic hazard assessment, which was initiated by the Japanese government after the 1995 Kobe earthquake, failed to foresee the Tohoku earthquake. However, the above results suggest the Tohoku earthquake could be foreseen with respect to at least its location and extent, if we monitored GPS data. In addition, the above method can be applicable to consider the Mmax of an other subduction zone, because the size of the Tohoku earthquake is propably the Mmax in the subduction zone along the Japan Trench. We first inspected the back slip rate distribution by Koketsu, Yokota, Kato and Kato (2012) carefully, and found similar areas of large back slip rate along the southernmost Kuril Trench and the Sagami Trough. The former area is as large as that along the Japan Trench. Therefore, the Mmax along the southernmost Kuril Trench should be around the M of the Tohoku earthquake, though Nanayama et al. (2003) estimated it to be 8.4 from tsunami deposit surveys. The latter area looks like the source region of the 1703 Genroku earthquake. Since this region is twice larger than that of the 1923 Kanto earthquake, the Mmax along the Sagami Trough should be around 8.1. We secondly recovered annual rates of back slip using GPS data in southwestern Japan. The resultant distribution shows a long area of large back slip rate along the Nankai Trough. This area looks larger than the source region of the 1707 Hoei earthquake. Accordingly, the Mmax along the Nankai Trough should be greater than 8.6, which was estimated by Utsu (1999) for the Hoei earthquake.

  5. Focal depth, magnitude, and frequency distribution of earthquakes along oceanic trenches

    NASA Astrophysics Data System (ADS)

    Hammed, O. S.; Popoola, O. I.; Adetoyinbo, A. A.; Awoyemi, M. O.; Badmus, G. O.; Ohwo, O. B.

    2013-11-01

    The occurrence of earthquakes in oceanic trenches can pose a tsunami threat to lives and properties in active seismic zones. Therefore, the knowledge of focal depth, magnitude, and time distribution of earthquakes along the trenches is needed to investigate the future occurrence of earthquakes in the zones. The oceanic trenches studied, were located from the seismicity map on: latitude +51 to +53 and longitude -160 to 176 (Aleutian Trench), latitude +40 to +53 and longitude +148 to +165 (Japan Trench), and latitude -75 to -64 and longitude -15 to +30 (Peru-Chile Trench). The following features of seismic events were considered: magnitude distribution, focal depth distribution, and time distribution of earthquake. The results obtained in each trench revealed that the earthquakes increased with time in all the regions. This implies that the lithospheric layer is becoming more unstable. Thus, tectonic stress accumulation is increasing with time. The rate of increase in earthquakes at the Peru-Chile Trench is higher than that of the Japan Trench and the Aleutian Trench. This implies that the convergence of lithospheric plates is higher in the Peru-Chile Trench. Deep earthquakes were observed across all the trenches. The shallow earthquakes were more prominent than intermediate and deep earthquakes in all the trenches. The seismic events in the trenches are mostly of magnitude range 3.0-4.9. This magnitude range may indicate the genesis of mild to moderate tsunamis in the trench zone in near future once sufficient slip would occur with displacement of water column.

  6. Counter-intuitive Behavior of Subduction Zones: Weak Faults Rupture, Strong Faults Creep

    NASA Astrophysics Data System (ADS)

    Wang, K.; Gao, X.; Bilek, S. L.; Brown, L. N.

    2014-12-01

    Subduction interfaces that produce great earthquakes are often said to be "strongly coupled", and those that creep are said to be "weakly coupled". However, the relation between the strength and seismogenic behavior of subduction faults is far from clear. Seismological and geodetic observations of earthquake rupture usually provide information only on stress change, not fault strength. In this study, we infer fault strength by calculating frictional heating along megathrusts and comparing results with heat flow measurements. We find that stick-slip megathrusts that have produced great earthquakes such as at Japan Trench and northern Sumatra have very low apparent friction coefficients (~ 0.02 - 0.03), but megathrusts that creep such as at Northern Hikurangi and Manila Trench have higher values (up to ~0.13). The differnce cannot be explained by coseismic dynamic weakening of the stick-slip megathrusts, because the average stress drop in great earthquakes is usually less than 5 MPa, equivalent to a coseismic reduction of apparent friction coefficient by less than ~0.01. Therefore our results indicate differences in the static strength of different subduction faults. Why are the creeping faults stronger? We think it is related to their creeping mechanism. Very rugged subducting seafloor tends to cause creep and hinder great earthquake rupture (Wang and Bilek, 2014). In contrast, all giant earthquakes have occurred at subduction zones with relatively smooth subducting seafloor. Large geometrical irregularities such as seamounts generate heterogeneous structure and stresses that promote numerous small and medium size earthquakes and aseismic creep. The creeping is a process of breaking and wearing of geometrical irregularities in a deformation zone and is expected to be against relatively large resistance (strong creep). This is different from the creeping of smooth faults due to the presence of weak fault gouge (weak creep) such as along the creeping segment of the San Andreas fault. The general correlation between subducting seafloor ruggedness, creeping, and greater heat dissipation, if further verified, provides a new perspective in assessing earthquake and tsunami hazards for risk mitigation. Wang, K., and S. L. Bilek. Tectonophysics 610, 1-24 (2014).

  7. Digging a Paleoseismic Trench

    USGS Multimedia Gallery

    Tom Fumal and Jim Lienkamper of the USGS place shims inside the trench at Tule Pond in Fremont, CA. These shims will add support to the trenches walls and allow scientists to work inside it safely....

  8. Earthquake hazards on the cascadia subduction zone.

    PubMed

    Heaton, T H; Hartzell, S H

    1987-04-10

    Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. PMID:17789780

  9. Dynamic evolution in a Cretaceous high-P/T subduction channel evidenced by the juxtaposition of amphibolite blocks with different P-T paths: an example from the Kamuikotan belt, northern Japan

    NASA Astrophysics Data System (ADS)

    Okamoto, A.; Takeshita, T.

    2013-12-01

    A subduction channel developed at the boundary between a subducting oceanic plate and an overlying plate could be geologically defined as the place, where accretionary sediments were dragged down to great depth with an oceanic plate to suffer a high-P/T type metamorphism, and transformed to high-P/T metamorphic rocks (e.g. blueschist and eclogite). In the study area, while typical high-P/T metamorphic rocks (blueschist), which originated from Cretaceous accretionary sediments, amphibolites and metacherts also occur as tectonic blocks in mlange surrounded by either serpentinite or pelitic matrix, which originally suffered intermediate-P/T type metamorphism, but later the same high-P/T type metamorphism as the sediments did. In this research, we have analyzed mineral assemblages in these amphibolites and metacherts, and conducted micro-chemical analyses of compositional zoning in amphibole and garnet from these rocks with an EPMA. As a result, compositional zoning in some constituent amphibole can be divided into 3 types. Type I is a dominant type, where actinolite is overgrown by glaucophane, indicating pressure increase. Type II, which has been found in only one sample, is defined as the compositional zoning in amphibole consisting of magnesiohornblende, actinolite and glaucophane from core to rim. The compositional zoning shows a change of the temperature gradient from low-P/T (or intermediate-P/T) type to high-P/T type, which could reflect a cooling of the subduction channel with time from the onset of subduction to a steady state. Type III is characterized by the compositional zoning in amphibole from tschermakite to glaucophane-magnesioriebeckite. This also shows a cooling of the subduction channel with time. In this sample, garnet also shows a compositional zoning from a Mn-rich and Ca-poor inner core to a Mn-poor and Ca-rich outer core, which is surrounded by a Mn-rich rim, showing a compositional discontinuity across the core-rim boundary. The compositional zoning in garnet indicates an increase in both temperature and pressure during the initial growth, followed by temperature decrease during the later growth, comparable with the P-T paths inferred from the amphibole zoning. These different types of compositional zonings in amphibole and garnet show different P-T paths. Hence, these amphibolite tectonic blocks with different temperature-pressure-time paths could have been juxtaposed perhaps by large-scale ductile flow in the ancient subduction channel, which occurred during the initial stage of subduction from the onset to a steady state. This kind of dynamic evolution in subduction channel at the initial stage has now been reported from other well-known high-pressure terrains (e.g. Sambagawa, Japan; Saman, Dominican Repblic).

  10. Arc Evolution in Response to the Subduction of Buoyant Features

    NASA Astrophysics Data System (ADS)

    Jenkins, Luke; Fourel, Loic; Goes, Saskia; Morra, Gabriele

    2015-04-01

    The subduction of buoyant features such as aseismic ridges or oceanic plateaux has been invoked to explain arc deformation, flat subduction and increase in seismic coupling. Other studies have challenged these ideas, attributing a larger role to the overriding plate. However, many open questions remain about the dynamics of the relative simple case of a single freely subducting plate. How big does a plateau need to be to change the arc shape? What is the control of plate's strength on the impact of buoyant features? How do the velocities adapt to the subduction of less dense material? In the present study, we propose a systematic approach in order to tackle these questions. We use a new 3-D coupled fluid-solid subduction model where the interaction between the slab and the isoviscous mantle is only calculated on the slab surface, significantly increasing computational efficiency. The oceanic plate rheology is visco-elasto-plastic and its top surface is free. We find that arc shape is significantly altered by the subduction of buoyant plateaux. Along the subduction plane through the plateau and depending on its size, the dip angle and the retreat velocity significantly decrease. Flat subduction is obtained in the case of large and strongly buoyant plateau/ridge. An interesting feature is that retreat velocity increases right after the plateau or ridge has finished subducting in order to catch up with the rest of the plate. The gradient in retreat velocity obtained along the trench may cause the slab to have a heterogeneous response to ridge push, eventually leading to slab advance where buoyant material is present. We apply our models to the Izu-Bonin-Marianas (IBM) trench and propose that subduction of the buoyant Caroline Island Ridge at the southern edge of the Mariana trench can explain both trench motion history and the current morphology of the IBM slab as imaged by seismic tomography.

  11. Simulation of tectonic evolution of the Kanto basin of Japan since 1 Ma due to subduction of the Pacific and Philippine sea plates and collision of the Izu-Bonin arc

    NASA Astrophysics Data System (ADS)

    Hashima, Akinori; Sato, Toshinori; Sato, Hiroshi; Asao, Kazumi; Furuya, Hiroshi; Yamamoto, Shuji; Kameo, Koji; Miyauchi, Takahiro; Ito, Tanio; Tsumura, Noriko; Kaneda, Heitaro

    2015-04-01

    The Kanto basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the collision of the Izu-Bonin arc with the Japanese island arc. Geomorphological, geological, and thermochronological data on long-term vertical movements over the last 1 My suggest that subsidence initially affected the entire Kanto basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modelled the tectonic evolution of the Kanto basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the arc-arc collision process has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0-0.5 Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following a change in plate motion. Observed changes in the subsidence/uplift pattern are better explained by scenario (2), suggesting that recent (<1 My) deformation in the Kanto basin shows a lag in crustal response to the shift in plate motion. We also calculated recent stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions.

  12. Scattering of trapped P and S waves in the hydrated subducting crust of the Philippine Sea plate at shallow depths beneath the Kanto region, Japan

    NASA Astrophysics Data System (ADS)

    Takemura, Shunsuke; Yoshimoto, Kazuo; Tonegawa, Takashi

    2015-12-01

    We performed a detailed analysis of seismograms obtained during intraslab earthquakes beneath the Kanto region and revealed a strong lateral variation in the waveforms of high-frequency trapped P and S waves propagating through the subducting crust of the Philippine Sea plate. Significantly distorted spindle-shaped trapped P and S waves with large peak delays were observed in areas where the Philippine Sea plate is at shallow depths beneath the Kanto region. In order to interpret these seismic observations, in relation to the structural properties of the crust of the Philippine Sea plate, we conducted finite difference method simulations of high-frequency seismic wave propagation using various possible heterogeneous velocity structure models. Our simulation successfully reproduced the observed characteristics of the trapped waves and demonstrated that the propagation of high-frequency P and S waves is significantly affected by small-scale velocity heterogeneities in the subducting crust. These heterogeneities can be traced to a depth of approximately 40 km, before disappearing at greater depths, a phenomenon that may be related to dehydration in the subducting crust at shallower depths.

  13. Mid-mantle anisotropy beneath Japan and South America from source-side shear wave splitting

    NASA Astrophysics Data System (ADS)

    Lynner, C.; Long, M. D.

    2014-12-01

    Measurements of seismic anisotropy such as shear wave splitting are commonly used to constrain deformation in the upper mantle; however, observations of anisotropy at mid-mantle depths are relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) in the northwest Pacific (Japan and Izu-Bonin) and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from seismic sources deeper than 350km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, splitting as large as 1 sec is consistently observed, indicative of seismic anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as 600km argues for a contribution from anisotropy in the uppermost lower mantle, as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike (opposite of what is observed in the shallower sub-slab mantle) and do not appear to depend on the propagation direction of the waves. Beneath South America, splitting directions vary from trench-parallel to trench-perpendicular and depend on propagation direction (that is, whether the shear waves travel in an up-dip or down-dip direction). This difference may reflect the different deformation styles of each slab as it interacts with the transition zone discontinuities; the subducting Pacific plate beneath Japan stagnates atop the 660km discontinuity, while the subducting Nazca plate beneath South America penetrates into the lower mantle. While the elasticity and fabric development for phases that are present in the transition zone and uppermost lower mantle remain imperfectly known, our results provide unequivocal evidence for the presence of anisotropy at mid-mantle depths in the vicinity of subducting slabs.

  14. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief

    NASA Astrophysics Data System (ADS)

    Bassett, Dan; Watts, Anthony B.

    2015-05-01

    An ensemble averaging technique is used to remove the long-wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within fore arcs, many of which are attributed to the tectonic structure of the subducting plate. The residual-gravimetric expression of subducting fracture zones extends >200 km landward of the trench axis. The bathymetric expression of subducting seamounts with height ?1 km and area ?500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0 <17 km). Subducting seamounts are similar in wavelength, amplitude, and morphology to unsubducted seamounts. Morphology, spatial distributions, and reduced levels of seismicity are considered inconsistent with mechanical models proposing wholesale decapitation, and the association of subducting seamounts with large-earthquakes. Subducting aseismic ridges are associated with uplift and steepening of the outer fore arc, a gradual reduction in residual bathymetric expression across the inner fore arc, and a local increase in the width and elevation of the volcanic-arc/orogen. These contrasting expressions reflect the influence of margin-normal variations in rigidity on where and how the upper plate deforms, both to accommodate subducting relief and in response to stresses transmitted across the plate interface. The outer fore arc and arc have lower rigidity due to fracturing and thermal weakening, respectively. Similar associations with complex earthquakes and fault creep suggest aseismic ridge subduction may also be accommodated by the development and evolution of a broad fracture network, the geometrical strength of which may exceed the locking strength of a smooth fault.

  15. Great (≥Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor

    USGS Publications Warehouse

    Scholl, David W.; Kirby, Stephe H.; von Huene, Roland; Ryan, Holly F.; Wells, Ray E.; Geist, Eric L.

    2015-01-01

    However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.

  16. The earthquake cycle in subduction zones

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

    A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

  17. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge. PMID:8662577

  18. Bathymetry of Mariana Trench-Arc System and Formation of the Challenger Deep as a Consequence of Weak Plate Coupling

    NASA Astrophysics Data System (ADS)

    Gvirtzman, Z.; Stern, R. J.

    2004-12-01

    The Challenger Deep in the southernmost Mariana Trench (western Pacific Ocean) is the deepest point on the earth's surface (10,920 m below sea level). Its location within a subduction trench, where one plate bends and descends below another, is not surprising. However, why is it located in the southernmost Mariana Trench and not at its central part, where the rate of subduction is higher, where the lithosphere is the oldest (and densest) on Earth, and where the subducted lithosphere pulling down is the longest in Earth (~1000 km or more according to seismic tomography)? We suggest that although subduction rate and slab age generally control trench depth, here, the width of the Plate Coupling Zone is more important and counteracts trench deepening. Beneath the central Marianas the subducted slab is attached to the upper plate along a 150-km-wide surface that holds the shallow portion of the subducted plate nearly horizontal, in spite of its great load and, thus, counters trench deepening. In contrast, along the south Mariana Trench the subducted length of the lithosphere is much shorter, but its attachment to the upper plate is only along a relatively narrow, 50-km-wide, surface. In addition, a tear in the slab beneath this region helps it to sink rapidly through the mantle and this combination of circumstances allows the slab to roll back, steepen, and form the deepest trench on Earth. In a wider perspective, the interrelations shown here between trench deepening, ridge shallowing, slab steepening, and forearc narrowing may shed light on other subduction zones located near edges of rapidly retreating slabs.

  19. Rheological effects on slab stagnation and trench rollback

    NASA Astrophysics Data System (ADS)

    Cizkova, Hana; Bina, Craig

    2013-04-01

    Trench rollback has been a widely discussed phenomenon in recent years, and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the (upper) mantle. Here we concentrate on the effects of rheology in controlling the rollback and associated stagnation of slabs in the transition zone. We perform numerical simulations of slab evolution in a 2D Cartesian model with strongly nonlinear rheology combining diffusion creep, dislocation creep and a power-law stress limiter. Decoupling of the subducting and overriding plates is facilitated by a low-viscosity crustal layer prescribed on top of the subducting plate. We investigate models with the age of the subducting plate varying between 70 Myr and 150 Myr at the trench. We study the effects of the yield stress of the stress-limiting rheology (0.2-1 GPa) and of the crustal strength. We demonstrate that retrograde trench migration develops in most models considered, regardless of the subducting plate age or prescribed strength. Rollback then mostly produces slabs that are horizontally deflected at the 660-km phase boundary and remain subhorizontal at the bottom of the transition zone. Slab morphologies are in agreement with stagnant, horizontally deflected structures reported in the transition zone by seismic tomography. Furthermore, if the strength of the slab is limited to less than 0.5 GPa, the slab experiences a significant amount of horizontal buckling. Both subducting plate velocity and trench rollback velocity then exhibit periodic time variations with dominant periods of around 20 Myr with rollback velocity maxima occurring at plate velocity minima and vice versa. These oscillations are reflected also in dip-angle variations that may further influence, for example, the exhumation of high-pressure metamorphic rocks. The amplitude of the rollback velocity is sensitive to several model parameters. As one might expect, it increases with the age of the subducting plate, thus reflecting its increasingly negative buoyancy. On the other hand, rollback velocity decreases if we increase the viscosity of the crust and strengthen the coupling between the subducting and overriding plates. High friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation. Also, the reduction in extra negative buoyancy associated with the 410-km exothermic phase transition suppresses trench rollback. The interpretation of the effects that control slab rollback and stagnation may be rather complex in strongly nonlinear rheological models, where, for example, the buoyancy effects may be counteracted by associated yield-stress weakening.

  20. Flexural ridges, trenches, and outer rises around coronae on Venus

    NASA Technical Reports Server (NTRS)

    Sandwell, David T.; Schubert, Gerald

    1992-01-01

    Flexural signatures outboard of Venusian coronal rims are examined with the purpose of inferring the thickness of the planet's elastic lithosphere. Topographic profiles of several prominent coronae which display clear trench and outer rise signatures are presented. Via a thin elastic plate flexure model to characterize the shape of the trench and outer rise, Venusian flexures are found to be similar in both amplitude and wavelength to lithospheric flexures seaward of subduction zones on earth. It is shown that circumferential fractures are concentrated in areas where the topography is curved downward, in good agreement with the high tensile stress predicted by the flexure models. Two scenarios for the development of the ridge-trench-outer rise flexural topography and circumferential fractures of coronae are presented. The first scenario involves reheating and thermal subsidence of the lithosphere interior to the corona, while the second involves expansion of the corona interior and roll back of the subducting lithosphere exterior to the corona.

  1. Unrevealing the History of Earthquakes and Tsunamis of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Ramirez-Herrera, M. T.; Castillo-Aja, M. D. R.; Cruz, S.; Corona, N.; Rangel Velarde, V.; Lagos, M.

    2014-12-01

    The great earthquakes and tsunamis of the last decades in Sumatra, Chile, and Japan remind us of the need for expanding the record of history of such catastrophic events. It can't be argued that even countries with extensive historical documents and tsunami sand deposits still have unsolved questions on the frequency of them, and the variables that control them along subduction zones. We present here preliminary results of a combined approach using historical archives and multiple proxies of the sedimentary record to unrevealing the history of possible great earthquakes and their tsunamis on the Mexican Subduction zone. The Mexican subduction zone extends over 1000 km long and little is known if the entire subduction zone along the Middle American Trench behaves as one enormous unit rather than in segments that rupture at different frequencies and with different strengths (as the short instrumental record shows). We searched on historical archives and earthquake databases to distinguish tsunamigenic events registered from the 16th century to now along the Jalisco-Colima and Guerrero-Oaxaca coastal stretches. The historical data referred are mostly from the 19th century on since the population on the coast was scarce before. We found 21 earthquakes with tsunamigenic potential, and of those 16 with doubtful to definitive accompanying tsunami on the Jalisco-Colima coast, and 31 tsunamigenic earthquakes on the Oaxaca-Guerrero coast. Evidence of great earthquakes and their tsunamis from the sedimentary record are scarce, perhaps due poor preservation of tsunami deposits in this tropical environment. Nevertheless, we have found evidence for a number of tsunamigenic events, both historical and prehistorical, 1932 and 1400 AD on Jalisco, and 3400 BP, 1789 AD, 1979 ad, and 1985 AD on Guerrero-Oaxaca. We continue working and a number of events are still to be dated. This work would aid in elucidating the history of earthquakes and tsunamis on the Mexican subduction zone.

  2. Variability in forearc deformation during subduction: Insight from geodynamic models and application to the Calabria subduction zone

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter; Duarte, Joao

    2015-04-01

    In nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that impact the style of subduction and overriding plate deformation. Here we present 3D dynamic analogue models of subduction, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, with a special focus on overriding plate deformation. Our models indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of backarc basins (e.g., Tyrrhenian Sea, Aegean Sea, Scotia Sea) is generally expected to occur at a comparable location (300-500 km from the trench), irrespective of the boundary condition. Furthermore, our models indicate that the style of forearc deformation (shortening or extension) is determined by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed set-up is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature and accretionary wedge configuration. Furthermore, it provides explanation for the natural observations of both backarc extension in the Tyrrhenian Sea and forearc extension in the Calabria region, which have been active since the Miocene. We explain the observations as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting forced subduction to be accommodated almost entirely by slab rollback (not trenchward overriding plate motion), while trench retreat was accommodated almost entirely by backarc and forearc extension (not trenchward overriding plate motion), similarly to our SP-OP-Fixed model. This tectonic setting induced strong trench suction, which caused the forearc extension in Calabria.

  3. Overriding plate deformation and variability of fore-arc deformation during subduction: Insight from geodynamic models and application to the Calabria subduction zone

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter P.; Duarte, Joo. C.

    2015-10-01

    In nature, subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that influence the style of subduction and overriding plate deformation. Here we present dynamic laboratory models of progressive subduction in three-dimensional space, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed, and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, particularly focusing on overriding plate deformation. The results indicate that the variation in far-field boundary conditions has an influence on the slab geometry, subduction partitioning, and trench migration partitioning. Our models also indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of back-arc basins (e.g., Tyrrhenian Sea, Aegean Sea, and Scotia Sea) is generally expected to occur at a comparable location (250-700 km from the trench), irrespective of the boundary condition. In addition, our models indicate that the style of fore-arc deformation (shortening or extension) is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed setup is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature, and accretionary configuration. Furthermore, the model can explain back-arc and fore-arc extension at the Calabria subduction zone since the latest middle Miocene as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting induced strong trench suction, driving fore-arc extension, and forced subduction to be accommodated almost entirely by slab rollback (not trenchward subducting plate motion), while trench retreat was accommodated almost entirely by back-arc and fore-arc extension (not trenchward overriding plate motion), comparable to our SP-OP-Fixed model.

  4. Morphology and origin of the Challenger Deep in the Southern Mariana Trench

    NASA Astrophysics Data System (ADS)

    Fujioka, Kantaro; Okino, Kyoko; Kanamatsu, Toshiya; Ohara, Yasuhiko

    2002-05-01

    A high resolution bathymetry survey reveals the detailed morphology of the Southern Mariana Trench. Distinct right stepping, N80E trending en echelon deeps were found on the trench bottom within the Challenger Deep. Horst and graben structures were revealed on the outer swell of the Southern Mariana Trench. These structures, as well as slope-failure on the inner and outer slopes of the trench are similar to features observed in other deep sea trenches. We propose here that the Southern Mariana Trench is a transform fault based on swath mapping, morphological analysis and tectonic interpretation. The en echelon deeps formed in a right-lateral strike-slip stress regime related to oblique Pacific/Caroline Plate subduction under the Southern Mariana Trench combined with the Mariana Trough backarc spreading. We confirm that the world's deepest point lies in the western portion of the Challenger Deep, among the en echelon deeps.

  5. Structure of the Ryukyu Subduction Zone at its Western end: Slab Buckling, Double Seismic Layer, and the Effect of Dehydration

    NASA Astrophysics Data System (ADS)

    Chou, H.; Kuo, B.; Hung, S.; Chiao, L.; Wu, Y.; Zhao, D.

    2004-12-01

    Network data of Taiwan and Japan were integrated to illuminate the collision-oblique subduction complex in a region within 100 km of the island's coast, which has been poorly resolved by either single network. We relocated 4814 events applying a series of 1-D velocity model inversion and double difference method. These processes reduce the variance of traveltime residuals by about 70% with respect to each network value. The relocated seismicity delineates better the curving of the trench-forarc system toward the island and the 15--20 km apart double seismogenic layer (DSL) within the slab of the subducted Philippine Sea Plate in the depth range of 30-70 km. Not revealed before is the seismicity distribution that clearly defines bulging of the slab near its western end continuing from 50 km depth to 90 km. The overall geometry and the focal mechanisms suggest that the slab buckles against the Eurasian plate under lateral compression while subducting obliquely toward the continent. Tomographic inversion of the data set further reduces variance by 61%. The images reveal that much of the DSL is punctuated by low velocity anomalies (LVA) on the upper layer. We propose two hypotheses to explain the buckling-DSL-LVA combination, based on a previous notion that the DSL results from lateral compression. It could be that diabase and olivine present different creep rheology for crust and mantle causing the double layer, and that dehydration of hydrous minerals triggers melting that lowers the seismic velocity. Lateral compression could have thickened the typical oceanic crust to accommodate the at least 15 km gap for DSL. Or, a non-basaltic section of crust, thick and low in velocity, is subducted, which causes the 15--20 km separated double layer and the low anomalies in tomography. Subduction of a buoyant segment of crust has various implications for the dynamics of this subduction-collision junction.

  6. Variations in oceanic plate bending along the Mariana trench

    NASA Astrophysics Data System (ADS)

    Zhang, Fan; Lin, Jian; Zhan, Wenhuan

    2014-09-01

    We quantify along-trench variations in plate flexural bending along the Mariana trench in the western Pacific Ocean. A 3-D interpreted flexural deformation surface of the subducting Pacific Plate was obtained by removing from the observed bathymetry the effects of sediment loading, isostatically-compensated topography based on gravity modeling, age-related lithospheric thermal subsidence, and residual short-wavelength features. We analyzed flexural bending of 75 across-trench profile sections and calculated five best-fitting tectonic and plate parameters that control the flexural bending. Results of analysis revealed significant along-trench variations: the trench relief varies from 0.9 to 5.7 km, trench-axis vertical loading (-V0) from -0.73×1012 to 3.17×1012 N/m, and axial bending moment (-M0) from 0.1×1017 to 2.7×1017 N. The effective elastic plate thickness seaward of the outer-rise region (TeM) ranges from 45 to 52 km, while that trench-ward of the outer-rise (Tem) ranges from 19 to 40 km. This corresponds to a reduction in Te of 21-61%. The transition from TeM to Tem occurs at a breaking distance of 60-125 km from the trench axis, which is near the outer-rise and corresponds to the onset of observed pervasive normal faults. The Challenger Deep area is associated with the greatest trench relief and axial vertical loading, while areas with seamounts at the trench axis are often associated with more subtle trench relief, smaller axial vertical loading, and greater topographic bulge at the outer-rise.

  7. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase 2): Volume 3

    SciTech Connect

    Payne, J.; Bandy, B.; Altis, S.; Lee, M.C.; Dwan, S.F.; Ku, K.; Hilde, T.W.C.

    1989-02-01

    The objectives of this study represent a continuation and refinement of the objectives addressed in Phase 1. This study focuses on trying to define the tectonics of sediment subduction at the trench axis through the use of accepted plate tectonic principles and the application of new subduction theory. The fundamental methods include: (1) compilation of all available bathymetric data from our Global Marine Geophysical Data Collection for all major ocean trenches, (2) generation of stacked bathymetric profiles and corresponding navigational maps, and structural maps, (3) selection and analysis of appropriate seismic reflection and refraction profiles and additional supporting data such as side-scan sonar, SEABEAM, magnetic, gravity and drilling data, and (4) detailed study of selected trench regions in which data quality and/or quantity is exceptional. Phase 2 of this project represents a unique compilation and synthesis of existing data for the world's deep ocean trenches. The analysis of data and discussion of results in the context of current literature aids our understanding of the sediment distribution and nature of sediment deformation through various stages of plate convergence, the determination of whether sediments are subducted or accreted, and the evaluation of the controlling factors for sediment subduction and/or accretion. A major emphasis in our analysis of the data was to try and map the seaward-of-the-trench distribution of faults and associated surface roughness. Illustrations and an extensive bibliography are included in the report.

  8. Bathymetry of Mariana trench-arc system and formation of the Challenger Deep as a consequence of weak plate coupling

    NASA Astrophysics Data System (ADS)

    Gvirtzman, Zohar; Stern, Robert J.

    2004-04-01

    The Challenger Deep in the southernmost Mariana Trench (western Pacific Ocean) is the deepest point on the Earth's surface (10,920 m below sea level). Its location within a subduction trench, where one plate bends and descends below another, is not surprising. However, why is it located in the southernmost Mariana Trench and not at its central part, where the rate of subduction is higher, where the lithosphere is the oldest (and densest) on the Earth, and where the subducted lithosphere pulling down is the longest in the Earth (1000 km or more according to seismic tomography)? We suggest that although subduction rate and slab age generally control trench depth, the width of the plate-coupling zone is more important. Beneath the central Marianas the subducted slab is attached to the upper plate along a 150-km-wide surface that holds the shallow portion of the subducted plate nearly horizontal, in spite of its great load and, thus, counters trench deepening. In contrast, along the south Mariana Trench the subducted length of the lithosphere is much shorter, but its attachment to the upper plate is only along a relatively narrow, 50-km-wide, surface. In addition, a tear in the slab beneath this region helps it to sink rapidly through the mantle, and this combination of circumstances allows the slab to steepen and form the deepest trench on the Earth. In a wider perspective, the interrelations shown here between trench deepening, ridge shallowing, slab steepening, and forearc narrowing may shed light on other subduction zones located near edges of rapidly steepening slabs.

  9. Spirit Digs a Trench

    NASA Technical Reports Server (NTRS)

    2004-01-01

    A view from the front hazard avoidance camera of NASA's Spirit rover on its 47th sol shows a trench excavated by the rover's left front wheel within the 'Laguna Hollow' area. The trench, dubbed 'Road Cut,' is 7 centimeters (3 inches) deep. The soil at this location is more cohesive than the material where Spirit's twin, Opportunity, dug its first trench at Meridiani. Spirit made 11 back-and-forth passes to dig this trench, and still did not produce as deep a hole as Opportunity dug in 6 passes. Scientists and engineers plan to begin up-close inspection of the soil in this trench on sol 48 by placing the microscopic imager on the floor and the walls before conducting Moessbauer and alpha particle x-ray spectrometer readings on some of the same points.

  10. Evolving force balance during incipient subduction

    NASA Astrophysics Data System (ADS)

    Gurnis, Michael; Hall, Chad; Lavier, Luc

    2004-07-01

    Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates. Subduction initiation is known to have occurred in a variety of tectonic settings: old fracture zones, transform faults, and extinct spreading centers and through polarity reversal behind active subduction zones. Although occurring within different tectonic settings, four known subduction initiation events (Izu-Bonin-Mariana (IBM) along a fracture zone, Tonga-Kermadec along an extinct subduction boundary, New Hebrides within a back arc, and Puysegur-Fiordland along a spreading center) were typified by rapid uplift within the forearc followed by sudden subsidence. Other constraints corroborate the compressive nature of IBM and Tonga-Kermadec during initiation. Using an explicit finite element method within a two-dimensional domain, we explore the evolving force balance during initiation in which elastic flexure, viscous flow, plastic failure, and heat transport are all considered. In order to tie theory with observation, known tectonic settings of subduction initiation are used as initial and boundary conditions. We systematically explore incipient compression of a homogeneous plate, a former spreading center, and a fracture zone. The force balance is typified by a rapid growth in resisting force as the plate begins bending, reaching a maximum value dependent on plate thickness, but typically ranging from 2 to 3 1012 N/m for cases that become self-sustaining. This is followed by a drop in stress once a shear zone extends through the plate. The formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall. Cumulative convergence, not the rate of convergence, is the dominant control on the force balance. Viscous tractions influence the force balance only if the viscosity of the asthenosphere is >1020 Pa s, and then only after plate failure. Following plate failure, buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction. The total work done is also influenced by the rate of lithospheric failure. A self-sustaining subduction zone does not form from a homogeneous plate. A ridge placed under compression localizes subduction initiation, but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate. The large initial bending resistance can be entirely eliminated in ridge models, explaining the propensity for new subduction zones to form through polarity reversals. A fracture zone (FZ) placed in compression leads to subduction initiation with rapid extension of the overriding plate. A FZ must be underthrust by the older plate for 100-150 km before a transition from forced to self-sustaining states is reached. In FZ models the change in force during transition is reflected by a shift from forearc uplift to subsidence. Subduction initiation is followed by trench retreat and back arc extension. Moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of Pacific subduction zones. The models provide an explanation for the compressive state of western Pacific margins before and during subduction initiation, including IBM and Tonga-Kermadec in the Eocene, and the association of active back arcs with young subduction zones. On the basis of our dynamic models and the relative poles of rotation between Pacific and Australia during the Eocene, we predict that the northern segment of the Tonga-Kermadec convergent margin would have initiated earlier with a progressive

  11. Spatial changes of inter-plate coupling inferred from sequences of small repeating earthquakes in Japan

    NASA Astrophysics Data System (ADS)

    Igarashi, Toshihiro

    2010-10-01

    We extract sequences of small repeating earthquakes to clarify inter-plate coupling of subducting plates over a large area of the Japanese Islands. As a result, many sequences are detected at the Philippine Sea plate subducting from the Ryukyu trench and Pacific plate subducting from the Kuril-Japan trench. The average slip-rates and standard deviations estimated from the sequences show substantial spatial changes of inter-plate coupling. The large deviations of slip-rates correspond to the occurrence of episodic slips such as after-slips following large earthquakes. Constant slip-rates approaching the relative plate motion indicate weak coupling areas. Slip deficits and sparse distributions of repeating groups suggest locked areas. In the Nankai trough, deep low-frequency earthquakes in the transition zone and burst-type repeating sequences within plates have not been located in the downdip direction of groups with slow slip-rates. This suggests that the space-time characteristics of inter-plate coupling affected these seismic events.

  12. Tsunami Numerical Simulation for Hypothetical Giant or Great Earthquakes along the Izu-Bonin Trench

    NASA Astrophysics Data System (ADS)

    Harada, T.; Ishibashi, K.; Satake, K.

    2013-12-01

    We performed tsunami numerical simulations from various giant/great fault models along the Izu-Bonin trench in order to see the behavior of tsunamis originated in this region and to examine the recurrence pattern of great interplate earthquakes along the Nankai trough off southwest Japan. As a result, large tsunami heights are expected in the Ryukyu Islands and on the Pacific coasts of Kyushu, Shikoku and western Honshu. The computed large tsunami heights support the hypothesis that the 1605 Keicho Nankai earthquake was not a tsunami earthquake along the Nankai trough but a giant or great earthquake along the Izu-Bonin trench (Ishibashi and Harada, 2013, SSJ Fall Meeting abstract). The Izu-Bonin subduction zone has been regarded as so-called 'Mariana-type subduction zone' where M>7 interplate earthquakes do not occur inherently. However, since several M>7 outer-rise earthquakes have occurred in this region and the largest slip of the 2011 Tohoku earthquake (M9.0) took place on the shallow plate interface where the strain accumulation had considered to be a little, a possibility of M>8.5 earthquakes in this region may not be negligible. The latest M 7.4 outer-rise earthquake off the Bonin Islands on Dec. 22, 2010 produced small tsunamis on the Pacific coast of Japan except for the Tohoku and Hokkaido districts and a zone of abnormal seismic intensity in the Kanto and Tohoku districts. Ishibashi and Harada (2013) proposed a working hypothesis that the 1605 Keicho earthquake which is considered a great tsunami earthquake along the Nankai trough was a giant/great earthquake along the Izu-Bonin trench based on the similarity of the distributions of ground shaking and tsunami of this event and the 2010 Bonin earthquake. In this study, in order to examine the behavior of tsunamis from giant/great earthquakes along the Izu-Bonin trench and check the Ishibashi and Harada's hypothesis, we performed tsunami numerical simulations from fault models along the Izu-Bonin trench. Tsunami propagation was computed by the finite-difference method of the non-liner long-wave equations with Corioli's force (Satake, 1995, PAGEOPH) in the area of 130 - 145E and 25 - 37N. The 15-seconds gridded bathymetry data are used. The tsunami propagations for eight hours since the faulting of the various fault models were computed. As a result, large tsunamis from assumed giant/great both interplate and outer-rise earthquakes reach the Ryukyu Islands' coasts and the Pacific coasts of Kyushu, Shikoku and western Honshu west of Kanto. Therefore, the tsunami simulations support the Ishibashi and Harada's hypothesis. At the time of writing, the best yet preliminary model to reproduce the 1605 tsunami heights is an outer-rise steep fault model which extends 26.5 - 29.0N (300 km of length) and with 16.7 m of average slip (Mw 8.6). We will examine tsunami behavior in the Pacific Ocean from this fault model. To examine our results, field investigations of tsunami deposits in the Bonin Islands and discussions on plate dynamics and seismogenic characteristics along the Izu-Bonin trench are necessary.

  13. Geological and historical evidence of irregular recurrent earthquakes in Japan.

    PubMed

    Satake, Kenji

    2015-10-28

    Great (M∼8) earthquakes repeatedly occur along the subduction zones around Japan and cause fault slip of a few to several metres releasing strains accumulated from decades to centuries of plate motions. Assuming a simple 'characteristic earthquake' model that similar earthquakes repeat at regular intervals, probabilities of future earthquake occurrence have been calculated by a government committee. However, recent studies on past earthquakes including geological traces from giant (M∼9) earthquakes indicate a variety of size and recurrence interval of interplate earthquakes. Along the Kuril Trench off Hokkaido, limited historical records indicate that average recurrence interval of great earthquakes is approximately 100 years, but the tsunami deposits show that giant earthquakes occurred at a much longer interval of approximately 400 years. Along the Japan Trench off northern Honshu, recurrence of giant earthquakes similar to the 2011 Tohoku earthquake with an interval of approximately 600 years is inferred from historical records and tsunami deposits. Along the Sagami Trough near Tokyo, two types of Kanto earthquakes with recurrence interval of a few hundred years and a few thousand years had been recognized, but studies show that the recent three Kanto earthquakes had different source extents. Along the Nankai Trough off western Japan, recurrence of great earthquakes with an interval of approximately 100 years has been identified from historical literature, but tsunami deposits indicate that the sizes of the recurrent earthquakes are variable. Such variability makes it difficult to apply a simple 'characteristic earthquake' model for the long-term forecast, and several attempts such as use of geological data for the evaluation of future earthquake probabilities or the estimation of maximum earthquake size in each subduction zone are being conducted by government committees. PMID:26392616

  14. Geochemical variations in Japan Sea back-arc basin basalts formed by high-temperature adiabatic melting of mantle metasomatized by sediment subduction components

    NASA Astrophysics Data System (ADS)

    Hirahara, Yuka; Kimura, Jun-Ichi; Senda, Ryoko; Miyazaki, Takashi; Kawabata, Hiroshi; Takahashi, Toshiro; Chang, Qing; Vaglarov, Bogdan S.; Sato, Takeshi; Kodaira, Shuichi

    2015-05-01

    The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr-Nd-Pb-Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan fore arc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea.

  15. Frictional properties of fault rocks along the shallow part of the Japan Trench décollement: insights from samples recovered during the Integrated Ocean Drilling Project Expedition 343 (the JFAST project)

    NASA Astrophysics Data System (ADS)

    Remitti, Francesca; Smith, Steven; Gualtieri, Alessandro; Di Toro, Giulio; Nielsen, Stefan

    2014-05-01

    The Japan Trench Fast Drilling Project (JFAST), Integrated Ocean Drilling Program (IODP) Expedition 343, successfully located and sampled the shallow slip zone of the Mw =9.0 Tohoku-Oki earthquake where the largest coseismic slip occurred (c. 50 m). Logging-while-drilling, core-sample observations and the analysis of temperature data recovered from a third borehole show that a thin (<5 m), smectite rich plate-boundary fault accommodated the large slip of the Tohoku-Oki Earthquake rupture, as well as most of the interplate motion at the drill site. Effective normal stress along the shallow plate-boundary fault is estimated to be c. 7 MPa. Single-velocity and velocity-stepping rotary-shear friction experiments on fault material were performed with the Slow to HIgh Velocity Apparatus (SHIVA) installed at INGV in Rome. Quantitative phase analysis using the combined Rietveld and R.I.R. method indicates that the starting material is mainly composed of smectite (56 wt%) and illite/mica (21 wt%) and minor quartz, kaolinite, plagioclase and K-feldspar. The amount of amorphous fraction has also been calculated and it is close to the detection limit. Each experiment used 3.5 g of loosely disaggregated gouge, following sieving to a particle size fraction <1 mm. Experiments were performed either 1) "room-dry" (40-60% humidity) at 8.5 MPa normal stress (one test at 12.5 MPa), or 2) "water-dampened" (0.5 ml distilled water added to the gouge layers) at 3.5 MPa normal stress. Slip velocities ranged over nearly seven orders of magnitude (10-5 - 3 m s-1). Total displacement is always less than 1 m. The peak and steady-state frictional strengths of the gouges are significantly lower under water-dampened conditions, with mean steady-state friction coefficients (μ, shear stress/normal stress) at all investigated velocities of 0.04<μ<0.1. This is consistent with the small measured frictional heat anomaly along the plate boundary fault ~1.5 years after the Tohoku-Oki earthquake. Under room-dry conditions the gouge material is velocity-strengthening at intermediate velocities (0.001 - 0.1 m s-1), but strongly velocity-weakening at > 0.1 m s-1. Instead, under water-dampened conditions, the gouge is velocity-neutral to velocity-weakening at all investigated velocities. In other words, the intermediate-velocity strengthening, which would probably act as a "barrier" to rupture propagation in the dry gouges, disappears in water-dampened gouges. This result is compatible with propagation of the Tohoku rupture to the trench, and also with large coseismic slip at shallow depths. Quantitative phase analysis using the combined Rietveld and R.I.R. method has been performed also on six post-experiment gouges for the determination of both the crystalline and amorphous fractions. Preliminary results show that the mineralogical assemblage is basically the same after the experiments, with both smectite and illite phases preserved, this suggests that the weakening mechanism operating in this material is active at low temperature.

  16. Anomalous heat flow from a Miocene ridge crest-trench collision, Antarctic Peninsula

    SciTech Connect

    Dougherty, M.E.; Von Herzen, R.P.; Barker, P.F.

    1986-01-01

    In January 1985 a marine heat-flow survey was carried out aboard the British Antarctic Survey research ship RRS Discovery southwest of the Anvers fracture zone where a ridge crest-trench collision occurred approximately 15 million years ago. Anomalously high heat flow has been discovered coming from the oceanic crust and continental margin to the west of the Antarctic Peninsula. The purpose of this study was to examine the thermal state of one section of the Antarctic Peninsula where young oceanic lithosphere has been subducted. At the time of arrival of the ridge crest to the trench, subduction and spreading of the ridge both stopped. A heat-flow anomaly should still be present around the subduction zone since newly formed crust was in the trench at the time of collision. Heat-flow patterns around trench arc systems subducting old ocean crust in the West Pacific show a distinctive low heat-flow zone centered around the trench axis. Locations of the heat-flow stations were chosen to determine best the thermal state of the surviving flank of the spreading center as well as that of the collision and subduction complex. A table gives a summary of the locations and heat-flow data for each successful measurement. Corrections to the raw gradients include adjustments for sedimentation and seasonal variations in bottom-water temperature.

  17. Rates of sediment recycling beneath the Acapulco trench: Constraints from (U-Th)/He thermochronology

    NASA Astrophysics Data System (ADS)

    Ducea, Mihai N.; Valencia, Victor A.; Shoemaker, Sarah; Reiners, Peter W.; Decelles, Peter G.; Campa, Maria Fernanda; MorN-Zenteno, Dante; Ruiz, Joaquin

    2004-09-01

    The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction erosion is an important process during the evolution of this margin. Turbidites derived from the uplifted forearc are the predominant sedimentary input into this trench, while pelagic sediments are subordinate. Apatite (U-Th)/He ages were obtained on 23 samples from two transects across the Sierra Madre del Sur (Acapulco and Puerto Escondido) and reveal slow cooling during the Miocene. (U-Th)/He ages range between 25 and 8 Ma and correlate inversely with elevation. Long-term erosional exhumation rates inferred from these ages range from 0.11 to 0.33 km/m.y., with higher rates in the range core, and suggest that the Sierra Madre del Sur has been a slowly decaying mountain range, since at least the early Miocene. Apparent Miocene-Pliocene sedimentation ("preservation") rates in the Acapulco trench derived from Deep Sea Drilling Project data are about an order of magnitude smaller than the Miocene forearc erosion rates estimated from (U-Th)/He ages, suggesting that the terrigenous input to the trench was almost entirely recycled via subduction erosion, at least during the Miocene. The Miocene subducted flux per unit length of the margin is about 30 km3/(km m.y.), or a subducted volume per unit time of 44 103 km3/m.y., when integrated over the length of the trench.

  18. Louisville seamount subduction and its implication on mantle flow beneath the central Tonga-Kermadec arc.

    PubMed

    Timm, Christian; Bassett, Daniel; Graham, Ian J; Leybourne, Matthew I; de Ronde, Cornel E J; Woodhead, Jon; Layton-Matthews, Daniel; Watts, Anthony B

    2013-01-01

    Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system. PMID:23591887

  19. Metamorphic Perspectives of Subduction Zone Volatiles Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.

    2008-12-01

    Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites indicates retention of seafloor N signatures and, in some cases, enrichments in sedimentary N due to forearc metamorphic fluid-rock interactions (Halama et al., this session). A global estimate of C cycling, using seafloor inputs (carbonate and organic matter) and estimates of volcanic CO2 outputs, indicates ~40% return (with large uncertainty) of the subducting C in volcanic gases. This imbalance appears plausible, given the evidence for deep carbonate subduction, in UHP marbles, and the preservation of graphite in UHP metasediments, together seemingly indicating that large fractions of subducting C survive forearc-to-subarc metamorphism. Estimates of return efficiency in the Central America arc, based on data for volcanic gases, are lower and variable along strike (12-29%), quite reasonably explained by de Leeuw et al. (2007, EPSL) as resulting from incomplete decarbonation of subducting sediment and AOC, fluid flow patterns expected given sediment section thickness, and varying degrees of forearc underplating. The attempts to mass-balance C and N across individual arc-trench systems demonstrate valuable integration of information from geophysical, field, petrologic, and geochemical observations. Studies of subduction-zone metamorphic suites can yield constraints on the evolution of deeply subducting rocks and the physicochemical characteristics of fluids released in forearcs and contributing to return flux in arc volcanic gases.

  20. Heterogeneous interplate coupling along the Nankai Trough, Japan, detected by GPS-acoustic seafloor geodetic observation

    NASA Astrophysics Data System (ADS)

    Yokota, Yusuke; Ishikawa, Tadashi; Sato, Mariko; Watanabe, Shun-ichi; Saito, Hiroaki; Ujihara, Naoto; Matsumoto, Yoshihiro; Toyama, Shin-ichi; Fujita, Masayuki; Yabuki, Tetsuichiro; Mochizuki, Masashi; Asada, Akira

    2015-12-01

    The recurring devastating earthquake that occurs in the Nankai Trough subduction zone between the Philippine Sea plate and the Eurasian plate has the potential to cause an extremely dangerous natural disaster in the foreseeable future. Many previous studies have assumed interplate-coupling ratios for this region along the trench axis using onshore geodetic data in order to understand this recursive event. However, the offshore region that has the potential to drive a devastating tsunami cannot be resolved sufficiently because the observation network is biased to the land area. Therefore, the Hydrographic and Oceanographic Department of Japan constructed a geodetic observation network on the seafloor along the Nankai Trough using a GPS-acoustic combination technique and has used it to observe seafloor crustal movements directly above the Nankai Trough subduction zone. We have set six seafloor sites and cumulated enough data to determine the displacement rate from 2006 to January 2011. Our seafloor geodetic observations at these sites revealed a heterogeneous interplate coupling that has three particular features. The fast displacement rates observed in the easternmost area indicate strong interplate coupling (>75%) around not only the future Tokai earthquake source region but also the Paleo-Zenisu ridge. The slow displacement rates near the trench axis in the Kumano-nada Sea, a shallow part of the 1944 Tonankai earthquake source region, show a lower coupling ratio (50% to 75%). The slow displacement rate observed in the area shallower than the 1946 Nankaido earthquake source region off Cape Muroto-zaki reflects weakening interplate coupling (about 50%) probably due to a subducting seamount. Our observations above the subducting ridge and seamount indicate that the effect of a subducting seamount on an interplate-coupling region depends on various conditions such as the geometry of the seamount and the friction parameters on the plate boundary.

  1. Provenance of Cretaceous trench slope sediments from the Mesozoic Wandashan Orogen, NE China: Implications for determining ancient drainage systems and tectonics of the Paleo-Pacific

    NASA Astrophysics Data System (ADS)

    Sun, Ming-Dao; Xu, Yi-Gang; Wilde, Simon A.; Chen, Han-Lin

    2015-06-01

    The Wandashan Orogen of NE China is a typical accretionary orogen related to Paleo-Pacific subduction. The Raohe Complex, as a major part of the orogen, consists of mid-Triassic to mid-Jurassic radiolarian chert and intraoceanic igneous rocks in an accretionary prism overlain by weakly sheared terrestrial-sourced clastic trench slope sediments. Sensitive high-resolution ion microprobe U-Pb dating and LA-MC-ICPMS Hf isotopic analysis of detrital zircons from the terrestrial-sourced Yongfuqiao Formation sandstone show that most zircons are Phanerozoic (90%): 140-150 Ma (10%), 180-220 Ma (25%), 240-270 Ma (15%), 300-360 Ma (15%), 391-395 Ma (3%), and 450-540 Ma (20%), whereas 10% are Precambrian in age. About 90% of the zircons have ɛHf(t) values ranging from +11.1 to -12.8. This suggests that the major provenance of the trench slope sediments was from the adjacent eastern segment of the Central Asian Orogenic Belt and the Jiamusi Block. The age of the Yongfuqiao Formation is constrained to the earliest Cretaceous, which represents the accretion time of the mid-Triassic to mid-Jurassic oceanic complexes. When compared with the Mino Complex in Japan and the Tananao Complex in Taiwan, three different provenances are identified suggesting three ancient drainage systems which transported sediments from NE China, North China, and South China to the Paleo-Pacific subduction-accretion system.

  2. Surface ruptures associated with the 2011 Mw 6.6 Fukushima Hamadori earthquake (northeast Honshu, Japan): normal faulting in trench-normal stretching forearc subsequent to the 2011 Great Tohoku megathrust earthquake

    NASA Astrophysics Data System (ADS)

    Maruyama, T.; Awata, Y.; Azuma, T.

    2011-12-01

    On 11 April 2011 a shallow large normal faulting earthquake with Mw 6.6 occurred in southern Fukushima Prefecture, located on forearc region of northeast Honshu, Japan, where significant trench-normal crustal stretching has occurred since the Great Tohoku megathrust earthquake (Mw 9.0) of 11 March 2011. The earthquake resulted in two distinct surface ruptures along the previously mapped active fault traces; NNW-SSE-trending Idosawa fault on the west and NW-SE-trending Yunotake fault on the east. In order to define map distribution, geometry, slip vector pattern and slip distribution along the surface breaks as well as to archive fragile offset features before land modification, we conducted field mapping along the entire traces of the ruptures and surveyed offset cultural features using a total station since 17 April, 6 days after the earthquake. Our field mapping revealed that i) the both ruptures are predominantly normal faulting with west- to southwest-side-down on the west-dipping fault planes, which is consistent with focal mechanisms of mainshock and principal aftershocks and crustal deformation pattern as inferred from GPS and InSAR data, ii) the fault displacement is concentrated on a distinct slip surface in mountainous area underlain by the basement metamorphic rocks, while is dispersed in broad deformation zone which comprises scarp with only small vertical displacement, crestal extensional graben and hanging-wall warping that consumes a large part of the net vertical displacement in the hilly lands and terrace surfaces where unconsolidated materials are accumulated, iii) although the rupture lengths along the Idosawa and the Yunotake faults are nearly same (13.5 km and 15.6 km, respectively), vertical displacement on the Idosawa fault (2.2 m at the maximum) are four times of that on the Yunotake fault (0.5 m), iii) azimuths of slip vectors vary systematically along the Idoawa fault (pure normal slip near the center and oblique slip near the lateral tips of the rupture), but are nearly invariable along the Yunotake fault (oblique normal slip). These remarkable differences in these rupture behaviors of two surface ruptures may reflect the property of seismogenic faulting, such as degree of fault maturity, fault geometry at depth and rupture directivity.

  3. Elastic Properties of Subduction Zone Materials in the Large Shallow Slip Environment for the Tohoku 2011 Earthquake: Laboratory data from JFAST Core Samples

    NASA Astrophysics Data System (ADS)

    Jeppson, T.; Tobin, H. J.

    2014-12-01

    The 11 March 2011 Tohoku-Oki earthquake (Mw=9.0) produced large displacements of ~50 meters near the Japan Trench. In order to understand earthquake propagation and slip stabilization in this environment, quantitative values of the real elastic properties of fault zones and their surrounding wall rock material is crucial. Because elastic and mechanical properties of faults and wallrocks are controlling factors in fault strength, earthquake generation and propagation, and slip stabilization, an understanding of these properties and their depth dependence is essential to understanding and accurately modeling earthquake rupture. In particular, quantitatively measured S-wave speeds, needed for estimation of elastic properties, are scarce in the literature. We report laboratory ultrasonic velocity measurements performed at elevated pressures, as well as the calculated dynamic elastic moduli, for samples of the rock surrounding the Tohoku earthquake principal fault zone recovered by drilling during IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST). We performed measurements on five samples of gray mudstone from the hanging wall and one sample of underthrust brown mudstone from the footwall. We find P- and S-wave velocities of 2.0 to 2.4 km/s and 0.7 to 1.0 km/s, respectively, at 5 MPa effective pressure. At the same effective pressure, the hanging wall samples have shear moduli ranging from 1.4 to 2.2 GPa and the footwall sample has a shear modulus of 1.0 GPa. While these values are perhaps not surprising for shallow, clay-rich subduction zone sediments, they are substantially lower than the 30 GPa commonly assumed for rigidity in earthquake rupture and propagation models [e.g., Ide et al., 1993; Liu and Rice, 2005; Loveless and Meade, 2011]. In order to better understand the elastic properties of shallow subduction zone sediments, our measurements from the Japan Trench are compared to similar shallow drill core samples from the Nankai Trough, Costa Rica, Cascadia, and Barbados ridge subduction zones. We find that shallow subduction zone sediments in general have similarly low rigidity. These data provide important ground-truth values that can be used to parameterize fault slip models addressing the problem of shallow, tsunamigenic propagation of megathrust earthquakes.

  4. From subduction to collision: results of French POP2 program on Taiwan-Philippine festoon

    SciTech Connect

    Blanchet, R.; Stephan, J.F.; Rangin, C.; Baladad, D.; Bouysse, Ph.; Chen, M.P.; Chotin, P.; Collot, J.Y.; Daniel, J.; Drouhot, J.M.; Marsset, B.; Pelletier, B.; Richard, M.; Tardy, M.

    1986-07-01

    A sea-beam, seismic, magnetic, and gravimetric survey was conducted with the R/V Jean-Charcot in three key regions off the Taiwan-Philippine festoon in the western Pacific: (1) Ryukyu active margin and its junction with Taiwan; (2) northern part of the Manila Trench and its junction with the Taiwan tectonic prism; and (3) southern termination of Manila Trench in front of Mindoro Island. Transitions between active subduction along the Manila Trench and collision of Taiwan and Mindoro, and relations between active subduction and extension in the Okinawa-Ryukyu and the northeastern Taiwan systems are particularly studied.

  5. Shear wave azimuthal anisotropy in the transition zone from oceanic to continental subduction along the western Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Evangelidis, Christos

    2015-04-01

    In western Greece, the Hellenic subduction system is separated by the Cephalonia Transform Fault (CTF), a dextral offset of ~100 km, into the northern and southern segments, which are characterized by different convergence rates and slab composition. Recent seismic data show that north of CTF there is a subducted continental lithosphere in contrast to the region south of CTF where the on-going subduction is oceanic. Shear wave splitting of SKS phases provide useful information for the upper mantle anisotropy. Its direction and strength is caused by the ongoing upper mantle flow that constrain the subduction geodynamics. I have now measured SKS splitting parameters from all broadband stations of the Hellenic Unified Seismic Network (HUSN) and some selected stations of the National Strong Motion Network, specially concentrated in the transitional area from oceanic to continental subduction. These measurements, combined with previously published observations, provide the most complete up-to-date spatial coverage for the area. Generally, the pronounced zonation of seismic anisotropy across the subduction zone, as inferred from other studies, is also observed here. Fast SKS splitting directions are trench-normal in the region nearest to the trench. The fast splitting directions change abruptly to trench-parallel above the corner of the mantle wedge and rotate back to trench-normal over the back-arc. Additionally, beneath western Greece, between the western Gulf of Corinth in the south and the Epirus-Thessaly area in the north a toroidal pattern emerges that possibly depicts a slab tear between the oceanic and the continental subducted slabs and a consequent toroidal asthenospheric flow.

  6. Subduction inputs within the NanTroSEIZE transect area: Results from Expedition 322 of the Integrated Ocean Drilling Program

    NASA Astrophysics Data System (ADS)

    Underwood, M.; Saito, S.; Kubo, Y.

    2009-12-01

    One of the essential ingredients in any comprehensive study of a seismogenic subduction zone is a thorough characterization of subduction inputs, including both sedimentary strata and upper igneous basement. Initial conditions must be known in detail before we can determine how frictional, hydrogeological, thermal, and geochemical properties evolve in the down-dip direction. The fundamental purpose of Expedition 322 of the Integrated Ocean Drilling Program was to gather such information just seaward of the trench in the Kumano Transect area of Nankai Trough (offshore SW Japan). The Kumano region is highlighted by a series of large seamounts that formed on the subducting plate within the Shikoku Basin. Total sediment thickness above basement highs (e.g., Kashinosaki Knoll) has been significantly reduced, and the condensed section contains mostly hemipelagic mud deposited by suspension fall-out. In stark contrast, areas of smooth basement and the flanks of basement highs were sites of turbidite sedimentation, particularly during the Miocene. Contents of expandable clay minerals also change progressively as a function of depositional age (more smectite in older strata). The Miocene turbidites and basal smectite-rich mudstones pass beneath the frontal accretionary prism, so those units are keys to the behavior of the deep subduction system. Fluid production should be enhanced by dehydration of the smectite-rich mudstones, and the sandy intervals should provide pathways for focused fluid flow beneath the plate-boundary fault. As the NanTroSEIZE project progresses into the stage of ultra-deep riser drilling, results from Expedition 322 will provide the initial conditions of strata (pre-subduction) to compare against the in situ properties of correlative units just above, within, and below the seismogenic megasplay fault and the plate-boundary fault.

  7. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase 2): Volume 1

    SciTech Connect

    Payne, J.; Bandy, B.; Altis, S.; Lee, M.C.; Dwan, S.F.; Ku, K.; Hilde, T.W.C.

    1989-02-01

    This is volume one of three volumes. The objectives of this study represent a continuation and refinement of the objectives addressed in Phase I. This study focuses on trying to define the tectonics of sediment subduction at the trench axis through the use of accepted plate tectonic principles and the application of new subduction theory. The fundamental methods include: (1) compilation of all available bathymetric data from our Global Marine Geophysical Data Collection for all major ocean trenches, (2) generation of stacked bathymetric profiles and corresponding navigational maps, and structural maps, (3) selection and analysis of appropriate seismic reflection and refraction profiles and additional supporting data such as side-scan sonar, SEABEAM, magnetic, gravity and drilling data, and (4) detailed study of selected trench regions in which data quality and/or quantity is exceptional. Phase II of this project represents a unique compilation and synthesis of existing data for the world's deep ocean trenches. The analysis of data and discussion of results in the context of current literature aids our understanding of the sediment distribution and nature of sediment deformation through various stages of plate convergence, the determination of whether sediments are subducted or accreted, and the evaluation of the controlling factors for sediment subduction and/or accretion. A discussion is included on forearc petroleum and natural gas hydrate resource potential. 128 figs.

  8. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase 2): Volume 2

    SciTech Connect

    Payne, J.; Bandy, B.; Altis, S.; Lee, M.C.; Dwan, S.F.; Ku, K.; Hilde, T.W.C.

    1989-02-01

    The objectives of this study represent a continuation and refinement of the objectives addressed in Phase 1. This study focuses on trying to define the tectonics of sediment subduction at the trench axis through the use of accepted plate tectonic principles and the application of new subduction theory. The fundamental methods include: (1) compilation of all available bathymetric data from our Global Marine Geophysical Data Collection for all major ocean trenches, (2) generation of stacked bathymetric profiles and corresponding navigational maps, and structural maps, (3) selection and analysis of appropriate seismic reflection and refraction profiles and additional supporting data such as side-scan sonar, SEABEAM, magnetic, gravity and drilling data, and (4) detailed study study of selected trench regions in which data quality and/or quantity is exceptional. Phase 2 of this project represents a unique compilation and synthesis of existing data for the world's deep ocean trenches. The analysis of data and discussion of results in the context of current literature aids our understanding of the sediment distribution and nature of sediment deformation through various stages of plate convergence, the determination of whether sediments are subducted or accreted, and the evaluation of the controlling factors for sediment subduction and/or accretion. A discussion on petroleum and natural gas hydrate resource potential is included.

  9. Structural interpretation and physical property estimates based on COAST 2012 seismic reflection profiles offshore central Washington, Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Webb, S. I.; Tobin, H. J.; Everson, E. D.; Fortin, W.; Holbrook, W. S.; Kent, G.; Keranen, K. M.

    2014-12-01

    The Cascadia subduction zone has a history of large magnitude earthquakes, but a near-total lack of plate interface seismicity, making the updip limit of the seismogenic zone difficult to locate. In addition, the central Cascadia accretionary prism is characterized by an extremely low wedge taper angle, landward vergent initial thrusting, and a flat midslope terrace between the inner and outer wedges, unlike most other accretionary prisms (e.g. the Nankai Trough, Japan). The Cascadia Open Access Seismic Transect (COAST) lines were shot by R/V Marcus Langseth in July of 2012 off central Washington to image this subduction zone. Two trench-parallel and nine trench-perpendicular lines were collected. In this study, we present detailed seismic interpretation of both time- and depth-migrated stacked profiles, focused on elucidating the deposition and deformation of both pre- and syn-tectonic sediment in the trench and slope. Distribution and timing of sediments and their deformation is used to unravel the evolution of the wedge through time. Initially, interpretation of the time-sections is carried out to support the building of tomographic velocity models to aid in the pre-stack depth migration (PSDM) of selected lines. In turn, we use PSDM velocity models to estimate porosity and pore pressure conditions at the base of the wedge and across the basal plate interface décollement where possible, using established velocity-porosity transforms. Interpretation in this way incorporates both accurate structural relationships and robust porosity models to document wedge development and present-day stress state, in particular regions of potential overpressure. Results shed light on the origin and evolution of the mid-slope terrace and the low taper angle for the forearc wedge. This work may shed light ultimately on the position of the potential updip limit of the seismogenic zone beneath the wedge.

  10. Very low frequency earthquakes off the Pacific coast of Tohoku, Japan

    NASA Astrophysics Data System (ADS)

    Matsuzawa, Takanori; Asano, Youichi; Obara, Kazushige

    2015-06-01

    We found very low frequency earthquakes (VLFEs) at a shallow subduction zone close to the Japan Trench off the Pacific coast of Tohoku, Japan. Centroid moment tensor solutions of VLFEs showed reverse fault mechanisms with a compression axis in the east-west direction. A cross-correlation analysis of seismograms with template events between 2005 and 2013 revealed three major VLFE clusters and their temporal evolution. A VLFE cluster in the central off-Tohoku region located in the large slip area of the 2011 Tohoku earthquake was detectable only before the Tohoku earthquake. However, VLFEs in the northern and southern off-Tohoku regions at the rim of the large slip area were activated after the Tohoku earthquake. The change in the activity may reflect the stress redistribution by the coseismic and/or afterslip processes of the Tohoku earthquake.

  11. Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling and application to the northern Caribbean plate boundary

    USGS Publications Warehouse

    ten Brink, U.; Lin, J.

    2004-01-01

    Strike-slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers. We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc region using three-dimensional models of static Coulomb stress change. Model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the Coulomb stress on strike-slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region. In contrast, subduction events with slip vectors perpendicular to the trench axis enhance the Coulomb stress on strike-slip faults farther back in the forearc, while reducing the stress adjacent to the trench. A significant contribution to Coulomb stress increase on strike-slip faults in the back region of the forearc comes from "unclamping" of the fault, i.e., reduction in normal stress due to thrust motion on the subduction interface. We argue that although Coulomb stress changes from individual subduction earthquakes are ephemeral, their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant. We use the Coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the Caribbean subduction zone. Subduction earthquakes with slip vectors nearly perpendicular to the Caribbean trench axis is dominant in the Hispaniola segment, where the strike-slip faults are more than 60 km inland from the trench. In contrast, subduction slip motion is nearly parallel to the Caribbean trench axis along the Puerto Rico segment, where the strike-slip fault is less than 15 km from the trench. This observed jump from a strike-slip fault close to the trench axis in the Puerto Rico segment to the inland faults in Hispaniola is explained by different distributions of Coulomb stress in the forearc region of the two segments, as a result of the change from the nearly trench parallel slip on the Puerto Rico subduction interface to the more perpendicular subduction slip beneath Hispaniola. The observations and modeling suggest that subduction-induced strike-slip seismic hazard to Puerto Rico may be smaller than previously assumed but the hazard to Hispaniola remains high. Copyright 2004 by the American Geophysical Union.

  12. Subduction Drive of Plate Tectonics

    NASA Astrophysics Data System (ADS)

    Hamilton, W. B.

    2003-12-01

    Don Anderson emphasizes that plate tectonics is self-organizing and is driven by subduction, which rights the density inversion generated as oceanic lithosphere forms by cooling of asthenosphere from the top. The following synthesis owes much to many discussions with him. Hinge rollback is the key to kinematics, and, like the rest of actual plate behavior, is incompatible with bottom-up convection drive. Subduction hinges (which are under, not in front of, thin leading parts of arcs and overriding plates) roll back into subducting plates. The Pacific shrinks because bounding hinges roll back into it. Colliding arcs, increasing arc curvatures, back-arc spreading, and advance of small arcs into large plates also require rollback. Forearcs of overriding plates commonly bear basins which preclude shortening of thin plate fronts throughout periods recorded by basin strata (100 Ma for Cretaceous and Paleogene California). This requires subequal rates of advance and rollback, and control of both by subduction. Convergence rate is equal to rates of rollback and advance in many systems but is greater in others. Plate-related circulation probably is closed above 650 km. Despite the popularity of concepts of plumes from, and subduction into, lower mantle, there is no convincing evidence for, and much evidence against, penetration of the 650 in either direction. That barrier not only has a crossing-inhibiting negative Clapeyron slope but also is a compositional boundary between fractionated (not "primitive"), sluggish lower mantle and fertile, mobile upper mantle. Slabs sink more steeply than they dip. Slabs older than about 60 Ma when their subduction began sink to, and lie down on and depress, the 650-km discontinuity, and are overpassed, whereas younger slabs become neutrally buoyant in mid-upper mantle, into which they are mixed as they too are overpassed. Broadside-sinking old slabs push all upper mantle, from base of oceanic lithosphere down to the 650, back under shrinking oceans, forcing rapid Pacific spreading. Slabs suck forward overriding arcs and continental lithosphere, plus most subjacent mantle above the transition zone. Changes in sizes of oceans result primarily from transfer of oceanic lithosphere, so backarcs and expanding oceans spread only slowly. Lithosphere parked in, or displaced from, the transition zone, or mixed into mid-upper mantle, is ultimately recycled, and regional variations in age of that submerged lithosphere may account for some regional contrasts in MORB. Plate motions make no kinematic sense in either the "hotspot" reference frame (HS; the notion of fixed plumes is easily disproved) or the no-net-rotation frame (NNR) In both, for example, many hinges roll forward, impossible with gravity drive. Subduction-drive predictions are fulfilled, and paleomagnetic data are satisfied (as they are not in HS and NNR), in the alternative framework of propulsionless Antarctica fixed relative to sluggish lower mantle. Passive ridges migrate away from Antarctica on all sides, and migration of these and other ridges permits tapping fresh asthenosphere. (HS and NNR tend to fix ridges). Ridge migration and spreading rates accord with subduction drive. All trenches roll back when allowance is made for back-arc spreading and intracontinental deformation. Africa rotates slowly toward subduction systems in the NE, instead of moving rapidly E as in HS and NNR. Stable NW Eurasia is nearly stationary, instead of also moving rapidly, and S and E Eurasian deformation relates to subduction and rollback. The Americas move Pacificward at almost the full spreading rates of passive ridges behind them. Lithosphere has a slow net westward drift. Reference: W.B. Hamilton, An alternative Earth, GSA Today, in press.

  13. High frequency waves guided by the subducted plates underneath Taiwan and their association with seismic intensity anomalies

    NASA Astrophysics Data System (ADS)

    Chen, K. H.; Kennett, B. L.; Furumura, T.

    2011-12-01

    Energy from seismic events traveling up a subduction zone reveal large-amplitude, high-frequency signal with sustained long coda. In Japan, such seismic waves guided by the high wave velocity and high Q plate lead to surprisingly large intensity in the forearc area, even if the events are not felt near the epicenter. Seismic events with guided wave characteristics can explain the anomalous ground shaking, and provide useful information on the plate configuration. Taiwan, situated at the plate boundary zone between the Eurasian plate (EP) and the Philippine Sea plates (PSP), exhibits a unique interaction between the EP and PSP. In northeast Taiwan, the PSP subducts beneath the rifted Eurasian plate margin along the Ryukyu Trench, whereas in southwest Taiwan the Eurasian plate subducts underneath PSP. The anomalous seismic intensity from intermediate-depth earthquakes should happen in Taiwan, if the subducted plates are acting as an efficient waveguide for high-frequency seismic waves. Here we investigate the possible relationship between anomalous PGA patterns and the trapping effect of the high frequency signal in the PSP/EP. M>5 earthquakes along the subducted PSP reveal depth dependent waveguide behavior, confirming an association with a wave guide effect in the subducting slab rather than localized site amplification effects. Comparison of the PGA patterns and the seismic characteristics suggests that the abnormal intensity from intermediate-depth events is likely to be a result of excitation of high-frequency signals while propagating along the PSP. The events in EP show an extension of stronger seismic intensity and faster propagation speed along the Longitudinal Valley. This poses the question whether such an elongation of the intensity contours is associated with a fold of the Eurasian plate crust underneath eastern Taiwan. If so, we expect the guided waves to be observed at stations along the east coast or on the eastern flank of Central Range. The seismic waveform characteristics, however, reveal partial guiding across the southern portion of Taiwan and along the east coast, suggesting some of the high frequency energy may couple into the crust due to either long travel distance or thick plate. By detection and quantification of the subduction zone guided waves, the geometry, thickness, velocity gradient, and heterogeneities of the plates can be further inferred through 2D and 3D finite-difference modeling and comparison with other well-established subduction zones. Additionally, knowledge of waveguide characteristics and modeled parameters that fit the observations are critical inputs to connecting with the seismic intensity anomalies for ground motion and earthquake hazard estimation.

  14. 3D Numerical simulations of oblique subduction

    NASA Astrophysics Data System (ADS)

    Malatesta, C.; Gerya, T.; Scambelluri, M.; Crispini, L.; Federico, L.; Capponi, G.

    2012-04-01

    In the past 2D numerical studies (e.g. Gerya et al., 2002; Gorczyk et al., 2007; Malatesta et al., 2012) provided evidence that during intraoceanic subduction a serpentinite channel forms above the downgoing plate. This channel forms as a result of hydration of the mantle wedge by uprising slab-fluids. Rocks buried at high depths are finally exhumed within this buoyant low-viscosity medium. Convergence rate in these 2D models was described by a trench-normal component of velocity. Several present and past subduction zones worldwide are however driven by oblique convergence between the plates, where trench-normal motion of the subducting slab is coupled with trench-parallel displacement of the plates. Can the exhumation mechanism and the exhumation rates of high-pressure rocks be affected by the shear component of subduction? And how uprise of these rocks can vary along the plate margin? We tried to address these questions performing 3D numerical models that simulate an intraoceanic oblique subduction. The models are based on thermo-mechanical equations that are solved with finite differences method and marker-in-cell techniques combined with multigrid approach (Gerya, 2010). In most of the models a narrow oceanic basin (500 km-wide) surrounded by continental margins is depicted. The basin is floored by either layered or heterogeneous oceanic lithosphere with gabbro as discrete bodies in serpentinized peridotite and a basaltic layer on the top. A weak zone in the mantle is prescribed to control the location of subduction initiation and therefore the plate margins geometry. Finally, addition of a third dimension in the simulations allowed us to test the role of different plate margin geometries on oblique subduction dynamics. In particular in each model we modified the dip angle of the weak zone and its "lateral" geometry (e.g. continuous, segmented). We consider "continuous" weak zones either parallel or increasingly moving away from the continental margins. Moreover, we tested the effect on subduction/exhumation dynamics of several values of the trench-parallel component of convergence-rate vector. Gerya T., Stöckhert B., Perchuk A.L. (2002). Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics, vol. 21, n. 6, 1056. Gerya, T. V., 2010. Introduction to numerical geodynamic modelling. Cambridge University Press, Cambridge. Gorczyk W., Guillot S., Gerya T.V., Hattori K. (2007a). Asthenospheric upwelling, oceanic slab retreat, and exhumation of UHP mantle rocks: insights from Greater Antilles. Geophysical research letters, vol. 34, L21309. Malatesta C., Gerya T., Scambelluri M., Federico L., Crispini L., Capponi G. (2012). Intraoceanic subduction of "heterogeneous" oceanic lithosphere in narrow basins: 2D numerical modeling. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.01.003

  15. Geophysical signature of hydration-dehydration processes in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-04-01

    Seismological and magneto-telluric tomographies are potential tools for imaging fluid circulation when combined with petrophysical models. Recent measurements of the physical properties of serpentine allow refining hydration of the mantle and fluid circulation in the mantle wedge from geophysical data. In the slab lithospheric mantle, serpentinization caused by bending at the trench is limited to a few kilometers below the oceanic crust (<5 km). Double Wadati-Benioff zones, 20-30 km below the crust, are explained by deformation of dry peridotites, not by serpentine dehydration. It reduces the required amount of water stored in solid phases in the slab (Reynard et al., 2010). In the cold (<700C) fore-arc mantle wedge above the subducting slab, serpentinization is caused by the release of large amounts of hydrous fluids in the cold mantle above the dehydrating subducted plate. Low seismic velocities in the wedge give a time-integrated estimate of hydration and serpentinization. Serpentinization reaches 50-100% in hot subduction, while it is below 10% in cold subduction (Bezacier et al., 2010; Reynard, 2012). Electromagnetic profiles of the mantle wedge reveal high electrical-conductivity bodies. In hot areas of the mantle wedge (> 700C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs, explaining the observed high conductivities. In the cold melt-free wedge (< 700C), high conductivities in electromagnetic profiles provide "instantaneous" images of fluid circulation because the measured electrical conductivity of serpentine is below 0.1 mS/m (Reynard et al., 2011). A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. Low-salinity fluids (? 0.1 m) released by slab dehydration evolve towards high-salinity (? 1 m) fluids during progressive serpentinization in the wedge. These fluids can mix with arc magmas at depths and account for high-chlorine melt inclusions in arc lavas. High electrical conductivities up to 1 S/m in the hydrated wedge of the hot subductions (Ryukyu, Kyushu, Cascadia) reflect high fluid concentration, while low to moderate (<0.01 S/m) conductivities in the cold subductions (N-E Japan, Bolivia) reflect low fluid flow. This is consistent with the seismic observations of extensive shallow serpentinization in hot subduction zones, while serpentinization is sluggish in cold subduction zones. Bezacier, L., et al. 2010. Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones. Earth and Planetary Science Letters, 289, 198-208. Reynard, B., 2012. Serpentine in active subduction zones. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.10.012. Reynard, B., Mibe, K. & Van de Moortele, B., 2011. Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones. Earth and Planetary Science Letters, 307, 387-394. Reynard, B., Nakajima, J. & Kawakatsu, H., 2010. Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones. Geophysical Research Letters, 37, L24309.

  16. The Role of Slab Windows in Subduction Cycles

    NASA Astrophysics Data System (ADS)

    Thorkelson, D. J.; Breitsprecher, K.

    2011-12-01

    Active continental margins are enduring features which commonly record a history of subduction spanning tens of millions of years. A subduction history is commonly divisible into distinct intervals of subduction activity separated by periods of non-subduction. The intervals of non-subduction are dominated by transform, transtensional or transpressional regimes. The recognition of subduction cycles as a normal pattern of active continental margins was an essential step forward in the understanding of ancient continental margin assemblages, plate evolution and global tectonics. The causes of interruptions of subduction are varied, and include collision of island arcs or oceanic plateaus, swerves in the motions of large plates, plate deformation and microplate formation, and subduction of oceanic spreading ridges. These processes punctuate subduction that may have occurred unbroken for millions or tens of millions of years, but do not necessarily lead to destruction of the continental margin as fundamentally convergent and active. The intersection of a mid-ocean spreading ridge with a subduction zone brings two distinctive tectono-magmatic systems together at the same location. The style of ridge-subduction zone interaction varies considerably, depending on factors such as the obliquity of ridge-trench intersection, relative plate motions, plate integrity and thermal conditions. Where the ridge intersects the trench, a triple junction exists which, in most cases, migrates along the continental margin. The two oceanic plates that flank the spreading ridge naturally have different motion vectors relative to the overriding plate, and as the triple junction migrates, a given part of the continental margin will be in contact with one plate, and at a later time, the other plate. One or both of the oceanic plates may be convergent with the continent but in all cases a gap in the extent of the subducted slab, termed a slab window, will develop beneath the continent in the region near the triple junction. The slab window is a product of simple geometrical divergence between the oceanic plates in concert with more cryptic processes including thermal erosion and physical degradation of the subducting slab edge(s). Slab windows and their geological products are thereby linked to one of the most common interruptions to subduction beneath active continental margins, i.e, where one oceanic plate replaces another at the locus of a migrating ridge-trench-trench or ridge-trench-transform triple junction. Slab windows have played an important role in the evolution of many continental (and oceanic) convergent margins, most notably the west coast of the Americas during the Cenozoic. All of these windows have been, or are, involved with the replacement of a metasomatized mantle wedge by drier asthenosphere, modification or elimination of the volcanic arc, and changes to the regional structural and tectonic system. Despite the proliferation of slab windows, and the interruption of subduction for long intervals, the fundamental nature of the western margin of South, Central and North America as an enduring belt of plate convergence and subduction remains intact.

  17. Seismic anisotropy and texture development during early stages of subduction

    NASA Astrophysics Data System (ADS)

    Di Leo, Jeanette; Walker, Andrew; Li, Zhong-Hai; Wookey, James; Ribe, Neil; Kendall, J.-Michael; Tommasi, Andra

    2014-05-01

    Shear wave splitting measurements are frequently used to infer upper mantle flow trajectory, based on the fact that, under strain, olivine develops lattice-preferred orientation (LPO) textures in the convecting mantle. However, such inferences ought to be made carefully, since the relationship between splitting fast polarisation and olivine LPO depends on several factors, one of them being the deformation history of the volume of mantle in question. This is especially the case in regions such as subduction zones, where complex and time-dependent mantle flow occurs. Here, we present an integrated model to simulate strain-history-dependent LPO development and measure the resulting shear wave splitting in a subduction setting. We do this for a subduction model that approximates the geometry of the double-sided Molucca Sea subduction system in eastern Indonesia. We test a single-sided and a double-sided subduction case, and compare the results to shear wave splitting observations of this region. Since the subduction zone is fairly young, early textures from the slab's descent from the near-surface to the bottom of the mantle transition zone - which we simulate in our models - have not yet been overprinted by subsequent continuous flow. It further allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel sub-slab mantle flow. We simulate olivine LPO evolution in polycrystalline aggregates as they move and deform along pathlines extracted from a 3-D mantle flow model. Interactions between crystals are described using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. After calculating elastic properties associated with LPO textures, we estimate the resulting splitting parameters (fast direction ?, delay time ?t) for synthetic SKS phases. Our models demonstrate that complex, backazimuth-dependent behaviour in ? appears in even apparently simple models of subduction zone mantle flow. We also show that although a rear barrier amplifies trench-parallel sub-slab anisotropy due to mantle flow, it is not essential for producing trench-parallel fast directions. In a simple model of one-sided subduction and deformation dominated by the motion of dislocations belonging to the (010)[100] slip system, trench-parallel fast directions result from a combination of simple shear and deformation by axial compression in the sub-slab mantle.

  18. 'Snow White' Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This image was acquired by NASA's Phoenix Mars Lander's Surface Stereo Imager on Sol 43, the 43rd Martian day after landing (July 8, 2008). This image shows the trench informally called 'Snow White.'

    Two samples were delivered to the Wet Chemistry Laboratory, which is part of Phoenix's Microscopy, Electrochemistry, and Conductivity Analyzer (MECA). The first sample was taken from the surface area just left of the trench and informally named 'Rosy Red.' It was delivered to the Wet Chemistry Laboratory on Sol 30 (June 25, 2008). The second sample, informally named 'Sorceress,' was taken from the center of the 'Snow White' trench and delivered to the Wet Chemistry Laboratory on Sol 41 (July 6, 2008).

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  19. Track-and-Trench

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This image shows the tracks and trench marks made by the Mars Exploration Rover Opportunity at Meridiani Planum, Mars. The rover can be seen to the lower left of the lander. The trench is visible to the upper left of the rover, which has traveled a total of 35.3 meters (116 feet) since leaving the lander on sol 7 (January 31, 2004). On sol 23 (February 16, 2004), the rover used one of its wheels to dig a trench measuring approximately 10 centimeters (4 inches) deep, 50 centimeters (20 inches) long, and 20 centimeters (8 inches) wide. This vertically projected image was created using a combination of images from the rover's navigation camera and hazard-avoidance cameras.

  20. Phoenix's Snow White Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

    A soil sample taken from the informally named 'Snow White' trench at NASA's Phoenix Mars Lander work site produced minerals that indicate evidence of past interaction between the minerals and liquid water.

    This image was taken by the Surface Stereo Imager on Sol 103, the 103rd day since landing (Sept. 8, 2008).

    The trench is approximately 23 centimeters (9 inches) long.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by JPL, Pasadena, Calif. Spacecraft development was by Lockheed Martin Space Systems, Denver.

  1. An elastic plate model for interseismic deformation in subduction zones

    NASA Astrophysics Data System (ADS)

    Kanda, Ravi V. S.; Simons, Mark

    2010-03-01

    Geodetic observations of interseismic surface deformation in the vicinity of subduction zones are frequently interpreted using simple kinematic elastic dislocation models (EDM). In this theoretical study, we develop a kinematic EDM that simulates plate subduction over the interseismic period (the elastic subducting plate model (ESPM)) having only 2 more degrees of freedom than the well-established back slip model (BSM): an elastic plate thickness and the fraction of flexural stresses due to bending at the trench that are released continuously. Unlike the BSM, in which steady state deformation in both plates is assumed to be negligible, the ESPM includes deformation in the subducting and overriding plates (owing to plate thickness), while still preserving the correct sense of convergence velocity between the subducting and overriding plates, as well as zero net steady state vertical offset between the two plates when integrated over many seismic cycles. The ESPM links elastic plate flexure processes to interseismic deformation and helps clarify under what conditions the BSM is appropriate for fitting interseismic geodetic data at convergent margins. We show that the ESPM is identical to the BSM in the limiting case of zero plate thickness, thereby providing an alternative motivation for the BSM. The ESPM also provides a consistent convention for applying the BSM to any megathrust interface geometry. Even in the case of nonnegligible plate thickness, the deformation field predicted by the ESPM reduces to that of the BSM if stresses related to plate flexure at the trench are released either continuously and completely at shallow depths during the interseismic period or deep in the subduction zone (below 100 km). However, if at least a portion of these stresses are not continuously released in the shallow portion of the subduction zone (via seismic or aseismic events), then the predicted surface velocities of these two models can differ significantly at horizontal distances from the trench equivalent to a few times the effective interseismic locking depth.

  2. Subslab seismic anisotropy and mantle flow in the western Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Peng, C. C.; Kuo, B. Y.; Chen, C. W.

    2014-12-01

    We present source-side anisotropy for a few subduction zones in an attempt to map the mantle flow beneath the slab. Shear-wave splitting parameters of S were measured at stations towards the back of the subduction with the receiver-side anisotropy removed. We examined the observed fast directions against tilting/rotation of olivine fabric relative to the geometry of the subduction. We found that at the SW edge of the Ryukyu subduction zone the olivine fabric in the subslab mantle must rotate clockwise by 25 degrees from the slab subduction trajectory to explain the observed pattern of shear-wave splitting. This rotation echoes the deformation model of the slab when it is impinging against the Eurasian lithosphere. In the Vanuatu (New Hebrides) subduction zone, the olivine fabric may rotate dramatically to accommodate the rapid retreat of the trench and flipping of subduction polarity in the past a few Mys.

  3. 3D Finite-Difference Modeling of Scattered Teleseismic Wavefields in a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Morozov, I. B.; Zheng, H.

    2005-12-01

    For a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the zone underlying the trench result in subhorizontally-propagating waves that could be difficult to distinguish from converted P- and S- wave backscattered from the surface. Because back-scattered modes often provide the most spectacular images of subducting slabs, it is important to understand their differences from the arrivals scattered from the trench zone. To investigate the detailed teleseismic wavefield in a subduction zone environment, we performed a full-waveform, 3-D visco-elastic finite-difference modeling of teleseismic wave propagation using a Beowulf cluster. The synthetics show strong scattering from the trench zone, dominated by the mantle and crustal P-waves propagating at 6.2-8.1.km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered modes, and also have similar amplitudes. Although their amplitude decay characters are different, with the uncertainties in the velocity and density structure of the subduction zone, unambiguous distinguishing of these modes appears difficult. However, under minimal assumptions (in particular, without invoking slab dehydration), recent observations of receiver function amplitudes decreasing away from the trench favor the interpretation of trench-zone scattering.

  4. Precise hypocenter distribution and earthquake generating and stress in and around the upper-plane seismic belt in the subducting Pacific slab beneath NE Japan

    NASA Astrophysics Data System (ADS)

    Kita, S.; Okada, T.; Nakajima, J.; Matsuzawa, T.; Uchida, N.; Hasegawa, A.

    2007-12-01

    1. Introduction We found an intraslab seismic belt (upper-plane seismic belt) in the upper plane of the double seismic zone within the Pacific slab, running interface at depths of 70-100km beneath the forearc area. The location of the deeper limits of this belt appears to correspond to one of the facies boundaries (from jadeite lawsonite blueschist to lawsonite amphibole eclogite) in the oceanic crust [Kita et al., 2006, GRL]. In this study, we precisely relocated intraslab earthquakes by using travel time differences calculated by the waveform cross-spectrum analysis to obtain more detailed distribution of the upper plane-seismic belt within the Pacific slab beneath NE Japan. We also discuss the stress field in the slab by examining focal mechanisms of the earthquakes. 2. Data and Method We relocated events at depths of 50-00 km for the period from March 2003 to November 2006 from the JMA earthquake catalog. We applied the double-difference hypocenter location method (DDLM) by Waldhauser and Ellsworth (2000) to the arrival time data of the events. We use relative earthquake arrival times determined both by the waveform cross-spectrum analysis and by the catalog-picking data. We also determine focal mechanisms using the P wave polarity. 3. Spatial distribution of relocated hypocenters In the upper portion of the slab crust, seismicity is very active and distributed relatively homogeneously at depths of about 70-100km parallel to the volcanic front, where the upper-plane seismic belt has been found. In the lower portion of slab crust and/or the uppermost portion of the slab mantle, seismicity is spatially very limited to some small areas (each size is about 20 x 20km) at depths around 65km. Two of them correspond to the aftershock area of the 2003 Miyagi (M7.1) intraslab earthquake and that of the 1987 Iwaizumi (M6.6) intraslab earthquake, respectively. Based on the dehydration embrittelment hypothesis, the difference of the spatial distribution of the seismicity in the slab should correspond to the difference of the spatial distribution of the hydrated minerals and their dehydration reactions. In the upper slab crust, the upper-plane seismic belt is found because the hydrated minerals could be distributed homogeneously and the dehydration reaction (from jadeite lawsonite blueschist to lawsonite amphibole eclogite [Hacker et al., 2003b]) occurs perhaps largely at depth of 70-100km. Our result also suggests that in the lower portion of the slab crust and/or the uppermost portion of the slab mantle, the hydrated minerals could be inhomogeneously distributed and the seismicity occurs at depths around 65km, where another dehydration reaction may exist. 4. Characteristics of the focal mechanisms We examined the stress distribution within the slab by using focal mechanisms of the upper plane, interplane and lower plane events. From the plate interface to about 20 km below it, downdip-compressional (DC) type events are dominant. Below 20km from the plate interface, downdip-tensional (DT) type events are dominant. Many of interplane events have DC type focal mechanisms because of their locations in the uppermost portions of the slab mantle. These results indicate that the stress neutral plane from the DC type to DT type could be located at depth of about 20km from the plate interface.

  5. All the way up and deep down: new insights on the seismogenic portion of subduction megathrusts from recent giant earthquakes and thermal modeling

    NASA Astrophysics Data System (ADS)

    Gutscher, Marc-Andre; Duarte, Joao C.; Schellart, Wouter P.

    2013-04-01

    Until less than 10 years ago, there was a fairly broad consensus that seismogenic rupture could only occur between the forearc basement and the downgoing oceanic plate. This conceptual model considered that the mantle wedge was serpentinized and weak and likewise that the shallowest portion of the forearc, typically the accretionary wedge, was composed of high-porosity overpressured sediments, and that neither of these domains were capable of storing and releasing elastic stress and thus contribute to seismogenic rupture. This paradigm has been challenged by the detailed observations following the series of great megathrust earthquakes starting with the M9.1 Sumatra-Andaman Dec. 2004 earthquake and ending with the most recent M9.0 Tohoku Mar. 2011 earthquake. Deep crustal seismic surveys as well as aftershock distribution and focal mechanism studies now provide compelling evidence that seismogenic rupture commonly extends beneath the entire accretionary wedge and right up to the deep-sea trench, with low-angle thrust type focal mechanisms throughout this zone. Conversely, the down-dip limit of the seismogenic zone for both NW Sumatra and NE Japan clearly extends to well below the tip of the mantle wedge. Numerical modeling of forearc thermal structure for these two zones, considering the 100-150°C and 350-450°C isotherms as proxies for the up-dip and down-dip limits, respectively, successfully predicts the very wide extent (200 km downdip width) of the NW Sumatra seismogenic zone. For NE Japan, the thermal model successfully predicts the downdip limit, but the updip limit near the trench is more problematical. Using the same low values of interplate shear stress for both Sumatra and Japan, thermal modeling predicts a position of about 80km inboard from the trench for the 100°C isotherm along the subduction megathrust. However, both the distribution of thrust type aftershocks and published slip models indicate that the Tohoku earthquake ruptured up to the trench (where preliminary thermal models predict a temperature of only about 10°C at the decollement). We propose the hypothesis that a much higher degree of effective friction and strong shear heating along the oceanic basement - forearc basement contact could provide an explanation for this apparent paradox. Indeed, the Japan forearc has very little sediment at the trench (typically about 0.5 km) and is considered a non-accretionary (erosive) margin and thus has very different rheological properties than the NW Sumatra forearc. The hypothesis of higher effective friction and elevated shear heating for this margin configuration will be explored in greater detail in future work.

  6. Numerical modeling of the deformations associated with large subduction earthquakes through the seismic cycle

    NASA Astrophysics Data System (ADS)

    Fleitout, L.; Trubienko, O.; Garaud, J.; Vigny, C.; Cailletaud, G.; Simons, W. J.; Satirapod, C.; Shestakov, N.

    2012-12-01

    A 3D finite element code (Zebulon-Zset) is used to model deformations through the seismic cycle in the areas surrounding the last three large subduction earthquakes: Sumatra, Japan and Chile. The mesh featuring a broad spherical shell portion with a viscoelastic asthenosphere is refined close to the subduction zones. The model is constrained by 6 years of postseismic data in Sumatra area and over a year of data for Japan and Chile plus preseismic data in the three areas. The coseismic displacements on the subduction plane are inverted from the coseismic displacements using the finite element program and provide the initial stresses. The predicted horizontal postseismic displacements depend upon the thicknesses of the elastic plate and of the low viscosity asthenosphere. Non-dimensionalized by the coseismic displacements, they present an almost uniform value between 500km and 1500km from the trench for elastic plates 80km thick. The time evolution of the velocities is function of the creep law (Maxwell, Burger or power-law creep). Moreover, the forward models predict a sizable far-field subsidence, also with a spatial distribution which varies with the geometry of the asthenosphere and lithosphere. Slip on the subduction interface does not induce such a subsidence. The observed horizontal velocities, divided by the coseismic displacement, present a similar pattern as function of time and distance from trench for the three areas, indicative of similar lithospheric and asthenospheric thicknesses and asthenospheric viscosity. This pattern cannot be fitted with power-law creep in the asthenosphere but indicates a lithosphere 60 to 90km thick and an asthenosphere of thickness of the order of 100km with a burger rheology represented by a Kelvin-Voigt element with a viscosity of 3.1018Pas and μKelvin=μelastic/3. A second Kelvin-Voigt element with very limited amplitude may explain some characteristics of the short time-scale signal. The postseismic subsidence is conspicuous over Thailand and Malaysia (Satirapod et al., ASR, 2012). A low viscosity wedge, with a viscosity of the order of 3. 1018 Pas is necessary to explain data in the middle-field (volcanic arc area). Post-seismic slip on the fault plane (15% of the cosismic slip) in the months after the earthquakes explains near-field deformations. The creep law and geometry deduced from postseismic data can be used to predict deformations through the seismic cycle. Far away (500 to 1500km) sizable (5mm/yr to 1cm/yr) interseismic horizontal velocities are expected. Although one should not deny the presence of long-term intraplate geologic deformations, the seismic cycle contributes significantly to the intraplate compressive preseismic deformations in the Sunda and Amurian plates. The interseismic peak in vertical velocity, predicted by elastic backslip models over the end of the locked portion of the interface can be, in viscoelastic models, pushed over the continentward border of the LVW. This may explain the pattern of vertical velocities in Northern Honshu previous to Tohoku earthquake. The deviatoric stresses associated with the seismic cycle add up to the long-term tectonic stresses and are predicted to induce a peak in extensional stress in the subducting and overriding plates with a time delay which increases with the distance to the subduction zone.

  7. Trenching the Trough

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This animation shows the Mars Exploration Rover Opportunity digging a trench near the trough dubbed 'Anatolia' with its left front wheel on sol 73. It was taken by the rover's hazard-avoidance camera.

    The trench was dug so that Opportunity would be able to place its Moessbauer spectrometer on a soil target (the pile of material on the right side of the trench) during a four-day flight software update. The rover's alpha particle X-ray spectrometer was pointed at the sky at this time taking calibration measurements.

    Spirit performed a similar operation during its flight software update, but its Moessbauer was placed on a rock dubbed 'Route 66.' Since there are no rocks at Opportunity's current location, rover team members chose a patch of soil.

    The trench itself is 95 centimeters (38 inches) long by 16 centimeters (6 inches) wide by 11 centimeters (4 inches) deep. It is the deepest hole dug by either Spirit or Opportunity to date.

  8. Snow White 5 Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This image was acquired by NASA's Phoenix Mars Lander's Robotic Arm Camera on the 35th Martian day of the mission, or Sol 34 (June 29, 2008), after the May 25, 2008, landing. This image shows the trench informally called 'Snow White 5.' The trench is 4-to-5 centimeters (about 1.5-to-1.9 inches) deep, 24 centimeters (about 9 inches) wide and 33 centimeters (13 inches) long.

    Snow White 5 is Phoenix's current active digging area after additional trenching, grooming, and scraping by Phoenix's Robotic Arm in the last few sols to trenches informally called Snow White 1, 2, 3, and 4. Near the top center of the image is the Robotic Arm's Thermal and Electrical Conductivity Probe.

    Snow White 5 is located in a patch of Martian soil near the center of a polygonal surface feature, nicknamed 'Cheshire Cat.' The digging site has been named 'Wonderland.'

    This image has been enhanced to brighten shaded areas.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  9. A Model for the Termination of the Ryukyu Subduction Zone against Taiwan A Triple Junction of Collision, Subduction/Separation and Subduction Boundaries

    NASA Astrophysics Data System (ADS)

    Wu, F. T.; Liang, W.; Lee, J. C.

    2006-12-01

    In the vicinity of Taiwan, the Philippine Sea plate moves in the direction of N50^{O}W to collide with the Eurasian continent and at the same time subducts toward the north along the Ryukyu subduction zone. The geometry of the junction between the Ryukyu subduction and Taiwan is obscured by intense deformation near the island. Based on tomographic velocity images for the subduction zone and relocated seismicity the junction is determined to be near the Longitudinal Valley at the latitude of about 23.7^{O}N, i.e., the northern part of the Coastal Range overlies the subduction zone. North of this latitude the subduction becomes deeper and only the portion of the Eurasian lithosphere that is in contact with the subduction zone is engaging in collision. Above the subduction zone the Eurasian plate undergoes stretching due to the shortening of the plate on the Taiwan side and the lack of shortening of the Eurasian plate above the subducting Philippine Sea plate. When the subducting Philippine Sea reaches the depths of the lower part of the lithosphere or asthenosphere then the Eurasian plate is no longer under compression and a bight in the surface structure is created. Adding to these actions is the opening of the Okinawa Trough and the resulting southward migration of the Ryukyu Trench. Because of the opening of Okinawa this junction is a dynamically changing triple junction that involves collision between about 23^{O}N and 23.7^{O}N, collision and separation, with normal and strike-slip motion along the boundary, to its north and subduction along the Ryukyu Trench. The triple junction migrates southward with time. The plate configuration in this junction resembles that in the eastern syntaxis of Himalaya where the Indian plate moves northwestward to collide along the Himalayan front and subducts under Northeastern India and Myanmar. While the Taiwan collision began about four million years the Indian collision has probably gone on for at least a few tens of million years. The Taiwan situation can provide a scenario of the earlier development of the Indian/Eurasian collision regime.

  10. A Regime Diagram for Subduction

    NASA Astrophysics Data System (ADS)

    Stegman, D. R.; Farrington, R.; Capitanio, F. A.; Schellart, W. P.

    2009-12-01

    Regime diagrams and associated scaling relations have profoundly influenced our understanding of planetary dynamics. Previous regime diagrams characterized the regimes of stagnant-lid, small viscosity contrast, transitional, and no-convection for temperature-dependent (Moresi and Solomatov, 1995), and non-linear power law rheologies (Solomatov and Moresi, 1997) as well as stagnant-lid, sluggish-lid, and mobile-lid regimes once the finite strength of rock was considered (Moresi and Solomatov, 1998). Scalings derived from such models have been the cornerstone for parameterized models of thermal evolution of rocky planets and icy moons for the past decade. While such a theory can predict the tectonic state of a planetary body, it is still rather incomplete in regards to predicting tectonics. For example, the mobile-lid regime is unspecific as to how continuous lithospheric recycling should occur on a terrestrial planet. Towards this goal, Gerya et al., (2008) advanced a new regime diagram aiming to characterize when subduction would manifest itself as a one-sided or two-sided downwelling and either symmetric or asymmetric. Here, we present a regime diagram for the case of a single-sided, asymmetric type of subduction (most Earth-like type). Using a 3-D numerical model of a free subduction, we describe a total of 5 different styles of subduction that can possibly occur. Each style is distinguished by its upper mantle slab morphology resulting from the sinking kinematics. We provide movies to illustrate the different styles and their progressive time-evolution. In each regime, subduction is accommodated by a combination of plate advance and slab rollback, with associated motions of forward plate velocity and trench retreat, respectively. We demonstrate that the preferred subduction mode depends upon two essential controlling factors: 1) buoyancy of the downgoing plate and 2) strength of plate in resisting bending at the hinge. We propose that a variety of subduction regimes are generated primarily as a product of two mechanisms. The first mechanism is that of the competition between the weight of the slab and the strength of the plate, which can be understood in terms of the applied bending moment, and this competition results in a particular radius of curvature (for which we provide a simple scaling theory). The second mechanism is the interaction between the slab and the more viscous lower mantle, which produces each regime's distinct slab morphology. Thus, the emergence of five distinct styles of subduction is a direct consequence of the presence of the modest barrier to flow into the lower mantle. Although only 2 of these styles presently operate on Earth, the possibility exists that other modes may have been the predominant mode in the past. Based on these models, we propose that the lithosphere is the primary factor in describing key elements of the plate tectonics system over time, rather than the convecting mantle. We discuss the various factors that may have influenced secular changes in Earth's tectonic behavior, some of which may have interesting consequences for the geochemical evolution of the Earth.

  11. The Pliny-Strabo trench region: A large shear zone resulting from slab tearing

    NASA Astrophysics Data System (ADS)

    Özbakır, Ali D.; Şengör, A. M. C.; Wortel, M. J. R.; Govers, R.

    2013-08-01

    The eastern part of the Hellenic subduction zone is composed of the Pliny and Strabo "trenches" that have been regarded as a zone of convergence between the subducting African lithosphere and the overriding Anatolian-Aegean plate. In the Pliny and Strabo "trenches", the oblique relative plate motion is generally thought to be accommodated by a typical strain partitioning consisting of strike-slip and convergence components. Notwithstanding the occurrence of strike-slip motion parallel with the Pliny-Strabo "trenches", trench-normal thrusting is not observed so far. Therefore, we conducted a detailed analysis to investigate the deformation mechanisms of the eastern part of the Hellenic Trench system. Our analyses of offshore faulting and mechanisms of earthquakes in the overriding Aegean lithosphere show that the region of the Pliny and Strabo "trenches" obeys the mechanics of the sinistral shear zone model of Tchalenko (1970). We propose that the trench perpendicular convergence is taken up by the Rhodes fold and thrust belt, which has been postulated off the southeast coast of Rhodes. Several regional P-wave tomography results give indications of a slow seismic anomaly under this zone, which is interpreted as a tear between the Hellenic and Cyprus subduction zones. The primary reason for such tear and its propagation is the ongoing rollback of the subducted part of the African lithosphere, also referred to as "the Aegean slab". Our work elucidates the surface expression of this tearing process in the form of the development of a shear zone between the Aegean lithosphere in the NW and the African lithosphere in the SE, the Pliny-Strabo Shear Zone.

  12. Estimation of Interplate Coupling in the Central and Eastern of Java trench from CGPS Observation

    NASA Astrophysics Data System (ADS)

    Irwan, Meilano; Endra, Gunawan; Susilo, Susilo; Joni, Effendi; Hasanuddin Z., Abidin; Dina, Sarsito

    2015-04-01

    We used three-component surface velocities in the Central and Eastern Java to estimate plate coupling on the subduction interface at the Java subduction zone. The observation period starting from 2009 to 2013 with more than 40 CGPS observation stations. The results show a heterogeneous distribution of interplate coupling from Central to the East of Java trench. Strong coupling at a depth of 10-30 km with rate 3-4 cm/yr is estimated at the south of East Java. The downdip limits of the coupled areas is estimated at 50km. This slip deficit on subduction interface has important implication for seismic hazard of Java Island.

  13. Temperature Models for the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Manea, V. C.; Kostoglodov, V.; Currie, C.; Manea, M.; Wang, K.

    2002-12-01

    It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by a temperature. Therefore, we have developed four two-dimensional steady state thermal models for Jalisco, Guerrero, Oaxaca and Chiapas. The updip limit of the seismogenic zone is taken between 100 C and 150 C, while the downdip limit is thought to be at 350 C because of the transition from stick-slip to stable-sliding. The shape of the subducting plate is inferred from gravity and seismicity. The convergence velocity between oceanic and continental lithospheric plates is taken as the following: 5 cm/yr for Jalisco profile, 5.5 for Guerrero profile, 5.8 for Oaxaca profile, and 7.8 for Chiapas profile. The age of the subducting plates, since they are young, and provides the primary control on the forearc thermal structure, are as the following: 11 My for Jalisco profile, 14.5 My for Guerrero profile, 15 My for Oaxaca profile, and 28 My for Chiapas profile. We also introduced in the models a small quantity of frictional heating (pore pressure ratio 0.98). The value of 0.98 for pore pressure ratio was obtained for the Guerrero profile, in order to fit the intersection between the 350 C isotherm and the subducting plate at 200 Km from trench. The value of 200 km coupling zone from trench is inferred from GPS data for the steady interseismic period and also for the last slow aseismic slip that occurred in Guerrero in 2002. We have used this value of pore pressure ratio (0.98) for all the other profiles. For the others three profiles we obtained the following coupling extents: Jalisco - 100 km, Oaxaca - 170 km and Chiapas - 125 km (from the trench). Independent constrains of the thermal models come from the surface thermal measurements (offshore - Prol-Ledesma et al (1989) and onshore - Ziagos et al. (1985)). Unfortunately these measurements are very sparse, and present an important dispersion and have large uncertainties. In our models, all profiles show a decrease in heat flow from the trench towards the continent, which is characteristic for subduction zones. We also have included a mantle wedge flow current in order to keep a constant depth to the lithosphere-asthenosphere boundary. This mantle wedge convection provides an increase in heat flow near the volcanic arc which is consistent with the surface observations. Our models indicate that the seismogenic zone in Mexico comprised between 100 C and 350 C is in good agreement with the coseismic rupture width inferred from the megathrust earthquake aftershocks and seismic models of rupture. These thermal models will be used to calculate the thermal stresses induced by the subducting plate.

  14. Collapse of the northern Jalisco continental slope:Subduction erosion, forearc slivering, or subduction beneath the Tres Marias escarpment?

    NASA Astrophysics Data System (ADS)

    Bandy, W. L.; Mortera-Gutierrez, C. A.; Ortiz-Zamora, G.; Ortega-Ramirez, J.; Galindo Dominguez, R. E.; Ponce-Nez, F.; Prez-Caldern, D.; Rufino-Contreras, I.; Valle-Hernndez, S.; Prez-Gonzlez, E.

    2010-12-01

    The Jalisco subduction zone exhibits several interesting characteristics. Among these is that convergence between the Rivera and North American plate is highly oblique, especially north of 20N, the obliquity progressively increasing to the NW. By analogy to other better studied subduction zones, this distribution of forces should produce a NW-SE extension in the overriding plate, especially north of 20N. This has led to the proposal that the trench perpendicular Bahia de Banderas is an expression of this extension [Kostoglodov and Bandy, JGR, vol. 100, 1995]. To further investigate this proposal, multibeam bathymetric data and seafloor backscatter images, seismic reflection sub-bottom profiles and marine magnetic data were collected during the MORTIC08 campaign of the B.O. EL PUMA in March 2009. The bathymetric data provides for 100% coverage (20 to 200 meter spacing of the actual measured depth value depending on the water depth) of the continental slope and trench areas north of 20N. These data indicate that a marked change occurs in the morphology of the continental slope at 20N. To the north the slope consists of a broad, fairly flat plain lying between a steep lower inner trench slope to the west and a steep, concave seaward, escarpment to the east. In contrast, to the south the continental slope exhibits a more gradual deepening until the steep lower inner trench slope. A prominent submarine canyon deeply incises the continental slope between these two morphotectonic domains. This canyon appears to represent the boundary between two NW-SE diverging forearc blocks or slivers, consistent with the presence of oblique convergence. In contrast, the broad, fairly flat plain is better explained by subsidence induced by subduction erosion (i.e. erosion of the base of the overriding plate underneath the continental slope area). The shoaling of the trench axis northward towards the Puerto Vallarta Graben and subsequent deepening may be related to subduction of the Rivera Plate beneath the Tres Marias Escarpment.

  15. Oceanic plate weakened by flexural bending-induced faulting in the outer rise region of the Mariana subduction zone

    NASA Astrophysics Data System (ADS)

    Zhang, F.; Lin, J.; Zhan, W.

    2013-12-01

    Strong flexural bending near trenches could significantly weaken oceanic plates through development of trench-parallel extensional normal faults. We assessed the oceanic plate weakening near the outer rise region of the Mariana subduction zone by analyzing and modeling the plate deformation caused by flexural bending. We first obtained a 3-D deformation surface of the subducting plate by removing from seafloor bathymetry the topographic effects of sediments, seamounts, and age-related thermal subsidence. We then calculated theoretical models of plate deformation and inverted for along-trench changes in the vertical force and bending moment at the trench axis, as well as spatial variations in the effective elastic thickness of the subducting plate, that best explain the observations. We found that to replicate simultaneously the observed steep slope of the seafloor near the trench axis and the long-wavelength flexural profiles seaward of the outer rise region, the effective elastic thickness of the plate must change significantly. The best-fitting models reveal that the effective elastic thickness is about 45-55 km seaward of the outer rise (TeMax), but is reduced to only 19-40 km trench-ward of the outer rise region (TeMin); the transition from TeMax to TeMin occurs at Xr =70-120 km away from the trench axis. The resultant reduction in the calculated effective elastic thickness, i.e., 1 - (TeMin /TeMax), is in the range of 20-60%, being the greatest near the Challenger Deep area, where the plate deforms significantly within a narrow distance from the trench axis and the trench axis is the deepest. Our results revealed that reduction in Te along the Mariana trench does not exceed 60%, implying that an elastic core remains in the subducting plate despite pervasive faulting caused by flexural bending near the trench axis.

  16. Subduction Initiation Along the Macquarie Ridge Complex?

    NASA Astrophysics Data System (ADS)

    Coffin, M. F.; Meckel, T. P.; Meckel, T. P.; Mosher, S.; Mosher, S.; Massell, C.; Daczko, N.; Daczko, N.; Wertz, K.; Wertz, K.; Bernadel, G.; Symonds, P.

    2001-12-01

    The Macquarie Ridge Complex (MRC) extends for ca. 1500 km between New Zealand's South Island and the Indian-Antarctic-Pacific triple junction, and comprises the boundary between the Australian (Indian) and Pacific plates. Motion along this boundary has changed from divergence to dominantly strike slip, with areas of transpression and transtension, since ca. 10 Ma. The arcuate MRC displays unique bathymetry among submarine ridges worldwide, with four distinct segments (from north to south, Puysegur, McDougall, Macquarie, and Hjort) characterized by alternating ridge-trough polarity. A major fault zone on the crest or flank of the bathymetric ridge is continuous along the entire length of the MRC. Intermediate depth earthquakes, compressional focal mechanisms, and a single, small calc-alkaline volcano (Solander Island) suggest that subduction may be initiating in southernmost New Zealand (Fiordland) and the Puysegur region. Marine geophysical data show one or more major thrust faults along the Puysegur trough in addition to the strike slip fault along the Puysegur ridge, implying strain partitioning. The McDougall and Macquarie segments are characterized by shallow focus earthquakes and strike slip focal mechanisms; the ridges and troughs may be explained by past thrust faulting, but any evidence for subduction or initiation thereof is absent. In the Hjort region, a well-developed trench complements the crestal fault zone, again implying strain partitioning, although all earthquakes appear to be shallow. Unsampled seamounts paralleling the Hjort trench and ridge may be related to subduction, or may have been produced by hotspot activity. The morphology of the MRC integrates the changes in relative motion between the Australian (Indian) and Pacific plates since ca. 10 Ma, and the MRC presents a case study for possible models of subduction initiation.

  17. Arc-arc collision ongoing in the southernmost part of the Kuril trench region revealed from integrated analyses of the 1998-2000 Hokkaido Transect seismic data

    NASA Astrophysics Data System (ADS)

    Iwasaki, T.; Tsumura, N.; Ito, T.; Sato, H.; Kurashimo, E.; Hirata, N.; Arita, K.; Noda, K.; Fujiwara, A.; Abe, S.; Kikuchi, S.; Suzuki, K.

    2014-12-01

    The oblique subduction of the Pacific plate beneath the southernmost part of the Kuril trench is generating a unique tectonic environment in the Hokkaido Island, Japan. In this area, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc from the east at the time of middle Miocene to form the Hidaka collision zone (HCZ). This collision has been acting as a responsible factor for the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and the development of the thick foreland fold-and-thrust belt. A multi-disciplinary project of the 1998-2000 Hokkaido Transect, crossing the northern part of the HCZ in EW direction, collected high-quality seismic data on a 227-km seismic refraction/wide-angle reflection profile and three seismic reflection lines. Reprocessing/reinterpretation for this data set revealed detailed collision structure ongoing in the northern part of the HCZ. The westward obduction of the Kuril arc crust was clearly imaged along the HMT. This obduction starts at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). In the west of the HMT, we recognize the gently eastward dipping structure, representing the fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT. The most important finding from our reprocessing is a series of reflection events at a 30-45 km depth below the obducted Kuril arc crust, which probably correspond to the lower crust/Moho within the NE Japan arc descending down to the east under the collision zone. The wide-angle reflection data indicate that the subducted NE Japan arc meets the Kuril arc 30-40 km east of the HMT at a depth of 30 km. This structural geometry well explained a weak but coherent seismic phase observed at far offsets (120-180 km) on the wide-angle reflection line. The obtained structure shows the complicated collision style where the upper 30-km Kuril arc crust is thrust up with significant deformation. At the moment, we cannot find out the strong evidence of crustal delamination. This is in a marked contrast with the case of the southern part of the HCZ, where the upper 23-km crust is obducted at about 20 km distance from the HMT, while the remaining lower crust is descending down to the subducted Pacific plate.

  18. Friction and stress coupling on the subduction interfaces

    NASA Astrophysics Data System (ADS)

    Tan, E.; Lavier, L.; van Avendonk, H.

    2011-12-01

    At a subduction zone, the down-going oceanic plate slides underneath the overriding plate. The frictional resistance to the relative motion between the plates generates great earthquakes along the subduction interface, which can cause tremendous damage in the civil life and property. There is a strong incentive to understand the frictional strength of the subduction interface. One fundamental question of mechanics of subuction is the degree of coupling between the plates, which is linked to the size of earthquakes. It has been noted that the trench-parallel (along-strike) gravity variation correlates positively with the trench-parallel topography anomaly and negatively with the activity of great earthquake (Song and Simons, 2003). Regions with a negative trench-parallel gravity anomaly are more likely to have great earthquakes. The interpretation of such correlation is that strong coupling along subduction interface will drag down the for-arc region of the overriding plate, which generates the gravity and topography anomalies, and could store more strain energy to be released during a great earthquake. We developed a 2D numerical thermo-mechanical code for modeling subduction. The numerical method is based on an explicit finite element method similar to the Fast Lagrangian Analysis of Continua (FLAC) technique. The constitutive law is visco-elasti-plastic with strain weakening. The cohesion and friction angle are reduced with increasing plastic strain after yielding. To track different petrologic phases, Lagrangian particles are distributed in the domain. Basalt-eclogite, sediment-schist and peridotite-serpentinite phase changes are included in the model. Our numerical models show that the degree of coupling negatively correlates with the coefficient of friction. In the low friction case, the subduction interface has very shallow dipping angle, which helps to elastically couple the downing plate with the overriding plate. The topography and gravity anomalies of the low friction case also indicate strong coupling between plates.

  19. Intrinsic and Extrinsic Factors in Subduction Dynamics

    NASA Astrophysics Data System (ADS)

    Billen, Magali; Arredondo, Katrina

    2014-05-01

    Since the realization that tectonic plates sink into the mantle, in a process we now call subduction, our understanding of this process has improved dramatically through the combined application of observations, theory and modeling. During that time independent research groups focusing on different aspects of subduction have identified factors with a significant impact on subduction, such as three-dimensionality, slab rollback, rheology of the slab and mantle and magnitude of phase changes. However, as each group makes progress we often wonder how these different factors interact as we all strive to understand the real world subduction system. These factors can be divided in two groups: intrinsic factors, including the age of the slab, its thermal structure, composition, and rheology, and extrinsic factors including others forces on plates, overall mantle flow, structure of the overriding plate, rheology of the mantle and phase changes. In addition, while modeling has been a powerful tool for understanding subduction, all models make important (but often necessary) approximations, such as using two dimensions, imposed boundary conditions, and approximations of the conservation equations and material properties. Here we present results of a study in which the "training wheels" are systematically removed from 2D models of subduction to build a more realistic model of subduction and to better understand how combined effects of intrinsic and extrinsic factors contribute to the dynamics. We find that a change from the Boussinesq to the extended Boussinesq form of the conservation equations has a dramatic effect on slab evolution in particular when phase changes are included. Allowing for free (dynamically-driven) subduction and trench motion is numerically challenging, but also an important factor that allows for more direct comparison to observations of plate kinematics. Finally, compositional layering of the slab and compositionally-controlled phase changes also have a strong effect on the rate of subduction and small-scale buckling and folding of the slab. These studies suggest that the evolution of slabs can differ significantly from more simplified models, and therefore a better understanding of the underlying physical controls on slab dynamics requires more realistic models.

  20. Phoenix's La Mancha Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This false color image, taken by NASA's Phoenix Mars Lander's Surface Stereo Imager, was taken on the 131st Martian day, or sol, of the mission (Oct. 7, 2008). The image shows color variations of the trench, informally named 'La Mancha,' and reveals the ice layer beneath the soil surface. The trench's depth is about 5 centimeters deep.

    The color outline of the shadow at the bottom of the image is a result of sun movement with the combined use of infrared, green, and blue filters.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  1. Phoenix's 'Dodo' Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This image was taken by NASA's Phoenix Mars Lander's Robotic Arm Camera (RAC) on the ninth Martian day of the mission, or Sol 9 (June 3, 2008). The center of the image shows a trench informally called 'Dodo' after the second dig. 'Dodo' is located within the previously determined digging area, informally called 'Knave of Hearts.' The light square to the right of the trench is the Robotic Arm's Thermal and Electrical Conductivity Probe (TECP). The Robotic Arm has scraped to a bright surface which indicated the Arm has reached a solid structure underneath the surface, which has been seen in other images as well.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  2. Snow White Trenches

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This image was acquired by NASA's Phoenix Mars Lander's Surface Stereo Imager on the 25th Martian day of the mission, or Sol 24 (June 19, 2008), after the May 25, 2008, landing. This image shows the trenches informally called 'Snow White 1' (left) and 'Snow White 2' (right). The trench is about 5 centimeters (2 inches) deep and 30 centimeters (12 inches) long.

    'Snow White' is located in a patch of Martian soil near the center of a polygonal surface feature, nicknamed 'Cheshire Cat.' The 'dump pile' is located at the top of the trench, the side farthest away from the lander, and has been dubbed 'Croquet Ground.' The digging site has been named 'Wonderland.'

    This image has been enhanced to brighten shaded areas.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  3. Three-dimensional dynamic laboratory models of subduction with an overriding plate and variable interplate rheology

    NASA Astrophysics Data System (ADS)

    Duarte, Joo C.; Schellart, Wouter P.; Cruden, Alexander R.

    2013-10-01

    Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flow force the OP to deform. We present results of 3-D dynamic laboratory models of subduction that include an OP. We introduce new interplate materials comprising homogeneous mixtures of petrolatum and paraffin oil to achieve progressive subduction. The rheology of these mixtures is characterized by measurements using a strain rate controlled rheometer. The results show that the strength of the mixture increases with petrolatum content, which can be used as a proxy for the degree of mechanical coupling along the subduction interface. Results of subduction experiments are presented with different degrees of mechanical coupling and the influence this has on the dynamics and kinematics of subduction. The modelling results show that variations in the degree of mechanical coupling between the plates have a major impact on subduction velocities, slab geometry and the rate of OP deformation. In all experiments the OP is displaced following trench migration and experiences overall extension localized in the plate interior. This suggests that OP deformation is driven primarily by the toroidal component of subduction-related mantle return flow. The subduction rate is always very slow in experiments with medium mechanical coupling, and subduction stops prematurely in experiments with very high coupling. This implies that the shear forces along the plate interface in natural subduction zone systems must be relatively low and do not vary significantly. Otherwise a higher variability in natural subduction velocities should be observed for mature, non-perturbed subduction zones. The required low shear force is likely controlled by the rheology of highly hydrated sedimentary and basaltic rocks.

  4. Seismic anisotropy around subduction zones: Insights from three-dimensional modeling of upper mantle deformation and SKS splitting calculations

    NASA Astrophysics Data System (ADS)

    Faccenda, M.; Capitanio, F. A.

    2013-01-01

    Inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive. To assist the interpretation of the seismic signal and its relation with the mantle circulation pattern, we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3-D numerical models of Earth-like subduction. This computational strategy accounts for the non-steady-state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered. In the subduction models, a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate. The sub-slab upper mantle is characterized by two domains with different fabrics: at shallow depth, the mantle entrained with the subducting slab develops trench-perpendicular directed anisotropy due to simple shear deformation, while in the deeper mantle, slab rollback induces pure shear deformation causing trench-parallel extension and fast seismic directions. Subducting plate advance favors the development of the fabric in the entrained mantle domain, while slab retreat increases the trench-parallel anisotropy in the deeper upper mantle. In the deeper domain, the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center. As such, strong trench-parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates. The synthetic SKS splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain, with trench-perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges. Instead, above relatively narrow, retreating slabs (≤600 km and low subduction partitioning ratio [SPR]), azimuths are trench parallel due to the strong anisotropy in the lower sub-slab domain. In all models, the anisotropy in the back arc and on the sides of the subducting plate is, respectively, trench perpendicular and sub-parallel to the return flow at depth. Results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally.

  5. Water-rich bending faults at the Middle America Trench

    NASA Astrophysics Data System (ADS)

    Naif, Samer; Key, Kerry; Constable, Steven; Evans, Rob L.

    2015-09-01

    The portion of the Central American margin that encompasses Nicaragua is considered to represent an end-member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge.

  6. New Seafloor Map of the Puerto Rico Trench Helps Assess Earthquake and Tsunami Hazards

    NASA Astrophysics Data System (ADS)

    ten Brink, Uri; Danforth, William; Polloni, Christopher; Andrews, Brian; Llanes, Pilar; Smith, Shepard; Parker, Eugene; Uozumi, Toshihiko

    2004-09-01

    The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure 1). The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S. Virgin Islands, where 4 million U.S. citizens reside. Widespread damage in Puerto Rico and Hispaniola from an earthquake in 1787 was estimated to be the result of a magnitude 8 earthquake north of the islands. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918. Large landslide escarpments have been mapped on the seafloor north of Puerto Rico, although their ages are unknown. The Puerto Rico Trench is atypical of oceanic trenches. Subduction is highly oblique (10-20) to the trench axis with a large component of left-lateral strike-slip motion. Similar convergence geometry is observed at the Challenger Deep in the Mariana Trench, the deepest point on Earth. In addition to its extremely deep seafloor, the Puerto Rico Trench is also characterized by the most negative free-air gravity anomaly on Earth, -380 mGal, located 50 km south of the trench, where water depth is 7950 m (Figure 2). A tilted carbonate platform provides evidence for extreme vertical tectonism in the region. This platform was horizontally deposited over Cretaceous to Paleocene arc rocks starting in the Late Oligocene. Then, at 3.5 Ma, the carbonate platform was tilted by 4 toward the trench over a time period of less than 40 kyr, such that its northern edge is at a depth of 4000 m and its reconstructed elevation on land in Puerto Rico is at +1300 m (Figures 1 and 2).

  7. An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala ( Pacific).

    USGS Publications Warehouse

    Ambos, E.L.; Hussong, D.M.; Holman, C.E.

    1985-01-01

    Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench. Approximately 170 events were recorded by 3 or more seismometers, and almost half were located with statistical hypocentral errors of <10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone NW of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Most magnitudes ranged between 3.0 and 4.0 mb, and the threshold magnitude of locatable events was about 2.8 mb. Two distinct composite focal mechanisms were determined. One appears to indicate high- angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone NW of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. Projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling.-from Authors

  8. Nationwide tsunami hazard assessment project in Japan

    NASA Astrophysics Data System (ADS)

    Hirata, K.; Fujiwara, H.; Nakamura, H.; Osada, M.; Ohsumi, T.; Morikawa, N.; Kawai, S.; Aoi, S.; Yamamoto, N.; Matsuyama, H.; Toyama, N.; Kito, T.; Murashima, Y.; Murata, Y.; Inoue, T.; Saito, R.; Akiyama, S.; Korenaga, M.; Abe, Y.; Hashimoto, N.

    2014-12-01

    In 2012, we began a project of nationwide Probabilistic Tsunami Hazard Assessment (PTHA) in Japan to support various measures (Fujiwara et al., 2013, JpGU; Hirata et al., 2014, AOGS). The most important strategy in the nationwide PTHA is predominance of aleatory uncertainty in the assessment but use of epistemic uncertainty is limited to the minimum, because the number of all possible combinations among epistemic uncertainties diverges quickly when the number of epistemic uncertainties in the assessment increases ; we consider only a type of earthquake occurrence probability distribution as epistemic uncertainty. We briefly show outlines of the nationwide PTHA as follows; (i) we consider all possible earthquakes in the future, including those that the Headquarters for Earthquake Research Promotion (HERP) of Japanese Government, already assessed. (ii) We construct a set of simplified earthquake fault models, called "Characterized Earthquake Fault Models (CEFMs)", for all of the earthquakes by following prescribed rules (Toyama et al., 2014, JpGU; Korenaga et al., 2014, JpGU). (iii) For all of initial water surface distributions caused by a number of the CEFMs, we calculate tsunamis by solving a nonlinear long wave equation, using FDM, including runup calculation, over a nesting grid system with a minimum grid size of 50 meters. (iv) Finally, we integrate information about the tsunamis calculated from the numerous CEFMs to get nationwide tsunami hazard assessments. One of the most popular representations of the integrated information is a tsunami hazard curve for coastal tsunami heights, incorporating uncertainties inherent in tsunami simulation and earthquake fault slip heterogeneity (Abe et al., 2014, JpGU). We will show a PTHA along the eastern coast of Honshu, Japan, based on approximately 1,800 tsunami sources located within the subduction zone along the Japan Trench, as a prototype of the nationwide PTHA. This study is supported by part of the research project on research on evaluation of hazard and risk of natural disasters, under the direction of the HERP of Japanese Government.

  9. Core-log integration for rock mechanics using borehole breakouts and rock strength experiments: Recent results from plate subduction margins

    NASA Astrophysics Data System (ADS)

    Saito, S.; Lin, W.

    2014-12-01

    Core-log integration has been applied for rock mechanics studies in scientific ocean drilling since 2007 in plate subduction margins such as Nankai Trough, Costa Rica margin, and Japan Trench. State of stress in subduction wedge is essential for controlling dynamics of plate boundary fault. One of the common methods to estimate stress state is analysis of borehole breakouts (drilling induced borehole wall compressive failures) recorded in borehole image logs to determine the maximum horizontal principal stress orientation. Borehole breakouts can also yield possible range of stress magnitude based on a rock compressive strength criterion. In this study, we constrained the stress magnitudes based on two different rock failure criteria, the Mohr-Coulomb (MC) criteria and the modified Wiebols-Cook (mWC) criteria. As the MC criterion is the same as that under unconfined compression state, only one rock parameter, unconfined compressive strength (UCS) is needed to constrain stress magnitudes. The mWC criterion needs the UCS, Poisson's ratio and internal frictional coefficient determined by triaxial compression experiments to take the intermediate principal stress effects on rock strength into consideration. We conducted various strength experiments on samples taken during IODP Expeditions 334/344 (Costa Rica Seismogenesis Project) to evaluate reliable method to estimate stress magnitudes. Our results show that the effects of the intermediate principal stress on the rock compressive failure occurred on a borehole wall is not negligible.

  10. Spirit Shadow over Laguna Trench

    NASA Technical Reports Server (NTRS)

    2004-01-01

    NASA's Mars Exploration Rover Spirit casts a shadow over the trench that the rover is examining with tools on its robotic arm. Spirit took this image with its front hazard-avoidance camera on Feb. 21, 2004, during the rover's 48th martian day, or sol. It dug the trench with its left front wheel the preceding sol. Plans call for Spirit to finish examining the trench on sol 50.

  11. Trench Left By Sampler Scoop

    NASA Technical Reports Server (NTRS)

    1976-01-01

    A shallow 12-inch-long trench was dug by Viking 2 s surface sampler scoop yesterday (September 12) on Mars. The trench is difficult to see in this photo because it is in the shadow of a rock (out of view to the right). The sampler scoop stopped operating sometime after soil was excavated from the trench and delivered to Viking 2 s biology instrument.

  12. Opening and closing slab windows in congested subduction zones

    NASA Astrophysics Data System (ADS)

    Moresi, Louis

    2013-04-01

    Subduction zones often try to swallow buoyant material which is embedded in the oceanic lithosphere: plume material or hotspot residues, oceanic plateaux, and fragments of continental material. This often results in the formation of a slab window and it has been shown (Mason et al, 2010; Betts et al, 2012) that this window strongly influences the subsequent evolution of the slab and the advance/retreat rate of the trench. The buoyant material typically pushes the trench into a local state of advance, and the creation of the slab window allows the rest of the trench to retreat as the mantle behind the slab flows in through the window. This situation is inherently unstable: if the buoyancy anomaly is finite in size, then the retreating trench will soon move behind the anomaly and juxtapose negatively buoyant oceanic lithosphere with active subduction. This creates the potential to close the slab window and, in doing so, transfer the buoyant material to the over-riding plate. Models show that this closure of the window initially occurs through a lateral rollback process followed by a catastrophic re-initiation of subduction behind the colliding buoyant anomaly. This rollback leaves a characteristic, tightly rolled remnant in the mantle and significant rotation in the over-riding plate and the newly-docked block. The over-riding plate is thrown into extension perpendicular to the original orientation of the trench. This same situation applies at the late-stages of a closing ocean due to the passive margin geometry and the presence of debris collected from the closing ocean floor and it seems likely that these models can also be applied to the complicated geometry of subduction in such environments. Mason, W. G.; Moresi, L.; Betts, P. G. & Miller, M. S. Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones Tectonophysics, 2010, 483, 71-79 P. Betts, W. Mason, L. Moresi, The influence of mantle plumes on subduction zone dynamics, Geology, 40, 739-742 (2012)

  13. Magnitude and location of historical earthquakes in Japan and implications for the 1855 Ansei Edo earthquake

    USGS Publications Warehouse

    Bakun, W.H.

    2005-01-01

    Japan Meteorological Agency (JMA) intensity assignments IJMA are used to derive intensity attenuation models suitable for estimating the location and an intensity magnitude Mjma for historical earthquakes in Japan. The intensity for shallow crustal earthquakes on Honshu is equal to -1.89 + 1.42MJMA - 0.00887?? h - 1.66log??h, where MJMA is the JMA magnitude, ??h = (??2 + h2)1/2, and ?? and h are epicentral distance and focal depth (km), respectively. Four earthquakes located near the Japan Trench were used to develop a subducting plate intensity attenuation model where intensity is equal to -8.33 + 2.19MJMA -0.00550??h - 1.14 log ?? h. The IJMA assignments for the MJMA7.9 great 1923 Kanto earthquake on the Philippine Sea-Eurasian plate interface are consistent with the subducting plate model; Using the subducting plate model and 226 IJMA IV-VI assignments, the location of the intensity center is 25 km north of the epicenter, Mjma is 7.7, and MJMA is 7.3-8.0 at the 1?? confidence level. Intensity assignments and reported aftershock activity for the enigmatic 11 November 1855 Ansei Edo earthquake are consistent with an MJMA 7.2 Philippine Sea-Eurasian interplate source or Philippine Sea intraslab source at about 30 km depth. If the 1855 earthquake was a Philippine Sea-Eurasian interplate event, the intensity center was adjacent to and downdip of the rupture area of the great 1923 Kanto earthquake, suggesting that the 1855 and 1923 events ruptured adjoining sections of the Philippine Sea-Eurasian plate interface.

  14. Snow White Trench (Animation)

    NASA Technical Reports Server (NTRS)

    2008-01-01

    [figure removed for brevity, see original site] Click on image for animation

    This animation shows the evolution of the trench called 'Snow White' that NASA's Phoenix Mars Lander began digging on the 22nd Martian day of the mission after the May 25, 2008, landing.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  15. Earthquake Recurrence along the Kuril Trench: A New View from Paleoseismology

    NASA Astrophysics Data System (ADS)

    Satake, K.; Nanayama, F.

    2003-12-01

    Paleoseismological data along the Pacific coast of eastern Hokkaido indicate that unusual earthquakes have repeated at about 500 year interval with the most recent event in the 17th century. Along the Kuril trench, interplate earthquakes with rupture length of 100-200 km occurred in 1952 (Mw 8.1) and 1973 (Mw 7.8), as well as 1843 (M 8.0) and 1894 (M 7.9), which have been considered characteristics of this subduction zone. We review paleoseismological data, examine coastal deformation and tsunami inundation from fault models, and propose a model of earthquake recurrence in the Kuril subduction zone. Pleistocene marine terraces on the Pacific coast show slight net uplift, at an average of 0.1-0.4 mm/yr in the past several hundred thousand years, whereas tide-gauge data show gradual subsidence of 8-9 mm/yr since 1900. Infrequent unusual event (Armageddon) has been inferred (Ikeda, 1996) to resolve this conflict. Holocene stratigraphic and microfossil studies have indicated sea-level changes in the last 3 ka (e.g., Sawai, 2001). Each event is marked by an abrupt upward change from brackish bay deposits to freshwater peat. The youngest change has been dated in the 17th century with an estimated uplift amount of 0.5-1m (Atwater et al., 2003). Such evidence has been found along the 100 km long coast and recurred up to seven times in the last 2.5 ka (Kelsey et al., 2002). Extensive tsunami deposits indicate large prehistoric tsunamis (Nanayama et al., 2003). At Kiritappu, for instance, sand sheets extend 3 km inland, much further than historic tsunamis. Ten sheets of tsunami deposits indicate recurrence of such unusual tsunami with an average recurrence interval of about 500 years. The most recent event occurred in the 17th century. Historic documents in Honshu rules out unusual tsunamis that would cause damage along the Sanriku coast. Tsunami damage from the 1611 and 1677 earthquakes, both along the Japan trench, have been documented along the Sanriku coast. We modeled and examined three types of earthquakes: Armageddon, interplate events, and tsunami earthquakes. The fault extends down to 85 km depth in the Armageddon model, and would cause the coastal uplift. Interplate earthquake fault, down to 50 km depth, would cause slight subsidence of coast. The ocean bottom deformation from the tsunami earthquakes is limited near the trench axis. We also varied fault length along the trench axis as 200 km (single segment) and 300 km (multi-segment). Tsunami numerical modeling from these fault models calculates coastal tsunami heights for the Hokkaido and Honshu coasts and inundation for selected sites where the tsunami deposits were mapped. Only multi-segment fault can explain the tsunami deposits and lack of documented damage on Sanriku coast. The coastal uplift seems to be caused by postseismic deformation along the deeper extent of such infrequent multi-segment interplate earthquakes. To explain the observed uplift, however, postseismic slip larger than the coseismic slip in seismogenic zone is needed.

  16. Developing framework to constrain the geometry of the seismic rupture plane on subduction interfaces a priori - A probabilistic approach

    USGS Publications Warehouse

    Hayes, G.P.; Wald, D.J.

    2009-01-01

    A key step in many earthquake source inversions requires knowledge of the geometry of the fault surface on which the earthquake occurred. Our knowledge of this surface is often uncertain, however, and as a result fault geometry misinterpretation can map into significant error in the final temporal and spatial slip patterns of these inversions. Relying solely on an initial hypocentre and CMT mechanism can be problematic when establishing rupture characteristics needed for rapid tsunami and ground shaking estimates. Here, we attempt to improve the quality of fast finite-fault inversion results by combining several independent and complementary data sets to more accurately constrain the geometry of the seismic rupture plane of subducting slabs. Unlike previous analyses aimed at defining the general form of the plate interface, we require mechanisms and locations of the seismicity considered in our inversions to be consistent with their occurrence on the plate interface, by limiting events to those with well-constrained depths and with CMT solutions indicative of shallow-dip thrust faulting. We construct probability density functions about each location based on formal assumptions of their depth uncertainty and use these constraints to solve for the ‘most-likely’ fault plane. Examples are shown for the trench in the source region of the Mw 8.6 Southern Sumatra earthquake of March 2005, and for the Northern Chile Trench in the source region of the November 2007 Antofagasta earthquake. We also show examples using only the historic catalogues in regions without recent great earthquakes, such as the Japan and Kamchatka Trenches. In most cases, this method produces a fault plane that is more consistent with all of the data available than is the plane implied by the initial hypocentre and CMT mechanism. Using the aggregated data sets, we have developed an algorithm to rapidly determine more accurate initial fault plane geometries for source inversions of future earthquakes.

  17. Velocities of Subducted Sediments and Continents

    NASA Astrophysics Data System (ADS)

    Hacker, B. R.; van Keken, P. E.; Abers, G. A.; Seward, G.

    2009-12-01

    The growing capability to measure seismic velocities in subduction zones has led to unusual observations. For example, although most minerals have VP/ VS ratios around 1.77, ratios <1.7 and >1.8 have been observed. Here we explore the velocities of subducted sediments and continental crust from trench to sub-arc depths using two methods. (1) Mineralogy was calculated as a function of P & T for a range of subducted sediment compositions using Perple_X, and rock velocities were calculated using the methodology of Hacker & Abers [2004]. Calculated slab-top temperatures have 3 distinct depth intervals with different dP/dT gradients that are determined by how coupling between the slab and mantle wedge is modeled. These three depth intervals show concomitant changes in VP and VS: velocities initially increase with depth, then decrease beyond the modeled decoupling depth where induced flow in the wedge causes rapid heating, and increase again at depth. Subducted limestones, composed chiefly of aragonite, show monotonic increases in VP/ VS from 1.63 to 1.72. Cherts show large jumps in VP/ VS from 1.55-1.65 to 1.75 associated with the quartz-coesite transition. Terrigenous sediments dominated by quartz and mica show similar, but more-subdued, transitions from ~1.67 to 1.78. Pelagic sediments dominated by mica and clinopyroxene show near-monotonic increases in VP/ VS from 1.74 to 1.80. Subducted continental crust that is too dry to transform to high-pressure minerals has a VP/ VS ratio of 1.68-1.70. (2) Velocity anisotropy calculations were made for the same P-T dependent mineralogies using the Christoffel equation and crystal preferred orientations measured via electron-backscatter diffraction for typical constituent phases. The calculated velocity anisotropies range from 5-30%. For quartz-rich rocks, the calculated velocities show a distinct depth dependence because crystal slip systems and CPOs change with temperature. In such rocks, the fast VP direction varies from slab-normal at shallow depths through trench-parallel at moderate depths to down-dip approaching sub-arc depths. Vertically incident waves have VP/ VS of 1.7-1.3 over the same range of depths, waves propagating up dip have VP/ VS of 1.7-1.3, and waves propagating along the slab at constant depth have VP/ VS of 1.7-1.45. These remarkably low VP/ VS ratios are due to the anomalous elastic behavior of quartz. More aluminous lithologies have elevated VP/ VS ratios: 1.85 for slab-normal waves, 1.75 for trench-parallel waves, and 1.65 for down-dip waves. Subducted continental crust that is too dry to transform to high-pressure minerals has relatively ordinary VP/ VS ratio of 1.71-1.75 for vertically incident waves, 1.6-1.7 for waves propagating up dip, and 1.65-1.75 for waves propagating along the slab. Thus, subducted mica-rich sediments can have high VP/ VS ratios, whereas quartzose lithologies generate low VP/ VS ratios.

  18. An oceanic plateau subduction offshore Eastern Java

    NASA Astrophysics Data System (ADS)

    Shulgin, A.; Kopp, H.; Mueller, C.; Planert, L.; Lueschen, E.; Flueh, E. R.; Djajadihardja, Y.

    2010-12-01

    The area offshore Java represents one of a few places globally where the early stage of subduction of an oceanic plateau is observed. We study the little investigated Roo Rise oceanic plateau on the Indian plate, subducting beneath Eurasia.Our study area is located south of eastern Java and covers the edge of the Roo Rise plateau, the Java trench and the entire forearc section. For the first time the detailed deep structure of the Roo Rise is studied, subduction of which has a significant effect on the forearc dynamics and evolution and the increase of the geohazards risks. The tsunamogenic earthquakes of 1994 and 2006 are associated with the oceanic plateau edge been subducted. We present integrated results of a refraction/wide-angle reflection tomography, gravity modeling, and multichannel reflection seismic imaging using data acquired in 2006 along a corridor centered around 113E and composed of a 340 km long N-S profile and a 130 km long E-W oriented profile. The composite structural models reveal the previously unresolved deep geometry of the collision zone and the structure of the oceanic plateau. The crustal thickness of the Roo Rise plateau is ranging from 18 to 12 km. The structure of the upper crust of the incoming oceanic plate shows the extreme degree of fracturing in its top section, and is associated with a plate bending. The forearc Moho has a depth range from 16 to 20 km. The gravity modeling requires a sharp crustal thickness increase below Java. Within our profiles we do not recover any direct evidence for the presence of the bathymetric features on the oceanic plate currently present below the accretionary prism, responsible for the tsunamogenic earthquake triggering. However vertical variations of the forearc basement edge are observed on the trench-parallel profile, which opens a discussion on the origin of such basement undulations, together with a localized patchy uplift character of the forearc high.The complex geometry of the backstop suggests two models for the structural formation within this segment of the margin. The subducting plateau is affecting the stress field within the accretionary complex and the backstop edge, which favors the initiation of large, potentially tsunamogenic earthquakes such as the 1994 Mw=7.8 tsunamogenic event.

  19. On the importance of plumes to initiate subduction and plate tectonics

    NASA Astrophysics Data System (ADS)

    Davaille, Anne

    2015-04-01

    Understanding the details of plate failure and the initiation of subduction remains a challenge due to the complexity of mantle rocks. We carried out experiments on convection in aqueous colloidal dispersions heated from below, and dried and cooled from above. The rheology of these fluids depends strongly on solid particle fraction fp, being Newtonian at low fp, and presenting memory, yield stress, elasticity, and brittle properties as fp increases. Such a behaviour is analogue to the rheology of mantle rocks as temperature decreases. When drying is sufficiently rapid in the laboratory, a visco-elasto-plastic skin ("lithosphere") forms on the fluid surface. Depending on its rheology, and on the different scales of convection existing in our laboratory mantle, we observed different modes of one-sided subduction initiation. However, not all of them lead to continuous plate tectonics. If subduction is definitely a necessary condition for plate tectonics, it is not sufficient. Amongst the different modes of subduction initiation, we observed two of them where one-sided subduction was induced by the impingement of a hot plume under the skin, the trench being localized on the rim of the plume impingement zone under the lithosphere. Then depending on the lithospheric rheology, the nascent subduction can then either stop as the result of subducted plate necking, or continue to sink smoothly. Due to the brittle character of the skin, the subduction trench will never describe a complete circle, but several tears and/or transform faults will develop as subduction and roll back proceed. Inspection of the geological record on Earth suggests that such a strong association between plumes and subduction may have been instrumental in the nucleation and growth of cratons, the onset of continuous plate tectonics, and present-day initiation of subduction around some large oceanic plateaus.

  20. Seismic velocity structure and deformation due to the collision of the Louisville Ridge with the Tonga-Kermadec Trench

    NASA Astrophysics Data System (ADS)

    Stratford, W.; Peirce, C.; Paulatto, M.; Funnell, M.; Watts, A. B.; Grevemeyer, I.; Bassett, D.

    2015-03-01

    New marine geophysical data recorded across the Tonga-Kermadec subduction zone are used to image deformation and seismic velocity structures of the forearc and Pacific Plate where the Louisville Ridge seamount chain subducts. Due to the obliquity of the Louisville Ridge to the trench and the fast 128 mm yr-1 south-southwest migration of the ridge-trench collision zone, post-, current and pre-seamount subduction deformation can be investigated between 23S and 28S. We combine our interpretations from the collision zone with previous results from the post- and pre-collision zones to define the along-arc variation in deformation due to seamount subduction. In the pre-collision zone the lower-trench slope is steep, the mid-trench slope has 3-km-thick stratified sediments and gravitational collapse of the trench slope is associated with basal erosion by subducting horst and graben structures on the Pacific Plate. This collapse indicates that tectonic erosion is a normal process affecting this generally sediment starved subduction system. In the collision zone the trench-slope decreases compared to the north and south, and rotation of the forearc is manifest as a steep plate boundary fault and arcward dipping sediment in a 12-km-wide, 2-km-deep mid-slope basin. A 3 km step increase in depth of the middle and lower crustal isovelocity contours below the basin indicates the extent of crustal deformation on the trench slope. At the leading edge of the overriding plate, upper crustal P-wave velocities are 4.0 km s-1 and indicate the trench fill material is of seamount origin. Osbourn Seamount on the outer rise has extensional faulting on its western slope and mass wasting of the seamount provides the low Vp material to the trench. In the post-collision zone to the north, the trench slope is smooth, the trench is deep, and the crystalline crust thins at the leading edge of the overriding plate where Vp is low, 5.5 km s-1. These characteristics are attributed to a greater degree of extensional collapse of the forearc in the wake of seamount subduction. The northern end of a seismic gap lies at the transition from the smooth lower-trench slope of the post-collision zone, to the block faulted and elevated lower-trench slope in the collision zone, suggesting a causative link between the collapse of the forearc and seismogenesis. Along the forearc, the transient effects of a north-to-south progression of ridge subduction are preserved in the geomorphology, whereas longer-term effects may be recorded in the 80 km offset in trench strike at the collision zone itself.

  1. Strong mechanical coupling along the central Andes: implications for trench curvature, shortening, and topography

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Iaffaldano, G.; di Giuseppe, E.; Corbi, F.; Faccenna, C.; Bunge, H.

    2010-12-01

    The Andean system, where the Nazca plate undergoes continental South America, is often regarded as the archetype of convergent margin where spatial and temporal correlations between the development of trench curvature, shortening of the overriding plate, and topography uplift stand out from the geologic record. Despite the large amount of observations available, the details of those links are still matter of debate. There are, nevertheless, distinctive evidences suggesting that the degree of mechanical coupling between converging plates - that is the amount of resistive force mutually transmitted between plates and opposite to their respective motions - may significantly vary along the Andean margin at present-day. Here we present laboratory experiments of analog subduction that for the first time explicitly relate trench curvature, shortening, and the distribution of topographic volume along the convergent margins to lateral variations in mechanical coupling between subducting and overriding plates. The ability of the overriding plate to slide above the subducting one is significantly inhibited by strong mechanical coupling. This inference applies in particular to the central Andean margin as opposed to its northern and southern limbs. Consequently, the South American plate shortens more, and the trench advances less than elsewhere along the margin, generating the peculiar shape observed along Andes at present-day. The presence of the overriding plate and its degree of coupling with the subducting slab impact the evolution of convergent systems perhaps more that previously thought.

  2. Reactivation of an old plate interface as a strike-slip fault in a slip-partitioned system: Median Tectonic Line, SW Japan

    NASA Astrophysics Data System (ADS)

    Sato, Hiroshi; Kato, Naoko; Abe, Susumu; Van Horne, Anne; Takeda, Tetsuya

    2015-03-01

    In models for strain-partitioning at obliquely-convergent plate boundaries, trench-parallel slip occurs on a vertical fault. Trench-parallel slip at the Nankai subduction zone, SW Japan, is mapped along the Median Tectonic Line (MTL) which dips approximately 40°N. To understand its structural context and how the MTL functions in this slip-partitioned system, we collected a set of three seismic profiles in the Kii peninsula south of Osaka, using a multi-scale acquisition strategy that provides increasingly fine resolution. To understand its fault kinematics, we analyzed microseismic activity in two locations on the fault, using source data from Japan's Hi-net monitoring network. Structural details suggest that the MTL functioned as a megathrust during subduction of the Cretaceous Sanbagawa HP metamorphic belt. Its current pattern of microseismicity shows that it behaves as a strike-slip fault with no indication of a vertical fault at or around its surface trace. Thus, trench-parallel slip at the Nankai is now accommodated on an inclined fault plane in an unusual form of partitioning. This system appears to have developed out of a two-phase tectonic history in which a thrust structure that formed under initial-phase compressive stresses has been reactivated as a strike-slip fault under subsequent-phase shear stresses. Its unusual kinematics show that shear failure can occur on an existing non-vertical fault plane at a regional scale in preference to the rupture of a new ideal (vertical) fault plane.

  3. Subduction Erosion Processes Along the Northwestern Margin of South America

    NASA Astrophysics Data System (ADS)

    Collot, J.; Sage, F.; Calahorrano, A.; Agudelo, W.; Ribodetti, A.

    2007-12-01

    Subduction erosion is one of the dominant processes that shape convergent margins. Mechanisms favoring subduction erosion occur at both highly- and weakly-coupled margins. Multibeam bathymetry and MCS data collected along the Ecuador-SW Colombia trench show an erosional margin fronted by a narrow wedge of imbricated slope sediment. Ubiquitous arcuate slump scarps on the relatively steep inner trench slope denote frequent slope instabilities along a margin that consists of a trenchward-tilted oceanic basement and fore-arc basin, overlain by slope sediments. PSDM seismic reflection sections across three segments of the Ecuador-SW Colombia margin show that physical conditions enabling basal erosion at the plate interface vary along the margin. In southern Ecuador, seaward of the Gulf of Guayaquil, margin extensional deformation suggests a low- friction plate interface. There, the shallow segment of the subduction channel (SC) is roofed by a strong reflector, and dominated by high excess pore pressure that peaks to 40 MPa. Such pore pressure implies that a permeability barrier prevent fluids from migrating upward. Therefore, breaking the permeability barrier, likely during a megathrust slip, would release over-pressured fluids and allow basal erosion by hydrofracturation In central Ecuador, where the Carnegie Ridge enters subduction, margin extensional deformation indicates a low- friction plate interface. The deeper section of the margin basement gradually thins seaward and disappears ~13 km from the trench indicating basal erosion. PSDM sections image a 3D patchiness across the plate interface implying rapid variations in mechanical coupling and erosion processes. Basement weakening, which results from over pressured fluids in the SC, is marked by enhanced reflectivity at the base of the upper plate. Moreover, at the basement apex, basement breakup is caused by superposition of compressional and extensional fault systems. In northern Ecuador, the seaward section of the 2 km-thick fore-arc basin is sharply tilted trenchward at the inner-trench slope break, thus reflecting subduction erosion of the outer margin wedge. Compressive deformation suggests relatively high-friction plate interface. A strong reflector does not roof the thick, presumably water-rich subduction channel, suggesting that over-pressured fluids are not confined in the SC, but pervasively invade the overlaying outer wedge. Moreover, a crustal splay fault associated with low rock velocities is interpreted as a major conduit for fluid flow. The overall low velocity of outer wedge rocks suggest that they are altered by fluids, and therefore considered weak and easy to break up. Furthermore, diffuse shearing in relation with slip hardening is thought to occur along the upper segment of the megathrust. Therefore, faulting, rock alteration, hydrofracturation and diffuse shearing along the interplate fault would favor basal erosion of the outer wedge.

  4. Subduction Erosion Processes Along the Northwestern Margin of South America

    NASA Astrophysics Data System (ADS)

    Collot, J.; Sage, F.; Calahorrano, A.; Agudelo, W.; Ribodetti, A.

    2004-12-01

    Subduction erosion is one of the dominant processes that shape convergent margins. Mechanisms favoring subduction erosion occur at both highly- and weakly-coupled margins. Multibeam bathymetry and MCS data collected along the Ecuador-SW Colombia trench show an erosional margin fronted by a narrow wedge of imbricated slope sediment. Ubiquitous arcuate slump scarps on the relatively steep inner trench slope denote frequent slope instabilities along a margin that consists of a trenchward-tilted oceanic basement and fore-arc basin, overlain by slope sediments. PSDM seismic reflection sections across three segments of the Ecuador-SW Colombia margin show that physical conditions enabling basal erosion at the plate interface vary along the margin. In southern Ecuador, seaward of the Gulf of Guayaquil, margin extensional deformation suggests a low- friction plate interface. There, the shallow segment of the subduction channel (SC) is roofed by a strong reflector, and dominated by high excess pore pressure that peaks to 40 MPa. Such pore pressure implies that a permeability barrier prevent fluids from migrating upward. Therefore, breaking the permeability barrier, likely during a megathrust slip, would release over-pressured fluids and allow basal erosion by hydrofracturation In central Ecuador, where the Carnegie Ridge enters subduction, margin extensional deformation indicates a low- friction plate interface. The deeper section of the margin basement gradually thins seaward and disappears ~13 km from the trench indicating basal erosion. PSDM sections image a 3D patchiness across the plate interface implying rapid variations in mechanical coupling and erosion processes. Basement weakening, which results from over pressured fluids in the SC, is marked by enhanced reflectivity at the base of the upper plate. Moreover, at the basement apex, basement breakup is caused by superposition of compressional and extensional fault systems. In northern Ecuador, the seaward section of the 2 km-thick fore-arc basin is sharply tilted trenchward at the inner-trench slope break, thus reflecting subduction erosion of the outer margin wedge. Compressive deformation suggests relatively high-friction plate interface. A strong reflector does not roof the thick, presumably water-rich subduction channel, suggesting that over-pressured fluids are not confined in the SC, but pervasively invade the overlaying outer wedge. Moreover, a crustal splay fault associated with low rock velocities is interpreted as a major conduit for fluid flow. The overall low velocity of outer wedge rocks suggest that they are altered by fluids, and therefore considered weak and easy to break up. Furthermore, diffuse shearing in relation with slip hardening is thought to occur along the upper segment of the megathrust. Therefore, faulting, rock alteration, hydrofracturation and diffuse shearing along the interplate fault would favor basal erosion of the outer wedge.

  5. Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

    USGS Publications Warehouse

    Gamage, S.S.N.; Umino, N.; Hasegawa, A.; Kirby, S.H.

    2009-01-01

    We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ???30?? at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28-32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the double-planed deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending-unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10-23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock. ?? 2009 The Authors, Journal compilation ?? 2009 RAS.

  6. Scientists Examine Challenges and Lessons From Japan's Earthquake and Tsunami

    NASA Astrophysics Data System (ADS)

    Showstack, Randy

    2011-03-01

    A week after the magnitude 9.0 great Tohoku earthquake and the resulting tragic and damaging tsunami of 11 March struck Japan, the ramifications continued, with a series of major aftershocks (as Eos went to press, there had been about 4 dozen with magnitudes greater than 6); the grim search for missing people—the death toll was expected to approximate 10,000; the urgent assistance needed for the more than 400,000 homeless and the 1 million people without water; and the frantic efforts to avert an environmental catastrophe at Japan's damaged Fukushima Daiichi Nuclear Power Station, about 225 kilometers northeast of Tokyo, where radiation was leaking. The earthquake offshore of Honshu in northeastern Japan (see Figure 1) was a plate boundary rupture along the Japan Trench subduction zone, with the source area of the earthquake estimated at 400-500 kilometers long with a maximum slip of 20 meters, determined through various means including Global Positioning System (GPS) and seismographic data, according to Kenji Satake, professor at the Earthquake Research Institute of the University of Tokyo. In some places the tsunami may have topped 7 meters—the maximum instrumental measurement at many coastal tide gauges—and some parts of the coastline may have been inundated more than 5 kilometers inland, Satake indicated. The International Tsunami Information Center (ITIC) noted that eyewitnesses reported that the highest tsunami waves were 13 meters high. Satake also noted that continuous GPS stations indicate that the coast near Sendai—which is 130 kilometers west of the earthquake and is the largest city in the Tohoku region of Honshu—moved more than 4 meters horizontally and subsided about 0.8 meter.

  7. The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

    NASA Astrophysics Data System (ADS)

    Guillaume, B.; Husson, L.; Funiciello, F.; Faccenna, C.

    2013-07-01

    We designed three-dimensional dynamically self-consistent laboratory models of subduction to analyse the relationships between overriding plate deformation and subduction dynamics in the upper mantle. We investigated the effects of the subduction of a lithosphere of laterally variable buoyancy on the temporal evolution of trench kinematics and shape, horizontal flow at the top of the asthenosphere, dynamic topography and deformation of the overriding plate. Two subducting units, which correspond to a negatively buoyant oceanic plate and positively buoyant continental one, are juxtaposed via a trench-perpendicular interface (analogue to a tear fault) that is either fully-coupled or shear-stress free. Differential rates of trench retreat, in excess of 6 cm yr-1 between the two units, trigger a more vigorous mantle flow above the oceanic slab unit than above the continental slab unit. The resulting asymmetrical sublithospheric flow shears the overriding plate in front of the tear fault, and deformation gradually switches from extension to transtension through time. The consistency between our models results and geological observations suggests that the Late Cenozoic deformation of the Aegean domain, including the formation of the North Aegean Trough and Central Hellenic Shear zone, results from the spatial variations in the buoyancy of the subducting lithosphere. In particular, the lateral changes of the subduction regime caused by the Early Pliocene subduction of the old oceanic Ionian plate redesigned mantle flow and excited an increasingly vigorous dextral shear underneath the overriding plate. The models suggest that it is the inception of the Kefalonia Fault that caused the transition between an extension dominated tectonic regime to transtension, in the North Aegean, Mainland Greece and Peloponnese. The subduction of the tear fault may also have helped the propagation of the North Anatolian Fault into the Aegean domain.

  8. Overriding plate thickness control on subducting slab curvature

    NASA Astrophysics Data System (ADS)

    Holt, A.; Buffett, B. A.; Becker, T. W.

    2014-12-01

    The curvature of subducting lithosphere controls deformation due to bending at the trench, which results in a force that dissipates gravitational potential energy and may affect seismic coupling. We use 2-D, thermo-mechanical subduction models to explore the dependence of the radius of curvature on the thickness of the subducting and overriding plates for models with both viscous and effectively plastic lithospheric rheologies. Such a plastic rheology has been shown to reproduce the bending stresses/moment computed using a kinematic strain rate description and a laboratory derived composite rheology. Laboratory and numerical models show that the bending geometry of subducting slabs with a viscous rheology is strongly dependent on slab thickness; thicker plates have a larger radius of curvature. However, the curvature of subducting plates on Earth, illuminated by the distribution of earthquake hypocenters, shows little to no dependence on the plate thickness or age. Such an observation is instead compatible with plates that have a plastic rheology. Indeed, our numerical models show that the radius of curvature of viscous plates has a stronger dependence on subducting plate thickness than in equivalent plastic models. In viscous plates, the bending moment produces a torque, which balances the torque exerted by buoyancy. However, for the plastic plate case the bending moment saturates at a maximum value and so cannot balance the gravitational torque. The saturation of bending moment means that, (a) the radius of curvature of the bending region is not constrained by this torque balance, and, (b) other forces are required to balance the gravitational torque. We explore the role that the overriding plate could play in controlling the subducting plate curvature in plastic plate models where the bending stresses have saturated. For such plates, we find that increasing the thickness of the overriding plate causes the radius of curvature to increase. The same correlation is found in real subduction zones when the radius of curvature is compared with near-trench overriding lithospheric thickness. We suggest that the thickness of the overriding plate, through controlling the depth extent of the slab suction caused by the strong overriding plate, exerts a primary control on the curvature of subducting lithosphere.

  9. Seafloor Geodesy for Approaching Great Earthquakes Around Japan

    NASA Astrophysics Data System (ADS)

    Fujita, M.; Sato, M.; Ishikawa, T.; Watanabe, S. I.; Yokota, Y.

    2014-12-01

    Seafloor geodesy has been developed as an application of space geodetic technique for the purpose of investigating geodynamic phenomena having their major information in offshore regions. One of such targets is the occurrence of disastrous earthquakes in plate subduction areas. Japan, among others, has repeatedly experienced offshore megathrust earthquakes because of its tectonic location, where multiple plates interact with each other. Most recently, an earthquake of M9.0 occurred off the Pacific coast of east Japan in 2011 with a subsequent huge tsunami, which totally devastated coastal areas and claimed nearly 20,000 lives including those still missing. We, the group of Japan Coast Guard (JCG), have developed a seafloor geodetic technique combining the GPS positioning and underwater acoustic ranging, which is able to measure the position of the seafloor reference point consisting of multiple acoustic transponders with a precision of a few centimeters. We have deployed our seafloor reference points over two regions on the Pacific side of Japan; one is the region along the Japan trench off the eastern coast where the huge 2011 event occurred and another is the region along the Nankai Trough off the southern coast where earthquakes of around M8 have repeated every 100-150 years. With these measurements, we have so far successfully obtained important results providing exclusive information for elucidating the plate boundary behavior causing huge earthquakes. In particular, in the region off east Japan, we have revealed different phases of seafloor movements during the period between several years before and after the 2011 event. They include linear intraplate movements with several centimeters per year before the event, which were the first significant offshore geodetic signals detected around Japan, as well as the coseismic displacements of unprecedentedly huge amount over 20 m close to the epicenter and subsequent postseismic movements with various characteristics depending on the area. These seafloor data contribute to the understandings of physical processes relevant to the event beneath the seafloor in this region, which are unable to be clarified only by the terrestrial data. The presentation summarizes efforts in the field of seafloor geodesy in Japan with a focus on the results obtained by the JCG group.

  10. Mw 8.6 Sumatran earthquake of 11 April 2012: rare seaward expression of oblique subduction

    USGS Publications Warehouse

    Ishii, Miaki; Kiser, Eric; Geist, Eric L.

    2013-01-01

    The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ~20-50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004.

  11. Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

    NASA Astrophysics Data System (ADS)

    Ratheesh Kumar, R. T.; Windley, B. F.; Rajesh, V. J.; Santosh, M.

    2013-12-01

    We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0 < Te < 3 km) in that region, where the NER is close to the Andaman trench (north of 10N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te < Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

  12. Interplate coupling at oblique subduction zones: influence on upper plate erosion.

    NASA Astrophysics Data System (ADS)

    Malatesta, Cristina; Gerya, Taras; Crispini, Laura; Federico, Laura; Scambelluri, Marco; Capponi, Giovanni

    2014-05-01

    In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45 with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where the sedimentary infill of the trench is almost nonexistent, short-term shallow coupling occurs and friction between the frontal sector of the overriding plate and the downgoing plate triggers upper-plate bending. In this sector, after the early short-term coupling, the overriding plate is hereafter decoupled from the subducting slab. Moving along trench-strike, where sediments amount increases, the upper plate couples with the subducting plate and is dragged coherently downwards. If a large amount of sediments is stored in the trench the overriding plate is scraped off and incorporated as fragments along the plate interface. Our results suggest that a) one main parameter controlling coupling at convergent plate margins is the occurrence and the amount of sediment at the trench; b) the upper plate margin is dragged to depth or destroyed only where sediments thickness at the trench is large enough to promote interplate coupling, suggesting that a variation of sediment amount along the trench-axis influences the amount and style of transport of upper-plate material in the mantle.

  13. Serpentinization in the trench-outer rise region offshore of Nicaragua: constraints from seismic refraction and wide-angle data

    NASA Astrophysics Data System (ADS)

    Ivandic, Monika; Grevemeyer, Ingo; Bialas, Joerg; Petersen, C. Joerg

    2010-03-01

    Recent seismic evidence suggested that most oceanic plate hydration is associated with trench-outer rise faulting prior to subduction. Hydration at trenches may have a significant impact on the subduction zone water cycle. Previous seismic experiments conducted to the northwest of Nicoya Peninsula, Northern Costa Rica, have shown that the subducting Cocos lithosphere is pervasively altered, which was interpreted to be due to both hydration (serpentinization) and fracturing of the crustal and upper-mantle rocks. New seismic wide-angle reflection and refraction data were collected along two profiles, running parallel to the Middle American trench axis offshore of central Nicaragua, revealing lateral changes of the seismic properties of the subducting lithosphere. Seismic structure along both profiles is characterized by low velocities both in the crust and upper mantle. Velocities in the uppermost mantle are found to be in the range 7.3-7.5kms-1 thus are 8-10 per cent lower than velocities typical for unaltered peridotites and hence confirm the assumption that serpentinization is a common process at the trench-outer rise area offshore of Nicaragua. In addition, a prominent velocity anomaly occurred within the crust beneath two seamounts. Here, velocity reduction may indicate increased porosity and perhaps permeability, supporting the idea that seamounts serve as sites for water percolation and circulation.

  14. 'Dodo' and 'Baby Bear' Trenches

    NASA Technical Reports Server (NTRS)

    2008-01-01

    NASA's Phoenix Mars Lander's Surface Stereo Imager took this image on Sol 11 (June 5, 2008), the eleventh day after landing. It shows the trenches dug by Phoenix's Robotic Arm. The trench on the left is informally called 'Dodo' and was dug as a test. The trench on the right is informally called 'Baby Bear.' The sample dug from Baby Bear will be delivered to the Phoenix's Thermal and Evolved-Gas Analyzer, or TEGA. The Baby Bear trench is 9 centimeters (3.1 inches) wide and 4 centimeters (1.6 inches) deep.

    The Phoenix Mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  15. Phoenix Deepens Trenches on Mars

    NASA Technical Reports Server (NTRS)

    2008-01-01

    The Surface Stereo Imager on NASA's Phoenix Mars Lander took this false color image on Oct. 21, 2008, during the 145th Martian day, or sol, since landing. The white areas seen in these trenches are part of an ice layer beneath the soil.

    The trench on the upper left, called 'Upper Cupboard,' is about 60 centimeters (24 inches) long and 3 centimeters (1 inch) deep. The trench in the middle, called 'Ice Man,' is about 30 centimeters (12 inches) long and 3 centimeters (1 inch) deep. The trench on the right, called 'La Mancha,' is about 31 centimeters (12 inches) and 5 centimeters (2 inches) deep.

    The Phoenix mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  16. Phoenix Deepens Trenches on Mars

    NASA Technical Reports Server (NTRS)

    2008-01-01

    The Surface Stereo Imager on NASA's Phoenix Mars Lander took this false color image on Oct. 21, 2008, during the 145th Martian day, or sol, since landing. The bluish-white areas seen in these trenches are part of an ice layer beneath the soil.

    The trench on the upper left, called 'Dodo-Goldilocks,' is about 38 centimeters (15 inches) long and 4 centimeters (1.5 inches) deep. The trench on the right, called 'Upper Cupboard,' is about 60 centimeters (24 inches) long and 3 centimeters (1 inch) deep. The trench in the lower middle is called 'Stone Soup.'

    The Phoenix mission is led by the University of Arizona, Tucson, on behalf of NASA. Project management of the mission is by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Spacecraft development is by Lockheed Martin Space Systems, Denver.

  17. Deformation of Japan as measured by improved analysis of GEONET data

    NASA Astrophysics Data System (ADS)

    Owen, S. E.; Dong, D.; Webb, F. H.; Newport, B. J.; Simons, M.

    2006-12-01

    The Japan subduction zone represents a complex set of plate interfaces with significant trench-parallel variability in great earthquakes and transient deep slip events. Within the Japan arc the Nankai segment of the Eurasian-Philippine plate boundary is one of the classic subduction zone segments that last produced a set of temporally linked great earthquakes in the 1940's. Recently, down-dip of the Nankai seismogenic portion of the plate interface, transient slip events and seismic tremor events were observed. Through analysis of the GEONET GPS data, the spatial and higher frequency temporal characteristics of transient slip events can be captured. We describe our analysis methods, the spatial filtering technique that has been developed for use on large networks, a periodic signal filtering method that improves on commonly-used sinusoidal function models, and the resultant velocities and time series. Our newly developed analysis method, the GPS Network Processor, gives us the ability to process large volumes of data extremely fast. The basis of the GPS Network Processor is the JPL-developed GIPSY-OASIS GPS analysis software and the JPL-developed precise point positioning technique. The Network Processor was designed and developed to efficiently implement precise point positioning and bias fixing on a 1000-node (2000 cpu) Beowulf cluster. The entire 10 year ~1000-station GEONET data set can be reanalyzed using the Network Processor in a matter of days. This permits us to test different processing strategies, each with potentially large influence on our ability to detect strain transients from the subduction zones. For example, we can test different ocean loading models, which can effect the diurnal positions of coastal GPS sites by up to 2 cm. We can also test other potentially important factors such as using reprocessed satellite orbits and clocks, the parameterization of the tropospheric delay, or the implementation of refined solid body tide estimates. We will present the results of tests that we have run to date at the meeting.

  18. Opportunity Trenches Martian Soil

    NASA Technical Reports Server (NTRS)

    2004-01-01

    The Mars Exploration Rover Opportunity dragged one of its wheels back and forth across the sandy soil at Meridiani Planum to create a hole (bottom left corner) approximately 50 centimeters (19.7 inches) long by 20 centimeters (7.9 inches) wide by 9 centimeters (3.5 inches) deep. The rover's instrument deployment device, or arm, will begin studying the fresh soil at the bottom of this trench later today for clues to its mineral composition and history. Scientists chose this particular site for digging because previous data taken by the rover's miniature thermal emission spectrometer indicated that it contains crystalline hematite, a mineral that sometimes forms in the presence of water. The brightness of the newly-exposed soil is thought to be either intrinsic to the soil itself, or a reflection of the Sun. Opportunity's lander is in the center of the image, and to the left is the rock outcrop lining the inner edge of the small crater that encircles the rover and lander. This mosaic image is made up of data from the rover's navigation and hazard-avoidance cameras.

  19. Tectonic erosion as a possible driving force of blueschist exhumation in the Cretaceous forearc of central Hokkaido, Japan.

    NASA Astrophysics Data System (ADS)

    Ueda, H.

    2008-12-01

    Introduction In an oceanic subduction zone, where surface erosion was less effective and underlying slab was less buoyant, driving forces of unroofing and lifting-up for the blueschist exhumation are not still well understood. In the Kamuikotan Zone of central Hokkaido, Japan, ca. 125 Ma blueschist unit was exhumed and unconformably overlain by ca. 105-110 Ma forearc basin deposits. There is no trench-fill deposits coeval to the exhumation event, suggesting that the exhumation occurred in a non-accretionary stage. This paper presents a new idea to explain how the blueschist exhumation occurred in an non-accretionary margin, based on field observations of a unit underlying the blueschist. Geologic background The focused unit (Shizunai Unit) is an accretionary complex, which structurally underlies the blueschist facies metabasite unit. It is a structural pile of metabasites and pelagic to trench-fill sedimentary rocks. It wholly suffered a very low-grade HP metamorphism (lawsonite-albite facies or lower) dated as 105-115 Ma. Mudstones yield middle Early Cretaceous radiolarians (ca. 130-135 Ma). The forearc basin deposits unconformably overlying the exhumed rocks are fluvial to shallow-marine, whereas those in more trench-ward areas are deep-sea turbidites. Debris-flow and slump deposits are widespread over the basin, suggesting unstable slopes dipping to the trench, probably resulting from the uplift of the blueschists. Re-accretion of tectonically eroded rocks The Shizunai unit contains a nappe sub-unit consisting mostly of sedimentary rocks. It is an imbricate stack of sedimentary sequences from hemipelagic tuffaceous mudstone to terrigenous trench-fill turbidite. They represent upper horizons of trench strata, and lack rocks of the lower horizons (chert and metabasites). Stratigraphic upper parts of each sequence frequently intercalate debrites and olistostrome, implying syn- sedimentary imbricate thrusting. Deformation structures with bedding-parallel shortening at the earliest, unconsolidated stage are common in turbidites. These characteristics (imbricate upper trench strata, syn- sedimentary tectonics, and lateral compression just after sedimentation) suggest that the clastic nappe was originally formed as a frontal accretionary unit at the toe of the wedge. It then subducted to a depth of 15-20 km during the late Early Cretaceous. The subduction of a frontal accretionary unit means tectonic erosion. Whereas its incorporation into the high-pressure subduction complex means underplating re-accretion. Exhumation induced by tectonic erosion During the exhumation event, two types of material transport synchronously occurred. One is the rearward and downward transport along the base of the wedge as tectonic erosion and re-accretion, with mean vertical velocity of -0.5 to -1.3 mm/y. The other is upward transport at the rear of the wedge as the blueschist exhumation, with mean vertical velocity of 1 to 2 mm/y. These similar but opposite vertical velocities suggest that the two modes of material transport compensate by each other as a corner-flow transport. Steepening of the forearc slope is explained by removal of frontal wedge materials by tectonic erosion, and releasing them at the rear side. Unroofing by gravity slides inferably occurred under the oversteepened slope. If significant amounts of frontal materials are tectonically eroded and released at the depths, blueschists could thus be exhumed as a consequence of induced corner flow.

  20. Stress states at the subduction input site, Nankai Subduction Zone, using anelastic strain recovery (ASR) data in the basement basalt and overlying sediments

    NASA Astrophysics Data System (ADS)

    Yamamoto, Yuzuru; Lin, Weiren; Oda, Hirokuni; Byrne, Timothy; Yamamoto, Yuhji

    2013-07-01

    The three-dimensional orientations of stress and stress magnitudes in the basement basalt and overlying sediments at the subduction input site, IODP Site C0012, have been determined using anelastic strain recovery (ASR) analyses. The ASR results in the sedimentary sequence indicate that ?1 is nearly vertical. The magnitudes of ?2 and ?3 are very similar, indicating that the stress state in the sedimentary sequence is "at rest". On the other hand, ASR analyses in the basement basalt show that ?1 is nearly horizontal and oriented NE-SW, almost parallel (or slightly oblique) to the trench axis. ?3 plunges moderately to the NW. The stress state of the basement basalts suggests a strike-slip or thrust (reverse fault) regime, which is very different from a "state at rest" condition, which is the theoretical stress condition for the ocean floor far from a subduction zone. The basement basalt at the subduction input site (C0012) has experienced trench-parallel compression and trench-normal extension, consistent with the focal mechanisms of earthquakes in the vicinity. The estimated stress magnitudes show only small variations between the principal stresses, implying that the directions of principal stress could be easily rotated in association with any tectonically induced local stress variation. The stress orientation in the basement basalt seems to be the result of hinge extension during bending of the Philippine Sea Plate, either in association with subduction or with the formation of an anticline during intraoceanic thrusting.

  1. Structural and thermal control of seismic activity and megathrust rupture dynamics in subduction zones: Lessons from the Mw 9.0, 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Satriano, Claudio; Dionicio, Viviana; Miyake, Hiroe; Uchida, Naoki; Vilotte, Jean-Pierre; Bernard, Pascal

    2014-10-01

    The 2011 Tohoku megathrust earthquake ruptured a vast region of the northeast Japan Trench subduction zone in a way that had not been enough anticipated by earthquake and tsunami risk scenarios. We analyzed the Tohoku rupture combining high-frequency back-projection analysis with low frequency kinematic inversion of the co-seismic slip. Results support the to-day well-accepted broadband characteristics of this earthquake. Most of the seismic moment is released during the first 100 s, with large co-seismic slip (up to 55 m) offshore Miyagi in a compact region on the landward side of the trench. Coherent high-frequency radiation areas and relatively low co-seismic slip are a distinctive signature of the slab-mantle interface. The broadband characteristics of the Tohoku rupture are interpreted, integrating the seismic activity and structure information on the NE Japan forearc region, as a signature of along-dip segmentation and segment interactions, that result from thermal structure, plate geometry, material composition and fracture heterogeneities along the plate boundary interface. Deep mantle corner flow and low dehydration rates along the cold subduction slab interface lead to an extended seismogenic slab-mantle interface, with strong bi-material contrast controlling larger propagation distance in the downdip preferred rupture direction. Off Miyagi, plate bending below the mantle wedge, ∼142.3°E at ∼25 km depth, is associated with the eastern limit of the deep M7-8-class thrust-earthquakes, and of the strongest coherent high-frequency generation areas. The region of the slab-crust interface between the mantle wedge limit, ∼142.7°E at ∼20 km depth, and a trenchward plate bending, ∼143.2°E at ∼15 km, acted as an effective barrier resisting for many centuries to stress-loading gradient induced by deep stable sliding and large earthquakes along the slab-mantle interface. The 2011 Tohoku earthquake, whose hypocenter is located on the east side of the mantle wedge limit, released the accumulated stress in this region and succeeded to overcome the plate bending, driving the upper plate boundary interface to slip co-seismically, regardless its frictional property, thanks to a combination of dynamic effects associated with bi-material rupture directivity and stress changes induced by reflection from the surface of waves released by the unstable slip. This conceptual framework provides elements for reappraisal of long-term seismic activity and occurrence of rare and extreme tsunamigenic megathrust in other subduction zones, like those of North-Central Lesser Antilles, Central and Northern Chile.

  2. Central Andean Giant Ore Deposits: Links to Forearc Subduction Erosion, Shallowing Subduction and Thickening Crust

    NASA Astrophysics Data System (ADS)

    Kay, S. M.; Mpodozis, C.

    2013-05-01

    An outstanding question on the Central Andean margin is the relationship between tectonic processes like ebbing arc volcanism, shallowing of the subducting slab and crustal thickening, and the origin of giant porphyry and epithermal Cu, Au and Ag deposits. Another potentially important factor in forming these major mineral deposits is forearc subduction erosion, which is postulated to have removed up to ~250 km of Central Andean forearc crust since the Jurassic. Geochemical and geophysical studies provide insights into possible links. Evidence for partial melts of removed and subducted forearc crust reaching the arc magma source and thus the magmas that host the ore deposits comes from the chemistry of late Neogene volcanic rocks on both the northern and southern margin of the Chilean-Pampean flat-slab (28-33S), where the frontal arc was displaced ~50 km into the foreland between ~10 and 3 Ma. This chemical evidence consists of transient ultra-steep REE patterns, elevated Mg, Cr and Ni contents and steps in isotopic ratios that are particularly notable in the glassy adakitic 8-3 Ma (Pircas Negras) andesites on the northern flat-slab margin at 27-28S. Well constrained reconstructions of the margin near 26-28S that assume a sustained 300 km wide arc-trench gap and ~50 km of forearc removal suggest an accelerated average forearc subduction erosion rate over 150 km3/my/km between 8 and 3 Ma. Noting that the late Miocene arc is now at least ~ 260 km from the trench from 26S to 34S and that the active arc extrapolates through the amagmatic flat-slab region (28-33S) at 300 km from the trench, accelerated forearc removal could be inferred from ~34S to 26S at ~10 to 3 Ma. Geophysical evidence for forearc crust entering the mantle wedge as the flatslab shallowed could come from low Vp/Vs seismic ratios in the mantle wedge under the flatslab, which Wagner et al. (2010) attribute to orthopyroxene. Formation of this orthopyroxene could be explained by forearc crust reacting with the mantle wedge. Thus, the slab shallowing, crustal thickening and forearc subduction erosion in the flatslab region, which began at ca 20-18 Ma and accelerated after 11-10 Ma could have set the stage for the formation of the Los Pelambres, Rio Blanco and El Teniente giant Cu porphyries between ~ 11-4 Ma. The backarc 8-6 Ma Bajo de la Alumbrera Cu-Au district near 27S, also formed east of the migrating volcanic arc on the northern flatslab margin at this time. This deposit is notable for now being above a high Qp mantle seismic anomaly overlying the slab, which is at a depth of ~150 km. Elsewhere, Ag-Zn mineralization in the ~14-12 Ma Potosi district near 19.5S in the Altiplano backarc, which has been suggested to have occurred in the early stages of steepening of a shallow slab, would potentially predate flushing of eroded forearc material from an expanding mantle wedge. In the same vein, a lack of known big Cu-Au-Ag deposits associated with the late Neogene giant plateau ignimbrite complexes, considered to be fomed over steepening subduction zones characterized by low Vp and Vs and high Qp tomographic seismic anomalies, could also partially reflect loss of forearc subducted components from an expanding wedge.

  3. Control of paleoshorelines by trench forebulge uplift, Loyalty Islands

    NASA Astrophysics Data System (ADS)

    Dickinson, William R.

    2013-07-01

    Unlike most tropical Pacific islands, which lie along island arcs or hotspot chains, the Loyalty Islands between New Caledonia and Vanuatu owe their existence and morphology to the uplift of pre-existing atolls on the flexural forebulge of the New Hebrides Trench. The configuration and topography of each island is a function of distance from the crest of the uplifted forebulge. Both Maré and Lifou are fully emergent paleoatolls upon which ancient barrier reefs form highstanding annular ridges that enclose interior plateaus representing paleolagoon floors, whereas the partially emergent Ouvea paleoatoll rim flanks a drowned remnant lagoon. Emergent paleoshoreline features exposed by island uplift include paleoreef flats constructed as ancient fringing reefs built to past low tide levels and emergent tidal notches incised at past high tide levels. Present paleoshoreline elevations record uplift rates of the islands since last-interglacial and mid-Holocene highstands in global and regional sea levels, respectively, and paleoreef stratigraphy reflects net Quaternary island emergence. The empirical uplift rates vary in harmony with theoretical uplift rates inferred from the different positions of the islands in transit across the trench forebulge at the trench subduction rate. The Loyalty Islands provide a case study of island environments controlled primarily by neotectonics.

  4. Shallow subduction, ridge subduction, and the evolution of continental lithosphere

    SciTech Connect

    Helmstaedt, H.; Dixon, J.M.; Farrar, E.; Carmichael, D.M.

    1985-01-01

    Subduction of oceanic lithosphere beneath continental crust at a shallow angle has occurred throughout the Phanerozoic Eon. Ridge subduction often follows shallow subduction and causes bimodal volcanism and crustal rifting, forming back-arc basins. Recent models for Archean plate tectonics propose very fast rates of spreading (400-800 km/Ma) and convergence, and sinking rates comparable to or slower (<10 km/Ma) than those of today. As faster convergence and slower sinking correspond to subduction at shallower angles, shallow subduction and ridge subduction must have been ubiquitous during the Archean permobile regime. This is compatible with a back-arc-basin origin for Archean greenstone belts. The common coexistence of tholeiitic and calc-alkaline igneous rocks in Archean greenstone belts, also implies ridge subduction. The authors envisage a transition, between 2.4 and 1.8 Ga., from a regime dominated by shallow subduction and repeated ridge subduction to one of normal plate tectonics with steeper subduction. Spreading rates decreased; continental plates became larger and stable shelves could develop at trailing margins. Shallow subduction became the exception, restricted to episodes of abnormally fast convergence; nevertheless, the long span of post-Archean time makes it unlikely that any part of the continental crust has escaped shallow subduction and ridge subduction. These processes recycle much volatile-rich oceanic crust into the sub-continental upper mantle, thereby underplating the crust, effecting upper-mantle metasomatism and affecting intraplate magmatism.

  5. Dynamic buckling of subducting slabs reconciles geological and geophysical observations

    NASA Astrophysics Data System (ADS)

    Lee, Changyeol; King, Scott D.

    2011-12-01

    Ever since the early days of the development of plate tectonic theory, subduction zones have been engrained in geological thinking as the place where steady, linear slabs descend into the mantle at a constant, uniform dip angle beneath volcanic arcs. However, growing evidence from geological and geophysical observations as well as analog and numerical modeling indicates that subducting slabs buckle in a time-dependent manner, in contrast to the steady-state, linear cartoons that dominate the literature. To evaluate the implication of time-dependent slab buckling of geological events, we conduct a series of 2-D numerical dynamic/kinematic subduction experiments by varying the viscosity increase across the 660 km discontinuity and the strength of the subducting slab. Our results show that slab buckling is a universal figure in all the experiments when rate of the trench migration ( Vtrench) is relatively slow ( Vtrench| < 2 cm/a) and viscosity increases across the 660 km discontinuity are greater than a factor of 30. Slab buckling is expressed as alternate shallowing and steepening dip of the subducting slab (from ~ 40 to ~ 100) which is correlated with increasing and decreasing convergent rate of the incoming oceanic plate toward the trench. Further, the slab buckling in our experiments is consistent with the previously developed scaling laws for slab buckling; using reasonable parameters from subducted slabs the buckling amplitude and period are ~ 400 km and ~ 25 Myr, respectively. The slab buckling behavior in our experiments explains a variety of puzzling geological and geophysical observations. First, the period of slab buckling is consistent with short periodic variations (~ 25 Myr) in the motions of the oceanic plates that are anchored by subduction zones. Second, the scattered distributions of slab dips (from ~ 20 to ~ 90) in the upper mantle are snapshots of time-dependent slab dip. Third, the current compressional and extensional stress environments in back-arcs are well correlated with shallowing and steeping slab dip resulting from slab buckling. Fourth, the temporal evolution of stress environments in the Andes is well correlated with alternate slab dip. These correlations indicate that time-dependent slab buckling is a major factor controlling subduction zone dynamics.

  6. Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause

    NASA Astrophysics Data System (ADS)

    ten Brink, Uri

    2005-06-01

    The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. ago over a time period as short as 14-40 kyr. I explain these vertical movements by a sudden increase in the slab's descent angle that caused the trench to subside and the island to rise. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5W and trend southwestwardly toward eastern Puerto Rico. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount.

  7. Seismicity of the Incoming Plate at the Mariana Trench Located Using an Ocean Bottom Seismograph Array

    NASA Astrophysics Data System (ADS)

    Jasperson, H.; Wiens, D. A.; Lizarralde, D.

    2014-12-01

    We locate earthquakes occurring near the Mariana Trench using an ocean bottom seismograph (OBS) array to study the role of incoming plate seismicity in hydrating the mantle as well as to constrain seismicity at the updip end of the Mariana subduction thrust. The array consisted of 20 broadband OBSs, and 5 temporary broadband seismographs deployed on nearby islands from February 2012 to February 2013. The OBS water depths were limited to 6 km or less, so they were deployed surrounding the trench and 5 hydrophones were deployed in the water column near the trench axis. Preliminary results indicate that seismicity in the incoming plate begins about 120 km west of the trench axis, with the highest seismicity levels found near the trench axis itself. Many of the incoming plate earthquakes locate near large fault scarps identified in seafloor bathymetry. In the forearc, Mariana shallow thrust seismicity begins where the plate interface is at 15-20km depth. There is a strong contrast in velocity structure across the trench, which causes difficulty in determining precise locations and depths. Further work will clarify the depths and focal mechanisms of the recorded earthquakes.

  8. Influence of subduction history on South American topography

    NASA Astrophysics Data System (ADS)

    Flament, Nicolas; Gurnis, Michael; Mller, R. Dietmar; Bower, Dan J.; Husson, Laurent

    2015-11-01

    The Cenozoic evolution of South American topography is marked by episodes of large-scale uplift and subsidence not readily explained by lithospheric deformation. The drying up of the inland Pebas system, the drainage reversal of the Amazon river, the uplift of the Sierras Pampeanas and the uplift of Patagonia have all been linked to the evolution of mantle flow since the Miocene in separate studies. Here we investigate the evolution of long-wavelength South American topography as a function of subduction history in a time-dependent global geodynamic model. This model is shown to be consistent with these inferred changes, as well as with the migration of the Chaco foreland basin depocentre, that we partly attribute to the inboard migration of subduction resulting from Andean mountain building. We suggest that the history of subduction along South America has had an important influence on the evolution of the topography of the continent because time-dependent mantle flow models are consistent with the history of vertical motions as constrained by the geological record at four distant areas over a whole continent. Testing alternative subduction scenarios reveals flat slab segments are necessary to reconcile inferred Miocene shorelines with a simple model paleogeography. As recently suggested, we find that the flattening of a subduction zone results in dynamic uplift between the leading edge of the flat slab segment and the trench, and in a wave of dynamic subsidence associated with the inboard migration of the leading edge of flat subduction. For example, the flattening of the Peruvian subduction contributed to the demise of Pebas shallow-water sedimentation, while continental-scale tilting also contributed to the drainage reversal of the Amazon River. The best correlation to P-wave tomography models for the Peruvian flat slab segment is obtained for a case when the asthenosphere, here considered to be 150 km thick and 10 times less viscous than the upper mantle, is restricted to the oceanic domain.

  9. Strength and survival of subducted lithosphere during the Laramide orogeny

    SciTech Connect

    Spencer, J.E. )

    1993-04-01

    The strength of subducted ocean lithosphere is influenced primarily by two competing processes. During subduction brittle rock strength increases because of increasing compressive stress across fracture surfaces which increases frictional resistance to sliding. The strength of rocks hot enough to be in the plastic deformation regime decreases primarily because of heat conducted from the overriding plate and the asthenosphere. A one-dimensional finite-element heat-flow program was used to simulate subduction in two dimensions where conductive heat flow parallel to the slab and to the upper plate could be neglected. Temperatures determined with this method, and pressures based on depth, were then used to calculate the form of the brittle-plastic failure envelope for subducted lithosphere. An olivine flow law and strain rate of 10[sup [minus]15] s[sup [minus]1] were used for the plastic part of the failure envelope. The failure envelope was then used to calculate slab-parallel compressive strength and maximum sustainable bending moment. Modeling of Maramide subduction beneath southwestern North America, using slab ages and subduction rates for the Farallon plate from Engebretson et al., suggests that the subducted slab will not retain much strength beyond 1,000 to 1,200 km inland unless the thickness of the North American lithosphere, and depth to the top of the slab, are significantly less than 200 km. Slab survival for distances of 1000 km seems assured. Survival for much greater distances is possible. The slab is predicted to have been up to several times stronger beneath southwestern North America than at the trench because much rock remains in the brittle regime and is under high confining pressure.

  10. Deep electrical resistivity structure of Costa Rican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Worzewski, T.; Jegen, M.; Brasse, H.; Taylor, W.

    2009-04-01

    The water content and its distribution play an important role in the subduction process. Water is released from the subducting slab in a series of metamorphic reactions and the hydration of the mantle wedge may trigger the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). We will show numerical modeling studies which are used to determine the resolution and sensitivity of the MT response to fluids in the crust and subducting slab under the special condition of a coastal setting. In 2007-2008 we conducted a long-period magnetotelluric investigations in northwestern Costa Rica on- and offshore, where the Cocos Plate subducts beneath the Carribean plate. Eleven marine magnetotelluric Stations newly developed and constructed by IFM-GEOMAR and University of Kiel were deployed on the 200 km long marine extension of the profile for several months. We will present the data and its processing, as well as our attempts to eliminate motion induced noise observed on some stations on the cliffy shelf due to tidal waves hitting the shelf and trench parallel- and perpendicular currents. The marine profile was extended landwards by the Free University of Berlin over length of 160 kilometers with further 18 stations. We present preliminary modeling results of land data, which revealed interesting features, inter alia a possible image of fluid release from the downgoing slab in the forearc, as well as ongoing modeling of the combined on- and offshore data sets.

  11. Geoid anomalies in the vicinity of subduction zones

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1980-01-01

    The regional geoid of the southwest Pacific is matched reasonably well by results from a model of the upper mantle density structure (including slabs) associated with subduction zones of the region. Estimates of the geoid are obtained from Geos-3 and Seasat radar altimeter data. These data are very well suited to the task of detecting intermediate wavelength (600-4000 km) geopotential variations. Actually, subducting slabs can be expected to produce primarily intermediate and longer wavelength variations. Gravimetric profiles across trench/island arc complexes resolve primarily short wavelengths. The model represents subducting slabs as thin surfaces of anomalous mass per unit area. These surfaces are positioned using published seismicity results which detail the configuration of the Benioff zones. Crustal effects are ignored. Effects due to the contrast between the young thermal lithosphere of the behind-arc regions (marginal basins) and the older lithosphere seaward of the trench are modelled. Results indicate that the New Hebrides slab possesses an average areal density anomaly of about 300,000 gm/sq cm. This is about three times that which is estimated for the Tonga-Kermadec slab. Additional modelling suggests that slabs worldwide may be an important source of large, long wavelength gravity highs; i.e., they may contribute substantially to geopotential power of harmonic degree as low as three or four up to twenty or more.

  12. Thermal modelling of the Laramide orogeny: testing the flat-slab subduction hypothesis

    NASA Astrophysics Data System (ADS)

    English, Joseph M.; Johnston, Stephen T.; Wang, Kelin

    2003-09-01

    The Laramide orogeny is the Late Cretaceous to Palaeocene (80-55 Ma) orogenic event that gave rise to the Rocky Mountain fold and thrust belt in Canada, the Laramide block uplifts in the USA, and the Sierra Madre Oriental fold and thrust belt in Mexico. The leading model for driving Laramide orogenesis in the USA is flat-slab subduction, whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards, producing basement-cored block uplifts and arc magmatism in the foreland. The thermal models presented here indicate that arc magma generation at significant distances inboard of the trench (>600 km) during flat-slab subduction is problematic; this conclusion is consistent with the coincidence of volcanic gaps and flat-slab subduction at modern convergent margins. Lawsonite eclogite xenoliths erupted through the Colorado Plateau in Oligocene time are inferred to originate from the subducted Farallon slab, and indicate that the Laramide flat-slab subduction zone was characterised by a cold thermal regime. Thermal modelling indicates that this regime can be produced by flat-slab subduction of old (>50 Myr) oceanic lithosphere at high convergence rates. In the Canadian and Mexican portions of the Laramide orogen, the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat-slab subduction in these regions. It is proposed that subduction of an oceanic plateau/aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat-slab subduction in the USA. These findings cast doubt on the flat-slab model as a primary means of driving Laramide orogenesis along its entire length, and instead point to the need for an alternative mechanism for Cordilleran-wide Laramide orogenesis.

  13. Monitoring of seafloor crustal deformation using GPS/Acoustic technique along the Nankai Trough, Japan

    NASA Astrophysics Data System (ADS)

    Yasuda, K.; Tadokoro, K.; Ikuta, R.; Watanabe, T.; Fujii, C.; Matsuhiro, K.; Sayanagi, K.

    2014-12-01

    Seafloor crustal deformation is crucial for estimating the interplate locking at the shallow subduction zone and has been carried out at subduction margins in Japan, e.g., Japan Trench and Nankai Trough [Sato et al., 2011; Tadokoro et al., 2012]. Iinuma et al. [2012] derived slip distributions during the 2011 Tohoku-Oki earthquake using GPS/Acoustic data and on-land GPS data. The result showed that maximum slip is more than 85 m near the trench axis. The focal area along the Nankai trough extended to the trough axis affected this earthquake by cabinet office, government of Japan. ?We monitored seafloor crustal deformation along the Nankai trough, Japan. Observation regions are at the eastern end of Nankai trough (named Suruga trough) and at the central Nankai trough. We established and monitored by two sites across the trough at each region. In the Suruga trough region, we repeatedly observed from 2005 to 2013. We observed 13 and 14 times at a foot wall side (SNE) and at a hanging wall side (SNW), respectively. We estimated the displacement velocities with relative to the Amurian plate from the result of repeated observation. The estimated displacement velocity vectors at SNE and SNW are 428 mm/y to N943?W direction and 3911 mm/y to N849?W direction, respectively. The directions are the same as those measured at the on-land GPS stations. The magnitudes of velocity vector indicate significant shortening by approximately 4 mm/y between SNW and on-land GPS stations at hanging wall side of the Suruga Trough. This result shows that the plate interface at the northernmost Suruga trough is strongly locked. In the central Nankai trough region, we established new two stations across the central Nankai trough (Both stations are about 15km distance from trough) and observed only three times, August 2013, January 2014, and June 2014. We report the results of monitoring performed in this year.

  14. Seismic properties of the Nazca oceanic crust in southern Peruvian subduction system

    NASA Astrophysics Data System (ADS)

    Kim, YoungHee; Clayton, Robert W.

    2015-11-01

    The horizontal Nazca slab, extending over a distance of ∼800 km along the trench is one of enigmatic features in Peruvian subduction zone. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment. We use data from the recent seismic experiment in southern Peru to find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of ∼20-40% near the top plate interface along- and perpendicular-to the trench from ∼40-120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of ∼800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel).

  15. The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

    NASA Astrophysics Data System (ADS)

    Guillaume, B.; Husson, L.; Funiciello, F.; Faccenna, C.

    2013-04-01

    We design three-dimensional dynamically self-consistent laboratory models of subduction to analyze the relationships between overriding plate deformation and subduction dynamics in the upper mantle. We investigate the effects of the subduction of a lithosphere of laterally variable buoyancy on the temporal evolution of trench kinematics and shape, horizontal flow at the top of the asthenosphere, dynamic topography and deformation of the overriding plate. The interface between the two units, analogue to a trench-perpendicular tear fault between a negatively buoyant oceanic plate and positively buoyant continental one, is either fully-coupled or shear-stress free. Differential rates of trench retreat, in excess of 6 cm yr-1 between the two units, trigger a more vigorous mantle flow above the oceanic slab unit than above the continental slab unit. The resulting asymmetrical sublithospheric flow shears the overriding plate in front of the tear fault, and deformation gradually switches from extension to transtension through time. The consistency between our models results and geological observations suggests that the Late Cenozoic deformation of the Aegean domain, including the formation of the North Aegean Trough and Central Hellenic Shear zone, results from the spatial variations in the buoyancy of the subducting lithosphere. In particular, the lateral changes of the subduction regime caused by the Early Pliocene subduction of the old oceanic Ionian plate redesigned mantle flow and excited an increasingly vigorous dextral shear underneath the overriding plate. The models suggest that it is the inception of the Kefalonia Fault that caused the transition between an extension dominated tectonic regime to transtension, in the North Aegean, Mainland Greece and Peloponnese. The subduction of the tear fault may also have helped the propagation of the North Anatolian Fault into the Aegean domain.

  16. Reevaluating plate driving forces from 3-D models of subduction

    NASA Astrophysics Data System (ADS)

    Stegman, D. R.; Freeman, J.; Schellart, W. P.; Moresi, L.; May, D.; Turnbull, R.

    2004-12-01

    Subducting lithospheric slabs mechanically attached to tectonic plates provide the main driving force for surface plate motion. Numerical models historically simulate slab dynamics as a 2-D process and further simplify the problem into either a density driven model (no heat transfer) or a corner-flow problem (thermal convection) [Christensen, 2001; Enns et al., (in revision); van Keken, 2003]. Recent 3-D global models of density driven flow incorporating a history of plate motion (Conrad and Lithgow-Bertelloni, 2002) have succussfully ruled out slab "suction" (basal shear traction induced by downward flow of the slabs) as a major driving force, but exact partitioning of the remaining forces acting on the slab remain unconstrained. A survey of trenches around the world reveals that over half of the slabs presently subducted in the upper mantle have a discontinuous edge (either a slab tip on a young slab, or the side edge of a slab with finite width) around which mantle can flow: prime examples being slabs in the Mediterranean and Carribean. However, even slabs with a wide lateral extent (and where a 2-D approximation may seem appropriate), show signs of having 3-D complexity. For example, on the surface Tonga appears relatively symmetric, but when the history of subduction is considered, the slab has a twisted, 3-D structure due to significant eastward retreat of just the northern part of an originally N-S oriented trench edge. Similarly the widest slabs, South American and Kamchatka, show seismic anisotropy attributed to trench parallel mantle flow (Russo and Silver, 1994; Peyton, et al., 2001, respectively), while the Aleutian trench has oblique subduction varying in magnitude from west to east, and medium width Central American slab likely has a slab window allowing 3-D flow (Johnston and Thorkelson, 1997). Recent laboratory experiments of subduction have demonstrated the full complexity of flow occuring in 3-D geometry (Kincaid and Griffiths, 2003; Schellart, 2004), owing to the analog slab having a lateral extent smaller than the width of the box. These experiments clearly show subduction of a finite-width slab will generate a flow of material from behind the slab around both the side edges and under the nose of the slab into the mantle wedge. This rollback induced flow establishes a positive feedback with backward hinge migration on the surface, and has significant consequences for the composition and dynamics of the mantle wedge. Here we present results of 3-D numerical experiments aimed to quantify the partitioning between different forces acting on such a slab. These experiments include a high viscosity slab (relative to background mantle), a high viscosity lower mantle and a computational domain large enough so that the flow induced by subduction of a finite-width slab is not constrained by the side or bottom boundaries. We provide a self-consistent force balance and integrate the forces acting over the different portions of the slab, thereby partitioning such forces into specific components. We quantify the force due to rollback-induced flow, and signify its importance as a driving force relative to the other forces present: a net slab pull force, a force responsible for bending the slab at the subduction hinge, and a resistive force due to shear traction on the upper, lower, and nose (if present) surfaces of the subducted slab.

  17. Seismicity and tectonics of the subducted Cocos Plate

    NASA Astrophysics Data System (ADS)

    Burbach, George Vanness; Frohlich, Cliff; Pennington, Wayne D.; Matumoto, Tosimatu

    1984-09-01

    We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best-defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike-slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller-scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone. Appendices 2, 3, and 4 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84-009; $2.50.

  18. Trench-Parallel Flow Beneath the Nazca Plate from Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Russo, R. M.; Silver, P. G.

    1994-02-01

    Shear-wave splitting of S and SKS phases reveals the anisotropy and strain field of the mantle beneath the subducting Nazca plate, Cocos plate, and the Caribbean region. These observations can be used to test models of mantle flow. Two-dimensional entrained mantle flow beneath the subducting Nazca slab is not consistent with the data. Rather, there is evidence for horizontal trench-parallel flow in the mantle beneath the Nazca plate along much of the Andean subduction zone. Trench-parallel flow is attributable to retrograde motion of the slab, the decoupling of the slab and underlying mantle, and a partial barrier to flow at depth, resulting in lateral mantle flow beneath the slab. Such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the Pacific basin to the growing mantle reservoir beneath the Atlantic basin. Trench-parallel flow may explain the eastward motions of the Caribbean and Scotia sea plates, the anomalously shallow bathymetry of the eastern Nazca plate, the long-wavelength geoid high over western South America, and it may contribute to the high elevation and intense deformation of the central Andes.

  19. Seismicity, topography, and free-air gravity of the Aleutian-Alaska subduction zone

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Blakely, R. J.; Scholl, D. W.; Ryan, H. F.

    2011-12-01

    The Aleutian-Alaska subduction zone, extending 3400 km from the Queen Charlotte Fault to Kamchatka, has been the source of six great megathrust earthquakes in the 20th Century. Four earthquakes have ruptured the 2000-km-long Aleutian segment, where the Cenozoic Aleutian arc overlies the subducting Pacific plate. These include the 1946 M 8.6 earthquake off Unimak Is., the 1957 M 8.6 and 1986 M 8.0 earthquakes off the Andreanoff Is., and the 1965 M 8.7 Rat Is. earthquake. The source regions of these earthquakes inferred from waveform inversions underlie the well-defined Aleutian deep-sea terrace. The deep-sea terrace is about 4 km deep and is underlain by Eocene arc framework rocks, which extend nearly to the trench. It is bounded on its seaward and landward margins by strong topographic and fee-air gravity gradients. The main asperities (areas of largest slip) for the great earthquakes and nearly all of the Aleutian thrust CMT solutions lie beneath the Aleutian terrace, between the maximum gradients. Similar deep-sea terraces are characteristic of non-accretionary convergent margins globally (75% of subduction zones), and, where sampled by drilling (e.g., Japan, Peru, Tonga, Central America), are undergoing sustained subsidence. Sustained subsidence requires removal of arc crust beneath the terrace by basal subduction erosion (BSE). BSE is in part linked to the seismic cycle, as it occurs in the same location as the megathrust earthquakes. Along the eastern 1400 km of the Alaskan subduction zone, the Pacific plate subducts beneath the North American continent. The boundary between the Aleutian segment and the continent is well defined in free-air gravity, and the distinctive deep-sea terrace observed along the Aleutian segment is absent. Instead, the Alaskan margin consists of exhumed, underplated accretionary complexes forming outer arc gravity highs. Superimposed on them are broad topographic highs and lows forming forearc basins (Shumagin, Stevenson) and islands (Kodiak, Shumagin). Two great earthquakes ruptured much of this segment: the 1938 M 8.3 earthquake SW of Kodiak and the 1964 M 9.2 earthquake, which ruptured 800 km of the margin between Prince William Sound and Kodiak Island. Large slip during the 1938 event occurred under the Shumagin and Tugidak basins, but slip in 1964 is thought to have occurred on asperities under Prince William Sound and the outer arc highs off Kodiak. Seismic profiling and industry drilling indicates sustained subsidence has also occurred along the Alaska margin. BSE is probably occurring there, but the terrace structure is buried by the high sedimentation rate. At present, the inherited accretionary structures, the ongoing collision of the Yakutat terrane, and uncertainties in finite fault modeling obscure correlation of slip with topographic and gravity signatures in the 1964 source region.

  20. Tracing slab inputs along the Izu-Bonin-Marianas subduction zone: results from volatile emissions

    NASA Astrophysics Data System (ADS)

    Fischer, T. P.; Hilton, D. R.; Shaw, A. M.; Hauri, E. R.; Kazahaya, K.; Mitchell, E.; Shimizu, A.; de Moor, M.; Sharp, Z. D.

    2005-12-01

    The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the island of Honshu, Japan to Guam and is a type example of an intra-oceanic convergent margin. Subduction began 45 Ma ago and IBM subducts the oldest seafloor on Earth. A number of parameters vary systematically along the strike of the arc: the slab is steeply plunging in the S and gently dipping in the N; the age of the subducted crust varies from Mid-Jurassic in the S to Mid-Cretaceous in the N. Other parameters remain constant: crustal thickness (~20 km); no accretionary prism; no sediment fill in the trench. The sediment outboard of the arc is characterized based on ODP sites 801 (Marianas) and 1149 (Izu islands). 200 m of volcaniclastics are overlain by a 100 m of pelagic clay and chert in the S. In the N, volcaniclastics are lacking and the 400 m sediment sequence is dominated by 200 m of cherts, overlain by 40 m of pelagic clay and 120 m of volcanic ash and diatom/radiolarian clay. There is also a distinct layer (3 m) of hydrothermally altered MORB in the S. Thus, the IBM system is an ideal location to study the inputs and outputs of the subduction factory and to understand the processes occurring within the factory itself. We collected hydrothermal gas samples from 4 volcanic centers in the Marianas (Alamagan, Pagan, Agrigan, Uracas) and 6 centers in the Izu arc (Aogashima, Hachijojima, Niijima, Shikinejima, Oshima, Hakone). With the exception of Uracas (140C) and a well on Hachijojima (170C), all gas discharges were at or below the boiling temperature of water. As is typical for arc-related samples, the major gases are dominated by H2O, CO2 and S species. We see the following variations in N2/Ar and N2/He ratios of non-air contaminated samples along the arc: Agrigan clearly shows a mantle wedge signature of low N2/Ar (70) and N2/He (210) and negative ?15N (- 2.0 ). All other centers have N2/He ratios characteristic of that resulting from the addition of N from subducted sediments (1000 to 2500). Most Izu samples also show N2/Ar ratios higher than air (up to 210). Helium isotopes of Mariana samples are MORB-like (7.4 to 7.9 RA), whereas CO2/3He varies from 10.1 to 10.7 x 109 with ?13C between -0.5 to - 0.7 permil. Based on N-CO2-He-Ar sytematics, the Izu section of the arc has a signature characteristic of subducted sediment derived fluids. The Mariana section (Agrigan in particular), shows a volatile signature that suggests contribution dominantly from the altered oceanic basement. This is in contrast to studies based on trace elements and radiogenic isotopes that identify Agrigan as the `sediment endmember' of the Mariana arc. Analyses of stable and noble gas isotopes of the samples are currently underway to further constrain the source of volatiles discharging along the arc.

  1. Nature of the Moho in Japan and Kamchatka

    NASA Astrophysics Data System (ADS)

    Iwasaki, Takaya; Levin, Vadim; Nikulin, Alex; Iidaka, Takashi

    2013-04-01

    The Kamchatka peninsula and the islands of Japan are located along the eastern margin of the Asian continent. The natures of the Moho in these areas have been studied for decades, with a variety of geophysical methods, including active and passive seismic methods, gravity and other techniques. The Moho and the upper mantle structures in the NE Japan and SE Japan Arcs have been investigated well both from active and passive seismic source studies. The Moho depth in the NE Japan is ranging from 30 to 40 km. Almost parallel to the present volcanic front, there exists a belt of deep Moho (34-36 km) with a lower Pn velocity (7.5-7.7 km/s). Amplitude analysis of the PmP phase indicates that the Moho beneath the NE Japan Arc is not a simple velocity contrast, but rather a gradual transition. Toward the backarc side, remarkable crustal thinning is recognized. Actually, the Moho depth decreases from 35 km beneath the central part of NE Japan to 18 km beneath the Sea of Japan. This thinning is evident in the upper crust, while the lower crust remains constant in thickness. This may be explained by the continuous magmatic underplating beneath the rifted crust or deformation under a simple shear mode, allowing the lower crustal thickness to remain unchanged. The Moho in the SW Japan Arc is also at a depth of 30-40 km. The Pn velocity is 7.7-7.8 km/s, slightly higher than that in the NE Japan, although this value was mostly sampled in the eastern half of the SW Japan Arc where the recent volcanic activity has been less effective. Fluids expelled from the subducted oceanic lithosphere (the PHS plate) play an important role in controlling the structure of the mantle wedge. As these fluids leak into the mantle wedge they induce serpentinization there, transforming original mantle materials to those of lower velocity and higher Vp/Vs. The crustal thinning of the SW Japan Arc is characterized by notable decrease in upper crustal thickness, which is similar to the case of the NE Japan Arc. The Moho and uppermost mantle structures beneath the southern part of the Kamchatka have a lot of similarities to those beneath the NE Japan Arc. Earlier DSS investigations and converted wave analyses show that Moho is situated at a depth of 38-40 km along the east coast of Kamchatka, that is beneath the volcanic front, but decreases to about 32 km near the west coast. Moho depth values based on modern receiver function methodology are also ranging from 31 to over 38 km. Moho is a fairly simple boundary under the western coast of Kamchatka, while in the Central Kamchatka Depression and especially along the eastern coast it is likely gradational. Uppermost mantle material beneath the Moho is complex, with additional impedance contrasts that are likely anisotropic in their properties being present under the entire Kamchatka peninsula. The dominant anisotorpy-inducing fabric varies from site to site along the west coast, but is almost universally trench-normal along the east coast. The seismic velocities beneath Kamchatka are very low (7.4-7.8 km/s for P-wave and 4.1-4.2 km/s for S wave). Also, gradual structural change is recognized around the Moho beneath the active volcanoes. These features are quite similar to those in NE Japan Arc.

  2. Interseismic Strain Along the Sumatra Subduction Zone: A Case for a Locked Fault Portion Extending Well Below the Forearc Moho

    NASA Astrophysics Data System (ADS)

    Simoes, M.; Avouac, J.; Cattin, R.; Henry, P.; Natawidjaja, D. H.

    2003-12-01

    A current and most accepted view about the seismogenic zone along subduction zones is that the downdip extent of the locked fault portion would correspond either to the 350° C isotherm if this temperature is reached above the Moho, or to the intersection with the forearc Moho for colder subduction zones [Oleskevich et al., 1999]. This limit would reflect the transition from slip-weakening friction to aseismic stable-sliding, or else ductile flow. In the first case, when the downdip end of the locked zone is temperature-controlled, stable-sliding of quartzo-feldspathic rocks would be the controlling factor, while the systematic presence of serpentinite or other hydrated minerals is advocated to explain aseismic interplate slip in the mantle at temperatures much less that the 750° C needed for ductile flow of mantle rocks [Peacock and Hyndman, 1999]. Here, we consider the case of the Sumatra subduction zone where the ~53 Myr Indian oceanic crust subducts below an island-arc characterized by a relatively thin crust, with a Moho depth estimated to ~23 km. We model interseismic deformation from a creeping dislocation embedded in an elastic half-space, using the back-slip approach. In addition to recently published GPS velocities, we take advantage of recent data on the pattern and rate of interseismic uplift that have been obtained from the study of coral growth [Natawidjaja, 2003; Sieh et al., 1999]. These data are found to put tight constraints on the horizontal position of the downdip limit of the locked fault zone, at 127 +/- 4/2 km from the trench. This position corresponds to a depth between 40 and 58 km and to a temperature of 269° C +/- 155° C, when compared with thermal modeling. So, in this particular setting, the locked fault portion extends well into the mantle. However, temperature is not high enough to advocate stable sliding or ductile flow of unaltered or altered mantle rocks. This case appeals to some reappraisal of the physical control on the depth of the locked fault zone along subduction zones. References. Natawidjaja, D.H., Neotectonics of the Sumatran fault and paleogeodesy of the Sumatran subduction zone., PhD thesis, California Institute of Technology, Pasadena, 2003. Oleskevich, D.A., R.D. Hyndman, and K. Wang, The updip and downdip limit to great subduction earthquakes : thermal and structural models of Cascadia, South Alaska, SW Japan and Chile., Journal of Geophysical Research, 104 (B7), 14965-14991, 1999. Peacock, S.M., and R.D. Hyndman, Hydrous minerals in the mantle wedge and the maximum depth of subdcution thrust earthquakes., Geophysical Research Letters, 26 (16), 2517-2520, 1999. Sieh, K., S.N. Ward, D. Natawidjaja, and B.W. Suwargadi, Crustal deformation at the Sumatra subduction zone revealed by coral data., Geophysical Research Letters, 26 (20), 3141-3144, 1999.

  3. Revisiting the physical characterisitics of the subduction interplate seismogenic zones

    NASA Astrophysics Data System (ADS)

    Heuret, Arnauld; Lallemand, Serge; Funiciello, Francesca; Piromallo, Claudia

    2010-05-01

    Based on the Centennial earthquake catalog, the revised 1964-2007 EHB hypocenters catalog and the 1976-2007 CMT Harvard catalog, we have extracted the hypocenters, nodal planes and seismic moments of worldwide subduction earthquakes for the 1900-2007 period. For the 1976-2007 period, we combine the focal solutions provided by Harvard and the revised hypocenters from Engdahl et al. (1998). Older events are extracted from the Centennial catalogue (Engdahl and Villasenor, 2002) and they are used to estimate the cumulated seismic moment only. The selection criteria for the subduction earthquakes are similar to those used by Mc Caffrey (1994), i.e., we test if the focal mechanisms are consistent with 1/ shallow thrust events (depth > 70 km, positive slips, and at least one nodal plane gets dip < 45°), and, 2/ the plate interface local geometry and orientation (one nodal plane is oriented toward the volcanic arc, the azimuth of this nodal plane ranges between ± 45° with respect to the trench one, its dip ranges between ± 20° with respect to the slab one and the epicentre is located seaward of the volcanic arc). Our study concerns segments of subduction zones that fit with estimated paleoruptures associated with major events (M > 8). We assume that the seismogenic zone coincides with the distribution of 5.5 < M < 7 subduction earthquakes. We provide a map of the interplate seismogenic zones for 80% of the trench systems including dip, length, downdip and updip limits, we revisit the statistical study done by Pacheco et al. (1993) and test some empirical laws obtained for example by Ruff and Kanamori (1980) in light of a more complete, detailed, accurate and uniform description of the subduction interplate seismogenic zone. Since subduction earthquakes result from stress accumulation along the interplate and stress depends on plates kinematics, subduction zone geometry, thermal state and seismic coupling, we aim to isolate some correlations between parameters. The statistical analysis reveals that: 1- vs, the subduction velocity is the first order controlling parameter of seismogenic zone variability, both in term of geometry and seismic behaviour; 2- steep dip, large vertical extent and narrow horizontal extent of the seismogenic zone are associated to fast subductions, and cold slabs, the opposite holding for slow subductions and warm slabs; the seismogenic zone usually ends in the fore-arc mantle rather than at the upper plate Moho depth; 3- seismic rate () variability is coherent with the geometry of the seismogenic zone:  increases with the dip and with the vertical extent of the seismogenic zone, and it fits with vs and with the subducting plate thermal state; 4- mega-events occurrence determines the level of seismic energy released along the subduction interface, whatever  is; 5- to some extent, the potential size of earthquakes fits with vs and with the seismogenic zone geometry, but second order controlling parameters are more difficult to detect; 6- the plate coupling, measured through Upper Plate Strain, is one possible second order parameter: mega-events are preferentially associated to neutral subductions, i.e. moderate compressive stresses along the plate interface; high plate coupling (compressive UPS) is thought to inhibit mega-events genesis by enhancing the locking of the plate interface and preventing the rupture to extend laterally. This research was supported as part of the Eurohorcs/ESF — European Young Investigators Awards Scheme (resp. F.F.), by funds from the National Research Council of Italy and other National Funding Agencies participating in the 3rd Memorandum of Understanding, as well as from the EC Sixth Framework Programme.

  4. Continental response to active ridge subduction

    NASA Astrophysics Data System (ADS)

    Haschke, M.; Sobel, E. R.; Blisniuk, P.; Strecker, M. R.; Warkus, F.

    2006-08-01

    Apatite fission track ages from a ~2000 m elevation transect from the Patagonian fold and thrust belt (47.5S) allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction. Accelerated cooling started at 17 Ma, predating the onset of ridge collision (14-10 Ma), and was followed by reheating between 10 and 6 Ma. Thermal modeling favors reheating on the order of 60C at ~28C/Ma due to east-migration of a slab window after the ridge-trench collision. Final rapid cooling since 4 Ma of ~18C/Ma (geothermal gradient of 14C/km) correlates with the presence of an orographic barrier and >1 km rock uplift in this region between 17.1 and 6.3 Ma. Increased precipitation and erosion since 4 Ma caused asymmetric exhumation, with 3-4 km on the leeside. Repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the Chile Triple Junction.

  5. A precise bathymetric map of the world's deepest seafloor, Challenger Deep in the Mariana Trench

    NASA Astrophysics Data System (ADS)

    Nakanishi, Masao; Hashimoto, Jun

    2011-12-01

    Data from three bathymetric surveys by R/V Kairei using a 12-kHz multibeam echosounder and differential GPS were used to create an improved topographic model of the Challenger Deep in the southwestern part of the Mariana Trench, which is known as the deepest seafloor in the world. The strike of most of the elongated structures related to plate bending accompanied by subduction of the Pacific plate is N70°E and is not parallel to the trench axis. The bending-related structures were formed by reactivation of seafloor spreading fabric. Challenger Deep consists of three en echelon depressions along the trench axis, each of which is 6-10 km long, about 2 km wide, and deeper than 10,850 m. The eastern depression is the deepest, with a depth of 10,920 ± 5 m.

  6. Sedimentation and deformation in the Amlia Fracture Zone sector of the Aleutian Trench

    USGS Publications Warehouse

    Scholl, D. W.; Vallier, T.L.; Stevenson, A.J.

    1982-01-01

    A wedge-shaped, landward thickening mass of sedimentary deposits composed chiefly of terrigenous turbidite beds underlies the west-south west-trending Amlia sector (172??20???-173??30???W) of the Aleutian Trench. Pacific oceanic crust dips northward beneath the sector's sedimentary wedge and obliquely underthrusts (30?? off normal) the adjacent Aleutian Ridge. The trench floor and subsurface strata dip gently northward toward the base of the inner trench slope. The dip of the trench deposits increases downsection from about 0.2?? at the trench floor to as much as 6-7?? just above basement. The wedge is typically 2-2.5 km thick, but it is thickest (3.7-4.0 km) near the base of the inner slope overlying the north-trending Amlia Fracture Zone and also east of this structure. Slight undulations and relatively abrupt offsets of the trench floor reflect subsurface and generally west-trending structures within the wedge that are superimposed above ridges and swales in the underlying oceanic basement. The southern or seaward side of some of these structures are bordered by high-angle faults or abrupt flexures. Across these offsets the northern side of the trench floor and underlying wedge is typically upthrown. West-flowing turbidity currents originating along the Alaskan segment of the trench (1200 km to the east) probably formed the greater part of the Amlia wedge during the past 0.5 m.y. The gentle northward or cross-trench inclination of the trench floor and underlying wedge probably reflects regional downbending of the oceanic lithosphere and trench-floor basement faulting and rotation. Much of the undulatory flexuring of the trench wedge can be attributed to differential compaction over buried basement relief. However, abrupt structural offsets attest to basement faulting. Faulting is associated with extensional earthquakes in the upper crust. The west-trending basement offsets are probably normal faults that dip steeply south or antithetic to the north dip of the subducting oceanic crust. Up-to-arc extensional faulting can be attributed to the downbending of the Pacific plate into the Aleutian subduction zone. The rupturing direction and dip is controlled by zones of crustal weakness that parallel north Pacific magnetic anomalies, which were formed south of a late Cretaceous-early Tertiary spreading center (Kula-Pacific Ridge). The strike of these anomalies is fortuitously nearly parallel to the Amlia sector. The up-to-arc fracturing style may locally assist in elevating blocks of trench deposits to form the toe of the trench's landward slope, which is in part underlain by a compressionally thickened accretionary mass of older trench deposits. Compressional structures that can be related to underthrusting are only indistinctly recorded in the turbidite wedge that underlies the trench floor. ?? 1982.

  7. Genesis of jadeite-quartz rocks in the Yorii area of the Kanto Mountains, Japan

    NASA Astrophysics Data System (ADS)

    Fukuyama, Mayuko; Ogasawara, Masatsugu; Horie, Kenji; Lee, Der-Chuen

    2013-02-01

    This paper reports the results of U-Pb dating and REE (rare earth element) analysis of zircons separated from jadeite-quartz rocks within serpentinite mlanges in the Yorii area of the Kanto Mountains, Japan. These rocks contain jadeite, albite, and quartz, with minor aegirine-augite, zircon, monazite, thorite, allanite, and titanite. Mineral textures provide evidence of a jadeite + quartz = albite reaction during formation of these jadeite-quartz rocks. Zircon crystals separated from the jadeite-quartz rocks can be split into two distinct types, here named Types I and II, based on their morphology and REE concentrations. Type I zircons are prismatic and have fluid, jadeite, quartz, and albite inclusions. Those show positive Ce and negative Eu anomalies and HREE (heavy rare earth element) enriched chondrite normalized REE patterns and have higher REE concentrations than those generally found in magmatic zircons. Type I zircons would have precipitated from a fluid. Mineralogical observation provides that Type I zircon crystallized at the same timing of the formation of the jadeite-quartz rocks. Type II zircons are porous and have REE patterns indicative of a hydrothermal zircon. Both types of zircons are fluid-related. Type I zircons yield U-Pb ages of 162.2 0.6 Ma, with an MSWD (mean square weighted deviation) of 1.4. At this time, Japan was still a part of the eastern margin of the Asian continent, with the subduction of the oceanic paleo-Pacific Plate leading to the formation of the Jurassic Mino-Tanba-Chichibu accretionary complex in Japan. The age data indicate that the jadeite-quartz rocks formed in a deep subduction zone environment at the same time as the formation of the Jurassic accretionary complex in a shallower near-trench subduction zone environment. The jadeite-quartz rocks contain high concentrations of Zr and Nb, with low LILE (large ion lithophile elements) concentrations, suggesting that the HFSE (high field strength elements) can be concentrated into jadeite-quartz rocks prior to a fluid moving up into the mantle wedge. Typical arc volcanic rocks are depleted in the HFSE, suggesting that the high HFSE concentrations within jadeite-quartz rocks are consistent with fluids being st