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Sample records for japan trench subduction

  1. Marine magnetic anomaly and magnetization of subducting Pacific Plate around the Japan Trench

    NASA Astrophysics Data System (ADS)

    Fujiwara, T.

    2013-12-01

    We studied marine magnetic anomaly in the northwestern margin of the Pacific Plate off Japan to examine whether the magnetic anomaly varies due to tectonic phenomenon caused by the plate subduction. For the sake of this study, we newly collected magnetic data aboard JAMSTEC cruises in the seaward area where was sparsely surveyed, and made a magnetic anomaly map by compilation of our data, data published by Geological Survey of Japan, and data from NGDC. The seafloor of the seaward slope of the Japan Trench is characterized by a series of parallel magnetic anomalies (Japanese Lineation Set) during M11-M7 (135-127 Ma). The anomalies are well lineated and have high-amplitudes of ~500-1000 nT peak-to-trough. The amplitudes of the anomalies gradually decay to the landward from the trench axis associated with the plate subduction. Equivalent magnetization was calculated from the magnetic anomaly to correct for effects of seafloor topography and increasing depth of subducting plate. Densely distributed seismic survey profiles in the study area enabled us to constrain the depth of the plate. On the seaward trench slope from the trench axis to a distance of ca. 100 km, horst-graben structure is developed and large steps grow associated with plate bending and normal faulting, which would cause some kind of destruction and mechanical disorganization of the magnetic layer by faulting. However, the magnetization is not influenced apparently there. The magnetization gradually decreases as the plate subduction proceeded. The apparent decay could reflect destruction and mechanical disorganization and/or chemical demagnetization of the topmost part of the oceanic crust along the plate boundary. The magnetization in reverse polarity decays larger than that in normal polarity. The result is indicative of reduction of remanence in the oceanic crust and induced magnetization possibly due to serpentinized uppermost mantle.

  2. 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.

  3. Combined wide-angle and multichannel seismic survey at an asperity of subduction earthquakes in the Japan Trench

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Miura, S.; Smith, A. J.; Sato, T.; Tsuru, T.; Fujie, G.; Ito, A.; Takahashi, N.; Suyehiro, K.; Kaneda, Y.; Hino, R.; Mochizuki, K.; Kasahara, J.; Kanazawa, T.

    2003-12-01

    Recent progress of seismic wave form inversion revealed overlapping asperities of large earthquakes in the Japan Trench, e.g., the 1994 Sanriku-Haruka-Oki earthquake (M=7.5) and 1968 Tokachi-Oki earthquake (M=7.9). It is also recognized that the epicenters (initial break) of both earthquakes are situated at the trench-ward edge of the asperities, and the aftershocks were observed only around the asperities. Investigating structural factor controlling these asperity and seismicity pattern is believed to provide important and fundamental information to the physics of earthquakes. In August, 2003, we acquired wide-angle and multichannel seismic data covering the entire asperity region. The purpose of this study is to image structures of the asperity, and investigate the structures affecting the distribution of the seismicity pattern. Part of MCS data was processed onboard. We made CDP stacks by applying multiple suppression and poststack time migration. From the migration section of the dip profile, we recognized several key structures related to a subduction process; e.g., 1) the top of subducted oceanic crust can be traced up to 80 km from the trench axis. This reflector is recognized at 10 s twt to 70 km from the axis, rising up to 9s at 75 km then again down to 10 s farther than 75 km, 2) from 30 to 50km and 70 to 75 km, a weak reflector which is parallel to the top of the oceanic crust is identified. This might be the base of subducted crust, 3) located 35 to 45 km from the trench axis are several sub-parallel reflectors at 1 s above the top of the oceanic crust. These sub-parallel reflectors are also recognized at a central part of a strike profile, however there reflectors are not clear at the northern and southern end of the profile.

  4. Accelerated pacific plate subduction following interplate thrust earthquakes at the Japan trench

    NASA Astrophysics Data System (ADS)

    Heki, Kosuke; Mitsui, Yuta

    2013-02-01

    Interplate thrust earthquakes are usually followed by afterslips, and they let the fore-arc move slowly trenchward. However, we do not know if the subducting oceanic plate is accelerated landward after such earthquakes. The westward velocity of Global Positioning System (GPS) stations in NE Japan show gradient decreasing from east to west reflecting the E-W contractional strain built up by the inter-plate coupling. Here we show that such coupling significantly enhanced (∼1.5 times) after the 2003 Tokachi-Oki earthquake (Mw 8.0), Hokkaido, in the segments adjacent to the ruptured fault. The coupling seems to be further enhanced (∼3 times) after the 2011 Tohoku-Oki earthquake (Mw 9.0). It is unlikely that interplate friction suddenly increased over such a large region, and relatively strong pre-2003 coupling there would not allow such enhancements even if full coupling is attained. Hence they are attributable to the temporary acceleration of the Pacific Plate subduction. We propose a simple 2-dimensional model in which down-dip acceleration of the slab let the force balance rapidly recover promoted by a thin low-viscosity layer on the slab surface. The accelerated subduction would account for temporary activations of regional interplate seismicity after megathrust earthquakes.

  5. 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.

  6. 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 décollement 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 décollement 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 décollement suggests that rupture to the trench may be characteristic of this margin.

  7. The alkaline magma squeezed upward by the plate flexure prior to subduction off the Chile and Japan Trenches

    NASA Astrophysics Data System (ADS)

    Hirano, N.; Machida, S.; Abe, N.

    2010-12-01

    The petit-spot monogenetic volcanoes on the NW Pacific Plate represent a new type of volcanism on their tectonic settings (Hirano et al., 2006). The most important feature of petit-spot volcanoes is that they do not derive their heat supply from the deep mantle (in contrast to hotspot volcanoes), despite their occurrence as intra-plate volcanoes. Instead, the magma probably originates within the asthenosphere and erupts along fractures in the lithosphere where it is flexed prior to subduction. Although it is clear that the surface morphology and distribution of petit-spot volcanoes are influenced by cracks in the lithospheric that reach the surface, it remains uncertain whether petit-spot volcanoes form wherever the plate flexes and fractures. The project “Expedition of petit-spot VI” was carried out off the Chile Trench on March 2009 using R/V Mirai in order to find such young volcanoes. The area, off Valparaiso, Chile, is characterized by trench-parallel normal faults (horsts and graben) resulting from extensional bending of the subducting Nazca Plate. An important difference between the area with trench-parallel normal faults and other parts of the subducting plate is the presence of tiny knolls. Dredged rocks at the knoll are highly vesicular, and fresh specimens with quench features are associated with lava lobes and breccias within the pelagic sediments. The rock and bathymetry are similar to the petit-spot volcanoes on the NW Pacific Plate. Therefore, the widespread occurrence of petit-spot is indicated by the discovery of petit-spots at the Nazca Plate. The most important process to occur the petit-spot volcanoes could be the plate flexures and their tectonic forces.

  8. 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.

  9. 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.

  10. Deep scientific dives in the Japan and Kuril Trenches

    NASA Astrophysics Data System (ADS)

    Cadet, Jean Paul; Kobayashi, Kazuo; Lallemand, Serge; Jolivet, Laurent; Aubouin, Jean; Boulègue, Jacques; Dubois, Jacques; Hotta, Hiroshi; Ishii, Teruaki; Konishi, Kenji; Niitsuma, Nobuaki; Shimamura, Hideki

    1987-05-01

    In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible "Nautile". The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2-3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.

  11. Trench motion, slab geometry and viscous stresses in subduction systems

    NASA Astrophysics Data System (ADS)

    Royden, Leigh H.; Husson, Laurent

    2006-11-01

    A semi-analytic, 3-D model for subduction within a Newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress, trench motion and slab geometry in subduction systems. Although negative slab buoyancy provides the basic driving force for subduction, slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow (<100-150 km) mantle and undersupported by viscous stresses at greater depth in the upper mantle. These deeper parts of the subduction system act as an `engine' for subduction while shallower parts act as a `brake' on trench motion; trench migration rates and slab geometry reflect a competition between these two effects. During steady-state subduction, trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of ~2 × 1020 Pa s above 300 km and ~5 × 1020 Pa s below. Steady-state slab dip increases as slab density decreases, especially for very low-density slabs, which dip significantly more steeply than high-density slabs. The horizontal velocity at the top of the lower mantle, measured relative to the foreland, has a very large effect on trench migration rates, rivalling or even exceeding that of slab buoyancy. Slab width, parallel to the trench, also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab. The stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere. Very stiff slabs, with elastic plate thicknesses more that ~40 km or viscosities in the range of 1025-1026 Pa s, subduct significantly more slowly than weak slabs, with trench migration rates in the range of half to a third that of weak slabs. Large, unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate, indicating that local geology can exert important constraints on slab dynamics. During non-steady-state subduction, rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere. In the absence of other driving forces for convergence, trench migration rates can change by a factor of two or more in as little as 2-3 Myr, for example, from 35 to 70 mm yr-1 when an oceanic piece of slab follows a continental one into the subduction system. Subduction of variable-buoyancy lithosphere is accompanied by changes in slab dip with depth and through time.

  12. The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

    USGS Publications Warehouse

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

    1987-01-01

    This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V "Jean Charcot". The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65?? oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary? ?? 1987.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. Aftershocks of the December 7, 2012 intraplate doublet near the Japan Trench axis

    NASA Astrophysics Data System (ADS)

    Obana, Koichiro; Kodaira, Shuichi; Nakamura, Yasuyuki; Sato, Takeshi; Fujie, Gou; Takahashi, Tsutomu; Yamamoto, Yojiro

    2014-12-01

    On December 7, 2012, a pair of large Mw 7.2 intraplate earthquakes occurred near the Japan Trench axis off Miyagi, northeast Japan. This doublet consisted of a deep reverse-faulting event followed by a shallow normal-faulting event. Aftershock observations using conventional and newly developed ultra-deep ocean bottom seismographs in the trench axis area showed that the shallow normal-faulting event occurred in the subducting Pacific plate just landward of the trench axis. The shallow normal-faulting aftershock activity indicated that in-plate tension in the incoming/subducting Pacific plate extends to a depth of at least 30 km, which is deeper than before the 2011 Tohoku-Oki earthquake, whereas in-plate compression occurs at depths of more than 50 km. Hence, we concluded that the neutral plane of the in-plate stress is located between depths of 30 and 50 km near the trench axis.

  18. 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.

  19. 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.

  20. Oblique subduction modelling indicates along-trench tectonic transport of sediments.

    PubMed

    Malatesta, Cristina; Gerya, Taras; Crispini, Laura; Federico, Laura; Capponi, Giovanni

    2013-01-01

    Convergent plate margins are currently distinguished as 'accretional' or 'erosional', depending on the tendency to accumulate sediments, or not, at the trench. Accretion and erosion can coexist along the same margin and we have noticed that this mostly occurs where subduction is oblique. Here we show that at oblique subduction zones, sediments that enter the trench are first buried, and later migrate laterally parallel to the trench and at various depths. Lateral migration of sediments continues until they reach a physical barrier where they begin to accumulate. The accretionary wedge size decreases along the trench moving away from the barrier. We therefore suggest that the gradual variation of the accretionary wedge size and sediment amount at the trench along one single subduction zone, as observed in many active plate margins worldwide, can be explained by the lateral tectonic migration of sediments driven by obliquity of subduction as well. PMID:24030161

  1. JDASH - Japan Trench Deep-sea Research for Assessing Shallow Seismic Slips and Their History

    NASA Astrophysics Data System (ADS)

    Hino, R.; Kodaira, S.; Kanamatsu, T.; Shinohara, M.; Ito, Y.

    2014-12-01

    We started a multidisciplinary research project to elucidate the history of recurring tsunamigenic slips on the shallowest portion of the plate boundary fault along the Japan Trench before the 2011 earthquake and to understand post-seismic transient processes since the massive rupture of 2011. In the 5-year project, we will: 1) Conduct high-resolution seismic surveys and use the resultant data to both elucidate the spatial extent of past shallow seismic slip events and identify deformation structures characteristic of the events. 2) Determine the timing of past slip events by collecting sub-seafloor sedimentary core samples from the ultra-deep environment near the Japan Trench and identifying and dating earthquake-generated turbidite layers. 3) Identify postseismic deformation processes on the shallow fault by using dense arrays of broadband seismic sensors and monitoring seafloor deformation near the trench axis. The geodetic monitoring includes continuous measurement of baseline lengths across the trench axis, which reflects relative motion of the overriding North American (or Amur) plate and the subducting Pacific plate. Systematic studies on past shallow seismic slip events will improve our understanding of the space-time history of large earthquakes, which has mostly derived from geological studies on tsunami deposits along coasts. The space-time history of large earthquakes will provide clues to understanding why earthquakes of M <8 have been frequent and those of M~9 infrequent in the same subduction system, a fundamental question raised by the 2011 Tohoku earthquake. The post-2011 deformation near the trench will put strong constraint on the frictional behavior of the updip end of the megathrust. Since shallow slip events increase size of tsunamis associated with megathrust earthquakes, history of past events is crucial for evaluating tsunami hazard in regions facing to subduction zones. This project will establish a methodology to clarify reliable reccurence history of shallow seismic slip events in subduction zones in the world.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. Seismological evidence of fault ruptures reaching the trench axis in the Japan Trench

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Nakamura, Y.; Miura, S.; Kasaya, T.; Kanamatsu, T.; Ikehara, K.; Moore, G. F.

    2013-12-01

    The 2011 Tohoku-Oki earthquake in the Japan Trench is characterized by extremely large slip (more than 50 m) close to the trench. This is clearly indicated by data from repeated bathymetric and seismic surveys, which 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. One of the most important observations from those data is that the cumulative displacement observed in the deformed sediment mass is estimated to be more than 1 km, which is far larger than the slip of a single earthquake. This means that the deformation structure in the trench-filling sediment may preserve a long term record of earthquake slips reaching to the trench axis. In other words, it can be possible by using seismic data, to map an area where co-seismic slip reaches the axis. In order to test this idea, we have started a high-resolution seismic imaging project along the entire Japan Trench axis. Through the summer of 2013, the survey has been completed from 38 o N to 40o N. Onboard processed seismic sections show strong along-trench variation of structures of the trench-filling sediment. The deformed sediment mass decreases in size northward from 38o N and becomes obscure at around 38.5o N where a horst is situated in the trench axis. Although 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 spatial distribution and temporal variation of slip to the trench, we will integrate the seismic images with geological studies, such as piston-coring data obtained in the trench. Furthermore, in order to know even longer records of earthquake slips and evidences of seismic fault motions (i.e., high velocity slip) along faults reaching at the trench axis, we plan to propose a future along-trench axis drilling transect down to the bottom of the trench filling sediment mass.

  7. 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.

  8. 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.

  9. Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong

    2016-02-01

    We determined P-wave tomographic images by inverting a large number of arrival-time data from 2749 local earthquakes and 1462 teleseismic events, which are used to depict the three-dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ˜19°N and ˜21°N. The subducted buoyant plateau is possibly oriented toward NW-SE, and the subducted plate at ˜21°N is slightly steeper than that at ˜19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ˜50 km at ˜18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low-velocity zone is revealed at depths of 20-200 km beneath the Manila Accretionary Prism at ˜22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction.

  10. 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.

  11. 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.

  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. Visual observations of the sea floor subduction line in the middle-america trench.

    PubMed

    Heezen, B C; Rawson, M

    1977-04-22

    Four dives were made to the floor of the Middle-America Trench with the U.S. Navy's deep research submersible DSV Turtle. The area investigated is located between Costa Rica and the Cocos Ridge where the depth of the trench floor does not exceed the 2000-meter capability of the submersible. At the axis of the trench floor a series of steep northeast-facing scarps 10 to 20 meters high lie parallel to the trench axis. Here oceanic crust appears to have been carried down by near-vertical normal vaults of small displacement. Between these small scarps and the landward wall of the trench a narrow line of recent deformation interrupts a smooth apron. Unconsolidated sediments are thrust in sharply serrated piles and cut by sharp-edged chasms. This line of deformation is interpreted as the present sea floor trace of crustal subduction. PMID:17776949

  14. Trench-parallel flow and seismic anisotropy in the Mariana and Andean subduction systems.

    PubMed

    Kneller, Erik A; van Keken, Peter E

    2007-12-20

    Shear-wave splitting measurements above the mantle wedge of the Mariana and southern Andean subduction zones show trench-parallel seismically fast directions close to the trench and abrupt rotations to trench-perpendicular anisotropy in the back arc. These patterns of seismic anisotropy may be caused by three-dimensional flow associated with along-strike variations in slab geometry. The Mariana and Andean subduction systems are associated with the largest along-strike variations of slab geometry observed on Earth and are ideal for testing the link between slab geometry and solid-state creep processes in the mantle. Here we show, with fully three-dimensional non-newtonian subduction zone models, that the strong curvature of the Mariana slab and the transition to shallow slab dip in the Southern Andes give rise to strong trench-parallel stretching in the warm-arc and warm-back-arc mantle and to abrupt rotations in stretching directions that are accompanied by strong trench-parallel stretching. These models show that the patterns of shear-wave splitting observed in the Mariana and southern Andean systems may be caused by significant three-dimensional flow induced by along-strike variations in slab geometry. PMID:18097407

  15. Subduction dynamics: From the trench to the core-mantle boundary

    NASA Astrophysics Data System (ADS)

    Kincaid, Chris

    1995-07-01

    Subduction occurs along convergent plate boundaries where one of the colliding lithospheric plates descends into the mantle. Subduction zones are recognized where plates converge at ˜2-15 cm/yr, although well developed trenches and volcanic arcs (e.g. the line of active volcanoes lying parallel to most ocean trenches, such as the Aleutian Islands in the North Pacific) occur when convergence rates are higher, 4-10 cm/yr. This report is meant to provide a brief review on the general topic of subduction dynamics. A recent spin on subduction studies is the growing realization that the need to understand this global Earth process may be argued not only on purely scientific grounds, but also in terms of societal relevance. While subducting slabs of oceanic lithosphere clearly provide the dominant driving force for mantle dynamics and plate tectonics, over half of the Earth's present 40,000 km of subduction zones are associated with continental margins where a large and rapidly increasing percentage of the Earth's population resides. Subductioninduced hazards along active continental margins include those associated with volcanic hazards (Blong, 1984; Tilling, 1989) such as lava flows, pyroclastic flows and ash fallout and tectonic processes, such as faulting, tsunamis and earthquakes. With regards to earthquake hazards, all of the great (magnitude >9) earthquakes in recorded history have occurred at subduction zones, with 50% of all energy released since 1900 being in four events (1964-Alaska; 1960-Chile; 1957- Aleutians; 1952-Kamchatka). Subduction zone hazards have significant impact on long time scales, such as contributions to global climate change (Robock, 1991; Simarski, 1992; Johnson, 1993; Bluth et al., 1993) and short time scales such as airline safety (Casadevall, 1992). Moreover, accretionary wedges are important in terms of resource potential and trenches have occasionally been suggested as nuclear waste disposal sites.

  16. An ongoing large submarine landslide at the Japan trench

    NASA Astrophysics Data System (ADS)

    Nitta, S.; Kasaya, T.; Miura, S.; Kawamura, K.

    2013-12-01

    This paper deals with an active submarine landslide on a landward trench slope in the Japan trench. Studied area is located on the upper terrace ranging from 400 to 1200 m in water depth, off Sendai, northeast Japan. We have surveyed in detail the seabed topography using a multi narrow beam (hereafter MBES) and a subbottom profiler (hereafter SBP) during the cruise MR12-E02 of R/V Mirai. The survey lines were 12 lines in N-S, and 3 lines in E-W, and situated in the region from 141°45'E, 37°40'N to 142°33'E, 38°32'N. Moreover, we used multi-channel seismic profile by the cruise KR04-10 of R/V Kairei in the interpretation of the SBP results. In general, horseshoe-shaped depressions of about 100 km wide along the trench slope are arrayed along the Japan trench. It has thought that they were formed by large submarine landslides, but we could not understand critically the relationship between the depressions and the submarine landslides. Based on the survey results, we found signals of an active submarine landslide in the depression as follows. 1) We observed arcuate-shaped lineaments, which are sub-parallel to a horseshoe-shaped depression. The lineaments concentrate in the south region from 38°N at about 20 km wide. These lineaments are formed by deformation structures as anticlines, synclines and normal fault sense displacements. 2) Most of the synclines and anticlines are not buried to form the lineaments. 3) Normal faults cutting about 1 km deep are observed in a multi-channel seismic profile. The normal faults are located just below the arcuate-shaped lineaments, and are tilted eastward being the downslope direction. It indicates a large submarine landslide. We concluded that the arcuate-shaped lineaments were generated by surface sediment movement with the submarine landsliding. We think that the submarine landslide of about 20 km wide and about 1 km thick move continuously down the landward trench slope. This would be the formation process of the horseshoe-shaped depression along the Japan trench.

  17. Development of Observatories for the Japan Trench Fast Drilling Project

    NASA Astrophysics Data System (ADS)

    Kyo, N.; Namba, Y.; Saruhashi, T.; Sawada, I.; Eguchi, N.; Toczko, S.; Kano, Y.; Yamano, M.; Muraki, H.; Fulton, P. M.; Brodsky, E. E.; Davis, E. E.; Sun, T.; Mori, J. J.; Chester, F. M.

    2012-12-01

    The Mw 9.0 Tohoku earthquake and accompanying tsunami produced the largest slip ever recorded in an earthquake and devastated much of northern Japan on March 11, 2011. The IODP proposal for JFAST (Japan Trench Fast Drilling project) planned to drill into the Tohoku subduction zone using the research ship Chikyu, measure the fault zone physical properties, recover fault zone material, and install an observatory to directly record the temperature anomaly caused by frictional slip during the earthquake. Considering the significant technical and operational challenges related to the great water depth of ~7,000 meters, and timing constraints, the observatory needed to be both robust and simple. After frequent discussions among scientists, engineers and operators, we decided to prepare two different types of observatories. 1. Autonomous MTL (Miniature Temperature Logger) observatory. The important temperature monitoring is accomplished by 55 MTLs attached to a string (Vectran rope) which is suspended inside a 4.5" casing in the borehole. The string latches at the top of the casing to allow retrieval using the remotely operated vehicle (ROV) Kaiko operated by JAMSTEC. This observatory avoids risks associated with a thermistor cable and wellhead data logger, and increases reliability by applying proven technologies. Perhaps most importantly, this configuration allows flexibility in defining the final depth distribution of the temperature sensors. This is advantageous since information of the exact depth of the fault zone will be known only after drilling and logging. Also, the judicious placement of weak links along the string helps to minimize possible loss of the entire sensor string if it is clamped by post-seismic movements that deform the casing. 2. Telemetered PT (Pressure and Temperature) observatory. Based on the previous deployment experience of the NanTroSEIZE C0010 observatory, we prepared another system that enables long term monitoring and repeated ROV data recovery. Two 0.25" stainless steel hydraulic lines are banded with protectors to the outside of 3.5" casing. The bottom ends of pressure lines terminate at permeable screens (mini-screens) that are exposed formation-fluid pressure. The mini-screens are positioned close to and above the fault. The top end of each pressure line is connected to a pressure logging package (including valves, pressure transducers, and data logger) at the wellhead. In addition, a temperature measurement string consisting of 21 channels of thermistors is installed inside the 3.5" casing. The thermistor string passes through a side entry port at the top of the casing where it is connected to a temperature logging package. The spacing intervals of the thermistors are fixed in the manufacturing process, but the total length is adjustable on board. Data recorded by both loggers can be recovered thorough wet mate connectors on the ROV, or the data loggers can be retrieved. Additionally, an acoustic modem installed onto the temperature logger can transfer some data to the receiver without a physical connection. During IODP Expeditions 343 and 343T, we successfully deployed the autonomous MTL observatory into an 854.81 meter deep borehole in 6,897.5 meter water depth. Unfortunately because of limited time and technical difficulties with drilling the second hole, we could not deploy the telemetered PT observatory.

  18. Slow slip near the trench at the Hikurangi subduction zone, New Zealand.

    PubMed

    Wallace, Laura M; Webb, Spahr C; Ito, Yoshihiro; Mochizuki, Kimihiro; Hino, Ryota; Henrys, Stuart; Schwartz, Susan Y; Sheehan, Anne F

    2016-05-01

    The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world's largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist. PMID:27151867

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

    NASA Astrophysics Data System (ADS)

    Čížková, 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.

  20. 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 (15-19.5ºN), 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 28º (south of the Scaborough Seamount Chain) to 12º (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.

  1. 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.

  2. 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.

  3. 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.

  4. Complex submarine landsliding processes caused by subduction of large seamounts along the Middle America Trench

    NASA Astrophysics Data System (ADS)

    Harders, Rieka; Ranero, Cesar R.; Weinrebe, Wilhelm; von Huene, Roland

    2014-05-01

    Subduction of kms-tall and tens-of-km wide seamounts cause important landsliding events at subduction zones around the word. Along the Middle America Trench, previous work based on regional swath bathymetry maps (with 100 m grids) and multichannel seismic images have shown that seamount subduction produces large-scale slumping and sliding. Some of the mass wasting event may have been catastrophic and numerical modeling has indicated that they may have produced important local tsunamis. We have re-evaluated the structure of several active submarine landlide complexes caused by large seamount subduction using side scan sonar data. The comparison of the side scan sonar data to local high-resolution bathymetry grids indicates that the backscatter data has a resolution that is somewhat similar to that produced by a 10 m bathymetry grid. Although this is an arbitrary comparison, the side scan sonar data provides comparatively much higher resolution information than the previously used regional multibeam bathymetry. We have mapped the geometry and relief of the head and side walls of the complexes, the distribution of scars and the different sediment deposits to produce a new interpretation of the modes of landsliding during subduction of large seamounts. The new higher resolution information shows that landsliding processes are considerably more complex than formerly assumed. Landslides are of notably smaller dimensions that the lower resolution data had previously appear to indicate. However, significantly large events may have occur far more often than earlier interpretations had inferred representing a more common threat that previously assumed.

  5. Deformation of oceanic lithosphere before subduction, a case study at the northern Manila trench

    NASA Astrophysics Data System (ADS)

    Chang, E.; Hsueh, H.; Mozziconacci, L.

    2012-12-01

    Although the reality of subduction has been greatly strengthened by recent investigations, there is little information dealing with the mechanisms by which material is subducted or accreted at the convergent area. An attempt to observe the lithospheric deformation at the northern Manila trench zone has been made, by means of the seismic analysis with the data acquired by a small OBS (ocean bottom seismometer) network. Along the Manila trench, the South China Sea plate subducts eastward beneath the Philippine Sea plate at south Taiwan. The global teleseismic observations report that the shallow interface seismicity is rather in fragmentary distribution in the northern Manila subduction system. In order to study the local seismic parameters, an OBS experiment was conducted with 9 short-period seismometers deployed at the northernmost Manila Trench (20.5°~21°N, 119.8°~120.2°E) from November 20th to November 25th. Over 2,700 events were retrieved and the focal mechanisms of the events with the clear waveforms have been determined. The focal depths range from the seafloor to about 40 km in depth. Two distinct seismic clusters are recognized in different spatial geometrical distributions. One is formed vertically and parallel to the neighboring Manila trench, whereas the other is a deep horizontal plane lying at a depth of about 30 km. The 1-D seismic velocity model demonstrates that the local Moho is taken place at about 16 km, which is thicker than the typical oceanic lithosphere. In order to extract more information on the offshore seismic sources, we performed the near-field waveform inversion program FASTMECA, to determine focal mechanisms with the events of ML >2.5. Focal mechanism solutions are mainly strike-slip in type but also reveal an important component in normal faulting. Actually a few reverse events are observed as well at south of the study area. Such a mixing of focal mechanisms reflects not only the bending of the plate but also the local adjustment subject to the spatial shortening of the plate before subduction. However, it is difficult to draw a clear tectonic cause for the origin of the flat seismicity distributed at depth in the study area.

  6. 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.

  7. 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.

  8. Subduction erosion: the cause of sediment-starved trenches and the birth of new forearc above the seismogenic interface?

    NASA Astrophysics Data System (ADS)

    Vannucchi, P.; Morgan, J.; Sak, P. B.; Balestrieri, M.

    2013-12-01

    Subduction erosion is usually thought to occur at ';sediment starved' margins with trench sediments limited to a layer <400m thick. In the region of CRISP drilling along the Osa Peninsula in Central America, trench sediment thicknesses vary from 100-200m. Here, a pulse of extreme subduction erosion occurred at ~2.5Ma. This episode was linked to the rapid formation of a deep (~1 km) sediment-filled forearc basin where sediment accumulation reached a peak rate of 1035 m/Myr. The most recent sediments in this basin do not come from high Talamanca inland from Osa, but instead from the nearshore Osa mélange within the Osa Peninsula. While rapid sediment accumulation has occurred within this forearc basin, the adjacent trench has remained unfilled, as indicated by IODP Site U1381, 5 km outboard of the trench, where a thin (96 m) veneer of recent to middle Miocene (Serravallian) sediment mantling the aseismic Cocos Ridge was recovered. Thus the highly erosive margin off the Osa Peninsula has had a large volume of sediment delivered offshore, but this sediment never reached the trench and was instead captured within a rapidly subsiding forearc basin. From this example, it is clear that sediment-starved trenches do not necessarily imply low rates of sediment supply to the margin. At this margin, the thin sediment layer in the trench is not the controlling factor for subduction erosion, but rather the consequence of rapid subduction erosion that created a rapidly subsiding forearc basin. A further consequence for Osa is that the forearc is rapidly being transformed from eroded material to the new basin fill. If this material differs rheologically from ';old forearc', this may have further implications for changing seismic characteristics at erosive subduction margins.

  9. Results of trench perpendicular wide angle seismic transects across the Manila subduction zone offshore southern Taiwan

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Multi-channel seismic reflection and wide-angle seismic data collected in 2009 aboard the R/V Marcus Langseth as part of the TAIGER program delineate the crustal structure of the Manila subduction zone in the northern South China Sea. As part of that project, we recorded marine seismic data using a deployment of ocean-bottom-seismometers (OBS) from the U.S. instrument pool and National Taiwan Ocean University. The region between northern Luzon and southern Taiwan evolves from oceanic subduction to incipient arc-continent collision. This presentation focuses on results of 2 offshore transects across the Manila subduction zone offshore southern Taiwan. Our goal here is to document the transition from pure oceanic subuction in the south to incipient arc-continent collision in the north, an understanding of which is integral for future geodynamic modeling of the advanced arc-continent collision in the north. The northern transect, line T2 is located at 21.4° N and used 30 OBSs. Line T1 was located at 20.5° N and used 27 OBSs across the Manila subduction zone. Data quality is extremely variable due to the local geology and quality of seafloor coupling at each instrument. Preliminary travel-time tomography of transect T2 shows a 10-15 km thick Eurasian crust with crustal velocities of 5-7.5 km/sec entering the Manila trench suggesting thinned continental crust, serpentinized upper mantle, or both in this region. The model shows the accretionary prism to be cored by high velocity material (6-7 km/sec) that may be the result of accretion of crustal material from the subducting Eurasian slab. We also observe asymmetric crustal thickening beneath the Gagua Ridge that is potentially a result of failed subduction of the Philippine Sea Plate westward along the Gagua Ridge. The wide-angle data is complimented by MCS reflection data to constrain sediment thickness, top of the crystalline basement, and moho. Preliminary work is in progress with transect T1 which will be presented along with transect T2 to provide an along strike comparison of the Manila subduction zone at both latitudes. We suspect that there may be more typical oceanic crust subducting in the south and primarily sedimentary material in the prism along transect T1.

  10. 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.

  11. 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 Ω 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.

  12. Crustal structure and deformation associated with seamount subduction at the north Manila Trench represented by analog and gravity modeling

    NASA Astrophysics Data System (ADS)

    Li, Fucheng; Sun, Zhen; Hu, Dengke; Wang, Zhangwen

    2013-12-01

    We investigated the deformation in the accretionary wedge associated with subducted seamounts in the northern Manila Trench by combining observations from seismic profiles and results from laboratory sandbox experiments. From three seismic reflection profiles oriented approximately perpendicular to the trench, we observed apparent variations in structural deformation along the trench. A number of back-thrust faults were formed in the accretionary wedge where subducted seamounts were identified. In contrast, observable back-thrusts were quite rare along the profile without seamounts, indicating that seamount subduction played an important role in deformation of the accretionary wedge. We then conducted laboratory sandbox experiments to investigate the effects of subducted seamounts on the structural deformation of the accretionary wedge. From the analog modeling results we found that seamount subduction could cause well-developed back-thrusts, gravitational collapse, and micro-fractures in the wedge. We also found that a seamount may induce normal faults in the wedge and that normal faults may be eroded by subsequent seamount subduction. In addition, we constrained the crustal structure of the South China Sea plate from modeling free-air gravity data. The dip angle of the subducting plate, which was constrained by hypocenters of available earthquakes, increased from south to north in the northern Manila Trench. We found a laterally heterogeneous density distribution of the oceanic crust according to the gravity data. The density of subducted crust is ~2.92 g/cm3, larger than that of the South China Sea crust (2.88 g/cm3).

  13. Hadal disturbance and radionuclide profiles at the deepest Japan Trench, northeastern Japan

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    Four months after the 2011 Tohoku-Oki earthquake, we carried out a video survey and collected sediment core collection from the hadal region (~7,600 m water depth) of the Japan Trench using an autonomous instrument. Fine material remained suspended at ~50 m above the seabed presumably induced by turbidities released during the central earthquake and the following aftershocks. Elevated levels of Cs-137 (T1/2=30 y) and excess Pb-210 (T1/2=22.3 y) concentrations suggested that 30 cm thick sediment layer had accumulated at the trench base (7,553 m) after the mainshock. However, no Cs-134 (T1/2=2 y) fallout from the Fukushima Dai-ichi nuclear disaster was detected. In contract, inspection of a nearby sediment site (7,261 m) 4.9 km away from the central trench site revealed fewer disturbances as reflected by a recent deposition of only 4 cm sediment, but here we encountered recent Cs-134 fallouts from the top 0-1 cm depth. We propose that the apparent lack of Cs-134 in the central trench is coursed by settlement of turbidites containing Cs-137 from past atmospheric fallout and higher excess Pb-210. The fast transport of the Cs-134 to the hadal slope sediment is presumably induced by enhanced scavenging and the vertical transport associated to an intensified diatom blooming occurring just at the time of the Fukushima disaster.

  14. 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.

  15. Physical properties and Consolidation behavior of sediments from the N. Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Valdez, R. D., II; Lauer, R. M.; Ikari, M.; Kitajima, H.; Saffer, D. M.

    2013-12-01

    Sediment hydraulic properties, consolidation state, and ambient pore pressure development are key parameters that affect fluid migration, deformation, and the slip behavior and mechanical strength of subduction zone megathrusts. In order to better understand the dynamics and mechanisms of large subduction earthquakes, Integrated Oceanic Drilling Program (IODP) Expedition 343, drilled into the toe of the Japan Trench subduction zone in a region of large shallow slip in the M 9.0 Tohoku earthquake, as part of the Japan Trench Fast Drilling Project (J-FAST). Here, we report on two constant rate of strain (CRS) uniaxial consolidation experiments and two triaxial deformation experiments on bedded claystone and clayey mudstone core samples collected from the frontal prism and subducted sediment section cored at Site C0019, 2.5 km landward of the Japan Trench, from depths of 697.18 and 831.45 mbsf. The goals of our experiments were: (1) to define the hydraulic and acoustic properties of sediments that host the subduction megathrust fault that slipped in the M 9.0 Tohoku earthquake; and (2) to constrain in-situ consolidation state and its implications for in-situ stress. The permeability-porosity trends are similar for the two samples, and both exhibit permeability that decreases systematically with increasing effective stress and decreasing porosity, and which varies log-linearly with porosity. Permeabilities of material from the frontal prism decrease from 5×10-18 m2 at 5 MPa effective stress, to 3.0×10-19 m2 at 70 MPa, and porosities decrease from 51% to 29%, while permeabilities of the subducted sediment sample decrease from 5×10-18 m2 at 5 MPa to 3.6×10-19 m2 at 90 MPa, and porosities decrease from 49% to 36%. In-situ permeabilities for the prism and underthrust sediment samples, estimated using laboratory defined permeability-porosity relationships, are 4.9×10-18 m2 and 3.7×10-18 m2, respectively. Elastic wavespeeds increase systematically with increasing effective stress. P-wave velocities (Vp) in the frontal prism sample increase from 2.1 km/s at 8 MPa to 2.7 km/s at 55 MPa effective stress, and velocities in the underthrust sediment sample increase from 2.3 km/s at 6 MPa to 3.0 km/s at 76.5 MPa. Estimated in-situ Vp for the frontal prism and underthrust sediment sample are 2.1 km/s and 2.4 km/s, respectively. This is slightly higher than both the logging while drilling (LWD) measurements and shipboard velocity measurements on discrete samples. We also estimated pre-consolidation pressures (Pc) for each sample using the work-stress method. Comparing Pc with the present day in-situ vertical stress calculated from shipboard bulk density data, we find that both samples are severely overconsolidated. We report this in terms of overconsolidation ratio (OCR), defined as the ratio of Pc to the in-situ stress expected for the case of normal consolidation. Values of OCR for the prism and underthrust samples are 3.95 and 4.28, respectively. This overconsolidation is broadly consistent with fully drained (non-overpressured) conditions, and may reflect uplift and unroofing of the sediments following peak burial greater than their current depth, a significant contribution from lateral tectonic stresses leading to an effective stress far greater than expected for the case of uniaxial burial, or cementation that leads to apparent overconsolidation.

  16. 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.

  17. Diving surveys of small seamounts on the outer rise of the Japan Trench, and replacement of benchmarks for seafloor geodesy

    NASA Astrophysics Data System (ADS)

    Fujimoto, H.; Kirby, S.; Abe, N.; Hino, R.; Kido, M.; Osada, Y.; Tsushima, H.

    2007-12-01

    Geodetic experiments and diving surveys of the outer rise of the Japan Trench off Northeastern Japan were carried out during the Kairei KR07-07 cruise in June 2007 using the ROV KAIKO 7000 II. The cruise aimed at three objectives. The first was diving surveys of small seamounts on the outer rise. A new type of seafloor volcanism called 'petit spot' was recently found near the Japan Trench (Hirano et al., 2006). Since each of diving surveys to three seamounts found fresh basalts, the survey area can be a potential locality for another petit spot volcanic field (Abe et al., this meeting). The second objective was diving surveys of the seafloor in the source region of the 2005 M7.1 outer rise earthquake. It was the largest outer rise earthquake recorded off the Japan Trench since the 1933 M8.4 Sanriku- oki tsunami earthquake. Although we could not find any indication of deformation on the seafloor during the survey, we are interested in the observation that the aftershock distribution (Hino et al., this meeting) was overlapped with the area of the small seamounts. The seamounts may have resulted from young volcanism like 'petit spot' caused by oceanic plate flexure (Hirano et al., 2006). Bathymetric maps show graben structures near the small seamounts sub-parallel to the trench axis suggesting normal faults in the outer rise region. We can suppose that the normal faults can be another mechanism for the young volcanism near the Japan Trench, or that the 'petit spot' volcanism may have induced the large intra-plate normal fault earthquake. Anyway the seamounts can be related to intra-plate earthquakes. The third objective was renewal of acoustic seafloor benchmarks deployed on the outer rise. Three precision acoustic transponders (PXPs) were deployed in 2002 to observe the motion of the pacific plate near the subduction plate boundary, and somehow exhausted the batteries after a few short observations. Each of two PXPs was replaced with a new one after cm-order observation of the relative positions. A new PXP was deployed from the sea surface near the third PXP. We plan to combine the geodetic observations with the old PXP net with future works based on the data of precise relative positioning.

  18. 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.

  19. Detecting Seismic Signatures in the Rock Record at the Japan Trench

    NASA Astrophysics Data System (ADS)

    Rabinowitz, H. S.; Savage, H. M.; Polissar, P. J.; Plank, T. A.; Rowe, C. D.; Kirkpatrick, J. D.

    2014-12-01

    IODP Expedition 343 (JFAST) drilled through the plate boundary at the Japan Trench where potentially several megathrust earthquakes have occurred, most recently the 2011 Mw 9.1 Tohoku-oki earthquake. Here, we investigate structural features of the plate boundary at the JFAST site to determine their seismic history. Using trace element geochemistry as a tool to fingerprint sedimentary units, we develop a stratigraphy of the JFAST core that shows significant plate boundary complexity. To determine the seismic history of the faults identified in the core, we use a novel method to detect temperature rise along faults: the thermal destruction of organic molecules. We interpret the destruction of alkenones, a common biomarker found in marine sediments, along faults to be a result of coseismic heating. In order to constrain initial alkenone concentration, we use our trace element stratigraphy to correlate individual JFAST samples with their protolith in the DSDP Site 436 reference core. We find that at least three faults demonstrate significant destruction of alkenones, implying that these faults have experienced shallow seismic slip. In the past, frictional work has been difficult to measure on faults due to a paucity of available coseismic temperature proxies preserved in the rock record. By coupling experimentally determined reaction kinetics of alkenone destruction with models of temperature rise during earthquakes, we convert the alkenone destruction measurements into estimates of maximum temperature rise on faults in the JFAST core. This novel measurement allows us to explore energy budgets at the subduction interface.

  20. 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.

  1. 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.

  2. Supercycles along the Japan Trench and Foreseeability of the 2011 Tohoku Earthquake

    NASA Astrophysics Data System (ADS)

    Koketsu, K.; Yokota, Y.

    2011-12-01

    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. The national program of seismic hazard assessment, which was initiated by the Japanese government after the 1995 Kobe earthquake, failed to foresee this earthquake, because no supercycle of megathrust events had been identified along the Japan trench. For example, the program identified a regular cycle of six M7 to 8 characteristic earthquakes in the land side of the Miyagi-oki region, and only reported the high probability of having another M7 earthquake there. The Japanese government also built nation-wide dense arrays of seismometers and GPS receivers after the Kobe earthquake. Nishimura et al. recovered annual rates of back slip, which is the drag of the overriding plate by interplate coupling, using GPS data during a calm period before the Tohoku earthquake. We then recovered coseismic slips through a joint inversion of ground motion and GPS data during the earthquake. The distributions of recovered coseismic slips and back slip rates bear a close resemblance to each other. An area of large back slip rate was previously thought to be related to a regular cycle of characteristic earthquakes. However, our result demonstrates that the area is related to a supercycle of megathrust earthquakes. From the coseismic slips and back slip rates in the Miyagi-oki region, we calculated the coseismic moment release and moment accumulation rate of the Tohoku earthquake to be 143 x 10**20 Nm and 0.434 x 10**20 Nm/year, respectively. Since characteristic earthquakes occasionally release some part of accumulated seismic moment, those in the Miyagi-oki region were compiled. We then calculated the moment releases by them to be 47 x 10**20 Nm. These moment releases and accumulation rate lead to a supercycle period of 438 years. However, this period is too short, if the 869 Jogan earthquake is the only documented event to have occurred with a possible magnitude and location similar to that of the Tohoku earthquake. Within the compilation, the 1611 Keicho earthquake can be a hidden candidate between the 869 Jogan and 2011 Tohoku earthquakes. Extensive tsunami damage caused by this earthquake was documented over the Tohoku district. The time series, which was drawn using the moment releases and accumulation rate, is mostly controlled by the moment releases of megathrust earthquakes. Supercycles were found in the Mentawai region along the Sunda trench, but the rupture pattern of two large earthquakes in 2007 does not completely coincide with the back slip distribution. Similar disagreement was also found for the 2010 Maule earthquake in the Chilean subduction zone. These imply that they are not megathrust earthquakes of supercycles. Therefore, we cannot foresee characteristic earthquakes in regular cycles using a back slip distribution, but the 2011 Tohoku earthquake could be foreseen with respect to at least its location and extent, if we monitored GPS array data considering their relation to megathrust earthquakes of supercycles.

  3. Confirmation that Large-Magnitude Megathrust Earthquakes Are Linked to the Subduction of Thick, Laterally Continuous Bodies of Trench Sediment

    NASA Astrophysics Data System (ADS)

    Scholl, D. W.; Kirby, S. H.; Keranen, K. M.; Blakely, R. J.; Wells, R. E.

    2010-12-01

    THE HYPOTHESIS: Ruff (1989) surmised that subduction of a thick section of trench-floor sediment would construct a laterally homogenous layer between upper and lower plates that would smooth the roughness of subducted sea-floor relief and strength-coupling asperities. During a megathrust earthquake (Eq), an even distribution of interplate strength (coupling) running parallel to the subduction zone (SZ) would favor long trench-parallel ruptures. Rupture zones exceeding about 250 km in length are characteristic of great (Mw 8.0 and higher) and giant (Mw 8.5 and higher) megathrust Eqs. Ruff observed that roughly half of all recorded megathrust Eqs of Mw 8.2 and larger broke adjacent to sediment-flooded trenches, thus suggesting a link between subduction of thick sediment sequences and rupture areas of high magnitude Eqs. TESTING THE HYPOTHESIS: We examined Ruff’s conjecture by compiling a database of well-documented instrumentally and historically recorded great and giant megathrust Eqs. We compared this listing with the global distribution of trench-axis sediment bodies that have along-trench dimension of 250 km or longer. Epicenters of great and giant Eqs were plotted along trench sectors classified as having a Very Thin (less than 0.5 km), Thin (0.5-1.0 km), Thick (1.0-3.0 km), or Very Thick (greater than 3.0 km) section of sediment entering the SZ. The highest Mw Eq recorded in each sediment sector (27 in all, 15 of which are thickly sedimented) was tabulated. Using only instrumentally recorded Eqs plus the geologically well-vetted Cascadia rupture of 1700 (19 events), trench sectors with axial deposits thicker than 1.0 km are associated with the occurrence of: 53 percent of Mw8.0 and larger (10 of 19), 67 percent of Mw8.3 and larger (6 of 9), 75 percent of Mw8.5 and larger (6 of 8), 80 percent of Mw9.0 and larger (4 of 5), 100 percent of Mw larger than 9.0 (3 of 3). Combining instrumental and historic EQs (27 events) changes the corresponding occurrence percentages to 55, 59, 57, 80, and 100. CONCLUSION: As noted by Ruff (1989), a number of physical parameters (e.g., subducted seamounts and ridges) contribute to the locations and rupture lengths of great and giant megathrust Eqs. But the observations listed above make it clear that subduction of a thick, laterally continuous section of sediment is a major determinant. Presumably, thickness is relative, i.e., it need only be adequate to smooth subducting relief sufficient to reduce lateral patchiness of asperities. The Ruff conjecture that sediment subduction of adequate thickness promotes large-magnitude megathrust rupturing seems to be confirmed. [Ruff, L., 1989, Do trench sediments affect great earthquakes occurrence in subduction zones, Pure and Applied Geophysics, v. 129, Nos. 1/2, p. 263-282].

  4. 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

  5. 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.

  6. Seismic consequences of warm versus cool subduction metamorphism: examples from southwest and northeast japan

    PubMed

    Peacock; Wang

    1999-10-29

    Warm and cool subduction zones exhibit differences in seismicity, seismic structure, and arc magmatism, which reflect differences in metamorphic reactions occurring in subducting oceanic crust. In southwest Japan, arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth; in northeast Japan, arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth. Thermal-petrologic models predict that oceanic crust subducting beneath southwest Japan is 300 degrees to 500 degrees C warmer than beneath northeast Japan, resulting in shallower eclogite transformation and slab dehydration reactions, and possible slab melting. PMID:10542143

  7. 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.

  8. 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.

  9. 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.

  10. 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 López" and southern "Puerto López-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.

  11. 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.

  12. Faulting within the Pacific plate at the Mariana Trench: Implications for plate interface coupling and subduction of hydrous minerals

    NASA Astrophysics Data System (ADS)

    Emry, Erica L.; Wiens, Douglas A.; Garcia-Castellanos, Daniel

    2014-04-01

    We investigate faulting within the incoming Pacific plate at the Mariana subduction trench to understand stresses within the bending plate, regional stresses acting upon the plate interface, and the extent of possible faulting-induced mantle serpentinization. We determine accurate depths by inverting teleseismic P and SH waveforms for earthquakes occurring during 1990-2011 with Global Centroid Moment Tensor (GCMT) solutions. For earthquakes with Mw 5.0+, we determine centroid depths and source time functions and refine the fault parameters. Results from Central Mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the Moho. At the Southern Mariana Trench, extensional earthquakes continue to 5 km below the Moho. One compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here. In addition, we model the stress distribution within the Pacific plate along two bathymetric profiles extending seaward from the Mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces. Results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the Pacific plate at Southern Mariana is experiencing larger, compressional stresses, possibly due to greater interplate coupling. Additionally, we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle, then the top 5-15 km of the Pacific plate mantle are partially serpentinized, and a higher percentage of serpentinization is located near the Central Mariana trench where extensional events extend deeper.

  13. 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.

  14. 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.

  15. Constraints on Subduction Zone Coupling along the Philippine and Manila Trenches based on GPS and Seismological Data

    NASA Astrophysics Data System (ADS)

    Hamburger, M. W.; Johnson, K. M.; Nowicki, M. A. E.; Bacolcol, T. C.; Solidum, R., Jr.; Galgana, G.; Hsu, Y. J.; Yu, S. B.; Rau, R. J.; McCaffrey, R.

    2014-12-01

    We present results of two techniques to estimate the degree of coupling along the two major subduction zone boundaries that bound the Philippine Mobile Belt, the Philippine Trench and the Manila Trench. Convergence along these plate margins accommodates about 100 mm/yr of oblique plate motion between the Philippine Sea and Sundaland plates. The coupling estimates are based on a recently acquired set of geodetic data from a dense nationwide network of continuous and campaign GPS sites in the Philippines. First, we use a kinematic, elastic block model (tdefnode; McCaffrey, 2009) that combines existing fault geometries, GPS velocities and focal mechanism solutions to solve for block rotations, fault coupling, and intra-block deformation. Secondly, we use a plate-block kinematic model described in Johnson (2013) to simultaneously estimate long-term fault slip rates, block motions and interseismic coupling on block-bounding faults. The best-fit model represents the Philippine Mobile Belt by 14 independently moving rigid tectonic blocks, separated by active faults and subduction zones. The model predicts rapid convergence along the Manila Trench, decreasing progressively southwards, from > 100 mm/yr in the north to less than 20 mm/yr in the south at the Mindoro Island collision zone. Persistent areas of high coupling, interpreted to be asperities, are observed along the Manila Trench slab interface, in central Luzon (16-18°N) and near its southern and northern terminations. Along the Philippine Trench, we observe ~50 mm/yr of oblique convergence, with high coupling observed at its central and southern segments. We identify the range of allowable coupling distributions and corresponding moment accumulation rates on the two subduction zones by conducting a suite of inversions in which the total moment accumulation rate on a selected fault is fixed. In these constrained moment inversions we test the range of possible solutions that meet criteria for minimum, best-fit, and maximum coupling that still fit the data, based on reduced chi-squared calculations. In spite of the variable coupling, the total potential moment accumulation rate along each of the two subduction zones is estimated to range from 3.98 x 1019 to 2.24 x 1020 N-m yr-1, equivalent to a magnitude Mw 8.4 to 8.9 earthquake per 100 years.

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. 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.

  1. 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.

  2. 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.

  3. Crustal accretion in the Manila trench accretionary wedge at the transition from subduction to mountain-building in Taiwan

    NASA Astrophysics Data System (ADS)

    Lester, Ryan; McIntosh, Kirk; Van Avendonk, Harm J. A.; Lavier, Luc; Liu, C.-S.; Wang, T. K.

    2013-08-01

    New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc-continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10-14 km thick outboard of the trench, but thins to 6 km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10 km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0 km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc-continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin.

  4. Supraslab earthquake clusters above the subduction plate boundary offshore Sanriku, northeastern Japan: Seismogenesis in a graveyard of detached seamounts?

    NASA Astrophysics Data System (ADS)

    Uchida, Naoki; Kirby, Stephen H.; Okada, Tomomi; Hino, Ryota; Hasegawa, Akira

    2010-09-01

    Thousands of offshore repeating earthquakes with low-angle thrust focal mechanisms occur along the subduction plate boundary of NE Japan. Double-difference relocation methods using P- and S-wave arrivals reveal clusters of events above these repeating events. To assure good depth control we restrict our study to events that are close to seismic stations. These "supraslab" earthquake clusters are regional features at depths of 25 to 50 km, and most of these clusters are below the depth of the forearc Moho, which we determined from converted waves. Seismicity over this depth range does not occur under the inland area of NE Japan except just below the vicinity of the arc volcanoes. Re-entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past. Our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the Pacific plate during slab descent, driven by the resistance of seamounts to subduction. Detachment during slab descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth. High fluid pressure produced during dehydration of clay minerals and other low-temperature hydrous minerals could enable detachment at depths. Seamount crust is thus accreted to forearcs, possibly leading to a long-term component of near-coastal uplift. Supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system.

  5. Variation in deformation of the South Panama Accretionary Prism: Response to oblique subduction and trench sediment variation

    NASA Astrophysics Data System (ADS)

    Mackay, Mary E.; Moore, Gregory F.

    1990-08-01

    Migrated single-channel seismic lines and Sea-MARC II side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south Panama, a region previously believed to be a transform margin. The eastern flank of the Coiba Ridge is being subducted in this region, creating the requisite geometry for along-strike variation in trench sediment thickness and type. The regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east. In the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments. A wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area. The eastward increase in sediment thickness correlates with the following changes in the accretionary prism: (1) decrease in initial surface slope; (2) broadening of the inner trench slope; (3) increase in thrust spacing; (4) steepening of frontal thrusts; and (5) fold development. Each of the responses reflects, in varying degrees, the increase in volume and changing physical properties of the accreted material, as well as the changing shear strength of the décollement. The increase in overburden pressure and compaction in the deeper sediments, together with the change in lithology from oceanic to trenchfill sediments, should produce an eastward increase in sediment shear strength both within the wedge and along its base. Because the décollement will tend to form where the pore fluid pressure-overburden ratio is at a maximum, the shear strength within the wedge should increase relative to basal shear strength. Decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base.

  6. Pore pressure evolution at the plate interface along the Cascadia subduction zone from the trench to the ETS transition zone

    NASA Astrophysics Data System (ADS)

    Skarbek, R. M.; Rempel, A. W.; Schmidt, D. A.

    2010-12-01

    Pore fluid pressures in subduction zones are a primary control on fault strength and slip dynamics. Numerous studies document elevated pore pressures in the outer wedge along several margins. Seismic observations and the occurrence of non-volcanic tremor provide additional evidence for the presence of near-lithostatic pore pressures at the plate interface far down-dip from the trench (~35 km depth). Here we use numerical models in one and two dimensions to evaluate the pore pressure and compaction state of sediments on the subducting Juan de Fuca plate in Cascadia from the trench to the ETS zone. 2-D models allow pressure to diffuse vertically and also laterally normal to strike of the megathrust; 1-D models simulate only vertical diffusion. Model parameters are chosen with reference to two strike-normal profiles: one through central Oregon and one through the Olympic Peninsula of Washington. We examine temporal variations in sediment input to the trench and consider implications for fault strength and permeability as well as the down-dip extent to which compactive dewatering can be considered a significant fluid source. In 1-D, we use a general and fully nonlinear model of sediment compaction derived without making any assumptions regarding stress-strain or porosity-permeability relations and allowing finite strains. In contrast, most previous models of fluid flow in subduction zones have used linear models of diffusion that rely on assumptions of constant sediment permeability and infinitesimal strains for their formulation. Our nonlinear finite-strain model remains valid at greater depths, where stresses and strains are large. Boundary conditions in 1-D are constrained by pore pressure estimates along the megathrust fault that are based on seismic velocities (e.g. Tobin and Saffer, 2010) and data from consolidation tests conducted on sediments gathered during ODP Leg 204 (Tan, 2001). Initial conditions rely on input sediment thickness; while sediment thickness at the trench in Cascadia is fairly well constrained (~1-3 km) by seismic studies, it is less clear how much of the section is frontally accreted and how much is subducted with the downgoing plate. Along the Washington profile, Batt et al. (2001) estimated that 80-100% of the incoming sediment is frontally accreted, based on comparisons between accretionary flux at the trench and erosional flux in the Olympic Mountains. We assume that similar values hold for the Oregon profile as well. Values of permeability along the plate interface are extracted from 1-D models and used to parameterize 2-D models. 2-D modeling is motivated by the need to examine time dependency of sediment influx, as well as the influence of splay faults within the accretionary wedge. Preliminary results indicate that fluid flux resulting from sediment compaction is complete well up-dip of the ETS zone, where the magnitude of fluid flux associated with mineral dehydration reactions becomes more significant. Ongoing work is centered on incorporating the effects of dehydration fluid sources within our models of pore pressure evolution and examining the implications of our results on the dynamics of slow slip events.

  7. Crustal structure and seismicity associated with seamount subduction: Preliminary results from the Tonga-Kermadec Trench - Louisville Ridge collision zone

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    The Tonga-Kermadec trench, which separates the subducting Pacific plate from the overthrusting Indo-Australian plate, is intersected at ~26° S by the Louisville Ridge seamount chain. The collision zone is characterized by a 3000 m reduction in trench depth, a 15° anticlockwise rotation of the trench axis, a 20 % reduction in the width of extensional bend faulting on the Pacific plate and a rough, hummocky, forearc on the Indo-Australian plate. These morphological characteristics are accompanied by a 40 % reduction in seismicity compared to regions immediately to the north and south. The influence of subducting seamounts on megathrust processes is not limited to their immediate vicinity and there is evidence of morphological and seismological manifestations in the forearc with wavelengths similar to the wavelengths of the flexural moats and bulges that flank these features. The Louisville Ridge collision zone has been the subject of three marine geophysical surveys conducted onboard R/V Sonne in 2004, 2007 and 2011. Swath bathymetry data were collected throughout all surveys and the collated dataset reveals a pronounced forearc high reaching a depth of 2700 m, located ~80 km and at an azimuth of 305° from Osbourn, the oldest known seamount within the Louisville chain. The bathymetric high correlates with a free-air gravity and magnetic anomaly high (50 mGal and 200 nT peaks respectively) and all three datasets reveal both a flanking depression, approximately one third of the amplitude of the peak, and an outer high. Within the region of reduced seismicity, a preliminary forward velocity model derived from strike-parallel wide-angle seismic data, suggest a north-south reduction in depth to the forearc Moho and a local crustal thickening in the vicinity of the high. We present here a preliminary analysis of new and existing seismic, swath and potential field datasets from the Louisville Ridge collision zone. In addition to the anomalous forearc structure introduced above, we discuss the role that subducting seamounts may play in controlling the seismological and structural segmentation of convergent plate boundaries.

  8. 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.

  9. Changes in the crust and upper mantle near the Japan-Bonin Trench

    NASA Astrophysics Data System (ADS)

    Houtz, R.; Windisch, C.; Murauchi, S.

    1980-01-01

    Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multi-channel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.05 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n = 47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11-11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reduced T2/X2 analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well-observed mantle reflection and its strong 8.2-km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3-km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor. The peculiar double reflector near mantle and the marked change in velocity structure and upper mantle reflectivity at the edge of the outer slopes seem to confirm the changes in upper mantle refraction velocity reported by Talwani et al. (1977) in the Curacao trench, Caribbean Sea, but our evidence is not conclusive. In spite of the ambiguity it is clear that velocity structures near active trenches cannot be confidently defined by correlating sections with similar velocities.

  10. 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.

  11. Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/ Vs in the transparent frontal prism

    NASA Astrophysics Data System (ADS)

    Nakamura, Yasuyuki; Kodaira, Shuichi; Cook, Becky J.; Jeppson, Tamara; Kasaya, Takafumi; Yamamoto, Yojiro; Hashimoto, Yoshitaka; Yamaguchi, Mika; Obana, Koichiro; Fujie, Gou

    2014-12-01

    Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average Vp/ Vs in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.

  12. 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.

  13. Seismological detection of low-velocity anomalies surrounding the mantle transition zone in Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Liu, Zhen; Park, Jeffrey; Karato, Shun-ichiro

    2016-03-01

    In the Japan subduction zone, a locally depressed 660 discontinuity has been observed beneath northeast Asia, suggesting downwelling of materials from the mantle transition zone (MTZ). Vertical transport of water-rich MTZ materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions, causing seismic low-velocity anomalies. Melt layers implied by low-velocity zones (LVZs) above the 410 discontinuity have been detected in many regions, but seismic evidence for partial melting below the 660 discontinuity has been limited. High-frequency migrated Ps receiver functions indicate LVZs below the depressed 660 discontinuity and above the 410 discontinuity in the deep Japan subduction zone, suggesting dehydration melting induced by water transport out of the MTZ. Our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle.

  14. 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.

  15. Microbial Community Structure of the Japan Trench Cold Seeps Sediment Determined by Phospholipid Analysis

    NASA Astrophysics Data System (ADS)

    Chan, O.; Fang, J.; Kato, C.

    2004-12-01

    Seafloor cold seeps support some of the most prolific and diverse ecosystems on Earth. A multitude of microbial habitats are associated with cold seeps. The seeping fluids are enriched in reduced chemical species such as sulfide and methane. These reduced species are utilized by microorganisms to gain energy from the reduction of sulfate and oxidation of methane, the so-called anaerobic oxidation of methane. The Japan Trench is characterized by abundant chemosynthesis-based communities associated with cold seeps. Chemosynthetic communities of Maorithyas hadalis and Calyptogena phaseoliformis have been discovered at depths of over 7,000 m. In this project, sediment samples were collected from communities dominated by thyasirid bivalve Maorithyas hadalis and the vesicomyid clam Calyptogena phaseoliformis and analyzed for phospholipid fatty acids (PLFA). Our objectives were to determine and compare the microbial biomass and community structure of the two sites with different megafaunal species. Result showed the average estimated microbial biomass was 2.97*109 and 4.78*109 cells (g dry wt)-1 for Calyptogena and Maorithyas sediment, respectively. Fatty acids ranging from 12 to 22 carbons were detected. The PLFA profiles suggest the presence of methanotrophic bacteria, sulfur-oxidizing bacteria as well as sulfate-reducers. The polyunsaturated fatty acids (C 18:2 and C20:5) also allow us to trace the possible source of the sediment to the piezophilic bacteria. The assemblage of fatty acids indicates the presence of complex microbial communities in the cold seeps sediments of the Japan Trench.

  16. 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

  17. 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

  18. 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.

  19. 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.

  20. Permeability anisotropy in marine mudstones in the Nankai Trough, SW Japan: Implications for hypothesized lateral fluid flow and chemical transport outboard of the trench

    NASA Astrophysics Data System (ADS)

    Saffer, D. M.; McKiernan, A. W.; Skarbek, R. M.

    2008-12-01

    Characterizing dewatering pathways and chemical fluxes near and outboard of subduction trenches is important toward understanding early sediment dewatering and devolatilization. Quantifying fluid flow rates also constrains the hydraulic gradients driving flow, and thus ultimately hold implications for pore pressure distribution and fault mechanical strength. We focus on the well-studied Nankai Trough offshore SW Japan, where drilling has sampled the sedimentary section at several boreholes from ~11 km outboard of the trench to 3 km landward. At these drillsites, &δ37Cl data and correlation of distinct extrema in downhole chloride profiles have been interpreted to reflect substantial horizontal fluid flow to >10 km outboard of the trench within the ~400 m-thick, homogeneous Lower Shikoku Basin (LSB) facies mudstone. The estimated horizontal velocities are 13 ± 5 cm yr-1; the flow is presumably driven by loading during subduction, and mediated by either permeable conduits or strong anisotropy in permeability. However, the pressure gradients and sediment permeabilities necessary for such flow have not been quantified. Here, we address this problem by combining (1) laboratory measurement of horizontal and vertical sediment permeability from a combination of constant rate of strain (CRS) consolidation tests and flow-through measurements on core samples; and (2) numerical models of fluid flow within a cross section perpendicular to the trench. In our models, we assign hydrostatic pressure at the top and seaward edges, a no-flow condition at the base of the sediments, and pore pressures ranging from 40%-100% of lithostatic at the arcward model boundary. We assign sediment permeability on the basis of our laboratory measurements, and evaluate the possible role of thin permeable conduits as well as strong anisotropy in the incoming section. Our laboratory results define a systematic log-linear relationship between sediment permeability and porosity within the LSB mudstones. The overall variation in permeability for our suite of samples is ~1 order of magnitude. Notably, horizontal permeabilities fall within the range of measured vertical permeabilities, and indicate no significant anisotropy. Using laboratory-derived permeability values, simulated horizontal flow rates range from 10-4 to 10-1 cm yr-1, and decrease dramatically with distance seaward of the trench. With permeability anisotropy of 1000x (i.e. kh = 1000kv), simulated flow rates peak at 3 cm yr-1 at the trench, and decrease to 3x10-1 cm yr-1 by 10 km seaward. These flow rates are substantially lower than those inferred from the geochemical data and also lower than the plate convergence rate of 4 cm yr-1, such that net transport of fluids out of the subduction zone is not likely. If discrete conduits are included in our models, permeabilities of ~10-114m2 are required to sustain the inferred flow rates. However, no potential conduits in the LSB were observed by coring or logging- while-drilling. In contrast, net egress of fluids - and associated chemical transport and pressure translation - are plausible at margins where continuous permeable strata are subducting. Overall, our results highlight a major discrepancy between constraints on fluid flow derived from physical hydrogeology and inferences from geochemical data. In this case, we suggest that the chemical signals may be affected by other processes such as in situ clay dehydration and down-section chemical variations.

  1. 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

  2. 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.

  3. Episodic subsidence and active deformation of the forearc slope along the Japan Trench near the epicenter of the 2011 Tohoku Earthquake

    NASA Astrophysics Data System (ADS)

    Arai, Kohsaku; Inoue, Takahiko; Ikehara, Ken; Sasaki, Tomoyuki

    2014-12-01

    To investigate the present-day geological deformation occurring off the Pacific coast of Tohoku, Japan, we obtained high-resolution multichannel seismic reflection and bathymetric data in 2007. The study area is located along the Japan Trench near the epicenter of the 2011 off the Pacific coast of Tohoku Earthquake. The seismic profiles do not show active structures indicative of compressional stress on the forearc slope. Instead, recent tectonic deformation is characterized by extensional subsidence and the occurrence of normal faults within a series of small basins on the slope to a water depth of ∼3000 m. These isolated basins are thickest (∼525 m, ∼0.7 s two-way travel time) in regions underlying areas of flat bathymetry. The isolated basins range in width from several to 40 km and are covered by stratified sediments onlapping at the termination of concave-down reflectors. The seismic units below the basins show continuous, subparallel internal reflectors, suggesting that the subsidence-related deformation started abruptly and recently. The thickness of sediments overlying the unconformity at the top of the Cretaceous is roughly constant in the upper slope area. However, lenticular internal reflection patterns occur locally. The lenticular sedimentary units are similar to the isolated basins in terms of their widths and internal reflection patterns. We infer that episodes of extensional deformation of the overriding plate, in the form of isolated basins, have occurred over geological time. We suggest that the geological structures of the forearc slope along the Japan Trench are typical of those resulting from subduction erosion and propose that the episodic subsidence accompanied by normal faulting is the most recent deformation.

  4. 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

  5. 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.

  6. Diverse magmatic effects of subducting a hot slab in SW Japan: Results from forward modeling

    NASA Astrophysics Data System (ADS)

    Kimura, Jun-Ichi; Gill, James B.; Kunikiyo, Tomoyuki; Osaka, Isaku; Shimoshioiri, Yusuke; Katakuse, Maiko; Kakubuchi, Susumu; Nagao, Takashi; Furuyama, Katsuhiko; Kamei, Atsushi; Kawabata, Hiroshi; Nakajima, Junichi; van Keken, Peter E.; Stern, Robert J.

    2014-03-01

    response to the subduction of the young Shikoku Basin of the Philippine Sea Plate, arc magmas erupted in SW Japan throughout the late Cenozoic. Many magma types are present including ocean island basalt (OIB), shoshonite (SHO), arc-type alkali basalt (AB), typical subalkalic arc basalt (SAB), high-Mg andesite (HMA), and adakite (ADK). OIB erupted since the Japan Sea back-arc basin opened, whereas subsequent arc magmas accompanied subduction of the Shikoku Basin. However, there the origin of the magmas in relation to hot subduction is debated. Using new major and trace element and Sr-Nd-Pb-Hf isotope analyses of 324 lava samples from seven Quaternary volcanoes, we investigated the genetic conditions of the magma suites using a geochemical mass balance model, Arc Basalt Simulator version 4 (ABS4), that uses these data to solve for the parameters such as pressure/temperature of slab dehydration/melting and slab flux fraction, pressure, and temperature of mantle melting. The calculations suggest that those magmas originated from slab melts that induced flux melting of mantle peridotite. The suites differ mostly in the mass fraction of slab-melt flux, increasing from SHO through AB, SAB, HMA, to ADK. The pressure and temperature of mantle melting decreases in the same order. The suites differ secondarily in the ratio of altered oceanic crust to sediment in the source of the slab melt. The atypical suites associated with hot subduction result from unusually large mass fractions of slab melt and unusually cool mantle temperatures.

  7. Decoupling of Pacific subduction zone guided waves beneath central Japan: Evidence for thin slab

    NASA Astrophysics Data System (ADS)

    Padhy, Simanchal; Furumura, Takashi; Maeda, Takuto

    2014-11-01

    The fine-scale seismic structure of the northeast Japan subduction zone is studied based on waveform analyses of moderate-sized (M4.5-6), deep-focus earthquakes (h >350 km) and the finite difference method (FDM) simulation of high-frequency (up to 8 Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source-receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high-frequency coda can be captured with the two-dimensional (2-D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high-frequency S wave due to low-Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high-frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered-Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high-frequency slab-guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.

  8. Imaging the subducting slabs and mantle upwelling under the Japan Islands

    NASA Astrophysics Data System (ADS)

    Zhao, Dapeng; Yanada, T.; Hasegawa, A.; Umino, N.; Wei, Wei

    2012-08-01

    A high-resolution P-wave tomography of the crust and mantle down to 700 km depth beneath the Japan Islands is determined using a large number of high-quality arrival-time data from local earthquakes and teleseismic events simultaneously. The tomography shows that the Philippine Sea slab is subducting aseismically down to 430 km depth under southwest Japan, though the seismicity within the slab ends at 180 km depth. A low-velocity (low-V) zone in the mantle wedge under Tohoku and Kyushu is found to extend westward from the volcanic front to the backarc under the Japan Sea and East China Sea. Significant low-V anomalies are revealed in the deep portion of the mantle wedge (400-500 km depth) above the Pacific slab under southwest Japan, which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the Pacific slab. Low-V anomalies appear at 420-700 km depths beneath the Pacific slab under eastern Japan, which may reflect hot mantle upwelling associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.

  9. 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 (N302.0°E) 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.

  10. 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.

  11. Subduction mega-thrust beneath Mt. Fuji, central Japan

    NASA Astrophysics Data System (ADS)

    Sato, H.; Ishiyama, T.; Iwasaki, T.; Abe, S.; Kato, N.; Imaizumi, T.; Hirata, N.

    2012-12-01

    The Philippine Sea plate (PHS) is being subducted beneath Honshu, associated with the buoyant subduction of the Izu-Bonin arc. Many scientists estimated the plate boundary along the northwestern part of the Izu collision zone, however, covered by volcanic products from Mt. Fuji and Hakone volcanoes, no active fault system is recognized. To reveal the location of plate boundary mega-trust and to evaluate the seismic hazards produced by these active faults, we performed deep and shallow high -resolution seismic reflection profiling across the flank of Mt. Fuji and Hakone volcanoes. Deep seismic data were acquired for 34-km-long seismic line, using four vibroseis trucks and explosives (<50 kg), 780 fixed channels. Shallow high-resolution seismic reflection data were collected across the frontal part of the fault system, using Mini-vib (IVI) and a 200 channels recording system. On the deep seismic section, westward dipping reflectors are dominant beneath the Hakone volcano on the PHS and extend to the west at the depth of 7 km beneath sub-horizontal reflectors. The top surface of the west dipping reflectors is interpreted as a plate boundary mega-thrust. The velocity profile obtained by refraction tomography suggests that the high velocity zone on the hanging wall and low velocity westward dipping layer in the footwall, which corresponds the volcanic products of Hakone volcano. The hanging-wall unit consists of the accreted arc crust from the Izu-Bonin arc, Quaternary coarse trough fill and Quaternary volcanic products. On the seismic section, the vertical offset of the top of Vp 5.4 km/sec zone is 2.5 km. Probable Quaternary coarse trough fill, deposited in the trough between the Izu-Bonin arc and Honshu arc, distributed on the mega-thrust forming wedge-shaped geometry. The high-resolution seismic section suggests that the plate boundary fault zone consists of several branching faults. The frontal thrust controlled the thickness of the deposits, probably younger than 300 ka, for 1-km-vertical offset, suggesting that the net slip rate of the major thrust is about 10 mm/y. Based on morphotectonic observation and high-resolution shallow seismic sections, it is highly probable that the thrust displaced the Gotemba debris avalanche deposits dated 2.9 ka (Miyachi et al., 2004). From the seismic hazard point of view, such large slip rate of this thrust indicates that the estimated magnitude of earthquake reaches to be M8-. As the seismogenic source fault is located beneath Mt. Fuji, strong ground motions produced by the movement of this fault, may cause the debris avalanche of the flank of Mt. Fuji and it has potential to produce devastative damage to the cities distributed on the flank of Mt. Fuji. Further research will be needed to obtain more precise estimate the seismic hazards produced by this mega-thrust.

  12. 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.

  13. 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

  14. Three-dimensional electromagnetic imaging of upwelling fluids in the Kyushu subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Hata, Maki; Oshiman, Naoto; Yoshimura, Ryokei; Tanaka, Yoshikazu; Uyeshima, Makoto

    2015-01-01

    A three-dimensional (3-D) lithospheric-scale electrical resistivity model, developed using network-magnetotelluric (network-MT) data, contains structures associated with arc magmatism beneath Kyushu Island in the Southwest Japan arc. Kyushu Island, where the Philippine Sea Plate (PSP) subducts beneath the Eurasian plate, can be divided into northern and southern volcanic regions separated by a nonvolcanic region. Many active Quaternary volcanoes occur along the volcanic front (VF) associated with the PSP in the two volcanic regions. Our 3-D electrical resistivity model shows three different shapes of upwelling fluid-like conductive anomalies, indicative of either slab-derived aqueous fluid and/or partial melt beneath the volcanic and nonvolcanic regions. A conductive anomaly in the northern volcanic region, located at some distance from the subducting PSP, extends from the surface to depths of <100 km, whereas another conductive anomaly in the southern volcanic region, located along the subducting PSP at >70 km depth, extends from the surface to depths of >100 km. In the nonvolcanic region, the upper region of a relatively conductive anomaly extends upward to a depth of ~50 km along the subducting plate. The degrees of magmatism and the relative contribution of slab-derived fluids to the magmatism vary spatially in the one nonvolcanic and two volcanic regions.

  15. 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.

  16. 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.

  17. 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.

  18. Sedimentation Triggered by the 2011 Tohoku Megathrust Earthquake along the Japan Trench

    NASA Astrophysics Data System (ADS)

    McHugh, C. M.; Kanamatsu, T.; Cormier, M. H.; Seeber, L.; Bopp, R.; Ikehara, K.; Usami, K.

    2014-12-01

    Recent developments in the field of subaqueous paleoseismology have provided information about the sedimentation record of earthquakes and about the long-term seismicity of fault systems. In 2013, the Japan Agency for Marine-Earth Science and Technology conducted expeditions NT13-02 and NT13-19 to the 2011 Tohoku Mw 9.0 megathrust earthquake and tsunami source, with R/V Natushima in 800-5,900 m water depth. The goal was identifying earthquake-triggered deposits and mapping their spatial and temporal distribution, as a strategy to recognize the sedimentary signature of Tohoku-like events and measure recurrence intervals for seismic hazard assessment. Twenty-four piston cores, 3 to 6 m long, were recovered during the NT13-19 expedition along a 300 km-long portion of the mid-slope terrace. This elongated structure is parallel to the strike of the Japan Trench, and located landward of the frontal prism where deformation is most intense. Faults, sometimes forming steep scarps, define small (5km long) confined basins that were targeted for coring. Radioisotopes 137Cs and xs210Pb measured in the cores mark the 2011 Tohoku earthquake-related sedimentation. Detection of 134Cs and enrichment of 137Cs provided a 2011 Fukushima reactor signature, which was found in the upper 5cm of several cores and was buried 10-15cm in others. Very high activities of xs210Pb were measured in the upper half-meter of the majority of the cores providing evidence of very recent depositional events that we are linking to the 2011 earthquake. We envision the shaking by the earthquake fluidized a layer of surface sediment, which then moved downslope and was deposited where surface slope decreased. These sediments also incorporated 137Cs derived from global fallout over the past half century. These deposits can be recognized in the cores because they are homogeneous and lack bioturbation. The thickest ones (~1m) have soft sediment deformation features at their base. Along the mid-slope terrace, they are thicker in the region where maximum 2011 rupture displacement was documented. Several older event horizons separated by bioturbated sediments are recognized in the cores. Usami et al. (2014) used tephra chronology to estimate an average recurrence interval of 100-500 yrs in most cores, although some cores suggest recurrence of 1500-2000 yrs.

  19. 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.

  20. Active intraplate deformation as geodynamic responses to oblique shallow subduction of a flat slab: example from central and southwest Japan

    NASA Astrophysics Data System (ADS)

    Ishiyama, Tatsuya; Sato, Hiroshi

    2015-04-01

    Subduction of a flat slab has been recognized as one of the primary driving mechanism of wide intracontinental subsidence farther away from the subduction leading edge in many subduction margins. In most cases, however, quantitative and qualitative limitations on chronological constraints prevent comprehensive understanding of these geodynamic linkages. In this study, we show distinct, geologic and seismic evidence for spatial and temporal correlation between plate subduction and intercontinental deformation, mainly driven by dynamic interaction between subducting Philippine Sea (PHS) plate and overriding continental crusts of central and southwest Japan (Eurasian plate) along the Nankai-Tonankai subduction zone since Pliocene. Based on analyses of Pliocene to Pleistocene tectonic histories by use of rich dataset of Neogene stratigraphy, drainage network evolution, and shallow to deep seismic reflection profiles, depocenters of wide sedimentary basins and active thrusting have migrated northward since ca. 5 Ma to present from forearc to backarc of the southwest Japan arc. Median tectonic line, active dextral strike-slip fault as a forearc sliver along the Nankai, is located north of the upward extension of the downdip limit of the interseismic locked zone. Southwest Japan north of the MTL, underlain by the subducting slab with steady state slip (Nakanishi et al., 2002; Kodaira et al., 2004), appears tectonically less inactive than central Japan and has behaved as a less deformed rigid block. Contrastingly, Quaternary active intraplate deformation has been prominent north of the inactive MTL above a shallow flat segment of the PHS plate along the Tonankai. Deep seismic reflection profile images upward corrugated very shallow PHS slab being contact with continental lower crust beneath actively deforming area. We interpreted temporal and spatial correlation of oblique subduction of the shallow and flat, corrugated PHS slab as an essential mechanical role to enhance downward drag of the overriding plate and synchronous strong compressional stress field in the crust. More westerly PHS subduction since middle Pleistocene, suggested by unconformity in the forearc basin deposits and change of sense of fault slip along the active MTL cause stronger horizontal stress in the overriding plate, consistent with increasing geologic slip rate on active structures with this plate configurations. In shorter timescales, during four repetitions of the Nankai subduction zone earthquakes since 17th century, numbers of intraplate large (M>6.5) earthquakes occurred above or near the PHS flat slab are much larger than other surrounding regions. This also may suggest mechanical link between subduction processes and seismicity in the overriding plate.

  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. Bending-related Topographic Structures of the Subducting Plate in the Northwestern Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Nakanishi, M.; Manabe, I.; Oikawa, M.

    2012-12-01

    We present the elongated topographic structures associated with bending of the subducting oceanic plate along the western Kuril, Japan and Izu-Ogasawara trenches using multibeam bathymetric data. The trench-outer rise earthquake near the Japan Trench occurred in the same day after the 2011 Tohoku Earthquake. Several studies pointed out high occurrence probability of trench-outer rise earthquake after the 2011 Tohoku Earthquake in near future. Trench-outer rise earthquakes occur by reactivation or creation of normal faults caused as the oceanic lithosphere approaches a subduction zone and bends into the deep-sea trench. Bending-related faults in the oceanward trench slope are ubiquitous structures of oceanic plates incoming to trenches. In general, the faults are thought to be formed parallel or subparallel to the bending axis of the incoming plate, namely the trench axis. Oceanward slopes of several trenches have bending-related structure with a strike different from the trench axes (Masson, 1991; Kobayashi et al., 1998; Ranero et al., 2003). In these areas, abyssal hill fabric was reactivated instead of the creation of new faulting parallel to the trench axis. The Mesozoic Pacific Plate is subducting along the Kuril, Japan, Izu-Ogasawara, and Mariana trenches (Nakanishi et al. 1992). Kobayashi et al. (1998) investigated the bending-related structures of the oceanward trench slope of the western Kuril and northern Japan trenches using the multibeam data. They concluded that the abyssal hill fabric is revalidated when abyssal hill fabric trend within 30 degree of trench axes. To examine controlling factors for strikes of bending-related structures, it is indispensable to describe oceanic spreading fabric and to identify magnetic anomaly lineations. The oceanic spreading fabric consists of inherited abyssal hill fabric and other preexisting weak zones related to seafloor spreading process, which are fracture zones, non-transform offsets, and so on. The new bathymetric map demonstrated that most of bending-related topographic structures exist in the oceanward trench slopes deeper than 5600 m. The map revealed that bending-related topographic structures are developed parallel to the trench axis or inherited oceanic spreading fabric. Detailed identification of magnetic anomalies near the Japan Trench revealed curved lineations and discontinuity of lineations associated with propagation ridges. Comparison between the detailed bathymetric and magnetic anomaly lineation maps elucidated that abyssal hill fabrics were reactivated where the angle between abyssal hill fabrics and trench axis is less than about 30 degree.

  3. Spatial variation of attenuation factor in subduction zone of Philippine Sea slab around Kyushu Island Japan

    NASA Astrophysics Data System (ADS)

    Parithusta, Rizkita; Matsumoto, Satoshi; Shimizu, Hiroshi

    2010-05-01

    Kyushu Island, in south-western part of Japan is characterized by subduction from Philippine Sea Slab and Eurasian Plate (Amurian); volcanic front seen in islands arcs runs through the central part of Kyushu Island. In Kyushu, shallow and intermediate-depth earthquakes occur robustly through a depth of about 200 km. We estimated attenuation structure beneath Southern Part of Japan, at subduction zone of Philippine Sea Slab by applying modified coda normalization method (Eq.1) proposed by Parithusta, et.al. (2008*). The method estimates relative source spectra by taking spectral ratio in coda waves between two events at first. From a lot of the spectral data, those can be estimated with higher stability through singular value decomposition. After that, the relative source effect between event pair can be eliminated by the solution from ratios between direct wave spectra for many event pairs. We confirmed the estimation of source factor by assessment with empirical method, the result show that estimates of source factor almost satisfy empirical relation between magnitude and energy relation. The attenuation factor can be obtained from a relation below; ( ) Edij(tij,?-)- -1 dn = ln Edi'j(ti'j,?) = - ?Q(?) (tij - ti'j)+ const.... (1) Where: Ed denotes Direct S-wave power spectrum and Q is attenuation factor at target area; t is lapse time from origin time. Subscript i,jdenote identification number for event and station, respectively. Q-1 factor can be estimated from decay with ?tii'j(= tij - ti'j). By using this method, we obtained frequency dependent Q-1 value with smaller estimation error than previous study carried by Matsumoto et.al (2007). We used waveform data from earthquakes occurred in Philippine Sea Slab, recorded by Hi-net and Kyushu University seismic networks. Window length adopted here is 2.5 seconds for taking spectrum. The results shows the Q-1 values around Bungo-Suido area, northern part of Kyushu. Q-1 values are plotted in seven depth ranges as a function of frequency. The Q-1 values are obtained in a range from 10-4 to 10-1. This range is similar to that estimated in other studies. These results suggest that attenuation at depths 30-60 km is high but decreases markedly within 75 km depth, high attenuation is also observed at depth greater than 90 km. We found that Q-1 value in this study has frequency dependency, which decrease gradually with frequency. Keywords: Attenuation, Coda, Philippine Sea Slab, Subduction, Kyushu Island. *Presented on Japan Geosciences Union Makuhari -May 2008

  4. Preliminary geodetic strain measurement from the South Fiordland Region of New Zealand using repeat GPS surveys: Implications for subduction on the Puysegur Trench

    NASA Astrophysics Data System (ADS)

    Pearson, Chris

    Comparing 1996 GPS data and data collected only one year earlier demonstrates that 50% of the instantaneous relative plate motion is accommodated between the west coast of the South Island and the eastern side of the Waiau Basin, a distance of about 75 km. The observed relative velocity vectors are nearly parallel to the inferred orientation of the plate boundary and rotated more than 30° clockwise compared to the relative plate motion vector, a difference that is significant at the 95% level of confidence. Modeling shows that the observed deformation rates are consistent with a model where nearly all the relative plate motion occurs on the plate boundary thrust as defined by the Benioff Zone associated with subduction on the Puysegur Trench.

  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. Trench-parallel Anisotropy in Subduction Zones: Evaluating the Contributions of Olivine Fabric Transitions and Flow Around Slab Edge in Numerical Flow Models

    NASA Astrophysics Data System (ADS)

    Montesi, L. G.; Behn, M. D.; Long, M. D.; Miller, K. J.

    2009-12-01

    Mantle circulation in the mantle wedge is often described based on corner flow theory, modified to include the temperature dependence of mantle viscosity. However, corner flow is only a 2D abstraction, whereas subduction zones are fundamentally 3D objects, featuring slab edges and trench curvature. Seismic anisotropy, often detected using the shear wave splitting methods developed by Paul Silver and coworkers, is now commonly used to map the mantle flow field, and has demonstrated clearly that simple consideration of corner flow is not enough. Trench parallel anisotropy contradicts direct corner flow models if the mantle adopts the type of olivine fabric most commonly observed in natural samples, the A-type fabric. Therefore, several alternative views of anisotropy development have proposed, including two that we address in this work: 1) The high stresses and water content of the mantle wedge produce an alternative fabric, B-type, for which the fast axis of olivine is perpendicular to flow lines. The difficulties with this proposition are that stresses in the wedge may not be high enough and the strain necessary to replace the A-fabric of mantle entering the wedge may be too great to occur in the wedge. 2) Flow around slab edges from the subslab to the wedge domains induces trench parallel flow in addition to corner flow, aligning the fast axis of olivine with the slab. Difficulties with this model include the low intensity of this flow observed in the absence of lateral obstacles to flow, like the tank walls of analogue experiments, and the long distance along the trench where this flow must penetrate. However, this model can explain the dependence of the intensity of anisotropy with the ratio of trench migration velocity to convergence velocity, and the presence of sub-slab anisotropy. Subslab anisotropy is more consistently trench-parallel than wedge anisotropy and is more intense when the rate of slab advance or retreat is high (Long and Silver, 2008). To date, flow around slab edges associated with slab advance or retreat is the only explanation proposed to explain the characteristics of sub-slab anisotropy. A new and efficient empirical method of modeling the development of anisotropy for different fabrics in numerical mantle flow model (see poster by Miller and Montési in DI07), provides us with the tools necessary to compare the anisotropy patterns expected by each model. We can follow flow trajectories in the mantle wedge and around slab edge and track fabric evolution as the A- or D-type fabric of dry incoming mantle is progressively replaced by a C- or B- type fabric in the mantle wedge, following hydration by slab-released fluids. Thus, it is possible to determine for each trench advance or retreat rate where slab parallel anisotropy is most likely due to slab-parallel flow or B-type fabrics, and under which circumstances a region of trench-perpendicular anisotropy may be expected away from slab edges.

  7. 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 mélange, which has a complicated accretionary structure, and is poorly constrained by isotopic ages. The mélange 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.

  8. Subduction zone structures and slip behavior in megathrust

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Nakanishi, A.; Nakamura, Y.

    2014-12-01

    Earthquake, tsunami and geodetic data show that co-seismic slips of a large megathrust earthquakes do not uniformly propagate along a plate boundary. For example, a clear segmentation of slip zones of magnitude-8 class megathrust earthquakes are well recognized in the Nankai Trough. Moreover, a lateral variation of the slips are revealed even in one segment. In the Japan Trench, the most characteristic slip behavior of the 2011 Tohoku-oki earthquake is an extremely large slip reaching to the trench axis, but geodetic, tsunami or teleseismic show the slip was heterogeneous near the trench. In order to examine whether those complex slip distributions are attributed by any distinct structural factor, we have been carried out active-source seismic surveys in the subduction seismogenic. In the Nankai Trough, large-scale subducted seamounts, ridges and doming structure intruded in an overriding accretion wedge are imaged. Comparing co-seismic slip distribution of the 1944 Tonankai and the 1946 Nankai earthquakes with the seismic images, we concluded that those structures are key factors to control the slip distributions. In the central part of the Japan Trench area, we fund a rough basement geometry is overprinted on the horst-and-graben structure. Those complex geometry of basement cause a strong lateral variation of the thickness of subducting pelagic/hemi-pelagic sediment. Many geological studies suggest that properties of the plate-boundary sediment attribute the large slip near the trench. We therefore plan to acquire additional high-resolution seismic data in the entire Japan Trench in order to examine a role of incoming sediment on the large slip to the trench axis. In this presentation we present an overview of the structural factors controlling slips in megathrust earthquakes, including new data acquired in the Nankai Trough and Japan Trench.

  9. 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.

  10. 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.

  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. 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.

  13. Numerical simulations of temperature distributions associated with subduction of the Philippine Sea plate, southwest Japan

    NASA Astrophysics Data System (ADS)

    Yoshioka, Shoichi; Suminokura, Yoichiro; Matsumoto, Takumi; Nakajima, Junichi

    2010-05-01

    Large megathrust earthquakes have occurred repeatedly along the Nankai Trough with recurrence interval of about 90 to 150 years, which have been caused by plate motion of the Philippine Sea (PHS) plate in the NW direction subducting beneath southwest Japan. Deep low-frequency earthquakes have occurred beneath Shikoku and the Kii Peninsula. These earthquakes that have occurred in the convergent plate boundary have close relation to thermal state produced by plate subduction. The PHS plate embraces the Shikoku Basin in its northern part. The Kinan seamount chain is located in the central part of the Shikoku Basin. This is the fossil ridge which had been spreading in the ENE-WSW direction. The fossil ridge and its surrounding region are subducing along the Nankai Trough, and the direction of the plate motion of the PHS plate is considered to be changed to the current direction at about 3 Ma (Takahashi, 2004). We constructed a 2-D thermal convection model to simulate temperature field associated with subduction of the PHS plate along the Nankai Trough. Then, we evaluated the reliability of the calculated temperature field, by comparing it with observed heat flow data. In this study, we constructed the numerical model, taking account of spatio-temporal change of the age of the PHS plate, kinematics of the past and present plate motion of the PHS plate, and the up-to-date shape of the upper surface of the PHS plate. We calculated temperature distribution and heat flow along three profiles passing through northern Kyushu, Shikoku, and the Kii Peninsula, and compared these results with the observed heat flow data. We used Hi-net heat flow data (Matsumoto, 2009) as well as borehole and heat probe (Tanaka et al., 2004) and BSR (Ashi et al., 1999, 2002) data. The calculated heat flow fits well with the observation for all the three profiles within the range of horizontal distance of about 100km landward from the trough axis. But the observation values increased gradually at about 100km, and decreased at more landward. On the other hand, the calculated results tended to decrease gradually toward just above the mantle wedge when we only considered the effect of plate subduction. To explain high heat flow values obtained by Hi-net, we took into account the effect of large-scale hot plume in our model, which was indicated by seismic tomography results. We also incorporated the effect of yield stress and thinner conductive continental plate into our model. The calculated results showed higher heat flow values with short wavelength, which was consistent with the observation. This suggests that high heat flow values observed by Hi-net may be explained by the existence of large-scale hot plume reaching shallow depths in addition to plate subduction.

  14. Deep seismic reflection profiling of the subduction megathrust across the Sagimi trough and Tokyo bay, Japan

    NASA Astrophysics Data System (ADS)

    Sato, Hiroshi; Iwasaki, Takaya; Abe, Susumu; Saito, Hideo; Kawanaka, Taku; Hirata, Naoshi

    2010-05-01

    Beneath the metropolitan Tokyo, the Philippine Sea plate, in particular the fore arc portion of the Izu-Bonin island arc, has been subducted. Subduction megathrust beneath Tokyo generated M-8 class earthquakes, such as the 1923 Kanto (M7.9) and 1703 Genroku (M8.0) earthquakes. Due to the buyant subduction of the Izu-Bonin arc, the megathrust lies very shallow part of the crust. The Kozu-Matsuda fault, probable spray fault from the megathrust, emerged at the surface. In 2009, we acquired the deep seismic reflection data across the toe of the thrust system to reveal the connectivity of the probable spray fault to the megathrust. Together with the deep seismic section acquired in 2003, we show a 120-km-long deep seismic reflection profile from the front to 30 km in depth and discuss the geometry and characteristics of the thrust system. We performed deep seismic profiling across the Sagami trough for a 70-km-long seismic line in September 2009, using two ships for offshore seismic data acquisition: a gun-ship with a 3020 cu. inch air-gun and a cable-ship with a 2-km-long, streamer cable and a 480 cu. inch air-gun. The seismic signals were recorded at Miura and Izu peninsulas located both ends of the seismic line. At both sides of the onshore line, off-line recorders were deployed along total 20-km-long seismic lines at a 50m interval. Seismic reflection data were acquired by different offset of ships making large-offset gathers. The northeast end of the seismic line connected with the 2003 Tokyo bay seismic line (Sato et al., 2005: Science). The obtained seismic sections portray the detailed geometry of the spray faults, suggesting an emergent thrust with 4 km thick landward dipping strata. It merges to the megathrust at 6-7 sec (TWT). Judging from the geometry of fault-related fold in the trough fill sediments, the tip of the megathrust is located at 3 sec (TWT) beneath the trough axis. According to the co-seismic crustal deformation, the slip of the 1923 Kanto earthquake occurred along the main megathrust. According to the paleoseismic trenching survey of the spray fault (the Kozu-Matsuda fault, KMF), KMF displaced from AD 1100 to 1350 (Kanagawa Pref., 2005). Shimazaki et al. (2009: JpGU meeting) found the tsunami sediments correlatable to the 1923 Kanto, the 1703 Genroku and 1293 seismic event. Judging from the connectivity of KMF to the megathrust, the seismic event of AD 1293 was caused by displacement of the megathrust and out-of-sequence spay fault (KMF). From the coseismic crustal deformation and seismic waveforms of the 1923 Kanto earthquake, the locations of asperities were well determined (Sato et al., 2005). The distribution of slip deficit on the plate interface determined by GPS (Sagiya and Sato, 2005: Seismol. Soc. Jpn. meeting) accords well to the estimated asperity zone. On the seismic reflection profile, the asperity zone (stack plate interface) is marked by poor reflection from the fault surface and the plate interface is clearly identified as strong reflectors at the deeper steady creeping zone.

  15. Permeability in sediments and their role in large slip near the surface of the plate boundary fault in the Japan Trench

    NASA Astrophysics Data System (ADS)

    Tanikawa, W.; Hirose, T.; Mukoyoshi, H.; Tadai, O.; Lin, W.

    2013-12-01

    Fluid transport properties such as permeability, porosity, and specific storage are significant parameters that affect earthquake dynamic process. Thermal pressurization model (Mitsui et al., 2012, Earth and Planetary Science Letters) and shallow strong patch model (Kato and Yoshida, 2011, Geophysical Research Letters) were proposed to explain the giant earthquake in the Tohoku area, and transport property around the plate boundary fault is an important factor that impact on both models. Therefore we measured the transport properties of shallow sediments sampled around the plate boundary near the Japan Trench in the IODP expedition 343 at confining pressures up to 40 MPa. The permeabilities of samples from the shallow plate boundary fault at 820 mbsf were very low at 10 -20 m2, equivalent to a hydraulic diffusivity of 10-10 m2/s. Permeability in the core of the fault zone at the plate boundary were lower than those in the immediately overlying and underling sediments and the surrounding intact sediment, suggesting that the plate boundary fault can act as a barrier for fluid flow. Low permeability and high specific storage in the shallow plate boundary fault create a strong potential for dynamic fault weakening due to fluid pressurization with frictional heating, even when the initial shear stress is low. Our investigation supports the hypothesis that thermal pressurization on the fault plane induced the extremely large slip in the shallow part of the subduction zone during the Tohoku earthquake. As the fault zone has a lower permeability than the surrounding sediments and a higher clay content, pore pressure generation at depth by dehydration of clay minerals can explain formation of the shallow strong patch on the fault more reasonably than continuous fluid influx from the subducting oceanic crust proposed by Yoshida and Kato (2011, Geophysical Research Letters). Although there are many possible mechanisms of fault weakening, thermal pressurization can act relatively efficiently as slip begins, even at shallow depths. Therefore thermal pressurization is the most likely trigger mechanism for the large shallow displacement of the Tohoku earthquake.

  16. 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.

  17. 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.

  18. Bathymetric patterns of α and β 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 α 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 (β) 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 (δ 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.

  19. 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.

  20. Numerical simulations of temperature distributions associated with subduction of the Philippine Sea plate in southwest Japan

    NASA Astrophysics Data System (ADS)

    Suminokura, Y.; Yoshioka, S.; Matsumoto, T.; Nakajima, J.

    2009-12-01

    Large megathrust earthquakes have occurred repeatedly along the Nankai Trough with recurrence interval of about 90 to 150 years, which have been caused by plate motion of the Philippine Sea (PHS) plate in the NW direction subducting beneath southwest Japan. Deep low-frequency earthquakes have occurred beneath Shikoku and the Kii Peninsula. These earthquakes that have occurred in the convergent plate boundary have close relation to thermal state produced by plate subduction. The PHS plate embraces the Shikoku Basin in its northern part. The Kinan seamount chain is located in the central part of the Shikoku Basin. This is the fossil ridge which had been spreading in the ENE-WSW direction. The fossil ridge and its surrounding region are subducing along the Nankai Trough, and the direction of the plate motion of the PHS plate is considered to be changed to the current direction at about 3 Ma (Takahashi, 2004). We constructed a 2-D thermal convection model to simulate temperature field associated with subduction of the PHS plate along the Nankai Trough (Torii and Yoshioka, 2007). Then, we evaluated the reliability of the calculated temperature field, by comparing it with observed heat flow data. In this study, we constructed the numerical model, taking account of spatio-temporal change of the age of the PHS plate, kinematics of the past and present plate motion of the PHS plate, and the up-to-date shape of the upper surface of the PHS plate. We calculated temperature distribution and heat flow along three profiles passing through northern Kyushu, Shikoku, and the Kii Peninsula, and compared these results with the observed heat flow data. We used Hi-net heat flow data (Matsumoto, 2007) as well as borehole and heat probe (Tanaka et al., 2004) and BSR (Ashi et al., 1999, 2002) data. The calculated heat flow fit well with the observation for all the three profiles within the range of horizontal distance of about 100km landward from the trough axis. But the observation value increased gradually at about 100km, and decreased at more landward. On the other hand, the calculated results tended to decrease gradually toward just above the mantle wedge associated with subduction of the slab. The observed heat flow increased and decreased from fore arc to back arc along the profile passing through northern Kyushu. The calculated results in northern Kyushu had a tendency that was similar to that in Shikoku and the Kii Peninsula. In addition, the calculated values for all the three profiles were less than those of the observed Hi-net heat flow data. More detailed analyses and examinations of the observed data are necessary to explain the high heat flow obtained by Hi-net. There may be effective heat transport and/or internal heating which are not considered in our model. We will mention such models in our presentation.

  1. Small-scale spatial variation in near-surface turbidites around the JFAST site near the Japan Trench

    NASA Astrophysics Data System (ADS)

    Yoshikawa, Shuro; Kanamatsu, Toshiya; Kasaya, Takafumi

    2016-03-01

    This paper aims to improve our understanding of the depositional processes associated with turbidites related to recent earthquake events. A series of short sediment cores (ca. 20-30 cm long) were recovered from the landward slope of the Japan Trench around JFAST (Japan Trench Fast Drilling Project) site C0019 by a remotely operated vehicle, KAIKO 7000 II, and the sample sites were accurately located using an LBL (long base line) acoustic navigation system. The properties of the cores were analyzed using visual observations, soft X-ray radiographs, smear slides, measurement of anisotropy of magnetic susceptibility, and analysis of radioactive elements (134Cs, 137Cs, and excess 210Pb). For the first time, small-scale (ca. 200-1000 m) spatial variations in recent earthquake-triggered deep-sea turbidites, the formation of which was probably linked to the 2011 Tohoku-oki earthquake, are described. We also examine the submarine landslide that probably generated the sediment unit below the turbidites, which is thought to be an important process in the study area. The spatial distribution and characteristics of the near-surface seismoturbidite obtained immediately after the earthquake, presented here, will enable precise calibration of offshore evidence of recent earthquakes, and thus facilitate the use of the sedimentary archive for paleoseismic interpretations. Furthermore, although sampling for turbidite seismology on steep slopes has not been widely performed previously, our results suggest that the recent event deposits may be continuously tracked from the slope to the basin using a combination of the present sampling method and conventional large-scale investigation techniques.

  2. 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.

  3. Characterization of event deposits induced by Tohoku-oki Earthquakes in the Japan Trench using paleo and rockmagnetic techniques

    NASA Astrophysics Data System (ADS)

    Kanamatsu, T.; Ikehara, K.; Usami, K.; Fink, H.; Strasser, M.

    2013-12-01

    A study on differences in bathymetric data between before and after 2011 Tohoku-Oki earthquake revealed a large coseismic displacement of the overriding plate, and a topographic high formation in the trench axis (e.g. Strasser et al., 2013). In order to define the sediment deposition or disturbance occurred by these events, sediment piston cores were collected from the surface sediment around the topographic high in the trench axis. Intervals in the upper several ten-cm of recovered cores consist of turbidite units, which are considered to have been formed just after the earthquake. Other turbidite units are also recognized in the older than 2011 event, and they are regarded as evidences of past-other Tohoku earthquakes. Rockmagnetic studies on samples were carried out to analyze their depositional process. Several upward decreasing patterns of magnetic susceptibility in the core tops are interpreted as repeating turbidite cycles of 2011 event. Anisotropy of magnetic susceptibility data show that most Kmin axes are normal to the horizontal plane, and suggest that all intervals were not formed by chaotic deposition (e.g. debris flow). Besides Kmax directions are parallel to the horizontal plane and those alignments generally show clusters in two major directions, which are perpendicular each other. It is interpreted that such variation was induced by changing turbidity current state. Paleomagnetic directions in the turbidite intervals display large swinging patterns, probably due to DRM. However, the records in background intervals reveal a consistent inclination variation within obtained cores. Preliminary interpretation for this record is that the trend represents a record of secular geomagnetic variation in a time span. Parameters of magnetic grain size (e.g. kARM/k) indicate that the grain distribution of background sediment and those of older turbidite are clearly distinct, but the distribution of 2011 event is very similar to those of background sediments. Because the trench sediment is generally very fine and it is hard to recognize the sedimentary characteristics visually, paleomagnetic and rock magnetic characterizations are useful to analyze their depositional history. Figuring out this process is important not only for understanding the deposition of 2011 event but for understanding the older event deposits took place before in the Japan trench.

  4. 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.

  5. Glacial-interglacial trench supply variation, spreading-ridge subduction, and feedback controls on the Andean margin development at the Chile triple junction area (45-48°S)

    NASA Astrophysics Data System (ADS)

    Bourgois, Jacques; Guivel, Christele; Lagabrielle, Yves; Calmus, Thierry; BoulèGue, Jacques; Daux, ValéRie

    2000-04-01

    During the Chile triple junction (CTJ) cruise (March-April 1997), EM12 bathymetry and seismic reflection data were collected in the vicinity of the Chile triple junction (45-480S), where an active spreading ridge is being subducted beneath the Andean continental margin. Results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction-erosion to subduction-accretion. The Andean continental margin and the Chile trench exhibit a strong segmentation which reflects the slab segmentation and the Chile triple junction migration. Three segments were identified along the Andean continental margin: the presubduction, the synsubduction, and the postsubduction segments, from north to south. Both climate-induced variations of the sediment supply to the trench and the tectonic reorganization at the Nazca-Antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin. Along the survey area we infer the succession of two different periods during the last glacial-interglacial cycle: a glacial period with ice-rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice cap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction-accretion: (1) along the presubduction segment after the last deglaciation and (2) along the postsubduction segment after the interglacial episode at 130-117 ka. Conversely, a nonaccretion or subduction-érosion mode characterized the presubduction and postsubduction segments during glacial maximums. The major effects of subduction of the buoyant Chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the Nazca-Antarctica plate boundary. We suggest that a postsubduction westward jump of the Chile ridge occurred during the past 780 kyr. It produced slab fragmentation and individualization of an ephemeral microplate north of the Taitao fracture zone: the Chonos microplate. In 780 kyr, two episodes of subduction-accretion separated by an episode of subduction-erosion occurred in relation with the Chonos microplate individualization and subduction. The current northward migration of the triple junction along the Chonos microplate-South America plate boundary introduces a sharp change in the tectonic mode from subduction-erosion to the north to subduction-accretion to the south. The data collected along the Taitao ridge have revealed the complex three-dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite: the Taitao Ridge ophiolite. No connection exists between the Taitao Ridge ophiolite and the Bahia Barrientos ophiolite cropping out onland in the Taitao peninsula.

  6. The huge shallow slip during the 2011 Tohoku-Oki earthquake as a result of very low coseismic shear strength of the Japan Trench décollement material

    NASA Astrophysics Data System (ADS)

    Ujiie, K.; Tanaka, H.; Saito, T.; Tsutsumi, A.; Mori, J. J.; Kameda, J.

    2013-12-01

    Megathrust earthquakes commonly occur in subduction zones at depths where there is strong coupling between the plates and long-term strain accumulation. Unconsolidated sediments in the shallow plate-boundary décollement were thought to slip aseismically and have low levels of coupling. However, the 2011 Tohoku-Oki earthquake (Mw9.0) produced unprecedented slip of >50 m near the Japan Trench, resulting in the devastating tsunami. IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST) successfully drilled the décollement in the maximum slip area of the 2011 earthquake. The décollement mostly consists of highly sheared pelagic clays. To investigate the mechanisms of the huge shallow seismic slip, we conducted high-velocity (1.3 m/s) friction experiments on the Japan Trench décollement material at normal stresses of ~2.0 MPa and displacements of ~60 m. To simulate both permeable and impermeable conditions during high-velocity shearing, the water-saturated gouge was placed between a pair of solid cylinders of porous Berea sandstone and Indian gabbro, respectively. The results show rapid slip weakening properties with very low peak and steady-state shear strength. The steady-state values for the effective coefficient of friction at normal stress of 2 MPa are 0.2 and 0.1 for the permeable and impermeable tests, respectively. The steady-state shear stress is independent of normal stress, suggesting the fluid-like behavior of the gouge during high-velocity shearing. The fluid-like behaved gouge is also supported by microstructural observations showing the evidence of fluidization effects such as injection structures and mixing flow. The axial displacement data indicate that the specimen compacted and dilated during permeable and impermeable tests, respectively. For the same amount of displacement, the temperature in the gouge is always smaller for the impermeable tests compared to the permeable tests. These results indicate that high-velocity weakening is more pronounced in the impermeable tests due to more effective thermal pressurization than in the permeable tests. Similar behaviors were also obtained from the high-velocity friction experiments on the Nankai Trough décollement material. However, when we compare the data obtained under the same experimental conditions for the two different regions, the décollement material from the Japan Trench has overall lower effective coefficient of friction than material from the Nankai Trough. The weaker décollement during seismic slip is likely due to higher smectite content in the Japan Trench (80%) than that in the Nankai Trough (30%). The presence of a smectite-rich décollement with very low effective coefficient of friction is incompatible with the idea that large strain accumulates in the region of the plate-boundary during the interseismic period. Our results indicate that large slip result from an extremely low dynamic shear strength due to the abundance of smectite and thermal pressurization effects. Large coseismic displacement could be promoted even in unstrained portions at shallow depths as the earthquake rupture propagates through the smectite-rich fault material particularly under fluid-saturated, impermeable conditions. This provides an explanation for the very large slip and resultant tsunami during the 2011 earthquake.

  7. Hydrodynamics of tsunamis in subduction zones. The differences between the Chile 2010 and Japan 2011 tsunamis

    NASA Astrophysics Data System (ADS)

    Monardez, P.; Salinas, R.; Comte, D.

    2012-04-01

    Tsunamis due to large earthquakes in subduction zones have different hydrodynamic behaviors, depending on the location, the bathymetry and the geometry of the rupture associated to the large earthquake. When the width of the rupture (related to the length of the tsunami) is larger than its distance to the shore, the hydrodynamics in the near zone is completely different than the alternate case. In the first case, the earthquake triggers a tsunami composed by one or a group of a few waves with a few minutes in between propagating from the rupture, which reach the coast a few minutes after the earthquake. In the second case, the earthquake triggers a deformation in the water surface which cannot create a complete tsunami wave; there is not enough distance to complete it. Then, a succession of secondary effects are triggered, which are composed by several floods, up to seven or eight, separated several minutes (up to forty or more) and propagate parallel to the coast, which can be even perpendicular to the coast. This case is still poorly understood, even it has been observed and described in the literature over the past three centuries. The difference in hydrodynamic behavior was evidenced in the tsunamis of February of 2010 in Chile and March of 2011 in Japan. In this work we show a theory, which has been validated by field observations and numerical simulations based only on the hydrodynamics of the area, that explains the phenomena and it has been extended to other historical tsunamis in Chile. The effects of the Chile 2010 tsunami in the near field zone were complex. The small township of Cobquecura, located at 20 km from the epicenter, did not suffer major damage from the tsunami. The major port zone of Talcahuano at 100 km from the epicenter, received four destructive waves every forty minutes approximately, and lasted three hours after the occurrence of the earthquake, while the bay of San Vicente, adjacent to the above, only suffered a minor, but abrupt, rise in the sea level about 20 minutes after the end of the earthquake. Flux in general was reported to be parallel to the coast, from the north. In the case of Japan 2012 tsunami, the first wave arrived to shore from 1 to 50 min after the earthquake, depending on the distance to the rupture. This first wave was in the order of a few centimeters. The maximum wave arrived from 30 minutes to two hours after the earthquake, with high waves larger than 3 m, with flux perpendicular/diagonal to the coast.

  8. 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.

  9. Upper Mantle Shear Wave Anisotropy for Stations in Mexico and its Relationship to Subduction at the Middle America Trench

    NASA Astrophysics Data System (ADS)

    van Benthem, S. A.; Valenzuela, R. W.

    2007-05-01

    We have calculated the splitting parameters that describe upper mantle shear wave anisotropy under stations in continental Mexico located over the subducting Cocos Plate. SKS and SKKS arrivals recorded on both the radial and transverse horizontal components were used. The splitting parameters which quantify anisotropy are the delay time (δt) and the fast polarization direction (φ). The anisotropy is calculated using the approach by Silver and Chan [1991]. A time segment containing the SKS arrival is selected from both horizontal components. The space of possible solutions is then searched in one-degree intervals with φ ranging between 0 and 180°. Specifically, the coordinate axes are rotated every 1 degree increment and the autocorrelation and crosscorrelation between the components is calculated. For each value of φ, the solution space is also searched in 0.05 s increments. Next the eigenvalues corresponding to each δt and φ combination are calculated. In the presence of noise, the desired solution will be given by the matrix which is most nearly singular. In order to check our results, we apply a correction in the amount of the measured δt and φ to the original records and then rotate them to make sure that the anisotropy disappears. The shapes and the difference in the arrival times of the fast and slow waves are compared to make sure that the result is robust. As a further check, the polarization of the particle motion for the radial and transverse components before and after correction is plotted. The records used were taken from Mexico's Servicio Sismológico Nacional broadband network [Singh et al., 1997]. The orientation of the fast polarization direction, φ, can be explained by the absolute motion of the North American plate for some of the stations. Most of the stations, however, require a different explanation for the orientation of φ. For example, the orientation of φ for stations Platanillo (PLIG), Yautepec (YAIG), and Popocatépetl (PPIG) is aligned with the direction of absolute motion for the North American plate [Van Benthem, 2005]. The orientation of φ under Cayaco (CAIG) is close to the orientation of the relative plate motion vector between the Cocos and North American plates. The only measurement available under Ciudad Universitaria (CUIG) indicates that the orientation of φ is consistent with the extensional regime in the Trans- Mexican Volcanic Belt, with normal faults running east-west.

  10. 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.

  11. 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.

  12. Melting of a subducting slab and production of high-mg andesite magmas: Unusual magmatism in SW Japan at 13∼15 Ma

    NASA Astrophysics Data System (ADS)

    Furukawa, Y.; Tatsumi, Y.

    Characteristic high-Mg andesite magmas were produced in the SW Japan arc at 13∼15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction. High-Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. Detailed Structure of the Brittle-Ductile Transition Zone on the Subducting Plate Boundary Beneath the Southern Part of Kii Peninsula, Southwestern Japan

    NASA Astrophysics Data System (ADS)

    Kurashimo, E.; Kato, A.; Iidaka, T.; Iwasaki, T.; Hirata, N.; Ito, K.; Yamazaki, F.; Miyashita, K.; Nakagawa, S.; Obara, K.; Kasahara, K.

    2005-12-01

    The Nankai trough region, where the Philippine Sea Plate is subducted beneath the southwestern Japan arc, is a well-known seismogenic zone of interplate earthquakes (e.g. the 1944 Tonankai Earthquake (M=7.9) and the 1946 Nankai Earthquake (M=8.0)) . A detailed crustal and upper mantle structures of the subducting Philippine Sea Plate and the overlying SW Japan arc is inevitably important to constrain the physical process of earthquake occurrence. Resent results of seismic experiments have gradually revealed the relation between the crustal structure and the seismogenic zone. However, there is still little known about the physical properties of the deeper part of the plate boundary, especially the transition zone on the subducting plate. To reveal the detailed structure of the transition zone on the subducting plate, we conducted a deep seismic profiling in the southern part of Kii Peninsula, SW Japan. In this experiment, 280 seismometers were deployed on a 60-km-long line in the east-west direction with about 200 m spacing, on which three explosives shots were fired as controlled seismic sources. Charge size of the shots is 100 kg. Each seismograph system consisted of a 4.5 Hz, vertical component seismometer and a single channel data recorder. The recorder has 24-bit analogue-to-digital converter and records data at 250 Hz sampling rate. We obtained high signal-to-noise ratio data along the entire length of the profile. The most remarkable feature of the record sections is that extremely high amplitude reflections, which are interpreted as a reflected wave from the top of the subducting Philippine Sea plate, can be recognized. The seismic reflection method was applied to these data to obtain a detailed and clear image of deeper structure. The stacked image shows several features of the deeper part of the crust including the subducting plate boundary at 9 sec in two way travel time. The reflectivity of the subducting plate boundary changes laterally.

  18. 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.

  19. Deep recycling of oceanic asthenosphere material during subduction

    NASA Astrophysics Data System (ADS)

    Liu, Lijun; Zhou, Quan

    2015-04-01

    Uncertainties in the origin and composition of oceanic asthenosphere lead to different views on its temporal evolution upon subduction. We investigate the evolution of asthenosphere material during subduction using high-resolution geodynamic models. In contrast to some earlier models suggesting that limited amount of asthenosphere material can be entrained during subduction, we find that much of the subslab mantle (>100 km thick) could recycle into the deep mantle following the slab, even if this mantle layer remains buoyant and less viscous during entrainment. Our results support the hypothesis that observed trench-parallel subslab seismic anisotropy is a downward continuation of the anisotropic asthenosphere. The entrainment of sometimes buoyant asthenosphere material provides a new mechanism for reducing slab dip angle which is consistent the shallower Pacific slab underneath Japan than that farther south. Episodic release of entrained buoyant materials during subduction can also explain enigmatic intraplate volcanism, such as the Changbaishan volcano in Northeast China.

  20. 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 Kármán 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.

  1. 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.

  2. 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.

  3. Circum-Pacific modes of subduction, collision, and metallogenesis

    SciTech Connect

    Nishiwaki, C.; Uyeda, S.

    1986-07-01

    Tectonic processes in trench-arc-back-arc regions, as depicted on the Plate-Tectonic Map of the Circum-Pacific Region, are controlled by different modes of subduction. In one end member, the Chilean or high-stress subduction zone, the stress regime in the overriding lithosphere is compressive; whereas in the other end member, the Mariana or low-stress subduction zone, extensional tectonics prevails. The two modes are characterized by porphyry copper and massive sulfide metallogenesis, respectively. In both modes, sediment that fills grabens on the subducting plate may be subducted. When a large buoyant feature such as drifting continental crust arrives at the trench, collision-accretion tectonics with a strong compressive stress ensues. In such a collision zone, however, buoyant subduction of the light crust continues to a considerable extent, such as in the doubling of crust under Tibet and the subduction of the Izu block under central Japan. When continental crust and oceanic sediments subduct, they can begin melting at low temperature and shallow depth, generating more felsic granitoids than those that originate at greater depth under andesitic volcanic arcs. In the northwest Pacific, felsic granitic arcs are extensive, mostly S type, and ilmenite bearing, and they are accompanied by the world's largest tin and tungsten belt. They contrast with, but are parallel to many andesitic volcanic arcs characterized by rich copper-zinc-gold metallogenesis. The authors speculate that the extensive tin-tungsten granitic arcs have their genesis in the buoyant subduction, remelting, and large-scale anataxis of sediment-dominated crust as a result of collision-accretion tectonism.

  4. 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)

  5. 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 mélange 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 mélange zones (Mugi mélange, Yokonami mélange, 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 mélange 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 220°C and 80 to 210 MPa. For shear veins, P-T conditions are approximately 185 to 270°C 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 210°C, 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.

  6. 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.

  7. 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 mélanges that have experienced maximum burial temperatures of ~250 -320°C, [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.

  8. 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.

  9. 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.

  10. Shear Veins Under High Pore Pressure Condition Along Subduction Interface: Yokonami Mélange, 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 mélange, 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 mélange is mainly sandstones surrounded by foliated black shales with minor components of basalts, cherts, tuffs, and limestones, representing tectonic mélange textures. Shear veins cutting mélange 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 mélange 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 mélange 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 mélange 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.

  11. 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.

  12. 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.

  13. 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.

  14. Sub-slab anisotropy beneath the Sumatra and circum-Pacific subduction zones from source-side shear wave splitting observations

    NASA Astrophysics Data System (ADS)

    Lynner, Colton; Long, Maureen D.

    2014-06-01

    the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid Earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub-slab region. Recently, the source-side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub-slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub-slab mantle. Here we present source-side shear wave splitting measurements for the Central America, Alaska-Aleutians, Sumatra, Ryukyu, and Izu-Bonin-Japan-Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu-Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska-Aleutian regions. We find that plate motion parallel fast splitting directions in the sub-slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (>95 Ma) is associated with trench parallel splitting while younger lithosphere (<95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source-side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at midmantle depths beneath these regions.

  15. 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.

  16. 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.

  17. 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.

  18. 3D Seismic Velocity Structure Around Philippine Sea Slab Subducting Beneath Kii Peninsula, Japan

    NASA Astrophysics Data System (ADS)

    Shibutani, T.; Imai, M.; Hirahara, K.; Nakao, S.

    2013-12-01

    Kii Peninsula is a part of the source area of Nankai Trough megaquakes and the region through which the strong seismic waves propagate to big cities in Kansai such as Osaka, Kyoto, Nara, Kobe, and so on. Moreover, the rupture starting point is thought to be possibly at off the peninsula. Therefore, it is important for simulations of the megaquakes and the strong motions to estimate accurately the configuration of the Philippine Sea slab and the seismic velocity structure around the slab and to investigate properties and conditions of the plate boundary surface. Deep low frequency events (DLFEs) are widely distributed from western Shikoku to central Tokai at 30 - 40 km depths on the plate boundary (Obara, 2002). Results from seismic tomography and receiver function analyses revealed that the oceanic crust of the Philippine Sea plate had a low velocity and a high Vp/Vs ratio (Hirose et al., 2007; Ueno et al., 2008). Hot springs with high 3He/4He ratios are found in an area between central Kinki and Kii Peninsula despite in the forearc region (Sano and Wakita, 1985). These phenomena suggest the process that H2O subducting with the oceanic crust dehydrates at the depths, causes the DLFEs, and moves to shallower depths. We carried out linear array seismic observations in the Kii Peninsula since 2004 in order to estimate the structure of the Philippine Sea slab and the surrounding area. We have performed receiver function analyses for four profile lines in the dipping direction of the slab and two lines in the perpendicular direction so far. We estimated three dimensional shapes of seismic velocity discontinuities such as the continental Moho, the upper surface of the oceanic crust and the oceanic Moho (Imai et al., 2013, this session). In addition, we performed seismic tomography with a velocity model embedded the discontinuities and observed travel times at stations in the linear arrays, and successfully estimated 3D seismic velocity structure around the Philippine Sea slab beneath the Kii Peninsula in higher resolutions. The results show that in the vicinity of the areas of the DLFEs low velocity anomalies (LVAs) are distributed from the oceanic crust to the mantle wedge. These LVAs are thought to be due to fluids discharged from hydrous minerals in the oceanic crust by dehydration that occurs at 30 - 40 km depths on the plate boundary. Other strong LVAs (with 5 % velocity perturbation or more) are widely distributed in the lower crust beneath northern Wakayama Prefecture where the seismicity in the upper crust is high. Since the latter LVAs continue to deeper in the mantle wedge than the former LVAs, the origin of the LVAs in the two regions might be different. No matter what the origin is, the latter LVAs beneath the northern Wakayama area are probably due to fluids too. Then the high seismicity in the area can be explained by the reduction of the effective normal stress on the fault planes due to the increase of the pore pressure in the micro cracks caused by the fluids from the LVAs.

  19. Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

    NASA Astrophysics Data System (ADS)

    Han, Shuoshuo; Carbotte, Suzanne M.; Canales, Juan Pablo; Nedimović, Mladen R.; Carton, Hélène; Gibson, James C.; Horning, Greg W.

    2016-03-01

    We present prestack time-migrated multichannel seismic images along two cross-plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50-70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120-150 km and ~65-80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6-7 km into the mantle are observed. If attributed to serpentinization, ~0.12-0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper-middle crust. The regional difference in subduction bend-faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate-depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°-40°) lower crustal reflections are imaged in ~6-8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.

  20. 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.

  1. 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

  2. Substrate-specific pressure-dependence of microbial sulfate reduction in deep-sea cold seep sediments of the Japan Trench

    PubMed Central

    Vossmeyer, Antje; Deusner, Christian; Kato, Chiaki; Inagaki, Fumio; Ferdelman, Timothy G.

    2012-01-01

    The influence of hydrostatic pressure on microbial sulfate reduction (SR) was studied using sediments obtained at cold seep sites from 5500 to 6200 m water depth of the Japan Trench. Sediment samples were stored under anoxic conditions for 17 months in slurries at 4°C and at in situ pressure (50 MPa), at atmospheric pressure (0.1 MPa), or under methanic conditions with a methane partial pressure of 0.2 MPa. Samples without methane amendment stored at in situ pressure retained higher levels of sulfate reducing activity than samples stored at 0.1 MPa. Piezophilic SR showed distinct substrate specificity after hydrogen and acetate addition. SR activity in samples stored under methanic conditions was one order of magnitude higher than in non-amended samples. Methanic samples stored under low hydrostatic pressure exhibited no increased SR activity at high pressure even with the amendment of methane. These new insights into the effects of pressure on substrate specific sulfate reducing activity in anaerobic environmental samples indicate that hydrostatic pressure must be considered to be a relevant parameter in ecological studies of anaerobic deep-sea microbial processes and long-term storage of environmental samples. PMID:22822404

  3. Substrate-specific pressure-dependence of microbial sulfate reduction in deep-sea cold seep sediments of the Japan Trench.

    PubMed

    Vossmeyer, Antje; Deusner, Christian; Kato, Chiaki; Inagaki, Fumio; Ferdelman, Timothy G

    2012-01-01

    The influence of hydrostatic pressure on microbial sulfate reduction (SR) was studied using sediments obtained at cold seep sites from 5500 to 6200 m water depth of the Japan Trench. Sediment samples were stored under anoxic conditions for 17 months in slurries at 4°C and at in situ pressure (50 MPa), at atmospheric pressure (0.1 MPa), or under methanic conditions with a methane partial pressure of 0.2 MPa. Samples without methane amendment stored at in situ pressure retained higher levels of sulfate reducing activity than samples stored at 0.1 MPa. Piezophilic SR showed distinct substrate specificity after hydrogen and acetate addition. SR activity in samples stored under methanic conditions was one order of magnitude higher than in non-amended samples. Methanic samples stored under low hydrostatic pressure exhibited no increased SR activity at high pressure even with the amendment of methane. These new insights into the effects of pressure on substrate specific sulfate reducing activity in anaerobic environmental samples indicate that hydrostatic pressure must be considered to be a relevant parameter in ecological studies of anaerobic deep-sea microbial processes and long-term storage of environmental samples. PMID:22822404

  4. 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.

  5. 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 décollement 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 μs = 0.2- 0.26), in contrast to the much stronger wall rocks (μs > ∼ 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.

  6. 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.

  7. 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

  8. 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.

  9. Hydration of the incoming plate in the Kuril subduction zone

    NASA Astrophysics Data System (ADS)

    Fujie, G.; Kodaira, S.; Yamashita, M.; Sato, T.; Takahashi, T.; Takahashi, N.; Noguchi, N.

    2010-12-01

    Water supplied from the subducting oceanic plate by dehydration is inferred to cause seismicity and magmatism in subduction zones. It is important, therefore, to reveal the distribution of water within the incoming plate for understanding seismic and volcanic activities in subduction zones. In 2009 and 2010, to reveal the detailed seismic structure and hydration process within the incoming plate, we conducted a wide-angle seismic survey in the Kuril subduction zone, where the old Pacific plate formed in the eastern Pacific ridge is subducting from south to north beneath the island arc of Japan. We designed a north-south 500km-long seismic experimental line to be perpendicular to the Kuril trench. The northern end of our line is located at about 30km south of the trench axis and well-developed horst and graben structure is observed around the northern end. We deployed 80 Ocean Bottom Seismometers (OBSs) at intervals of 6km and shot a large tuned airgun array towed by R/V Kairei. In addition, we obtained MCS reflection data using a 444-channel hydrophone streamer (6km long) along the same line. We modelled both P-wave and S-wave velocity structures by the traveltime inversion using refraction, reflection and PS-conversion traveltimes. Our results show that P-wave velocity beneath the well-developed horst and graben structure is about 5% lower than that in the south of outer rise. This is consistent with a previous structure study in the Chili subduction zone that shows the P-wave velocity in the vicinity of the trench axis is lower than that of normal oceanic plate. More notable feature of our results is the regional variations of Vp/Vs. The S-wave velocity, as well as P-wave velocity, gradually decreases toward the trench axis. However Vp/Vs is not uniform; Vp/Vs immediately beneath the sediments is remarkably high beneath the well-developed horst and graben structure, and Vp/Vs decreases with depth (high Vp/Vs is confined to the top of the oceanic plate). Since the high Vp/Vs implies the high degree of crustal hydration, one plausible explanation for our Vp/Vs model is that normal faults related to the well-developed horst and graben structure provide pathways for water percolation from sediments to oceanic crust, and lead to crustal hydration of the incoming plate.

  10. 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).

  11. 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.

  12. 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.

  13. 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.

  14. The Genesis of the Abukuma Adakites, Northeast Japan Resulting From the Interaction Between Hot Asthenospheric Mantle and Subducting Slab: A Numerical Model Study

    NASA Astrophysics Data System (ADS)

    Lee, C.; Lim, C.

    2013-12-01

    The geochemistry of the transient Miocene adakites (~16 Ma) in the Abukuma Mountains, Northeast Japan shows that the adakites were generated by the partial melting of the subducted oceanic crust. However, the very old age of the converging oceanic plate which cannot yield high slab temperatures enough for the partial melting poses a problem for the genesis of the adakites. Other possible geneses such as the partial melting of the lower crust, flat subduction and/or transient cold plume are not relevant to the genesis of the adakites. Instead, it is thought that the injection of the upwelling hot asthenospheric mantle to the mantle wedge caused by the East Sea (Japan Sea) opening heats the cold subducting slab hotter enough for the partial melting of the oceanic crust. Although the hypothesis is promising, quantitative evaluation of the interaction between the cold Pacific slab and hot asthenospheric mantle has not been carried out. Thus, we conducted a series of 2-dimensional kinematic-dynamic subduction model experiments to evaluate the thermal structures of the subducting slab, essential for the partial melting of the oceanic crust. Since time-dependence is crucial for the transient adakites, the time-evolving convergence rate and slab age of the incoming Pacific plate for the last 65 Ma constrained from a recent plate reconstruction model are implemented in the numerical models with the transient hot asthenospheric mantle. The convergence rate and slab age are implemented along the oceanward wall boundary and updated each time step. The mantle potential temperature of 1350 °C and the mantle adiabat of 0.35 °C/km are used. The transient injection of the hot asthenospheric mantle to the mantle wedge is implemented as a function of depth- and time-dependent normal temperature distribution along the arcward wall boundary and updated each time step. The peak temperature of the hot asthenospheric mantle is assumed as 1550 °C at 100 km depth and the standard temperature deviation corresponding to the thickness of the hot asthenospheric mantle is varied as 20, 30 and 40 km. The peak time of the hot asthenospheric mantle is assumed as 22.5 Ma, corresponding to the initiation of the East Sea opening, and the standard time deviation is varied as 2.5 Ma, 5 Ma and 10 Ma. Results show that the transient injection of the hot asthenospheric mantle is required for the partial melting of the oceanic crust. Larger thickness of the hot asthenospheric mantle expressed as larger standard temperature deviation results in larger slab temperatures. Longer injection of the hot asthenospheric mantle results in longer and larger slab temperatures. All the experiments show that the peak slab temperatures over the wet solidus of the oceanic crust are attained on ~17-18 Ma, consistent with the ~16 Ma adakites in the Abukuma Mountains. However, a longer span of the injection of the hot asthenospheric mantle except for the standard time deviation of 2.5 Ma results in a longer span of the slab temperatures over the wet solidus from ~12.5 to ~22.5 Ma, inconsistent with the short eruption span of the Miocene adakites on ~16 Ma. This indicates that the injection of the upwelling hot asthenospheric mantle occurred very shortly (< 5 Ma) when the East Sea opened.

  15. 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.

  16. 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…

  17. Potential Fields Illuminate Earthquake Sources in Subduction-Margin Settings

    NASA Astrophysics Data System (ADS)

    Blakely, R. J.; Wells, R. E.

    2008-05-01

    Potential-field anomalies can be used to map the geologic structure of earthquake source regions in subduction zones and thus provide promise for assessing future earthquake hazards. Satellite free-air gravity anomalies over subduction zones consist of an offshore trench-parallel gravity low and a subparallel coastal gravity high that reflect the topography and structure of the inner trench slope and coast ranges, respectively. Large coseismic slip in shallow megathrust earthquakes correlates with gravity lows centered on large forearc sedimentary basins along the deep sea terrace offshore. The trench-parallel gravity low, the basins, and the earthquakes are all thought to be related to the resistance to slip along the plate boundary, and basin-centered gravity lows in similar settings may be the likely source of asperities in future earthquakes. Landward of the shallow megathrust, magnetic anomalies provide clues to processes occurring within the subducting slab and overlying mantle wedge. In some subduction zones, water released from the transformation of basalt to eclogite hydrates overlying sub-continental mantle, producing serpentinite, and embrittles the downgoing slab, promoting intraslab earthquakes. The 1970 central Peru earthquake (MW 7.5 to 8.0) and the 1949 Olympia, Washington, earthquake (MW 7.1) are recent examples. Thermal models indicate that the hydrated mantle wedge in most subduction zones is below the Curie temperature of magnetite. If serpentinite is sufficiently abundant, hydrated mantle wedges will produce long-wavelength magnetic anomalies observable at the earth's surface. A crust-mantle model of the Cascadia subduction margin based on magnetic, gravity, and seismic data, is consistent with the presence of significant volumes of hydrated mantle. The advent of new global magnetic databases may allow us to map hydrated mantle worldwide. The World Digital Magnetic Anomaly Map and the CHAMP satellite magnetic field, processed to emphasize sources at mantle depths, show evidence for hydrated mantle at many subduction margins of the world, including Cascadia, northeast Japan, the Aleutians, southern Mexico, and central America. All of these subduction zones have thermal characteristics believed to be conducive for intraslab earthquakes. On the other hand, the Peru, Chile, and Nankai subduction margins, also known to be susceptible to intraslab earthquakes, have only minor forearc magnetic anomalies, indicating complexities in the relationship between hydrated mantle and intraslab seismogenesis. In the shallow crust of the forearc, magnetic fields illuminate seismically active crustal faults activated by stresses induced by the subducting plate. The Seattle fault, source of a MW 7 earthquake 1100 ka, and the Nojima Fault, source of the MW 6.9 Kobe earthquake in 1995, are well-known examples. In Cascadia, where bedrock often is concealed by vegetation and young glacial deposits, a three-pronged approach has proven useful in mapping and characterizing crustal faults: (1) Laser terrain mapping (lidar) reveals scarps cutting late- Pleistocene glacial surfaces. (2) In almost every case, these lidar scarps correspond with short-wavelength, low- amplitude magnetic anomalies in high-resolution aeromagnetic surveys, which in turn facilitate geologic mapping of the faults away from scarps and help to characterize faults at depth. (3) The lidar and aeromagnetic data together provide targets for follow-on trench excavations that yield temporal information on past earthquakes needed to assess earthquake hazards.

  18. Oceanic Plate Bending Along the Manila Trench

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    We quantify along-trench variations in plate flexural bending along the Manila trench in the South China Sea. A 3-D interpreted flexural deformation surface of the subducting South China Sea 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 21 across-trench profile sections along the Manila trench and then calculated five best-fitting tectonic and plate parameters that control the flexural bending for each of the across-trench profile sections. Results of analysis revealed significant along-trench variations: The trench relief of the Manila trench varies from 0.8 to 2.2 km, trench-axis vertical loading (-V0) from -0.4x1012 to 1.21x1012 N/m, and axial bending moment (-M0) from 0.005x1017 to 0.6x1017 N. The effective elastic plate thickness seaward of the Manila outer-rise region (TeM) ranges from 30 to 40 km, while that trench-ward of the outer-rise (Tem) ranges from 11 to 30 km. This corresponds to a reduction in Te of 26-63% for the Manila trench. The transition from TeM to Tem occurs at a breaking distance of 50-120 km from the Manila trench axis. The axial vertical loading, bending moment, and the effective elastic thickness of the Manila trench are much smaller than the Mariana trench (Zhang et al., 2014). The contrast in the flexural bending between the Mariana and Manila trenches might be related to the difference in the ages of the subducting plates and other tectonic variables. Zhang, F., Lin, J., Zhan, W., 2014. Variations in oceanic plate bending along the Mariana trench, Earth Planet. Sci. Lett. 401, 206-214. doi: 10.1016/j.epsl.2014.05.032

  19. Approximate General Coulomb Model for Accretionary Prisms: An Integrated Study of the Kumano Transect, Nankai Subduction Zone, Southwest Japan

    NASA Astrophysics Data System (ADS)

    Skarbek, Rob; Ikari, Matt; Hüpers, Andre; Rempel, Alan; Wilson, Dean; Kitajima, Hiroko

    2014-05-01

    In accretionary wedges, the mechanical and hydrologic properties along splay faults and the plate boundary fault at the base of the wedge are intimately related to properties within the wedge itself, as well as to sedimentation and/or mass wasting at the wedge surface, and accretionary flux at the wedge toe; Coulomb wedge theories tie these processes together and have been successful in their application to convergent margins. Most such theories assume for the sake of simplicity that mechanical parameters (e.g. bulk density, compressibility, frictional strength) and pore pressure are constant throughout the overlying wedge. However, the values of these parameters must necessarily change with depth and distance from the trench. Here, we derive a model for a fully general Coulomb wedge, parameterized using data specific to the Kumano transect at Nankai, to better understand the location of the basal plate interface and the properties of material composing an actively accretionary prism. We use shear strength data collected for incoming sediments at Integrated Ocean Drilling Program Site C0011 of the NanTroSEIZE project to parameterize the wedge's coefficient of friction. Preliminary results of models where the friction coefficient of the wedge decreases with depth, with other parameters constant and zero cohesion, indicate that including depth dependent frictional strength in the wedge decreases the taper angle of the wedge, with the effect becoming more pronounced with distance from the trench. This model will be further refined by including seismically and numerically determined spatial variations in fluid pressure within the wedge, as well as detailed locations of the upper and basal wedge surfaces along the Kumano transect determined from 3-D seismic data.

  20. Changbaishan volcanism in northeast China linked to subduction-induced mantle upwelling

    NASA Astrophysics Data System (ADS)

    Tang, Youcai; Obayashi, Masayuki; Niu, Fenglin; Grand, Stephen P.; Chen, Yongshun John; Kawakatsu, Hitoshi; Tanaka, Satoru; Ning, Jieyuan; Ni, James F.

    2014-06-01

    Volcanism that occurs far from plate margins is difficult to explain with the current paradigm of plate tectonics. The Changbaishan volcanic complex, located on the border between China and North Korea, lies approximately 1,300 km away from the Japan Trench subduction zone and is unlikely to result from a mantle plume rising from a thermal boundary layer at the base of the mantle. Here we use seismic images and three-dimensional waveform modelling results obtained from the NECESSArray experiment to identify a slow, continuous seismic anomaly in the mantle beneath Changbaishan. The anomaly extends from just below 660 km depth to the surface beneath Changbaishan and occurs within a gap in the stagnant subducted Pacific Plate. We propose that the anomaly represents hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific Plate and is now escaping through a gap in the subducting slab. We suggest that this subduction-induced upwelling process produces decompression melting that feeds the Changbaishan volcanoes. Subduction-induced upwelling may also explain back-arc volcanism observed at other subduction zones.

  1. 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

  2. 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.

  3. 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.

  4. 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 (~250°C). 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 >350°C, 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.

  5. Global overview of subduction seismicity

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Presti, D.; Heuret, A.; Piromallo, C.

    2013-12-01

    In the framework of the EURYI Project ';Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modelling', we propose the first global overview of subduction seismicity. Previous studies have been focused on interplate seismicity, intraslab seismicity, upper plate deformation, or relation between interplate and intraslab seismicity, but the three components of subduction seismicity have been never approached in an systematic and exhaustive study. To allow such a study, nodal planes and seismic moments of worldwide subduction-related earthquakes heve been extracted by EHB hypocenter and CMT Harvard catalogues for the period 1976 - 2007. Data were collected for centroid depths between sea level and 700 km and for magnitude Mw 5.5. For each subduction zone, a set of trench-normal transects were constructed choosing a 120km width of the cross-section on each side of a vertical plane and a spacing of 1 degree along the trench. For each of the 505 resulting transects, the whole subduction seismogenic zone was mapped as focal mechanisms projected on to a vertical plane after their faulting type classification according to the Aki-Richards convention. Transect by transect, fist the seismicity that can be considered not related to the subduction process under investigation was removed, then was selected the upper plate seismicity (i.e. earthquakes generated within the upper plate as a result of the subduction process). After deletion from the so obtained event subset of the interplate seismicity as identified in the framework of this project by Heuret et al. (2011), we can be reasonably confident that the remaining seismicity can be related to the subducting plate. Among these earthquakes we then selected the shallow (0-70 km), intermediate (70-300 km) and deep (300-660 km) depth seismicity. Following Heuret et al. (2011), the 505 transects were merged into 62 larger segments that were ideally homogeneous in terms of their seismogenic zone characteristics. For each subduction around the world, interplate, intraslab and upper plate seismicity have been estimated and compared to each other through several parameters (seismic rate, moment released rate, maximal expressed magnitude) order to obtain a snapshot on the general behaviour of global subduction-related seismicity. In a second step, the seismological parameters have been compared to long-term geodynamical parameters (e.g., subduction velocity, plate and trench absolute motions, slab age, thermal parameter and geometry, sediment thickness at trench) with the aim to find possible cause-effect relationships.

  6. 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.

  7. 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.

  8. Effects of Subduction Parameters on the Style of Dynamic Buckling of Subducting Slabs

    NASA Astrophysics Data System (ADS)

    Lee, C.; King, S. D.

    2011-12-01

    Buckling of subducting slabs has been suggested to explain the apparent thickening of the subducting slab in the shallow lower mantle, constrained by seismic tomography images. Previous analog/numerical model experiments show that the buckling of subducting slabs develops due to viscosity increases across the upper-lower mantle boundary and/or phase transformations. Our 2-d numerical model studies conducted before show that buckling of subducting slab is fairly consistent with the scaling laws derived for buckling of falling fluids. However, effects of diverse subduction parameters on the style of dynamic buckling of subducting slabs are still not well known. Therefore, we conduct a series of 2-d numerical model experiments allowing dynamic subduction to evaluate the effects of subduction parameters including: 1) viscosity increases across the upper-lower mantle boundary, 2) strength of subducting slab, 3) phase transformations from olivine to wadsleyite (~410 km depth) and from ringwoodite to perovskite plus magnesiowüstite (~660 km depth) in the mantle, 4) trench migrations/mantle wind, and 5) mantle compressibility. Results of the experiments can be summarized below; 1) Higher viscosity increases across the upper-lower mantle boundary create more cycles of slab buckling, slow subduction rate and longer subduction life. 2) Stronger subducting slab creates longer periods of slab buckling, slow subduction rate and longer subduction life. 3) The phase transformation from olivine to wadsleyite is crucial in development of slab buckling. However, the phase transformation from ringwoodite to perovskite plus magnesiowüstite only contributes a minor role in development of slab buckling. 4) Even low migration rate/mantle wind (~1cm/year) significantly reduces buckling of subducting slab. Stagnant slab is well established if higher migration rate/mantle wind is applied with phase transformations and/or higher viscosity increases across the upper-lower mantle boundary. 5) Mantle compressibility develops irregular buckling of subducting slab compared with the experiments without compressibility. However, the effect of mantle compressibility is minor. Most of the experiments which allow buckling of subducting slab are fairly consistent with the scaling laws. However, slab buckling is significantly reduced by trench migration/mantle wind. Since most of the subduction zones have been experiencing trench migration, slab buckling observed in the mantle implies that buckling of subducting slab results from relatively stable trench such as the Mariana subduction zone.

  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. Japan

    ERIC Educational Resources Information Center

    Hawkins, John N.

    1986-01-01

    Analyzes the intergroup relations in Japanese society and Japan's educational system. Challenges the view that Japan is a homogeneous society by presenting the various forms of discrimination against Koreans, Ainu, and the burakumin. Suggests that despite ostracism and isolation, groups can affect public policy and achieve social advancement. (SA)

  11. 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…

  12. Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan

    NASA Astrophysics Data System (ADS)

    Kawahara, Hirokazu; Endo, Shunsuke; Wallis, Simon R.; Nagaya, Takayoshi; Mori, Hiroshi; Asahara, Yoshihiro

    2016-06-01

    Large parts of the shallow mantle wedge are thought to be hydrated due to release of fluids from the subducting slab and serpentinization of the overlying mantle rocks. If serpentinization proceeds under low SiO2 activity, brucite can be a major phase in the low-temperature (< 450 °C) part of the serpentinized mantle wedge, but only very few natural examples have been documented. A combined petrological, geochemical, and geological study shows that brucite is widely distributed in the wedge mantle-derived Shiraga metaserpentinite body in the Sanbagawa metamorphic belt of SW Japan. Thermodynamic modeling combined with bulk rock composition and point counting indicates that the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~ 10-15 vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m-thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H2O budget and mechanical properties of serpentinite; these should be taken into consideration when examining the behavior of the shallow mantle wedge.

  13. 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.

  14. Plumes, plateaux and congestion in subduction zones

    NASA Astrophysics Data System (ADS)

    Moresi, Louis; Betts, Peter; Miller, Meghan; Willis, David

    2014-05-01

    The geologic record provides numerous examples where buoyant plumes, and their associated plateaux, have disrupted convergent plate margins. These interactions have produced a variety of responses in the overriding plate including transient episodes of arc magmatism, 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 numerical models of plume heads of variable dimension and buoyancy interacting with a subducting slab. 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 and plateau 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. We discuss geological evidence for the processes observed in our models.

  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. Seismic behavior and geodetic locking in areas of rough seafloor subduction

    NASA Astrophysics Data System (ADS)

    Bilek, S. L.; Wang, K.

    2012-12-01

    Subduction of rough seafloor occurs throughout the global subduction zones. This rough seafloor introduces heterogeneity in the fault zone that will affect the geodetic coupling and earthquake behavior in these regions. It is often hypothesized that large earthquakes are linked to the presence of subducted seamounts, even as recently as for the 2011 M=9 Tohoku earthquake. However, detailed study of the northern Japan subduction zone suggests fairly smooth incoming seafloor in the main Tohoku slip zone, with very rough seafloor subducting near the southern terminus of the 2011 rupture. This rough zone, which includes a large seamount, had been geodetically defined as a partially locked, or creeping, portion of the megathrust. These rough zones also produce smaller magnitude earthquakes rather than the M 8 or 9 earthquakes observed in zones with smoother incoming plate. We propose here that megathrust fault creep is fairly common in areas of rough incoming plate, and that it is increasingly being illuminated by denser seismic and geodetic observations. Segments of the Costa Rica margin show heterogeneity in earthquake magnitude and geodetic coupling that mimic the heterogeneity in the incoming Cocos Plate. The Nazca Ridge subducting offshore Peru has repeatedly served as a rupture barrier for adjacent great earthquakes, and recent geodetic modeling suggests the megathrust is primarily creeping where the ridge subducts. Along the northern Hikurangi margin, several large seamounts subduct in the region of frequent small-medium sized earthquakes. The subduction fault is geodetically shown to undergo significant creeping that is episodically manifested as near-trench slow slip events. The classic end-member Marianas subduction zone is another example of a subduction zone with very heterogeneous incoming plate that is likely creeping, producing only small magnitude earthquakes. These rough areas are in contrast to subduction zones with fairly smooth incoming plate, such as Chile, Alaska, Cascadia, Sumatra, and Nankai, which have a history of great earthquakes and little to no creep during the interseismic period. To explain the seismic and geodetic observations of these areas, we suggest a model where the rough seafloor subducts predominantly aseismically. Deformation and small magnitude earthquakes occur in the structurally complex fault zone and within fracture networks in the upper and lower plates. The complex structure and heterogeneous stresses of this environment provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures.

  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. 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

  19. Trench Connection

    PubMed Central

    Jamieson, Alan J.; Fujii, Toyonobu

    2011-01-01

    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

  20. Stress analysis using micro-fault and mineral veins along subduction interface within seismogenic zones, example from the Yokonami me Llange, the Cretaceous Shimanto Belt, SW Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, Y.; Takagi, M.; Kirikawa, T.; Taketo, K.

    2006-12-01

    The fluid flow along subduction zones has strong effects on rock strength, material and heat transfer, and change in rock physical property due to chemical rock-fluid interaction. The effects of fluid might be related to the seismogenic failure along subduction interface. The aim of this study is to understand thefluid pressure and stress state on the microfault within the accreionary complexes. They should be one of the important clues to understand the effect of fluid on rock mechanics. In this study, the distribution of mineral veins and vein volume were examined in detail and stress state were also examined by micro-fault analysis, calcite twin piezometer and fluid inclusion in the Yokomani melange, the Cretaceous Shimanto belt, SW Japan, which is an ancient accretionary complex on land. Generally, the mineral vein consists of quartz and calcite, and the calcite twin develops within the calcite well. Some experiments suggested that the density of the calcite twin depends on the differential stress. The measured calcite twin density in the study area indicates that the differential stress is about 270MPa in average, and ranges about 220-340MPa. In addition, micro-fault inversion analysis revealed that the about EW compresstional stress with 0.1 of stress ratio. Fluid inclusion analysis indicated that the P-T condtion of the vein formation was about 160MPa and 180 degree C. From those data and a Coulomb's failure envelope, stress condition was estimated. When the fluid pressure is existed at the time of cracking, sigma1 is about 560MPa, sigma3 is about 290MPa and the average stress is about 430MPa. While, when the fluid pressure was not considered, sigma1 is about 400MPa, sigma3 is about 130MPa and the average stress is about 270MPa. Assuming the mean stress was close to the lithostatic pressure, it is too deep when the fluid pressure was under consideration. On the other hand, at the case of the no consideration of fluid pressure, mean stress is about 192MPa which is about 7.8kmbsf, which is the good agree ment with metamorphic grade of the melange zone. Those result suggest that the fluid might migrated passively into cracks and has high pore pressure ratio to the lithostatic pressure.

  1. Analysis of Oblique Plate Convergence along the Manila Trench and the Philippine Trench

    NASA Astrophysics Data System (ADS)

    Hamburger, M. W.; Galgana, G. A.; Bacolcol, T.; McCaffrey, R.; Yu, S.

    2010-12-01

    The Manila Trench, a >1,200 km long, seismically active N-S trending trench located along the western margin of the Philippine archipelago, acts as the main convergence zone between the Philippine Mobile Belt (PMB) and the Sunda Block (a mobile fragment of the Eurasian Plate). We compare the ongoing subduction along the Manila Trench with that occurring along the opposing Philippine Trench/East Luzon Trough in the east, the boundary which separates the Philippine Mobile Belt from the Philippine Sea Plate. We use joint inversions of published geodetic velocity fields and focal mechanism data to obtain best-fit kinematic block models. From seismicity data, known geometries of faults and subduction dip angles, we construct models of tectonic blocks and their boundaries. We model the Manila and Philippine Trenches as opposite-dipping planes that confine rotating elastic blocks composing the Philippine Mobile Belt. We find that the convergence rate along the Manila Trench decreases progressively southwards, from >70 mm/yr near 19° N, to less than 20 mm/yr at its southern termination at Mindoro Island (~13° N). The systematic slowing reflects the ongoing collision process between Mindoro and the Palawan block; this region acts as a fulcrum, resulting in as much as 7°/My counterclockwise rotation of blocks of the Luzon arc with respect to the Sunda block. The near-orthogonal convergence along the Manila Trench contrasts with the ~40 mm/yr oblique convergence at the Philippine Trench, where the convergence angle changes from nearly westward in the southern PMB to NW near its northern termination in SE Luzon. We find that the Manila Trench is relatively poorly coupled compared to the moderately coupled Philippine Trench. Based on the rates and directions of convergence, we find that the northern Philippine Fault system accommodates the shear component of convergence along the two margins of the PMB, resulting in strain partitioning. The lower inferred seismic coupling along the Manila Trench as compared to the Philippine Trench may explain its relative low rate of historical seismic moment release in earthquakes. However, additional GPS sites close to the Manila and Philippine trenches are needed to reliably resolve coupling rates along these subduction boundaries.

  2. 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.

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

    NASA Astrophysics Data System (ADS)

    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.

  4. 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

  5. Geometrical effects of a subducted seamount on stopping megathrust ruptures

    NASA Astrophysics Data System (ADS)

    Yang, Hongfeng; Liu, Yajing; Lin, Jian

    2013-05-01

    have numerically simulated dynamic ruptures along a "slip-weakening" megathrust fault with a subducted seamount of realistic geometry, demonstrating that seamounts can act as a barrier to earthquake ruptures. Such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level, for larger seamount height-to-width ratio, and for shorter seamount-to-nucleation distance. As the seamount height increases from 0 to 40% of its basal width, the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as ~20%. We further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone, coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level. These results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions, including the case of the thrust fault being lubricated by seamount-top fluid-rich sediments, as suggested from observations in the Japan and Sunda Trenches.

  6. 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.

  7. Block movement and internal deformation of the Nankai forearc sliver associated with oblique subduction of the Philippine Sea plate in southwest Japan

    NASA Astrophysics Data System (ADS)

    Ichitani, S.; Tabei, T.; Kubo, A.

    2012-12-01

    We investigate block movement and internal deformation of the Nankai forearc sliver in southwest Japan. The forearc has been deformed by oblique subduction of the Philippine Sea plate: interseismic crustal shortening in the direction of plate convergence and long-term lateral movement along the Median Tectonic Line (MTL) that divides the forearc from the rest of the overriding plate. In this study, we decompose crustal deformation field into these two components using three-dimensional surface displacement rates from nationwide continuous GPS array and supplementary campaign networks across the MTL. We use the following procedures: (1) we correct the original velocity data to remove the forearc movement, assuming that the forearc moves at a constant rate of 3-10 mm/yr along the MTL but its fault plane is fully locked from surface to a depth of 15 km. (2) Using the corrected velocity data, we estimate interseismic slip deficit distribution on the plate interface reproduced by more than 500 triangular elements. Then the site velocities calculated from the above plate coupling distribution are compared with the original velocities. (3) Residuals between the original and calculated velocities illustrate the forearc lateral movement and the locking effect of the MTL fault plane. Now we can use the residual velocity field to estimate slip-locking distribution on the MTL fault plane. (4) We check the first-assumed constant rate of the forearc movement by comparing it with the estimated slip deficit rate on the MTL. In this analysis the optimal rate of the forearc movement is estimated as large as 5-6 mm/yr. In the eastern Shikoku the slip pattern is nearly pure strike-slip at a rate of 2-4 mm/yr. In contrast significant normal component is recognized together with strike-slip component of about 5 mm/yr in the western Shikoku.

  8. Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Kita, Saeko; Matsubara, Makoto

    2016-03-01

    The three-dimensional seismic attenuation structure (frequency-independent Q) beneath southwestern Japan was analyzed using t* estimated by applying the S coda wave spectral ratio method to the waveform data from a dense permanent seismic network. The seismic attenuation (Qp-1) structure is clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to approximately 50 km. At depths of 5 to 35 km, the seismic attenuation structure changes at the Median tectonic line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High-Qp zones within the lower crust of the overlying plate are found just above the slip regions at the centers of the long-term slow-slip events (SSEs) beneath the Bungo and Kii channels and central Shikoku. Beneath central Shikoku, within the overlying plate, a high-Qp zone bounded by low-Qp zones is located from the land surface to the plate interface of the subducting plate. The high-Qp zone and low-Qp zones correspond to high-Vp and low-Vp zones of previous study, respectively. The boundaries of the high- and low-Qp zones are consistent with the segment boundaries of tremors (segment boundaries of short-term SSEs). These results indicated that the locations of the long- and short-term SSEs could be limited by the inhomogeneous distribution of the materials and/or condition of the overlying plate, which is formed due to geological and geographical process. The heterogeneity of materials and/or condition within the fore-arc crust possibly makes an effect on inhomogeneous rheological strength distribution on the interface.

  9. Structure and properties of the lithosphere subducting beneath Indonesia, consequences on subduction

    NASA Astrophysics Data System (ADS)

    Jacob, Jensen; Dyment, Jerome

    2014-05-01

    We make inferences on the structure, age and physical properties of the subducting northern Wharton Basin lithosphere by (1) modeling the structure and age of the lithosphere subducted under the Sumatra trench through two- and three-plate reconstructions involving Australia, Antarctica, and India, and (2) superimposing the resulting fracture zones and magnetic isochrons to the geometry of the subducting plate as imaged by seismic tomography. This model provides an effective means to study the effect of varying physical properties of the subducting lithosphere on the subduction along the Sumatra trench. The age of the oceanic lithosphere determines its thickness and buoyancy, then its ability to comply with or resist subduction. The "subductability" of the lithosphere is the extra weight applied on the asthenosphere by the part of the bulk lithospheric density exceeding the asthenospheric density. A negative subductability means that the bulk lithospheric density is lower than the asthenospheric density, i.e. the plate will resist subduction, which is the case for lithosphere younger than ~23 Ma. The area off Sumatra corresponds to oceanic lithosphere formed between 80 and 38 Ma, with a lower subductability than other areas along the Sunda Trench. The spreading rate at which the oceanic lithosphere was formed has implications of the structure and composition of the oceanic crust, and therefore on its rheology. In a subduction zone, the contact between the subducting and overriding plates is considered to be the top of the oceanic crust and the overlying sediments. The roughness of this interface and the rheology of its constitutive material are essential parameters constraining the slip of the downgoing plate in the seismogenic zone, and therefore the characteristics of the resulting earthquakes. Whereas the rough topography of a slow crust may offer more asperities than the smooth topography of a fast crust, the weak rheology of serpentines in a slow crust would favor a regular slip, unlike the brittle magmatic rocks of the fast crust and the underlying dry olivine mantle. The presence of peculiar features such as fracture zones, seamounts, or oceanic plateaus also affects the seismic segmentation of the subduction zone at different scales. Many seamounts have been mapped in the Wharton Basin between 10°S and 15°S, and similar seamounts belonging to the same province may have existed further north and subducted in the Sunda Trench from southern Sumatra to Java and eastward. Conversely, the Roo Rise, a larger plateau located south of Eastern Java, may resist the subduction, as suggested by the geometry of the Sunda Trench in this area, diverting from the regular arc by a maximum of 60 km.

  10. 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] Kárason & 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.

  11. 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.

  12. 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.

  13. Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin

    NASA Astrophysics Data System (ADS)

    Gutscher, Marc-André; Spakman, Wim; Bijwaard, Harmen; Engdahl, E. Robert

    2000-10-01

    The cause and geodynamic impact of flat subduction are investigated. First, the 1500 km long Peru flat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three-dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the flat slab segment, with possible slab detachment occurring down dip as well. Other flat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. In nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and flat subduction, we conclude that flat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250-800 km from the trench is on average 3-5 times greater above flat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.

  14. 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.

  15. Geochemical Tracing of Mantle Flow above Subduction Zones

    NASA Astrophysics Data System (ADS)

    Pearce, J. A.; Barry, T. L.; Millar, I. L.; Leat, P. T.; Stern, R. J.

    2004-12-01

    Geochemical tracing may be used to track mantle flow above and behind subduction zones and so provide an independent test of the applicability of seismic anisotropy measurements. The theory is that, if mantle flow is accompanied by decompression, then extraction of small degree melts from multi-component mantle leads to compositional gradients in the mantle in both isotope and trace element space. These gradients may be obtained by inverting geochemical data from the products of mantle melting. If mantle flow is accompanied by addition of a subduction fluid, then simultaneous melting and subduction component addition may also produce compositional gradients. Numerical experiments enable compositional gradients to be quantified in terms of the extent of melt extraction, mantle temperature and other variables. In addition, isotope and trace element systematics provide evidence for the provenance of the mantle entering the subduction system, with Hf isotopes and immobile trace elements providing a means of establishing provenance even from magmas generated directly above the dehydrating subducted plate. This work focuses on a series of geochemical maps which enable mantle flow to be traced for range of oceanic arc basin systems (Izu-Bonin-Mariana, Tonga-Vanuatu, Scotia, Manus) and, provisionally, some continental systems (Japan, Cascades). Using maps based on geochemical proxies for melt extraction (such as Ta/Yb), subduction-addition (such as Th/Ta) and mantle provenance (such as epsilon-Hf v epsilon Nd), it is possible to demonstrate the existence of a wide range of mantle flow regimes. Thus, the Izu and Japan systems appear to be characterised by simple trench-orthogonal flow, the Mariana system by dispersion away from several separate centers of mantle upwelling, the Tonga-Vanuatu system by unidirectional flow from beneath the Pacific plate in the north, and the Scotia system by bi-directional flow from both north and south. In a number of these cases, isotopic fingerprinting using immobile isotope ratios is critical for establishing the ultimate source of the mantle: for example, the mantle entering the Scotia system may be seen to originate from the Atlantic Bouvet domain rather than the Atlantic Tristan or the Pacific domains. These results are broadly in keeping with seismic anisotropy measurements to date while providing greater coverage but, of course, geochemical tracing is restricted to areas of magmatic activity. Integration of geophysical and geochemical methods may therefore be necessary to provide the maximum information on mantle flow.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

    Čížková, 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.

  18. 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.

  19. 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.

  20. GPS Monitoring of Subduction Zone Deformation in Costa Rica

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul

    1997-01-01

    The subduction of the Cocos plate beneath Costa Rica is among the highest convergence rates in the world. The high subduction rate and nearness of the Nicoya Peninsula, Costa Rica to the Middle America Trench (MAT) provide a unique opportunity to map variations in interseismic strain of the crust above the seismogenic zone in response to variations in seismic coupling.

  1. 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.

  2. 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.

  3. Microstructures, Chemical Composition, and Viscosities of Fault-generated Friction Melts in the Shimanto Accretionary Complex, Southwest Japan: Implication for Dynamics of Earthquake Faulting in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Ujiie, K.; Yamaguchi, H.

    2004-12-01

    The pseudotachylytes (PT) were recently found in the Cretaceous Shimanto accretionary complex of eastern and western Shikoku, southwest Japan, but their microstructures under a backscattered electron image, chemical composition, and effects of frictional melting on co-seismic slip in the accretionary prism remains poorly understood. The PT bearing fault is the 1-2 m thick roof thrust of a duplex structure, which bounds the off-scraped coherent turbidites above from the imbricated melange below without a thermal inversion across the fault. The fault zone consists of foliated cataclasite of sandstone-shale melange in origin and dark veins. The PT commonly occurs as brecciated fragments in dark veins. The PT matrix is transparent under plane-polarized light and is optically homogeneous under cross-polarized light, similar to glass matrix. Under a backscattered electron image, the PT clearly shows the evidences for frictional melting and subsequent rapid cooling: rounded and irregularly shaped grains and vesicles in matrix and fracturing associated with grain margins. These textural features of the PT are very similar to those of experimentally generated PT. The EPMA analysis indicates that chemical composition of the PT matrix corresponds to illite with 5.7-9.9 wt% H2O and that partially melted grains are dominated by orthoclase and quartz. This indicates that the temperatures of the PT melt could reach the breakdown temperatures of orthoclase (1150 C) and quartz (1730 C), greater than the maximum temperature recorded in host rocks (170-200 C). We calculated the viscosity of friction melt, based on the chemical composition of the PT matrix and the volume fraction and aspect ratio of grains in the PT. We considered both Arrhenian and non-Arrhenian models for viscosity calculation. Our result demonstrates that the melt viscosity is much lower than PT in continental plutonic and metamorphic rocks: 10^3 Pa s (Arrhenian model) and 10^2 Pa s (non-Arrhenian model) even at 700 C and 10 Pa s (both models) at 1200 C. The extremely low melt viscosity is caused primarily by the formation of liquids (release of OH-) from hydrous illite, and secondarily by small volume fraction (< 20%) of grains in the PT. Because illite is commonly present in accretionary prisms, generation of a low viscosity melt from illite would lead to fault lubrication and hence control the efficiency of stored strain energy release and earthquake magnitude in subduction zones.

  4. P-wave tomography for 3-D radial and azimuthal anisotropy of Tohoku and Kyushu subduction zones

    NASA Astrophysics Data System (ADS)

    Wang, Jian; Zhao, Dapeng

    2013-06-01

    We determined high-resolution P-wave tomography for 3-D radial and azimuthal anisotropy of the Tohoku and Kyushu subduction zones using a large number of high-quality arrival-time data of local earthquakes recorded by the dense seismic network on the Japan Islands. Trench-normal P-wave fast-velocity directions (FVDs) are revealed in the backarc mantle wedge in both Tohoku and Kyushu, which are consistent with the model of slab-driven corner flow. Trench-parallel FVDs with amplitude <4 per cent appear in the forearc mantle wedge under Tohoku and Kyushu, suggesting the existence of B-type olivine fabric there. Trench-parallel FVDs are also visible in the mantle wedge under the volcanic front in Tohoku but not in Kyushu, suggesting that 3-D flow may exist in the mantle wedge under Tohoku and the 3-D flow is affected by the subduction rate of the oceanic plate. Negative radial anisotropy (i.e. vertical velocity being faster than horizontal velocity) is revealed in the low-velocity zones in the mantle wedge under the arc volcanoes in Tohoku and Kyushu as well as in the low-velocity zones below the Philippine Sea slab under Kyushu, which may reflect hot upwelling flows and transitions of olivine fabrics with the presence of water in the upper mantle. Trench-parallel FVDs and positive radial anisotropy (i.e. horizontal velocity being faster than vertical velocity) are revealed in the subducting Pacific slab under Tohoku and the Philippine Sea slab under Kyushu, suggesting that the slabs keep their frozen-in anisotropy formed at the mid-ocean ridge or that the slab anisotropy is induced by the lattice-preferred orientation of the B-type olivine.

  5. 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.

  6. Post-seismic deformation of the 2011 Tohoku earthquake, Japan

    NASA Astrophysics Data System (ADS)

    Kyriakopoulos, C.; Masterlark, T.; Chini, M.; Bignami, C.; Stramondo, S.

    2012-04-01

    The Mw 9.0 Tohoku earthquake on March 11, 2011 occurred near the northeast coast of Honshu, Japan. The earthquake resulted from a thrust faulting on the subduction zone boundary between the Pacific and North America plates. Surface displacements due to the Tohoku-Oki earthquake were observed by more than 1200 continuously recording Global Positioning System (GPS) sites, installed and operated by the Geodetic Survey of Japan (GSI). For the first time, in a megathrust event, the displacement above the hypocenter is detected from 5 GPS installed in the seafloor (Sato et., al 2011), giving new insights into the megathrust mechanism. The link, i.e. Green's Functions, between the surface displacement and the model parameters is obtained from a 3D Finite Element (FE) model for the 11 March earthquake. Several geophysical features of the Japan trench are implemented into the FE model. The Subducting slab geometry is implemented from USGS and Gavin Hayes Slab 1.0 project. Bathymetry and topography from the ETOPO Global Relief Project (NOAA) are implemented as well. Moreover, the model is designed to simulate coseismic and postseismic (poroelastic) deformation while simultaneously account for the known geologic structure and geophysical context (Zhao et al., 1992 ) of the Japanese subduction zone. The postseismic deformation is explored using the postseismic GPS vectors available in the literature. Furthermore, we applied DInSAR (Differential SAR interferometry) to infer the post-seismic deformation field by exploiting the available SAR images acquired by the ENVISAT satellite.

  7. The earliest mantle fabrics formed during subduction zone infancy

    NASA Astrophysics Data System (ADS)

    Harigane, Y.; Michibayashi, K.; Morishita, T.; Tani, K.; Dick, H. J.; Ishizuka, O.

    2013-12-01

    Harzburgites obtained from the oldest crust-mantle section in the Philippine Sea plate along the landward slope of the southern Izu-Ogasawara Trench in Izu-Bonin-Mariana arc, that explored by Dive 7K417 of the ROV Kaiko 7000II during R/V Kairei cruise KR08-07, and Dredge 31 of R/V Hakuho-Maru cruise KH07-02, operated by the Japan Agency for Marine-Earth Science and Technology. Harzburgites preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The main constituent minerals of harzburgites are olivine (15.6%), orthopyroxene (Opx; 13.1%) and spinel (0.5%), along with serpentine (70.8%) as a secondary mineral. Microstructure shows inequigranular interlobate (or protogranular) textures. There is no secondary deformation such as porphyroclastic or fine-grained textures. The secondary serpentine shows undeformed mesh texture in the harzburgites. Harzburgites have crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The mineral chemistry in harzburgites have high olivine forsterite (90.6-92.1 mol.%) and NiO (~0.4 wt%) contents, low Opx Al2O3 (<~1.5 wt%) and Na2O (<0.03 wt%), and high spinel Cr# (65-67). This has the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. Therefore, we propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.

  8. 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)

  9. 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.

  10. Digging a Paleoseismic Trench

    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....

  11. Numerical simulation of crustal deformation near the trench axis

    NASA Astrophysics Data System (ADS)

    Watanabe, T.; Tadokoro, K.; Yasuda, K.; Fujii, C.; Matsuhiro, K.

    2014-12-01

    The Philippine Sea plate subducts beneath southwest Japan along the Nankai Trough with a rate of about 4-6 cm/yr, where megathrust earthquakes have repeatedly occurred with recurrence intervals of about 100-150 years. It is known that these earthquakes often ruptured adjacent segments and brought the Japanese Islands serious damage like the 2011 Tohoku earthquake. Especially the damage caused by tsunami which arises from co-seismic slip at the shallow part of plate boundary is serious. Thus, it is important to investigate the status of strain accumulation near the trench axis in inter-seismic period. For this issue, we started seafloor geodetic observations in 2013 using a GPS/Acoustic technique at two or three sites across the trench axis, which are located in the shallow extension of the rupture area of the 1944 Tonankai earthquake. Therefore, on the basis of these observations, we investigated the crustal movement at the shallow part of plate boundary through numerical simulation using finite element method. Until now back slip model have been used to interpret the crustal deformation derived from onshore geodetic observations such as GPS, and then this model achieved a good deal of successful results. However, in late years it is doubted for crustal deformation at the shallow part of plate boundary whether it is appropriate to use back slip model there. In this study, we assumed frictionless plate boundary from the trench to a certain depth and assigned back slip to plate boundary deeper than a certain depth as boundary conditions. The frictionless up-dip segment is dragged passively toward the deep part by the back slip of the down-dip segment. This model reproduces the unlocked zone at the shallow depth and locked zones at the deep depth. We investigated the horizontal and vertical displacement profiles on the surface accompanied with the variation of the depth of the up-dip limit assigning back slip. This result showed that the peaks of horizontal displacement profiles correspond to the locations of starting point of back slip. In addition, horizontal displacement near the trench axis increases as the depth of the up-dip limit of back slip become shallow. For vertical displacements, we cannot detect the significant differences among the models.

  12. Estimation of coupling factor in the northern part of Ryukyu trench by using 2D FEM

    NASA Astrophysics Data System (ADS)

    Nakao, S.; Maeno, S.; Goto, K.

    2009-12-01

    Philippine Sea plate (PH) subducts beneath Ryukyu Arc, where Nansei Islands are located, at the Ryukyu trench. Coupling factor has been studied in the subduction zones around the Japan Island since GEONET GPS network of GSI, Japan were deployed. The coupling factors of zero and 13 % in this subduction zone were estimated. The northern part of Okinawa trough spreads in the direction of East-West. It is difficult to make a model of subduction zone that spreading of Okinawa trough is taken into account. We make 2-dimensional Finite Element (FEM) model to calculate the displacement pattern on the plate. The 2-D model is taken into account for the subduction of PH plate and spreading of Okinawa trough. GeoFEST FEM program (Parker et al, 2008) is used in this calculation. We also try to estimate coupling factor by trial and error mothod. We make two cross sections which are perpendicular to the trench axis of Ryukyu trench. One is located on Tanegashima Island and the other is Amami-Oshima Island. The boundary of subducted PH plate is determined based on the hypocenter distribution by Goto et al. (2008). The thickness of Ryukyu Arc is 25km based on the structure of seismic velocity by Iwasaki et al. (1990). Elastic constants of crust and mantle are adopted by Suito et al. (2002). The direction of movement of PH plate with respect to the Ryukuy Arc is derived from Euler vectors: one is Amur plate with respect to PH plate (Miyazaki and Heki, 2001) and the other is the direction of Ryukyu Arc with respect to Amur plate (Nishimura et al., 2004). The direction of subduction of PH plate with respect to Ryukyu Arc is calculated by using Euler vectors which estimated by Nishimura et al. (2004). Spreading rate of Okinawa trough is given at upper part of the western edge of the 2-D model. Amount of spreading of Okinawa trough is calculate at the western potion of Ryukyu Arc by the Euler vector of Nishimura et al. (2004). The depth of coupling region of plates is from 20 to 30km and 10 to 35 km in Tanegashima and Amami-Oshima cross section, respectively. Horizontal displacement rate observed at eight GEONET sites are estimated by the least squares method using the coordinate solution of F2 analysis of GSI at Tanegashima cross section. In Amami-Oshima cross section horizontal displacement rates at six GEONET sites are estimated. Annual and semi-annual components are also estimated when horizontal displacement rates are estimated. The coordinate system of these displacement rates is transformed to that with respect to the Ryukyu Arc by using absolute Euler vector of Amur plate and relative Euler vector of Ryukyu Arc with respect to Amur plate (Nishimura et al., 2004). Horizontal displacement rates become smaller when the coupling factor is smaller at both cross sections. Horizontal displacements begin to be smaller at about 10km from the eastern edge of coupling region. Horizontal displacement rates are smallest at the Okinawa trough side. When the coupling factor is large, horizontal displacement becomes large. Coupling factors, 30% are estimated at both cross sections by using trial and error method. At Tanegashima cross section, the depth of coupling region is also changed at estimation of coupling factor, which is from 10 to 35 km.

  13. 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

  14. Operations Summary During Riserless Drilling to >7700 mbsl in the Japan Trench for IODP Expedition 343 & 343T: JFAST, and Discussion of the Relationship Between Drilling Parameters and Rock Damage.

    NASA Astrophysics Data System (ADS)

    Toy, V. G.; Maeda, L.; Toczko, S.; Eguchi, N.; Chester, F. M.; Mori, J. J.; Sawada, I.; Saruhashi, T.

    2014-12-01

    During IODP Expedition 343: The Japan Trench Fast Drilling Project (JFAST), two main boreholes were drilled from the D/V Chikyu in ~7000 m water depth. An uncored hole that penetrated to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]) was documented using logging while drilling (LWD) tools. From an adjacent partially cored hole drilled to 844.5 mbsf (TD = 7734 mbsl) 21 cores were acquired that spanned the two main fault targets. The operations lasted 88 days. The drilling operation was very technically challenging. The drill string had to be withdrawn a number of times due to high seas, and technical issues; five holes were drilled (one abandoned after spud-in) and reoccupied in >6800 m water depth. A simple observatory was deployed in the wellhead installed during Exp 343 during the follow-up Exp 343T. In certain intervals during coring we mostly recovered loose, subrounded fine gravel clasts of the two major lithologies penetrated to those depths (silt and mudstones). We have performed particle shape and size analysis on these gravel aggregates. Particle shape variations apparent visually are not clearly quantified by conventional 'shape descriptors'. Variations in particle size distributions are apparent and we will discuss whether these relate to variations in drilling parameters.

  15. 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).

  16. Postseismic Deformations Associated with Maule Earthquake and the Mechanical Properties of the Asthenosphere and Subduction Interface

    NASA Astrophysics Data System (ADS)

    Klein, E.; Fleitout, L.; Vigny, C.; Garaud, J. D.

    2014-12-01

    The interseismic and postseismic deformations preceding and following the large subduction earthquake of Maule (Chile, Mw8.8, 2010) have been closely monitored with GPS from 70 km up to 2000 km away from the trench. Post-seismic deformations exhibit a behavior generally similar to that already observed after the Tohoku-Oki earthquake. Like in Japan, vertical uplift is observed on the oceanward side of the volcanic arc. A moderate large scale subsidence is associated with sizeable horizontal deformation in the far-field (500-2000km from the trench). In addition, near-field data (70-200km from the trench) feature a rather complex deformation pattern. Further north, we observe an increase of the eastward velocities and a slight rotation toward the North. We use a 3D FE code (Zebulon Zset) to relate these deformations to the mechanical properties of the mantle. The mesh features a spherical shell-portion from the core-mantle boundary to the earth's surface, extending over more than 60 degrees in latitude and longitude. The overridding and subducting plates are elastic, and the asthenosphere is viscoelastic. We test the presence and shape of two low viscosity areas in the mantle : a low viscosity wedge (LVW) above the subducting plate extending beneath the volcanic arc, and a narrow low viscosity channel (LVCh) along the lower part of the subduction interface, and potentially deeper. All the viscoelastic regions feature a Burgers rheology and we invert for their mechanical properties and geometrical characteristics. Our best fitting models present, (i) an asthenosphere extending down to 270km, with a 'long-term' viscosity of the order of 3.1018 Pa.s; (ii) a LVCh along the plate interface extending from depths of 50 to 150 km with viscosities slightly below 1018 Pa.s; (iii) a LVW restricted to the base of the lithosphere below the volcanic arc, with viscosities of a few 1018 Pa.s. Increased horizontal velocities are due to relaxation in both the asthenosphere and the LVCh. A deep channel is necessary to produce enough uplift in the middle-field (200-500km from the trench). Some additional slip on the plate interface, at shallow depth, is also necessary to explain all the characteristics of the near-field displacements.

  17. 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 mélange 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; Samaná, Dominican Repúblic).

  18. 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

  19. Earthquake hazards on the cascadia subduction zone

    SciTech Connect

    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/sub 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/sub w/ 8) or a giant earthquake (M/sub w/ 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M/sub w/ less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M/sub 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. 35 references, 6 figures.

  20. Subduction zone plate bending earthquakes and implications for the hydration of the downgoing plate

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    The greatest uncertainty in the amount of water input into the Earth at subduction zones results from poor constraints on the degree and depth extent of mantle serpentinization in the downgoing slab. The maximum depth of serpentinization is thought to be partly controlled by the maximum depth of tensional earthquakes in the outer rise and trench and is expected to vary from subduction zone to subduction zone or even along-strike for a single subduction zone. We explore the maximum depth of extensional faulting on the incoming plate for various subduction zones in order to gain insight into the possible extent of slab serpentinization. We relocate trench events at island arc subduction zones using hypocentroidal decomposition to determine which earthquakes occurred within the incoming plate. For earthquakes with Mw ~5.5+, we determine accurate depths and refine the CMT focal mechanism by inverting teleseismic P and SH waveforms. Results from the Mariana outer rise indicate that extensional earthquakes occur in the Pacific plate at depths ranging from 10-20 km beneath the top of the crust, with the character of trench seismicity changing significantly between the northern and southern portions of the subduction zone. In comparision, results from the Aleutian subduction zone show extensional trench earthquakes occurring from 5-30 km below the surface of the subducting slab. Compressional incoming plate earthquakes occur only near the Alaskan Peninsula, possibly due to stronger coupling between the slab and overriding plate in this region. Further results from oceanic arc subduction zones will be presented and differences between subduction zones as well as along-strike differences in the character of trench seismicity will be highlighted. If the presence of extensional faulting indicates subducting lithosphere hydration, then we expect that as much as the top 30 km of the slab may be hydrated and that the degree of slab serpentinization may vary significantly between subduction zones, potentially affecting arc geochemistry, intermediate depth seismicity, and the subduction zone water budget.

  1. 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.

  2. Varying Structure and Physical Properties of the Lithosphere Subducting Beneath Indonesia, Consequences on the Subduction

    NASA Astrophysics Data System (ADS)

    Jacob, J.; Dyment, J.

    2013-12-01

    We make inferences on the structure, age and physical properties of the subducting northern Wharton Basin lithosphere by (1) modeling the structure and age of the lithosphere subducted under the Sumatra trench through three-plate reconstructions involving Australia, Antarctica, and India, and (2) superimposing the resulting fracture zones and magnetic isochrons to the geometry of the subducting plate as imaged by seismic tomography. The model of Pesicek et al. (2010) was digitized and smoothed in order to get a realistic topography of the subducting plate. The fracture zone and magnetic isochron geometry was draped on this topography assuming a N18°E direction of subduction. This model provides an effective means to study the effect of varying physical properties of the subducting lithosphere on the subduction along the Sumatra trench. 1) The age of the oceanic lithosphere determines its thickness and buoyancy, then its ability to comply with or resist subduction. We define the "subductability" of the lithosphere as the extra weight applied on the asthenosphere by the part of the bulk lithospheric density exceeding the asthenospheric density. A negative subductability means that the bulk lithospheric density is lower than the asthenospheric density, i.e. the plate will resist subduction, which is the case for lithosphere less than ~23 Ma. The area off Sumatra corresponds to oceanic lithosphere formed between 80 and 38 Ma, with a lower subductability than other areas along the Sunda Trench. 2) The spreading rate at which the oceanic lithosphere was formed has implications of the structure and composition of the oceanic crust, and therefore on its rheology. In a subduction zone, the contact between the subducting and overriding plates is often considered to be the top of the oceanic crust and the overlying sediments. The roughness of this interface and the rheology of its constitutive material are essential parameters constraining the slip of the down going plate in the seismogenic zone, and therefore the characteristics of the resulting earthquakes. Indeed the rough topography of a slow crust may offer more asperities, and therefore a more irregular slip, than the smooth topography of a fast crust. Conversely, the weak rheology of serpentines present in a slow crust would favor a regular slip, unlike the brittle magmatic rocks of the fast crust and the underlying dry olivine mantle. 3) Local features, including fracture zones and seamounts, may affect the seismic segmentation of the subduction zone. Many seamounts have been mapped in the Wharton Basin between 10°S and 15°S., their age decreasing from 136 Ma to the East to 47 Ma to the West, with anomalously younger ages in Christmas Island. Similar seamounts belonging to the same province may have existed further north and subducted in the Sunda Trench from southern Sumatra to Java and eastward. Conversely, the Roo Rise, a larger plateau located south of Eastern Java, may have more difficulty to enter the subduction, as suggested by the geometry of the Sunda Trench in this area, diverting from the regular arc by a maximum of 60 km. References Pesicek, J.D., C.H. Thurber, S. Widiyantoro, H. Zhang, H.R. DeShon, and E.R. Engdahl (2010), Sharpening the tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma, Geophys. J. Int., 182, 433-453.

  3. 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.

  4. 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.

  5. Simulation of tectonic evolution of the Kanto Basin of Japan since 1 Ma due to subduction of the Pacific and Philippine Sea plates and the 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

    2016-06-01

    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 repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1 My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled 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 latest collision of the Izu Peninsula block 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 the shift in plate motion. Observed changes in the uplift/subsidence 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 plate motion shift. We also calculated 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.

  6. 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.

  7. Temperature and melt fraction distributions in a mantle wedge determined from the electrical conductivity structure: Application to one nonvolcanic and two volcanic regions in the Kyushu subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Hata, Maki; Uyeshima, Makoto

    2015-04-01

    We propose a new method for estimating the temperatures and melt fractions of the upper mantle. Our method is based on connecting the electrical conductivity structure from geophysical observations with laboratory-determined relationships between the electrical conductivity and temperature of four nominally anhydrous minerals (olivine, orthopyroxene, clinopyroxene, and garnet) and basaltic melt. The temperatures are expressed as the upper limit temperatures using the Hashin-Shtrikman lower bound in solid phases and using the Hashin-Shtrikman upper bound in solid-liquid mixed phases. We apply the method to a nonvolcanic and two volcanic regions in the Kyushu subduction zone, southwest Japan. Our results suggest that the temperatures of the upper mantle are 1100-1450°C for dry mantle and 900-1350°C for wet mantle and that the melt fractions of the upper mantle are <20% beneath the two volcanic regions and <5% beneath the nonvolcanic region for both dry and wet mantle.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Real feature of seismicity around Palau trench region, western Pacific: Is Palau trench aseismic silent plate boundary?

    NASA Astrophysics Data System (ADS)

    Ishihara, Y.; Shito, A.; Tanaka, S.; Suetsugu, D.

    2012-12-01

    Palau islands locate around plate converging zone in the western Pacific region. In the east off the Palau islands, obvious trench topography is developed whose bathymetry reaches about 6000 meters. Palau trench locates at the west side of Yap trench. However tectonic activity is quite different in the both trenches. Yap trench has active seismic activity associate with subduction process. Plate motion model shows clear convergent relative motion between Pacific plate and Philippine Sea plate at Yap trench. On the other hand, Palau trench doesn't have active seismicity according to ISC catalogue. In ten years in 2000's, only three small earthquakes are reported in ISC catalogue. Historically any great earthquake also is not reported. Recent plate motion model shows very low convergent motion at Palau trench though developed trench structure. Our group operates broadband seismic station at Palau (station code: PALU) for about 15 years. In our instant monitoring, local earthquakes sometime are recognized. We operated additional stations in Palau islands for six months to detect local earthquake and to locate hypocenters. Our objective of the research is evaluation of real seismicity of Palau region and final major interest is to understand tectonic activity of Palau trench. We install minimum network for hypocenter locating in Koror and Babeldaob islands, Palau that its array dimension is about 20 km. We use broadband seismographs and high resolution data loggers with GPS clock and solar power generators. We succeeded continuous recording without any troubles and clips of mass position. By careful motoring, we pick up greater than 70 local earthquakes in only six months. And we also tried to read the P and S wave arrival times. We succeeded to locate 27 hypocenters. The number of seismic events is much higher than initial estimation. The hypocenters locate east coast side of Palau islands where is trench side. The overview of distribution is parallel to trench. Estimated depth is distributed from 20 to 30 km. The determination is inaccurate and sparse distributed, but simple seismograms mean that these are not shallow crustal event. P and S wave amplitude analysis says that dip-slip type fault mechanism is dominant. This seismic activity may be strongly related with subduction process. These earthquakes are magnitude of 2 to 3. Seismicity of Palau area is much higher than initial estimation based on earthquake catalogue. These results mean that Palau trench has latent active seismic process and suggest that the trench may have convergent plate process than general understanding.

  14. 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

  15. 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.

  16. 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, N80°E 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.

  17. Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Tanikawa, Wataru; Mukoyoshi, Hideki; Lin, Weiren; Hirose, Takehiro; Tsutsumi, Akito

    2014-12-01

    We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10-18 m2 at site C0004 and 9 × 10-18 m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10-6 m2 s-1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.

  18. 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.

  19. 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.

  20. Does subduction-induced mantle flow drive backarc extension?

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter P.; Strak, Vincent; Duarte, João C.

    2016-05-01

    Backarc extension is a characteristic feature of many narrow subduction zones. Seismological and geochemical studies imply the occurrence of mantle flow around the narrow subducting slabs. Previous 3D models suggested that backarc extension is related to subduction-induced toroidal mantle flow. The physical viability of this mechanism, however, has never been tested using laboratory-based geodynamic models. In this work, we present dynamic laboratory models of progressive subduction in three-dimensional (3D) space that were carried out to test this mechanism. To achieve this, we have used a stereoscopic Particle Image Velocimetry (sPIV) technique to map simultaneously overriding plate deformation and 3D subduction-induced mantle flow underneath and around an overriding plate. The results show that the strain field of the overriding plate is characterized by the localization of an area of maximum extension within its interior (at 300-500 km from the trench). The position of maximum extension closely coincides (within ∼2 cm, scaling to 100 km) with that of the maximum trench-normal horizontal mantle velocity and velocity gradient measured at a scaled depth of 15-25 km below the base of the overriding plate, and the maximum horizontal gradient of the vertical mantle velocity gradient. We propound that in narrow subduction zones backarc extension in the overriding plate is mainly a consequence of the trench-normal horizontal gradients of basal drag force at the base of the overriding plate. Such shear force gradients result from a horizontal gradient in velocity in the mantle below the base of the lithosphere induced by slab rollback. Calculations based on our models indicate a tensional horizontal trench-normal deviatoric stress in the backarc region scaling to ∼28.8 MPa, while the overriding plate trench-normal stress resulting from the horizontal component of the trench suction force is about an order of magnitude smaller, scaling to ∼2.4-3.6 MPa.

  1. Upper Pleistocene uplifted shorelines as tracers of (local rather than global) subduction dynamics

    NASA Astrophysics Data System (ADS)

    Henry, Hadrien; Regard, Vincent; Pedoja, Kevin; Husson, Laurent; Martinod, Joseph; Witt, Cesar; Heuret, Arnauld

    2014-08-01

    Past studies have shown that high coastal uplift rates are restricted to active areas, especially in a subduction context. The origin of coastal uplift in subduction zones, however, has not yet been globally investigated. Quaternary shorelines correlated to the last interglacial maximum (MIS 5e) were defined as a global tectonic benchmark (Pedoja et al., 2011). In order to investigate the relationships between the vertical motion and the subduction dynamic parameters, we cross-linked this coastal uplift database with the “geodynamical” databases from Heuret (2005), Conrad and Husson (2009) and Müller et al. (2008). Our statistical study shows that: (1) the most intuitive parameters one can think responsible for coastal uplift (e.g., subduction obliquity, trench motion, oceanic crust age, interplate friction and force, convergence variation, dynamic topography, overriding and subducted plate velocity) are not related with the uplift (and its magnitude); (2) the only intuitive parameter is the distance to the trench which shows in specific areas a decrease from the trench up to a distance of ∼300 km; (3) the slab dip (especially the deep slab dip), the position along the trench and the overriding plate tectonic regime are correlated with the coastal uplift, probably reflecting transient changes in subduction parameters. Finally we conclude that the first order parameter explaining coastal uplift is small-scale heterogeneities of the subducting plate, as for instance subducting aseismic ridges. The influence of large-scale geodynamic setting of subduction zones is secondary.

  2. 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, João. 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.

  3. 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.

  4. Subduction and exhumation of continental crust: insights from laboratory models

    NASA Astrophysics Data System (ADS)

    Bialas, Robert W.; Funiciello, Francesca; Faccenna, Claudio

    2011-01-01

    When slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone, they may be scraped off at shallow levels, subducted to depths of up to 100-200 km and then exhumed as high pressure (HP) and ultra-high pressure (UHP) rocks, or subducted and recycled in the mantle. To investigate the factors that influence the behaviour of subducting slivers of continental material, we use 3-D dynamically consistent laboratory models. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. A sliver of continental material, also composed of silicone putty, overlies the subducting lithosphere, separated by a syrup detachment. The density of the sliver, viscosity of the detachment, geometry of the subducting system (attached plate versus free ridge) and dimensions of the sliver are varied in 34 experiments. By varying the density of the sliver and viscosity of the detachment, we can reproduce a range of sliver behaviour, including subduction, subduction and exhumation from various depths and offscraping. Sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction, but weak enough to allow adequate sliver displacement or detachment for exhumation. Changes to the system geometry alter the slab dip, subduction velocity, pattern of mantle flow and amount of rollback. Shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation. Steeper slab dips allow more buoyancy force to be directed in the up-dip direction of the plane of the plate, and aide exhumation of subducted slivers. Slower subduction can also aide exhumation, but if slab dip is too steep or subduction too slow, the sliver will subduct to only shallow levels and not exhume. Smaller slivers are most easily subducted and exhumed and influenced by the mantle flow.

  5. Processes and Consequences of Deep Subduction

    NASA Astrophysics Data System (ADS)

    Lowman, Julian

    Oceanic lithosphere subduction is the phenomenon responsible for some of the most spectacular and powerful expressions of the ceaseless motion of the Earth's tectonic plates. Subduction is manifested at the Earth's surface by a global system of oceanic trenches, the deep abyssal scars in the ocean floor that extend over tens of thousands of kilometers and cut up to 4 km deep into the planet's surface. Subduction zones are also associated with regional gravity anomalies, island arc formation, and a large portion of the world's earthquakes and volcanoes, including the deepest and most energetic earthquakes, which have been observed at depths of 660-700 km. In addition to these dramatic surficial features, the process of subduction affects mineral physics, geochemistry, petrology, structural geology, and rock mechanics. The conditions associated with subduction provide a unique natural laboratory in which the entrainment of water and sediment with anomalously cold slabs of subducted oceanic lithosphere creates a dynamic and volatile environment as plates sink and encounter hot ambient mantle, mineralogical phase boundaries, and rheological transitions. Part of the difficulty in unravelling the complexity of these systems lies in understanding the feedback that occurs between phenomena operating at macroscopic and microscopic scales. For example, phase transformation rates affect buoyancy, and therefore subduction rates. This affects the regional thermal structure and thereby the phase transformations. Slab rheology and morphology may similarly be affected by mineral transformations and their kinetics.

  6. 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.

  7. Diverse melanges of an ancient subduction complex

    NASA Astrophysics Data System (ADS)

    Lash, Gary G.

    1987-07-01

    Three lithologically and structurally diverse melanges occur within an early Paleozoic (Early Middle Ordovician) subduction complex in the central Appalachian orogen. Type I melange, characterized by horizons of variably deformed sandstone and scaly mudstone that alternate with coherent sandstone-rich sequences, is interpreted to reflect accretion-related deformation of water-saturated trench deposits. Type II melange, composed of exotic radiolaria-bearing mudstone clasts in a scaly mudstone matrix, can be explained by remobilization and mixing of inner-trench slope sediments. Type III melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix. Evidence of forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type III melange formed from mud diapirism. The close association of these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older subduction complexes.

  8. 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 - 145°E and 25 - 37°N. 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 - 29.0°N (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.

  9. 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.

  10. 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.

  11. Subduction of oceanic asthenosphere: A critical appraisal in central Alaska

    NASA Astrophysics Data System (ADS)

    Song, Teh-Ru Alex; Kawakatsu, Hitoshi

    2013-04-01

    Song and Kawakatsu (2012) have shown that the sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subduction of the oceanic asthenosphere that has strong radial anisotropy. This model not only explains the non-intuitive trench-parallel splitting pattern in most of subduction zones, but also predicts the trench-normal behavior (fast polarization direction sub-parallel to the absolute plate motion of the incoming plate) observed in several shallow subduction zones. The general validity of such a scenario is crucial to fundamental understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern and mass transport near subduction zones. To validate this scenario, we examine SKS splitting patterns observed across the fore-arc in central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100 km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views to models of seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

  12. Crustal-scale seismic profiles across the Manila subduction zone: The transition from intraoceanic subduction to incipient collision

    NASA Astrophysics Data System (ADS)

    Eakin, Daniel H.; McIntosh, Kirk D.; Van Avendonk, H. J. A.; Lavier, Luc; Lester, Ryan; Liu, Char-Shine; Lee, Chao-Shing

    2014-01-01

    use offshore multichannel seismic (MCS) reflection and wide-angle seismic data sets to model the velocity structure of the incipient arc-continent collision along two trench perpendicular transects in the Bashi Strait between Taiwan and Luzon. This area represents a transition from a tectonic regime dominated by subduction of oceanic crust of the South China Sea, west of the Philippines, to one dominated by subduction and eventual collision of rifted Chinese continental crust with the Luzon volcanic arc culminating in the Taiwan orogeny. The new seismic velocity models show evidence for extended to hyperextended continental crust, ~10-15 km thick, subducting along the Manila trench at 20.5°N along transect T1, as well as evidence indicating that this thinned continental crust is being structurally underplated to the accretionary prism at 21.5°N along transect T2, but not along T1 to the south. Coincident MCS reflection imaging shows highly stretched and faulted crust west of the trench along both transects and what appears to be a midcrustal detachment along transect T2, a potential zone of weakness that may be exploited by accretionary processes during subduction. An additional seismic reflection transect south of T1 shows subduction of normal ocean crust at the Manila trench.

  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. 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.

  15. 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.

  16. 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 southern migration of the transition between forced and self-sustaining states.

  17. 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.

  18. 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.

  19. 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

  20. 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.

  1. 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.

  2. Subduction initiation triggered by mantle suction flow

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Sobolev, Stephan

    2014-05-01

    Subduction initiation at passive margins, despite its important role in Wilson cycle, is still a debatable issue. Several workers have suggested different mechanisms for triggering subduction initiation along passive margins. Here we propose a new triggering factor, which is suction mantle flow. We suggest that a suction mantle flow, which is induced from the neighboring subduction zones in the region, is able to push the oceanic plate into the mantle and convert the passive margin into an active plate boundary. Using coupled thermo-mechanical models, we show that a dipping shear zone develops along passive margin as a suction mantle flow exists either below the passive margin or below the oceanic plate near the future plate boundary. Few million years after formation of shear zone, oceanic plate descends into the mantle and subduction initiates. In our numerical experiments, the subduction initiation time varies, depending on several parameters including the magnitude, domain size and location of the suction mantle flow velocities, age of oceanic plate and strength of continental lithosphere. we find that subduction nucleation is followed by trench retreat if the suction mantle flow excites an oceanward flow below the continental plate.

  3. Dynamics of trench motion: insights from 3-d laboratory experiments

    NASA Astrophysics Data System (ADS)

    Bellahsen, N.; Funiciello, F.; Faccenna, C.

    2003-04-01

    We have performed 3-D laboratory experiments to test the influence of the geometric configuration on the velocity field of a slab bearing plate. We scale down our experiments to natural gravity field using silicone putty and honey to simulate the viscous behavior of slab and mantle, respectively. Our results show how the 3-D geometry of the system and the viscosity contrast between the lithosphere and the mantle control the geometry and kinematics of the subduction. We highlight how these parameters influence the way the velocity is subduction is partitioned trench and plate motion.

  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. Stress drops for intermediate-depth intraslab earthquakes beneath Hokkaido, northern Japan: Differences between the subducting oceanic crust and mantle events

    NASA Astrophysics Data System (ADS)

    Kita, Saeko; Katsumata, Kei

    2015-02-01

    Spatial variations in the stress drop for 1726 intermediate-depth intraslab earthquakes were examined in the subducting Pacific plate beneath Hokkaido, using precisely relocated hypocenters, the corner frequencies of events, and detailed determined geometry of the upper interface of the Pacific plate. The results show that median stress drop for intraslab earthquakes generally increases with an increase in depth from ˜10 to 157 Mpa at depths of 70-300 km. More specifically, median stress drops for events in the oceanic crust decrease (9.9-6.8 MPa) at depths of 70-120 km and increase (6.8-17 MPa) at depths of 120-170 km, whereas median stress drop for events in the oceanic mantle decrease (21.6-14.0 MPa) at depths of 70-170 km, where the geometry of the Pacific plate is well determined. The increase in stress drop with depth in the oceanic crust at depths of 120-170 km, for which several studies have shown an increase in velocity, can be explained by an increase in the velocity and a decrease in the water content due to the phase boundary with dehydration in the oceanic crust. Stress drops for events in the oceanic mantle were larger than those for events in the oceanic crust at depths of 70-120 km. Differences in both the rigidity of the rock types and in the rupture mechanisms for events between the oceanic crust and mantle could be causes for the stress drop differences within a slab.

  6. The role of hydrous phases in the formation of trench parallel anisotropy: Evidence from Rayleigh waves in Cascadia

    NASA Astrophysics Data System (ADS)

    Wagner, Lara S.; Fouch, Matthew J.; James, David E.; Long, Maureen D.

    2013-06-01

    cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research. We provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well-localized region of trench parallel fast directions in Rayleigh wave phase velocities near the Cascade arc at 45 to 66 s periods. We constrain the location of the anisotropic material to within or directly above the oceanic plate, using the depth sensitivity of Rayleigh waves as a function of frequency and the accurate slab imaging available for Cascadia from scattered wave studies. We infer that the likely source of trench-parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate. Similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  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. '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.

  13. 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.

  14. Heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle: evidence from South America, Izu-Bonin and Japan

    NASA Astrophysics Data System (ADS)

    Lynner, Colton; Long, Maureen D.

    2015-06-01

    Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ˜600 km 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 and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs.

  15. 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

  16. 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 (<700°C) 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 (> 700°C), 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 (< 700°C), 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. Revisiting Parts of the Makran Subduction Zone in Search of its Paleoearthquakes

    NASA Astrophysics Data System (ADS)

    Rajendran, C.

    2013-05-01

    In contrast to the subduction zones of southern Chile, Cascadia, Southern Alaska, Sumatra-Andaman and the Japan Trench that have generated the largest tsunamigenic earthquakes, the Makran subduction zone has not generated any megathrust earthquake. However, the 2004 earthquake and the unprecedented Indian Ocean tsunami that followed is a reminder that the tsunamigenic potential of a subduction zone cannot be evaluated solely on the basis of its current seismicity. The Mw 8.1 earthquake of 1945 and the consequent tsunami that originated on the eastern part of Makran are the only historically known hazardous events in this region; the history of its past earthquakes remains unexplored. Here we use tectonic proxies along the Makran Coast, available constraints on historical seismicity and the tell-tale characteristics of sea floor morphology to track its seismotectonic history. We explored the accessible coastal sites for evidence of past earthquake-related deformation and secondary features such as liquefaction. In the near-shore regions of the central part of Makran, near Chabahar, we found evidence of seismically induced liquefaction, possibly from the 1945 earthquake. Westward sites are remarkably devoid of liquefaction features, at least within the shallow stratigraphy. Whether it is an indication that no recent earthquakes have impacted the western part of Makran Coast region is an issue that needs to be explored. Elevated marine terraces on the western Makran and their uplift rates are indicative of long-term tectonic activity, comparable to that of the central part of Makran. The offshore data suggest occurrences of recently active submarine slumps on the eastern part of Makran, reflective of shaking events, from the great 1945 earthquake. The western segment appears to be potentially seismogenic, and is possibly going through a long interseismic period.

  4. 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.

  5. 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.

  6. 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.

  7. Outer rise earthquakes in front of the Northern Manila Trench

    NASA Astrophysics Data System (ADS)

    Hsueh, H.; Chang, E.

    2010-12-01

    At south Taiwan, the Eurasian Plate is subducting eastward beneath the Philippine Sea Plate along the Manila Trench. It connects with a volcanic arc, well-developed forearc basin and a Wadati-Benioff zone that eastern dips under Luzon. It is also the transition zone between the South China Sea basin and the southeast Eurasian continent. Particularly, the subdution along the north Manila Trench offshore west Luzon forms a broad bending of plate around 200N before it lasts to Taiwan. The tensional shallow earthquakes are well reported for this area which may associate with the plate bending in front of the trench as well as the back-arc spreading taken place at the forearc basins. The seismotectonic structure is complicated here and worth more probe into. However, the seismological observation has problems with inadequate coverage of the azimuth and incomplete records of medium to small earthquakes in offshore area. In this study we used the OBS (Ocean Bottom Seismometer) data which are acquired from nine OBS stations deployed along the north Manila Trench (about 20.50~210N, 119.80~120.20E), from November 20th to November 25th in 2006. Over 2,700 events have been detected in this short-term experiment. The identified hypocenters show that the local seismicity is parallel to the Manila Trench in a NW-SE-trending, which are located exactly at the outer rise in front of the Manila trench. The local 1D layered velocity model is also conducted for this area, which shows the depth of Moho is about 11.6 km. Further stress analysis conducted from the near-field focal mechanisms can reveal the main that the local events are rather in normal-faulting, which is typical for the outer rise earthquake. According to foregoing results, we will expect to model the geometry of the subducting slab beneath Manila Trench, plate motion and to discuss with the possible tectonic process, for ex. the tear at the continental-to-oceanic boundary at around 200N reported by other studies. Keywords: Ocean-Bottom seismograph, Outer rise, Velocity model, Manila Trench

  8. Subducted lithosphere, slab tearing and continental delamination under Taiwan: arc-continent collision at the junction of quasi-orthogonal subduction systems

    NASA Astrophysics Data System (ADS)

    Suppe, J.; Carena, S.; Wu, Y.; Ustaszewski, K. M.

    2011-12-01

    The present-day oblique arc-continent collision in Taiwan has a map-view reversal in subduction polarity, with the Eurasian plate subducting under the Philippine Sea plate south of Taiwan and the Philippine Sea plate subducting under the Eurasian continental margin northeast of Taiwan. Tomographic imaging of subducted lithosphere under Taiwan allows us to test models of collision and polarity reversal, because each model makes different predictions of the geometries of the subducted slabs. Furthermore the tomography reveals previously unpredicted phenomena, including the delamination of Eurasian continental mantle lithosphere with lateral inflow of hot upper mantle. The two subduction systems are quasi-orthogonal where they meet at Taiwan and therefore are not well-represented in the classic 2D cross-sectional models of arc-continent collision. The 3D models proposed for Taiwan recognize that either [1] the subduction reversal is inherited from a former trench-trench transform that is obliterated upon collision or [2] there is a progressive tearing of the Eurasian plate along the continental margin at the north end of the actively subducting Eurasian slab, which allows the quasi-orthogonal subduction reversal to translate stably with the oblique collision, with no true slab breakoff. The progressive tearing model [2] has a kinematically stable trench-trench juncture, whereas the transform obliteration model [1] is unstable, requiring a discontinuity in behavior, for example a change to [2]. More complex mixed models have also been proposed, all of which make testable predictions of subducted slab geometry. Modern global tomography, augmented with local and regional tomography near Taiwan, provides good imaging of the subducted Eurasian and Philippine-Sea lithosphere in the upper mantle, including the torn subducted edge of the Eurasian slab. Slab geometries are in close agreement with the progressive tearing of model [2] and in strong disagreement with the transform obliteration of model [1]. No subducted trench-trench transform is observed, instead the subducted western edge of the Philippine Sea plate continues northward ~900km, nearly to the latitude of Shanghai, in agreement with model [2]. Furthermore, no slab breakoff is observed. However, it is surprising that the active tear at the north end of the subducting Eurasian slab is not at the edge of the continent but ~250km to the north under the Eurasian continental shelf north of Taipei. The subducted continent-ocean boundary is clearly shown in the seismic velocities within the subducted Eurasian slab. Furthermore the tear does not propagate through the overlying continental shelf, therefore the continental mantle lithosphere and lower crust are subducting by delamination attached to the oceanic part of the Eurasian plate. Delamination occurs instantaneously adjacent to the active tear and involves the progressive lateral intrusion of hot mantle from the Okinawa-Ryukyu mantle wedge, which lies above the subducting Philippine-Sea plate east and northeast of Taiwan. Active magmatism is spatially associated with delamination in northernmost Taiwan, near Taipei.

  9. 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-Núñez, F.; Pérez-Calderón, D.; Rufino-Contreras, I.; Valle-Hernández, S.; Pérez-González, 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.

  10. 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.

  11. Numerical modeling of flat-slab subduction in South America: the influence of thick overriding lithosphere

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    How and why normally dipping subduction zones evolve into flat subduction ones has been long debated. For example, it was suggested that slab flattening in South America results from subduction of buoyant aseismic ridges, however analogue and numerical models suggest that their dimensions are too small to induce flattening of such large slab segments. Here we note that in this continent, flat subduction occurs where thick cratonic lithosphere is closer to the trench than in adjacent steep subduction segments. We develop time-dependent, 2-D numerical experiments to show that trenchward motion of thick cratonic lithosphere progressively closes the asthenospheric wedge and increases the suction forces leading, in some cases, to flat subduction. In particular, we model the last 30 Myr of subduction in the Chilean flat slab segment since there slab geometry, plate velocities and volcanism are well constrained. We demonstrate that trenchward motion of cratonic lithosphere, ~180-300 km thick, presently ~700-800 km away from the trench, reproduces a flat-slab geometry that fits the stress pattern, seismicity distribution, and the temporal and spatial evolution of volcanism in the region. Finally, we suggest that steep and flat subduction cycles hypothesized to be responsible for the Andean Neogene evolution result from subduction beneath variable-thickness continental lithosphere.

  12. Rollback subduction: the great killer of mantle plumes

    NASA Astrophysics Data System (ADS)

    Druken, K. A.; Kincaid, C. R.; Griffiths, R. W.

    2010-12-01

    Subduction driven mantle flow is shown to stall and decapitate buoyant upwellings, thereby severely limiting vertical heat and mass transport. Ongoing debate tends to focus on the expected surface expression of plumes rising independently of the background circulation, however we present 3-D laboratory results that suggest rollback subduction greatly alters this classic plume model. A Phenolic sheet and temperature dependent glucose fluid, are used to model the subducting plate and upper ~2000 km of the mantle, respectively. Experiments varied style and rate of rollback subduction as well as plume temperature and position. Results show that buoyant upwellings located as far as 1500 km behind the trench fall under two regimes, (I) plate dominated or (II) plume dominated. In either regime, down-dip sinking of the slab initially stalls vertical plume motion and the combination of down-dip sinking and trench rollback redistributes material throughout the system. Plumes with as much as 400°C excess temperature behave as passive features in the subduction-induced 3-D flow (Regime I). Less than 10% of plume material in this regime is capable of reaching zones for melt generation, with rollback subduction trapping or re-subducting the majority of plume material at depth. Only plumes of 600°C excess temperature (or more) are able to overcome the dominant 3-D flow and transport heat and mass to the surface (Regime II). Regardless of plume temperature, conduit velocities (proxy for melt generation) show cycles of high and low hotspot activity also due to distortion from subduction-induced flow. As a result of both the sinking and rollback motions, the temporal hotspot trend is variable and differs from conventional plate-conduit interaction.

  13. 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.

  14. Crustal structure and deformation at the northern Manila Trench between Taiwan and Luzon islands

    NASA Astrophysics Data System (ADS)

    Ku, Chia-Yen; Hsu, Shu-Kun

    2009-03-01

    The Philippine Sea Plate overrides the Eurasian Plate along the east-dipping Manila Trench between Taiwan and Luzon islands. From south to north, the plate convergence gradually evolves from normal subduction of the South China Sea lithosphere to initial collision of the Taiwan orogen. The subduction-related earthquakes become diffusive close to Taiwan; the accretionary prism is dramatically wider toward Taiwan. To understand the plate convergent features of this subduction-collision transition zone, we have analyzed twelve seismic reflection profiles across the Manila Trench between Luzon and Taiwan. The results show that the basement of northern South China Sea basin generally dips toward east and south. The northern Manila Trench accumulates more trench-fill sediments in the south than in the north. A sequence boundary t 0 is suggested to distinguish the hemipelagic and trench-fill sediments. Probably due to the collision in Taiwan, the sequence boundary t 0 displays more gentle slope or has been uplifted in the north. Structural analysis shows that the subducting crust in the northern Manila Trench area can be characterized by three distinctive zones: a normal fault zone (NFZ), a proto-thrust zone (PTZ) and a thrust zone (TZ). The NFZ is defined by the distribution of numerous normal faults in the top or upper portion of the subducting crust. The normal faults are gradually buried by trench-fill sediments when they are closer to the deep trench. It is suggested that normal fault may take place at the location where the crust starts to bend and induces gravity sliding of the upper sedimentary layers. Some buried normal faults could be reactivated to blind thrust faults because of stronger plate convergence near the accretionary prism. The PTZ is located between the NFZ and the frontal thrust of the accretionary prism; it contains blind thrust faults or folds instead of thrust faults. The successive distribution of the crustal structures of normal faults, blind thrust and thrust faults at the trench area suggests that the blind thrust faults develop along the location of pre-existing normal faults. The brittle deformation occurs at the lower part of the sedimentary layers probably because of stronger compaction and less water content; eventually blind thrust faults may propagate upward and become thrust faults at the seabed.

  15. 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.

  16. 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.

  17. Complex rupture processes of the Solomon Islands subduction zone earthquake and subduction controlled upper mantle structure beneath Anatolia

    NASA Astrophysics Data System (ADS)

    Biryol, Cemal Berk

    This dissertation explores subduction zone-related deformation both on short time scales in the form of subduction zone earthquakes and over larger time and geographical scales in the form of subduction rollback or detachment of the subducting lithosphere. The study presented here is composed of two parts. First, we analyzed the source-rupture processes of the April 1, 2007 Solomon Islands Earthquake (Mw=8.1) using a body-wave inversion technique. Our analysis indicated that the earthquake ruptured approximately 240 km of the southeast Pacific subduction zone in two sub-events. In the second part of this study, we used shear-wave splitting analysis to investigate the effects of the subducting African lithosphere on the upper-mantle flow field beneath the Anatolian Plate in the Eastern Mediterranean region. Our shear-wave splitting results are consistent with relatively uniform southwest-directed flow towards the actively southwestward-retreating Aegean slab. Based on spatial variations in observed delay times we identified varying flow speeds beneath Anatolia and we attribute this variation to the differential retreat rates of the Aegean and the Cyprean trenches. Finally, we used teleseismic P-wave travel-time tomography to image the geometry of the subducting African lithosphere beneath the Anatolia region. Our tomograms show that the subducting African lithosphere is partitioned into at least two segments along the Cyprean and the Aegean trenches. We observed a gap between the two segments through which hot asthenosphere ascends beneath the volcanic fields of western Anatolia. Our results show that the Cyprean slab is steeper than the Aegean slab. We inferred that this steep geometry, in part, controls the flow regime of asthenosphere beneath Anatolia causing variations in flow speeds inferred from shear-wave splitting analysis.

  18. 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.

  19. Slab detachment in laterally varying subduction zones: 3-D numerical modeling

    NASA Astrophysics Data System (ADS)

    Duretz, T.; Gerya, T. V.; Spakman, W.

    2014-03-01

    Understanding the three-dimensional (3-D) dynamics of subduction-collision systems is a longstanding challenge in geodynamics. We investigate the impact of slab detachment in collision systems that are subjected to along-trench variations. High-resolution thermomechanical numerical models, encompassing experimentally derived flow laws and a pseudo free surface, are employed to unravel lithospheric and topographic evolutions. First, we consider coeval subduction of adjacent continental and oceanic lithospheres (SCO). This configuration yields to two-stage slab detachment during collision, topographic buildup and extrusion, variable along-trench convergence rates, and associated trench deformation. The second setting considers a convergent margin, which is laterally limited by a transform boundary (STB). Such collisional system is affected by a single slab detachment, little trench deformation, and moderately confined upper plate topography. The effect of initial thermal slab age on SCO and STB models are explored. Similarities with natural analogs along the Arabia-Eurasia collision are discussed.

  20. 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).

  1. 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

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

    NASA Astrophysics Data System (ADS)

    Duarte, João 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.

  3. 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.

  4. 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.

  5. 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.

  6. Lithosphere tearing at STEP faults: response to edges of subduction zones

    NASA Astrophysics Data System (ADS)

    Govers, R.; Wortel, M. J. R.

    2005-07-01

    Slab edges are a relatively common feature in plate tectonics. Two prominent examples are the northern end of the Tonga subduction zone and the southern end of the New Hebrides subduction zone. Near such horizontal terminations of subduction trenches, ongoing tearing of oceanic lithosphere is a geometric consequence. We refer to such kinks in the plate boundary as a Subduction-Transform Edge Propagator, or STEP. Other STEPs are the north and south ends of the Lesser Antilles trench, the north end of the South Sandwich trench, the south end of the Vrancea trench, and both ends of the Calabria trench. Volcanism near STEPs is distinct from typical arc volcanism. In some cases, slab edges appear to coincide with mantle plumes. Using 3D mechanical models, we establish that STEP faults are stable plate tectonic features in most circumstances. In the (probably rare) cases that the resistance to fault propagation is high, slab break-off will occur. Relative motion along the transform segment of the plate boundary often is non-uniform, and the STEP is not a transform plate boundary in the (rigid) plate tectonics sense of the phrase. STEP propagation may result in substantial deformation, rotation, topography and sedimentary basins, with a very specific time-space evolution. Surface velocities are substantially affected by nearby STEPs.

  7. Turbidite facies in an ancient subduction complex: Torlesse terrane, New Zealand

    USGS Publications Warehouse

    MacKinnon, T.C.; Howell, D.G.

    1984-01-01

    The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns. ?? 1984 Springer-Verlag New York Inc.

  8. Are diamond-bearing Cretaceous kimberlites related to low-angle subduction beneath western North America?

    NASA Astrophysics Data System (ADS)

    Currie, Claire A.; Beaumont, Christopher

    2011-02-01

    Diamond-bearing Cretaceous kimberlites of western North America were emplaced 1000-1500 km inboard of the Farallon plate subduction margin and overlap with the development of the Western Interior Seaway, shut-down of the Sierra Nevada arc, and the Laramide orogeny. These events are consistent with a decrease in subduction angle along much of the margin, which placed the subducted Farallon plate in close proximity to the continental interior at the time of kimberlite magmatism. Our numerical models demonstrate that low-angle subduction can result from high plate convergence velocities and enhanced westward motion of North America. Further, rapid subduction allows hydrous minerals to remain stable within the cool interior of the subducting plate to more than 1200 km from the trench. Destabilization of these minerals provides a fluid source that can infiltrate the overlying material, potentially triggering partial melting and kimberlite/lamproite magmatism.

  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. 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.

  11. 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.

  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. Subduction of fracture zones

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Gerya, Taras; Manea, Marina; Zhu, Guizhi; Leeman, William

    2013-04-01

    Since Wilson proposed in 1965 the existence of a new class of faults on the ocean floor, namely transform faults, the geodynamic effects and importance of fracture zone subduction is still little studied. It is known that oceanic plates are characterized by numerous fracture zones, and some of them have the potential to transport into subduction zones large volumes of water-rich serpentinite, providing a fertile water source for magma generated in subduction-related arc volcanoes. In most previous geodynamic studies, subducting plates are considered to be homogeneous, and there is no clear indication how the subduction of a fracture zone influences the melting pattern in the mantle wedge and the slab-derived fluids distribution in the subarc mantle. Here we show that subduction of serpentinized fracture zones plays a significant role in distribution of melt and fluids in the mantle wedge above the slab. Using high-resolution tree-dimensional coupled petrological-termomechanical simulations of subduction, we show that fluids, including melts and water, vary dramatically in the region where a serpentinized fracture zone enters into subduction. Our models show that substantial hydration and partial melting tend to concentrate where fracture zones are being subducted, creating favorable conditions for partially molten hydrous plumes to develop. These results are consistent with the along-arc variability in magma source compositions and processes in several regions, as the Aleutian Arc, the Cascades, the Southern Mexican Volcanic Arc, and the Andean Southern Volcanic Zone.

  14. Supraslab earthquakes above the Pacific-plate slab offshore Sanriku, NE Japan

    NASA Astrophysics Data System (ADS)

    Uchida, N.; Kirby, S. H.; Okada, T.; Hino, R.; Hasegawa, A.

    2009-12-01

    Double-difference relocation methods using P and S wave arrivals for the forearc earthquakes offshore Sanriku indicate that clusters of earthquakes are located above small repeating earthquakes that delineate the upper boundary of the Pacific plate. To assure good depth control, we restrict our study to events that are close to seismic stations. These "supraslab" earthquake clusters are regional features at depths of 25 to 50 km. The depth distribution of forearc Moho estimated from SP converted waves from repeating earthquake sources shows that the Moho depths becomes shallower from near the coastline (-30km) to the approximate eastern limit of the mantle wedge (-20km) and most of these clusters are below the depth of the forearc Moho. Re-entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past along the Japan trench. Our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the Pacific plate during slab descent, driven by the resistance of seamounts to subduction caused by their relief. Detachment during descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth. Seamounts probably also fragment during subduction along weak seamount structures, such as former volcanic rifts and slump-block boundaries. Seamount crust is thus accreted to forearcs, possibly leading to a long-term component of near-coastal uplift. Supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system.

  15. Deformation Along the Western Hellenic Subduction Zone From Continuous GPS

    NASA Astrophysics Data System (ADS)

    Floyd, M. A.; Nocquet, J.; Billiris, H.; Paradissis, D.; England, P.; Parsons, B.

    2005-12-01

    In respect of the great Sumatra-Andaman earthquake and subsequent tsunami of 26 December 2004, the study of coupling at subduction zones is imperative to understanding the seismic---or otherwise---hazard posed to a region. The Mediterranean Sea includes the Hellenic subduction zone, which accommodates the ~35 mm/yr of convergence between the oceanic lithosphere of the African plate and the extending continental lithosphere of the Aegean region. How the convergence is accommodated through earthquakes here remains controversial. For example, Jackson & McKenzie [1988] concluded that earthquakes can account for ~10% of the relative motion, implying that the subduction zone is in a stable-sliding state. On the contrary, Pirazzoli et al. [1982], using the Holocene geological record, inferred that large earthquakes (M>8) have occurred along the arc. In the last years, the development of permanent GPS networks near subduction zones has provided new information on the mechanisms of strain accumulation and release along the plate interface, highlighting the existence of transient slow slip events that may account for a significant part of convergence. We present results from a permanent network, in operation since early 2003, covering the western Hellenic Arc. Initial results indicate that no deformation perpendicular to the trench can be detected, supporting the hypothesis that the trench is currently in a stable-sliding state. Comparisons to 1992--2000 campaign data (McClusky et al. [2000]) indicate similar results. However, significant (~1m) extension is found in a comparison between historic triangulation and recent campaign GPS studies (Davies, et al. [1997]), averaged over 100 years, yet no significant earthquake can be invoked to explain such a large trench-perpendicular extension of the overriding plate. The existence of transient slip events elsewhere, however, presents a potential explanation for this discrepency in that the subduction zone does undergo strain accumulation and release cycles, but with a signal that, thus far, has been undetectable by intermittent studies in the region.

  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. 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 23°S and 28°S. 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.

  18. 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.

  19. 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.

  20. Tsunami generation near Japan by Earthquakes Along-strike Single Segmentation and Along-dip Double Segmentation

    NASA Astrophysics Data System (ADS)

    Koyama, Junji; Tsuzuki, Motohiro

    2013-04-01

    After the 2011 Tohoku-oki megathrust earthquake of Mw9.0, we have proposed a hypothesis that megathrust earthquakes worldwide occur Along-dip Double Segmentation (ADDS) or Along-strike Single Segmentation (ASSS). The former is characterized by the apparent absence of earthquakes in the aligned seismic segments along the Japan trench as opposed to those along the Japan Islands that generate repeated smaller earthquakes (ADDS), where the 2011 Tohoku-oki megathrust occurred. Meanwhile, the latter is by a weak seismic activity before the main event all over the subduction zone, where we find aligned seismic segments along the subduction zone from the trench to the island-arc (ASSS). A typical example of ASSS is the Nankai trough, Japan, where future great earthquakes are expected. The 1960 and 2010 Chile megathrusts occurred in ASSS. In and near Japan, ADDS earthquake activity is restrictively found along the Pacific side of Hokkaido and Tohoku regions and the Hyuganada, Kyushu. The rest of seismic activity near Japan is classified into ASSS. Comparing tsunami magnitude m from local tsunami-wave heights and seismic moment Mo from long-period surface-waves of 61 earthquakes from 1923 in and near Japan, we found that tsunami-wave heights of ASSS earthquakes are almost two times larger than those of ADDS's. This is also confirmed by studying tsunami magnitude Mt evaluated from teleseismic tsunami-wave heights. The reason of this different excitation between ADDS and ASSS is considered to be due to either (1) shallower focal depths for ASSS give rise to larger ocean bottom deformation, resulting in larger tsunami excitation, (2) larger dip-angles of fault planes for ASSS, (3) three dimensional ocean-bottom structures, such as troughs, trenches and continental shelves, or (4) ocean bottom topography nearby causes the focusing of tsunami waves. (1) is the conclusion that we would like to derive. (2) Speaking about the effect of dip angles to the maximum ocean bottom deformations, the difference is about 30% in cases of reverse faults with dip angles of 30 and 60 degrees. (3) Both of earthquakes along the passive margin of the back-arc basin of the Japan sea and along the Nankai trough are classified into ASSS. (4) Both of local and teleseismic tsunami-wave heights do suggest the similar result, rejecting the local tsunami focusing. Therefore, we conclude that the larger tsunami excitation for ASSS earthquakes is due to larger amount of ocean bottom deformations than those for ADDS earthquakes or by the reason of (1) or by both the effects. Asperity for ADDS locates in the shallow part of the subduction zone along the trench, and it ruptures only in the case of megathrust events like as the 2011 Tohoku-oki earthquake. In estimating tsunami wave heights for future earthquakes, we have to take into account of this difference in tsunami excitations in the ADDS or ASSS zone.

  1. 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.

  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. Trenc