Sample records for japan trench subduction

  1. Subduction of the Daiichi Kashima Seamount in the Japan Trench

    USGS Publications Warehouse

    Lallemand, S.; Culotta, R.; Von Huene, R.

    1989-01-01

    In 1984-1985, the Kaiko consortium collected Seabeam, single-channel seismic and submersible sampling data in the vicinity of the Daiichi-Kashima seamount and the southern Japan trench. We performed a prestack migration of a Shell multichannel seismic profile, that crosses this area, and examined it in the light of this unusually diverse Kaiko dataset. Unlike the frontal structure of the northern Japan trench, where mass-wasting appears to be the dominant tectonic process, the margin in front of the Daiichi-Kashima shows indentation, imbrication, uplift and erosion. Emplacement of the front one-third of the seamount beneath the margin front occurs without accretion. We conclude that the Daiichi-Kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin. ?? 1989.

  2. Water supplement by silica diagenesis in cold subduction zone: an implication for the Japan Trench

    NASA Astrophysics Data System (ADS)

    Hina, S.; Hamada, Y.; Kameda, J.; Yamaguchi, A.; Kimura, G.

    2011-12-01

    The fluid existing at plate interfaces in subduction zone makes a strong effect on seismicity and fault slip of the plate boundary megathrusts. As a source of the fluid, pore fluid included in subducting sedimentsand dehydration reaction of clay minerals have been discussed in detail, however, dewatering from siliceous sediments such as diatom and radiolarian ooze are poorly investigated in spite of their major occurrence in old oceanic plate. Silica in the siliceous sediment is transformed from amorphous silica into quartz via cristobalite phase (Opal A ? Opal CT ? Quartz) releasing structured water. In this study, we evaluate the amount of dehydration from siliceous sediment in subducting plate. Silica diagenesis and dehydration are calculated quantitatively introducing reaction kinetics (Mizutani, 1970) and temperature profile models of the Japan Trench, a cold type subduction zone where the siliceous sediments subduct in (Peacock and Wang, 1999; Wang and Suyehiro, 1999; Wada and Wang, 2009; Kimura et al., submitted). As a result, through this diagenetic conversion, structured water of silica minerals is released as much as 140g/m^2/year at shallow plate boundary (~13km depth below the sea floor), where the temperature is about ~100 -~120°C. This water should generate an excess pore pressure which drops effective stress and rock strength along the décollement. Dehydration of silica can play an important role in slip propagation to shallow portion of plate boundary as the Grate Tohoku Earthquake (9 March 2011).

  3. Geochemical and biogeochemical observations on the biological communities associated with fluid venting in Nankai Trough and Japan Trench subduction zones

    Microsoft Academic Search

    J. Boulègue; E. L. Benedetti; D. Dron; A. Mariotti; R. Létolle

    1987-01-01

    We have studied the compositions and structure of the lamellibranchia Calyptogena sp associated with fluid venting in subduction zones off Japan. The gills of the Calyptogena are the habitat of symbiotic bacteria which develop owing to chemoautotrophy. They most probably assimilate hydrogen sulfide, methane and\\/or carbon dioxide and reduce nitrogen. The isotopic budget of methane uptake enables to establish the

  4. Intense PP reflection beneath the aseismic forearc slope of the Japan Trench subduction zone and its implication of aseismic slip subduction

    Microsoft Academic Search

    Kimihiro Mochizuki; Mikako Nakamura; Junzo Kasahara; Ryota Hino; Minoru Nishino; Asako Kuwano; Yasuyuki Nakamura; Tomoaki Yamada; Masanao Shinohara; Peyman Poor Moghaddam; Toshihiko Kanazawa

    2005-01-01

    Strong anticorrelation between intensity of plate boundary PP reflection and seismicity had been revealed by a seismic reflection-refraction survey conducted in 1996 in a seismic-aseismic boundary region on the forearc slope of the Japan Trench. Amplitude of the strong reflection was explained by the presence of a thin layer (?200 m) of low P wave velocity (3–4 km\\/s) at the

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

    NASA Astrophysics Data System (ADS)

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

    2013-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Cadet, Jean-Paul; Kobayashi, Kazuo; Aubouin, Jean; Boulègue, Jacques; Deplus, Christine; Dubois, Jacques; von Huene, Roland; Jolivet, Laurent; Kanazawa, Toshihiko; Kasahara, Junzo; Koizumi, Kinichiro; Lallemand, Serge; Nakamura, Yasuo; Pautot, Guy; Suyehiro, Kiyoshi; Tani, Shin; Tokuyama, Hidekazu; Yamazaki, Toshitsugu

    1987-05-01

    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 km 2 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?

  8. 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.

  9. Tsunami earthquakes and subduction processes near deep-sea trenches

    Microsoft Academic Search

    Yoshio Fukao

    1979-01-01

    A tsunami eqrthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep-sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October

  10. Precision evaluation for intensive GPS acoustic measurements along Japan trench

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    After the Tohoku-oki earthquake in 2011, researchers recognized the importance of the state of inter-plate coupling close to the trench for giant earthquakes, in where seafloor geodetic surveys were few or not available. To overcome this limitation, we have developed GPS/acoustic instrument for greater depth up to 6000m, which can cover the region close to the trench for most subduction zones. MEXT, Japan promotes to construct a network of GPS/acoustic survey sites along the Japan trench to elucidate post-seismic behavior after the giant earthquake. In 2012, research group in Tohoku and Nagoya Universities constructed 20 new survey sites along the Japan trench and started their initial positioning. Three to six transponders were installed for each site, which were 86 transponders in total. The network covers large portion of the Japan trench, mainly along the deep land-side of the trench, covering the region of expected significant afterslip as well as of the large coseismic slip. In this year second phase surveys are planned to detect displacement of roughly 1-yesr since the last survey. These are the first intensive surveys that we have never been experienced, and with new survey style. Therefore, in advance to the second phase survey, we summarize the first phase survey in 2012 in the presentation. As one of the most important key to obtain precise positioning of seafloor transponders is how to estimate horizontal variation of sound speed in ocean, which are neglected in the past analysis. For this purpose, some of the sites consist of six transponders, with which such variation can be potentially estimated. For this context, in the second phase surveys, we are going to introduce automatic surface vehicle to enable simultaneous measurement from two points from sea surface, which will provide information of the horizontal variation in sound speed even for three or four transponders. In addition we have made both moving and stationary surveys, in which we can apply couple of analytical techniques to compare the advantages to each other. In this talk, we report the summary of the first phase survey and comparison of the analytical techniques in the precision in positioning.

  11. 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.

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    1998-08-01

    Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70°E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of 39°00'N, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of 39°00'N. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (<10 km) roughness of plateaus and valleys edged by outward- and inward-facing fault escarpments. Throughout the surveyed areas, escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall, but with no distinct increase below that depth. No significant difference is recognized in fault throws towards and away from the trench. It can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis, which is caused by downward bending of the subducting lithosphere. The relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the East Pacific Rise crest. The maximum length and height of escarpments are small in the Kuril Trench compared with those in the Japan Trench, implying a difference in mechanical strength depending on the fault orientation. The crust is weakest along the inherited spreading fabric, second weakest probably along the non-transform offset direction and strongest in directions very oblique to these orientations. Seamounts appear to be more rigid than normal ocean crust, with no particular weak orientations, resulting in fewer but larger faults along the axis of plate bending, as most clearly represented in the subducting Daiichi-Kashima Seamount.

  14. 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.

  15. Seismo-turbidites in the Japan Trench inner slope

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  16. Deep scientific dives in the Japan and Kuril Trenches

    Microsoft Academic Search

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

    1987-01-01

    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

  17. 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.

  18. 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

  19. 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

  20. Trench-parallel flow in the mantle wedge: insights from integrating seismology and 3D subduction zone modeling

    NASA Astrophysics Data System (ADS)

    van Keken, P. E.; Kneller, E. A.; Syracuse, E.; Long, M.; Abt, D.; Abers, G.; Fischer, K.

    2007-12-01

    The mantle wedge in many subduction zones is characterized by a cold, low attenuation fore-arc and a hot, high attenuation arc and back-arc. The transition from fore-arc to arc mantle correlates in the Honshu, Nankai and possibly the Ryukyu subduction zones with a change in orientation of shear wave splitting from trench-parallel to trench-normal. For the Marianas, Andes and Central America subduction zones the trench-parallel splitting occurs further into the arc as well. The differences between these fundamental seismological observations suggest that multiple mechanisms can be responsible for the formation of trench-parallel anisotropy. Proposed mechanisms include fabric transitions in olivine due to changes in hydration and stress, melt alignment, and 3D flow. We use high resolution 2D and 3D dynamical models of these subduction zones to investigate the role of 3D flow and olivine fabric transitions in generating the observed patterns of anisotropy. The slab geometry is determined from available shallow geophysics and Benioff zone seismicity. For Ryukyu we obtain satisfactory splitting magnitudes for models that are based strictly on 2D corner flow with the B-type fabric in the cold fore-arc mantle. The widespread trench-parallel anisotropy in the Marianas and the 30-34S segment of the Andean subduction zone is better explained by 3D flow driven by along-trench pressure differences induced by changes in slab dip (Andes) or curvature (Marianas).Initial models for the Nicaragua-Costa Rica and Cascadia subduction zones incorporating present-day slab shape are dominated by 2D cornerflow and cannot explain the observed anisotropy. Future modeling will explore whether the time-dependent evolution of this subduction zone can create three-dimensional flow patterns with significant arc-parallel flow.

  1. 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.

  2. 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.

  3. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  5. Neogene evolution of lower trench-slope basins and wedge development in the central Hikurangi subduction margin, New Zealand

    NASA Astrophysics Data System (ADS)

    Bailleul, Julien; Chanier, Frank; Ferrière, Jacky; Robin, Cécile; Nicol, Andrew; Mahieux, Geoffroy; Gorini, Christian; Caron, Vincent

    2013-04-01

    Detailed analysis of the stratigraphic architecture and deformation of lower trench-slope sedimentary basins permits the tectonic evolution of subduction margins to be constrained. This study utilises offshore seismic reflection profiles and onshore outcrop data to examine the entire lower trench-slope of the Hikurangi subduction margin in the eastern North Island, New Zealand. Our results constrain the main spatial and temporal changes of facies and sedimentary units since about 25 Ma. We demonstrate that the geometries and locations of Miocene to Quaternary sedimentary basins are controlled by tectonic activity and reflect stages of subduction wedge development. Four types of sedimentary basins have been recognized: 1) flysch basins with local olistostromes at the front of seaward propagating thrust sheets; 2) 5-10 km wide turbidite-rich trench-slope basins between uplifting structural ridges (i.e. anticlines) associated with shortening within 100 km of the subduction front at the seafloor; 3) 30-40 km wide trench-slope basins associated with an upslope increase in thrust and ridge spacing; and 4) mixed siliciclastic-carbonate shelves formed in association with margin uplift after filling of the wider (30-40 km) trench-slope basins. The lateral and vertical successions of basin geometries and sedimentary infill are consistent with the overall progressive uplift of the subduction wedge. Formation of some of the wide trench-slope basins may be accompanied by significant local subsidence and normal faulting synchronous with active shortening at the subduction front. Margin-wide normal faulting during the Middle-Late Miocene may have formed due to upslope collapse related to tectonic erosion. All of the basins studied contain major unconformities at their base and top, with basin strata deposited over about 2-8 Myr. The short life span of these lower trench-slope sedimentary basins is consistent with a succession of short paroxysmal tectonic episodes rather than continuous deformation for the duration of subduction. Stratigraphic discontinuities within basins (e.g., facies changes and reversal of paleo-currents) also record short-term tectonic events (c. 1-2 Myr) on the basin-bounding structures and attest to the episodic nature of upper-plate deformation in response to continuous subduction beneath the active margin.

  6. 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.

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

    Microsoft Academic Search

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

    2005-01-01

    Detailed heat flow surveys on the oceanic trench slope offshore Nicaragua and Central Chile indicate heat flow values lower than the expected conductive lithospheric heat loss and lower than the global mean for crust of that age. Both areas are characterised by pervasive normal faults exposing basement in a setting affected by bending-related faulting due to plate subduction. The low

  8. 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.

  9. 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.

  10. Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust

    Microsoft Academic Search

    R. D. Hyndman; K. Wang; M. Yamano

    1995-01-01

    For coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault. In this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature. We have developed a transient thermal model for the Nankai subduction zone of southwest Japan to

  11. 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].

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    Investigations of Mass Transport Deposits (MTDs) and turbidite depositions in the confined North Ecuador subduction trench provide access to paleo-seismic information and insights into long-term mechanisms for frontal tectonic erosion at a convergent margin. The Ecuadorian margin is located along Northwestern South America, where the Nazca plate underthrusts eastward the South America plate with a 58 mm.yr-1 convergence rate. The studied trench has been the site of four great subduction earthquakes (7.7 ? Mw ? 8.8) during the 20th century, and 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 datings revealed that seven MTDs emplaced in distinct trench sub-basins since ~20 kyr, and 27 turbidites deposited in the southernmost trench sub-basin since ~4.7kyr. Our analysis shows that six MTDs 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 emplaced during five main events. Three were triggered locally and tentatively dated 5.2 kyr, 1.4 kyr and sub-actual, whereas four were emplaced in distinct trench sub-basins as a result of two regional events at 20 kyr and 13.7 kyr. None of the MTDs occurred during the fast stage of the last sea level rise (~14 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. Synchronous deposits in independent basins however suggest that at least part of the MTDs were likely triggered by earthquakes. Low sedimentation rates on the slope and high recurrence seismicity also support a seismic trigger for the 27 turbidites sampled in the trench. However, the large 1.4-12 kyr MTDs return time strongly contrasts with the 50-250 yr turbidites return time. The later is consistent with the 73 to >103 yr frequency of Mw?8.2 to ?8.8 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, which rate is estimated to be 9.7 km3/Myr/km, since at least 13 kyr.

  18. 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.

  19. 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.

  20. Heterogeneous structure across the source regions of the 1968 Tokachi-Oki and the 1994 Sanriku-Haruka-Oki earthquakes at the Japan Trench revealed by an ocean bottom seismic survey

    Microsoft Academic Search

    Tadaaki Hayakawa; Junzo Kasahara; Ryota Hino; Masanao Shinohara; Aya Kamimura; Minoru Nishino; Takeshi Sato; Toshihiko Kanazawa

    2002-01-01

    To study the physical properties along the subducting plate boundary at the Japan Trench, a seismic study was carried out using ocean bottom seismometers (OBSs) and artificial sources was carried out. The 250km long survey line with an almost N–S strike crosses the two major focal zones of the 1968 Tokachi-Oki earthquake and the 1994 Sanriku-Haruka-Oki earthquake. Ray tracing, and

  1. Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

    NASA Astrophysics Data System (ADS)

    Renard, Vincent; Nakamura, Kazuaki; Angelier, Jacques; Azema, Jacques; Bourgois, Jacques; Deplus, Christine; Fujioka, Kantaro; Hamano, Yozo; Huchon, Philippe; Kinoshita, Hajimu; Labaume, Pierre; Ogawa, Yujiro; Seno, Tetsuzo; Takeuchi, Akira; Tanahashi, Manabu; Uchiyama, Akinori; Vigneresse, Jean-Louis

    1987-05-01

    Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.

  2. Zenisu Ridge: a deep intraoceanic thrust related to subduction, off southwest Japan

    NASA Astrophysics Data System (ADS)

    Lallemant, Siegfried; Chamot-Rooke, Nicolas; Le Pichon, Xavier; Rangin, Claude

    1989-03-01

    During the French-Japanese Kaiko project, Seabeam, seismic and submersible observations were made in the eastern part of the Nankai subduction zone, close to the area of collision between the Izu-Bonin island arc and the Japan margin. The most prominent feature is the Zenisu Ridge, an elongated relief of the Philippine Sea plate running parallel to the Trench. Magnetic anomalies indicate that the crust of the Zenisu Ridge is a part of the Shikoku oceanic basin formed in the Early Miocene, 23 Ma ago and presumably uplifted at a later stage. Structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics. Mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust, both segments being of equal magnitude (vertical offset about 1 to 1.5 km). Geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth. The overall structure of the surrounding area reveals a compressive deformation zone widening toward the east, the magnitude of the compressive deformation decreasing westward as well as southward of the Zenisu Ridge.

  3. 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 -?t and ?-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 ?-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.

  4. Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events

    Microsoft Academic Search

    Dapeng Zhao; Akira Hasegawa; Hiroo Kanamori

    1994-01-01

    We have determined a detailed three-dimensional P wave velocity structure of the Japan subduction zone to 500-km depth by inverting local, regional, and teleseismic data simultaneously. We used 45,318 P wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the Japan Islands. The arrival times are recorded by the Japan University Seismic Network which covers the

  5. 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.

  6. Effects of ridge crest-trench interaction on Antarctic-Phoenix Spreading: Forces on a young subducting plate

    NASA Astrophysics Data System (ADS)

    Larter, Robert D.; Barker, Peter F.

    1991-11-01

    Precise measurements of spreading rates on marine magnetic profiles collected to the west of the Antarctic Peninsula have enabled some consideration of the forces governing plate motion, since Antarctic—Phoenix motion has been controlled by the local rather than global force balance over the last 35 m.y. The total effective driving force per unit length of trench is calculated to have ranged between 2.6 and 3.6×1012 N/m, which is much less than is commonly thought necessary to support subduction. Conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench. The low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed. Driving forces show a strong correlation with observed spreading rates, which indicates that resistive forces were largely velocity dependent. Fluid migration up the subduction zone may elevate temperatures in and around the shear zone, reducing resistive forces below the levels required by purely conductive models. Changes in convergence rate may affect the depths of both the brittle/ductile deformation boundary and the basalt/eclogite phase change, causing a negative feedback which would appear as a velocity-dependent resistive force. The different driving forces acting on the NE and SW parts of the Phoenix plate, as a consequence of older oceanic lithosphere at the trench in the NE, caused Antarctic-Phoenix spreading to take place about a near pole to the SW since 21 m.y. ago, and ultimately resulted in disruption of the Phoenix plate about 9 m.y. ago. Spreading rates decreased abruptly about 6 m.y. ago, probably because of E-W compression across the long transform faults bounding the Phoenix plate. However, spreading on the last three segments of the Antarctic-Phoenix Ridge continued at least until 4 m.y. ago. Either spreading stopped progressively from SW to NE, or the final stage took place about a very near pole to the SW. A magnetic quiet zone extends up to 95 km from the margin between the Tula Fracture Zone and the North Anvers Fracture Zone, and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision. The absence of a magnetic quiet zone associated with the most recent ridge crest-trench collisions suggests a change in sedimentary regime during the late Miocene. Anomalously fast apparent spreading rates between 23 and 21 m.y. ago are thought to indicate an error in this part of the magnetic reversal time scale.

  7. Along-Arc Variations in the Location of Frontal Volcanoes and the Orientation of Volcanic Cross-Chains in Subduction Zones: 3-D Mantle Wedge Flow and Sub-Arc Mantle Temperatures in the southern Kuril-NE Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Wada, I.; He, J.

    2014-12-01

    We develop a 3-D thermal model for a margin segment that extends from the southern Kuril Islands to NE Japan, using a realistic geometry for the subducting Pacific plate, and investigate the effect of 3-D mantle wedge flow on the mantle wedge temperature and its relation to the distribution of arc volcanoes. Mantle wedge flow is driven largely by viscous coupling between the subducting slab and the overlying mantle, and its flow pattern is influenced by the geometry of the subducting slab and the subduction direction relative to the trench. Along the southern Kuril-NE Japan margin, the slab takes a complex geometry with varying subduction obliquity. The 3-D modeling results show that in NE Japan, the directions of the mantle wedge in-flow and out-flow are E-W, nearly parallel to the subduction direction. However, in southern Kuril, due to oblique subduction, the mantle flows in from NE, obliquely to the subduction direction, and flows out parallel to the subduction direction. These mantle wedge flow patterns are consistent with those inferred from the measured seismic anisotropy of the mantle wedge. The predicted inflow directions in both NE Japan and southern Kuril correlate well with the E-W and NE-SW orientations of cross-arc chains, respectively, indicating that the cross-chain orientation may be guided by the mantle inflow direction. In southern Kuril, obliquity subduction and steeper slab dip results in slightly cooler mantle wedge than in Tohoku at shallow depths (<100 km depth). Further, the northerly mantle inflow in southern Kuril and the westerly inflow in NE Japan converge in the hinge zone where the slab bends to accommodate a dip direction change between southern Kuril and NE Japan, discouraging mantle inflow within the hinge zone and causing the mantle wedge to be relatively cold. The slab surface depths beneath the frontal volcanoes in southern Kuril and NE Japan are about 120 km and 100 km, respectively. The along-arc variation in the mantle wedge temperature likely affects the locations of melt generation and frontal volcanoes. We qualitatively examine whether the correlations between arc location, cross-chain orientation, slab geometry, and subduction obliquity that are found in southern Kuril and NE Japan are present elsewhere.

  8. Subduction zone megathrust ruptures and trench parallel gravity anomalies: Evaluating their relationship with relocated aftershocks and background seismicity

    NASA Astrophysics Data System (ADS)

    Teel, A.; Thurber, C. H.; Lin, G.

    2009-12-01

    Subduction zone megathrust rupture characteristics have been hypothesized to be related to physical conditions along the plate interface. Song and Simons (2003) and Wells et al. (2003) both find that rupture preferentially occurs in regions of strongly negative trench parallel gravity anomalies (TPGA). The inference is that TPGA reflects local variations in geologic structure which serve as a control on underlying seismic coupling and segmentation. A recent study by Llenos and McGuire (2007) uses estimated second moments of the slip distribution, inferred from Rayleigh waves, to create rupture models for 15 great subduction zone earthquakes. They find that for 11 of the 15 events studied, TPGA increases between the earthquake centroid and the edges of rupture, supporting the hypothesis of structural control on the boundaries of rupture. As an independent test of this relationship, we use a joint hypocenter determination method based on the JED algorithm described in Douglas (1967) to relocate earthquakes in 12 of the 15 regions studied by Llenos and McGuire (2007). Our results are consistent with the areal extent and directivity of rupture for the Llenos and McGuire (2007) models in 10 cases, but they are inconsistent with the location of the Llenos and McGuire (2007) rupture models in 9 cases. This indicates that their findings relating TPGA to rupture extent need revision. Additionally, we do not find seismicity to be correlated to TPGA. If ruptures tend to terminate in areas of increasing TPGA, one would expect to find seismicity exhibiting the same trend.

  9. 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

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  12. The Japan Trench Fast Drilling Project (JFAST): Investigating Large Slip of the 2011 Tohoku-Oki, Japan Earthquake with Seafloor Boreholes

    NASA Astrophysics Data System (ADS)

    Mori, J. J.; Chester, F.; Eguchi, N.; Toczko, S.; Brodsky, E. E.; Kodaira, S.

    2012-12-01

    The 2011 Tohoku-oki earthquake produced the largest fault slip ever recorded for an earthquake, up to 50 meters on the shallow portion of the subduction megathrust. This region of the plate boundary was thought to be an area of aseismic slip by many researchers, so the huge co-seismic displacements and resultant devastating tsunami were a shocking surprise to the seismological community. In response to the earthquake, IODP Expedition 343 (JFAST) was designed to investigate the physical conditions and rupture mechanisms that produced the large slip, using drilling of seafloor boreholes to the plate boundary decollement. During April/May and July 2012, three boreholes located at a site close to the Japan Trench about 90 km east of earthquake epicenter, successfully reached the plate boundary fault at depths of about 820 meters below seafloor. These boreholes enabled geophysical logging, core sampling and installation of a temperature observatory in the vicinity of the fault zone. Preliminary analyses of core samples obtained from the plate boundary decollement show a narrow zone (less than 5 meters) of highly deformed fabric in a clay layer. The pronounced localization of deformation within this material suggests coseismic weakening during past earthquakes. Estimates of the level of dynamic friction during the recent earthquake are expected from the temperature monitoring instrumentation that was installed during the expedition. These temperature data will be retrieved in several months. Further investigations of the physical, chemical, and mechanical properties of the fault zone material along with estimates of the local stress state from borehole breakouts, will be combined with the current results to explain mechanisms for the huge slip during the earthquake.

  13. 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

  14. 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

  15. 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.

  16. Along-trench Structural Variations of Downgoing Juan de Fuca Plate in relation to Regional Scale Segmentation of the Cascadia Subduction Zone: insights from Multichannel Seismic Imaging

    NASA Astrophysics Data System (ADS)

    Han, S.; Carbotte, S. M.; Canales, J.; Carton, H. D.; Nedimovic, M. R.

    2012-12-01

    The Cascadia subduction zone is regionally segmented in its structural characteristics, intra-slab seismicity and Episodic Tremor and Slip, and submarine morphology. The origin of these regional variations in subduction zone structure is not clear. Based on spatial correlations of the segmentation with the topography and geologic terranes of the overriding North America plate, an upper plate related origin has been proposed (e.g. Brudzinski & Allen 2007). However, characteristics of the downgoing Juan de Fuca (JdF) plate, including plate segmentation defined by pseudofaults, the distribution of buried seamounts, and changes in faulting and alteration of the plate may also contribute (Nedimovic et al. 2009). During the Cascadia Ridge-to-Trench experiment conducted in June-July 2012 aboard the R/V Langseth, Multichannel Seismic (MCS) data along a 450 km-long trench-parallel line ~10 km seaward of the Cascadia deformation front were collected. The major scientific objective of this line is to characterize JdF plate structure along an approximately constant age transect just seaward of the trench and to assess the contribution of the down-going plate to regional segmentation of the subduction zone. Preliminary brute stacks generated at sea show bright intracrustal reflectivity, including a series of antithetic shallowly dipping events, as well as clusters of southward and northward dipping events; there are marked changes in characteristics of this reflectivity along the Oregon and Washington margins. Several strike slip faults offsetting the sediment section and an ~6 km wide small seamount are also imaged. 2D MCS processing will be conducted on this line and the results will be presented at the meeting.

  17. 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 (?ST), subduction thrust curvature (CST), and trench curvature angle (?T). 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 (?ST ? 30°), low subduction thrust curvature (CST ? 2.0 × 10-13 m-2) and low trench curvature angles (-6.3° ? ?T ? 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?, ?ST, CST and ?T). 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  19. 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.

  20. Can Interseismic Geodetic Observations Resolve Persistent Rupture Asperities? A study of the Japan trench off Tohoku.

    NASA Astrophysics Data System (ADS)

    Kanda, R. V.; Hetland, E. A.; Simons, M.; Owen, S. E.; Webb, F. H.

    2008-12-01

    In the last century, several large (M > 7) earthquakes have occured on the megathrust interface along the Japan Trench, offshore of Japan's Tohoku region. Published earthquake source inversions based on seismological data suggest that some of these earthquakes have repeatedly ruptured the same region of the fault (i.e., asperities), while others have ruptured closely clustered asperities (e.g., Yamanaka and Kikuchi, 2004). For instance, the 1978, M 7.4 and the 2005, M 7.2 Miyagi-oki events are inferred to have ruptured the same asperity, while the 1968, M 7.9 Tokachi-oki event, and the 1994, M 7.5 Sanriku-oki event ruptured distinct asperities that are close to each other. In contrast, inversions of geodetic data from interseismic periods produce models that are locked over more spatially extensive regions (e.g., Suwa et al, 2003). These broad and smooth regions are in contrast to the smaller discrete asperities indicated by earthquake source studies, and may be a consequence of lack of model resolution and a resulting need for regularization that is inherent to the use of onshore geodetic data. Alternatively, the differences may imply the potential for a large earthquake in the future. Thus, the different levels of apparent coupling implied by these two classes of models have very different implications for regional seismic hazard. Here, we test the hypothesis that mechanical coupling on inferred asperities alone is sufficient to explain available geodetic observations or alternatively, that these data require additional regions on the megathrust to be coupled. To address this question, we use a 3-D mechanical model of stress-dependent interseismic creep along the megathrust, that is consistent with a given frictional rheology and the known spatio-temporal distribution of large earthquakes. These mechanical models predict that asperities are surrounded by a "halo" of very low creep-rates (a "stress-shadow" effect) late in the seismic cycle, which also results in a relatively smooth and long wavelength surface velocity field (see poster by Hetland et al. in this session). We test if this "physical" smoothing preserves any signature of the original asperities, in comparison to the artificial smoothing produced by model regularization in inversions of interseismic geodetic data. Underlying this analysis is the assumption that known asperities persist across multiple earthquake cycles.

  1. 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.

  2. 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.

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

    NASA Astrophysics Data System (ADS)

    Maeda, Takuto; Furumura, Takashi; Obara, Kazushige

    2014-07-01

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

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

    NASA Astrophysics Data System (ADS)

    Mori, J. J.

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Observation of seafloor crustal deformation is crucial for megathrust earthquake because most of the focal areas are located below seafloor. Seafloor crustal deformation can be observed GPS/Acoustic technique, and this technique has been carried out at subduction margins in Japan, e.g., Japan Trench, Suruga Trough, and Nankai Trough. At the present, the accuracy of seafloor positioning is one to several centimeters for each epoch. Velocity vectors at seafloor site are estimated through repeated observations. Co- and post- seismic slip distribution and interseismic deformation are estimated from results of seafloor geodetic measurement (e.g., Iinuma et al., 2012; Tadokoro et al., 2012). We repeatedly observed seafloor crustal deformations at two sites across the Suruga Trough from 2005 to investigate interplate locking condition at the focal area of the anticipated megathrust, Tokai, earthquake. We observed 12 and 16 times at an east site of the Suruga Trough (SNE) and at an west site of the Suruga Trough (SNW), respectively. We reinstalled seafloor benchmarks at both sites because of run out of batteries in 2012. We calculated and removed the bias between the old and new seafloor benchmarks. Furthermore, we evaluated two type of analysis. One is Fixed triangular configuration Analysis (FTA). When we determine the seafloor benchmark position, we fix the triangular configuration of seafloor units averaging all the measurements to improve trade-off relation between seafloor benchmark position and sound speed structure. Sound speed structure is assumed to be horizontal layered structure. The other one is Fixed Triangle and Gradient structure of sound speed structure (FTGA). We fixed triangular configuration same as FTA. Sound speed structure is assumed to have gradient structure. Comparing FTA with FTGA, the RMS of horizontal position analyzed through FTA is smaller than that through FTGA at SNE site. On the other hand, the RMS of horizontal position analyzed through FTA is larger than that through FTGA at SNW site. We estimated the displacement velocities with relative to the Amurian plate from the result of repeated observation. The estimated displacement velocity vectors at SNE and SNW are 42×8 mm/y to N94W direction and 46×13 mm/y to N77W direction, respectively. The directions are the same as those measured at the on-land GPS stations. The magnitudes of velocity vector indicate significant shortening by approximately 11 mm/y between SNW and on-land GPS stations at the western part of the Suruga Trough. We also calculated the theoretical surface deformation pattern to depict the interplate locking condition. These results show that the plate interface at the shallow zone of the northernmost Suruga trough is strongly locked.

  6. Unusually large earthquakes inferred from tsunami deposits along the Kuril trench

    Microsoft Academic Search

    Futoshi Nanayama; Kenji Satake; Ryuta Furukawa; Koichi Shimokawa; Brian F. Atwater; Kiyoyuki Shigeno; Shigeru Yamaki

    2003-01-01

    The Pacific plate converges with northeastern Eurasia at a rate of 8-9m per century along the Kamchatka, Kuril and Japan trenches. Along the southern Kuril trench, which faces the Japanese island of Hokkaido, this fast subduction has recurrently generated earthquakes with magnitudes of up to ~8 over the past two centuries. These historical events, on rupture segments 100-200km long, have

  7. Subduction dynamics at the middle America trench : new constraints from swath bathymetry, multichannel seismic data, and ¹?Be

    E-print Network

    Kelly, Robyn K

    2003-01-01

    The cosmogenic radionuclide ¹?Be is a unique tracer of shallow sediment subduction in volcanic arcs. The range in ¹?Be enrichment in the Central American Volcanic Arc between Guatemala and Costa Rica is not controlled by ...

  8. Tectonic erosion along the Japan and Peru convergent margins

    E-print Network

    Demouchy, Sylvie

    Tectonic erosion along the Japan and Peru convergent margins R. VON HUENE U.S. Geological Survey, M the Japan and Peru Trenches indicate rates of erosion comparable to well-known rates of accretion. Proposed of abundant sediment. Subduction of the oblique-trending Nazca Ridge off Peru pro- duced many similar

  9. Mapping the subducting Pacific slab beneath southwest Japan with Hi-net receiver functions [rapid communication

    NASA Astrophysics Data System (ADS)

    Niu, Fenglin; Levander, Alan; Ham, Sangwon; Obayashi, Masayuki

    2005-10-01

    We have used 4th root receiver function stacks, and pre-stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern Japan. Receiver functions were calculated from the quiet short-period seismograms recorded by a recently deployed borehole network, Hi-net. We found that a relatively broad frequency band can be retrieved from a short-period seismogram by a deconvolution of the instrument response. The quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments. We applied common-conversion-point gathering to the receiver-function data to image the P to S conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence. We found that the topographic anomalies of the 410- and 660-km discontinuities beneath southwest Japan have very different length scales. The former is characterized by a narrow, ˜150-200 km wide, topographic high, while the latter exhibits a broad, > 400 km wide, moderate topographic low together with a small-scale, larger-amplitude depression. A 2.5D pre-stack depth migration of the receiver functions shows the transition zone features clearly, as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660-km. These observations show that the subducted Pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone, and is flat lying either above or across the 660-km discontinuity. The flat lying slab is, however, restricted to the bottom of the transition zone, and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities.

  10. Fluid venting along Japanese trenches: tectonic context and thermal modeling

    NASA Astrophysics Data System (ADS)

    Henry, P.; Lallemant, S. J.; Le Pichon, X.; Lallemand, S. E.

    1989-03-01

    Large benthic chemosynthetic communities have been observed at four main locations during the Kaiko submersible dives in the Japanese trenches. They appear to be associated with venting along fractures. The first site for our observation was along the Japan and Kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench. The benthic communities there seem to be related to tension gashes parallel to the subduction vector. The other communities were found on the toe of the Nankai accretionary prism, along the frontal thrust and tension gashes. The temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water. As the anomaly has a small spatial extent and as the peak thermal gradient is high, the best fitting model is to be found in a vertical upward flow at a velocity of 100 m/yr in a cylindrical conduit leading out of an underlying shallow thrust.

  11. Automated Monitoring of Non-Volcanic Tremors in Southwest Japan Subduction Zone

    NASA Astrophysics Data System (ADS)

    Suda, N.; Ishihara, Y.; Nakata, R.; Kusumi, T.

    2005-12-01

    Activities of low-frequency tremors, which occur in the non-volcanic region of southwest Japan along the subduction zone of the Philippine Sea plate, often show slow migration of tremor sources with a speed of about 10 km/day and quasi-periodicity of occurrence with an interval of 2--6 months. Such characteristics seem to be related to fluid liberated from the subducting slab. Monitoring of tremor activities therefore may lead us to elucidation of fluid property and its role in the subduction zone. To monitor activities of low-frequency tremors in real-time, we have developed the automatic detection and hypocenter determination system using real-time data from the commutation-satellite seismic telemetry system. We divide the tremor region into 6 subregions, and use 8--12 vertical-component seismic data to monitor the tremor activity in each subregion. Before applying the detection method, we preprocess real-time data by applying the bandpass filter with a frequency band of 1--10 Hz, resampling them from 100 Hz to 20 Hz, calculating their envelopes, and finally applying the moving average with a time window of 3 seconds. The automatic detection method is based on a two-step numerical statistical test: the first test is for examining whether given two envelopes with a length of 2 minutes are correlated or not, and the second for examining whether an event occur or not in this time window using results of the first test. If an event is detected then the automatic hypocenter determination method is applied. In this method time lags for the maximum cross-correlations obtained in the detection method are utilized as traveltime difference data, and hypocenters are determined by minimizing the absolute residual using the simplex method. The results obtained by the automatic detection method are consistent to those determined by visual inspection of waveforms, and also the results obtained by the automatic hypocenter determination are consistent to those determined by the conventional traveltime analysis of envelope waveforms. Thus our system is useful for real-time monitoring of the tremors, and consequently will provide basic data for investigation of the subduction zone associated with great earthquakes.

  12. 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 (2? ±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.

  13. On the relationship of S-point and trench parallel mass transport at obliquely convergent subduction margins: insights from analogue modelling

    NASA Astrophysics Data System (ADS)

    Hoffman-Rothe, A.; Kukowski, N.; Lohrmann, J.; Adam, J.; Oncken, O.

    2003-04-01

    Strain partitioning is a common observation in obliquely convergent subduction zones. Partitioning results in trench parallel material transport that, in the case of localisation of deformation, may be accommodated along large-scale margin parallel strike-slip faults. Such strike slip faults are located rather far inland from the trench (100-300km; e.g. Peru, North-Chile, Sumatra). Field studies as well as numerical and analogue models have shown the influence of convergence obliquity as a first order condition for the onset and degree of strain partitioning in the convergent wedge. The effect of friction at the wedge base is under discussion. In this context, the existence and position of the singularity (S-point, i.e. S-line for the 3D case) on the plate interface, defining the location from which basal shear forces are not further transmitted into the overlying wedge, has to be accounted for. This singularity is an inescapable necessity of numerical and analogue models. It may have its natural analogy in the line of intersection of the subducting slab with some brittle-ductile boundary in the convergent wedge. To date analogue experiments show strain partitioning in the deforming accretionary prism above the S-line, while the hinterlandward wedge remains undeformed. This, however, is not in accordance with natural examples: the formation of strike-slips faults is triggered further inland and is separated from the position of the assumed S-line. We therefore designed a series of scaled 3D analogue sandbox experiments, for which obliquity of convergence, basal friction and the position of the S-line can be varied. Analysis of the 3D surface displacement with CAD and PIV (particle imaging velocimetry) is used to evaluate material transport and to differentiate kinematic and dynamic domains of the convergent wedge. We test the influence of the relative position of the S-line with respect to the deformation front and the effect of the existence or non-existence of a basal shear force in the hinterlandward wedge. This shows whether shear along the entire base of the brittle wedge is required to facilitate lateral material transport at greater distance from the trench.

  14. 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.

  15. Silent fault slip following an interplate thrust earthquake at the Japan Trench

    Microsoft Academic Search

    Kosuke Heki; Shin'ichi Miyazaki; Hiromichi Tsuji

    1997-01-01

    Recent global space geodetic measurements have revealed that the velocities of tectonic plates over timescales as short as a decade1 are consistent with models of velocities averaged over the past few million years. The slip inferred from interplate thrust earthquakes at deep sea trenches and and number of earthquakes, however, often falls short of that predicted from these observed plate

  16. Simulation of Long-Period Ground Motion From a Hypothetical Nankai Subduction Earthquake in Western Japan

    NASA Astrophysics Data System (ADS)

    Iwaki, A.; Iwata, T.

    2008-12-01

    The long-term occurrence potentials of the megathrust earthquakes in Nankai trough of the subducting Philippine Sea Plate are from 50% and 70% within 30 years from 2008 (The Headquarters for Earthquakes Research Promotion, 2008). The hypothetical source region of Nankai earthquake extends as wide as 300 km in the east-west direction; therefore it is likely to cause large long-period ground motion to many sedimentary basins in wide area of Japan. We study the propagation and generation of the long-period (3 - 20 s) ground motion in two sedimentary basins in western Japan: the Osaka basin and the Oita basin, located northeast and west of the source region, respectively. Among the sedimentary basins in western Japan, these are two deep basins in which long-period structures such as high rise buildings and oil storage tanks are at high risk of seismic hazard by long-period ground motion. We carried out a ground motion simulation of a hypothetical Nankai earthquake by a 3D finite-difference method (Pitarka, 1999) using 3D basin and crustal velocity structure models (Iwata et al., 2008; Oita Prefecture, 2008). The basin velocity structure models are validated by long-period ground motion simulations of observed earthquake records. The source model is based on Sekiguchi et al. (2008) that consists of five asperities and a background area, distributed on the top surface of Philippine Sea Plate of the crustal velocity model. We tried two source models with different hypocenter, one in which rupture propagates from east to west (east-hypo model), which is generally accepted as hypothetical source models of the Nankai earthquake, and another from west to east (west-hypo model). The total seismic moment is 7.4× 1021 Nm (MW8.5) for both the source models. In the Oita basin, the maximum peak ground velocity (PGV) reaches 100 cm/s in the east-hypo model simulation while it is roughly one fifth smaller in the west-hypo model simulation. The east-hypo model causes the directivity effect that enlarges the seismic waves generated by the asperities near to the Oita basin. On the other hand, the maximum PGV exceeds 120 cm/s for both east- and west-hypo models in the Osaka basin, and the difference between the two models is not as explicit as in the Oita basin because one asperity that is located southward of Osaka is the most responsible for the large ground motion in the Osaka basin for both source models. The simulated waveforms in the east-hypo model are comparable to the observed seismograms of the 1946 Nankai earthquake (MJMA8.0) recorded at Japan Meteorological Agency stations in western Japan, which indicates that the simulated ground motion in the basins in this study at least not excessively overestimating. Seismograms provided by Japan Meteorological Agency were used in this study.

  17. 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.

  18. Some aspects of the tectonics of subduction zones

    NASA Astrophysics Data System (ADS)

    Aubouin, Jean

    1989-03-01

    The main structures of a subduction zone are as follows. (1) On the outer wall: faults, formed either by reactivation of the structural grain of the oceanic plate, when the latter is slightly oblique to the trench, or by a new fault network parallel to the trench, or both. The width of the faulted zone is about 50 miles. (2) On the inner wall: either an accretionary prism or an extensional fault network, or both; collapsed structures and slumps are often associated, sometimes creating confusion with the accretionary structures. (3) The overall structure of the trench itself is determined by the shape of the edge of the continental crust or of the island arc. Its detailed structure, however, is related to the oceanic plate, namely when the structural grain of the latter is slightly oblique to the trench, which then takes an "en echelon" form. Collapsed units can fill up the trench which is, in that case, restricted to an irregular narrow depression; the tectonic framework of the trench can be buried under a sedimentary blanket when the sedimentation rate is high and the trench bottom is a large, flat area. Two extreme types of active margins can be distinguished: convergent compressive margins, when the accretionary mechanism is strongly active; and convergent extensional margins where the accretionary mechanism is absent or only weakly active. The status of a given margin between these two extreme types is related to the convergence rate of the plates, the dip of the subduction zone, the sedimentation activity and the presence of a continental obstacle, because oceanic seamounts and aseismic ridges are easily subducted. Examples are taken from the Barbados, Middle America, Peru, Kuril, Japan, Nankai, Marianna, Manila, New Hebredes and Tonga trenches.

  19. 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.

  20. 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.

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

    Microsoft Academic Search

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

    2004-01-01

    The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction erosion

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

    Microsoft Academic Search

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

    2004-01-01

    (1) The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction

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

    Microsoft Academic Search

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

    2010-01-01

    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

  5. 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.

  6. Simulation of Long-Period Ground Motion From a Hypothetical Nankai Subduction Earthquake in Western Japan

    Microsoft Academic Search

    A. Iwaki; T. Iwata

    2008-01-01

    The long-term occurrence potentials of the megathrust earthquakes in Nankai trough of the subducting Philippine Sea Plate are from 50% and 70% within 30 years from 2008 (The Headquarters for Earthquakes Research Promotion, 2008). The hypothetical source region of Nankai earthquake extends as wide as 300 km in the east-west direction; therefore it is likely to cause large long-period ground

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  8. Shear-wave splitting in the northeastern Japan and southwestern Kurile arcs and its implications for mantle corner flow

    Microsoft Academic Search

    S. Hori; J. Nakajima; J. Shimizu; A. Hasegawa

    2005-01-01

    This paper investigates shear-wave splitting beneath the southwestern (SW) Kurile arc (Hokkaido) and the northeastern (NE) Japan arc (Tohoku) using local S phases to constrain a spatial pattern of mantle flow generated by the subduction of the Pacific plate. This region includes the arc-arc junction, where the direction of the plate motion is changed relative to the trench axis, and

  9. Precise hypocenter distribution of deep low-frequency earthquakes and its relationship to the local geometry of the subducting plate in the Nankai subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Ohta, Kazuaki; Ide, Satoshi

    2011-01-01

    We determine the precise hypocenter distribution of deep low-frequency earthquakes (LFEs) in the Nankai subduction zone and compare it with the local geometry of the subducting Philippine Sea plate. We apply a new hypocenter determination method utilizing the summed cross correlation coefficient over many stations, termed a network correlation coefficient (NCC), to 112 LFEs in the western Shikoku and 1566 LFEs in the whole Nankai subduction zone. While the catalog depths are widely distributed in some regions, the relocated hypocenters in every region construct a plane surface several km above the oceanic Moho interface and quite consistent with the geometry of the oceanic Moho. This result strongly supports the hypothesis that LFEs in the Nankai subduction zone occur on the subducting plate boundary and are directly generated by shear slips. If LFEs are indeed direct indicators of the locations of the plate interface, they might be useful to investigate the minute structure of the plate interface. The thin distributions of LFEs indicate that the interface between the subducting and the overriding plates is a distinct very thin boundary, and not a distributed shear zone.

  10. 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.

  11. Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back arc rifting

    NASA Astrophysics Data System (ADS)

    Fabbri, Olivier; Fournier, Marc

    1999-06-01

    Compilation of earthquake focal mechanisms, morphological analysis, and examination of mesofractures help to clarify the Cenozoic and recent stress history in the southern Ryukyu arc between the Ryukyu trench and the southern Okinawa Trough. (1) An older, pre-Miocene, intermediate-type stress field (vertical stress component ?v = ?2) is attested by conjugate strike-slip faults and shear joints in Eocene rocks of Ishigaki Island. The ?1 axis trends N90°E to N110°E in present-day coordinates. (2) Normal faults and extensional joints in Mio-Pliocene strata of Iriomote and Yonaguni Islands and fault scarps in Pleistocene strata at Yonaguni allow us to reconstruct a Miocene to Pleistocene stress field of tensional type (?v = ?1). The ?3 axis trends about NW-SE. The effects of this stress field can be traced back in Eocene rocks of Ishigaki Island. (3) Evidence for the youngest, Holocene to present-day stress field comes from the analysis of the Harvard Moment Tensor Catalog (1976-1997) for shallow earthquakes, Quaternary fault scarps visible on the two islands of Miyako and Yonaguni, and extensional joints in Holocene rocks of Yonaguni. This stress field is tensional. Two directions of extension are inferred: a regional, arc-perpendicular (N-S) one and a local, arc-parallel (N60°E) one in the easternmost part of the area near Miyako. The record of Neogene to Quaternary tensional stress fields demonstrates that the extension in the Okinawa Trough can be traced until the arc region. Arc-oblique to arc-perpendicular stretching can be directly correlated with oceanward rifting in the back arc basin, while the arc-parallel stretching reflects the increase of the arc curvature. The predominance of extensional features, the absence of shortening or transcurrent deformation structures, and a weak interplate coupling do not support the forearc sliver model for the present-day southern Ryukyu arc.

  12. 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.

  13. 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.

  14. Intermediate and Deep Seismicity and Lateral Structure of Subducted Lithosphere in the Circum-Pacific Region

    NASA Astrophysics Data System (ADS)

    Burbach, G. Vanness; Frohlich, Cliff

    1986-11-01

    In this paper we present a region-by-region review of the Wadati-Benioff zone structure of most of the world's seismically active subduction zones, focusing primarily on the intermediate and deep seismicity. Lateral changes in Wadati-Benioff zone structure are common in every major subduction zone. In this study we use these changes to define possible boundaries between portions or "segments" of lithosphere with differing subduction geometries. Although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere, the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate, rather than tearing. We found the strongest evidence for the existence of tears where tears are geometrically necessary, such as where a transform boundary terminates a trench, as at the New Hebrides, Tonga, and South Sandwich Trenches. Weak evidence suggesting other tears does exist in some regions, such as Taiwan, the Japan/Izu-Bonin corner, and the Philippines. The causes of these changes in structure are, in most cases, unclear. Only about 34% of the possible segment boundaries coincide with subducting bathymetric features. Some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry. We have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and Wadati-Benioff zones to estimate the lateral strain in subduction zones throughout the circum-Pacific region. Although no observed subduction zone has a perfectly strain-free geometry, there is a broad range of geometries for which the lateral strain is small. Indeed, the observed geometry of most subduction zones involve relatively little lateral strain. Comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain, the mechanisms of intermediate and deep earthquakes show no effects of lateral stress, and downdip stresses are clearly dominant. In regions such as the Mariana Arc, where the model predicts very large lateral extension, lateral tension is very evident in the focal mechanisms. In regions such as the Hokkaido corner, where the modeling predicts large compressional strains, the plate appears to buckle, and bending stresses parallel to the trench are evident. In general this study finds that subducted lithosphere is remarkably cohesive and rigid, and only rarely deforms by breaking or stretching.

  15. Stress Drops of Earthquakes on the Subducting Pacific Plate in the South-East off Hokkaido, Japan

    NASA Astrophysics Data System (ADS)

    Saito, Y.; Yamada, T.

    2013-12-01

    Large earthquakes have been occurring repeatedly in the South-East of Hokkaido, Japan, where the Pacific Plate subducts beneath the Okhotsk Plate in the north-west direction. For example, the 2003 Tokachi-oki earthquake (Mw8.3 determined by USGS) took place in the region on September 26, 2003. Yamanaka and Kikuchi (2003) analyzed the slip distribution of the earthquake and concluded that the 2003 earthquake had ruptured the deeper half of the fault plane of the 1952 Tokachi-oki earthquake. Miyazaki et al. (2004) reported that a notable afterslip was observed at adjacent areas to the coseismic rupture zone of the 2003 earthquake, which suggests that there would be significant heterogeneities of strength, stress and frictional properties on the surface of the Pacific Plate in the region. In addition, some previous studies suggest that the region with a large slip in large earthquakes permanently have large difference of strength and the dynamic frictional stress level and that it would be able to predict the spatial pattern of slip in the next large earthquake by analyzing the stress drop of small earthquakes (e.g. Allmann and Shearer, 2007 and Yamada et al., 2010). We estimated stress drops of 150 earthquakes (4.2 ? M ? 5.0), using S-coda waves, or the waveforms from 4.00 to 9.11 seconds after the S wave arrivals, of Hi-net data. The 150 earthquakes were the ones that occurred from June, 2002 to December, 2010 in south-east of Hokkaido, Japan, from 40.5N to 43.5N and from 141.0E to 146.5E. First we selected waveforms of the closest earthquakes with magnitudes between 3.0 and 3.2 to individual 150 earthquakes as empirical Green's functions. We then calculated source spectral ratios of the 150 pairs of interested earthquakes and EGFs by deconvolving the individual S-coda waves. We finally estimated corner frequencies of earthquakes from the spectral ratios by assuming the omega-squared model of Boatwright (1978) and calculated stress drops of the earthquakes by using the model of Madariaga (1976). The estimated values of stress drop range from 1 to 10 MPa with a little number of outliers(Fig.(a)). Fig.(b) shows the spatial distribution of stress drops in south-east off Hokkaido, Japan. We found that earthquakes occurred around 42N 145E had larger stress drops. We are going to analyze smaller earthquakes and investigate the spatial pattern of the stress drop in the future. Fig. (a) Estimated values of stress drop with respect to seismic moments of earthquakes. (b) Spatial distribution of stress drops.

  16. 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.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

  19. 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.

  20. Salty Trench

    NASA Technical Reports Server (NTRS)

    2004-01-01

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

  1. Sediment trench fill effect on trench age-depth relation 

    E-print Network

    Rader, Brian Lynn

    1985-01-01

    accreted to the overriding plate. Assuming that accreted sediments have a lower density than the subducting and overriding plates, modeiing of variations in the trench free-air gravity minimum was chosen as a way to determine relative amounts of accreted... sediment. A general model, based on seismic refraction data, predicted an initial seaward, then shoreward, migration and a decrease in magnitude of the free-air gravity minimum as the amount of accreted sediment was increased. Mapping the free...

  2. 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.

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

    NASA Astrophysics Data System (ADS)

    Matsuzawa, T.; Iio, Y.

    2011-12-01

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

  4. Oblique subduction of a Newtonian fluid slab

    Microsoft Academic Search

    Zheng-Kang Shen

    1995-01-01

    A Newtonian fluid model is proposed to describe the oblique subduction of a planar 2-D slab. The slab is assumed to subduct in response to the ridge push force exerted along the trench, the slab pull force at the downdip of the slab, the gravitational body force within the slab, and the frictional resistance force at the upper surface of

  5. 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.

  6. Metamorphic rocks of the Yap arc-trench system

    Microsoft Academic Search

    J. Hawkins; R. Batiza

    1977-01-01

    The Yap trench-arc is a link between the Mariana and Philippine arcs; the latter are both loci of acive volcanism and seismicity but the Yap arc is formed of metamorphic rocks and has had few historic earthquakes. It does not appear to be an active subduction zone. The 8-9 km deep Yap trench has a steep west well, it has

  7. Earthquake Rupture Processes Along the Philippine Trench

    NASA Astrophysics Data System (ADS)

    Sevilla, W. I.; Ammon, C. J.

    2004-12-01

    Earthquakes along subduction zones exhibit spatial and temporal variations in rupture processes. Recent studies have demonstrated a systematic decrease in the moment-normalized rupture duration of earthquakes with depth along the plate interface. Several shallow earthquakes were observed to exhibit anomalously long rupture duration, comparable with the durations of tsunami earthquakes. These observations were suggested to be manifestations of subduction faults with frictional properties capable of generating tsunami earthquakes. The cause of the duration variation was hypothesized to be related to physical changes in properties of subducted sediment. The Philippine trench is a setting where we can study the variations of earthquake source rupture processes along the plate interface. The trench is young (< 5 Ma) with a poorly developed accretionary prism. Bathymetry, seismic reflection studies, and offshore drilling show little or no evidence of accretion of the materials from the subducting Philippine Sea plate. Eighty nine possible interplate earthquakes were selected from the Harvard CMT catalogue for the year 1989 to 2001. From these, about 30 events had adequate signal-to-noise ratios and well-constrained mechanisms to warrant further investigation. We used teleseismic broadband records of vertical component waveforms and applied multi-station deconvolution technique to extract the source time function and depth of each event. Our results showed no systematic trend of decreasing source duration with depth at the Philippine trench. The observed moment-normalized durations of all the shallow earthquakes are shorter than tsunami earthquakes. We observed a significant scatter in the relationship between source time and depth, which may reflect heterogeneity of the materials at the trench interface. The thickness of low-rigidity materials in the trench shallow region appears insufficient to affect the rupture durations and produce anomalously slow ruptures. These null results for the sediment-starved Philippine Trench support the earlier hypothesis that the variations of earthquake duration with depth in other subduction zones is related to sediment properties.

  8. 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.

  9. Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench

    E-print Network

    Constable, Steve

    Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench bending faults on the incoming oceanic plate of the Middle America Trench offshore Nicaragua have been of the Middle America Trench containing fewer bending faults have less fluid flux from the subducting slab

  10. 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.

  11. Japan.

    PubMed

    1989-02-01

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

  12. Time Sequence of Deep Low-frequency Tremors in the Southwest Japan Subduction Zone : Triggering Phenomena and Periodic Activity

    Microsoft Academic Search

    Kazushige OBARA

    Non-volcanic deep low-frequency tremors detected in southwest Japan are distributed in the forearc side along the strike of the descending Philippine Sea plate. The source depth of the tremor corresponds to the slab interface or the Moho discontinuity. The time sequence of the tremor activity is characterized by long durations from hours to weeks. The mobility and the successive occurrence

  13. Space-time Renewal Model for Repeating Earthquakes and Slow Slip before and after the Major Earthquakes in the Northeastern Japan Subduction Zone

    NASA Astrophysics Data System (ADS)

    Nomura, S.; Ogata, Y.; Uchida, N.

    2014-12-01

    Repeating earthquake sequences on the plate subduction zone represent the slip-rate histories around their fault patches. So they are useful resources for monitoring precursory aseismic slip of major earthquakes on plate boundaries. Repeating earthquakes are often modeled by renewal processes, point processes whose recurrence intervals are independent and identically distributed. However, their repeating intervals are greatly influenced by larger seismic events or aseismic slow slip, and hence we need to model such non-stationary behavior of repeating earthquakes. In this study, we propose a non-stationary space-time model for repeating earthquakes based on the model in Nomura et al. (2014) applied to the Parkfield catalog. We used the empirical relation between magnitudes and slip sizes of repeating earthquakes by Nadeau and Johnson (1998) to estimate the slip-rate histories in repeating sequences. The proposed model can estimate spatio-temporal variation in slip rate with smoothness restriction adjusted to optimize its Bayesian likelihood.We apply the proposed model to the large catalog of repeating earthquakes in Uchida and Matsuzawa (2013) on subduction zone of Pacific Plate in the northeastern Japan from 1993 to 2011 and estimate slip-rate history of the plate boundary. From this analysis, we discuss the characteristic changes in slip rate before and after the major earthquakes such as Sanriku-Haruka-Oki (1994 Mw7.6), Tokachi-Oki (2003 Mw8.0), Kushiro-Oki (2004 Mw7.1), Fukushima-Oki (2008 Mw6.9), Ibaraki-Oki (2008 Mw7.0) and Tohoku-Oki (2011 Mw9.0).

  14. Japan.

    PubMed

    1987-02-01

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

  15. 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.

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

    NASA Astrophysics Data System (ADS)

    Itaba, S.; Kimura, T.

    2013-12-01

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

  17. 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.

  18. 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…

  19. 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.

  20. 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

  1. 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.

  2. 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.

  3. 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.

  4. What really causes flat slab subduction?

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  5. 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.

  6. Collision and subduction structure of the Izu-Bonin arc, central Japan: Recent studies from refraction/wide-angle reflection analysis and seismic tomography

    NASA Astrophysics Data System (ADS)

    Arai, R.; Iwasaki, T.; Sato, H.; Abe, S.; Hirata, N.

    2009-12-01

    Since the middle Miocene, the Izu-Bonin arc has been colliding from south with the Honshu arc in central Japan associated with subduction of the Philippine Sea plate. This process is responsible for forming a complex crustal structure called the Izu collision zone. Geological studies indicate the several geological blocks derived from the Izu-Bonin arc, such as the Misaka Mountains (MM), the Tanzawa Mountains (TM) and the Izu Peninsula (IP), were accreted onto the Honshu crust in the course of the collision, forming several tectonic boundaries in and around this collision zone (e.g. Amano, 1991). Recent seismic experiments succeeded in revealing the deep crustal structure in the eastern part of the Izu collision zone by reflection analysis (Sato et al., 2005) and refraction/wide-angle reflection analysis (Arai et al., 2009). Although these studies delineate the collision boundary between the Honshu crust and TM, and the upper surface of the subducting Philippine Sea plate, the southern part of the profile including the Kozu-Matsuda Fault (KMF, the tectonic boundary between TM and IP) is not well constrained due to the poor ray coverage. Moreover, clear images of tectonic boundaries are not obtained for the central or western part of the collision zone. In order to construct the structure model dominated by collision and subduction for the whole part of the collision zone, we carried out the following two analyses: (1) refraction tomography of active source data including another profile line in the western part of the collision zone (Sato et al., 2006), and (2) seismic tomography combining active and passive source data. In the analysis (1), we applied first arrival seismic tomography (Zelt and Barton, 1998) to the refraction data .We inverted over 39,000 travel times to construct a P wave velocity model for the 75-km-long transect, and a fine-scale structure with strong lateral heterogeneity was recovered. We conducted checkerboard resolution test to evaluate a spatial resolution, and confirmed that the final model has an enough resolution down to the depth of 5 km. We also performed a Monte Carlo uncertainty analysis (Korenaga et al, 2000) to estimate the posteriori model variance, showing that most velocities are well constrained with standard deviation of less than 0.20 km/s. Our result strongly indicates the existences of low velocity zones (< 6.0 km/s) along the tectonic boundaries and high velocity bodies (> 6.0 km/s) just beneath MM and TM, which correspond to the middle crust of the Izu-Bonin arc (Kodaira et al., 2007). In the analysis (2), hypocenters and velocity structure were simultaneously determined based on the double-difference method (Zhang and Thurber, 2003). The hypocenter distribution and final velocity structure obtained indicate several interesting features, including low velocity sedimentary layer (< 6.0 km/s) along the KMF and prominent seismic activity in the middle-lower crust (6.0-6.8 km/s) in the Izu-Bonin arc (10-25 km depth beneath TM). These results give us very important constraints for the collision process ongoing in our research area.

  7. JAPAN

    NSDL National Science Digital Library

    Mr. Hughes

    2006-03-04

    DESK Standard: Know the physical, political, and economic features of Japan. . DATES: You can begin this activity on April 16. You should complete it by April 20. OBJECTIVE: During fourth grade, we have learned about the history and physical features of Utah. This activity will allow you to compare what you\\'ve learned about the state of Utah with ...

  8. 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.

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  11. 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.

  12. Observations of Repeated Seafloor Tilt Events atop the Inward Steep Slope of the Japan Trench Using New High-Resolution Accelerometers

    NASA Astrophysics Data System (ADS)

    Fukao, Y.; Sugioka, H.; Ito, A.; Shiobara, H.; Paros, J. M.

    2014-12-01

    Continuous observations of seismic events and seafloor movements were made from May 2013 to March 2014 atop the steeply rising morphological unit (often called the lower trench slope) located about 50 km landward of the trench axis and 50 km seaward of the epicenter of the great 2011 Tohoku-Oki earthquake, where the seafloor is locally dipping to the southeast at 14-15° . Measurements were made with a new Acceleration Ocean Bottom Seismometer (AOBS) system using a triaxial array of nano-resolution accelerometers having ranges of 20 m/sec2, parts-per-billion sensitivity and good long-term stability. Observations were compared to a nearby system that included a broadband seismometer, differential pressure gauge and ocean bottom tiltmeter. The AOBS measures seafloor tilting as a change of acceleration in the absolute gravity field and the records show repeated occurrences of approximately unidirectional tilting either to the northwest or southeast. Rapid (tens of second) tilting was always triggered by strong seismic shaking, including a tilt of 0.0016 radians triggered by an Mw7.1 earthquake 145 km distant from the station. Slow (hours) tilting occurs with much smaller amplitude but in a similar dip direction and is most often recovered by opposite tilting at a slower rate. Records of the nearby tiltmeter in their unsaturated cases showed consistent tilt motions. Traditional broadband seismometers and tiltmeters do not have the range to measure strong seismic events and traditional strong motion sensors do not have the sensitivity or stability to make good long-term geodetic measurements. The AOBS can measure both the statically invariant 1g Earth's gravity vector and the higher frequency components associated with strong seismic shaking. This invariance, in spite of large offsets in each component axis due to tilts, indicates that the observed offsets are real signals of trench slope-related processes. These observations demonstrate the usefulness of AOBS for high-resolution measurements of submarine earthquakes, tilts, slow-slip and perhaps even longer-term events.

  13. 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)

  14. 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.

  15. Sunda-Java trench kinematics, slab window formation and overriding plate deformation since the Cretaceous

    E-print Network

    Müller, Dietmar

    Sunda-Java trench kinematics, slab window formation and overriding plate deformation since along the 3200 km long Sunda-Java trench, one of the largest subduction systems on Earth. Combining window underlying the Java­South Sumatra region, and we propose that decreased mantle wedge viscosities

  16. Digging a Paleoseismic Trench

    USGS Multimedia Gallery

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

  17. Global trench migration velocities and slab migration induced upper mantle volume fluxes: Constraints to find an Earth reference frame based on minimizing viscous dissipation

    Microsoft Academic Search

    W. P. Schellart; D. R. Stegman; J. Freeman

    2008-01-01

    Since the advent of plate tectonics different global reference frames have been used to describe the motion of plates and trenches. The difference in plate motion and trench migration between different reference frames can be substantial (up to 4 cm\\/yr). This study presents an overview of trench migration velocities for all the mature and incipient subduction zones on Earth as calculated

  18. The Great 1933 Sanriku-oki Earthquake: Possible Compound Rupture of Outer Trench Slope and Triggered Interplate Seismicity

    NASA Astrophysics Data System (ADS)

    Uchida, N.; Kirby, S. H.; Umino, N.; Hino, R.; Okal, E. A.

    2013-12-01

    The 1933 Sanriku-oki earthquake offshore northern Honshu, Japan (Mw8.4) is the largest earthquake that occurred outer-rise/outer-trench-slope region. The spatial extent of the aftershocks and possibility of a triggered seismicity was estimated by using modern relocation method and velocity structure. Land-station based hypocenter determination by using 3D velocity structure was firstly applied to the off-Sanriku, near-trench region where systematic hypocenter shifts are recognized in the previous studies. The improvement of hypocenter locations near the trench were confirmed by examinations of recent earthquakes that are accurately located based on OBS data. The earthquakes after the 1933 Sanriku-oki earthquake are located about 200 km long region under the outer trench slope that is separated from the aftershock seismicity under the inner trench slope. The outer-trench-slope earthquakes are shallow (depth <=50km) and has V-shape distribution in the trench-normal cross-section. The aftershock distribution suggests shallow rupture area and possibly a compound rupture for the 1933 main shock. We found the V-shaped compound rupture model explains better the polarity of Tsunami waves at the Sanriku coast than a single west dipping fault. This indicates that the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks both in outer- and inner trench slope regions was confirmed by the investigation of dominant wave frequency which is seen in the recent precisely located earthquakes in the two regions (Gamage et al., 2009). The earthquakes under the inner trench slope were shallow (depth <=30km) and located where recent activity of interplate thrust earthquakes is high. This suggests the deformation of the 1933 outer-rise earthquake triggered the interplate earthquakes. Recent (2001-2012) seismicity around the source area by the same method show the seismicity at the outer trench-slope region of northern Honshu can be divided into several groups of earthquakes along the trench; one group roughly corresponds to the aftershock region of the 1933 earthquake. Comparison of the 1933 rupture dimension based on our relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited by the region where fault scarps are trench parallel and cross cutting seafloor spreading fabric. These suggest bending and structural segmentation largely controls the horizontal and vertical extent of the fault. The re-examined aftershock distribution in this study provides a constraint on the stress state of the subducting plate and water supply to deep earth. They also suggest triggered of interplate seismicity that imply the outer rise /outer trench slope earthquake is closely involved in the earthquake cycle of interplate earthquake.

  19. The Mariana Trench

    NSDL National Science Digital Library

    This resource provides an overview of the Mariana Trench, the deepest of 22 known deep-ocean trenches, located in the Pacific Ocean to the east and south of the Mariana Islands near Guam. Topics include the unique adaptations of creatures to the cold, darkness, and extreme pressure of the abyssal zone, the depth and physical properties of the trench itself, and a brief discussion of the tectonic forces that produced the trench-island arc system of the Mariana Arc. There is also a history of exploration of the trench, a discussion of measurements of ocean depth, and links to books, videos, and online references on related topics.

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. Constraints and Simple Models for Arc and Subduction Geotherms

    NASA Astrophysics Data System (ADS)

    Parmentier, E.; Kelemen, P.; Hacker, B.; Hirth, G.

    2005-12-01

    There are many constraints on geotherms beneath arcs [1]. Metamorphic PT estimates for arc lower crustal rocks yield Moho temperatures > 800 C at 1 GPa, consistent with estimated mantle equilibration for some primary arc magmas at 1300 C and 1.5 GPa, and with regionally high heat flow in the Oregon and NE Japan arcs. However, none of these data indicate whether high T is steady state, or the result of transient heating due to ascending magma. Slow P- and S-wave velocities immediately beneath arc Moho suggest partial melt is common in the shallow mantle. Topography and gravity constraints for arcs indicate that the uppermost 100 km of the mantle wedge beneath arcs is >10x weaker than the surrounding mantle at the same depth. The geophysical observations suggest that high T(P) is regionally extensive, and so probably continues for long periods. Many recent thermal models for subduction zones emphasize the important effect of temperature dependent viscosity, the possible consequences of stress dependent viscosity and 3D flow, and the importance of accurately tracking the subduction interface [2]. However, most models incorporate a rigid upper layer. Omitting this rigid layer and modeling the entire upper plate with a temperature dependent viscosit, yields a variable thickness thermal boundary layer [1,3]. Where the thermal boundary layer is thinnest, the PT constraints in the previous paragraph are satisfied at steady state. The predicted distance of the highest heatflow from the trench is uncertain, however, because the models do not fully treat the shallow geometry of the subducting plate. There are fewer constraints on subduction geotherms. Low heat flow in forearcs precludes substantial shear heating of the top of the subducting plate while it is colder than about 500 C [4]. However, recent work on viscous heating in narrow, pre-existing shear zones suggests that instabilities arising at temperatures > 500 C can heat a region by a few hundred C over hundreds of meters around the shear zone [5]. This could be important along the subduction zone, and within pre-existing shear zones in both footwall and hanging wall, beneath arcs where heat flow data do not constrain the amount of shear heating. High P, blueschist facies rocks, thought to record subduction conditions, generally record T(P) higher than in most thermal models. Many ultra-high pressure (UHP) terranes, subducted to > 2 GPa, record peak T at or above aqueous fluid saturated solidii for metabasalt and metasediment, hotter than in most thermal models. Some terranes record T(P) higher than fluid undersaturated solidii as well. Because exhumed high P and UHP rocks crossed the subduction interface to transfer from footwall to hanging wall, they may have undergone nearly isobaric, conductive heating at peak P. Transfer from footwall to hanging wall limits the utility of high P and UHP samples for tracking Benioff zone conditions, but illustrates possibilities for partial melting and/or diapirism of subducted metasediment and metabasalt [6]. 1. Kelemen et al AGU03 2. Van Keken et al G302, Conder et al GRL02, Conder PEPI 04, Kincaid and Griffiths G304 3. Rowland and Davies GRL99 4. Peacock AGU03 5. Kelemen and Hirth EOS04 6. Kelemen et al TOG03, Gerya and Yuen EPSL03, Gerya et al Geol04

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. Gravity Anomaly Between Immature And Mature Subduction Zones In The Western Pacific And Its Implications For Subduction Evolution

    NASA Astrophysics Data System (ADS)

    Kim, Y.; Lee, S.

    2008-12-01

    From immature to mature subduction zones, the western Pacific is a key area to investigate subduction evolution. Among them, the Yap and Mussau trenches located in the boundary of Caroline plate and the Hjort trench to the south of New Zealand are considered as immature subduction zones. The common geological features of immature subduction zones include: (1) a short trench-arc distance, (2) the lack of Wadati-Benioff zone, and (3) the absence of arc volcanism triggered by subducting slab. On the other hand, the Izu-Bonin- Mariana and Tonga-Kermadec trenches are well-developed or mature subduction zones characterized by active arc volcanism and deep earthquakes. We compare two end-members, immature and mature subduction zones, with gravity anomaly derived from satellite altimetry which has sufficient accuracy for this kind of regional study. The isostatic residual gravity anomalies show that the width of non-isostatically- compensated region of the mature subduction zone is substantially wider than that of immature ones. Moreover, when we removed the gravitational effects due to the seafloor from the free-air gravity anomaly, a large difference was found between the immature and mature subduction zones in the overriding plate side. In the mature subduction zones, a low gravity anomaly of ~200-250 mGals can be found in the overriding plate which differs from the immature subduction zones. We discuss the possible causes of the low gravity anomaly including: (1) serpentinization in the upper mantle; (2) presence of partial melt in the mantle wedge; (3) difference in the density structure between the overriding and subducting plates in terms of slab age and cooling history; and (4) thickened crust or anomalous crustal structure beneath the arc. Serpentinization cannot explain the low gravity anomaly at ~150-200 km from the trench. Also, the difference of gravity anomaly due to the difference of the slab age is insufficient to account for the total anomaly. In this study, we explore various causes of low-gravity-anomaly development associated with the evolution of subduction zones systematically with special attention on the dynamic topography and thermal models.

  11. Trench CCD image sensor

    Microsoft Academic Search

    T. Yamada; A. Fukumoto

    1989-01-01

    The authors describe and present simulation data on the device structure, process flow, and operation of the Trench CCD (charge coupled device), which is being developed to increase the resolution of solid-state image sensors. The device provides larger dynamic range, higher sensitivity, and no image lag together with great packing density. A charge transfer channel formed around a trench eliminates

  12. Earth Planets Space, 62, 665673, 2010 Construction of semi-dynamic model of subduction zone

    E-print Network

    Tackley, Paul J.

    , and we find that the effect of Earth curvature is small by comparing our model with a similar oneEarth Planets Space, 62, 665­673, 2010 Construction of semi-dynamic model of subduction zone of subduction, such as the change in plate boundaries and trench curvature with time (Schellart et al., 2007

  13. 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 (?, shear stress/normal stress) at all investigated velocities of 0.04 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.

  14. 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.

  15. Hazard Alert: Trenches

    MedlinePLUS

    ... Construction Chart Book, p. 39. CPWR. 2008. HAZARD ALERT Find out more about safe work in trenches: • ... about construction hazards. Get more of these Hazard Alert cards – and cards on other topics. Call 301- ...

  16. 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 ?15N 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 ?15NAir (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.

  17. Ancient subduction zone in Sakhalin Island

    NASA Astrophysics Data System (ADS)

    Rodnikov, A. G.; Sergeyeva, N. A.; Zabarinskaya, L. P.

    2013-07-01

    The northern part of Sakhalin Island is an area of recent intensive tectonic movements and hydrothermal processes, as well as a place of accumulation of useful minerals. The deep structure of the lithosphere beneath the region of the Neftegorsk earthquake of May 27, 1995 in North Sakhalin, which killed residents and caused significant destruction, is examined in this paper. Our geodynamic model shows that North Sakhalin consists of the North Sakhalin Basin, Deryugin Basin and an ophiolite complex located between them. The Deryugin Basin was formed in place of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin was formed on the side of the back-arc basin at that time. The ophiolite complex is fixed in the position of ancient subduction zone that was active in the Late Cretaceous-Paleogene. Approximately in the Miocene, the subduction of the Okhotsk lithosphere apparently ceased. The remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data. On the surface, the subduction zone is manifested as deep faults stretched along Sakhalin. It is probable that the Neftegorsk earthquake was a result of activation of this ancient subduction zone.

  18. Variable Rupture Mode at Subduction Zones Around the Pacific

    NASA Astrophysics Data System (ADS)

    Satake, K.

    2005-12-01

    The enormity of the 2004 Sumatra-Andaman earthquake, in comparison with 19th- and 20th-century earthquakes in its rupture area, serves as a reminder that a subduction zone may produce earthquakes larger than those in recorded in the past. Historical record and paleoseismological data show that variability in rupture mode is characteristic of some subduction zones. Infrequent, gigantic earthquakes predominate in geologic records, while historic data tell of more frequent, smaller earthquakes. This implies that along the Cascadia subduction zone, great (M > 8) earthquake can occur more frequently than estimated from paleoseismological record. Like the 2004 Sumatra-Andaman earthquake, the giant 1960 Chilean earthquake (Mw 9.5) was unusually large. Historical predecessors of the 1960 earthquake occurred in 1837, 1737, and 1575. However, midway along the 1960 rupture, only the 1575 event produced geologic records of subsidence and tsunami as obvious as those of 1960. The 1837 and 1737 ruptures were probably small, at least at this latitude (Cisternas et al., 2005). Along the Nankai trough of southwest Japan, recurrence of semi-regular earthquakes has been documented in the 1300 years' written history, with an indication of some variability. The easternmost Suruga trough was ruptured in 1854 but not in 1944, leaving a seismic gap for the anticipated Tokai earthquake. The 1707 earthquake ruptured both Nankai and Tokai sources that ruptured separately in 1854 and in 1944 and 1946. The 1605 earthquake seems to be an unusual tsunami earthquake. Near Tokyo, along the Sagami trough, historical records and marine terraces show two types of large earthquakes (1923 type and 1703 type; Shishikura, 2003); their average recurrence intervals are estimated geologically as several hundred years and a few thousand years, respectively. Earthquakes larger than Mw 8.2 can happen along the southern Kuril trench even though they are unknown from the 200-year written history of Hokkaido. Plate-boundary earthquakes close to M 8, at intervals of 100 years or less, had been considered characteristic in this subduction zone. The 2003 Tokachi-oki earthquake (M 8.0), for instance, was preceded by similar earthquakes, from slightly different source areas, in 1952 and 1843. However, tsunami deposits show that unusually large tsunamis repeated at intervals averaging about 500 yr, with the most recent event in the 17th century (Hirakawa et al., 2000; Nanayama et al., 2003). The inferred inundation area is much wider than those typical earthquakes, and is best explained by earthquakes that broke more than one of the historical segments. Only these multi-segment earthquakes triggered deep postseismic creep that produced decimeters of coastal uplift (Sawai et al., 2004).

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

    NASA Astrophysics Data System (ADS)

    Evangelidis, Christos

    2015-04-01

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

  20. Stress, triggered earthquakes, and modelled tsunamis on the Sunda Trench

    NASA Astrophysics Data System (ADS)

    Steacy, S.; Nalbant, S.; McCloskey, J.; Antonioli, A.; Piatenesi, A.; Sieh, K.; Cocco, M.

    2007-12-01

    The current earthquake sequence off the coast of Sumatra illustrates both the potential, and the difficulties, of using stress based models to assess the likely location of future large earthquakes. Following the 26 December 2004 event, we showed that stress had increased on the Sumatra Fault as well as further south along the subduction zone, and published concerns that both structures had the potential to experience large triggered earthquakes. The 28 March 2005 event occurred in the area of concern on the Sunda Trench although the Coulomb stress changes at the hypocenter were only on the order to 0.1 bar. The 2005 earthquake, in turn, increased stress along a large extent of the Sumatra Fault as well as further south on the subduction zone. The latter is of particular concern as paleogeodetic data show that the last large event under the northern Mentawai Islands was in 1797, while to the south the trench last ruptured in 1833. In order to assess the tsunami hazard in the region we modelled 100 possible earthquakes, and their tsunamigenic potential, along the subduction zone; all models had the common feature that they involved slip under Siberut Island in the northern Mentawais. On 12 September 2007, an M=8.4 earthquake occurred along the Sunda Trench. The hypocenter was south of the Mentawai islands and its location had not experienced a significant stress increase from the 2005 event. The earthquake re-ruptured a portion of the trench that failed in 1833 but did not propagate as far north as Siberut Island. Here we present an updated view of the Coulomb stresses along the Sunda Trench and discuss the potential for a further large event under Siberut Island; this area has now been loaded from both the north and the south. Additionally, we show results for modelled tsunamis resulting from possible future earthquakes in this region.

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

  2. 4-D Subduction Models Incorporating an Upper Plate

    NASA Astrophysics Data System (ADS)

    Stegman, D.; Capitanio, F. A.; Moresi, L.; Mueller, D.; Clark, S.

    2007-12-01

    Thus far, relatively simplistic models of free subduction have been employed in which the trench and plate kinematics are emergent features completely driven by the negative buoyancy of the slab. This has allowed us to build a fundamental understanding of subduction processes such as the kinematics of subduction zones, the strength of slabs, and mantle flow-plate coupling. Additionaly, these efforts have helped to develop appreciable insight into subduction processes when considering the energetics of subduction, in particular how energy is dissipated in various parts of the system such as generating mantle flow and bending the plate. We are now in a position to build upon this knowledge and shift our focus towards the dynamic controls of deformation in the upper plate (vertical motions, extension, shortening, and dynamic topography). Here, the state of stress in the overriding plate is the product of the delicate balance of large tectonic forces in a highly-coupled system, and must therefore include all components of the system: the subducting plate, the overriding plate, and the underlying mantle flow which couples everything together. We will present some initial results of the fully dynamic 3-D models of free subduction which incorporate an overriding plate and systematically investigate how variations in the style and strength of subduction are expressed by the tectonics of the overriding plate. Deformation is driven in the overriding plate by the forces generated from the subducting plate and the type of boundary condition on the non-subducting side of the overriding plate (either fixed or free). Ultimately, these new models will help to address a range of issues: how the overriding plate influences the plate and trench kinematics; the formation and evolution of back-arc basins; the variation of tractions on the base of the overriding plate; the nature of forces which drive plates; and the dynamics controls on seismic coupling at the plate boundary.

  3. 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.

  4. Mega-asperity model for the 2011 Tohoku-Oki earthquake, Japan

    NASA Astrophysics Data System (ADS)

    Kumagai, H.; Pulido Hernandez, N. E.; Fukuyama, E.; Aoi, S.

    2011-12-01

    We performed moment tensor inversion of the 2011 Mw 9.0 Tohoku-Oki earthquake using regional seismic waveforms, which indicates that the earthquake can be approximated by a point source. We used strong-motion seismograms from a nationwide broadband seismic network of the F-net installed and operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). At each F-net station, a velocity-type strong-motion seismometer is installed at the end of a 30-50 m long vault to record strong motions up to +-2 m/s. We selected five F-net stations with the hypocentral distances more than several hundred kilometers. We used a waveform inversion method that assumes a double-couple focal mechanism for a point source [Nakano et al., 2008, GJI]. Our inversion using the strong-motion seismograms band-pass filtered between 0.006 and 0.02 Hz indicates that the best-fit source location was obtained at 38.25 N, 143.25 E at a depth of 30 km, at which the normalized residual was 0.13. The estimated mechanism and moment magnitude (Mw = 9.0) are consistent with those estimated from global broadband seismographic network data by the global CMT project and the National Earthquake Information Center (NEIC) of the U.S. Geological Survey. We fitted the omega-square model to amplitude spectra of observed vertical displacement seismograms, which resulted in the corner frequency of around 0.017 Hz. This implies average slip of roughly 70 m over a circular fault with radius of 70 km. These results suggest that a strongly localized asperity (mega asperity) was ruptured during the earthquake. For the northeastern Japan subduction zone, where the old Pacific Sea plate subducts along the Japan Trench, the seismic coupling was believed to be very small. The occurrence of the mega earthquake is apparently not compatible with weak seismic coupling. However, as suggested by Scholz and Small [1997, Geology], the subduction of a large seamount increases the normal stress across the subduction interface, which produces local seismic coupling in decoupled plates. If we consider a subducted seamount as the mega asperity at the source of the Tohoku-Oki earthquake, the occurrence of the mega earthquake in the decoupled plates is reasonably explained. Asano et al. [2011, EPS] indicated that all the CMT solutions of aftershocks above the source region of the Tohoku-Oki earthquake were characterized by normal faulting. A similar aftershock pattern was observed during the 1994 Java earthquake (Mw7.6). This earthquake occurred in a weakly coupled plate interface and its large slip area was coincident with a well identified seamount in this region. We can find a velocity anomaly near the source region of the Tohoku-Oki earthquake in the velocity model of Miura et al. [2005, Tectonophyics], which was obtained from a seismic line experiment off-Miyagi perpendicular to the trench. This velocity anomaly may be related with the subduction process of a seamount at this region. Further extensive refraction and reflection seismic surveys along the Japan Trench would contribute to verify our mega-asperity model and to evaluate seismic and tsunami hazards along the Japan Trench.

  5. Shear stresses on megathrusts: Implications for mountain building behind subduction zones

    Microsoft Academic Search

    Simon Lamb

    2006-01-01

    Shear stresses ? on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia

  6. Shear stresses on megathrusts: Implications for mountain building behind subduction zones

    Microsoft Academic Search

    Simon Lamb

    2006-01-01

    Shear stresses tau on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia

  7. Significant foreshock activities of M>7.5 earthquakes in the Kuril subduction zone

    NASA Astrophysics Data System (ADS)

    Harada, T.; Yokoi, S.; Satake, K.

    2014-12-01

    In the Kuril subduction zone, some M>7.5 earthquakes are accompanied by significant foreshock activities, providing a good opportunity to understand the characteristics of foreshocks for large interplate events such as occur along the Japan Trench and Nankai Trough etc. Some preliminary results from our examination of the foreshock sequences are as follows. Relocated foreshocks tend to migrate with time toward the trench axis. Foreshock distributions of the interplate earthquakes do not overlap with the large coseismic slips (asperities) of the mainshocks. Foreshocks of the 2007 northern Kuril outer-rise event, however, were distributed on the entire rupture area. Foreshock sequences seem to be limited in the regions where the background seismicity rates are relatively high. The foreshock activities were found in the examination of the space-time pattern of M>7 events along the northern Japan to Kuril trench since 1913 (e.g. Harada, Satake, and Ishibashi, 2011:AGU, 2012:AOGS). The large earthquakes preceded by active foreshock sequences are: the 2006 (M8.3), 2007 (M8.1) offshore Simushir earthquakes, the 1963 (M8.5), 1991 (M7.6), 1995 (M7.9) offshore Urup events, the 1978 (M7.8) offshore Iturup events, the 1969 (M8.2) offshore Shikotan event. In contrast, M>7.5 interplate earthquakes offshore Hokkaido (1952 (M8.1), 1973 (M7.8), 2003 (M8.1)) and intraslab earthquakes (1958 (M8.3), 1978 (M7.8), 1993 (M7.6), 1994 (M8.3)) had few or no foreshocks. In the examination of the active foreshocks, we relocated foreshocks by the Modified JHD method (Hurukawa, 1995), compared relocated foreshock areas with mainshock coseismic slip distributions estimated by the teleseismic body-wave inversion (Kikuchi and Kanamori, 2003), and examined the relation between active foreshock sequences and regional background seismicity. This study was supported by the MEXT's "New disaster mitigation research project on Mega thrust earthquakes around Nankai/Ryukyu subduction zones".

  8. Trench-parallel anisotropy produced by foundering of arc lower crust.

    PubMed

    Behn, Mark D; Hirth, Greg; Kelemen, Peter B

    2007-07-01

    Many volcanic arcs display fast seismic shear-wave velocities parallel to the strike of the trench. This pattern of anisotropy is inconsistent with simple models of corner flow in the mantle wedge. Although several models, including slab rollback, oblique subduction, and deformation of water-rich olivine, have been proposed to explain trench-parallel anisotropy, none of these mechanisms are consistent with all observations. Instead, small-scale convection driven by the foundering of dense arc lower crust provides an explanation for the trench-parallel anisotropy, even in settings with orthogonal convergence and no slab rollback. PMID:17615354

  9. '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.

  10. Evolution and diversity of subduction zones controlled by slab width.

    PubMed

    Schellart, W P; Freeman, J; Stegman, D R; Moresi, L; May, D

    2007-03-15

    Subducting slabs provide the main driving force for plate motion and flow in the Earth's mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250-7,400 km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics-the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges. PMID:17361181

  11. LWD lithostratigraphy, physical properties and correlations across tectonic domains at the NanTroSEIZE drilling transect, Nankai Trough subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Tudge, J.; Webb, S. I.; Tobin, H. J.

    2013-12-01

    Since 2007 the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) has drilled a total of 15 sites across the Nankai Trough subduction zone, including two sites on the incoming sediments of the Philippine Sea plate (PSP). Logging-while-drilling (LWD) data was acquired at 11 of these sites encompassing the forearc Kumano Basin, upper accretionary prism, toe region and input sites. Each of these tectonic domains is investigated for changes in physical properties and LWD characteristics, and this work fully integrates a large data set acquired over multiple years and IODP expeditions, most recently Expedition 338. Using the available logging-while-drilling data, primarily consisting of gamma ray, resistivity and sonic velocity, a log-based lithostratigraphy is developed at each site and integrated with the core, across the entire NanTroSEIZE transect. In addition to simple LWD characterization, the use of Iterative Non-hierarchical Cluster Analysis (INCA) on the sites with the full suite of LWD data clearly differentiates the unaltered forearc and slope basin sediments from the deformed sediments of the accretionary prism, suggesting the LWD is susceptible to the subtle changes in the physical properties between the tectonic domains. This differentiation is used to guide the development of tectonic-domain specific physical properties relationships. One of the most important physical property relationships between is the p-wave velocity and porosity. To fully characterize the character and properties of each tectonic domain we develop new velocity-porosity relationships for each domain found across the NanTroSEIZE transect. This allows the porosity of each domain to be characterized on the seismic scale and the resulting implications for porosity and pore pressure estimates across the plate interface fault zone.

  12. All-Pass Receiver Function Imaging Of Seismic Discontinuities In The Crust And The Uppermost Mantle At The Japan Subduction Zone

    NASA Astrophysics Data System (ADS)

    Hirahara, K.; Tonegawa, T.; Shibutani, T.

    2005-12-01

    We show our recent advancement in receiver function (RF) analyses on the detailed structures of seismic discontinuities in the crust and the uppermost mantle beneath the Japan Islands, based on a large number of teleseismic waveforms which have been obtained mainly at short-period (Hi-net and J-array) seismic stations densely installed over the Japan Islands, whose number is reaching 1,000. In addition to the usual RF analyses, we apply all-pass RF analysis which has been proposed by Satoh (2005) for enhancement of P-to-S phases converted at the sediment/basement boundary in strong motion records. In our method modified from the original Satoh_fs approach, the usual RF with water level in the spectral domain is decomposed to minimum-phase-shift and all-pass functions, and a Gaussian low-pass filtering is applied to the all-pass part, then back to the time domain, which we call all-pass RF. All-pass RF has a linear phase, which reserves purely the Ps-P time, implying all-pass RF has a clear peak at Ps-P time. In comparison with the usual RF, Ps phases at the Moho are generally enhanced in all-pass RF. We have executed depth-migration of RFs assuming 1-D IASPEI or 3-D velocity models obtained by travel time tomography (Nakamura et al., 2000), and have obtained CDP stacked RF images along several profiles. We have applied SVD (singular value decomposition) filtering (Chevrot and Giardin, 2000) to RFs or depth-migrated RFs. In this study, we applied 1 Hz low-pass filtering. Then, we obtain the depth distribution of the Moho over the Japan Islands by careful examination of the constructed RF profiles and also by the method of Zhu and Kanamori (2000). In contrast to the Moho Ps phases, enhancement of later phases such as plate-related, 410 and 660 km discontinuities dose not seem to be effective in all-pass RF. Accordingly, we show both profiles to see the deeper discontinuities.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  15. Subslab seismic anisotropy and mantle flow at the SW edge of the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The SW edge of the Ryukyu subdution zone is at the subduction-collision junction of the Phillipine plate and the Eurasin lithosphere. Previous splitting results have shown a large scale trench-parallel flow in the subslab mantle of the Ryukyu subduction zone, but how these mantle flow interact with the Eurasin lithosphere mantle remains unknown. We present the shear wave splitting measurements at the SW edge of the Ryukyu subduction zone for SKS coming from remote events and S from slab events received by Australian stations, both sampling the mantle behind the Ryukyu slab. Splittings at Australian stations were corrected. Delay times for S, ranging from 1 to 4.5 s, are larger than that for SKS averaged on about 1.1 s. Fast directions to first order parallel the trench, being consistent with previous observations. A more complicated pattern is seen toward the SW edge of the Rykyu subduction zone, where the fast direction is transitional rotating from trench-parallel to trench-oblique/-perpendicular,. The pattern at the western boundary of measurement reflects the fabric of the orogenic belt of Taiwan, while the rotation away from trench-parallel occurs distant from the Eurasian lithosphere, indicating its mantle dynamic control. Synthetic waveforms for shear-wave splitting is employed to interpret how the subslab asthenosphere deform along with the subduction to shape the observed pattern.

  16. Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling and application to the northern

    E-print Network

    ten Brink, Uri S.

    Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling 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

  17. Thermal and metamorphic environment of subduction zone episodic tremor and slip

    Microsoft Academic Search

    Simon M. Peacock

    2009-01-01

    Episodic tremor and slip (ETS) have been detected in the Cascadia and southwest Japan subduction zones, where the subducting crust is relatively warm because of the young incoming lithosphere (<20 Ma) and modest plate convergence rates (?40–60 mm\\/a). In the southwest Japan subduction zone, low-frequency earthquakes occur on the plate interface at depths of 30–35 km beneath Shikoku where finite

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  19. Subduction Zone Processes and Implications for Changing Composition of the Upper and Lower Mantle

    NASA Astrophysics Data System (ADS)

    Morris, J. D.; Ryan, J. G.

    2003-12-01

    With ca. forty thousand kilometers of subduction zones and convergence rates from 30 km Ma-1 to 180 km Ma-1, subduction carries massive amounts of material into seafloor trenches, and beyond. Most of the subducting plate is made of mantle material returning to the depths from which it originated. The hydrated and altered upper oceanic section and the overlying sediments, however, carry a record of low-temperature interaction with the ocean, atmosphere, and continents. Subduction and recycling of these components into the mantle has the potential to change mantle composition in terms of volatile contents, heat-producing elements, radiogenic isotope systematics, and trace element abundances. Enrichments in volatile and potassium, uranium, and thorium contents could change the rheological, thermal, and geodynamical behavior of portions of the mantle. Changing isotope and trace-element systematics provide a means for tracking mantle mixing and the possible subduction modification of the deep mantle. A large number of studies point to possible contributions of subducted sediments and altered oceanic crust (AOC) to the mantle-source region for enriched mantle II (EMII) and high mu (HiMU) enriched oceanic island basalts. Transit through the subduction zone, however, changes the composition of the subducting sediment and AOC from that measured outboard of trenches.This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers: boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic 10Be, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments.The focus here is on subduction processes from trench to rear arc. This chapter starts with a brief discussion of recent thermal models for the downgoing plate and the prograde metamorphic mineralogy of the oceanic crust and sedimentary veneer; the reader is referred to Schmidt and Poli (Chapter 3.17), for an extensive discussion. In the next step it uses 10Be to estimate the absolute mass of sediments subducted to the volcanic arc, in comparison to that supplied to the subduction trenches. Flux balances for 10Be subducted in the sediments versus that erupted in the volcanic arc provide estimates of the fraction of 10Be extracted from the downgoing plate, which can be extrapolated to other elements (cf. Plank and Langmuir, 1993). It subsequently looks at chemical changes for selected elements across the subduction zone, using data from fore-arc serpentinite mud volcanoes, subduction-assemblage metamorphic rocks, high-pressure eclogites, and volcanic lavas from Kurile cross-arc transects, and examines boron-isotope systematics across the convergent margin. Lithium-isotope systematics and comparison of 10Be with uranium-series systematics sometimes delineate multiple stages of subduction modification of the mantle and pinpoint the compositional effects of prior subduction modification on the upper mantle. This contribution ends with estimates of the efficiency of arsenic, antimony, potassium, caesium, rubidium, barium, strontium, uranium, thorium, lead, cerium, samarium, neodymium, lutetium, and hafnium recycling from trench to rear arc, relative to that of boron and beryllium.2.11.2. Thermal Structure and Mineralogy of The Subducting PlateCentral to understanding the recycling of subducted elements in the arc or their subduction to the deep mantle is the temperature variation in the subducting slab, and the prograde mineral assemblages in the sediment, oceanic crust, and lithospheric mantle. Together, they determine where dehydration of the sediments, crust, and deeper subducting mantle occurs, a

  20. Stress and Strength of Seismogenic and Creeping Subduction Faults (Invited)

    NASA Astrophysics Data System (ADS)

    Wang, K.; Bilek, S. L.; Wada, I.; Gao, X.; Brown, L.

    2013-12-01

    Force balance studies of subduction zone forearcs constrained by earthquake focal mechanisms, active faulting, and topography suggest very weak subduction megathrusts. If represented by an effective coefficient of friction ?', the ratio of shear to normal stress at failure, the average ?' value of most megathrusts is about 0.03, seldom exceeding 0.06, an order of magnitude lower than fault strengths predicted by the Byerlee's law with hydrostatic pore fluid pressure. The ?' value required to explain heat flow observations using megathrust frictional heating modeling is usually also about 0.03, regardless of whether the megathrust is seismogenic or creeping. The mechanism for the weakness is not fully understood, although it must be a combined consequence of fault zone material, fault zone fabric, and pore fluid pressure. Prior to March 11, 2011, the Japan Trench was a rare exception where pervasive margin-normal compression of the upper plate made it difficult to infer megathrust strength. But wholesale stress reversal in much of the forearc due to the M 9 Tohoku earthquake dramatically verified the low-strength (?' = 0.03) prediction of Wang and Suyehiro (1999, GRL 26(35), 2307-2310). This value translates to depth-dependant shear strength of roughly 10 MPa at 10 km and 30 MPa at 30 km. With regard to how fault strength and stress affect earthquake processes, several issues deserve special attention. (1) There is little doubt that no megathrust is 'strongly' locked, but creeping megathrusts can be either weaker or stronger than locked faults. In fact, subduction of extremely rugged seafloor causes creeping, despite strong resistance caused by geometrical incompatibilities. Physical meanings of regarding locked and creeping faults as 'strongly coupled' and 'weakly coupled', respectively, are in serious question. (2) A ?' value of 0.03-0.05 is a spatial average. For a smooth fault, even small changes in pore fluid pressure alone can cause local deviations from this average by a factor of two or three. Locally high locking stresses and/or greater coseismic stress drops along smooth faults are by no means surprising and are not indicative of subducted topographic features such as seamounts. (3) Given the extremely low background strength and stress, the application of laboratory-observed high-rate (or dynamic) weakening of frictional contacts to natural subduction faults requires much careful thinking. In the lab, the contact is usually weakened from a static strength of ?' ~ 0.6 to dynamic values ~ 0.1. No natural subduction faults have an average static strength of 0.6, not even 0.1. Therefore such weakening can only take place on small fault patches of locally high strength and stress. If high-strength patches have ?' values much greater than 0.1 (to allow weakening to 0.1), most parts of the locked megathrust must have ?' values less than 0.03 (to yield an average ?' of 0.03).

  1. Topography of the 410 and 660 km discontinuities beneath the Korean Peninsula and southwestern Japan using teleseismic receiver functions

    NASA Astrophysics Data System (ADS)

    Lee, Sang-Hyun; Rhie, Junkee; Park, Yongcheol; Kim, Kwang-Hee

    2014-09-01

    Topography of the 410 and 660 km seismic upper mantle discontinuities beneath the Korean Peninsula and southwestern Japan were determined using teleseismic receiver functions. P receiver functions were migrated from delayed times to corresponding piercing (conversion) points of P-to-S converted phases, using one-dimensional (1-D) and three-dimensional (3-D) models. Receiver functions were then stacked using Common Conversion Point (CCP) techniques, to enhance signal-to-noise ratios and thereby reduce uncertainty (noise). The 410 and 660 km discontinuities were clearly imaged, as positively valued amplitude peaks of CCP stacked receiver functions in the study area. Topographic variations were roughly consistent with the low temperature of the subducting Pacific Plate. However, the complex structure of the subducting Pacific Plate produced distinct changes of upper mantle discontinuities, which cannot be explained by temperature variations alone. Depression of the 410 km discontinuity, observed in a wide region extending from the Korean Peninsula to Kyushu Island, may be related to trench rollback history. Furthermore, the topography of the 660 km discontinuity varies significantly with latitude. At latitudes higher than 38°N, its depth remains unchanged, despite the presence of the stagnant slab, while significant depression has been observed at latitudes below 36°N. This may have been caused by differences in the angles of subduction of the Japan slab and the Izu-Bonin slab. However, heterogeneity of the water content of slabs may also have contributed to this topographical difference.

  2. 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.

  3. Trench migration and upper plate strain over a convecting mantle

    NASA Astrophysics Data System (ADS)

    Husson, Laurent

    2012-12-01

    Trench motion and upper plate deformation ultimately respond to mantle flow. Herein I build upon the mantle flow model results of Conrad and Behn (2010) and compute the drag forces underneath all plates, and show that they control the dynamics of plates and plate boundaries. The small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics. Less intuitive is the fact that the interplay between the drag forces from the upper and lower plates, that amounts to -5 to 8.5 × 1012 N m-1 (per unit trench length), dictates both trench migration rates and upper plate deformation. At odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones (slab age, interplate friction, water content etc), I find that the intrinsic properties of subduction zones in fact only modulate this behavior. More specifically, the mean value of the integrated trenchward mantle drag force from the lower and upper plates (from -2 to 6.5 × 1012 N m-1) controls upper plate deformation. Conversely, it is the difference between the lower and upper plates mantle drag forces (from -3 to 10 × 1012 N m-1) that controls trench migration rates. In addition, I find that a minimum trenchward force of ˜2.5 × 1012 N m-1 must be supplied by mantle drag before trenches can actually advance, and before upper plates undergo compression. This force results from the default tendency of slabs to rollback when solely excited by their own buoyancy, and is thus the effective tensional force that slab pull exerts on the plate interface.

  4. Trench migration and upper plate strain over a convecting mantle

    NASA Astrophysics Data System (ADS)

    Husson, Laurent

    2013-04-01

    Trench motion and upper plate deformation ultimately respond to mantle flow. Herein I build upon the mantle flow model results of Conrad and Behn (2010) and compute the drag forces underneath all plates, and show that they control the dynamics of plates and plate boundaries. The small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics. Less intuitive is the fact that the interplay between the drag forces from the upper and lower plates, that amounts to -5 to 8.5 TN/m (per unit trench length), dictates both trench migration rates and upper plate deformation. At odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones (slab age, interplate friction, water content etc), I find that the intrinsic properties of subduction zones in fact only modulate this behavior. More specifically, the mean value of the integrated trenchward mantle drag force from the lower and upper plates (from -2 to 6.5 TN/m) controls upper plate deformation. Conversely, it is the difference between the lower and upper plates mantle drag forces (from -3 to 10 TN/m) that controls trench migration rates. In addition, I find that a minimum trenchward force of ~2.5 TN/m must be supplied by mantle drag before trenches can actually advance, and before upper plates undergo compression. This force results from the default tendency of slabs to rollback when solely excited by their own buoyancy, and is thus the effective tensional force that slab pull exerts on the plate interface.

  5. Dynamics of sediment subduction, melange formation, and prism accretion

    NASA Astrophysics Data System (ADS)

    Shreve, Ronald L.; Cloos, Mark

    1986-09-01

    The descending plate and overriding block in a subduction zone are analogous to the guide surface and slide block in a slipper bearing, and subducted sediment is analogous to the lubricant. Subduction is more complex and varied, however, because the overriding block is not rigid, the sediment is buoyant, underplating can occur, and sediment supply can vary widely. A model based on the bearing analogy but taking these differences into account makes detailed quantitative predictions for actual sites, which are illustrated by calculations for five diverse examples: Mariana, 16°N; Mexico, 17°N; Lesser Antilles, 13°N (Barbados); Alaska, 153°W (Kodiak); and Japan, 40°N. It requires as input the geometry of the overriding block and the top of the descending plate, the distribution of density and permeability of the overriding block, the speed of subduction, the density and rheological properties of the subducted sediment, and the rate of sediment input. Its predictions include the profile of thickness of the layer of subducted sediment (all sites; maximum of 360 m at Mariana, 5300 m at Japan), the velocities of flow in the layer (all sites), the shear stresses exerted on the walls (all sites; low beneath accretionary prisms, up to 6 MPa beneath Japan), the rate of offscraping (none at Mariana and late Tertiary Mexico; 85% of input at Lesser Antilles; includes melange at Japan), the distribution and rates of underplating (none at Mariana, extensive at Japan), the zones of possible subduction erosion (extensive at Mariana; local at the others), the amount of sediment subducted to the volcanic arc (all sites; 2% of input at Lesser Antilles, 100% at Mariana), the qualitative pattern of flow at the inlet (five basic patterns; all sites), the upward flow of melange in many instances (none at Mariana; extensive at Japan), and, under relatively rare conditions, the formation of large-scale melange diapirs (only at Lesser Antilles beneath Barbados Island).

  6. Evolution of texture and seismic anisotropy during subduction (Invited)

    NASA Astrophysics Data System (ADS)

    Di Leo, J. F.; Walker, A.; Li, Z.; Wookey, J. M.; Ribe, N. M.; Kendall, J. M.; Tommasi, A.

    2013-12-01

    Shear wave splitting fast directions are routinely used to infer upper mantle flow direction, based on the premise that olivine develops strain-induced lattice-preferred orientation (LPO) textures in the convecting mantle. However, such inferences ought to be made with caution, since the relationship between olivine LPO and splitting fast direction depends on many factors, including the entire deformation history of the volume of mantle in question. This is especially the case in regions where complex time-dependent mantle flow is expected, such as subduction zones. Here, we present an integrated model to simulate strain-history-dependent LPO development and estimate the resulting shear wave splitting in a subduction setting. We do this for a mantle flow model that approximates the geometry of the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case, and compare the results to recent shear wave splitting measurements of this region by Di Leo et al. (2012). As the subduction system is fairly young, early textures from the slab's descent from the near-surface to the bottom of the mantle transition zone (which we are able to simulate in our models) have not yet been overprinted by subsequent continuous steady-state flow. It also allows us to test the significance of the double-sided geometry, or, more generally, the need for a rear barrier to achieve trench-parallel sub-slab mantle flow. We simulate olivine LPO evolution caused by deformation of polycrystalline aggregates as they deform and move along pathlines extracted from a 3-D mantle flow model. Interactions between crystals are described using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. After calculating elastic properties associated with LPO at multiple depths, we estimate the resulting splitting parameters (fast direction ?, delay time ?t) for synthetic SKS phases. Our models show that complex, backazimuth-dependent behavior in ? appears in even apparently simple models of subduction zone mantle flow. We also demonstrate that although a rear barrier amplifies trench-parallel sub-slab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions. In a simple model of A-type olivine LPO and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and deformation dominated by axial compression in the sub-slab mantle.

  7. Role of hydrated mantle in the production of trench-parallel shear wave splitting signals

    NASA Astrophysics Data System (ADS)

    Kaminski, E.; Okaya, D. A.

    2009-12-01

    Since the pioneering work of Russo and Silver (1994), seismic anisotropy has been used extensively to study the geometry of mantle flow in subduction zones. The development of models that explain the formation of Lattice Preferred Orientation (LPO) in deformed polycrystals of olivine now allows a quantitative comparison between seismic observations and predictions of different flow models. In some places, subduction zones are characterized by observed directions of shear wave anisotropy that are parallel with the trench direction. Because of different causes of LPO formation, such a trench-parallel signature can be due to either trench parallel flow or trench normal flow in a hydrated mantle. In order to choose which of these cases is most representative for a particular location, independent constraints are required, for example, those based on geochemistry. In this study, we explored in detail the implications of water in the development of LPO and its associated seismic anisotropy in subduction zones. Specifically, we compared the production of seismic anisotropy of dry trench parallel flow to hydrated trench normal flow. In particular, we take into account the effect of a progressive hydration of the mantle towards the trench and the resulting evolution of LPO. We constructed mantle wedge flow models and calculated the associated LPO fields using dense spatial gridding for subsequent seismic modeling. The intensity and geometrical orientations of the LPO as a function of flow geometry and hydration suggest that upcoming teleseismic waves will illuminate very heterogeneous LPO. The wedge LPO was mapped into a 3D earth volume that was used by an anisotropic 3D finite difference wave propagation code that is able to import full 21-element elastic tensors. Upcoming SV plane waves were propagated through the wedge LPO and the resulting synthetic seismograms were analyzed with commonly available shear wave splitting code (Menke & Levin, 2003). From this analysis, we show that progressive hydration of the mantle towards the trench within trench-normal flow has a specific LPO signature that is distinctive from that made by trench parallel flow. In addition, the long seismic wavelengths average out sharp LPO structure near the wedge tip. Due to that same averaging, we found that the presence of water is much less efficient that trench parallel flow to generate trench parallel anisotropy.

  8. Phoenix's La Mancha Trench

    NASA Technical Reports Server (NTRS)

    2008-01-01

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

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

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

  9. 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).

  10. 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

  11. 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.

  12. 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.

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

    NASA Astrophysics Data System (ADS)

    Faccenda, M.; Capitanio, F. A.

    2012-06-01

    The dynamics of subduction can be indirectly constrained by studying the induced mantle flow. However, inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive. We calculated the strain-induced lattice preferred orientation (LPO) developed in 3-D models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench-perpendicular, and becomes trench-parallel deeper, where the toroidal flow accommodates slab retreat. Synthetic SKS splitting shows that in the fore-arc slab rollback favors trench-parallel polarization of the fast shear wave component, while plate advance enhances trench-perpendicular seismic anisotropy. The interference between these two competing mechanisms yields subslab delay times of 0.5 to 1.3 sec, comparable with those observed at most natural subduction zones. The magnitude of the subslab trench-parallel splitting is independent of the rate at which the slab migrates, instead it is proportional to the amount of retreat.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. Sediment loading at the southern Chilean trench and its tectonic implications

    NASA Astrophysics Data System (ADS)

    Contreras-Reyes, Eduardo; Jara, Jorge; Maksymowicz, Andrei; Weinrebe, Wilhelm

    2013-05-01

    Non erosive margins are characterized by heavily sedimented trenches which obscure the morphological expression of the outer rise; a forebulge formed by the bending of the subducting oceanic lithosphere seaward of the trench. Depending on the flexural rigidity (D) of the oceanic lithosphere and the thickness of the trench sedimentary fill, sediment loading can affect the lithospheric downward deflection in the vicinity of the trench and hence the amount of sediment subducted. We used seismic and bathymetric data acquired off south central Chile, from which representative flexural rigidities are estimated and the downward deflection of the oceanic Nazca plate is studied. By flexural modeling we found that efficient sediment subduction preferentially occurs in weak oceanic lithosphere (low D), whereas wide accretionary prisms are usually formed in rigid oceanic lithosphere (high D). In addition, well developed forebulges in strong oceanic plates behaves as barrier to seaward transportation of turbidites, whereas the absence of a forebulge in weak oceanic plates facilitates seaward turbidite transportation for distances >200 km.

  19. 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.

  20. 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.

  1. Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

    USGS Publications Warehouse

    Gamage, S.S.N.; Umino, N.; Hasegawa, A.; Kirby, S.H.

    2009-01-01

    We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ???30?? at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28-32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the double-planed deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending-unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10-23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock. ?? 2009 The Authors, Journal compilation ?? 2009 RAS.

  2. Age spectra of detrital zircon of the Jurassic clastic rocks of the Mino-Tanba AC belt in SW Japan: Constraints to the provenance of the mid-Mesozoic trench in East Asia

    NASA Astrophysics Data System (ADS)

    Fujisaki, Wataru; Isozaki, Yukio; Maki, Kenshi; Sakata, Shuhei; Hirata, Takafumi; Maruyama, Shigenori

    2014-07-01

    U-Pb ages of detrital zircon grains were determined from an upper Middle Jurassic siliceous mudstone and two lower Upper Jurassic sandstones of the Mino-Tanba belt, Southwest Japan, by Laser-ablation ICPMS. The age spectra of detrital zircon grains of the three analyzed samples show multiple age clusters: 175-198 Ma (Early Jurassic), 202-284 Ma (Permian to Triassic), 336-431 Ma (Silurian to Carboniferous), and 1691-2657 Ma (Neoarchean to Paleoproterozoic). As per the Precambrian grains, the prominent peak exists around 1800-2000 Ma in all analyzed samples. The age clusters of 175-198 Ma, 202-284 Ma, and 336-431 Ma suggest that pre-Middle Jurassic Japan has exposed older granitic batholiths. The corresponding batholiths occur in the Cathaysian part of South China block. In contrast, the absence of them in modern Japan suggests that these batholiths were totally consumed by post-Jurassic tectonic erosion. The Neoarchean to Paleoproterozoic detrital zircon grains were derived from South China, North China, or possibly both of them; nonetheless, the circumstantial geologic lines of evidence point to South China, in particular to Cathaysia, rather than North China.

  3. 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.

  4. An oceanic plateau subduction offshore Eastern Java

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  5. Subduction-induced mantle flow, finite strain, and seismic anisotropy: Numerical modeling

    NASA Astrophysics Data System (ADS)

    Li, Zhong-Hai; Di Leo, Jeanette F.; Ribe, Neil M.

    2014-06-01

    Surface measurements of shear wave splitting patterns are widely used to infer the mantle circulation around subducting slabs; however, the relation between mantle flow and seismic anisotropy is still elusive. Finite strain is a direct measurement of time-dependent deformation and has been proposed as a proxy for the crystal-preferred orientation (CPO) of mantle minerals. We have conducted a series of numerical models to systematically investigate the mantle flow, finite strain, olivine CPO, and SKS wave splitting in oceanic subduction zones with variable slab width. They demonstrate that the preferred orientations of olivine a axes generally agree with the long (extensional) axes of the finite strain ellipsoid (FSE), even in these very complex mantle flow fields; however, neither the a axis nor the FSE axes necessarily aligns with the instantaneous mantle velocity vector. We identify two domains with distinct deformation mechanisms in the central subplate mantle, where simple shear induced by plate advance dominates at shallow depths and produces trench-normal fast splitting, while pure shear induced by slab rollback dominates the deeper mantle and results in trench-parallel fast splitting. The SKS splitting patterns are thus dependent on the competing effects of these two mechanisms and also on the subduction partition ratio ? = Xp/Xt: trench parallel when ?< 1 and trench normal when ?>1. In addition, different mantle deformation mechanisms and SKS splitting patterns are observed in the mantle wedge and around the slab edges, which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones.

  6. History of subduction and back-arc extension in the Central Mediterranean

    E-print Network

    Becker, Thorsten W.

    of the subduction process are the development of dynamic topo- graphy at the trench (e.g. Stern & Holt 1994; Royden 1993), deep burial of crustal rocks and the formation of the arc mag- matism (e.g. Jacob et al. 1976

  7. Accelerated subduction of the Pacific Plate after mega-thrust earthquakes: evidence from GPS and GRACE

    NASA Astrophysics Data System (ADS)

    Heki, K.; Mitsui, Y.; Matsuo, K.; Tanaka, Y.

    2012-12-01

    Interplate thrust earthquakes are often followed by afterslips (Heki et al., 1997; Miyazaki et al., 2004; Ozawa et al., 2012). They let the fore-arc move slowly trenchward and accelerate plate convergence. Accelerated convergence of the oceanic side (including ocean floor and slab) has been suggested by changes of focal mechanisms of earthquakes within oceanic plates after mega-thrust events, i.e. change from trench-normal compression to tension in outer rise regions, and from down-dip tension to compression in intermediate depths (Lay et al., 1989). However, landward acceleration of the oceanic plate has never been observed geodetically due to the scarcity of appropriate islands on oceanic plates near trenches. The westward velocity of GPS stations in NE Japan show gradient decreasing from east to west reflecting the E-W compressional stress built up by the inter-plate coupling. We found that such coupling significantly enhanced after the 2003 Tokachi-Oki earthquake (Mw8.0), Hokkaido, in the segments adjacent to the ruptured fault. The coupling was further enhanced after the 2011 Tohoku-Oki earthquake (Mw9.0). Movement of the ocean floor benchmark after the 2011 event suggests that the current (i.e. 2011-2012) subduction of the Pacific Plate is about three times as fast as the geological average, e.g. NUVEL-1 (DeMets et al., 1990). Such a temporary acceleration of the subduction would be a response of the subducting slab to the sudden decrease of interplate coupling (decoupling); because slab-pull and ridge-push cannot change, viscous traction has to increase to recover the force balance. We will present a simple physical model assuming a thin low-viscosity layer on the slab surface that has enabled such a rapid adjustment. The accelerated subduction would account for high regional interplate seismicity after mega-thrust earthquakes, especially successive ruptures of remote segments, e.g. the 2003 Tokachi-Oki, 2006 Kuril, and 2011 Tohoku-Oki earthquakes. GRACE satellite gravimetry revealed coseismic gravity drops in the back-arc regions due to the dilatation of island arc lithosphere for the 2004 Sumatra-Andaman (Han et al., 2005), 2010 Maule (Heki and Matsuo, 2010), and 2011 Tohoku-oki (Matsuo and Heki, 2011) earthquakes. Postseismic slow gravity increase centered in the fore-arc region was first found for the 2004 Sumatra-Andaman earthquake (Ogawa and Heki, 2007). Here we show that similar postseismic gravity increases also followed the other two mega-thrust earthquakes. We assume that they also reflect accelerating subduction of oceanic plates, i.e. episodic convergence at the boundary propagates into oceanic plate interior by stress diffusion (Bott and Dean, 1973), and postseismic thickening of the coseismically thinned lithosphere causes the on-going gravity increases.

  8. 'Dodo' and 'Baby Bear' Trenches

    NASA Technical Reports Server (NTRS)

    2008-01-01

    NASA's Phoenix Mars Lander's Surface Stereo Imager took this image on Sol 11 (June 5, 2008), the eleventh day after landing. It shows the trenches dug by Phoenix's Robotic Arm. The trench on the left is informally called 'Dodo' and was dug as a test. The trench on the right is informally called 'Baby Bear.' The sample dug from Baby Bear will be delivered to the Phoenix's Thermal and Evolved-Gas Analyzer, or TEGA. The Baby Bear trench is 9 centimeters (3.1 inches) wide and 4 centimeters (1.6 inches) deep.

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

  9. Seafloor Geodesy for Approaching Great Earthquakes Around Japan

    NASA Astrophysics Data System (ADS)

    Fujita, M.; Sato, M.; Ishikawa, T.; Watanabe, S. I.; Yokota, Y.

    2014-12-01

    Seafloor geodesy has been developed as an application of space geodetic technique for the purpose of investigating geodynamic phenomena having their major information in offshore regions. One of such targets is the occurrence of disastrous earthquakes in plate subduction areas. Japan, among others, has repeatedly experienced offshore megathrust earthquakes because of its tectonic location, where multiple plates interact with each other. Most recently, an earthquake of M9.0 occurred off the Pacific coast of east Japan in 2011 with a subsequent huge tsunami, which totally devastated coastal areas and claimed nearly 20,000 lives including those still missing. We, the group of Japan Coast Guard (JCG), have developed a seafloor geodetic technique combining the GPS positioning and underwater acoustic ranging, which is able to measure the position of the seafloor reference point consisting of multiple acoustic transponders with a precision of a few centimeters. We have deployed our seafloor reference points over two regions on the Pacific side of Japan; one is the region along the Japan trench off the eastern coast where the huge 2011 event occurred and another is the region along the Nankai Trough off the southern coast where earthquakes of around M8 have repeated every 100-150 years. With these measurements, we have so far successfully obtained important results providing exclusive information for elucidating the plate boundary behavior causing huge earthquakes. In particular, in the region off east Japan, we have revealed different phases of seafloor movements during the period between several years before and after the 2011 event. They include linear intraplate movements with several centimeters per year before the event, which were the first significant offshore geodetic signals detected around Japan, as well as the coseismic displacements of unprecedentedly huge amount over 20 m close to the epicenter and subsequent postseismic movements with various characteristics depending on the area. These seafloor data contribute to the understandings of physical processes relevant to the event beneath the seafloor in this region, which are unable to be clarified only by the terrestrial data. The presentation summarizes efforts in the field of seafloor geodesy in Japan with a focus on the results obtained by the JCG group.

  10. Total volume and latitudinal variations of the sedimentary trench-fill off Central and Southern Chile: A record of short and long-term geologic processes

    NASA Astrophysics Data System (ADS)

    Geersen, J.; Voelker, D.; Contreras Reyes, E.

    2012-12-01

    The amount and composition of trench sediment that is underthrust in a subduction zone impacts on the physical state of the shallow megathrust. Knowledge about the sedimentary and tectonic state and the geologic history of a trench system is therefore an important step towards understanding the structural and seismotectonic evolution of a convergent margin. The Chilean Trench south of 32°S is a sediment-filled basin that stretches in N-S direction. Variations in trench-fill are directly linked to and controlled by a number of sedimentary-tectonic processes and their interplay over space and time. In this work we unravel the individual processes for a ~1500 km long segment of the Chilean trench (32°S-46°S) and investigate their temporal and spatial significance. We use swath bathymetric and reflection seismic data to produce grids of the seafloor and the surface of the subducting oceanic plate buried beneath the trench sediments. The volume of the sedimentary trench-fill and its along-strike variations are derived from the difference between the two grids. Our results indicate that the volumetric sediment distribution can be best explained by high sediment input in the south and active northward sediment transport along the trench during glacial periods. However, during interglacial periods, small-scale factors that locally modify the sediment input seem to dominate. For the future along-strike variations in sediment volume will be compared with variations in structure and seismic behavior of the margin.

  11. Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

    NASA Astrophysics Data System (ADS)

    Anderson, Megan L.; Zandt, George; Triep, Enrique; Fouch, Matthew; Beck, Susan

    2004-12-01

    We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30°S and 36°S. Beneath the southern and northwestern parts of the network, fast polarization direction ($\\varphi$) is consistently trench-parallel, while in the northeast $\\varphi$ is trench-normal the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S $\\varphi$ observed in the south and northwest; E-W $\\varphi$ result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab.

  12. Seismicity of the Incoming Plate at the Mariana Trench Located Using an Ocean Bottom Seismograph Array

    NASA Astrophysics Data System (ADS)

    Jasperson, H.; Wiens, D. A.; Lizarralde, D.

    2014-12-01

    We locate earthquakes occurring near the Mariana Trench using an ocean bottom seismograph (OBS) array to study the role of incoming plate seismicity in hydrating the mantle as well as to constrain seismicity at the updip end of the Mariana subduction thrust. The array consisted of 20 broadband OBSs, and 5 temporary broadband seismographs deployed on nearby islands from February 2012 to February 2013. The OBS water depths were limited to 6 km or less, so they were deployed surrounding the trench and 5 hydrophones were deployed in the water column near the trench axis. Preliminary results indicate that seismicity in the incoming plate begins about 120 km west of the trench axis, with the highest seismicity levels found near the trench axis itself. Many of the incoming plate earthquakes locate near large fault scarps identified in seafloor bathymetry. In the forearc, Mariana shallow thrust seismicity begins where the plate interface is at 15-20km depth. There is a strong contrast in velocity structure across the trench, which causes difficulty in determining precise locations and depths. Further work will clarify the depths and focal mechanisms of the recorded earthquakes.

  13. Overriding plate thickness control on subducting slab curvature

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The curvature of subducting lithosphere controls deformation due to bending at the trench, which results in a force that dissipates gravitational potential energy and may affect seismic coupling. We use 2-D, thermo-mechanical subduction models to explore the dependence of the radius of curvature on the thickness of the subducting and overriding plates for models with both viscous and effectively plastic lithospheric rheologies. Such a plastic rheology has been shown to reproduce the bending stresses/moment computed using a kinematic strain rate description and a laboratory derived composite rheology. Laboratory and numerical models show that the bending geometry of subducting slabs with a viscous rheology is strongly dependent on slab thickness; thicker plates have a larger radius of curvature. However, the curvature of subducting plates on Earth, illuminated by the distribution of earthquake hypocenters, shows little to no dependence on the plate thickness or age. Such an observation is instead compatible with plates that have a plastic rheology. Indeed, our numerical models show that the radius of curvature of viscous plates has a stronger dependence on subducting plate thickness than in equivalent plastic models. In viscous plates, the bending moment produces a torque, which balances the torque exerted by buoyancy. However, for the plastic plate case the bending moment saturates at a maximum value and so cannot balance the gravitational torque. The saturation of bending moment means that, (a) the radius of curvature of the bending region is not constrained by this torque balance, and, (b) other forces are required to balance the gravitational torque. We explore the role that the overriding plate could play in controlling the subducting plate curvature in plastic plate models where the bending stresses have saturated. For such plates, we find that increasing the thickness of the overriding plate causes the radius of curvature to increase. The same correlation is found in real subduction zones when the radius of curvature is compared with near-trench overriding lithospheric thickness. We suggest that the thickness of the overriding plate, through controlling the depth extent of the slab suction caused by the strong overriding plate, exerts a primary control on the curvature of subducting lithosphere.

  14. Mid-Ocean Ridge Subduction Offshore Alaska During the Cretaceous

    NASA Astrophysics Data System (ADS)

    Sdrolias, M.; Müller, R. D.; Gaina, C.; Torsvik, T.

    2007-12-01

    We present a framework for the tectonic development of the Arctic region through a set of regional plate and ocean floor reconstructions since the early Cretaceous. In order to understand the effect of time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries on Arctic basin evolution and reactivation through time, we reconstruct now subducted ocean floor, including portions of tectonic plates which have now entirely vanished, and restore their plate boundary configurations and subduction history. We reconstruct paleo-oceans by creating "synthetic plates", the locations and geometry of which are established on the basis of magnetic lineations and fracture zones, geological data and the rules of plate tectonics. The absolute position of the Pacific Plate and its surrounding plates is restored using a Pacific hotspot reference frame, whereas all other plates are reconstructed based on an African-Indian hotspot reference system. This approach is required because the Pacific Plate was entirely surrounded by subduction zones in the Cretaceous, and therefore Pacific Ocean plates cannot be related to other tectonic plates via relative plate motions. Our reconstructions reveal that the Izanagi-Farallon spreading ridge was subducted underneath Alaska from about 120-100Ma. Prior to 120 Ma the northern portion of the Izagani-Farallon plate boundary was a convergent boundary according to our reconstructions, implying that between 140 and 120 Ma a subducting slab was overridden by the Alaskan North Slope and possibly other associated terranes. The Izanagi-Farallon subduction zone (before 120 Ma) and mid-ocean ridge (after 120 Ma) was oriented roughly orthogonal to the overriding plate. Trench subduction would have been associated with negative dynamic topography on the overriding plate, whereas an eastward migrating slab window underneath North Slope and its border terranes may have resulted in asthenospheric upwelling and extension. Mid-Cretaceous (Aptian to Santonian) rocks are missing over much of the Alaska Peninsula, presumably eroded, and the widespread absence of rocks of this age suggests uplift and erosion of the entire terrane during a portion of Aptian to Santonian time. These observations generally support our model, but the relative roles of trench and ridge subduction for causing the widespread regional erosion or for triggering the opening of the Canada basin remain open.

  15. Multiscale Architecture of a Subduction Complex and Insight into Large-scale Material Movement in Subduction Systems

    NASA Astrophysics Data System (ADS)

    Wakabayashi, J.

    2014-12-01

    The >1000 km by >100 km Franciscan complex of California records >100 Ma of subduction history that terminated with conversion to a transform margin. It affords an ideal natural laboratory to study the rock record of subduction-interface and related processes exhumed from 10-70 km. The Franciscan comprises coherent and block-in-matrix (mélange) units forming a nappe stack that youngs structurally downward in accretion age, indicating progressive subduction accretion. Gaps in accretion ages indicate periods of non-accretion or subduction erosion. The Franciscan comprises siliciclastic trench fill rocks, with lesser volcanic and pelagic rocks and serpentinite derived from the downgoing plate, as well as serpentinite and felsic-intermediate igneous blocks derived as detritus from the upper plate. The Franciscan records subduction, accretion, and metamorphism (including HP), spanning an extended period of subduction, rather than a single event superimposed on pre-formed stratigraphy. Melanges (serpentinite and siliciclastic matrix) with exotic blocks, that include high-grade metamorphic blocks, and felsic-intermediate igneous blocks from the upper plate, are mostly/entirely of sedimentary origin, whereas block-in-matrix rocks formed by tectonism lack exotic blocks and comprise disrupted ocean plate stratigraphy. Mélanges with exotic blocks are interbedded with coherent sandstones. Many blocks-in-melange record two HP burial events followed by surface exposure, and some record three. Paleomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow fault zones of <100's of m thickness, and <50 m in best constrained cases; these zones lack exotic blocks. Large-scale displacements, whether paleomegathrust horizons, shortening within accreted nappes, or exhumation structures, are accommodated by discrete faults or narrow shear zones, rather than by significant penetrative strain. Exhumation of Franciscan HP units, both coherent and mélange, was accommodated by significant extension of the overlying plate, and possibly extension within the subduction complex, with cross-sectional extrusion, and like subduction burial, took place at different times.

  16. Active deformation along the Andaman-Nicobar subduction zone from seismic reflection studies

    NASA Astrophysics Data System (ADS)

    Moeremans, R. E.; Singh, S. C.

    2013-12-01

    The Andaman-Sumatra subduction zone is one of the most seismically active regions on Earth and is a prime example of oblique subduction. It is the result of the oblique convergence between the downgoing Indo-Australian and the overriding Eurasian plates, leading to slip partitioning into a trench-normal thrust component along the plate interface and a trench-subparallel strike-slip component along a sliver fault. The direction of convergence is 90° with respect to the trench near Java, reduces to 45° off of northern Sumatra, and becomes almost parallel to the trench along the Andaman-Nicobar portion of the subduction. Rates of subduction vary from 63 mm/yr off of Java, 50 mm/yr near Nias Island, 45 mm/yr northwest of Sumatra, and 39 mm/yr near the Andaman Islands. After the great December 2004 earthquake, the Sumatran section of the subduction zone was heavily investigated using marine geophysical studies, but the deformation processes in the Andaman-Nicobar region remain poorly understood due to the lack of data. Here, we present seismic reflection profiles from the Andaman-Nicobar region that cover the deformation front, the forearc high, and the forearc basin. We find that the presence of thick (> 3 s TWT) sediments lead to slip taking place predominantly along landward vergent frontal faults. The frontal fault vergence changes to seaward due to the thinning (< 2 s TWT) of the sediments in the region where the Ninetyeast ridge subducts. The presence of a thick (> 3 s TWT) 20 km-long unit of undeformed sediments, possibly resulting from the landward vergence of the frontal thrusts, suggests that ~40 km of the Ninetyeast ridge has subducted beneath the Andaman forearc. The forearc is widest between the Andaman and Nicobar Islands, likely due to the subduction of thick sediments. The forearc basin is bounded in the west by a series of backthrusts and is underlain by a continental crust, which was once a part of the Malay Peninsula. The forearc basin is crescent-shaped, and seems to be actively deforming. It is shallowest and least deformed where the Ninetyeast ridge is being subducted. Part of the backthrust motion seems to be accommodated in a seaward direction in the southern profiles but landward backthrusts become prevalent where the Ninetyeast ridge converges upon the trench. The strike-slip motion is taken up along the N-S trending Andaman-Nicobar fault.

  17. A precise bathymetric map of the world's deepest seafloor, Challenger Deep in the Mariana Trench

    NASA Astrophysics Data System (ADS)

    Nakanishi, Masao; Hashimoto, Jun

    2011-12-01

    Data from three bathymetric surveys by R/V Kairei using a 12-kHz multibeam echosounder and differential GPS were used to create an improved topographic model of the Challenger Deep in the southwestern part of the Mariana Trench, which is known as the deepest seafloor in the world. The strike of most of the elongated structures related to plate bending accompanied by subduction of the Pacific plate is N70°E and is not parallel to the trench axis. The bending-related structures were formed by reactivation of seafloor spreading fabric. Challenger Deep consists of three en echelon depressions along the trench axis, each of which is 6-10 km long, about 2 km wide, and deeper than 10,850 m. The eastern depression is the deepest, with a depth of 10,920 ± 5 m.

  18. Monitoring of seafloor crustal deformation using GPS/Acoustic technique along the Nankai Trough, Japan

    NASA Astrophysics Data System (ADS)

    Yasuda, K.; Tadokoro, K.; Ikuta, R.; Watanabe, T.; Fujii, C.; Matsuhiro, K.; Sayanagi, K.

    2014-12-01

    Seafloor crustal deformation is crucial for estimating the interplate locking at the shallow subduction zone and has been carried out at subduction margins in Japan, e.g., Japan Trench and Nankai Trough [Sato et al., 2011; Tadokoro et al., 2012]. Iinuma et al. [2012] derived slip distributions during the 2011 Tohoku-Oki earthquake using GPS/Acoustic data and on-land GPS data. The result showed that maximum slip is more than 85 m near the trench axis. The focal area along the Nankai trough extended to the trough axis affected this earthquake by cabinet office, government of Japan. ?We monitored seafloor crustal deformation along the Nankai trough, Japan. Observation regions are at the eastern end of Nankai trough (named Suruga trough) and at the central Nankai trough. We established and monitored by two sites across the trough at each region. In the Suruga trough region, we repeatedly observed from 2005 to 2013. We observed 13 and 14 times at a foot wall side (SNE) and at a hanging wall side (SNW), respectively. We estimated the displacement velocities with relative to the Amurian plate from the result of repeated observation. The estimated displacement velocity vectors at SNE and SNW are 42±8 mm/y to N94±3?W direction and 39±11 mm/y to N84±9?W direction, respectively. The directions are the same as those measured at the on-land GPS stations. The magnitudes of velocity vector indicate significant shortening by approximately 4 mm/y between SNW and on-land GPS stations at hanging wall side of the Suruga Trough. This result shows that the plate interface at the northernmost Suruga trough is strongly locked. In the central Nankai trough region, we established new two stations across the central Nankai trough (Both stations are about 15km distance from trough) and observed only three times, August 2013, January 2014, and June 2014. We report the results of monitoring performed in this year.

  19. Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench

    NASA Astrophysics Data System (ADS)

    Key, Kerry; Constable, Steven; Matsuno, Tetsuo; Evans, Rob L.; Myer, David

    2012-10-01

    Water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism. Geochemical indicators suggest that the Nicaraguan slab is anomalously wet, yet the mechanism of slab hydration remains poorly constrained. Extensional bending faults on the incoming oceanic plate of the Middle America Trench offshore Nicaragua have been observed to penetrate to mantle depths, suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle. Low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation. Here we use controlled-source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile crossing the trench offshore Nicaragua. Along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults. Furthermore, a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor. The observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes, which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle. This implies that most serpentinisation happens at the trench, with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration. This observation explains why the heavily faulted trench offshore Nicaragua is associated with an anomalously wet slab, whereas other sections of the Middle America Trench containing fewer bending faults have less fluid flux from the subducting slab.

  20. Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling

    NASA Astrophysics Data System (ADS)

    Espurt, Nicolas; Funiciello, Francesca; Martinod, Joseph; Guillaume, Benjamin; Regard, Vincent; Faccenna, Claudio; Brusset, Stéphane

    2008-06-01

    We present lithospheric-scale analog models, investigating how the absolute plates' motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long-term process (~10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at ~7 Ma following the beginning of buoyant slab segments' subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges' subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity.

  1. Dynamic buckling of subducting slabs reconciles geological and geophysical observations

    NASA Astrophysics Data System (ADS)

    Lee, Changyeol; King, Scott D.

    2011-12-01

    Ever since the early days of the development of plate tectonic theory, subduction zones have been engrained in geological thinking as the place where steady, linear slabs descend into the mantle at a constant, uniform dip angle beneath volcanic arcs. However, growing evidence from geological and geophysical observations as well as analog and numerical modeling indicates that subducting slabs buckle in a time-dependent manner, in contrast to the steady-state, linear cartoons that dominate the literature. To evaluate the implication of time-dependent slab buckling of geological events, we conduct a series of 2-D numerical dynamic/kinematic subduction experiments by varying the viscosity increase across the 660 km discontinuity and the strength of the subducting slab. Our results show that slab buckling is a universal figure in all the experiments when rate of the trench migration ( Vtrench) is relatively slow ( Vtrench| < 2 cm/a) and viscosity increases across the 660 km discontinuity are greater than a factor of 30. Slab buckling is expressed as alternate shallowing and steepening dip of the subducting slab (from ~ 40 to ~ 100°) which is correlated with increasing and decreasing convergent rate of the incoming oceanic plate toward the trench. Further, the slab buckling in our experiments is consistent with the previously developed scaling laws for slab buckling; using reasonable parameters from subducted slabs the buckling amplitude and period are ~ 400 km and ~ 25 Myr, respectively. The slab buckling behavior in our experiments explains a variety of puzzling geological and geophysical observations. First, the period of slab buckling is consistent with short periodic variations (~ 25 Myr) in the motions of the oceanic plates that are anchored by subduction zones. Second, the scattered distributions of slab dips (from ~ 20 to ~ 90°) in the upper mantle are snapshots of time-dependent slab dip. Third, the current compressional and extensional stress environments in back-arcs are well correlated with shallowing and steeping slab dip resulting from slab buckling. Fourth, the temporal evolution of stress environments in the Andes is well correlated with alternate slab dip. These correlations indicate that time-dependent slab buckling is a major factor controlling subduction zone dynamics.

  2. Nature of the Moho in Japan and Kamchatka

    NASA Astrophysics Data System (ADS)

    Iwasaki, Takaya; Levin, Vadim; Nikulin, Alex; Iidaka, Takashi

    2013-04-01

    The Kamchatka peninsula and the islands of Japan are located along the eastern margin of the Asian continent. The natures of the Moho in these areas have been studied for decades, with a variety of geophysical methods, including active and passive seismic methods, gravity and other techniques. The Moho and the upper mantle structures in the NE Japan and SE Japan Arcs have been investigated well both from active and passive seismic source studies. The Moho depth in the NE Japan is ranging from 30 to 40 km. Almost parallel to the present volcanic front, there exists a belt of deep Moho (34-36 km) with a lower Pn velocity (7.5-7.7 km/s). Amplitude analysis of the PmP phase indicates that the Moho beneath the NE Japan Arc is not a simple velocity contrast, but rather a gradual transition. Toward the backarc side, remarkable crustal thinning is recognized. Actually, the Moho depth decreases from 35 km beneath the central part of NE Japan to 18 km beneath the Sea of Japan. This thinning is evident in the upper crust, while the lower crust remains constant in thickness. This may be explained by the continuous magmatic underplating beneath the rifted crust or deformation under a simple shear mode, allowing the lower crustal thickness to remain unchanged. The Moho in the SW Japan Arc is also at a depth of 30-40 km. The Pn velocity is 7.7-7.8 km/s, slightly higher than that in the NE Japan, although this value was mostly sampled in the eastern half of the SW Japan Arc where the recent volcanic activity has been less effective. Fluids expelled from the subducted oceanic lithosphere (the PHS plate) play an important role in controlling the structure of the mantle wedge. As these fluids leak into the mantle wedge they induce serpentinization there, transforming original mantle materials to those of lower velocity and higher Vp/Vs. The crustal thinning of the SW Japan Arc is characterized by notable decrease in upper crustal thickness, which is similar to the case of the NE Japan Arc. The Moho and uppermost mantle structures beneath the southern part of the Kamchatka have a lot of similarities to those beneath the NE Japan Arc. Earlier DSS investigations and converted wave analyses show that Moho is situated at a depth of 38-40 km along the east coast of Kamchatka, that is beneath the volcanic front, but decreases to about 32 km near the west coast. Moho depth values based on modern receiver function methodology are also ranging from 31 to over 38 km. Moho is a fairly simple boundary under the western coast of Kamchatka, while in the Central Kamchatka Depression and especially along the eastern coast it is likely gradational. Uppermost mantle material beneath the Moho is complex, with additional impedance contrasts that are likely anisotropic in their properties being present under the entire Kamchatka peninsula. The dominant anisotorpy-inducing fabric varies from site to site along the west coast, but is almost universally trench-normal along the east coast. The seismic velocities beneath Kamchatka are very low (7.4-7.8 km/s for P-wave and 4.1-4.2 km/s for S wave). Also, gradual structural change is recognized around the Moho beneath the active volcanoes. These features are quite similar to those in NE Japan Arc.

  3. Geoid anomalies in the vicinity of subduction zones

    NASA Technical Reports Server (NTRS)

    Mcadoo, D. C.

    1980-01-01

    The regional geoid of the southwest Pacific is matched reasonably well by results from a model of the upper mantle density structure (including slabs) associated with subduction zones of the region. Estimates of the geoid are obtained from Geos-3 and Seasat radar altimeter data. These data are very well suited to the task of detecting intermediate wavelength (600-4000 km) geopotential variations. Actually, subducting slabs can be expected to produce primarily intermediate and longer wavelength variations. Gravimetric profiles across trench/island arc complexes resolve primarily short wavelengths. The model represents subducting slabs as thin surfaces of anomalous mass per unit area. These surfaces are positioned using published seismicity results which detail the configuration of the Benioff zones. Crustal effects are ignored. Effects due to the contrast between the young thermal lithosphere of the behind-arc regions (marginal basins) and the older lithosphere seaward of the trench are modelled. Results indicate that the New Hebrides slab possesses an average areal density anomaly of about 300,000 gm/sq cm. This is about three times that which is estimated for the Tonga-Kermadec slab. Additional modelling suggests that slabs worldwide may be an important source of large, long wavelength gravity highs; i.e., they may contribute substantially to geopotential power of harmonic degree as low as three or four up to twenty or more.

  4. The Java margin revisited: Evidence for subduction erosion off Java

    NASA Astrophysics Data System (ADS)

    Kopp, H.; Flueh, E. R.; Petersen, C. J.; Weinrebe, W.; Wittwer, A.; Scientists, Meramex

    2006-02-01

    The eastern Sunda margin off Indonesia (from central Java to Sumba Island) remains a little investigated subduction zone, contrary to its well-studied northwestern segment. Whereas large portions of the Sunda margin are considered a classical accretionary zone, subduction characteristics along the central Java sector indicate erosive processes as the dominant mode of mass transfer. The tectonic framework of the central Java margin, with a convergence rate of 6.7 cm/yr, insignificant sediment input and a pronounced seafloor roughness where the oceanic Roo Rise is subducting underneath Java, facilitates subduction erosion. Evidence for erosion comes from newly acquired geophysical data off central Java: local erosive processes in the wake of seamount subduction are documented by a high-resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars. Subduction of oceanic basement relief leads to large-scale uplift of the forearc, as recorded on a reflection seismic profile, and to a dismemberment of the previous outer forearc high, giving way to isolated topographic elevations. The broad retreat of the Java Trench and deformation front above the leading edge of the Roo Rise has exposed an area of approximately 25,000 km 2 of deeper seafloor formerly covered by the previous frontal prism. Frontal erosion coincides with a steepening of the lower slope angle in the central Java sector compared to the neighbouring segments. In global compilations, the key geological parameters of the central Java margin lie in the erosive regime, reflecting the interplay of basement relief subduction, negligible sediment supply and a high convergence rate on the evolution of the margin.

  5. Interseismic Strain Along the Sumatra Subduction Zone: A Case for a Locked Fault Portion Extending Well Below the Forearc Moho

    NASA Astrophysics Data System (ADS)

    Simoes, M.; Avouac, J.; Cattin, R.; Henry, P.; Natawidjaja, D. H.

    2003-12-01

    A current and most accepted view about the seismogenic zone along subduction zones is that the downdip extent of the locked fault portion would correspond either to the 350° C isotherm if this temperature is reached above the Moho, or to the intersection with the forearc Moho for colder subduction zones [Oleskevich et al., 1999]. This limit would reflect the transition from slip-weakening friction to aseismic stable-sliding, or else ductile flow. In the first case, when the downdip end of the locked zone is temperature-controlled, stable-sliding of quartzo-feldspathic rocks would be the controlling factor, while the systematic presence of serpentinite or other hydrated minerals is advocated to explain aseismic interplate slip in the mantle at temperatures much less that the 750° C needed for ductile flow of mantle rocks [Peacock and Hyndman, 1999]. Here, we consider the case of the Sumatra subduction zone where the ~53 Myr Indian oceanic crust subducts below an island-arc characterized by a relatively thin crust, with a Moho depth estimated to ~23 km. We model interseismic deformation from a creeping dislocation embedded in an elastic half-space, using the back-slip approach. In addition to recently published GPS velocities, we take advantage of recent data on the pattern and rate of interseismic uplift that have been obtained from the study of coral growth [Natawidjaja, 2003; Sieh et al., 1999]. These data are found to put tight constraints on the horizontal position of the downdip limit of the locked fault zone, at 127 +/- 4/2 km from the trench. This position corresponds to a depth between 40 and 58 km and to a temperature of 269° C +/- 155° C, when compared with thermal modeling. So, in this particular setting, the locked fault portion extends well into the mantle. However, temperature is not high enough to advocate stable sliding or ductile flow of unaltered or altered mantle rocks. This case appeals to some reappraisal of the physical control on the depth of the locked fault zone along subduction zones. References. Natawidjaja, D.H., Neotectonics of the Sumatran fault and paleogeodesy of the Sumatran subduction zone., PhD thesis, California Institute of Technology, Pasadena, 2003. Oleskevich, D.A., R.D. Hyndman, and K. Wang, The updip and downdip limit to great subduction earthquakes : thermal and structural models of Cascadia, South Alaska, SW Japan and Chile., Journal of Geophysical Research, 104 (B7), 14965-14991, 1999. Peacock, S.M., and R.D. Hyndman, Hydrous minerals in the mantle wedge and the maximum depth of subdcution thrust earthquakes., Geophysical Research Letters, 26 (16), 2517-2520, 1999. Sieh, K., S.N. Ward, D. Natawidjaja, and B.W. Suwargadi, Crustal deformation at the Sumatra subduction zone revealed by coral data., Geophysical Research Letters, 26 (20), 3141-3144, 1999.

  6. Seismicity, topography, and free-air gravity of the Aleutian-Alaska subduction zone

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Blakely, R. J.; Scholl, D. W.; Ryan, H. F.

    2011-12-01

    The Aleutian-Alaska subduction zone, extending 3400 km from the Queen Charlotte Fault to Kamchatka, has been the source of six great megathrust earthquakes in the 20th Century. Four earthquakes have ruptured the 2000-km-long Aleutian segment, where the Cenozoic Aleutian arc overlies the subducting Pacific plate. These include the 1946 M 8.6 earthquake off Unimak Is., the 1957 M 8.6 and 1986 M 8.0 earthquakes off the Andreanoff Is., and the 1965 M 8.7 Rat Is. earthquake. The source regions of these earthquakes inferred from waveform inversions underlie the well-defined Aleutian deep-sea terrace. The deep-sea terrace is about 4 km deep and is underlain by Eocene arc framework rocks, which extend nearly to the trench. It is bounded on its seaward and landward margins by strong topographic and fee-air gravity gradients. The main asperities (areas of largest slip) for the great earthquakes and nearly all of the Aleutian thrust CMT solutions lie beneath the Aleutian terrace, between the maximum gradients. Similar deep-sea terraces are characteristic of non-accretionary convergent margins globally (75% of subduction zones), and, where sampled by drilling (e.g., Japan, Peru, Tonga, Central America), are undergoing sustained subsidence. Sustained subsidence requires removal of arc crust beneath the terrace by basal subduction erosion (BSE). BSE is in part linked to the seismic cycle, as it occurs in the same location as the megathrust earthquakes. Along the eastern 1400 km of the Alaskan subduction zone, the Pacific plate subducts beneath the North American continent. The boundary between the Aleutian segment and the continent is well defined in free-air gravity, and the distinctive deep-sea terrace observed along the Aleutian segment is absent. Instead, the Alaskan margin consists of exhumed, underplated accretionary complexes forming outer arc gravity highs. Superimposed on them are broad topographic highs and lows forming forearc basins (Shumagin, Stevenson) and islands (Kodiak, Shumagin). Two great earthquakes ruptured much of this segment: the 1938 M 8.3 earthquake SW of Kodiak and the 1964 M 9.2 earthquake, which ruptured 800 km of the margin between Prince William Sound and Kodiak Island. Large slip during the 1938 event occurred under the Shumagin and Tugidak basins, but slip in 1964 is thought to have occurred on asperities under Prince William Sound and the outer arc highs off Kodiak. Seismic profiling and industry drilling indicates sustained subsidence has also occurred along the Alaska margin. BSE is probably occurring there, but the terrace structure is buried by the high sedimentation rate. At present, the inherited accretionary structures, the ongoing collision of the Yakutat terrane, and uncertainties in finite fault modeling obscure correlation of slip with topographic and gravity signatures in the 1964 source region.

  7. Tracing slab inputs along the Izu-Bonin-Marianas subduction zone: results from volatile emissions

    NASA Astrophysics Data System (ADS)

    Fischer, T. P.; Hilton, D. R.; Shaw, A. M.; Hauri, E. R.; Kazahaya, K.; Mitchell, E.; Shimizu, A.; de Moor, M.; Sharp, Z. D.

    2005-12-01

    The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the island of Honshu, Japan to Guam and is a type example of an intra-oceanic convergent margin. Subduction began 45 Ma ago and IBM subducts the oldest seafloor on Earth. A number of parameters vary systematically along the strike of the arc: the slab is steeply plunging in the S and gently dipping in the N; the age of the subducted crust varies from Mid-Jurassic in the S to Mid-Cretaceous in the N. Other parameters remain constant: crustal thickness (~20 km); no accretionary prism; no sediment fill in the trench. The sediment outboard of the arc is characterized based on ODP sites 801 (Marianas) and 1149 (Izu islands). 200 m of volcaniclastics are overlain by a 100 m of pelagic clay and chert in the S. In the N, volcaniclastics are lacking and the 400 m sediment sequence is dominated by 200 m of cherts, overlain by 40 m of pelagic clay and 120 m of volcanic ash and diatom/radiolarian clay. There is also a distinct layer (3 m) of hydrothermally altered MORB in the S. Thus, the IBM system is an ideal location to study the inputs and outputs of the subduction factory and to understand the processes occurring within the factory itself. We collected hydrothermal gas samples from 4 volcanic centers in the Marianas (Alamagan, Pagan, Agrigan, Uracas) and 6 centers in the Izu arc (Aogashima, Hachijojima, Niijima, Shikinejima, Oshima, Hakone). With the exception of Uracas (140C) and a well on Hachijojima (170C), all gas discharges were at or below the boiling temperature of water. As is typical for arc-related samples, the major gases are dominated by H2O, CO2 and S species. We see the following variations in N2/Ar and N2/He ratios of non-air contaminated samples along the arc: Agrigan clearly shows a mantle wedge signature of low N2/Ar (70) and N2/He (210) and negative ?15N (- 2.0 ‰). All other centers have N2/He ratios characteristic of that resulting from the addition of N from subducted sediments (1000 to 2500). Most Izu samples also show N2/Ar ratios higher than air (up to 210). Helium isotopes of Mariana samples are MORB-like (7.4 to 7.9 RA), whereas CO2/3He varies from 10.1 to 10.7 x 109 with ?13C between -0.5 to - 0.7 permil. Based on N-CO2-He-Ar sytematics, the Izu section of the arc has a signature characteristic of subducted sediment derived fluids. The Mariana section (Agrigan in particular), shows a volatile signature that suggests contribution dominantly from the altered oceanic basement. This is in contrast to studies based on trace elements and radiogenic isotopes that identify Agrigan as the `sediment endmember' of the Mariana arc. Analyses of stable and noble gas isotopes of the samples are currently underway to further constrain the source of volatiles discharging along the arc.

  8. Seismicity and tectonics of the subducted Cocos Plate

    NASA Astrophysics Data System (ADS)

    Burbach, George Vanness; Frohlich, Cliff; Pennington, Wayne D.; Matumoto, Tosimatu

    1984-09-01

    We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best-defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike-slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller-scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone. Appendices 2, 3, and 4 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84-009; $2.50.

  9. Parameters controlling dynamically self-consistent plate tectonics and single-sided subduction in global models of mantle convection

    NASA Astrophysics Data System (ADS)

    Crameri, Fabio; Tackley, Paul J.

    2015-05-01

    Recent advances in numerical modeling allow global models of mantle convection to more realistically reproduce the behavior at convergent plate boundaries; in particular, the inclusion of a free surface at the outer boundary has been shown to facilitate self-consistent development of single-sided subduction. This allows for a more extensive study of subduction in the context of global mantle convection, as opposed to commonly used regional models. Our first study already indicated important differences between mantle convection with single-sided subduction and mantle convection with double-sided subduction. Here we further investigate the effect of various physical parameters and complexities on inducing Earth-like plate tectonics and its evolution in time. Results reinforce the previous finding that using a free surface instead of a free-slip outer boundary dramatically changes subduction style, with free surface cases displaying many episodes of single-sided subduction, which leads to more realistic slab dip, stress state, trench retreat rate, and slab-induced mantle flow. Longevity of single-sided subduction is promoted by a layer of hydrated crust with a low yield strength to lubricate the subduction channel, a low-viscosity asthenosphere, and a high strength of the slab (determined by a combination of high-diffusion creep viscosity and intermediate friction coefficient), although its effective viscosity is in the observationally constrained range in the bending region. The time evolution displays interesting events including subduction polarity reversals, subduction shut-off, and slab break-off.

  10. Interpretation of interseismic deformations and the seismic cycle associated with large subduction earthquakes

    NASA Astrophysics Data System (ADS)

    Trubienko, Olga; Fleitout, Luce; Garaud, Jean-Didier; Vigny, Christophe

    2013-03-01

    The deformations of the overriding and subducting plates during the seismic cycle associated with large subduction earthquakes are modelled using 2D and 3D finite element techniques. A particular emphasis is put on the interseismic velocities and on the impact of the rheology of the asthenosphere. The distance over which the seismic cycle perturbs significantly the velocities depends upon the ratio of the viscosity in the asthenosphere to the period of the seismic cycle and can reach several thousand km for rheological parameters deduced from the first years of deformation after the Aceh earthquake. For a same early postseismic velocity, a Burger rheology of the asthenosphere implies a smaller duration of the postseismic phase and thus smaller interseismic velocities than a Maxwell rheology. A low viscosity wedge (LVW) modifies very significantly the predicted horizontal and vertical motions in the near and middle fields. In particular, with a LVW, the peak in vertical velocity at the end of the cycle is predicted to be no longer above the deep end of the locked section of the fault but further away, above the continentward limit of the LVW. The lateral viscosity variations linked to the presence at depth of the subducting slab affect substantially the results. The north-south interseismic compression predicted by this preliminary 2D model over more than 1500 km within the Sunda block is in good agreement with the pre-2004 velocities with respect to South-China inferred from GPS observations in Thailand, Malaysia and Indonesia. In Japan, before the Tohoku earthquake, the eastern part of northern Honshu was subsiding while the western part was uplifting. This transition from subsidence to uplift so far away from the trench is well fitted by the predictions from our models involving a LVW. Most of the results obtained here in a 2D geometry are shown to provide a good estimate of the displacements for fault segments of finite lateral extent, with a 3D spherical geometry, as long as the displacements during the seismic cycle are normalised by the coseismic displacement.

  11. Continental Subduction and Subduction Initiation Leading to Extensional Exhumation of Ultra-High Pressure Rocks During Ongoing Plate Convergence in Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Buck, W. R.; Petersen, K. D.

    2014-12-01

    Subduction of continental rocks is necessary to produce ultra-high pressure (UHP) rocks but the mechanism bringing them to the surface is disputed. A major question is whether this involves fairly small diapirs of crust that move up through the mantle or it involves an entire subducted plate that undergoes coherent 'reverse subduction' (sometimes called 'eduction'). Both mechanisms have been invoked to explain the only known region of on-going exhumation of UHP rocks, on the D'Entrecasteaux Islands of Papua New Guinea. Ductile flow fabrics in the island rocks have been used to argue for a diapiric model while constraints on the plate kinematics of the region require relatively large (>100 km) amounts of recent (>6 Myr) extension, supporting eduction as a primary mechanism. A self-consistent thermo-mechanical model of continental subduction shows that eduction can be accompanied by some ductile flow within the crust. Also we show, that subduction and stacking of continental crust can cause a subduction zone to lock up and lead to subduction initiation elsewhere. When this happens the region of earlier continental subduction can reverse direction causing exhumation of rocks from depth of ~100 km followed by localized extension and plate spreading. This can occur even if a region is in overall convergence. Applied to New Guinea our results are consistent with earlier suggestions that extension of the Woodlark Basin was caused by the initiation of the New Britain Trench, as indicated on the attached figure. We suggest that this subduction initiation event triggered eduction that led to exposure of the D'Entrcasteaux Islands and exhumation of the UHP rocks there. Our numerical results are broadly consistent with the recently refined seismic structure of the region around the islands. The model implies that the present-day basement of the ~70 km wide Goodenough Bay, south of the islands, was subducted then exhumed. This can be tested by drilling.

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

    NASA Astrophysics Data System (ADS)

    Faccenda, M.; capitanio, F. A.

    2012-12-01

    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. As the development of seismic anisotropy due to upper and lower plate motions occurs at depths and timescales such that it is not directly observable, numerical modelling provides a useful tool to investigate these processes. We computed the seismic anisotropy of dry olivine-enstatite aggregates due to strain-induced LPO in 3D mechanical models of dynamic subduction by using, respectively, D-Rex and Underworld. Subsequently, FSTRACK was used to compute seismogram synthetics and SKS splitting patterns. We found that for relatively narrow subducting plates, retreat motions are maximized producing strong subslab trench-parallel anisotropy. Here, synthetic data reproduce quite well the observations in analogous subduction systems like Calabria and South Sandwich, where the fast azimuths orients parallel to the trench in the forearc and follow the toroidal flow patterns on the slab edges. Furthermore, we found that the amount of anisotropy is proportional to the amount of subduction, while it does not depend on the rate at which the plate subducts. On the other hand, larger subducting plates subducts mainly by plate advance, favoring poloidal motions and trench-perpendicular anisotropy. Additional Earth-like plate geometries involving along-trench variation of the subducting plate age that induces differential slab retreat motions are considered. We also tested different olivine fabrics (A, B, C, E type), yielding distinct SKS splitting patterns that may help to constrain the composition of the upper mantle. Although more sophisticated numerical modelling taking into account temperature-dependent mantle rock rheologies and P-T derivatives of single crystal elastic tensors are needed in order to reproduce more realistically the whole range of SKS splitting patterns, our results suggest that the interpretation of seismic anisotropy, based on coupled flow models and LPO calculations, can be used to constrain the kinematic and dynamic behavior of the Earth's interior.

  13. Complex flow resulted from along-strike variations in slab length in the southern Chilean subduction zone

    NASA Astrophysics Data System (ADS)

    Lin, S.; Chung, S.

    2013-12-01

    Complicated patterns and trench-parallel components of seismic anisotropy in the central portion of the subduction zones have not been link to three-dimensional flow in general, as toroidal circulations may mostly occur in the vicinity of a plate edge. Nevertheless, recent dynamical models demonstrate that complex flow forms when lateral heterogeneities in the subduction zone are considered. Seismic studies often show gradual variations in slab length along strike. The southern Chilean subduction zone is a notable example. The Peru-Chile Trench roughly extending 5900 km is the longest trench worldwide. Most of the regions are distant from the subduction zone edges. In this study the effects of slab-length variations are examined with numerical experiments. The results are used to characterize the regional mantle circulations and the potential heat sources for the recent Patagonian plateau basalts in the southern South America. The model results show that rollback subduction induces both significant poloidal components and trench-parallel flow, extending over 1500 kilometers along the strike of the subduction zone. The trajectories of a set of passive tracers show intricate patterns such as helical streamlines, revealing complex mantle circulations in systems with the along-arc slab-length variations. The upwelling may lead to decompression melting of the heterogeneous asthenosphere to cause extensive lavas in the back arc. Mantle upwelling of the sub-slab mantle moving towards supra-slab regions is observed in the southernmost portion of the subduction system, but only for models with slab segments roughly deeper than 100 km depth. The results indicate that certain segments of the Antarctic slab may reach at least 100 km depth.

  14. New Insights of the Rivera and Cocos Plates Subduction Beneath the Jalisco Block

    NASA Astrophysics Data System (ADS)

    Escudero, C. R.; Gutierrez Pena, Q. J.; Ochoa, J.; Nunez-Cornu, F. J.; Barba, D. C., Sr.

    2014-12-01

    The subduction of the Rivera and Cocos Plate is a complex tectonic process that involves the limits between these two plates, changes in subduction angle and the transition between convergent and divergent tectonics settings. To obtain new insights of the Rivera and Cocos Plates geometry we analyze earthquakes recorded from 2006 to 2008 by the Mapping the Riviera Subduction Zone (MARS) as well as earthquakes recorded by the Jalisco Seismic Network (RESAJ). To obtain better results and definition of the subduction geometry we manually performed the detection and phase picking of the earthquakes. We also analyzed results from the Seismic Experiment to Characterize the Seismic and Tsunami Risk Associated to the Rivera Plate and Jalisco Block Structure (TSUJAL), this results include refraction-reflection lines perpendicular to the trench. We present seismicity maps along with a focal mechanism analysis of several earthquakes, as well as seismic refraction and reflection preliminary profile.

  15. Submarine mass wasting processes along slopes influenced by long-term tectonic erosion: The Middle America Trench

    NASA Astrophysics Data System (ADS)

    Harders, R.; Ranero, C. R.; Weinrebe, W.

    2011-12-01

    We have studied submarine land-sliding using a seafloor topography and side-scan sonar data along the continental slope of the Middle America Trench. This subduction zone is dominated by tectonic erosion. Studies during the last few decades have shown mass wasting structures at submarine slopes around the world's continental margins, hot-spot volcanic islands, and volcanic island arcs. At Atlantic margins slides initiate at low slope angles and appear triggered by high sediment accumulation rates. At volcanic islands large-scale land-sliding is caused by volcano sector collapse. At subduction zones with accretionary prisms, land-sliding seems associated to contractional tectonics and fluid seepage. Submarine mass movements at subduction zones dominated by tectonic erosion are comparatively limited. However, tectonic erosion is active in about 50% of the world subduction zones. Distinct failures have been studied at slopes in Peru, Costa Rica, Nicaragua and New Zealand but extensive surveys have not been obtained. We present a comprehensive data sets on seafloor mapping on a subduction zone dominated by tectonic erosion. The data covers much of the Middle America Trench (MAT) from the Mexico-Guatemala border to Costa Rica - Panama border. The goal of this contribution is to evaluate how long-term tectonics caused by subduction erosion preconditions the continental slope structure to modulate the generation of land-sliding. We show that changes in subduction erosion processes, interacting with the local topography of the subducting plate correlate to variations in the type and distribution of failures along the slope of the region.

  16. Structure and deformation of the Kermadec forearc in response to subduction of the Pacific oceanic plate

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

    The Tonga-Kermadec forearc is deforming in response to on-going subduction of the Pacific Plate beneath the Indo-Australian Plate. Previous research has focussed on the structural development of the forearc where large bathymetric features such as the Hikurangi Plateau and Louisville Ridge seamount chain are being subducted. Consequently, knowledge of the `background' forearc in regions of normal plate convergence is limited. We report on an ˜250-km-long multichannel seismic reflection profile that was shot perpendicular to the Tonga-Kermadec trench at ˜28°S to determine the lateral and temporal variations in the structure, stratigraphy and deformation of the Kermadec forearc resulting solely from Pacific Plate subduction. Interpretation of the seismic profile, in conjunction with regional swath bathymetry data, shows that the Pacific Plate exhibits horst and graben structures that accommodate bending-induced extensional stresses, generated as the trenchward dip of the crust increases. Trench infill is also much thicker than expected at 1 km which, we propose, results from increased sediment flux into and along the trench. Pervasive normal faulting of the mid-trench slope most likely accommodates the majority of the observed forearc extension in response to basal subduction erosion, and a structural high is located between the mid- and upper-trench slopes. We interpret this high as representing a dense and most likely structurally robust region of crust lying beneath this region. Sediment of the upper-trench slope documents depositional hiatuses and on-going uplift of the arc. Strong along-arc currents appear to erode the Kermadec volcanic arc and distribute this sediment to the surrounding basins, while currents over the forearc redistribute deposits as sediment waves. Minor uplift of the transitional Kermadec forearc, observed just to the north of the profile, appears to relate to an underlying structural trend as well as subduction of the Louisville Ridge seamount chain 250 km to the north. Relative uplift of the Kermadec arc is observed from changes in the tilt of upper-trench slope deposits and extensional faulting of the basement immediately surrounding the Louisville Ridge.

  17. Deformation of Forearcs during Aseismic Ridge Subduction

    NASA Astrophysics Data System (ADS)

    Zeumann, S.; Hampel, A.

    2014-12-01

    Subduction of aseismic oceanic ridges causes considerable deformation of the forearc region. To identify the crucial parameters for forearc deformation we created 3D finite-element models representing both erosive and accretive forearcs as well as migrating and non-migrating ridges. As natural examples we choose the Cocos ridge subducting stationary beneath the erosive margin of Costa Rica and the Nazca and Gagua Ridges that migrate along the erosive Peruvian margin and the accretive accretive Ryukyu margin, respectively. A series of models show that the deformation of the forearc depends on the ridge shape (height, width), on the frictional coupling along the plate interface and the mechanical strength of the forearc. The forearc is uplifted and moved sideward during ridge subduction. Strain components show domains of both, shortening and extension. Along the ridge axis, extension occurs except at the ridge tip, where shortening prevails. The strain component normal to the ridge axis reveals extension at the ridge tip and contraction above the ridge flanks. Shortening and extension increase with increasing ridge height. Higher friction coefficients lead to less extension and more shortening. Accretive wedges show larger indentation at the model trench. For stationary ridges (Cocos Ridge) the deformation pattern of the forearc is symmetric with respect to the ridge axis whereas for migrating ridges (Nazca Ridge, Gagua Ridge) the oblique convergence direction leads to asymmetric deformation of the forearc. In case of ridge migration, uplift occurs at the leading flank of the ridge and subsidence at the trailing flank, in agreement with field observations and analogue models. For a model with a 200-km-wide and 1500-m-high ridge (i.e. similar to the dimensions of the Nazca Ridge), the modelled uplift rate at the southern ridge flank of the ridge is ~1 mm/a, which agrees well with uplift rates of ~0.7 mm/a derived from the elevation of marine terraces in southern Peru.

  18. Seismic Anisotropy and SKS Splitting in the Sangihe Subduction Zone Predicted from 3-D Mantle Flow Models

    NASA Astrophysics Data System (ADS)

    Di Leo, J. F.; Li, Z.; Walker, A. M.; Wookey, J.; Kendall, J.; Ribe, N. M.; Tommasi, A.

    2012-12-01

    Observations of shear wave splitting are often interpreted as being due to strain-induced crystal alignment of olivine in the convecting upper mantle, and the polarization of the fast shear wave is frequently taken to directly indicate the direction of mantle flow. Caution must be exercised when making such inferences, as the relationship between olivine lattice-preferred orientation (LPO) and fast direction is dependent on many factors, including the entire deformation history. This is especially the case in regions where complex time-dependent mantle flow is expected, e.g., subduction zones. Observations of shear wave splitting at subduction zones are varied, ranging from trench-perpendicular to -parallel fast directions, or a combination of both. Rigorously interpreting this variety of observations requires modeling which properly accounts for LPO development in the near-slab mantle environment. To this end, we simulate olivine LPO evolution caused by defomation of polycrystalline aggregates as they deform and move along pathlines extracted from a 3-D mantle flow model at a subduction zone (Li & Ribe, 2012). The model is based on 3-D boundary-element numerical simulations of a dense fluid sheet (representing the slab) with a geometry approximating that of the Sangihe subduction zone in Indonesia, where trench-parallel fast directions have recently been measured and ascribed to trench-parallel sub-slab mantle flow (Di Leo et al., 2012). This subduction zone is unique in that it is part of the only double-sided subduction system on Earth. At the Sangihe trench, the Molucca Sea plate is subducting westwards beneath the Eurasian plate. However, this microplate is also subducting eastwards at the nearby Halmahera trench. To test whether the measured trench-parallel fast directions are due to sub-slab mantle flow, and whether this is only possible due to the double-sided geometry, we use two different flow models: one with single- and one with double-sided subduction. The effect of deformation and LPO development is simulated assuming the defomation of each crystal is governed by the motion of dislocations. Interactions between crystals are descibed using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. This approach is necessary for the Sangihe subduction zone, as it is fairly young and the slab has only just reached the bottom of the mantle transition zone. In older subduction zones, early textures may eventually be destroyed. After calculating elastic properties associated with LPO at multiple depths, we estimate the resulting splitting parameters (fast direction ?, delay time ?t) for synthetic SKS phases. We compare these measurements with splitting observations in the Sangihe subduction zone. Our models show that complex behavior in ? appears in even apparently simple models of subduction zone mantle flow and that making robust dynamic inferences requires proper consideration of the geometry of subduction.

  19. KSC Launch Pad Flame Trench Environment Assessment

    NASA Technical Reports Server (NTRS)

    Calle, Luz Marina; Hintze, Paul E.; Parlier, Christopher R.; Curran, Jerome P.; Kolody, Mark R.; Sampson, Jeffrey W.

    2010-01-01

    This report summarizes conditions in the Launch Complex 39 (LC-39) flame trenches during a Space Shuttle Launch, as they have been measured to date. Instrumentation of the flame trench has been carried out by NASA and United Space Alliance for four Shuttle launches. Measurements in the flame trench are planned to continue for the duration of the Shuttle Program. The assessment of the launch environment is intended to provide guidance in selecting appropriate test methods for refractory materials used in the flame trench and to provide data used to improve models of the launch environment in the flame trench.

  20. Evolution of the Nankai Trough décollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging

    Microsoft Academic Search

    Nathan L. Bangs; Thomas H. Shipley; Sean P. S. Gulick; Gregory F. Moore; Shinichi Kuromoto; Yasuyuki Nakamura

    2004-01-01

    We mapped the amplitude of the Nankai Trough subduction thrust seismic reflection from the trench into the seismogenic zone with three-dimensional seismic reflection data. The décollement thrust forms within the lithologically homogeneous Lower Shikoku Basin facies along an initially nonreflective interface. The reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and

  1. Deformation and mantle flow beneath the Sangihe subduction zone from seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Di Leo, J. F.; Wookey, J.; Hammond, J. O. S.; Kendall, J.-M.; Kaneshima, S.; Inoue, H.; Yamashina, T.; Harjadi, P.

    2012-03-01

    Subduction of oceanic lithosphere is the most direct feedback between the Earth's surface and deep interior. However, the detail of its interaction with the broader convecting mantle is still unclear. Mantle flow around subduction zones can be constrained using seismic anisotropy, but despite many such studies, a simple global picture is lacking. The Sangihe subduction zone (where the Molucca Sea microplate is subducting westward beneath the Eurasian plate) is part of the tectonically complex Sulawesi-Philippine region, and an ideal natural laboratory to study complex subduction processes. We investigate the anisotropic structure of the Sangihe subduction zone with shear wave splitting measurements of local S and SKS phases at two stations (MNI in Sulawesi, DAV in the Philippines), as well as downgoing S phases at five stations at teleseismic distances. Combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase. The broad depth distribution of local events (˜60-630 km) allows us to observe a change in splitting behaviour at ˜380 km depth: above, fast directions (?) are trench-parallel and delay times (?t) are ˜0.34-0.53 s with no increase with depth. We suggest this anisotropy is caused by aligned cracks, possibly melt-filled beneath the volcanic arc, and fossil anisotropy in the overriding plate. Below ˜380 km, ? is predominantly trench-normal and ?t are slightly higher (˜0.53-0.65 s). As no correlation is observed with inferred distance travelled inside the slab, we attribute this anisotropy to shear layers atop the slab, which are coherent from ˜200 to 400 km depth and perhaps extend into the transition zone. SKS and source-side measurements show larger ?t (˜1.53 and 1.33 s, respectively) and trench-parallel ?. Since these phases predominantly sample sub-slab mantle, we consider along-strike lateral flow associated with the double-sided subduction of the Molucca Sea microplate to be the most likely explanation. We thus infer three dominant regions of anisotropy at the Sangihe subduction zone: one within the overriding lithosphere, one along the slab-wedge interface, and one below the subducting Molucca Sea slab. The mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy. This study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems.

  2. Subduction at Convergent Boundary

    NSDL National Science Digital Library

    The representation depicts subduction. The narrated animated movie (simulation) shows subduction of the Indian Plate as the Indian Plate and the Eurasian Plate converge at the plate boundary. The segment begins showing a world view of the Earth's plates and zooms in on the highlighted Indian and Eurasian plate activity. The animation transitions to a cross-sectional view, giving an inside-the-Earth look at what happens as these plates converge. The movie can be viewed in two ways- in continuous play or step by step.

  3. Seismicity and Geometry Properties of the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Papadimitriou, E. E.; Karakostas, V. G.; Vallianatos, F.; Makropoulos, C.; Drakatos, G.

    2014-12-01

    Recent seismicity and fault plane solutions of earthquakes that occurred along the Hellenic Arc-Trench system are engaged for approximating the geometry of the subducted oceanic plate. Seismicity and focal mechanisms confirm the gentle subduction (~15o-20o) of the oceanic crust reaching a depth of 20 km at a distance of 100 km from the trench. The slab is then bending at larger angles, and in particular at ~45o up to the depth of 80 km and at ~65o up to the depth of 180 km, when seismicity ceased. This geometry of the slab is shown in a bunch of cross sections normal to the convergence strike, up to ~25o (east Crete Island). To the east the sparse inslab seismicity reveals an almost vertical dipping of the lower part (from 80 km downdip) of the descending slab. The slab interface that accommodates hazardous earthquakes is clearly nonplanar with the main seismic moment release taking place on its up-dip side. The fore-arc, upper plate seismicity, is remarkably low in comparison with both subduction and back arc seismicity, and confined inside a seismogenic layer having a width not exceeding the 20km. Offshore seismicity is spatially variable forming distinctive streaks thus revealing that parts of the oceanic crust are probably slipped aseismically. This observation along with the fact that coupling in the Hellenic arc is only about a tenth of the plate motion, imply the presence of areas of lower and higher coupling across the subduction interface. Areas of high coupling imply areas of the slab interface subjected to high normal forces and correlate with earthquake asperities. Although asperity distributions vary substantially through time, identification of such characteristics in the seismogenesis can have a significant impact in the seismic hazard assessment. This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the "THALES Program: SEISMO FEAR HELLARC" project.

  4. Arc-arc Collision Structure in the Southernmost Part of the Kuril Trench Region -Results from Integrated Analyses of the 1998-2000 Hokkaido Transect Seismic Data-

    NASA Astrophysics Data System (ADS)

    Iwasaki, Takaya; Tsumura, Noriko; Ito, Tanio; Sato, Hiroshi; Kurashimo, Eiji; Hirata, Naoshi; Arita, Kazunori; Noda, Katsuya; Fujiwara, Akira; Abe, Susumu; Kikkuchi, Shunsuke; Suzuki, Kazuko

    2015-04-01

    The Hokkaido Island, located in the southernmost part of the Kuril trench region, has been under a unique tectonic environment of arc-arc collision. Due to the oblique subduction of the Pacific (PAC) plate, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc from the east at the time of middle Miocene to form complicated structures in the Hidaka collision zone (HCZ), as characterized by the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and a thick foreland fold-and-thrust belt. In and around the HCZ, a series of seismic reflection/refraction experiments were undertaken from 1994 to 2000, which provided important structural features including crustal delamination in the southern HCZ and a thick fold-and-thrust belt with velocity reversals (low velocity layers) in the northern HCZ. Reprocessing/reinterpretation for these data sets, which started in 2012, is aimed to construct a more detailed collision model through new processing and interpretation techniques. 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. Our reanalyses revealed interesting 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 occurs at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). The CRS/MDRS processing to the reflection data firstly succeeded in imaging clear reflection events at a 30-45 km depth below the obducted Kuril arc crust. These events show an eastward dip, probably corresponding to the lower crust/Moho within the NE Japan arc descending down to the east under the collision zone. Gently eastward dipping structures above these events (in a depth range of 5-10 km) are interpreted to be fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT. The refraction/wide-angle reflection analysis revealed a series of eastward dipping interfaces at depths of 15-30 km east of the HMT, some of which show a very large Vp contrast exceeding 0.5-1.0 km/s. The subducted NE Japan arc meets the Kuril arc 20-40 km east of the HMT at a depth of 20-30 km. The above mentioned high Vp contrasts may result from the mixture of the upper crustal (low Vp) materials of the NE Japan arc and lower crustal (high Vp) materials of the Kuril arc. Seismic reflection image in the southern HCZ reprocessed by almost the same techniques confirms a clear crustal delamination, where the upper 23-km crust is thrust up along the HMT while the lower part of the crust descends down to the subducted PAC plate. At the moment, the results in the northern HCZ do not provide positive evidence on shallow crustal delamination as found in the case of the southern HCZ, suggesting regional difference in collision style along the HMT.

  5. 3D Thermal Structure of the Alaska-Aleutian Arc with Predictions for the Metamorphic Structure and Seismic Velocities in the Subducting Slab

    NASA Astrophysics Data System (ADS)

    Volk, K.; Van Keken, P. E.; Hacker, B. R.; Abers, G. A.

    2014-12-01

    An understanding of the thermal structure of subduction zones is important to gain insight into metamorphic dehydration reactions, the flow of fluids, generation of intermediate depth seismicity and the production of melts that generate arc volcanism. A tradition approach to subduction zone modeling relies on 2D cross-sections. While this approach appears adequate for subduction systems that have near-normal convergence and small changes in structure along-strike, it fails to fully represent the thermal and velocity structure of subduction systems with curved trenches or oblique subduction (van Keken & Bengston, JGR, 2003). Two-dimensional models will also likely fail near slab edges and in regions with strong changes in convergence parameters such as slab velocity, age of incoming slab, and variations in overriding plate structure. The Alaska-Aleutian arc is a prime example of a subduction zone with strong along-arc variations, which made it of key interest for interdisciplinary studies with the GeoPRISMS program (geoprisms.org). As the Pacific Plate subducts beneath North America, the Aleutian trench curves southward. Subduction also trends from trench normal near Alaska to nearly trench parallel in the far western Aleutians. These changes in the subduction zone geometry are accompanied by changes in volcanic activity, arc size, and changes in the arc to slab distance. We have developed a full 3D model of the Aleutian-Alaska subduction system using a kinematic slab surface (following the geometry from Syracuse and Abers, G-cubed, 2006). In a comparison of 2D models we found that where subduction is most oblique the thermal structure of 2D cross-sections can differ up to ~60°C depending on whether the cross-section is normal to the trench or parallel to convergence. Due to the predicted strong 3D flow in the mantle wedge the discrepancies between 2D and 3D models will be higher. We use the 3D model to investigate the nature of the metamorphic changes in the slab using Perplex modeling, which also provides insights into the dehydration of the slab and the seismic velocity structure. The resulting thermal-petrological-seismological structure will be made available for general use and comparisons seismological, petrological and geochemical investigations of this complicated subduction system.

  6. Global analysis of the effect of fluid flow on subduction zone temperatures

    NASA Astrophysics Data System (ADS)

    Rotman, Holly M. M.; Spinelli, Glenn A.

    2013-08-01

    Knowledge of the controls on temperature distributions at subduction zones is critical for understanding a wide range of seismic, metamorphic, and magmatic processes. Here, we present the results of ˜220 thermal model simulations covering the majority of known subduction zone convergence rates, incoming plate ages, and slab dips. We quantify the thermal effects of fluid circulation in the subducting crust by comparing results with and without advective heat transfer in the oceanic crustal aquifer. We find that hydrothermal cooling of a subduction zone is maximized when the subducting slab is young, slowly converging, steeply dipping, and the crustal aquifer is ventilated near the trench. Incoming plate age is one of the primary controls on the effectiveness of advective heat transfer in the aquifer, and the greatest temperature effects occur with an incoming plate <50 Ma. The thermal effects of fluid circulation decrease dramatically with increasing age of the incoming plate. Temperatures in the Cascadia, Nankai, southern Chile, Colombia/Ecuador, Mexico, and Solomon Islands subduction zones are likely strongly affected by fluid circulation; for these systems, only thermal models of Cascadia and Nankai have included fluid flow in subducting crust.

  7. Petrology of Subducted Slabs

    Microsoft Academic Search

    Stefano Poli; Max W. Schmidt

    2002-01-01

    The subducted lithosphere is composed of a complex pattern of chemical systems that undergo continuous and discontinuous phase transformation, through pressure and temperature variations. Volatile recycling plays a major geodynamic role in triggering mass transfer, melting, and volcanism. Although buoyancy forces are controlled by modal amounts of the most abundant phases, usually volatile-free, petrogenesis and chemical differentiation are controlled by

  8. Subduction modelling with ASPECT

    NASA Astrophysics Data System (ADS)

    Glerum, Anne; Thieulot, Cédric; Spakman, Wim; Quinquis, Matthieu; Buiter, Susanne

    2013-04-01

    ASPECT (Advanced Solver for Problems in Earth's ConvecTion) is a promising new code designed for modelling thermal convection in the mantle (Kronbichler et al. 2012). The code uses state-of-the-art numerical methods, such as high performance solvers and adaptive mesh refinement. It builds on tried-and-well-tested libraries and works with plug-ins allowing easy extension to fine-tune it to the user's specific needs. We make use of the promising features of ASPECT, especially Adaptive Mesh Refinement (AMR), for modelling lithosphere subduction in 2D and 3D geometries. The AMR allows for mesh refinement where needed and mesh coarsening in regions less important to the parameters under investigation. In the context of subduction, this amounts to having very small grid cells at material interfaces and larger cells in more uniform mantle regions. As lithosphere subduction modelling is not standard to ASPECT, we explore the necessary adaptive grid refinement and test ASPECT with widely accepted benchmarks. We showcase examples of mechanical and thermo-mechanical oceanic subduction in which we vary the number of materials making up the overriding and subducting plates as well as the rheology (from linear viscous to more complicated rheologies). Both 2D and 3D geometries are used, as ASPECT easily extends to three dimensions (Kronbichler et al. 2012). Based on these models, we discuss the advection of compositional fields coupled to material properties and the ability of AMR to trace the slab's path through the mantle. Kronbichler, M., T. Heister and W. Bangerth (2012), High Accuracy Mantle Convection Simulation through Modern Numerical Methods, Geophysical Journal International, 191, 12-29.

  9. Stress, strain, and B-type olivine fabric in the fore-arc mantle: Sensitivity tests using high-resolution steady-state subduction zone models

    Microsoft Academic Search

    E. A. Kneller; P. E. van Keken; I. Katayama; S. Karato

    2007-01-01

    Shear-wave splitting observations at several subduction systems show trench-parallel fast directions in the fore-arc mantle. The presence of B-type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low-temperature and hydrated conditions. Sensitivity tests are shown that provide insights into the distribution and magnitude of B-type fabric using two-dimensional, high-resolution, kinematic-dynamic subduction zone

  10. Neogene subduction beneath Java, Indonesia: Slab tearing and changes in magmatism

    NASA Astrophysics Data System (ADS)

    Cottam, Michael; Hall, Robert; Cross, Lanu; Clements, Benjamin; Spakman, Wim

    2010-05-01

    Java is a Neogene calc-alkaline volcanic island arc formed by the northwards subduction of the Indo-Australian Plate beneath Sundaland, the continental core of SE Asia. The island has a complex history of volcanism and displays unusual subduction characteristics. These characteristics are consistent with the subduction of a hole in the down going slab that was formed by the arrival of a buoyant oceanic plateau at the trench. Subduction beneath Java began in the Eocene. However, the position and character of the calc-alkaline arc has changed over time. An older Paleogene arc ceased activity in the Early Miocene. Volcanic activity resumed in the Late Miocene producing a younger arc to the north of the older arc, and continues to the present day. An episode of Late Miocene thrusting at about 7 Ma is observed throughout Java and appears to be linked to northward movement of the arc. Arc rocks display typical calc-alkaline characteristics and reflect melting of the mantle wedge and subducted sediments associated with high fluid fluxes. Between West Java and Bali the present arc-trench gap is unusually wide at about 300 km. Seismicity identifies subducted Indian Ocean lithosphere that dips north at about 20° between the trench and the arc and then dips more steeply at about 60-70° from 100 to 600 km depth. In East Java there is gap in seismicity between about 250 and 500 km. Seismic tomography shows that this gap is not an aseismic section of the subduction zone but a hole in the slab. East Java is also unusual in the presence of K-rich volcanoes, now inactive, to the north of the calc-alkaline volcanoes of the active arc. In contrast to the calc-alkaline volcanism of the main arc, these K-rich melts imply lower fluid fluxes and a different mantle source. We suggest that all these observations can be explained by the tearing of the subducting slab when a buoyant oceanic plateau arrived at the trench south of East Java at about 8 Ma. With the slab unable to subduct, continued convergence caused contractional deformation and thrusting in Java. The slab then broke in front of the plateau. The trench stepped back to the south by about 150 km and subduction resumed behind the plateau, causing a hole to develop in the subducting slab. As the hole passed beneath the arc, and fluid flux declined, normal calc-alkaline volcanism ceased. With the mantle wedge melt component ‘switched off' K-rich melts, produced from a deeper mantle component that remained undiluted, dominated arc volcanism. As the hole got deeper K-rich volcanism ceased. Normal, calc-alkaline, arc activity resumed when the untorn slab following the hole was subducted.

  11. Slab Roll-Back and Trench Retreat As Controlling Factor for Island-Arc Related Basin Evolution: A Case Study from Southern Central America

    NASA Astrophysics Data System (ADS)

    Brandes, C.; Winsemann, J.

    2014-12-01

    Slab roll-back and trench retreat are important factors for basin subsidence, magma generation and volcanism in arc-trench systems. From the sedimentary and tectonic record of the Central American island-arc it is evident that repeated slab roll-back and trench retreats occurred since the Late Cretaceous. These trench retreats were most probably related to the subduction of oceanic plateaus and seamounts. Evidence for trench retreats is given by pulses of uplift in the outer-arc area, followed by subsidence in both the fore-arc and back-arc basins. The first slab roll-back probably occurred during the Early Paleocene indicated by the collapse of carbonate platforms, and the re-deposition of large carbonate blocks into deep-water turbidites. At this time the island-arc was transformed from an incipient non-extensional stage into an extensional stage. A new pulse of uplift or decreased subsidence, respectively during the Late Eocene is attributed to subduction of rough crust, a subsequent slab detachment and the establishment of a new subduction zone further westward. Strong uplift especially affected the outer arc of the North Costa Rican arc segment. In the Sandino Fore-arc basin very coarse-grained deep-water channel-levee complexes were deposited. These deposits contain large well-rounded andesitic boulders and are rich in reworked shallow-water carbonates pointing to uplift of the inner fore-arc. Evidence for the subsequent trench retreat is given by an increased subsidence during the early Oligocene in the Sandino Fore-arc Basin and the collapse of the Barra Honda platform in North Costa Rica. Another trench retreat might have occurred in Miocene times. A phase of higher subsidence from 18 to 13 Ma is documented in the geohistory curve of the North Limon Back-arc Basin. After a short pulse of uplift the subsidence increased to approx. 300 m/myr.

  12. Recent Results of Hadal Investigations in the Southern Mariana Trench

    NASA Astrophysics Data System (ADS)

    Fryer, P. B.; Hellebrand, E.; Sharma, S. K.; Acosta-Maeda, T.; Jicha, B. R.; Cameron, J.

    2014-12-01

    The deepest parts of the southern Mariana Trench have variously been interpreted to 1) indicate strike-slip motion along the trench, 2) contain a series of 3 sediment ponds at greater than 10,900 m depth separated from one another by fault-controlled ridges on the subducting plate, and 3) have an even deeper feature in the western-most pond (Vitiaz Deep). Recent lander deployments in all three ponds and the Deepsea Challenger submersible dive by J. Cameron in 2012 showed that the deepest ponds within the Challenger Deep area have nearly unbroken, flat surfaces. One point explored showed veined serpentinite at a depth of 10,800+ m. The potential for active serpentinite-hosted seeps and vent communities was demonstrated for the Shinkai Vent Field at 5,800m depth. Rocks collected using the Wood Hole Oceanographic Institution's hybrid remotely operated vehicle, Nereus, in 2009 from deep (10,879 m) on the incoming plate south of the Challenger Deep, were recovered from the base of a fault scarp where large, columnar-jointed blocks are draped with sediment. Optical microscopy, electron-microprobe and Raman analysis show that they are partially altered massive diabase with altered interstitial glass and containing microbial tubules in vug-filling secondary phases. The chain of seamounts striking NNW, colinear with the Lyra Trough, has been interpreted as a boundary between the Pacific Plate and the seafloor north of the Caroline Ridge. Sediments, drilled from above postulated basement north of the Caroline Ridge are no older that Oligocene. Ar/Ar age dates completed for one rock collected by Nereus in 2009 give a weighted mean plateau age, based on two experiments, of 24.6 +/- 3.2 Ma. Thus, the igneous basement of the subducting plate south of the Challenger Deep is, far younger than the Jurassic Pacific Plate subducting further east. This represents a previously unidentified tectonic plate. With new vehicles and technologies the future for hadal exploration is ripe.

  13. Multiple Layers of Anisotropy in the Chile-Argentina Subduction Zone, South America

    NASA Astrophysics Data System (ADS)

    Anderson, M. L.; Zandt, G.

    2004-12-01

    We examine shear wave splitting in teleseismic and local phases to observe seismic anisotropy in part of the South American subduction zone. The data is from the CHARGE network, which traversed the Andes Mountains of Chile and Argentina across two transects between 30° and 36° S. Beneath the southern part of our network, fast polarization directions from teleseismic phases are consistently trench-parallel, while in the northeastern part of the network fast directions are trench-normal. This trend appears to be correlated with a changing geometry of the subducted slab. Subduction zones may exhibit multiple layers of mantle anisotropy, corresponding to the mantle wedge, subducting slab, and the asthenosphere in the upper mantle below the slab. Based on previous work, we have suggested that the largest source of the anisotropy sampled by teleseismic phases is localized below the subducting slab. Preliminary measurements of anisotropy in the mantle wedge (sampled by local S-waves) reveal quite variable patterns in azimuth and magnitude, similar to other subduction zones. There is also an observed variation in splitting parameters in the teleseismic events with backazimuth that may be explained by a combination of wedge and below-slab anisotropy. However, preliminary two layer models with reasonable values for the wedge and below-slab components do not generally fit observed trends with backazimuth, therefore there may be three layers of anisotropy: the mantle wedge, the subducting slab lithosphere, and an asthenospheric layer. Because the magnitude of anisotropy due to the slab and to the wedge is relatively small compared to total anisotropy, removing it's effect from the total anisotropy leaves an asthenospheric component that is very similar to the total observed anisotropy by teleseisms. Calculations attempting this removal to date have made small adjustments to the telseismic values that actually make the asthenospheric component look more consistent across the network.

  14. Estimation of regional resistivity structure beneath the Kyushu, southwestern Japan, as inferred from the Network-MT survey

    Microsoft Academic Search

    M. Hata; N. Oshiman; R. Yoshimura; Y. Tanaka; M. Uyeshima; M. Ichiki

    2008-01-01

    Network-MT observations, which use telephone line networks as long baseline telluric measurements (Uyeshima, 1990), were carried out in the Kyushu district, southwestern Japan, from 1993 to 1998. The Kyushu district is the typical high angle subduction zone in Japan, which the Philippine sea plate subducts beneath the Eurasian plate, and some active volcanoes (for example, the Aso volcano, the Kirishima

  15. Forearc Mass Removal and the Effects of Subduction Erosion off the Nicoya Peninsula of Costa Rica

    Microsoft Academic Search

    P. Vannucchi; C. R. Ranero; D. W. Scholl

    2001-01-01

    Since early-middle Miocene subduction erosion has been the dominant process controlling the tectonics off northern Costa Rica. Leg 170's Site 1042, located 7 km landward of the Middle America Trench (MAT), reached the acoustically defined Base Of Slope Sediment (BOSS) horizon at a depth of ~ 3900 mbsl and yielded a carbonate-cemented breccia, 16.5 myr-old, formed in a nearshore setting.

  16. Influences of recurrence times and fault zone temperatures on the age-rate dependence of subduction zone seismicity

    NASA Astrophysics Data System (ADS)

    McCaffrey, Robert

    1997-10-01

    Correlations among subduction zone seismicity, convergence rate and subducting plate age are reassessed considering the possible roles of both recurrence times and fault zone temperatures. Distributions of earthquakes with respect to subducting lithosphere age and convergence rate are grossly explained by a recurrence relation when ages and rates at the world's trenches are taken into account. Correlations between maximum earthquake size Mwmax and convergence rate occur because faster subduction lowers the average recurrence time, so that at random within a limited sampling time, faster subduction zones have larger earthquakes. Published empirical slopes of an assumed linear relationship between Mwmax and convergence rate are predicted to within 1 standard deviation by such a recurrence model. Recurrence predicts that Mwmax should be related to the logarithm of convergence rate and revised age-rate- Mwmax data agree with such a relationship. No resolvable global correlation between age and Mwmax is found. Hence mechanical explanations of subduction zone seismicity based on such correlations are not required. Predicted average steady state temperatures, based on age and dip of the subducting lithosphere and convergence rate, at most subduction zone thrust faults are within a small range of values (±50°C). The few warm outliers, that is, Cascadia, Mexico, and southernmost Chile (south of Chile Rise) that subduct very young lithosphere, also have low seismic coupling coefficients suggesting that high temperature may inhibit seismicity. Fault zone temperatures can explain global variations in seismic coupling coefficients as well as the slab anchor model of Scholz and Campos [1995]. Applying this to the Cascadia subduction zone, in contrast to conclusions drawn from mechanical models of subduction and consistent with many other observations, due to its extremely high temperature, Cascadia may be a region where aseismic subduction predominates.

  17. Influence of Soil Suction on Trench Stability

    Microsoft Academic Search

    Valerie Whenham; Monika De Vos; Christian Legrand; Robert Charlier; Jan Maertens; Jean-Claude Verbrugge

    A research project on the stability of temporary trenches in unsaturated soil is carried out in Belgium. The main objective\\u000a of the project is to evaluate the seasonal variations of suction in the soil and to quantify the consequences of these suction\\u000a variations on trench stability. Within the framework of the research, a full scale instrumented test trench with vertical

  18. Recent Intermediate Depth Earthquakes in El Salvador, Central Mexico, Cascadia and South-West Japan

    NASA Astrophysics Data System (ADS)

    Lemoine, A.; Gardi, A.; Gutscher, M.; Madariaga, R.

    2001-12-01

    We studied occurence and source parameters of several recent intermediate depth earthquakes. We concentrated on the Mw=7.7 salvadorian earthquake which took place on January 13, 2001. It was a good example of the high seismic risk associated to such kind of events which occur closer to the coast than the interplate thrust events. The Salvadorian earthquake was an intermediate depth downdip extensional event which occured inside the downgoing Cocos plate, next to the downdip flexure where the dip increases sharply before the slab sinks more steeply. This location corresponds closely to the position of the Mw=5.7 1996 and Mw=7.3 1982 downdip extensional events. Several recent intermediate depth earthquakes occured in subduction zones exhibiting a ``flat slab'' geometry with three distinct flexural bends where flexural stress may be enhanced. The Mw=6.7 Geiyo event showed a downdip extensional mechanism with N-S striking nodal planes. This trend was highly oblique to the trench (Nankai Trough), yet consistent with westward steepening at the SW lateral termination of the SW Japan flat slab. The Mw=6.8 Olympia earthquake in the Cascadia subduction zone occured at the downdip termination of the Juan de Fuca slab, where plate dip increases from about 5o to over 30o. The N-S orientation of the focal planes, parallel to the trench indicated downdip extension. The location at the downdip flexure corresponds closely to the estimated positions of the 1949 M7.1 Olympia and 1965 M6.5 Seattle-Tacoma events. Between 1994 and 1999, in Central Mexico, an unusually high intermediate depth seismicity occured where several authors proposed a flat geometry for the Cocos plate. Seven events of magnitude between Mw=5.9 and Mw=7.1 occured. Three of them were downdip compressional and four where down-dip extensional. We can explain these earthquakes by flexural stresses at down-dip and lateral terminations of the supposed flat segment. Even if intermediate depth earthquakes occurence could be favored by stress transfer between intermediate depth and interplate zone during the earthquake cycle, flexural stresses associated with bendings which are not only present at ``flat slab'' geometry but also at ``normal'' dipping subduction zone, seem to govern the location of intermediate depth seismicity and to explain their focal mechanisms in El Salvador, SW Japon, Cascadia and Central Mexico.

  19. Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone

    NASA Astrophysics Data System (ADS)

    Solomon, Evan A.; Kastner, Miriam; Wheat, C. Geoffrey; Jannasch, Hans; Robertson, Gretchen; Davis, Earl E.; Morris, Julie D.

    2009-05-01

    Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, ~ 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, ~ 0.5 km landward of the trench, in the décollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (~ 50%) and a subduction zone fluid originating within the forearc (~ 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the décollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of ~ 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes.

  20. Antigorite-induced seismic anisotropy and implications for deformation in subduction zones and the Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Shao, Tongbin; Ji, Shaocheng; Kondo, Yosuke; Michibayashi, Katsuyoshi; Wang, Qian; Xu, Zhiqin; Sun, Shengsi; Marcotte, Denis; Salisbury, Matthew H.

    2014-03-01

    The present study, which is a follow-up of the Journal of Geophysical Research paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic-preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c axis fabric, is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that Vp anisotropy decreases but shear wave splitting (?Vs) and Vp/Vs increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in Vp, Vs and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700°C in the region of interest.

  1. Cascadia Subduction Zone

    USGS Publications Warehouse

    Frankel, Arthur D.; Petersen, Mark D.

    2008-01-01

    The geometry and recurrence times of large earthquakes associated with the Cascadia Subduction Zone (CSZ) were discussed and debated at a March 28-29, 2006 Pacific Northwest workshop for the USGS National Seismic Hazard Maps. The CSZ is modeled from Cape Mendocino in California to Vancouver Island in British Columbia. We include the same geometry and weighting scheme as was used in the 2002 model (Frankel and others, 2002) based on thermal constraints (Fig. 1; Fluck and others, 1997 and a reexamination by Wang et al., 2003, Fig. 11, eastern edge of intermediate shading). This scheme includes four possibilities for the lower (eastern) limit of seismic rupture: the base of elastic zone (weight 0.1), the base of transition zone (weight 0.2), the midpoint of the transition zone (weight 0.2), and a model with a long north-south segment at 123.8? W in the southern and central portions of the CSZ, with a dogleg to the northwest in the northern portion of the zone (weight 0.5). The latter model was derived from the approximate average longitude of the contour of the 30 km depth of the CSZ as modeled by Fluck et al. (1997). A global study of the maximum depth of thrust earthquakes on subduction zones by Tichelaar and Ruff (1993) indicated maximum depths of about 40 km for most of the subduction zones studied, although the Mexican subduction zone had a maximum depth of about 25 km (R. LaForge, pers. comm., 2006). The recent inversion of GPS data by McCaffrey et al. (2007) shows a significant amount of coupling (a coupling factor of 0.2-0.3) as far east as 123.8? West in some portions of the CSZ. Both of these lines of evidence lend support to the model with a north-south segment at 123.8? W.

  2. New seafloor map of the Puerto Rico trench helps assess earthquake and tsunami hazards

    NASA Astrophysics Data System (ADS)

    Brink, Uri ten; 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 l). 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 [McCann et al., 2004]. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918 [Mercado and McCann, 1998]. Large landslide escarpments have been mapped on the seafloor north of Puerto Rico [Mercado et al., 2002; Schwab et al., 1991],although their ages are unknown.

  3. New seafloor map of the Puerto Rico Trench helps assess earthquake and tsunami hazards

    USGS Publications Warehouse

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

    2004-01-01

    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 l). 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 [McCann et al., 2004]. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918 [Mercado and McCann, 1998]. Large landslide escarpments have been mapped on the seafloor north of Puerto Rico [Mercado et al., 2002; Schwab et al., 1991],although their ages are unknown.

  4. Ophiolites of the deep-sea trenches of the western Pacific

    SciTech Connect

    Chudaev, O.

    1990-06-01

    Igneous and metamorphic rocks of ophiolites are widespread in the basement of the Izu-Bonin, Volcano, Mariana, Philippine, Yap, Palau, New Hebrides, West Melanesian, Tonga, and Mussau trenches. Ophiolite in the trenches includes (1) metamorphic rocks ranging from low-T and low P to high-T and moderate P; (2) serpentinites after harzburgite; (3) ultramafic-mafic layered series; (4) tholeiitic basalts, dolerites, and their metamorphosed varieties; and (5) rocks of the boninitic series. Two geodynamic stages could be distinguished in the formation of the ophiolites of the Western Pacific. At the rifting stage, mantle diapir ascended during rifting and magmatic series were formed. In this period, rocks underwent intense low-temperature metasomatic alterations in the zones of heated seawater circulation. At the compression stage (subduction zone) regional pressure metamorphism of the ophiolites superimposed on the low-temperature metasomatic processes.

  5. Shallow subduction zone earthquakes and their tsunamigenic potential

    NASA Astrophysics Data System (ADS)

    Polet, J.; Kanamori, H.

    2000-09-01

    We have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7.0 or greater, as well as some other interesting events, in the period range 1-20s, by computing moment rate functions of teleseismic P waves. After comparing the source spectral characteristics of `tsunami earthquakes' (earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude) with regular events, we identified a subclass of this group: `slow tsunami earthquakes'. This subclass consists of the 1992 Nicaragua, the 1994 Java and the February 1996 Peru earthquakes. We found that these events have an anomalously low energy release in the 1-20s frequency band with respect to their moment magnitude, although their spectral drop-off is comparable to those of the other earthquakes. From an investigation of the centroid and body wave locations, it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up-dip until the stable sliding friction regime of unconsolidated sediments stops the propagation. Sediment-starved trenches, e.g. near Jalisco, can produce very shallow slip, because the fault material supports unstable sliding. The slow tsunami earthquakes also ruptured up-dip; however, their centroid is located unusually close to the trench axis. The subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment. Ocean surveys show that in these regions the ocean floor close to the trench is highly faulted. We suggest that the horst-and-graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate, making shallow earthquake nucleation and up-dip propagation to the ocean floor possible. The rupture partly propagates in sediments, making the earthquake source process slow. Two factors have to be considered in the high tsunami-generating potential of these events. First, the slip propagates to shallow depths in low-rigidity material, causing great deformation and displacement of a large volume of water. Second, the measured seismic moment may not represent the true earthquake displacement, because the elastic constants of the source region are not taken into account in the standard CMT determination.

  6. Potential for generation of natural gas in sediments of the convergent margin of the Aleutian Trench Area

    SciTech Connect

    Kvenvolden, K.A.; von Huene, R.

    1983-01-01

    Sediment being subducted in the eastern part of the convergent margin of the Aleutian Trench has a potential to generate large volumes of natural gas, perhaps as much as 2.8 x 10/sup 6/ m/sup 3/ of methane per km/sup 3/ of sediment, even though the content of organic carbon in the sediment is very low, averaging about 0.4%. This high potential for gas generation results primarily from the enormous volume of sediment undergoing subduction. Along the eastern Aleutian Arc-Trench system a 3-km thick sheet of sediment is being subducted at a rate of about 60 km per million years. We estimate, based on considerations of the stability requirements for gas hydrates observed as anomalous reflectors in some of our seismic records, and on one measurement in a deep well, that the geothermal gradient in this region is about 30/sup 0/C/km. Such a gradient suggests a temperature regime in which the maximum gas generation in the subducting sediment occurs beneath the upper slope. Thus the sediment of the upper slope, as opposed to that of the shelf and lower slope, could be the most prospective for gas accumulation if suitable reservoirs are present. 40 refs., 11 figs., 3 tabs.

  7. Dynamics of free subduction from 3-D boundary element modeling

    NASA Astrophysics Data System (ADS)

    Li, Zhong-Hai; Ribe, Neil M.

    2012-06-01

    In order better to understand the physical mechanisms underlying free subduction, we perform three-dimensional boundary-element numerical simulations of a dense fluid sheet with thickness h and viscosity ?2 sinking in an `ambient mantle' with viscosity ?1. The mantle layer is bounded above by a traction-free surface, and is either (1) infinitely deep or (2) underlain by a rigid boundary at a finite depth H + d, similar to the typical geometry used in laboratory experiments. Instantaneous solutions in configuration (1) show that the sheet's dimensionless `stiffness' S determines whether the slab's sinking speed is controlled by the viscosity of the ambient mantle (S < 1) or the viscosity of the sheet itself (S > 10). Time-dependent solutions with tracers in configuration (2) demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides. The extra `edge drag' exerted by the flow around the sides causes transverse deformation of the slab, and makes the sinking speed of a 3-D slab up to 40% less than that of a 2-D slab. A systematic investigation of the slab's interaction with the bottom boundary as a function of ?2/?1 and H/h delineates a rich regime diagram of different subduction modes (trench retreating, slab folding, trench advancing) and reveals a new `advancing-folding' mode in which slab folding is preceded by advancing trench motion. The solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary.

  8. Challenges in hardening technologies using shallow-trench isolation

    Microsoft Academic Search

    M. R. Shaneyfelt; P. E. Dodd; B. L. Draper; R. S. Flores

    1998-01-01

    Challenges related to radiation hardening CMOS technologies with shallow-trench isolation are explored. Results show that trench hardening can be more difficult than simply replacing the trench isolation oxide with a hardened field oxide.

  9. The variations of long time period slow slip events along the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Tu, Y. T.; Heki, K.

    2014-12-01

    Slow slip events (SSEs) are a type of slow earthquakes that can be observed with Global Positioning System (GPS) networks in the world. Those events are detected on intensely coupled plate boundaries such as Cascadia subduction zone (Dragert et al., 2001), western North America, Mexico (Kostoglodov et al., 2003), Alaska (Ohta et al., 2007) and Tokai and Boso areas (Ozawa et al., 2002, 2003), central Japan and are considered to have relations to large subduction thrust earthquakes. However, in southwestern Ryukyu trench where most of researchers believe that it should be a decoupled plate boundary, SSEs recur regularly and are located at a patch that is as deep as 20 to 40 km (Heki and Kataoka, 2008). For comprehending the characteristics and time variations of SSEs in this area, the GEONET GPS data of 16 years are used in this study. During 1997 to 2014, more than thirty SSEs are identified near Hateruma Island, Ryukyu. The average recurrence interval is calculated to be 6.3 months and release seismic moment is Mw 6.6 on average. However, the values of recurrence interval are not invariable. From 1997 to 2002, interval period of SSEs is 7.5 months, but during 2002 to 2008, the interval period decreases suddenly to 5.5 months. After 2008, the value restores to 7.2 months again. Furthermore, the slip amount of SSEs in this area varies with time. From 1997 to 2002, the slip is 9.5 cm/year; and during 2002 to 2008, the value slightly increases to 10.5 cm/year. However, in 2008 to 2013, the slip drops to 6.6 cm/year, but accord to the trend of cumulative slip, the slip value would increase in 2014. Considering these data, we find the slip values increase conspicuously in 2002 and 2013. Coincidentally, one Mw 7.1 thrust earthquake occurred in 2002 and earthquake swarm activity started in the Okinawa trough approximately 50km north of the SSE patch. In 2013, another earthquake swarm activity occurred in nearly the same area as the 2002 activity. This suggests that the slip amount varies due to the earthquake swarm activities in Okinawa trough; the external stress perturbations such as magma injection and the Mw 7.1 earthquake could accelerate the slip amount of SSEs.

  10. An oceanic plateau subduction: A case study offshore Eastern Java.

    NASA Astrophysics Data System (ADS)

    Shulgin, Alexey; Kopp, Heidrun; Mueller, Christian; Planert, Lars; Lueschen, Ewald; Flueh, Ernst; Djajadihardja, Yusuf

    2010-05-01

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

  11. Mantle flow around the slab edge: Numerical simulations and their implications for the trench-parallel flow in the sub-slab mantle

    NASA Astrophysics Data System (ADS)

    Honda, S.

    2008-12-01

    A simple semi-dynamic numerical model of the subduction zone, incorporating the plate velocity and a moving plate boundary as a priori constraints, has been constructed. In order to obtain the feature similar to the subduction, the flow velocity on the top and the small region around the shallow plate boundary is kinematically given and the flow in other part is dynamically calculated. We applied this model to the study of mantle flow around the slab-edge. The plate configuration mimics the region around the junction of Aleutian Islands and Kamchatka, that is, the convergent-transform fault plate boundaries. For the case with the overlying plate being almost stationary, the lateral flow from the sub-slab mantle under the subducting slab to the mantle under the neighboring plate hardly exists, once the slab penetrates into the high viscosity layer where the downward flow encounters the resistance. Similar result was obtained for the case with advancing trench, that is, the trench moves toward the overlying plate. For the case with retreating trench, that is, the trench moves toward the subducting plate, such sub-slab mantle flow exists even after the penetration of slab into the high viscosity layer. However, its speed is significantly smaller than that of plate velocity. A significant lateral flow is observed when the high temperature anomaly, that is, buoyant and low viscosity block is carried toward the slab. These results may have important implications for the understanding of trench- parallel anisotropy of seismic velocity in the sub-slab mantle.

  12. The Tectonic Effect of The Rollback of The Hellenic Trenches On The Westward Motion of Anatolian

    NASA Astrophysics Data System (ADS)

    Mart, Y.; Ryan, W. B. F.

    The westward tectonic motion of Anatolia has been considered a prime example of tectonic escape, and it was generally accepted that the northwards motion of the Ara- bian Plate along the Dead Sea Transform Fault pressed against the eastern edge of the Anatolian Plate and pushed it westwards. Anatolia was presumably escaping from the northward drive of Arabia. However, recently acquired GPS measurements in Anato- lia show clearly that the rate of its westwards motion increases westwards, from East Anatolia to the Aegean Sea and the Hellenic Trenches. These findings stand in contrast with the escape tectonic model, according to which the westwards motion of Anatolia would have decreased due to friction. Consequently the tectonic interpretation of the GPS measurements is baffling. A series of laboratory centrifuge experiments, which studied the effect of seismic friction on various subduction-related phenomena seems to elucidate the Anatolian enigma. The experiments show that reduced seismic friction would increase the pull stresses of the over-riding slab seawards, which, in turn, would enhance subduction trench roll-back, and consequently augment the growth of the back-arc basin, and pull, stretch and thin the over-riding slab. The experiments indicate that the penetration of the subducting lithosphere into the asthenosphere generates flow that has a double effect U it drives the underthrust slab downwards faster, and it pulls the over-riding slab "seawards" and leads to arc migration in that direction. Seawards arc migration occurred repeatedly in the Mediterranean Sea. It was traced in the Alboran Sea, in the Tyrrhenian Sea and also in the Ionian Sea. Merging these observations and the experimental results leads us to suggest that the tectonics of Ana- tolia is driven by the northwards subduction of lithosphere of the Ionian Sea, which is lubricated by the thick sedimentary sequence that overlies the magmatic lower crust, and especially the thick series of Messinian evaporites. Consequently the subduction processes and the roll-back of the Hellenic Trenches pulls the Anatolian plate west- wards, a pull that diminishes eastwards gradually due to friction, as indicated by the GPS measurements. It seems that Anatolia does not escape from Arabia, but rather it is lured by Ionia.

  13. Coseismic and interseismic displacements at a subduction zone - a parameter study using finite-element modelling

    NASA Astrophysics Data System (ADS)

    Li, Tao; Hampel, Andrea

    2013-04-01

    Tide-gauge and geodetic measurements of coseismic and interseismic displacements in the forearc of subduction zones showed that the coastal region undergoes uplift during the interseismic phase and subsidence during the coseismic phase, while opposite vertical movements are observed in the neighbouring regions (e.g., Savage & Thatcher 1992; Hyndman & Wang 1995). Horizontal displacements during the interseismic phase are typically directed landward, whereas the forearc moves seaward during the earthquake (e.g., Klotz et al. 1999). Here we use two-dimensional finite-element modelling to evaluate how the friction coefficient along the plate interface, the length and the position of the downdip end of the locked zone affect the coseismic and interseismic displacements. Our model consists of a deformable, rheologically stratified upper plate and an undeformable oceanic plate, which rotates at a prescribed angular velocity (cf. Cailleau & Oncken, 2008). The frictional plate interface is divided - from the trench to the base of the continental lithosphere - into a seismogenic zone, a transition zone and a landward free slip zone. During an initial phase, the seismogenic zone is locked, which leads to the accumulation of elastic strain in the forearc. During the subsequent coseismic phase, the strain is released and causes sudden slip of several meters on the plate interface. During the next interseismic phase, the seismogenic zone is locked again. Our model results show patterns of vertical and horizontal displacements that are in general agreement with geodetically observed patterns. A sensitivity analysis reveals that the magnitude of the vertical displacements is strongly influenced by the friction coefficients of the seismogenic zone and the transition zone. The location of the zones of maximum interseismic uplift and coseismic subsidence in the coastal regions depends on the length and position of the locked zone. Preliminary results from three-dimensional models show that the lateral extent of the rupture zone also affects the direction of the coseismic surface displacements, as recorded by GPS stations during the 2011 Japan earthquake (Ozawa et al. 2011). References: Cailleau, B., O. Oncken (2008) Past forearc deformation in Nicaragua and coupling at the megathrust interface: Evidence for subduction retreat? Geochemistry, Geophysics, Geosystems, 9, Q03016, doi:10.1029/2007GC001754. Hyndman, R.D., K. Wang (1995) The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime. Journal of Geophysical Research, 100, 22133-22154. Klotz, J., D. Angermann, G.W. Michel, R. Porth, C. Reigber, J. Reinking, J. Reinking, J. Viramonte, R. Perdomo, V.H. Rios, S. Barrientos, R. Barriga, O. Cifuentes (1999) GPS-derived Deformation of the Central Andes Including the 1995 Antofagasta Mw=8.0 Earthquake. Pure and Applied Geophysics, 154, 709-730. Ozawa, S., T. Nishimura, H. Suito, T. Kobayashi, M. Tobita, T. Imakiire (2011) Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake. Nature, 475, 373-376. Savage, J.C., Thatcher, W. (1992) Interseismic deformation at the Nankai Trough, Japan, subduction zone. Journal of Geophysical Research, 97, 11117-11135.

  14. Terrane Stations: intra-oceanic subduction assembled western North America

    NASA Astrophysics Data System (ADS)

    Sigloch, K.; Mihalynuk, M. G.

    2012-12-01

    The western quarter of North America consists of accreted terranes, crustal blocks that were added to the margin in a series of collisions over the past 200 million years - but why? The most widely accepted explanation posits a scenario analogous to Andean subduction, with these terranes conveyed to the continental margin while the oceanic Farallon plate subducted under it. Yet purely Andean-style subduction under North America is questionable as a terrane delivery mechanism, since no comparable accretion sequence took place along the South American margin, and since North American terranes are of very varied provenance. We consider this geological question directly related to a geodynamical one: Why has it been so difficult to reconcile - even on the largest scale - the geometries and locations of slabs in the lower-mantle, as imaged by seismic tomography, with Cretaceous plate reconstructions of the North American west coast (unless anomalous mantle rheology or ad hoc shifts of absolute reference frame are invoked)? This problem was recognized soon after the discovery of the massive, lower-mantle "Farallon slabs" by Grand (1994), but has recently been aggravated by the discovery of additional, more westerly deep slabs (Sigloch et al. 2008), thanks to USArray. Not all of these slabs can be Farallon, unless very non-vertical and/or uneven slab sinking behavior is allowed for. As a joint solution, we offer a radical reinterpretation of paleogeography and test it quantitatively: The seas west of Cretaceous North America must have resembled today's western Pacific. The Farallon and two more plates subducted into the intra-oceanic trenches of a vast archipelago in the eastern Panthalassa (proto-Pacific) ocean, both from the east and the west. The trenches remained stationary throughout much of Jurassic and Cretaceous times, depositing the massive, near-vertical slab walls imaged in the lower mantle today. On their overriding plates, island arcs and subduction complexes nucleated, and assembled with exotic fragments - hence our designation of intra-oceanic trenches as "terrane stations". The archipelago was gradually overridden by North America on its westward journey away from Pangaea. Episodes of crustal accretion and Cordilleran mountain building (Sevier, Canadian Rocky Mountains, Laramide) occurred when the continental margin collided with various parts of the archipelago. Into this accretion sequence, we can fit today's interior Alaska, the Franciscan subduction complex, the Intermontane and Insular superterranes of British Columbia, and terranes of the Pacific Northwest, such as Siletzia. Geodynamically, our scenario is simpler than previous models in that it is consistent with purely vertical slab sinking. Sinking rates can be quantified from slab and plate geometries, and range between 9 and 12 mm/yr. References: S. P. Grand, Mantle shear structure beneath the Americas and surrounding oceans, Journal of Geophysical Research 99, 11,591-11,621 (1994). K. Sigloch, N. McQuarrie, G. Nolet, Two-stage subduction history under North America inferred from multiple-frequency tomography, Nature Geoscience 1, 458-462 (2008).

  15. Modeling of Mantle Convection in 3D Subduction Zones

    NASA Astrophysics Data System (ADS)

    Bengtson, A. K.; van Keken, P. E.; Lin, S.; Kneller, E. A.

    2010-12-01

    The influence of 3D subduction zone geometries on mantle convection patterns is not well characterized. Subducting plates with complex 3D geometries often exhibit seismological signatures that cannot be explained by 2D flow patterns [Hoernle et al. Nature 2008/2009; Long and Silver Science 2008; Kneller et al. Nature 2007]. We use finite element methods to study the lateral transport and thermal structure in 3D subduction zones and their evolution over time. Realistic 3D geometries were created from seismological observations for both the Marianas and Central America, building upon the work of Kneller et al. [Nature 2007; Geochem. Geophys. Geosyst. 2008]. Highly refined meshes were created using spherical geometry on a Cartesian mesh, and slab motion was prescribed in a kinematic manner. The finite element code Sepran was used to solve the Stokes and heat equations using diffusion creep laws for viscosity. We present the velocities, temperatures, and pressures in the mantle wedge for the Marianas and Central America regions. Comparisons are made between 2D and 3D flow patterns to test the dependence of flow on 3D geometries, in addition to dependence of flow on plate direction. Resulting thermal and lateral structures are important for understanding mantle and slab mineralogy and seismic signatures. We find the lateral transport is particularly important in regions of strong obliquity and trench curvature.

  16. Vp\\/Vs Ratio and Depth to Moho and the Subducting Cocos Slab across Northern Costa Rica estimated from Receiver Function Analysis

    Microsoft Academic Search

    L. Linkimer; S. Beck; S. Schwartz; G. Zandt; V. Levin

    2008-01-01

    Costa Rica is located near the southern end of the Middle American Trench (MAT) in a complicated tectonic setting controlled by the interaction of the Cocos, Caribbean, and Nazca plates. The oceanic Cocos plate subducts to the northeast underneath the Caribbean plate creating a volcanic arc located 150 km away from MAT. In Northern Costa Rica the arc basement is

  17. The Central Chilean Margin: Lower Plate Structure and Subduction Zone Geometry

    NASA Astrophysics Data System (ADS)

    Kopp, H.; Flueh, E. R.; Klaeschen, D.; Thierer, P.; Ranero, C.; Gaedicke, C.

    2003-04-01

    The central Chilean margin was the target of a combined on-/offshore seismic experiment using RV SONNE as platform for the marine data acquisition during cruise SO161. The along-strike segmentation of the margin results in areas of reduced slab dip ('Flat slab' segments). Segment boundaries frequently coincide with the active subduction of bathymetric features on the lower plate. There may exist a correlation between the buoyancy of the subducted seafloor relief and the occurrence of shallow subduction. The fundamental effects of ridge subduction on the margin suggest a linkage of the subducting Juan Fernandez chain on the Nazca plate to the flat slab segment of the central Chilean margin, which poses one aspect investigated in the scope of the SPOC project presented here. The study area covers the eastern part of the aseismic Juan Fernandez Ridge and the continental slope off Valparaiso. Four wide-angle profiles were laid out: Two profiles are W-E oriented and located across the subduction complex at latitudes 32S and 31S, respectively. A small accretionary prism has accumulated against a backstop of increased shear strength. The upper slope is underlain by continental basement. 'Normal' oceanic crust enters the trench except where the lower plate is altered by the O'Higgins seamount group which marks the easternmost termination of the Juan Fernandez Ridge. This hotspot ridge is currently entering the trench and poses the target of a tomographic investigation using two perpendicular, densely spaced wide-angle lines. The inversion uses a top-to-bottom approach using first arrivals as well as later phases and includes a mantle inversion to obtain upper mantle velocities. Extrusive type volcanism formed the O'Higgins volcanoes when the plate moved over the hotspot that is currently forming Alexander Selkirk Island at the western termination of Juan Fernandez Ridge. A localized crustal root has evolved, but a downflexing of the crust cannot be observed.

  18. What earthquakes say concerning residual subduction and STEP dynamics in the Calabrian Arc region, south Italy

    NASA Astrophysics Data System (ADS)

    Orecchio, B.; Presti, D.; Totaro, C.; Neri, G.

    2014-12-01

    By relocation of shallow and intermediate depth earthquakes and joint evaluation of already available and properly estimated waveform inversion focal mechanisms we investigate the location and shallow kinematics of the residual subducting slab in the Calabrian Arc region, that is the only, apparently still active segment of the old subduction front of the western Mediterranean. In agreement with high P-wave velocity anomaly found at intermediate depths by previous local earthquake tomography, our shallow-to-intermediate earthquake hypocentre distribution shows that the Ionian subducting slab is still in-depth continuous only in a small internal segment of the Arc, while detachment or break-off processes have already developed elsewhere along the Arc. At the same time, the space distribution and the waveform inversion focal mechanisms of the earthquakes occurring at shallow depth (<70 km) do not evidence Subduction Transform Edge Propagator (STEP) fault activity at the edges of the descending slab. In particular, no trace is found of dip-slip faulting along near vertical planes parallel to the slab edges, that is no seismic evidence is available of vertical motion between the subducting segment of the plate and the adjacent portion of it. Also, the seismicity distribution and mechanisms found at crustal depths in the study region do not match properly with the expected scenario of relative motion at the lateral borders of the overriding plate. Our earthquake locations and mechanisms together with GPS information taken from the literature highlight a residual, laterally very short subducting slab showing quasi-nil velocity of trench retreat and no present-day STEP activity, still capable however of causing strong normal-faulting earthquakes in the trench area through its gravity-induced shallow deformation in a weak-coupling scenario.

  19. Slab1.0: A three-dimensional model of global subduction zone geometries

    NASA Astrophysics Data System (ADS)

    Hayes, Gavin P.; Wald, David J.; Johnson, Rebecca L.

    2012-01-01

    We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of `average' active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested.

  20. Forearc deformation at a modern day obliquely subducting margin: The Central American Perspective

    NASA Astrophysics Data System (ADS)

    Morgan, J. P.; Vannucchi, P.

    2012-12-01

    Central America provides a cornucopia of examples of different modes of forearc and arc deformation and their potential links to contemporaneous arc volcanism. In this talk we will focus on two: (1) The existence of notable strain partitioning with strike-slip motion of a forearc sliver from Nicaragua northward coupled to contemporaneous arc-extension in Nicaragua. This mode may represent an ongoing example of a typical mode of arc-crust injection to grow continental crust. In Nicaragua, arc extension and associated intra-arc accretion has occurred coeval with long-term subduction erosion of the forearc that has been well documented N (Guatamala) and S (Nicoya, Costa Rica) of Nicaragua. As a consequence of this mode of arc crustal growth, Nicaragua is the region of the Middle American Arc with the smallest relief volcanoes, yet the largest rates of recent mid-crustal accretion within the Arc. (2) The existence of extremely variable-rate subduction erosion associated with the recent subduction of the aseismic Cocos Ridge offshore Osa, Costa Rica. Here recent IODP results indicate that the forearc has had an extremely unstable and fluctuating response to the subduction of several large-relief fracture zones and the large-scale bathymetric relief of a plume+spreading center-created aseismic ridge. The Osa forearc has 'yo-yoed' several thousands of meters up and down in the past 2.5 Ma, while undergoing significant subduction erosion associated with the deposition of >800 m of new sediments into an Osa forearc basin. Curiously, this mode of extreme sedimentation into an eroding forearc is not associated with a sediment pile at the trench axis — essentially none of the recent sediments travels all the way downslope to reach the trench axis. We will discuss some implications of these observations for the mechanics of subduction channel and forearc deformation processes.

  1. Sediment and Rock Samples Recovered from the Challenger Deep, Southern Mariana Trench

    NASA Astrophysics Data System (ADS)

    Fryer, P. B.

    2013-12-01

    Sediments collected with push cores during Nereus hybrid-ROV traverses within the trench axis of the Challenger Deep were squeezed onboard ship (R/V Kilo Moana) to extract pore fluids. The squeeze-cakes were analyzed by XRD, SEM, Raman spectroscopy and electron microprobe. The analysis reveals an assemblage of clays, and volcanic ash that contains plagioclase, clino-pyroxene, opaques and, glass. Chlorite is present as are hydrated iron oxides and fragments of diatoms. The sediment is predominantly very fine-grained and video from both the Nereus traverses across the Challenger Deep in 2009 and from the Deepsea Challenger submersible dive by James Cameron in 2012 indicate recent resurfacing of the trench axis. This is consistent with the high degree of deformation and frequency of earthquakes in the southern Mariana forearc north of the trench. An apron of self-derived talus blankets the lower part of the inner trench slope and fines from submarine landslides are the likely source of the trench axis sediment. Rock samples collected using Nereus from deep (10,879 m) on the incoming plate south of the Challenger Deep are partially altered microgabbros with interstitial glass containing microtubules similar to those observed in a variety of marine settings in lavas and hypabyssal igneous rocks. The tubules are presumed to be caused by tunneling of lithoautotrophic microbes into the glass. The rocks were collected from a site at the base of a fault scarp where large columnar-jointed blocks are draped with sediment. The igneous basement of the subducting plate south of the Challenger Deep is, as yet, undated, but it may be younger than the Jurassic Pacific Plate subducting beneath the southeastern Mariana forearc. There is the suggestion of a boundary between and the Pacific Plate and the shallower sea floor (Caroline plate?) subducing beneath the southernmost arm of the Mariana Trench. Early interpretations by Hegarty and Weissel (1988) and more recently by Lee (2004) are that the chain of seamounts striking NNW, colinear with the Lyra Trough, might mark such a boundary. Age dating of the microgabbro recovered with Nereus should help to solve this question.

  2. Laboratory experiments on subduction-induced circulation in the wedge and the evolution of mantle diapirs

    NASA Astrophysics Data System (ADS)

    Sylvia, R. T.; Kincaid, C. R.; Behn, M. D.; Zhang, N.

    2014-12-01

    Circulation in subduction zones involves large-scale, forced-convection by the motion of the down-going slab and small scale, buoyant diapirs of hydrated mantle or subducted sediments. Models of subduction-diapir interaction often neglect large-scale flow patterns induced by rollback, back-arc extension and slab morphology. We present results from laboratory experiments relating these parameters to styles of 4-D wedge circulation and diapir ascent. A glucose fluid is used to represent the mantle. Subducting lithosphere is modeled with continuous rubber belts moving with prescribed velocities, capable of reproducing a large range in downdip relative rollback plate rates. Differential steepening of distinct plate segments simulates the evolution of slab gaps. Back-arc extension is produced using Mylar sheeting in contact with fluid beneath the overriding plate that moves relative to the slab rollback rate. Diapirs are introduced at the slab-wedge interface in two modes: 1) distributions of low density rigid spheres and 2) injection of low viscosity, low density fluid to the base of the wedge. Results from 30 experiments with imposed along-trench (y) distributions of buoyancy, show near-vertical ascent paths only in cases with simple downdip subduction and ratios (W*) of diapir rise velocity to downdip plate rate of W*>1. For W* = 0.2-1, diapir ascent paths are complex, with large (400 km) lateral offsets between source and surfacing locations. Rollback and back-arc extension enhance these offsets, occasionally aligning diapirs from different along-trench locations into trench-normal, age-progressive linear chains beneath the overriding plate. Diapirs from different y-locations may surface beneath the same volcanic center, despite following ascent paths of very different lengths and transit times. In cases with slab gaps, diapirs from the outside edge of the steep plate move 1000 km parallel to the trench before surfacing above the shallow dipping plate. "Dead zones" resulting from lateral and vertical shear in the wedge above the slab gap, produce slow transit times. These 4-D ascent pathways are being incorporated into numerical models on the thermal and melting evolution of diapirs. Models show subduction-induced circulation significantly alters diapir ascent beneath arcs.

  3. Bending mechanics and mode selection in free subduction: a thin-sheet analysis

    NASA Astrophysics Data System (ADS)

    Ribe, Neil M.

    2010-02-01

    To elucidate the dynamics of free (buoyancy-driven) subduction of oceanic lithosphere, I study a model in which a 2-D sheet of viscous fluid with thickness h and viscosity ??1 subducts in an infinitely deep ambient fluid with viscosity ?1. Numerical solutions for the sheet's evolution are obtained using the boundary-element method (BEM), starting from an initial configuration comprising a short `protoslab' attached to a longer horizontal `plate' that is free to move laterally beneath an impermeable traction-free surface. Interpretation of the solutions using thin viscous sheet theory shows that the fundamental length scale controlling the subduction is the `bending length' lb, defined at each instant as the length of the portion of the sheet's midsurface where the rate of change of curvature is significant. Geophysically speaking, lb is the sum of the lengths of the slab and of the region seaward of the trench where flexural bulging occurs. The bending length in turn enters into the definition of the sheet's dimensionless `stiffness' S ? ?(h/lb)3, which controls whether the sinking speed of the slab is determined by the viscosity of the sheet itself (S >> 1) or by that of the ambient fluid (S <= 1). Motivated by laboratory observations of different modes of subduction (retreating versus advancing trench, folding versus no folding, etc.) in fluid layers with finite depth, I calculate numerically the dip ?D of the slab's leading end as a function of ? and the normalized depth D/h to which it has penetrated. The contours of the function ?D(?, D/h) strongly resemble the intermode boundaries in the laboratory-based regime diagram of Schellart, supporting the hypothesis that the mode of subduction observed at long times in experiments is controlled by the dip of the slab's leading end when it reaches the bottom of the layer. In particular, the BEM solutions explain why trenches advance in the laboratory only when ? lies in an intermediate range, and why they retreat when ? is either smaller or larger than this. Application of the BEM model to Wu et al.'s compilation of the minimum curvature radii of subducted slabs suggests ? ? [140, 510] for the Earth. This is too small to permit the laboratory-type `trench advancing' mode, in agreement with the lack of tomographic evidence for slabs that are `bent over backwards'.

  4. Detailed structural image around splay-fault branching in the Nankai subduction seismogenic zone: Results from a high-density ocean bottom seismic survey

    Microsoft Academic Search

    Ayako Nakanishi; Shuichi Kodaira; Seiichi Miura; Aki Ito; Takeshi Sato; Jin-Oh Park; Yukari Kido; Yoshiyuki Kaneda

    2008-01-01

    To investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone, it is important to know the detailed structure around the plate boundary and active splay-fault system. The Nankai Trough, southwestern Japan, is among the best studied subduction zones with splay faults in the world. This paper presents a detailed structural image around the splay fault in the coseismic

  5. Biannually repeating slow slip events in an uncoupled segment of the Ryukyu Arc, SW Japan

    NASA Astrophysics Data System (ADS)

    Kataoka, T.; Heki, K.

    2007-12-01

    At the Ryukyu Trench, SW Japan, the Philippine Sea Plate (ph) subduct toward NW. The overriding Ryukyu Arc is composed of three nearly rigid blocks with little interseismic elastic deformation. The South Ryukyu Block (sr), the westernmost part of Japan, is known for extremely fast plate convergence (~12 cm/yr) by the the ph subduction and the oceanward movement of the arc due to active opening of the Okinawa Trough. There ph and sr are 'decoupled' i.e. large interplate thrust events do not occur. Yarai et al. (2004) found repeating slow slip events (SSE) there by analyzing the movement of Hateruma, the southernmost island of the arc, by GPS observations. Here we studied these SSE from the movements of six GPS points in five islands on sr during 1997-2007, and report their time constants, recurrence intervals, consistency between slips and plate convergence, predictability of events. The events occur mostly in spring and fall, but this apparent seasonality is caused by their rather regular biannual recurrences. The time constants ranged between 0.1 and 0.15 years, twice as long as those in Cascadia although their recurrence intervals (~6 months) were only ~1/2 of those in Cascadia (~13 months). SSE signals were clearly seen both in horizontal and vertical coordinates of GPS points in the islands Ishigaki and Iriomote, as well as Hateruma, and we estimated the fault parameters of the SSE as the ph-sr interplate events. Their slip directions were consistent with the plate convergence direction, and the typical amount of slip (~ 6 cm, Mw ~6.6) coincided with the convergence in 1/2 year. The depths of the faults were 20-40 km, which corresponds to the transient depth between seismogenic and freely sliding zones. This situation is similar to Shikoku and Cascadia except that there are no large thrust events at 'seismogenic zones' in Ryukyu. The amounts of slips had significant correlation with the time lengths until the next events, suggesting the time-predictable characteristics of the events. These SSE are unique in two points, i.e. (1) the first repeating SSE found in an uncoupled subduction zone, (2) best recorded SSE sequence with nearly 20 events and 3-D crustal movement data associated with the events.

  6. Plate interaction in the NE Caribbean subduction zone from continuous GPS observations

    USGS Publications Warehouse

    ten Brink, Uri S.; Lopez-Vegas, Alberto M.

    2012-01-01

    Kinematic similarities between the Sumatra and Puerto Rico Trenches highlight the potential for a mega-earthquake along the Puerto Rico Trench and the generation of local and trans-Atlantic tsunamis. We used the horizontal components of continuous GPS (cGPS) measurements from 10 sites on NE Caribbean islands to evaluate strain accumulation along the North American (NA) - Caribbean (CA) plate boundary. These sites move westward and slightly northward relative to CA interior at rates ?2.5 mm/y. Provided this motion originates in the subduction interface, the northward motion suggests little or no trench-perpendicular thrust accumulation and may in fact indicate divergence north of Puerto Rico, where abnormal subsidence, bathymetry, and gravity are observed. The Puerto Rico Trench, thus, appears unable to generate mega-earthquakes, but damaging smaller earthquakes cannot be discounted. The westward motion, characterized by decreasing rate with distance from the trench, is probably due to eastward motion of CA plate impeded at the plate boundary by the Bahamas platform. Two additional cGPS sites in Mona Passage and SW Puerto Rico move to the SW similar to Hispaniola and unlike the other 10 sites. That motion relative to the rest of Puerto Rico may have given rise to seismicity and normal faults in Mona Rift, Mona Passage, and SW Puerto Rico.

  7. Comparing different tectonic reconstructions of the westernmost Mediterranean based on 3D fully dynamic numerical subduction modelling.

    NASA Astrophysics Data System (ADS)

    Chertova, Mariya; Spakman, Wim; Geenen, Thomas; van den Berg, Arie; van Hinsbergen, Douwe J. J.

    2013-04-01

    Since ~30Ma ago the westernmost Mediterranean region (Betic-Rif-Alboran region) has undergone a long and complicated subduction evolution comprising rollback and lithosphere tearing processes. A number of geodynamic reconstructions have been proposed for this region which differ in length, position, and width of the initial subduction zone, as well as in the initial amount of the subducted slab, position of transform faults, the major direction of slab rollback, or even in initial direction of the subduction. Proposed tectonic reconstructions are purely kinematic based on plate reconstruction and describing the motions of subduction trenches and geological fragments. Here we model the subduction processes that possibly underlie the tectonic reconstructions such as slab rollback, lithosphere tearing, back-arc opening and slab stagnation. Detailed seismic tomography images of this region allow us to compare our results of the 3D subduction modelling with the present day slab position and shape in the mantle which may help to discriminate between proposed tectonic reconstructions. We create a 3D numerical subduction model of the region incorporating rheological and paleogeographic data and corresponding to the past ~30Ma of tectonic evolution. We implement visco-plastic rheology consisting of diffusion and dislocation creep using a stress limiter approach to control lithosphere strength in our model. Selective weakening of lithospheric material at continent-ocean boundaries is (optionally) used to allow for lithosphere tearing. To minimize the influence of the side-boundaries of the 3D model on the subduction process, we implement "open" side boundaries (Chertova et al. 2012). We use constraints from plate motion models to incorporate Europe(Iberia)-Africa convergence. The different tectonic reconstructions of the Western Mediterranean region are based on different interpretations of the initial lateral extent of the subduction trench and the initial amount of subduction at ~30Ma. We use this as initial condition on our modeling and predict present-day slab position and -geometry, which can be compared to the tomographic image of the slab. As the main result, we present a 3D numerical fully dynamic model of the evolution of this region, which correlates with geological, tectonic, paleogeography and seismic tomography data. We demonstrate that tectonic reconstructions based on initially short (~50-100 km) NW dipping subduction restricted to the Balearic margin shows a better correlation with present-day mantle structure than slab predictions from other reconstructions.

  8. Time-Varying Subduction and Rollback Velocities in Slab Stagnation and Buckling

    NASA Astrophysics Data System (ADS)

    Cizkova, H.; Bina, C. R.

    2012-12-01

    Models of subducting slabs exhibit buckling behavior in or below the mantle transition zone for certain combinations of dynamical parameters [Christensen, 2001]. Slabs that descend below the transition zone exhibit buckling instabilities in the lower mantle in response to viscosity increases [Ribe et al., 2007]. Numerical experiments for such vertically descending slabs exhibit buckling for large (>30x) viscosity increases and slow (imposed) trench migration, manifesting as temporal oscillations in dip angle and convergence rate [Lee and King, 2011]. Here we focus upon buckling behavior in subducting slabs that "stagnate" [Fukao et al., 2009] via subhorizontal deflection in the transition zone (due to some combination of buoyancy variations, viscosity contrasts, and slab rollback). We perform numerical experiments using a 2D Cartesian model of the mantle. Subducting and overriding plates are 5000 km long, separated by a 15-km thick weak crustal layer [Chertova et al., 2012]. The age of the subducting plate at the trench is 100 Ma or 150 Ma; yield stress that controls slab strength in the upper mantle varies between 0.2 GPa and 1 GPa. Analysis of the evolution of subduction velocities (after the tip of the slab descends below 400 km) reveals temporal oscillations of order 10 cm/yr with periods of order ~20 Myr (similar to the ~25 Myr periods obtained [Lee and King, 2011] for vertically descending slabs) in response to repeated slab buckling within the transition zone. Periods of these oscillations increase with increasing slab strength controlled by the yield stress, while the amplitude of the plate velocity peaks is higher for weaker slabs. Our models also yield corresponding oscillations in slab rollback velocities, which are an order of magnitude smaller and phase shifted (by ~180°). Rollback velocities are higher for older and thus more negatively buyoant slabs. Our results may shed more light on the conditions needed to produce flat lying slabs as observed in some parts of the mantle transition zone. \\vspace Chertova, M., Geenen, T., van den Berg, A., Spakman, W., 2012. Using open sidewalls for modelling self-consistent lithosphere subduction dynamics, Solid Earth Discuss. 4, 707-744. \\vspace Christensen, U., 2001. Geodynamic models of deep subduction, Phys. Earth Planet. Int. 127, 25-34. \\vspace Fukao, Y., 2009. Stagnant slab: A review, Ann. Rev. Earth Planet. Sci. 37, 19-46. \\vspace Lee, Ch., King, S., 2011. Dynamic buckling of subducting slabs reconciles geological and geophysical observations. Earth Planet. Sci. Lett. 312, 360-370. \\vspace Ribe, N.M., Stutzmann, E., Ren, Y., van der Hilst, R.D., 2007. Buckling instabilities of subducted lithosphere beneath the transition zone. Earth Planet. Sci. Lett. 254, 173-179.

  9. Subduction Processes Off Chile: Preliminary Geophysical Results of Sonne Cruise So-161(2+3)

    NASA Astrophysics Data System (ADS)

    Reichert, Chr.; Spoc Scientific Shipboard Party, The

    Within the scope of the multi-disciplinary SPOC project (Subduction Processes Off Chile) some 8.700 km were recently acquired using marine magnetic, gravity and swath bathymetric methods whereof 5.200 km were surveyed with multi-channel seismics, including three seismic lines with simultaneous onshore observations. The goal of the project is to identify the variety of subduction features and accompanying conditions along the Central Chile segment of the collision zone between the Nazca and S-American plates, i.e. between Coquimbo and Valdivia. An additional line was achieved just south of Chiloe Island entering the submerged Central Valley. The present status of data allows the following observations: the oceanic crust is seg- mented by features like the Juan Fernandez Ridge, the Mocha and Valdivia Fracture Zones, accumulations of seamounts and by distinct parallel fault patterns with various azimuths. From north to south the subduction-induced inclination of the oceanic crust toward the trench decreases while the trench proper widens and the margin slope an- gle increase significantly. In the north clearly extensional normal faulting occurs on the upper and middle slopes, very similar to that observed off North Chile where sub- duction erosion is active. While in the north no bottom simulating reflectors could be observed they occur in the south as local patches. The slope area in the south is over- printed by a faint lineation pattern with a dominant azimuth of some 120 degrees that neatly fits with a mapped pattern onshore with the same azimuth. The pronounced forearc basins in the south are often accompanied by outer arc highs and strikingly narrow accretionary wedges. The latter are in contrast with the high convergence rate of more than 8 cm/yr and the relatively thick trench filling of up to more than 2.000 m. Thus, a non-accretionary subduction type is suggested, here.

  10. Crustal Structure Beneath the Kanto-Koshinetsu Region, Central Japan Inferred from Receiver Functions

    Microsoft Academic Search

    T. Igarashi; N. Hirata

    2005-01-01

    We applied the receiver function technique using multiple-taper method to estimate crust, uppermost mantle, and the subducting plate structure in the Kanto-Koshinetsu region, central Japan. This region is characterized by many faults and tectonic lines, and moreover the Philippine Sea plate subducts northward and the Pacific plate subducts westward. We analyzed P-wave seismograms recorded by regional seismographic networks from August

  11. Crust and subduction zone structure of Southwestern Mexico

    NASA Astrophysics Data System (ADS)

    Suhardja, Sandy Kurniawan; Grand, Stephen P.; Wilson, David; Guzman-Speziale, Marco; Gomez-Gonzalez, Juan Martin; Dominguez-Reyes, Tonatiuh; Ni, James

    2015-02-01

    Southwestern Mexico is a region of complex active tectonics with subduction of the young Rivera and Cocos plates to the south and widespread magmatism and rifting in the continental interior. Here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the MARS (MApping the Rivera Subduction zone) array to investigate crustal structure as well as the nature of the subduction interface near the coast. The array was deployed in the Mexican states of Jalisco, Colima, and Michoacan. Crustal thickness varies from 20 km near the coast to 42 km in the continental interior. The Rivera plate has steeper dip than the Cocos plate and is also deeper along the coast than previous estimates have shown. Inland, there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern Jalisco and Michoacan is likely supported by buoyant mantle. High crustal Vp/Vs ratios (greater than 1.82) are found beneath the trenchward edge of magmatism including below the Central Jalisco Volcanic Lineament and the Michoacan-Guanajuato Volcanic Field implying a new arc is forming closer to the trench than the Trans Mexican Volcanic Belt. Elsewhere in the region, crustal Vp/Vs ratios are normal. The subducting Rivera and Cocos plates are marked by a dipping shear wave low-velocity layer. We estimate the thickness of the low-velocity layer to be 3 to 4 km with an unusually high Vp/Vs ratio of 2.0 to 2.1 and a drop in S velocity of 25%. We postulate that the low-velocity zone is the upper oceanic crust with high pore pressures. The low-velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition.

  12. Provenance of Miocene submarine fans in the Shikoku Basin: Results from NanTroSEIZE and implications for stratigraphic correlation of subduction inputs

    NASA Astrophysics Data System (ADS)

    Pickering, K. T.; Underwood, M.; Moore, G. F.

    2013-12-01

    Seismo-stratigraphy, coring and LWD during IODP Expeditions 319, 322, and 333 (Sites C0011 / C0012) show three Miocene submarine fans in the NE Shikoku Basin, with broadly coeval deposits at ODP Site 1177 and DSDP Site 297, NW Shikoku Basin. Pickering et al. (2013) have shown that the sediment dispersal patterns for these fans have major implications for paleogeographies at that time. The oldest, Middle Miocene Kyushu Fan is the finest grained, has a sheet-like geometry, and was fed by quartz-rich sediment gravity-flows derived mostly from an ancestral landmass in the East China Sea. This likely sediment provenance is further supported by U-Pb zircon and fission track analysis of both zircons and apatites from sediments taken from the forearc and trench of the Nankai Trough, together with rivers from southwest Japan, that point to the influence of the Yangtze River in supplying into the Shikoku Basin prior to rifting of the Okinawa Trough at 10 to 6 Ma (Clift et al. 2013). During prolonged hemipelagic mud deposition at C0011-C0012 (12.2 to 9.1 Ma), sand supply continued at Sites 1177 and 297. Sand delivery to much of the Shikoku Basin, however, probably halted during a phase of sinistral strike-slip and oblique plate motion, after which the Daiichi Zenisu Fan (9.1 to 8.0 Ma) was fed by submarine channels. The youngest fan (Daini Zenisu; 8.0 to 7.6 Ma) has sheet-like geometry with thick-bedded, coarse-grained pumiceous sandstones. The pumice fragments were fed from a mixed provenance that included the collision zone of the Izu-Bonin and Honshu arcs. The shift from channelized to sheet-like flows was probably favored by renewal of relatively rapid northward subduction, which accentuated the trench as a bathymetric depression. Understanding the stratigraphic position and 3-D geometry of the sandbodies has important implications for stratigraphic correlation throughout the northern Shikoku Basin, together with subduction-related processes, including the potential for focused fluid flow and fluid overpressures above and below the plate-boundary fault. References Pickering, K.T., Underwood, M.B., Saito, S., Naruse, H., Kutterolf, S., Scudder, R., Park, J.-O., Moore, G.F. & Slagle, A. 2013. Geochemistry, Geophysics, Geosystems, 14, doi:10.1002/ggge.20107 Clift, P.D., Carter, A., Nicholson, U. & Masago, H. 2013. Tectonics, doi: 10.1002/tect.20033

  13. Progressive deformation of the Chugach accretionary complex, Alaska, during a paleogene ridge-trench encounter

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy M.; Bradley, Dwight C.; Haeussler, Peter

    1997-02-01

    The Mesozoic accretionary wedge of south-central Alaska is cut by an array of faults including dextral and sinistral strike-slip faults, synthetic and antithetic thrust faults, and synthetic and antithetic normal faults. The three fault sets are characterized by quartz ± calcite ± chlorite ± prehnite slickensides, and are all relatively late, i.e. all truncate ductile fabrics of the host rocks. Cross-cutting relationships suggest that the thrust fault sets predate the late normal and strike-slip fault sets. Together, the normal and strike-slip fault system exhibits orthorhombic symmetry. Thrust faulting shortened the wedge subhorizontally perpendicular to strike, and then normal and strike-slip faulting extended the wedge oblique to orogenic strike. Strongly curved slickenlines on some faults of each set reveal that displacement directions changed over time. On dip-slip faults (thrust and normal), slickenlines tend to become steeper with younger increments of slip, whereas on strike-slip faults, slickenlines become shallower with younger strain increments. These patterns may result from progressive exhumation of the accretionary wedge while the faults were active, with the curvature of the slickenlines tracking the change from a non-Andersonian stress field at depth to a more Andersonian system (? 1 or ? 2 nearly vertical) at shallower crustal levels. We interpret this complex fault array as a progressive deformation that is one response to Paleocene-Eocene subduction of the Kula-Farallon spreading center beneath the accretionary complex because: (1) on the Kenai Peninsula, ENE-striking dextral faults of this array exhibit mutually cross-cutting relationships with Paleocene-Eocene dikes related to ridge subduction; and (2) mineralized strike-slip and normal faults of the orthorhombic system have yielded 40Ar/ 39Ar ages identical to near-trench intrusives related to ridge subduction. Both features are diachronous along-strike, having formed at circa 65 Ma in the west and 50 Ma in the east. Exhumation of deeper levels of the southern Alaska accretionary wedge and formation of this late fault array is interpreted as a critical taper adjustment to subduction of progressively younger oceanic lithosphere yielding a shallower basal décollement dip as the Kula-Farallon ridge approached the accretionary prism. The late structures also record different kinematic regimes associated with subduction of different oceanic plates, before and after ridge subduction. Prior to triple junction passage, subduction of the Farallon plate occurred at nearly right angles to the trench axis, whereas after triple junction migration, subduction of the Kula plate involved a significant component of dextral transpression and northward translation of the Chugach terrane. The changes in kinematics are apparent in the sequence of late structures from: (1) thrusting; (2) near-trench plutonism associated with normal + strike-slip faulting; (3) very late gouge-filled dextral faults.

  14. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

    Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration. PMID:17784486

  15. Submarine slope failures along the convergent continental margin of the Middle America Trench

    NASA Astrophysics Data System (ADS)

    Harders, Rieka; Ranero, CéSar R.; Weinrebe, Wilhelm; Behrmann, Jan H.

    2011-06-01

    We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well-defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated.

  16. Improved seismic velocity structure in southwestern Japan using pronounced sP phase

    Microsoft Academic Search

    T. Hayashida; F. C. Tajima; J. J. Mori

    2010-01-01

    In southwestern Japan the Philippine Sea plate (PHSP) subducts along the Nankai trough and this subduction causes the megathrust earthquakes in the Nankai seismic zone as well as large intraslab and inland earthquakes in the vicinity. The dip angle of the PHSP varies significantly along strike. In this region the sP phase is widely observed and its amplitude sometimes becomes

  17. Preliminary results of three-dimensional stress orientation in the accretionary prism in Nankai Subduction Zone, Japan by anelastic strain recovery measurements of core samples retrieved from IODP NanTroSEIZE Site C0009

    NASA Astrophysics Data System (ADS)

    Lin, W.; Byrne, T. B.; Yamamoto, Y.

    2010-12-01

    During IODP Expedition 319, the first riser-drilling borehole in ocean was penetrated by D/V CHIKYU at Site C0009 in the Nankai convergent margin, Japan. From 0 mbsf (meters below seafloor) to 1285 mbsf, the borehole crossed the Kumano forearc basin and from 1285 mbsf to the bottom depth of 1604 mbsf, the Nankai accretionary prism. In a short depth range of 84.20 m from 1509.7 to 1593.9 mbsf, core samples were retrieved by rotary core barrel drilling. We collected 3 whole-round core samples for measurements of anelastic strain recovery (ASR) by the same methods of sample preparation and anelastic strain data acquisition conducted in the previous Stage-1 expeditions of the same NanTroSEIZE drilling program (Byrne et al., 2009; GRL, Vol.36, L23310). Anelastic normal strains, measured every ten minutes in nine directions, including six independent directions, were used to calculate the anelastic strain tensors. All three samples showed coherent strain recovery over a long period more than 1 month. The three samples were from C0009A (3R,1531 mbsf; 4R, 1540 mbsf and 8R, 1577 mbsf, respectively) in lithologic Unit IV interpreted as accretionary prism or deformed slope sediments. All samples are composed of silty clays or hemipelagic muds with relatively high porosities (30%~). The ASR measurement results in Kumano Forearc Basin obtained from C0002 (Byrne et al., 2009) showed the maximum stress orientation is nearly vertical and a normal stress regime. However, the ASR results in the accretionary prism from C0009 show that the maximum principal stress axes plunge gently or are nearly horizontal and the stress regimes appear to be strike-slip or thrust (reverse fault) types. The maximum horizontal principal stress orientaions obtained from the ASR tests also show very good consistency with the stress orientaions determined from borehole breakouts in the same borehole and the same depth range (Lin et al., 2010; GRL, Vol.37, L13303). These results suggest that three-dimensional maximum principal stress (Sigma 1) and the stress regimes change with depth and/or formation. Possibly, the depth range around 1500 mbsf may be a transition zone of stress regime from normal faulting above to thrust faulting below.

  18. Combining seismic reflection and refraction data to investigate tectonic features of the Manila Trench offshore southern Taiwan

    NASA Astrophysics Data System (ADS)

    Chen, Yi-Ping; Liu, Char-Shine

    2015-04-01

    Disastrous earthquakes (Mw>8) were mostly megathrust earthquakes that slipped along plate boundaries as stresses can be easily accumulated in the megathrust fault zone between two plates. Some large thrust faults, called splay faults, have been suggested to emerge from the megathrust fault to the seafloor. The splay fault may enhance tsunami generation by raising the fault plane angle from a low angle megathrust fault to a high angle splay fault, which could increase the vertical displacement of the seafloor once the fault is activated. The Luzon subduction zone has been regarded as one of the high tsunami risk zones. South of Taiwan, the Luzon subduction zone consists of four morphotectonic units from west to east: the Manila Trench, the Hengchun Ridge (accretionary wedge), the North Luzon Trough (forearc basin) and the Luzon volcanic arc. The accretionary wedge can be further divided into a lower slope domain and an upper slope domain by a splay fault. This splay fault separates a folds and thrusts dominated lower slope domain of the accretionary wedge from an intensely deformed upper slope domain. This splay fault system extends from offshore southern Taiwan to offshore southwestern Taiwan in a SSE to NNW direction, and may connect to the Chi-Shan fault onshore. It has been suggested to be a major branch of the megathrust system in the Luzon subduction zone. In this study, we analyze several large-offset multi-channel seismic profile data collected during the TAIGER survey in 2009 across the Manila trench between 18.5°N to 21°N. Special processing procedures to attenuate multiples and to enhance deep signals on seismic reflection profile data have been performed to image tectonic features of the Luzon subduction zone. Velocity structural models from ocean bottom seismometer (OBS) data are constructed for depth conversion. Finally, we map the geometries of decollement, subducting oceanic basement, splay faults, and other structural features across the Manila trench. Our results suggest (1) the eastward dipping decollement steps down to basement at about 20 km from trench axis; and (2) there are 2 or 3 splay faults emerging from the megathrust fault zone from different branching points. The existence of these splay faults may suggest that this area has high potential of seismic and tsunami threats.

  19. Subduction zone guided waves: 3D modelling and attenuation effects

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2013-12-01

    Waveform modelling is an important tool for understanding complex seismic structures such as subduction zone waveguides. These structures are often simplified to 2D structures for modelling purposes to reduce computational costs. In the case of subduction zone waveguide affects, 2D models have shown that dispersed arrivals are caused by a low velocity waveguide, inferred to be subducted oceanic crust and/or hydrated outer rise normal faults. However, due to the 2D modelling limitations the inferred seismic properties such as velocity contrast and waveguide thickness are still debated. Here we test these limitations with full 3D waveform modelling. For waveguide effects to be observable the waveform must be accurately modelled to relatively high frequencies (> 2 Hz). This requires a small grid spacing due to the high seismic velocities present in subduction zones. A large area must be modelled as well due to the long propagation distances (400 - 600 km) of waves interacting with subduction zone waveguides. The combination of the large model area and small grid spacing required means that these simulations require a large amount of computational resources, only available at high performance computational centres like the UK National super computer HECTOR (used in this study). To minimize the cost of modelling for such a large area, the width of the model area perpendicular to the subduction trench (the y-direction) is made as small as possible. This reduces the overall volume of the 3D model domain. Therefore the wave field is simulated in a model ';corridor' of the subduction zone velocity structure. This introduces new potential sources of error particularly from grazing wave side reflections in the y-direction. Various dampening methods are explored to reduce these grazing side reflections, including perfectly matched layers (PML) and more traditional exponential dampening layers. Defining a corridor model allows waveguide affects to be modelled up to at least 2 Hz (needed for dispersion analysis) for the large model area that is considered. Simulations with a variety of quality factors (Q) at different parts of the subduction zone have been run to investigate how seismic attenuation affects the observed dispersed waveforms. We show that the low Q in the mantle wedge can improve the fit of the dispersed waveforms. A low Q in the low velocity waveguide structure however means that the delayed high frequency energy has very low amplitude, and so is not seen clearly at the surface. The Q of the low velocity crustal waveguide must therefore be greater than 250, suggesting that melting does not occur in the subducted oceanic crust at depths of 220 km or less. The velocity contrast seen at these depths must therefore be due to compositional variations. Benchmarking 2D elastic models with the 3D case shows that 2D models give a good approximation of 3D subduction zone waveguide structure. Visco-elastic simulations show that attenuation in the mantle wedge affects the observed dispersion, but the low velocity waveguide itself does not have significantly reduced Q. This work is an example of how the increasing computing power coupled with well-defined model boundaries can allow high resolution 3D modelling to be applied to specific structures of interest.

  20. Cyclic stressing and seismicity at strongly coupled subduction zones

    USGS Publications Warehouse

    Taylor, M.A.J.; Zheng, G.; Rice, J.R.; Stuart, W.D.; Dmowska, R.

    1996-01-01

    We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

  1. Modeling of LNG spills into trenches.

    PubMed

    Gavelli, Filippo; Chernovsky, Melissa K; Bullister, Edward; Kytomaa, Harri K

    2010-08-15

    A new method for the analysis of LNG spills into trenches has been developed to quantify vapor dispersion hazard distances. The model uses three steps to capture the behavior of an LNG spill into a trench. The first is to analytically calculate the evolving LNG flow, the second to calculate the vaporization rate along the trench, and the third is to calculate the dispersion of the vapors using a CFD model that has been validated for this application in the presence of complex geometries. This paper presents case studies that show the effect of wind perpendicular and parallel to the large aspect ratio trenches on vapor dispersion. The case studies also demonstrate the effect of complex terrain and obstacles such as vapor fences on vapor dispersion. The simulations show that wind direction relative to the trench has a significant effect on cloud shape, height, and maximum downwind distance. The addition of vapor fences to mitigate vapor dispersion hazards from an LNG spill into the LNG containment trench is shown to be effective. PMID:20447763

  2. Necessity of the Ridge for the Flat Slab Subduction: Insights from the Peruvian Flat Slab

    NASA Astrophysics Data System (ADS)

    Knezevic Antonijevic, S.; Wagner, L. S.; Beck, S. L.; Long, M. D.; Zandt, G.; Tavera, H.

    2014-12-01

    Flattening of the subducting plate has been linked to the formation of various geological features, including basement-cored uplifts, the cessation of arc volcanism, ignimbrite flare-ups, and the formation of high plateaus and ore deposits [Humphreys et al., 2003; Gutscher et al., 2000; Rosenbaum et al., 2005]. However, the mechanism responsible for the slab flattening is still poorly understood. Here we focus on the Peruvian flat slab, where the Nazca plate starts to bend at ~80 km depth and travels horizontally for several hundred kilometers, at which point steep subduction resumes. Based on a 1500 km long volcanic gap and intermediate depth seismicity patterns, the Peruvian flat slab appears to have the greatest along-strike extent and, therefore, has been suggested as a modern analogue to the putative flat slab during the Laramide orogeny in the western United States (~80-55 Ma). Combining 3D shear wave velocity structure and Rayleigh wave phase anisotropy between ~10° and 18° S, we find that the subducting Nazca plate is not uniformly flat along the entire region, but fails to the north of the subducting Nazca Ridge. Our results show that, in combination with trench retreat, rapid overriding plate motion, and/or presence of a thick cratonic root, the subduction of buoyant overthickened oceanic crust, such as the Nazca Ridge, is necessary for the formation and sustainability of flat slabs. This finding has important implications for the formation of flat slabs both past and present.

  3. Boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite

    Microsoft Academic Search

    Toshio Nakano; Eizo Nakamura

    2001-01-01

    In order to understand the behavior of boron (B) and its isotope fractionation during subduction zone metamorphism, B contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the Sambagawa Metamorphic Belt, central Shikoku, Japan. No systematic changes in whole-rock B content and isotope composition of the metasediments were observed among the different

  4. Postseismic deformation after Maule earthquake and the mechanical properties of the asthenosphere and subduction interface

    NASA Astrophysics Data System (ADS)

    Klein, Emilie; Fleitout, Luce; Vigny, Christophe

    2015-04-01

    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 Aceh and Tohoku-Oki earthquakes: 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. A 3D FE code (Zebulon Zset) is used to relate these deformations to the mechanical properties of the mantle and of the subduction interface. 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 3.1018Pa.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.

  5. Flexural modeling of circum-Pacific trench - outer rise systems and its implications for mantle rheology

    NASA Astrophysics Data System (ADS)

    Hunter, J.; Watts, A. B.; Bassett, D.

    2014-12-01

    Oceanic flexure studies suggest that the long-term strength and effective elastic thickness, Te, of oceanic lithosphere increases with age due to thermal cooling. A recent study at trench - outer rise systems (Bry and White, 2007, JGR), however, has questioned whether such a relationship exists. In order to reassess this issue, we model trench-perpendicular, ensemble-averaged profiles of satellite-derived free-air gravity anomalies at the trench - outer-rise of circum-Pacific subduction zones. A broken elastic plate model is used with an inverse approach, in which we iterate for Te and invert for the applied vertical force and bending moment at the trench using damped least squares. We first model the profiles using a plate of constant Te. Results show that, to first order, oceanic lithosphere does indeed strengthen with age. However, there is scatter, which we attribute to variations in plate curvature and axial loading, and the effects of hotspot volcanism. Comparisons between our results and those of Bry and White reveal that the discrepancy can be explained by the different ways the two studies treat the regional long-wavelength gravity field, the position of the plate break and the applied bending moment. In many subduction zones, a constant Te plate cannot fully explain the amplitude and wavelength of the bulge and the high curvature of the outer trench slope. We therefore model the profiles with a plate that is allowed to weaken trenchward of the outer-rise. The weakening is attributed to inelastic yielding, evidence of which is manifest in swath bathymetry and seismicity data. In an attempt to constrain experimentally derived low temperature flow laws for olivine, we compare our inverted Te values with the predictions of viscoelastic-plastic yield strength envelopes derived from recently published flow laws. We test six flow laws, finding that Mei et al (2010, JGR) is too strong to fit our inverted Te values. This is in accordance with the findings of Zhong and Watts (2013, JGR), who modelled Hawaiian Island loading using a 3D multilayered viscoelastic plate, suggesting that their result maybe generally true for oceanic lithosphere.

  6. Overriding plate deformation and its energy dissipation in three-dimensional subduction models

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter; Duarte, Joao

    2015-04-01

    Analogue and numerical models of subduction have been used to investigate overriding plate deformation during subduction. However, most models either exclude an overriding plate or impose an external force/velocity. Here, we present three-dimensional buoyancy-driven laboratory subduction models including an overriding plate to study the progressive deformation of the overriding plate during subduction. Considering there is uncertainty in the effective viscosity ratio between the subducting plate and sub-lithospheric upper mantle (?SP/?UM), a variability in overriding plate thickness (TOP), and complexity of the far-field plate boundary conditions in natural subduction zones, we investigate models in which we vary ?SP/?UMfrom 157 to 560, TOP from 1.0 cm to 2.5 cm (scaling to 50-125 km in nature), and far-field plate boundary conditions of the overriding plate and subducting plate. Our results show that the variability of these three parameters has an influence on the patterns of overriding plate deformation. Furthermore, we have used the subduction models to quantify the force (FOPD) that drives overriding plate deformation and the involved energy dissipation rate (?OPD) during such deformation, and we compare them with the negative buoyancy (FBU) and the total potential energy release rate (?BU) of the subducted slab, respectively. In our models of narrow subduction zones (15 cm in experiment, scaling to 750 km in nature) the overriding plate always experiences overall extension during trench retreat. Overall, FOPD/FBU has average values of 0.5-2.5%, with a maximum of 5.0% and ?OPD/?BUhas average values of 0.10-0.30%, with a maximum of 0.70%, which indicate that only a small portion of the negative buoyancy of the subducted slab is used to deform the overriding plate and an even smaller percentage of the slab's potential energy is consumed during overriding plate deformation. In addition, our results show that 2-30% of the overriding plate energy dissipation is dissipated in the forearc region and 14-42% in the region of maximum backarc extension. Finally, our calculated force to deform overriding plate is of comparable magnitude as the ridge push force in nature.

  7. Slab Pull and the Seismotectonics of Subducting Lithosphere (Paper 6R0645)

    NASA Astrophysics Data System (ADS)

    Spence, William

    1987-02-01

    This synthesis links many seismic and tectonic processes at subduction zones, including great subduction earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity, providing details on how subduction occurs. Compression is localized across a locked interface thrust zone, because both the ridge push and the slab pull forces are resisted there. The slab pull force increases with increasing plate age; thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone, the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate. The 1960 Chile earthquake (Mw 9.5), the largest earthquake to occur in historic times, began its rupture at an interface bounding oceanic plate <30 m.y. old. However, this rupture initiation was associated with the locally oldest subducting lithosphere (weakest coupling); the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere, terminating near the subducting Chile Rise. Prior to a great subduction earthquake, the sinking subducted slab will cause increased tension at depths of 50-200 km, with greatest tension near the shallow zone resisting plate subduction. Plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260-350 km, provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle. This explains K. Mogi's observations of M ? 7 thrust earthquakes at depths of 260-350 km, immediately downdip and within 3 years prior to five great, shallow earthquakes of northern Japan. The slab pull model explains the lower layer of double seismic zones as due to tension from the deeper, sinking plate and the upper layer as due to localized in-plate compression, as plate motion is resisted by the bounding mantle. Just downdip of the interface thrust zone, there occurs an aseismic 20°-50° dip increase of subducted plate. This slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change. Resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone.

  8. Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.; Pecher, Ingo; Kukowski, Nina; Hampel, Andrea

    2003-06-01

    Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700-850 m) likely due to preferential basal erosion under the center of Lima Basin. Long-term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr-1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr-1) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4-11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long-term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust.

  9. Ridge subduction at an erosive margin: The collision zone of the Nazca Ridge in southern Peru

    NASA Astrophysics Data System (ADS)

    Hampel, Andrea; Kukowski, Nina; Bialas, Joerg; Huebscher, Christian; Heinbockel, Raffaela

    2004-02-01

    The 1.5-km-high, obliquely subducting Nazca Ridge and its collision zone with the Peruvian margin have been imaged by wide-angle and reflection seismic profiles, swath bathymetry, and gravity surveying. These data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust. The lowermost layer contributes 10-12 km to the total crustal thickness of the ridge. The sedimentary cover is 300-400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges. At the collision zone of ridge and margin, the following observations indicate intense tectonic erosion related to the passage of the ridge. The thin sediment layer on the ridge is completely subducted. The lower continental slope is steep, dipping at ˜9°, and the continental wedge has a high taper of 18°. Tentative correlation of model layers with stratigraphy derived from Ocean Drilling Program Leg 112 cores suggests the presence of Eocene shelf deposits near the trench. Continental basement is located <15 km landward of the trench. Normal faults on the upper slope and shelf indicate extension. A comparison with the Peruvian and northern Chilean forearc systems, currently not affected by ridge subduction, suggests that the passage of the Nazca Ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long-term erosive regime.

  10. Ridge subduction as mechanism for initiation of rifting in South China: Implications from numerical modeling results

    NASA Astrophysics Data System (ADS)

    Zuo, X.; Chan, L. S.; Pubellier, M.

    2012-12-01

    The opening of South China Sea was preceded by extension in the northern margin during the Late Cretaceous-Cenozoic. Recent studies have pointed to possible subduction of an oceanic ridge in East Asia, which necessarily implies the subduction of a slab with increasing density over time. In this study, we used a numerical procedure to simulate the kinematics of the subduction with increasing slab density. A thermo-dynamical modelling program basing on Fast Lagrangian Analysis Continua method (FLAC) was adopted to compute the stress and strain configuration of the trench-backarc region. Varying slab angles, thermal gradient, convergence velocity and density at 670 km discontinuity were used as input parameters. The modelling results reveal the dominance of horizontal compression during the early stage of the subduction, which reverts to a horizontal extension in the back-arc region. Mantle upwelling appears to be the process governing the crustal extension. The results show that the slab angle is the key factor controlling the roll-back of the subducted slab, which in turn controls the development of mantle upwelling. A relatively high convergence velocity is shown to produce a strong coupling of the subducting slab with the overriding plate, and a higher mantle thermal gradient would facilitate the rolling back of the slab. This process probably accounts for the initiation of the extensional regime in the South China Block during the late stage of the Jurassic magmatism. The extension continued into the Cenozoic and eventually led to the full opening of the South China Sea. The study also points to a close association between ridge subduction and back-arc crustal extension.

  11. The fate of salt in the Cyprus subduction zone

    NASA Astrophysics Data System (ADS)

    Reiche, Sönke; Hübscher, Christian; Ehrhardt, Axel; Klimke, Jennifer

    2014-05-01

    The area between Cyprus and Hecataeus Rise to the north and Eratosthenes Seamount (ESM) further south is presently accommodating plate tectonic convergence between Africa and Anatolia. A number of studies have focused on the convergence history, especially after drilling close to the plate boundary in the course of ODP Leg 160. Even though drilling at Site 968 has reached late Messinian Lago Mare deposits, little information on deeper trench strata exist, owing to limited penetration of previously published seismic data. Here we show results from bathymetric data and a dense grid of seismic lines collected during research cruises MSM14/2 and MSM14/3 in 2010 across the Cyprus trench, shedding new light on the tectonostratigraphic evolution of the plate boundary. Evaporites of locally more than 1.5 km thickness occupy the northern trench area. Between Cyprus and ESM evaporites are heavily deformed and appear to be thrust southward over Pliocene-Quaternary strata. Thus significant post-Messinian tectonic shortening at the plate boundary was accommodated by allochthonous salt advance towards the ESM which is currently being thrust beneath the island of Cyprus. Such observations may provide an example of how salt efficiently escapes the subduction cycle. In contrast, evaporites between ESM and Hecataeus Rise have not experienced sufficient shortening for initiating allochthonous salt advance, even though compression-related thickening is clearly evident. The observed pattern of intensively deformed salt between Cyprus and ESM and moderately deformed evaporites in the eastern trench area is believed to reflect a predominately N-S-oriented post-Messinian convergence direction. Such implications raise the question about a fairly recent coupling between the motion of Cyprus and Anatolia. Along the entire study area, the southward salt limit coincides with the seafloor stepping down towards the ESM, suggesting thickening and ESM-directed advance of the evaporites to have caused at least the northern part of the circum-ESM depression. Evaporites are locally covered by up to 600 m thick Late Messinian Lago Mare deposits. This spatially limited sediment package is only observed directly south of Cyprus, pointing towards Cyprus-derived sediment delivery into the trench area during the final stages of the Messinian Salinity Crisis. Forming an intact, fairly undeformed roof above underlying mobile evaporites, large parts of the Lago Mare unit and overlying sediments of Pliocene-Quaternary age have apparently been rafted south during allochthonous salt advance.

  12. Nappes, tectonics of oblique plate convergence, and metamorphic evolution related to 140 million years of continuous subduction, Franciscan Complex, California

    SciTech Connect

    Wakabayashi, J. (Earth Sciences Associates Inc., Palo Alto, CA (United States))

    1992-01-01

    This paper presents a new synthesis of Franciscan Complex tectonics, with the emphasis on the pre-San Andreas fault history of these rocks. Field relations suggest that the Franciscan is characterized by nappe structures that formed during sequential accretion at the trench. The presence of these structures along with other field relations, including the lack of evidence for large offset of conglomerate suites, indicates that strike-slip fault systems of large displacement ({gt}500 km) did not cut the Franciscan Complex during subduction. Regional geology and comparisons to modern arc-trench systems suggest that strike-slip faulting associated with oblique subduction took place inboard (east) of the Franciscan in the vicinity of the magmatic arc. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism. Franciscan Complex tectonic history is presented in this paper.

  13. Eocene deep-sea communities in localized limestones formed by subduction-related methane seeps, southwestern Washington

    SciTech Connect

    Goedert, J.L. (Natural History Museum of Los Angeles County, CA (USA)); Squires, R.L. (California State Univ., Northridge (USA))

    1990-12-01

    Densely populated communities of soft-bottom-dwelling taxa similar to those found today along subduction zones off the coasts of Japan and Oregon have been discovered in very localized deep-water limestones of late middle to late Eocene age along the southwestern margin of Washington. Subduction was prevalent in this area during this time, and compressive forces squeezed subsurface methane-rich waters onto the ocean floor, where opportunistic bivalves (especially Modiolus, Calyptogena, and Thyasira), vestimentiferan tube worms, serpulid tube worms, siliceous sponges, very small limpets, trochid and turbinid archaeogastropods, and other macrobenthos colonized. These assemblages are the earliest recorded biologic communities formed in response to methane seeps in subduction zones.

  14. How weak is the subduction zone interface?

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    Several lines of evidence suggest that subduction zones are weak and that the unique availability of water on Earth is a critical factor in the weakening process. We have evaluated the strength of subduction zone interfaces using two approaches: (i) from empirical relationships between shear stress at the interface and subduction velocity, deduced from laboratory experiments; and (ii) from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength. Our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low (less than ~35 MPa). To account for this requirement, we propose that there is a feedback mechanism between subduction velocity, water released from the subducting plate, and weakening of the fore-arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally.

  15. Thick Consolidated Sediments Allow Rupture to Reach the Trench During December 2004 Sumatra-Andaman Great Earthquake

    NASA Astrophysics Data System (ADS)

    Gulick, S. P.; Austin, J. A.; Bangs, N. L.; Martin, K. M.; McNeill, L. C.; Henstock, T.; Bull, J. M.; Dean, S. M.; Djajadihardja, Y.; Permana, H.

    2009-12-01

    Slip inversions for the 2004 Sumatran-Andaman great earthquake (Mw 9.2) suggest that rupture propagated up dip to as far west as the Sunda Trench. Because earthquake moment magnitude scales with the subsurface rupture area, the moment magnitude varies with the updip extent of the rupture. The magnitude of the 2004 event, and aftershock distribution, suggest an updip limit of seismogenesis at or seaward of the Trench, but this result is surprising given the unlithified, weak sediments generally present at the distal toes of convergent margin accretionary prisms. We present new seismic reflection images collected as part of a UK-US-Germany-Indonesia experiment aboard the R/V Sonne. The data were acquired using a 2.4 km streamer and 5500 cubic inch G-gun array imaging the outer region of the 2004 rupture area; these data show extremely thick (>5 km), accreting sedimentary strata within which lie discrete, coherent stratigraphic blocks along mostly landward-vergent thrusts. The décollement lies near the base of this incoming sediment section, likely composed primarily of Nicobar/Bengal Fan turbidites. Available scientific ocean drilling results confirm that these sediments are silt- and occasionally sand-rich. We suggest that advanced dewatering and lithification of these thick turbidites, which experienced substantial burial depths, have made them more competent than most trench-fill sediments elsewhere; this inherent strength of the incoming section explains observed preservation of stratigraphic packages that are present up to at least 3 km shallower and 40 km landward of the deformation front. The combination of a deeply buried décollement and advanced lithification of the Trench sediments may allow seismogenic rupture to continue up-dip as far as the Sunda Trench. Potential for extended updip rupture may similarly exist along other subduction margins whose trenches contain significant thickness of sand- and silt-rich sediment and a deeply buried plate boundary thrust.

  16. A comparison of seismicity in world's subduction zones: Implication by the difference of b-values

    NASA Astrophysics Data System (ADS)

    Nishikawa, T.; Ide, S.

    2013-12-01

    Since the pioneering study of Uyeda and Kanamori (1979), it has been thought that world's subduction zones can be classified into two types: Chile and Mariana types. Ruff and Kanamori (1980) suggested that the maximum earthquake size within each subduction zone correlates with convergence rate and age of subducting lithosphere. Subduction zones with younger lithosphere and larger convergence rates are associated with great earthquakes (Chile), while subduction zones with older lithosphere and smaller convergence rates have low seismicity (Mariana). However, these correlations are obscured after the 2004 Sumatra earthquake and the 2009 Tohoku earthquake. Furthermore, McCaffrey (2008) pointed out that the history of observation is much shorter than the recurrence times of very large earthquakes, suggesting a possibility that any subduction zone may produce earthquakes larger than magnitude 9. In the present study, we compare world's subduction zones in terms of b-values in the Gutenberg-Richer relation. We divided world's subduction zones into 146 regions, each of which is bordered by a trench section of about 500 km and extends for 200 km from the trench section in the direction of relative plate motion. In each region, earthquakes equal to or larger than M4.5 occurring during 1988-2009 were extracted from ISC catalog. We find a positive correlation between b-values and ages of subducting lithosphere, which is one of the two important variables discussed in Ruff and Kanamori (1980). Subduction zones with younger lithosphere are associated with high b-values and vice versa, while we cannot find a correlation between b-values and convergence rates. We used the ages determined by Müller et al. (2008) and convergence rate calculated using PB2002 (Bird, 2003) for convergence rate. We also found a negative correlation between b-values and the estimates of seismic coupling, which is defined as the ratio of the observed seismic moment release rate to the rate calculated from plate tectonic velocities (Scholz and Campos, 2012). Lithosphere age also has a weak negative correlation with the degree of seismic coupling. Based on differences in b-values for the types of faulting, Schorlemmer et al. (2005) suggested that b-value depends inversely on differential stress. This idea, taken together with correlations in the present study, suggests a model where the buoyancy of subducting slabs which depends on the lithosphere age determines stress state and the b-value in each sunbduction zone. The stress state also controls the seismic coupling. This model is basically consistent with the idea of Ruff and Kanamori (1980). Subduction zones with younger and lighter lithosphere are in a compressive stress state and associate with high coupling and small b-values (Chile), while those with older and heavier lithosphere are in a tensional stress state and correlate with low coupling and large b-values (Mariana). Subduction zones such as Nicaragua and El Salvador where b-values are much higher than the expectation from the above correlations may be explained by considering the fact that local tectonics affects the seismic coupling (LaFemina et al., 2009; Scholz and Campos, 2012).

  17. Evolution and hydration of the Juan de Fuca crust and uppermost mantle: a plate-scale seismic investigation from ridge to trench

    NASA Astrophysics Data System (ADS)

    Carbotte, S. M.; Canales, J.; Carton, H. D.; Nedimovic, M. R.; Han, S.; Marjanovic, M.; Gibson, J. C.; Janiszewski, H. A.; Horning, G.; Delescluse, M.; Watremez, L.; Farkas, A.; Biescas Gorriz, B.; Bornstein, G.; Childress, L. B.; Parker, B.

    2012-12-01

    The evolution of oceanic lithosphere involves incorporation of water into the physical and chemical structure of the crust and shallow mantle through fluid circulation, which initiates at the mid-ocean ridge and continues on the ridge flanks long after crustal formation. At subduction zones, water stored and transported with the descending plate is gradually released at depth, strongly influencing subduction zone processes. Cascadia is a young-lithosphere end member of the global subduction system where relatively little hydration of the downgoing Juan de Fuca (JdF) plate is expected due to its young age and presumed warm thermal state. However, numerous observations support the abundant presence of water within the subduction zone, suggesting that the JdF plate is significantly hydrated prior to subduction. Knowledge of the state of hydration of the JdF plate is limited, with few constraints on crustal and upper mantle structure. During the Cascadia Ridge-to-Trench experiment conducted in June-July 2012 over 4000 km of active source seismic data were acquired as part of a study of the evolution and state of hydration of the crust and shallow mantle of the JdF plate prior to subduction at the Cascadia margin. Coincident long-streamer (8 km) multi-channel seismic (MCS) and wide-angle ocean bottom seismometer (OBS) data were acquired in a two-ship program with the R/V Langseth (MGL1211), and R/V Oceanus (OC1206A). Our survey included two ridge-perpendicular transects across the full width of the JdF plate, a long trench-parallel line ~10 km seaward of the Cascadia deformation front, as well as three fan lines to study mantle anisotropy. The plate transects were chosen to provide reference sections of JdF plate evolution over the maximum range of JdF plate ages (8-9 Ma), offshore two contrasting regions of the Cascadia Subduction zone, and provide the first continuous ridge-to-trench images acquired at any oceanic plate. The trench-parallel line was designed to characterize variations in plate structure and hydration linked to JdF plate segmentation for over 450 km along the margin. Shipboard brute stacks of the MCS data reveal evidence for reactivation of abyssal hill faulting in the plate interior far from the trench. Ridgeward-dipping lower crustal reflectors are observed, similar to those observed in mature Pacific crust elsewhere, as well as conjugate reflectivity near the deformation front along the Oregon transect. Bright intracrustal reflectivity is also observed along the trench-parallel transect with marked changes in reflectivity along the Oregon and Washington margins. Initial inspection of the OBS record sections indicate good quality data with the expected oceanic crustal and upper mantle P-wave arrivals: Ps and Pg refractions through sedimentary and igneous layers, respectively, PmP wide-angle reflections from the crust-mantle transition zone, and Pn upper mantle refractions. The Pg-PmP-Pn triplication is typically observed at 40-50 km source-receiver offsets. Pn characteristics show evidence for upper mantle azimuthal anisotropic propagation: along the plate transects Pn is typically weaker and difficult to observe beyond ~80 km offsets, while along the trench-parallel transect Pn arrivals have higher amplitude and are easily observed up to source-receiver offsets of 160-180 km. An overview on the Cascadia Ridge to Trench data acquisition program and preliminary results will be presented.

  18. 'Dodo-Goldilocks' Trench Elevation Map

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This color-coded elevation map shows the 'Dodo-Goldilocks' trench dug by the Robotic Arm on NASA's Phoenix Mars Lander. The trench, originally two separate trenches nicknamed 'Dodo' (left) and 'Goldilocks' (right), became one after further digging on the 18th Martian day, or Sol 18, of the mission (June 12, 2008). The trench is 7 to 8 centimeters (2.7 to 3 inches) at its deepest (blue). Because the terrain itself is inclined at a 14-degree angle, the highest areas (pink) are about 20 centimeters (7.8 inches) above the lowest areas.

    The trench is 22 centimeters (8.7 inches) wide and 35 centimeters (13.8 inches) long. Its deepest portion is closest to the lander.

    This picture was taken by Phoenix's Surface Stereo Imager on Sol 19 (June 13, 2008).

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

  19. Slab roll-back and trench retreat as controlling factor for basin subsidence in southern Central America

    NASA Astrophysics Data System (ADS)

    Brandes, Christian; Winsemann, Jutta

    2015-04-01

    Slab roll-back and trench retreat are important factors for basin subsidence, magma generation and volcanism in arc-trench systems. Based on the sedimentary and tectonic record of the southern Central American island-arc we conclude that repeated phases of slab roll-back and trench retreats occurred the arc-trench system since the Late Cretaceous. These trench retreats were most probably related to the subduction of oceanic plateaus and seamounts and effected both the fore-arc and back-arc evolution. We used numerical basin modelling techniques to analyse the burial history of fore-arc and back-arc basins in Central America and combined the results with field data of the sedimentological evolution of the basin-fills. From the basin models, geohistory curves were extracted for the fore-arc and back-arc basins to derive the subsidence evolution. The Sandino Fore-arc Basin is characterized by low subsidence during the first 40 Myr. Since the Late Cretaceous the basin has a linear moderate subsidence with a phase of accelerated subsidence in the Oligocene. In the North and South Limón Back-arc Basin, subsidence started at approximately the same time as in the Sandino Fore-arc Basin. The North and South Limón Basins show a linear subsidence trend in the Paleocene and Eocene. Evidence for trench retreats is given by pulses of uplift in the outer-arc area, followed by subsidence in both the fore-arc and back-arc basins. The first slab roll-back probably occurred during the Early Paleocene. This is indicated by the collapse of carbonate platforms, and the re-deposition of large carbonate blocks into deep-water turbidites. A new pulse of uplift or decreased subsidence, respectively during the Late Eocene is attributed to subduction of rough crust. A subsequent slab detachment and the establishment of a new subduction zone further westward was described by Walther et al. (2000). Strong uplift affected the entire fore-arc area, which led to the deposition of very coarse-grained deepwater channel-levee complexes in the Sandino Fore-arc Basin. The channel-fills are rich in reworked shallow-water carbonates that points to strong uplift of the inner fore-arc. A subsequent trench retreat is indicated by an increased subsidence during the Early Oligocene in the Sandino Fore-arc Basin and the collapse of the Barra Honda carbonate platform in North Costa Rica. Another trench retreat might have occurred in Miocene times (Cailleau and Oncken, 2008). A phase of higher subsidence from 18 to 13 Ma is documented in the geohistory curve of the North Limon Back-arc Basin. After a short pulse of uplift the subsidence increased to approx. 300 m/myr (Brandes et al., 2008). References: Brandes C., Astorga A., Littke R. and Winsemann J. (2008) Basin modelling of the Limón Back-arc Basin (Costa Rica): burial history and temperature evolution of an island-arc related basin system. Basin Research 20, 1, 119-142. Cailleau, B. and Oncken, O. (2008) Past forearc deformation in Nicaragua and coupling at the megathrust interface: Evidence for subduction retreat. Geochemistry, Geophysics, Geosystems 9, Q07S24, doi:10.1029/2007GC001754. Walther, C.H.E., Flueh, E.R., Ranero, C.R., von Huene, R. and Strauch, W. (2000) Crustal structure across the Pacific margin of Nicaragua: evidence for ophiolithic basement and a shallow mantle sliver. Geophysical Journal International 141, 759-777.

  20. Links Between Geologic Conditions and Lateral Seismicity Variations in Circum-Pacific Subduction Zones (Invited)

    NASA Astrophysics Data System (ADS)

    Bilek, S. L.; Stankova-Pursley, J.

    2009-12-01

    In general, the vast majority of global seismicity occurs within the upper 60 km of subduction zones. However, focused examination of earthquake locations within any given subduction zone shows a complex distribution, with high concentrations of seismicity adjacent to relatively quiet zones. In addition, there is a wide spectrum of earthquake rupture characteristics found in subduction zones, including complex slip patterns featuring multiple asperities and slip over a range of time periods. Features such as subducting bathymetry, characteristics of the incoming plate, and forearc structure can all impact locations and rupture characteristics of the shallow earthquakes. Here we review observations of along-strike variations of seismicity distributions and rupture characteristics in several subduction zones around the Pacific and discuss possible factors in causing these lateral transitions. For instance, in regions such as Central and South America and the western Pacific margins of Japan and Kuriles, along-strike changes in the subducting plate, such as temperature, plate origins, presence of ridges or fracture zones, produce along strike variations in the location and slip distribution of earthquakes. Individual earthquakes in Costa Rica, Peru, and Chile were likely influenced by the past subduction of seamounts, ridges, and fracture zones, although rupture histories determined for these events suggest these subducted features act as either rupture asperities or barriers. Small-scale variations are also observed, such as factor of 2 variations in earthquake apparent stress within a roughly 100-150 km segment along the Nicoya Peninsula in Costa Rica that correspond to variations in temperature, geodetically determined plate coupling, and past earthquake history. Along the Alaska-Aleutian subduction zone, lateral variation within forearc blocks appears to impart along-strike variations in the seismicity.

  1. Fault trends on the seaward slope of the Aleutian Trench: Implications for a laterally changing stress field tied to a westward increase in oblique convergence

    USGS Publications Warehouse

    Mortera-Gutierrez, C. A.; Scholl, D. W.; Carlson, R.L.

    2003-01-01

    Normal faults along the seaward trench slope (STS) commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone. This is not the circumstance for the Aleutian Trench, where the direction of convergence gradually changes westward, from normal to transform motion. GLORIA side-scan sonar images document that the Aleutian STS is dominated by faults striking oblique to the trench, west of 179??E and east of 172??W. These images also show a pattern of east-west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies. The stress-strain field along the STS is divided into two domains west and east, respectively, of 179??E. Over the western domain, STS faults and nodal planes of earthquakes are oriented oblique (9??-46??) to the trench axis and (69??-90??) to the magnetic fabric. West of 179??E, STS fault strikes change by 36?? from the E-W trend of STS where the trench-parallel slip gets larger than its orthogonal component of convergence. This rotation indicates that horizontal stresses along the western domain of the STS are deflected by the increasing obliquity in convergence. An analytical model supports the idea that strikes of STS faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending. For the eastern domain, most nodal planes of earthquakes strike parallel to the outer rise, indicating bending as the prevailing mechanism causing normal faulting. East of 172??W, STS faults strike parallel to the magnetic fabric but oblique (10??-26??) to the axis of the trench. On the basis of a Coulomb failure criterion the trench-oblique strikes probably result from reactivation of crustal faults generated by spreading. Copyright 2003 by the American Geophysical Union.

  2. Finding of a huge coral reef sliding down to the bottom of the Palau Trench

    NASA Astrophysics Data System (ADS)

    Fujioka, K.; Kitazato, H.; Wada, H.

    2006-12-01

    We found a huge limestone block from the Palau Trench bottom, southern Philippine Sea by submersible Shinkai 6500. The limestone consists of the shallow marine coral reef similar with that of the present coral reefs of the Palau Islands. The site of a huge limestone body is located at the southern part of the Palau Trench at the water depth of 6400 m. The size of coral reef is confirmed to be of 2km x 2 km x 1 km by submersible observation but bathymetric survey confirm the distribution of the coral reef to be of 20 km x 10 km x 3 km. The limestone of the coral reef shows striations by the fall down blocks. The surface of the limestone is dissolved nature because of the depth being deeper than that of the Carbonate Compensation Depth (CCD), ca. 4200 m in the western Pacific. The limestone body is intercalated by a black sediment and is covered by both calcareous planktonic and benthic foraminifers which indicate the very shallow marine environment. Age of the limestone is middle Miocene by the Sr isotope age determination as well as fossils in the limestone itself. The bathymetric survey revealed a huge horseshoe morphology now forming a submarine canyon structure nearby the limestone site. Gravity and magnetic survey show the notable anomaly for several seamounts on the Caroline Plate. We had a scenario that the coral reef was once exposed on land along the Palau arc then collapsed and sledded down to the trench bottom by the tectonic erosion of the forearc of the Palau Trench due to the subduction of seamounts on the Caroline Plate at sometime during Pleistocene. In the Palauan people have legends of their history making storyboards which tell us a story that the Palau Island was sinking.

  3. Three-dimensional Euler deconvolution and tectonic interpretation of marine magnetic anomaly data in the Puerto Rico Trench

    NASA Astrophysics Data System (ADS)

    Muszala, S. P.; Grindlay, N. R.; Bird, R. T.

    1999-12-01

    During the summer of 1996 single-channel-seismic, magnetic, gravity, Hydrosweep bathymetric and HMR1 sidescan data were collected north of the island of Puerto Rico to constrain the geologic and tectonic setting of the Puerto Rico trench. Magnetic data from this cruise are merged with other available data, then processed and interpreted with the aid of the Euler deconvolution method. The area north of the island of Puerto Rico is divided into three magnetic anomaly zones. Zone 1 is dominated by northwest to southeast trending magnetic anomalies. Zone 2 consists of an east-west region of relatively low amplitude anomalies and occurs south of zone 1. Zone 3 is dominated by the highest magnetic values in the study area and the source region is roughly centered under the island of Puerto Rico. Respectively, these zones approximate three geologic provinces of the Puerto Rico trench composed of Early Cretaceous ocean crust with sedimentary cover, a blueschist belt and an Oligocene-Cretaceous island arc with a limestone cap. Also mapped are the Main Ridge fracture zone and the Fourth of July fracture zone, both on the North American plate, which correlate with the Main Ridge and Fourth of July Ridge on the Caribbean plate. The 3-D Euler deconvolution facilitates the identification of new faults as well as the mapping of known faults also evident in the seismic and bathymetric data. A model concerning the formation of the Puerto Rico trench is proposed that incorporates the existence of strike-slip faults and the fracture zones associated with the subduction of the North America Plate. Evidence from this study support a tectonic interpretation of subduction followed by more recent strike-slip faulting that is accompanied by only a minimal amount of subduction.

  4. Chicxulub Ejecta Impact Trenches And Terminal Impact

    NASA Astrophysics Data System (ADS)

    Page, R. L.

    2013-05-01

    DISCOVERY During a Pacific voyage from E. Australia to the US on a small sailing research vessel what appears to be two Chicxulub impact trenches were discovered in the ocean floor at depths of 4,627m and 3,519m. DESCRIPTION Trench A begins at a depth of 4,627m, 704km from the Chicxulub impact and is 18km long, 5km wide and 225m deep. Trench B begins at a depth of 3,519m, 732km from the Chixculub impact and is 23km long, 7 km wide and 400m deep. At the end of Trench B is what appears to be a debris deposit 5km long. Their relationship to the Chicxulub impact seems confirmed by their central axis, which when extended intersect at the Chicxulub impact at N Lat 21.33, W Long 89.5. Down range 286km from the end of Trench B is what appears to be the terminal impact of the object that created the two trenches. This is in the form of several large boulders, small seamounts, and islands in shallower depths indicating breakup of the object 1040km from the Chicxulub impact. The trenches are in an area of the Caribbean where currents prevented them from being silted in, preserving their physical form on the ocean floor. The object that created the trenches could have been large ejecta from the impact or possibly part of the asteroid that separated before impact or upon impact and carried on 1000km down range. The trajectory of both trenches is an upward angle of about 3 degrees. This indicates that the trajectory of the object was at a low angle, very high velocity and was deflected slightly upward upon impact with the ocean floor 4,627 below sea level. RESEARCH The first two phases of 10 phases consisting of mapping, exploration, research, and documentation of the impacts have been completed. Phase 1 consisted of assembling available sea floor data of the area of the impacts. Phase 2 consisted of selecting aerial and under water images from Google Earth, preparing bathymetric mapping from a GEBCO_08. BODC raster analysis with 50m contour intervals of the impact area, preparing the Trajectory Section, and locating 12km wide cross sections of the trenches at 4km intervals. Future phases may consist of Telepresence Enabled Exploration of sea floor mapping, seismic mapping, photographic imaging, sea floor samples, core drilling, animated 3D imagery, a documentary film and presentation to the scientific community. GLOBAL SIGNIFICANCE This research will contribute to the knowledge of the phenomenon of the Chicxulub Impact, related impacts and potential identification of its composition. When mapping the 4 asteroids with impact ages of approximately 65 Ma with the trench impact location we find that 3 of the asteroids, Chicxulub, Mexico 150 km diameter; Eagle Butte, Canada 10 km diameter; Vista Alegre, Brazil 9.5 km diameter are in the same orbital plane as the Trench site. This suggests that the smaller asteroids, Eagle Butte and Vista Alegre, separated from the larger Chicxulub Asteroid upon entry and impacted at different locations but on the same orbital plane. The impact of Comet Shoemaker-Levy on Jupiter had 21 impacts over 6 days as the smaller comets separated as it entered the gravitational field of Jupiter. AUTHOR/PRINCIPAL INVESTIGATOR ROBERT L. PAGE, PROF EMERITUS, KANSAS STATE UNIVERSITY JARED PUMPHERY, ILLUSTRATIONS, OCEANOGRAPHIC BATHYRMETRY, MLA, KANSAS STATE UNIVERSITY

  5. Phoenix Deepens Trenches on Mars (3D)

    NASA Technical Reports Server (NTRS)

    2008-01-01

    The Surface Stereo Imager on NASA's Phoenix Mars Lander took this anaglyph on Oct. 21, 2008, during the 145th Martian day, or sol. Phoenix landed on Mars' northern plains on May 25, 2008.

    The trench on the upper left, called 'Dodo-Goldilocks,' is about 38 centimeters (15 inches) long and 4 centimeters (1.5 inches) deep. The trench on the right, called 'Upper Cupboard,' is about 60 centimeters (24 inches) long and 3 centimeters (1 inch) deep. The trench in the lower middle is called 'Stone Soup.'

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

  6. Phoenix Deepens Trenches on Mars (3D)

    NASA Technical Reports Server (NTRS)

    2008-01-01

    The Surface Stereo Imager on NASA's Phoenix Mars Lander took this anaglyph on Oct. 21, 2008, during the 145th Martian day, or sol. Phoenix landed on Mars' northern plains on May 25, 2008.

    The trench on the upper left, called 'Upper Cupboard,' is about 60 centimeters (24 inches) long and 3 centimeters (1 inch) deep. The trench in the middle,called 'Ice Man,' is about 30 centimeters (12 inches) long and 3 centimeters (1 inch) deep. The trench on the right, called 'La Mancha,' is about 31 centimeters (12 inches) and 5 centimeters (2 inches) deep.

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

  7. Puerto Rico Trench: Cruise Summary and Results

    NSDL National Science Digital Library

    Uri Brink

    This text reports on the two expeditions of the National Oceanic and Atmospheric Administration's (NOAA) Office of Ocean Exploration to map the Puerto Rico Trench, the deepest part of the Atlantic Ocean. Multibeam bathymetry and acoustic backscatter data were acquired as the expedition mapped the Caribbean plate, an entire tectonic province of the Earth. The featured section of the report is on the trench area and contains a three dimensional bathymetrical map of the trench. Other sections include observations of marine mammals, an introduction to and background information about the members of the expedition, a slide show that shows daily activities, and a summary of the trip containing a warning of the possibility of a tsunami producing underwater landslides in the area. The site is enhanced with photographs and maps.

  8. Snow White Trench Prepared for Sample Collection

    NASA Technical Reports Server (NTRS)

    2008-01-01

    The informally named 'Snow White' trench is the source for the next sample to be acquired by NASA's Phoenix Mars Lander for analysis by the wet chemistry lab.

    The Surface Stereo Imager on Phoenix took this shadow-enhanced image of the trench, on the eastern end of Phoenix's work area, on Sol 103, or the 103rd day of the mission, Sept. 8, 2008. The trench is about 23 centimeters (9 inches) wide.

    The wet chemistry lab is part of Phoenix's Microscopy, Electrochemistry and Conductivity suite of instruments.

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

  9. Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica

    NASA Astrophysics Data System (ADS)

    van Avendonk, H. J. A.; Holbrook, W. S.; Lizarralde, D.; Denyer, P.

    2011-06-01

    The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galápagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from ˜7.9 km/s offshore Nicoya Peninsula to ˜6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of ˜12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.

  10. Determining Earthquake Recurrence Intervals from Trench Logs

    NSDL National Science Digital Library

    Patricia Cashman

    Trench logs of the San Andreas Fault at Pallett Creek, CA are the data base for a lab or homework assignment that teaches about relative dating, radiometric dating, fault recurrence intervals and the reasons for uncertainty in predicting geologic phenomena. Students are given a trench log that includes several fault strands and dated stratigraphic horizons. They estimate the times of faulting based on bracketing ages of faulted and unfaulted strata. They compile a table with the faulting events from the trench log and additional events recognized in nearby trenches, then calculate maximum, minimum and average earthquake recurrence intervals for the San Andreas Fault in this area. They conclude by making their own prediction for the timing of the next earthquake. While basically an exercise in determining relative ages of geologic horizons and events, this assignment includes radiometric dates, recurrence intervals, and an obvious societal significance that has been well received by students. With minor modifications, this exercise has been used successfully with elementary school students through university undergraduate geology majors. Less experienced students can work in groups, with each group determining the age of a single fault strand; combining the results from different groups and calculating recurrence intervals can then be done as a class activity. University students in an introductory geology course for non-majors can add their data from the trench log to an existing table with other faulting events already provided. The exercise can be made more challenging for advanced students by using logs from several different trenches, requiring students to design the table themselves, and giving students the uncertainties for the radiometric dates rather than simple ages for the strata. Most studentsâat all levelsâare initially frustrated by their inability to determine an exact date of faulting from the available data. They gain a new appreciation for the task of the geoscientist who attempts to relate geologic phenomena to the human, rather than geologic, time scale.

  11. An overview of the submarine morphology of the Ecuador-South Colombia convergent margin: implication for mass transfer and age of the Carnegie Ridge subduction

    NASA Astrophysics Data System (ADS)

    Collot, Jean-Yves; Michaud, François; Alvarado, Alexandra; Marcaillou, Boris; Sosson, Marc; Ratzov, Gueorgui; Migeon, Sébastien; Alcinoe, Calahorrano; Andres, Pazmiño.

    2010-05-01

    A compilation of swath bathymetric data from the Ecuador South-Colombia subduction zone allows a detailed characterization of the geomorphology of the trench and margin seafloor. These data are used together with seismic reflection profiles to evaluate the age and the effects of the Carnegie ridge (CR) subduction, and thus determine the modes of mass transfer along the margin. The outer trench wall shows a well-developed flexural bending-fault pattern that cuts across the CR, and progressively rotates clockwise from south to north, parallel to the overall trench orientation, which varies from N to NE-trending. The first order segmentation of the Nazca plate due to the Carnegie Ridge and Grijalva Fracture Zone is reflected on the inner trench wall geomorphology, thus defining southern, central and northern margin segments. Sediment is transported from the Andes to the trench along the Guayaquil canyon across the southern margin segment, and along the Esmeraldas and Patia-Mira canyon systems across the northern margin segment, thus providing 0.8 km and up to 4.8 km trench fill, respectively. In contrast, little terrestrial sediment has been deposited in the shallow trench of the central margin segment. The overall morphological character of the central margin segment, which is characterized by a generally steep slope, mass wasting and a small frontal prism is compatible with an erosive margin, thus supporting negative mass transfer. Extensive mass wasting affected specifically the region of the northern central margin segment associated with the subduction of the northern flank of the CR, and the region that straddles the central and southern margin segments related to the subduction of the Grijalva Fracture Zone. Tectonic accretion is, however, active in the southern and northern margin segments in the form of the Guayaquil and Colombia accretionary wedges, which indicate positive mass transfer. According to GPS-based plate kinematics motions, we interpret the areas of extensively eroded margin slope along both the northern and southern regions of the central margin segment to result from the southward migration of the CR. This interpretation supports that the CR has been subducting since ~4-5 Myr.

  12. Fore-arc deformation at the transition between collision and subduction: Insights from 3-D thermomechanical laboratory experiments

    NASA Astrophysics Data System (ADS)

    Boutelier, D.; Oncken, O.; Cruden, A.

    2012-04-01

    Three-dimensional thermomechanical laboratory experiments of arc-continent collision investigate the deformation of the fore arc at the transition between collision and subduction. The deformation of the plates in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. In our experiments, the largest along-strike gradient of trench-perpendicular compression does not produce sufficiently localized shear strain in the transition zone to form a strike-slip system because of the fast propagation of arc lithosphere failure. Deformation is continuous along-strike, but the deformation mechanism is three-dimensional. Progressive along-strike structural variations arise because coupling between neighboring segments induces either advanced or delayed failure of the arc lithosphere and passive margin. The modeling results suggest that orogenic belts should experience deeper subduction of continental crust and hence higher-pressure metamorphism where the two plates first collided than elsewhere along the plate boundary where collision subsequently propagated. Furthermore, during the initial stage of collision the accretionary wedge is partially subducted, which leads to lubrication of the interplate zone and a reduction of shear traction. Therefore, a large convergence obliquity angle does not produce a migrating fore-arc sliver. Rather, the pressure force generated by subduction of the buoyant continental crust causes fore-arc motion. It follows that convergence obliquity during collision does not yield trench-parallel deformation of the fore arc and its influence on the collision process is limited. However, convergence obliquity may control the geometry of the active margin during the oceanic subduction stage prior to collision, and inherited structures may influence the propagation mechanism.

  13. Evaluation of in situ smectite dehydration as a pore water freshening mechanism in the Nankai Trough, offshore southwest Japan

    Microsoft Academic Search

    Demian M. Saffer; Alexander W. McKiernan

    2009-01-01

    Pore water freshening has been observed within sediments near the trench at numerous subduction zones. Constraining the relative contributions of long-distance updip flow of freshened fluids and in situ clay dehydration holds important implications for margin-scale fluid flow but remains unresolved because the evolution of pore water chemistry expected due to in situ clay dehydration and the budget of fresh

  14. Trench Reveals Two Faces of Soils

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This approximate true-color image mosaic from the panoramic camera on the Mars Exploration Rover Opportunity shows a trench dug by the rover in the vicinity of the 'Anatolia' region. Two imprints from the rover's Mossbauer spectrometer instrument were left in the exposed soils. Detailed comparisons between soils exposed at the surface and those found at depth reveal that surface soils have higher levels of hematite while subsurface soils show fine particles derived from basalt. The trench is approximately 11 centimeters deep. This image was taken on sol 81 with the panoramic camera's 430-, 530- and 750-nanometer filters.

  15. Phoenix's La Mancha Trench in 3D

    NASA Technical Reports Server (NTRS)

    2008-01-01

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

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

  16. Variations in Oxygen Fugacity among Forearc Peridotites from the Tonga Trench

    NASA Astrophysics Data System (ADS)

    Birner, S.; Warren, J. M.; Cottrell, E.; Lopez, O. G.; Davis, F. A.; Falloon, T.

    2013-12-01

    The Tonga subduction zone is an extension-dominated, non-accreting convergent plate margin in the South Pacific, characterized by rapid slab rollback [1]. It is unusual among subduction zones in that forearc peridotites, thought to be pieces of lithospheric mantle originating from the overriding plate, have been dredged from the trench. These spinel peridotites appear in dredges along almost 1000 km of the trench's length, from near the Samoa hotspot in the north to the Louisville seamounts in the south, and have been dredged from 4-9 km depth. The samples are very depleted, consisting entirely of dunites and harzburgites, with no observed lherzolites. Low modal abundances of orthopyroxene and high spinel Cr# (Cr/(Cr+Al)) also indicate large degrees of melt extraction. While some samples have been variably altered by hydrothermal processes, a large fraction of them are remarkably unaltered, making them ideal targets for geochemical investigation. Oxygen fugacity is an important geochemical control on phase stability, the composition of volatiles, and the position of the mantle solidus, thus rendering it critical to the understanding of mantle processes. Previous studies have suggested that subduction zone processes result in arc magmas with increased oxygen fugacity (fO2) relative to ridge magmas [2], but few direct observations of mantle wedge fO2 are available. In order to investigate the oxygen fugacity of the Tonga peridotites, mineral major element compositions were determined via electron microprobe. Fe3+/?Fe ratios were calculated for the spinel phase, calibrated with spinel standards of known Fe3+/?Fe ratio provided by B.J. Wood to the Smithsonian following the procedure of Lopez et al [3]. Oxygen fugacity was calculated according to the olivine-orthopyroxene-spinel oxybarometer method of Wood et al. [4]. Results from five dredges along approximately 600 km of trench showed oxygen fugacity values of 1 log unit above the QFM buffer, compared to the global ridge peridotite average of QFM -1. A sixth dredge from the middle of the sample area showed significantly more reduced values of QFM -2. Interactions with oxidized fluids in the mantle wedge have been proposed as a mechanism for oxidizing forearc peridotites relative to ridge peridotites. The additional observation of small length-scale variations in fO2 suggests that the interaction of fluids with the mantle is not a pervasive process, leading to the observed heterogeneity in oxygen fugacity values. [1] Wright et al., Marine Geophys Res (2000); [2] Kelley et al., Science (2009); [3] Lopez et al., EOS Transactions, T51D-2632 (2012); [4] Wood et al., Science (1990).

  17. Subduction zones beneath Indonesia imaged by Rayleigh wave phase velocity tomography

    NASA Astrophysics Data System (ADS)

    Liu, F.; Yang, T.; Harmon, N.

    2013-12-01

    Situated at the junction of several tectonic plates including Indian-Australia, Eurasia, and Philippine Sea, the Indonesian archipelago is one of the most tectonically complex regions on earth with subductions, collisions and accretions occurring along and within its boundaries. A high-resolution lithospheric and upper mantle model, therefore, is needed to understand these complex processes beneath this region. We present a phase velocity model derived from teleseismic Rayleigh waves recorded at seismic stations in this region. We use the modified version of the two-plane wave tomography, in which the non-planar effects of surface wave propagation such as multipathing and scattering are accounted for by two plane wave interference and using of finite frequency kernels. We measure the amplitudes and phases at 16 individual periods ranging from 20s to 150s for the fundamental mode of Rayleigh waves at over 30 stations. 254 earthquakes are selected from global events greater than Ms 5.5 in the distance range of 25°- 150°. To account for the wavefield inconsistencies among stations for each earthquake due to the large scale of our study region, we divide the seismic array into 4 groups of stations in the two-plane wave parameter inversion. The phase velocity maps from our preliminary results show coherent features between adjacent periods. The most dominant structure in phase velocity maps for all periods is the strong fast-velocity belts beneath Sunda Trench, Java Trench, Timor Trough and the trenches around Celebes Sea, which shift gradually toward the subduction directions. The strength of the high velocity anomaly varies among trenches, likely suggesting the different age of subducting slabs. In addition, a velocity contrast in the middle of Borneo appears to mark the Lupar Line, a boundary between the stable Sundaland continental core and fragments of ophiolitic and Asian continental material accreted to Borneo during the Cretaceous. The 3-D shear wave structure derived from these 2-D phase velocity maps at different periods, which is in progress, certainly will sharpen the images of the complex subduction system, unraveling more geodynamic processes in this region.

  18. 46. BASE OF UMBILICAL MAST FROM UMBILICAL MAST TRENCH. ERECTION ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    46. BASE OF UMBILICAL MAST FROM UMBILICAL MAST TRENCH. ERECTION AND RETRACTION CYLINDERS BETWEEN MAST AND TRENCH WALL. - Vandenberg Air Force Base, Space Launch Complex 3, Launch Pad 3 East, Napa & Alden Roads, Lompoc, Santa Barbara County, CA

  19. Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

    USGS Publications Warehouse

    Choy, G.L.; Kirby, S.H.

    2004-01-01

    The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (?? a = ??Es/Mo, where E s is radiated energy and Mo is seismic moment) of all globally distributed shallow (depth, ?? 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower ??a (< 1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average ?? a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average ??a with focal mechanism and tectonic regime suggests that the level of ?? a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high ??a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Mc (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than Mw. This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release. ?? 2004 RAS.

  20. Long-term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

    NASA Astrophysics Data System (ADS)

    Raimbourg, Hugues; Augier, Romain; Famin, Vincent; Gadenne, Leslie; Palazzin, Giulia; Yamaguchi, Asuka; Kimura, Gaku

    2014-06-01

    The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic mélange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a "simple" giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.

  1. Low on-Resistance SOI Dual-Trench-Gate MOSFET

    Microsoft Academic Search

    Xiaorong Luo; T. F. Lei; Y. G. Wang; G. L. Yao; Y. H. Jiang; K. Zhou; P. Wang; Z. Y. Zhang; Jie Fan; Q. Wang; R. Ge; Bo Zhang; Zhaoji Li; Florin Udrea

    2012-01-01

    A low specific on-resistance $(R_{{\\\\rm on}, {\\\\rm sp}})$ integrable silicon-on-insulator (SOI) MOSFET is proposed, and its mechanism is investigated by simulation. The SOI MOSFET features double trenches and dual gates (DTDG SOI): an oxide trench in the drift region, a buried gate inset in the oxide trench, and another trench gate (TG) extended to a buried oxide layer. First, the

  2. Development of common conversion point stacking of receiver functions for detecting subducted slabs

    NASA Astrophysics Data System (ADS)

    Abe, Y.; Ohkura, T.; Hirahara, K.; Shibutani, T.

    2010-12-01

    In subduction zones, the subducting slabs are thought to convey fluid into the mantle wedge to cause arc volcanism (Hasegawa et al., 2008. Iwamori, 2007). Kawakatsu & Watada (2007) examined the Pacific slab subducting beneath northeast Japan with receiver function (RF) analysis, and revealed where the hydrated oceanic crust and the serpentinized mantle wedge exist. In the other subduction zones, it is also essential to examine subducting slabs for better understanding of water transportation and volcanic activities. In this study, we develop a new method to migrate RFs in order to examine subducting slabs with high dip angle (Abe et al., submitted to GJI) and apply this method to examination of the Philippine Sea slab (PHS). The RF technique is one of the useful methods to obtain seismic velocity discontinuities. Ps phases converted at discontinuities in a teleseismic coda can be detected by RF analysis. RFs are usually converted to depth domain assuming a 1-d velocity structure, and the geometry of discontinuities is obtained (e.g. Yamauchi et al., 2003). In subduction zones, however, subducting slabs usually dip, and we should take into account the refraction of seismic waves at dipping interfaces. Therefore, we use the multi-stage fast marching method (FMM, de Kool et al., 2006) to convert RFs into depth domain. We stack transverse RFs, since polarity of them does not change depending on their dip angles and they are better at detecting phases converted at dipping interfaces than radial RFs. We have confirmed that this method works properly with synthetic test. We apply our method to waveform data observed in Kyushu, Japan, where PHS is subducting toward WNW and the Wadati-Benioff zone dips at 30° at depths up to 80 km, and dips at 70° at depths between 80 km and 170 km. We obtain a vertical section, on which RF amplitude is projected, across central part of Kyushu perpendicular to the depth contour of the Wadati-Benioff zone. On the section, positive peaks of RFs corresponding to the bottom of the hydrated oceanic crust or the serpentinized mantle wedge, are successfully detected along the Wadati-Benioff zone at depths up to 90 km. Fluid is expected to be conveyed by PHS up to this depth in this region. Acknowledgment. We use waveform data observed by the National Research Institute for Earth Science and Disaster Prevention, Japan Meteorological Agency (JMA), Kyushu Univ. and Kagoshima Univ. We also use the hypocentral data of JMA. We calculate travel time fields with FMTOMO which was coded by Nick Rawlinson.

  3. Evidence for shallow dehydration of the subducting plate beneath the Mariana forearc: New insights into the water cycle at subduction zones

    NASA Astrophysics Data System (ADS)

    Ribeiro, J.; Stern, R. J.; Kelley, K. A.; Shaw, A. M.; Martinez, F.; Ohara, Y.

    2014-12-01

    Water is efficiently recycled at subduction zones. It is fluxed from the surface into the mantle by the subducted plate and back to the surface or crust through explosive arc volcanism and degassing. Fluids released from dehydrating the subducting plate are transfer agents of water. Geophysical modeling [1] and the geochemistry of arc glasses [2] suggest that at cold-slab subduction zones, such as the Mariana convergent margin, the downgoing plate mostly dehydrates beneath the volcanic arc front (? ~ 80 -100 km depth to slab) to trigger volcanism. However, there is a gap in our understanding of the water fluxes released beneath forearcs, as examples of forearc magmatism are extremely rare. Here, we investigate the Southernmost Mariana Forearc Rift (SEMFR), where MORB-like spreading occurred unusually close to the trench, sampling slab-derived aqueous fluids released at ~ 30 to 100 km depth from the subducted plate. Examining the trace element and water contents of olivine-hosted melt inclusions and glassy rinds from the young (2 - 4 Ma) and fresh SEMFR pillowed basalts provide new insights into the global water cycle. SEMFR lavas contain ~2 wt % H2O, and the olivine-hosted melt inclusions have the highest subduction-related H2O/Ce ratios (H2O/Ce = 6000 - 19000) ever recorded in arc magmas (H2O/Ce < 10600 and global averaged H2O/Ce < 3000). Our findings show that (i) slab-derived fluids released beneath forearcs are water-rich compared to the deeper fluids released beneath the arc system; and (ii) cold downgoing plates lose most of their water at shallow depths (~ 70 - 80 km slab depth), suggesting that water is efficiently recycled beneath the forearc (? 90%). 1. Van Keken, P.E., et al., Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide. Journal of Geophysical Research: Solid Earth, 2011. 116(B1): p. B01401, DOI: 10.1029/2010jb007922. 2. Ruscitto, D.M., et al., Global variations in H2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes. Geochemistry Geophysics Geosystems, 2012. 13(3): p. Q03025, DOI: 10.1029/2011gc003887.

  4. 2D/3D Numerical Models of the Taiwan Orogen: Oblique Arc-Continent Collision overlying Orthogonal Subduction Systems

    NASA Astrophysics Data System (ADS)

    Kanda, R. V.; Suppe, J.; Wu, J. E.

    2013-12-01

    Recent plate-tectonic reconstructions based on mapping of subducted slabs imaged by state-of-the-art tomographic models, and constrained by paleomagnetic data demonstrate that the Philippine Sea Plate (PSP) was originally part of the Sunda Plate (SP). These reconstructions show that the PSP has moved northward with Australia across 25° of latitude since the early Eocene (~ 43 Ma). Most of this motion of the PSP was accommodated on the north and east by overriding a southward subducting East Asian Sea (EAS) ocean basin that was contiguous with the present-day Eurasian Plate (EP). On the western margin of the PSP, this northward advance was accommodated by a N-S transform system. Ages of the Luzon volcanic arc suggest that by early Miocene (~ 15-20 Ma), the EP seafloor west of this transform started subducting eastwards, and highly obliquely, underneath a NNW moving PSP that was detached from the SP. Further, by late Miocene (~10 Ma), northward subduction of the PSP along the present Ryukyu Trench began as a result of arc-continent collision of the PSP along the Eurasian continental margin and flipping of subduction polarity due to slab break-off of the south-subducting EAS. A significant rotation of the PSP-EP convergence to the present more northwesterly direction occurred only over the last ~2 Ma. This present-day juxtaposition of orthogonal subduction polarities beneath Taiwan can be understood in terms of a margin-parallel lithospheric STEP fault, that accomplishes the progressive SW extension of the Ryukyu Trench (RT), and also marks the northern limit of the EP subduction. The torn edge of the Eurasian lithosphere is imaged tomographically. Further support for this tearing comes from our newly developed multi-resolution stress maps based on focal-mechanism inversions and the seismicity distribution. Our inferred stress orientations indicate orthogonal contact between the subducting PSP and the Eurasian lithospheres, resulting in present-day E-W strike-parallel compression and horizontal flexure in the PSP above 100 km depth. Here, we present first-order 2.5D/3D lithospheric scale models of the Taiwan orogen resulting from the progressive deformation of the Eurasian margin and based on the above plate motion history. These models are also constrained by large-scale geologic and slab structure as well as 3D geophysical data: focal-mechanism based stress orientations and geodetic strain-rates. We use a particle-tracer based 3D Lagrangian-Eulerian code, SULEC, that can model the evolution of finite plastic and viscoelastic deformation. Our hierarchical modeling approach involves first using intuition building 2D models having simplified versions of the above spatio-temporal constraints, before considering more complex 3D setups. For simplicity, we start our models from the time of initiation of PSP subduction along the RT (~ 10 Ma), and pre-existing slabs in the upper-mantle. Our models address: (a) the timing of subduction flipping from southwards to northwards at the Ruykyu Trench; (b) the tearing of the EP lithosphere as a STEP fault; (c) the mechanism(s) by which the subducting PSP 'slid' under the EP continental margin as far north as Shanghai; and (d) the role of pre-existing subducting slabs along the PSP's western and eastern edges on the recent sudden change to northwesterly convergence.

  5. Challenges in hardening technologies using shallow-trench isolation

    Microsoft Academic Search

    M. R. Shaneyfelt; P. E. Dodd; B. L. Draper; R. S. Flores

    1998-01-01

    Challenges related to radiation hardening CMOS technologies with shallow-trench isolation are explored. It is shown that developing a radiation-hardened CMOS technology with shallow trench isolation is more complex than using a traditional hardened field oxide as the trench insulator. We illustrate the use of device simulations in concert with measurements on test structures to provide detailed physical insight into methods

  6. Progressive accretion in the Middle America Trench, Southern Mexico

    Microsoft Academic Search

    J. Casey Moore; Joel S. Watkins; Thomas H. Shipley; Steven B. Bachman; Floyd W. Beghtel; Arif Butt; Borys M. Didyk; Jeremy K. Leggett; Neil Lundberg; Kenneth J. McMillen; Nobuaki Niitsuma; Les E. Shephard; Jean-Francois Stephan; Herbert Stradner

    1979-01-01

    The lithology and age distribution of a seismic unit characterised by landward dipping reflectors suggest progressive underthrusting and uplift of trench deposits along the inner slope of the Middle America Trench. The fastest uplift and deformation rates, and the most rapid change in tilt of dipping reflectors coincide near the base of the trench slope and diminish rapidly landwards.

  7. Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand.

    PubMed

    Wannamaker, Philip E; Caldwell, T Grant; Jiracek, George R; Maris, Virginie; Hill, Graham J; Ogawa, Yasuo; Bibby, Hugh M; Bennie, Stewart L; Heise, Wiebke

    2009-08-01

    Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike-slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike-slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models. PMID:19661914

  8. Tectonic Constraints on the Geomorphology of the Chile Trench and Submarine Axial Channel from 34-36°S

    NASA Astrophysics Data System (ADS)

    Hass, B.; Trehu, A. M.

    2013-12-01

    High-resolution multibeam bathymetry and seismic reflection data collected in May, 2012 off the Chilean coast between 34-35°S are analyzed to interpret tectonic controls on the geomorphology of the trench, axial channel, and accretionary prism. Data were collected up-dip of the region of greatest slip during the 2010 Maule Mw 8.8 earthquake as part of an investigation of the stress regime in this region in the aftermath of this major subduction thrust event. The Maule earthquake contrasts with the 2011 Tohoku earthquake in that slip during the earthquake was arrested ~30 km down-dip from the deformation front. The new data cover a segment of the trench in which the axial channel appears to be unusually shallow, wide and meandering compared to the channel north or south of this region. We hypothesize that the unusual morphology of the axial channel in this region is in response to an incoming normal fault block that effectively decreases the width of the trench. This is therefore an ideal place to study the effects of converging high-relief morphological features on the trench and axial channel. This segment of the accretionary complex also exhibits interesting morphological features. Three unnamed canyons originating at the mouths of Rio Maule, Rio Huenchullami, and Rio Mataquillo enter the trench between 34°S and 36°S. The canyon off Rio Huenchullami appears to be deflected northward along a strike-slip fault parallel to the deformation front. A large sediment wedge at the base of the canyon off of Rio Maule is interpreted to be a fan lobe. Variable sediment thickness, as determined from seismic reflection data, is explored as a potential factor influencing whether sediment is frontally accreted or basally underplated at the deformation front. Sediments play a key role in the coupling of the plate interface at convergent margins, and consequently affect the nature of earthquakes and tsunamis generated there. Studying the processes that control sediment thickness, distribution, and mass transfer will therefore help improve our understanding of subduction zone seismicity. ChilePEPPER: Michael Tryon (SIO); Eduardo Contreras-Reyes, Andrei Maksymowitz, Javier Ruiz, Emilio Vera (Un. de Chile Santiago) 3D perspective view of trench bathymetry illustrating unusual axial channel morphology. Looking down from SW at 225km, 4x vertical exaggeration.

  9. 300 Area Process Trenches Closure Plan

    SciTech Connect

    Luke, S.N.

    1994-08-15

    Since 1987, Westinghouse Hanford Company has been a major contractor to the US Department of Energy, Richland Operations Office and has served as co-operator of the 300 Area Process Trenches, the waste management unit addressed in this closure plan. For the purposes of the Resource Conservation and Recovery Act, Westinghouse Hanford Company is identified as ``co-operator.`` The 300 Area Process Trenches Closure Plan (Revision 0) consists of a Resource Conservation and Recovery Act Part A Dangerous Waste Permit Application, Form 3 and a Resource Conservation and Recovery Act Closure Plan. An explanation of the Part A Permit Application, Form 3 submitted with this document is provided at the beginning of the Part A Section. The closure plan consists of nine chapters and six appendices. The 300 Area Process Trenches received dangerous waste discharges from research and development laboratories in the 300 Area and from fuels fabrication processes. This waste consisted of state-only toxic (WT02), corrosive (D002), chromium (D007), spent halogenated solvents (F001, F002, and F003), and spent nonhalogented solvent (F005). Accurate records are unavailable concerning the amount of dangerous waste discharged to the trenches. The estimated annual quantity of waste (item IV.B) reflects the total quantity of both regulated and nonregulated waste water that was discharged to the unit.

  10. Spatial Heterogeneity of the Mantle Wedge Structure Corresponding to Interplate Coupling in NE Japan Forearc Region

    NASA Astrophysics Data System (ADS)

    Yamamoto, Y.; Hino, R.; Suzuki, K.; Ito, Y.; Yamada, T.; Shinohara, M.; Kanazawa, T.; Aoki, G.; Tanaka, M.; Uehira, K.; Fujie, G.; Kaneda, Y.; Takanami, T.; Sato, T.

    2008-12-01

    The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the NE Japan arc. Interplate coupling along the plate interface is estimated to be strong by a backslip modeling of the land GPS observation in the middle to southern part of the arc [Suwa et al., 2006]. However, the off-Miyagi region and the off-Fukushima region show different characteristics of the interplate seismic activity. In the off Miyagi region, the large earthquakes with thrust mechanisms have occurred at an interval of about 40 years, and an interplate earthquake of M 7.2 occurred in this region on 16 August 2005. In the off-Fukushima region, few large interplate earthquakes have occurred while the background microseismicity is very high. In order to clarify differences in the seismic velocity structures, corresponding to the differences in the seismic activity between these regions, we estimated a 3D seismic velocity by the double-difference tomography method using both land station data and offshore station data. In our results, most of the relocated hypocenters are along the plate boundary. The subducting oceanic crust and the mantle wedge of the overriding plate were imaged as the landward dipping low velocity layer and the high velocity layer above it, respectively. In the mantle wedge, there are some velocity variations. Comparing spatial extents of the rupture areas of the 1978 and 2005 earthquakes [Yamanaka and Kikuchi, 2004; Yaginuma, 2006] and the velocity structure, we found that the location of high Vp, high Vs and low Vp/Vs anomaly corresponds to the rupture areas of the large interplate earthquakes. The high velocities and low Vp/Vs feature is interpreted as the non-serpentinized mantle wedge, and this may be the reason why M7 earthquakes repeatedly occurred in off-Miyagi region. In the off-Fukushima forearc region, high Vp/Vs area is found in the tip of this low velocity mantle wedge. This low Vp/Vs area is corresponding to the low backslip area revealed by GPS study [Iinuma et al. 2007]. We think that serpentinized mantle wedge delimited the extent of the seismogenic zone. The size of the seismogenic part is smaller in the off-Fukushima region with lower seismic coupling than in the off-Miyagi region.

  11. Two Remarkable Depth Phases Observed in Southwest Japan for Intraslab Earthquakes Within the Philippine Sea Plate

    Microsoft Academic Search

    T. Miyoshi; Y. Sekiguchi; K. Ishibashi; K. Obara

    2008-01-01

    Many distinct later phases are observed by local seismological networks for intraslab earthquakes that occur within the Philippine Sea plate, which is being subducted beneath southwest Japan. In this study, we analyze two remarkable later phases, identified between the P and S wave arrivals, which we refer as X1 and X2. We investigate NIED's Hi-net (High Sensitivity Seismograph Network, Japan)

  12. Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion

    Microsoft Academic Search

    Katsuhiko Shiomi; Kazushige Obara; Haruo Sato

    2006-01-01

    The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure,

  13. Probing the Detailed Seismic Velocity Structure of Subduction Zones Using Advanced Seismic Tomography Methods

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Thurber, C. H.

    2005-12-01

    Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S-P times in a manner similar to double-difference tomography. Obtaining a reliable Vp/Vs model of the subduction zone is more helpful for understanding its mechanical and petrologic properties. Our applications of the original version of double-difference tomography to several subduction zones beneath northern Honshu, Japan, the Wellington region, New Zealand, and Alaska, United States, have shown evident velocity variations within and around the subducting slab, which likely is evidence of dehydration reactions of various hydrous minerals that are hypothesized to be responsible for intermediate depth earthquakes. We will show the new velocity models for these subduction zones by applying our advanced tomographic methods.

  14. Deformation and stress change associated with plate interaction at subduction zones: a kinematic modelling

    NASA Astrophysics Data System (ADS)

    Zhao, Shaorong; Takemoto, Shuzo

    2000-08-01

    The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip-slip is imposed on the down-dip extension of the locked plate interface, or by the backslip model in which a normal slip is imposed on the locked plate interface. It is found that these two models, although totally different in principle, produce similar patterns for the vertical deformation at a subduction zone. This suggests that it is almost impossible to distinguish between these two models by analysing only the interseismic vertical deformation observed at a subduction zone. The steady-slip model cannot correctly predict the horizontal deformation associated with plate coupling at a subduction zone, a fact that is proved by both the numerical modelling in this study and the GPS (Global Positioning System) observations near the Nankai trough, southwest Japan. It is therefore inadequate to simulate the effect of the plate coupling at a subduction zone by the steady-slip model. It is also revealed that the unphysical assumption inherent in the backslip model of imposing a normal slip on the locked plate interface makes it impossible to predict correctly the horizontal motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages. If the analysis made in this work is proved to be correct, some of the previous studies on interpreting the interseismic deformation observed at several subduction zones based on these two models might need substantial revision. On the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic/mechanical conditions of the plate coupling implied by the present plate tectonics, a synthesized model is proposed to simulate the kinematic effect of the plate interaction during interseismic stages. A numerical analysis shows that the proposed model, designed to simulate the motion of a subducted slab, can correctly produce the deformation and the main pattern of stress concentration associated with plate coupling at a subduction zone. The validity of the synthesized model is examined and partially verified by analysing the horizontal deformation observed by GPS near the Nankai trough, southwest Japan.

  15. Flexural modelling of circum-Pacific trench - outer-rise systems and its implications for mantle rheology

    NASA Astrophysics Data System (ADS)

    Hunter, Johnny; Watts, Tony; Bassett, Dan

    2014-05-01

    The strength of the lithosphere is determined by its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. In oceanic regions, it is widely accepted that Te increases as a function of age at the time of loading due to thermal cooling of the lithosphere. A recent trench - outer-rise study, however, has questioned whether such a simple relationship exists. In order to reassess the relationship between strength and age, we use trench-normal, ensemble-averaged profiles of satellite-derived free-air gravity anomalies to model the trench - outer-rise of circum-Pacific subduction zones. A broken elastic plate model is used, with a finite difference solution that allows Te to vary as a function of distance from the trench. We use an inverse approach, iterating Te values and solving for a vertical shear force and a bending moment. We first model the profiles using a plate of constant Te. Results show that lithosphere younger than 100 Ma clearly strengthens with age. For example, the Middle America trench (6 - 29 Ma) has a mean Te of 14.7 ± 2.0 km, the Aleutian trench (42 - 63 Ma) has a mean Te of 29.8 ± 3.3 km, and the Kuril trench (97 - 129 Ma) has a mean Te of 41.0 ± 3.8 km. For lithosphere older than 100 Ma, however, the relationship is not as clear. For many subduction zones, a plate of constant Te cannot fit the wavelength of the bulge and the high curvature of the seaward wall of the trench, suggesting localized weakening. We therefore model the profiles with a plate that is allowed to weaken trenchward of the outer-rise. This provides significantly improved fits to observations. We attribute this apparent weakening primarily to inelastic yielding - a combination of brittle fracture of the upper lithosphere and ductile flow of the lower lithosphere - due to high curvatures. Evidence for this can be seen in swath bathymetry and seismicity data, which reflect zones of pervasive extensional faulting in the trench outer-rise region. Curvatures and strain rates derived from best-fit models are used, together with brittle-elastic-plastic yield strength envelopes, to calculate Te. The calculated and inverted Te values are then compared in order to constrain experimentally-derived low-temperature plasticity laws. We find that when inelastic yielding is taken into account, the flow law of Mei et al. (2010) accounts well for the inverted values. This result contrasts with that of Zhong and Watts (2013) who used a non-linear viscoelastic model to fit the seismically-constrained flexure beneath the Hawaiian Islands, finding that the Mei et al (2010) flow law produced a lithosphere that was too strong to fit observations. We discuss here the possible causes of the discrepancy between the trench - outer rise and Hawaiian Islands results and examine their implications for lithosphere rheology.

  16. Bathymetric Expression of the Antarctica-South America-Scotia Triple Junction in the Southern Chile Trench

    NASA Astrophysics Data System (ADS)

    Stock, J. M.

    2013-05-01

    At the southern Chile Trench, the Antarctica plate subducts beneath the South American continent at the western end of the South America - Scotia plate boundary. Antarctica-South America convergence is ~18 mm/yr, nearly orthogonal to the trench. Behavior of the Scotia plate in this region is less well known because of low seismicity and lack of detailed surveys, but its northern boundary is thought to lie in the WNW-trending Strait of Magellan along the left-lateral Magallanes-Fagnano fault zone, which has a GPS-derived slip rate >4 mm/yr farther east (Smalley et al., 2003; Mendoza et al., 2011). In a simple kinematic model, the Magallanes fault zone continues WNW from the Strait of Magellan to the trench, forming an unstable Fault-Trench-Trench triple junction, with slow Antarctica-South America convergence north of the triple junction, and even slower and more oblique Antarctica-Scotia convergence south of the triple junction. We collected multibeam bathymetry here on cruise Nathaniel B Palmer 0602 to seek evidence of the triple junction location and changes in morphology along the trench on either side. We surveyed the margin northwest of the western opening of the Strait of Magellan, from the abyssal seafloor (>4200 m water depth) to water depths of ~200 m, between latitudes 51°40' S and 52°35' S. Numerous submarine channels are seen on the continental slope. The two widest slope channels are 1) a western continuation of the Strait of Magellan and 2) another channel 30 km to the N which may be deflected in the mid-slope along the trace of the Magallanes fault. The steepest west-facing slopes, several hundred meters high, are present just east of the sediment-filled trench. The trend and character of the base of the slope changes at about 52°10' S, with a more regular, roughly linear morphology south of this latitude, contrasting with sets of 2-3 irregular bathymetric scarps or steps separated by horizontal distances of 1-3 km, north of this latitude. These differences may be caused by the change in convergence rate and direction at the triple junction, with the straighter southern section of trench resulting from the more oblique Antarctica - Scotia convergence. The trace of the Magallanes fault zone does not obviously displace the base of the slope. Thus, sedimentation and mass wasting on the slope, and processes at the base of the slope (including earthquakes or creep on the subduction interface, mass wasting, and mass transport by deep ocean currents) may dominate in controlling the slope morphology. Additionally, the triple junction may have diffuse deformation, with Scotia-South America motion reaching the southern Chile Trench over a distributed region (e.g., Pelayo and Wiens, 1989) involving other faults that could reach the southern Chile Trench in unsurveyed zones north or south of our study area.

  17. A tale of two arcs? Plate tectonics of the Izu-Bonin-Mariana (IBM) arc using subducted slab constraints

    NASA Astrophysics Data System (ADS)

    Wu, J. E.; Suppe, J.; Renqi, L.; Kanda, R. V. S.

    2014-12-01

    Published plate reconstructions typically show the Izu-Bonin Marianas arc (IBM) forming as a result of long-lived ~50 Ma Pacific subduction beneath the Philippine Sea. These reconstructions rely on the critical assumption that the Philippine Sea was continuously coupled to the Pacific during the lifetime of the IBM arc. Because of this assumption, significant (up to 1500 km) Pacific trench retreat is required to accommodate the 2000 km of Philippine Sea/IBM northward motion since the Eocene that is constrained by paleomagnetic data. In this study, we have mapped subducted slabs of mantle lithosphere from MITP08 global seismic tomography (Li et al., 2008) and restored them to a model Earth surface to constrain plate tectonic reconstructions. Here we present two subducted slab constraints that call into question current IBM arc reconstructions: 1) The northern and central Marianas slabs form a sub-vertical 'slab wall' down to maximum 1500 km depths in the lower mantle. This slab geometry is best explained by a near-stationary Marianas trench that has remained +/- 250 km E-W of its present-day position since ~45 Ma, and does not support any significant Pacific slab retreat. 2) A vanished ocean is revealed by an extensive swath of sub-horizontal slabs at 700 to 1000 km depths in the lower mantle below present-day Philippine Sea to Papua New Guinea. We call this vanished ocean the 'East Asian Sea'. When placed in an Eocene plate reconstruction, the East Asian Sea fits west of the reconstructed Marianas Pacific trench position and north of the Philippine Sea plate. This implies that the Philippine Sea and Pacific were not adjacent at IBM initiation, but were in fact separated by a lost ocean. Here we propose a new IBM arc reconstruction constrained by subducted slabs mapped under East Asia. At ~50 Ma, the present-day IBM arc initiated at equatorial latitudes from East Asian Sea subduction below the Philippine Sea. A separate arc was formed from Pacific subduction below the East Asian Sea. The Philippine Sea plate moved northwards, overrunning the East Asian Sea and the two arcs collided between 15 to 20 Ma. From 15 Ma to the present, IBM arc magmatism was produced by Pacific subduction beneath the Philippine Sea.

  18. Update Japan.

    ERIC Educational Resources Information Center

    Hoopes, Aaron

    This book is a guide intended for persons planning on relocating to Japan. Following a chapter on background information, 13 additional chapters lead the reader step-by-step through the relocation process. These chapters include: before leaving, on arrival, language, culture, doing business in Japan, household pointers and everyday life, schools…

  19. Subduction Drive of Plate Tectonics

    Microsoft Academic Search

    W. B. Hamilton

    2003-01-01

    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

  20. Structural, geochronological, magnetic and magmatic constraints of a ridge collision/ridge subduction-related ophiolite

    NASA Astrophysics Data System (ADS)

    Anma, Ryo

    2013-04-01

    A mid-oceanic ridge system subducts underneath South American plate at latitude 46S off Chilean coast, forming a ridge-trench-trench type triple junction. At ~ 6 Ma, a short segment of the Chile ridge system subducted in south of the present triple junction. This ridge subduction event resulted in emplacement of a young ophiolite (5. 6 to 5. 2 Ma) and rapid crustal uplift (partly emerged after 4.9 Ma), and synchronous magmatism. This ophiolite, namely the Taitao ophiolite, provides criteria for the recognition of ridge collision/ridge subduction-related ophiolites. Aiming to establish recognition criteria, we studied distribution of structures, magnetic properties, geochemical characteristics, and radiometric ages of the Taitao ophiolite and related igneous rocks. The Taitao ophiolite exhibits a classic Penrose-type stratigraphy: ultramafic rocks and gabbros (collectively referred as plutonic section hereafter) in the south, and sheeted dike complex (SDC) and volcanic sequences in the north. Composite foliations developed in the plutonic section, which were folded. SDC were exposed in two isolated blocks having orthogonal strikes of dike margins. Geochemically, gabbros have an N-MORB composition whereas basalts of the volcanic sequence have an E-MORB composition. U-Pb ages of zircons separated from gabbros, SDC and sediments interbeded with billow lavas implied that the center of magmatic activities migrated from the plutonic section to volcanic section during ~5.6 Ma and ~5.2 Ma. Zircon fission track ages of gabbros coincide with U-Pb ages within error range, implying rapid cooling. Demagnetization paths for SDC and lavas form a straight line, whereas those from the plutonic section are Z-shaped and divisble into two components: low coercivity and high coercivity. Restored orientation of gabbro structures imply that the magnetization acquired while gabbroic structures were folding. Thus, magma genesis and emplacement of the plutonic section of ophiolite took place almost instantaneously. The ophiolite is surrounded by synchronous (5.7 Ma to 5.2 Ma) granitic intrusions with various compositions. Our data indicates that the granitic melts started forming near the conjunction of the subducting ridge and transform fault. Generation of granitic melts continued as the spreading center of the same segment subducted, due to partial melting of the oceanic crust and subducted sediments at amphibolite-facies conditions. The obduction of the Taitao ophiolite also accompanied volcanism in the Chile Margin that migrates from west (5.2 Ma) to east (4.6 Ma) at a rate of 5 cm/y as a fracture zone subducted. A ridge collision/ridge subduction-related ophiolite has a short-life. The most intrinsic recognition criteria for such ophiolite must be hot emplacement of plutonic rocks, that represent magmatism at the axial magma chamber in the spreading ridge environment, into cold forearc region, which results in rapid cooling of the deep plutonic section (U-Pb ages coincide with cooling ages within an error range), and pervasive high temperature ductile deformation (after magmatic flow) throughout gabbro. Folding continued until the rocks were cooled to Curie T of magnetite (~580C) in the case of the Taitao ophiolite. A ridge collision/ridge subduction-related ophiolite may accompany block rotation of volcanic sequence, because of high viscosity contrast between hot plutonic section and overriding volcanic section that has already cooled and solidified. It may also accompany acidic intrusions with various compositions and basaltic volcanisms due to subduction of fracture zones.

  1. Fluid pathways in subduction zones

    NASA Astrophysics Data System (ADS)

    Spiegelman, M. W.; van Keken, P. E.; Hacker, B. R.

    2009-12-01

    A large amount of water captured in the oceanic crust and mantle is recycled in subduction zones. Upon compaction and heating most fluids are expelled, but a significant amount of water can be carried in hydrated mineral phases and point defects. While the qualitative role of volatiles and dehydration reactions is well appreciated in the mechanisms for intermediate depth seismicity, mantle wedge melting and arc volcanism, the quantitative details of the metamorphic reactions and the pathways of fluids and melts in the slab are poorly understood. We provide finite element models, combined with thermodynamic and mineralogical constraints, to estimate the water release and migration from the subducting slab to overlying arc. We use models from a selection of warm (e.g., Cascadia), cold (Central Honshu) and intermediate (Nicaragua) subduction zones, using slab geometries constrained from seismological observations. The fluid release is predicted from the breakdown of hydrated phases in sediments, oceanic crust and slab mantle. We use newly developed high resolution models for the flow of these released fluids that take into account permeability and compaction pressures. While the detailed structure depends on the chosen rheology and permeability, we find that for reasonable assumptions of permeability, a significant amount of fluids can travel through the wedge along nearly vertical pathways at rates and paths, consistent with geochronological and geochemical constraints. For models considered to date, we find that the principal source of fluids that feed the wedge come from the hydrated oceanic crust and particularly the hydrated slab mantle. Fluids released from the sediments and shallow crust, tend to travel along high permeability zones in the subducting slab before being released to hydrate the cold corner of subduction zones, suggesting that the cold and hydrated forearc region that is imaged in many subduction zones is maintained by an active hydrological cycle. For these materials to be involved in wedge melting, however, would require entrainment of the rehydrated material into the wedge flow. Current fluid flow models only include dehydration reactions and not melting, thus fluid pathways are for the hydrous fluid only. We plan to add a reactive flow formulation to these models that includes consistent hydration, dehydration and melting reactions, thus allowing for a more complete description of chemical transport through the subduction system. Nicaragua Model

  2. Diversity of tsunamigenic earthquakes along the Sunda subduction zone: 2004- 2006

    NASA Astrophysics Data System (ADS)

    Geist, E. L.

    2006-12-01

    In the past three years, three major tsunamigenic earthquakes have occurred along the Sunda subduction zone that spanned a wide range of tsunami generating capacity. In this study, the efficiency of tsunami excitation is measured by average and maximum observed local runup relative to the scalar seismic moment of the earthquake. The overall severity of the tsunami from the Mw~9.2 December 26, 2004 Sumatra-Andaman earthquake closely matched that expected from an earthquake of this magnitude. In contrast, the March 28, 2005 Nias earthquake was deficient in tsunami excitation for an earthquake of Mw=8.7. The tsunami from the latest earthquake along the subduction zone, the Mw=7.7 July 17, 2006 Java event, was greater than expected and in fact, was greater in terms of average and maximum runup than the March 2005 event. Seismogenic tsunamis are primarily generated from coseismic vertical displacement of the seafloor in response to slip on a fault. The variation in tsunamis from these three earthquakes can be explained by dip-directed variations in the distribution of slip or moment density over the rupture area. Moment density concentrated up dip along the interplate thrust results in four effects that contribute to increased tsunami excitation: (1) a strong reduction in shear modulus in the shallow part of subduction zone results in greater slip for a given moment density; (2) the circumstance of seafloor rupture results in a traction free boundary condition that increases the amount of slip near the trench, in comparison to an imbedded rupture; (3) shallower fault depth below the seafloor results in greater vertical displacement for a given amount of slip; and (4) an increase in the water depth over tsunami generation regions near the trench results in greater amplification of tsunami waves during shoaling according to Green's Law. The difference, therefore, in tsunami excitation from the March 2005 and July 2006 earthquakes can be ascribed to the fact that most of the energy release for the former event was concentrated in the downdip region of the interplate thrust, whereas initial seismic data suggests that most of the energy release for the latter was near the trench. For very long ruptures such as for the December 2004 earthquake, there also can be significant along-strike variability in the rupture process that affects the detailed distribution of local tsunami runup. A large amount of slip near the trench in the southern part of the rupture zone, as deduced from deep-ocean satellite altimetry measurements, resulted in the region of highest runup observed in the western Aceh Province. In contrast, slip along other regions of the long rupture was concentrated in the down dip extent of rupture and was less efficient in generating the tsunami. While we can start to understand the details of tsunami generation in hindsight, the more fundamental issue is understanding the physics of subduction earthquakes to explain why slip and moment release is distributed in particular patterns. This involves understanding "pre-seismic" effects such as the variation in frictional properties with depth and how stresses along the fault have evolved in response to previous earthquakes and tectonic loading. Understanding the dynamics of subduction earthquakes is equally important, in determining, for example, under what conditions earthquakes may rupture to the seafloor and dynamic branching onto secondary thrusts. A better understanding of subduction earthquake physics, combined with new deep-ocean measurements of tsunamis from an expanded network of bottom pressure sensors near the world's subduction zones, can lead to improvements in our ability to assess the hazards from future tsunamis.

  3. Subduction erosion off central Java: transition from accretion to erosion manifested by wide-angle seismic studies

    NASA Astrophysics Data System (ADS)

    Wittwer, A.; Flueh, E.; Rabbel, W.; Wagner, D.

    2006-12-01

    In this study, offshore wide-angle data acquired by ocean bottom instruments of a combined onshore- offshore investigation of the tectonic framework of central Java will be presented. The joint interdisciplinary project MERAMEX (Merapi Amphibious Experiment) was carried out in order to characterize the subduction of the Indo-Australian plate beneath Eurasia. The interpretation of three wide-angle data profiles, modelled with forward raytracing, indicates that the subduction of the Roo Rise with its thickened oceanic crust strongly influences the subduction zone. The dip angle of the downgoing oceanic plate is 10° and its crustal thickness increases to the east from 8 km to 9 km between both dip profiles off central Java. Large scale forearc uplift is manifested in isolated forearc highs, reaching water depths of only 1000 m compared to 2000 m water depth off western Java, and results from oceanic basement relief subduction. A broad band of seamounts trends E-W at approximately 10°S. Its incipient subduction off central Java causes frontal erosion of the margin here and leads to mass wasting due to oversteepening of the upper trench wall. A suite of wide-angle profiles off southern Sumatra to central Java indicates a clear change in the tectonic environment between longitude 108°E and 109°E. The well-developed accretionary wedge off southern Sumatra and western Java changes into a small frontal prism with steep slope angles of the upper plate off central Java.

  4. Possibility of existence of serpentinized material at the Izu-Bonin subduction plate boundary around 31N using Q structure by FDM-simulation

    Microsoft Academic Search

    A. Kamimura; J. Kasahara

    2003-01-01

    At the Izu-Bonin subduction zone (IBSZ), there is a chain of serpentine seamounts at the forearc slope of trench axis, and few large earthquakes occurred at shallow depth (400km). To elucidate these characteristics we carried out a seismic refraction-reflection study at the forearc slope of the IBSZ around 31N using 22 OBSs and chemical explosives and airguns as seismic sources

  5. Geometry of the Philippine Sea plate subducting beneath the southwestern Nankai seismogenic zone

    NASA Astrophysics Data System (ADS)

    Nakanishi, A.; Kodaira, S.; Fujie, G.; Obana, K.; Takahashi, T.; Yamamoto, Y.; Sato, T.; Kashiwase, K.; Fujimori, H.; Kaneda, Y.

    2010-12-01

    In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. Possibility of a megathrust earthquake along the Nankai Trough from Tokai to the Hyuga-nada, east off the Kyushu Island, Japan, is recently pointed out. To understand rupture synchronization and segmentation of the Nankai megathrust earthquake, it is important to know the geometry of the subducting Philippine Sea plate along the Nankai Trough. To obtain the deep subduction structure from the Hyuga-nada (off the Kyushu) to off the Shikoku area, the large-scale high-resolution wide-angle seismic study was conducted in 2008 and 2009. In this study, 160 and 200 ocean bottom seismographs were deployed, and a tuned airgun system (7800 cu. in.) shot every 200m along 11 profiles. Long-term observation was conducted for ~9 months by 21 OBSs off the Shikoku area. This research is part of ‘Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes’ funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Structural images beneath the Hyuga-nada clearly indicate the structural variation of the subducting Philippine Sea Plate from the subducting oceanic crust of Nankai Trough to the thick crust of the Kyushu Palau Ridge. The structural boundary between the oceanic crust and the Kyushu Palau Ridge is considered to lead to the southwestern rim of the coseismic slip zone of the 1968 Hyuga-nada earthquake (Mw7.5) (Yagi et al., 1999). This structural boundary may control the southwestern end of the megathrust earthquake of the Nankai Trough from the Tokai to Hyuga-nada. Moreover, geometry of the Philippine Sea plate estimated based on the structural images is shown in this presentation. Previously obtained seismic data is also used to make precise and detailed geometry of the subducting plate.

  6. World War I: Trenches on the Web

    NSDL National Science Digital Library

    Trenches on the Web, provided by Mike Iavarone, is an excellent resource for both high school classrooms and university students. The core of the site is the Reference Library, which features annotated and hyperlinked timelines, atlases of participating nations, a Special Features section containing a large number of contributed pieces on a wide range of related topics, a map room, and a photo archive. The thumbnail biographies section is under continuing development, and teachers are encouraged to design class projects to contribute pieces on well- or little-known war figures. Trenches on the Web also offers theme-based tours of the site, a discussion forum, and a frequently updated What's New section for veteran users. In addition, the many images and maps create a visually engaging interface. The breadth of information and opportunities for class projects make this one of the best online resources for teaching and learning about the First World War.

  7. 'Snow White' Trench After Scraping (Stereo View)

    NASA Technical Reports Server (NTRS)

    2008-01-01

    This 3D view from the Surface Stereo Imager on NASA's Phoenix Mars Lander shows the trench informally named 'Snow White.' This anaglyph was taken after a series of scrapings by the lander's Robotic Arm on the 58th Martian day, or sol, of the mission (July 23, 2008). The scrapings were done in preparation for collecting a sample for analysis from a hard subsurface layer where soil may contain frozen water.

    The trench is 4 to 5 centimeters (about 2 inches) deep, about 23 centimeters (9 inches) wide and about 60 centimeters (24 inches) long.

    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. Lithospheric normal faulting beneath the Aleutian trench

    Microsoft Academic Search

    Katsuyuki Abe

    1972-01-01

    The focal process of the Rat Island earthquake of March 30, 1965, which occurred beneath the Aleutian trench, is studied on the basis of the long-period surface-wave data and the spatial distribution of the aftershocks. The Rat Island earthquake is represented by a normal faulting with some left-lateral strike-slip component. The spatial distribution of the aftershocks shows a remarkable plane-like

  9. Using Satellite Gravity to Map and Model Forearc Basins and Thickness of Trench Sediment Worldwide: Implications for Great Earthquakes

    NASA Astrophysics Data System (ADS)

    Blakely, R. J.; Scholl, D. W.; Wells, R. E.; von Huene, R.; Barckhausen, U.

    2006-12-01

    There is growing evidence that historic great earthquakes (M>8) favor segments of subduction zones that exhibit key geologic factors, such as high sediment influx into the trench (e.g., Ruff, 1989), the presence of young accretionary prisms (von Huene and Scholl, 1991), the presence of trench-slope forearc basins (Wells et al., 2003; Song and Simons, 2003), and the mineralogical structure of the upper plate. The USGS Tsunami Sources Working Group (http://walrus.wr.usgs.gov/tsunami/workshop/index.html) recently described and quantified these factors for all eastern Pacific subduction margins. Although the level of knowledge of subduction zones world-wide is highly uneven, free-air gravity anomalies observed at satellite altitudes provide a consistent measure of some of these geologic factors. Satellite gravity demonstrates, for example, that regions of greatest slip during past megathrust earthquakes around the circum-Pacific spatially correlate with forearc basins and their associated deep-sea terrace gravity lows, with amplitudes typically >20 mGal. Basins may evolve because interseismic subsidence, possibly linked to basal erosion of the forearc by the subducting plate, does not fully recover after earthquakes. By inference, therefore, forearc basin gravity lows should be predictors of the location of large moment release during future great earthquakes. Moreover, great earthquakes have a statistical propensity to occur at trenches with excess sediments, in contrast to trenches dominated by horst-and-graben bathymetry. After removing the effects of bathymetric depth, low densities associated with trench fill are evident in satellite gravity anomalies and thus permit identification of trench segments with high sediment influx. Additional studies using satellite gravity anomalies may lead to new avenues in understanding the geologic processes that accompany great megathrust earthquakes, but we must confirm the ability of satellite gravity data to serve as a suitable proxy for high-quality marine gravity data. A new compilation of shipboard gravity data from west of Central America (Barckhausen et al., 1998; 2003) affords an excellent opportunity to make this determination. A statistical comparison of satellite and shipboard datasets offshore Nicaragua, Costa Rica, and Panama found average agreement to within 1 mGal and absolute agreement to within 5 mGal everywhere, except very near the coast where errors can be significantly larger. The Sandino forearc basin offshore Nicaragua, for example, is well imaged by satellite gravity anomalies. A preliminary model based strictly on satellite gravity anomalies indicates that the Sandino basin is 7 to 9 km deep, assuming an average density of 2520 kg/m3 for basin fill, a depth that is consistent with multichannel seismic-reflection studies. Most important to future studies of megathrust earthquakes, our comparison shows that conclusions regarding forearc basins would be virtually the same whether drawn from satellite or high-quality shipboard gravity data.

  10. Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

    NASA Astrophysics Data System (ADS)

    Handy, Mark R.; Ustaszewski, Kamil; Kissling, Eduard

    2014-09-01

    Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust-mantle decoupling during Adria-Europe collision. In particular, the switch from south-directed European subduction to north-directed "wrong-way" Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench-trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps-Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust-mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.

  11. The preliminary results of new submarine caldera on the west of Kume-jima island, Central Ryukyu Arc, Japan

    NASA Astrophysics Data System (ADS)

    Harigane, Y.; Ishizuka, O.; Shimoda, G.; Sato, T.

    2014-12-01

    The Ryukyu Arc occurs between the islands of Kyushu and Taiwan with approximately 1200 km in the full length. This volcanic arc is caused by subduction of the Philippine Sea plate beneath the Eurasia Plate along the Ryukyu trench, and is composed of forearc islands, chains of arc volcanoes, and a back-arc rift called Okinawa Trough. The Ryukyu Arc is commonly divided into three segments (northern, central and southern) that bounded by the Tokara Strait and the Kerama Gap, respectively (e.g., Konishi 1965; Kato et al., 1982). Sato et al. (2014) mentioned that there is no active subaerial volcano in the southwest of Iotori-shima in the Central Ryukyu Arc whereas the Northern Ryukyu Arc (i.e., the Tokara Islands) has active frontal arc volcanoes. Therefore, the existence of volcanoes and volcanotectonic history of active volcanic front in the southwestern part of the Central Ryukyu Arc are still ambiguous. Detailed geophysical and geological survey was mainly conducted using R/V Kaiyou-maru No.7 during GK12 cruise operated by the Geological Survey of Japan/National Institute of Advanced Industrial Science and Technology, Japan. As a result, we have found a new submarine volcanic caldera on the west of Kume-jima island, where located the southwestern part of Central Ryukyu Arc. Here, we present (1) the bathymetrical feature of this new submarine caldera for the first time and (2) the microstructural and petrological observations of volcanic rocks (20 volcanic samples in 13 dredge sites) sampled from the small volcanic cones of this caldera volcano. The dredged samples from the caldera consist of mainly rhyolite pumice with minor andesites, Mn oxides-crust and hydrothermally altered rocks. Andesite has plagioclase, olivine and pyroxene phenocrysts. Key words: volcanic rock, caldera, arc volcanism, active volcanic front, Kume-jima island, Ryukyu Arc

  12. Drainage Asperities on Subduction Megathrusts

    NASA Astrophysics Data System (ADS)

    Sibson, R. H.

    2012-12-01

    Geophysical observations coupled with force-balance analyses suggest that the seismogenic shear zone interface of subduction megathrusts is generally fluid-overpressured to near-lithostatic values (?v = Pf/?v > 0.9) below the forearc hanging-wall, strongly modulating the profile of frictional shear resistance. Fluid sources include the accretionary prism at shallow levels and, with increasing depth, metamorphic dehydration of material entrained within the subduction shear zone together with progressive metamorphism of oceanic crust in the downgoing slab. Solution transfer in fine-grained material contained within the deeper subduction shear zone (150 < T < 350°C) likely contributes to hydrothermal sealing of fractures. A dramatic difference may therefore exist between low prefailure permeability surrounding the megathrust and high postfailure fracture permeability along the rupture zone and adjacent areas of aftershock activity. Observed postseismic changes in the velocity structure of the fore-arc hanging-wall led Husen and Kissling (2001) to propose massive fluid loss across the subduction interface following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture. Such trans-megathrust discharges represent a variant of 'fault-valve' action in which the subduction interface itself acts as a seal trapping overpressured fluids derived from metamorphic dehydration beneath. In low-permeability assemblages the maximum sustainable overpressure is limited by the activation or reactivation of brittle faults and fractures under the prevailing stress state. Highest overpressures tend to occur at low differential stress in compressional stress regimes. Loci for fluid discharge are likely determined by stress heterogeneities along the megathrust (e.g. the hangingwall of the rupture at its downdip termination). Discharge sites may be defined by swarm aftershocks defining activated fault-fracture meshes. However, fluid loss across a subduction interface will be enhanced when the stress-state in the forearc hanging-wall switches from compressional reverse-slip faulting before failure to extensional normal-slip faulting postfailure, as occurred during the 2011 Mw9.0 Tohoku megathrust rupture. Mean stress and fault-normal stress then change from being greater than vertical stress prefailure, to less than vertical stress postfailure. Postfailure reductions in overpressure are expected from a combination of poroelastic effects and fluid loss through fault-fracture networks, enhancing vertical permeability. Mineralised fault-fracture meshes in exhumed fore-arc assemblages (e.g. the Alaska-Juneau Au-quartz vein swarm) testify to the episodic discharge of substantial volumes of hydrothermal fluid (< tens of km3). Localized drainage from the subduction interface shear zone increases frictional strength significantly, giving rise to a postfailure strength asperities. Anticipated strength increases from such fluid discharge depends on the magnitude of the drop in overpressure but are potentially large (< hundreds of MPa). Time to the subsequent failure is then governed by reaccumulation of fluid overpressure as well as shear stress along the subduction interface.

  13. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

    Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10-200 km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up to faulting pattern needs to be established, especially for water cycling to the transition zone in the core of the slab.

  14. Continental collision and slab break-off: modelling results and implications for topography, trench migration and magmatism

    NASA Astrophysics Data System (ADS)

    Van Hunen, J.; Allen, M. B.; Bottrill, A.; Magni, V.; Kaislaniemi, L.; Neill, I.

    2012-12-01

    The Cenozoic closure of Tethys and collision of Africa and Eurasia is characterized by observables such as significant topographic features, trench migration, and collision-related magmatism. Slab break-off is dynamically expected to occur, and has been suggested on the basis of tomographic images. While often proposed to (partly) explain these observables, slab break-off has not been directly observed. Here, dynamical models of subduction-collision and subsequent slab break-off are discussed to provide more insight into the dynamics of continental collision and slab break-off. These will then be used to relate plate tectonics to surface observations. In particular, slab break-off has been invoked to explain collision-related magmatism. Modelling results provide temporal constraints, since they show how collision of mature oceanic plates like the former Tethys ocean can delay slab break-off until 20-25 Myrs after initial collision. This suggests that slab break-off for the Arabia-Eurasia collision was unlikely to occur before 10-15 Ma, correlating with an upsurge in magmatism on the Turkish-Iranian plateau after this time. Suture-parallel migration of the slab break-off tear has a model speed of 10-15 cm/yr in this case, but can be significantly faster if the closed ocean basin is relatively young. These values fit well with estimates from recent depocentre migrations in southern Italy. Modelling results further suggest slab steepening following the onset of continental collision, which creates a temporary depression above the mantle wedge, well before the onset of slab break-off. Such depression is in line, both temporally and spatially, with Upper Oligocene-Lower Miocene carbonate strata deposited to the NE of the Arabia-Eurasia suture. Such slab steepening furthermore induces trench advance (i.e. trench motion towards the overriding plate), which is in agreement with recorded trench migration along collision zones such as Arabia-Eurasia and India-Eurasia. External forcing (such as mantle flow, or effects of nearby mantle plumes or slabs) is often invoked to explain such trench advance, but our modelling, in which such external forcing is absent, indicates that intrinsic forces might play an important role too. Finally, we discuss implications for the enigmatic magmatism observed along the Arabia-Eurasia collision zone. The composition of the volcanic centres ranges from OIB-like alkali basalts to mature continental arc lavas. Their composition, and spatial and temporal distribution, suggests a correlation with earlier subduction events, but a wide range of geodynamical processes (including local mantle upwellings, lithosphere-asthenosphere boundary instabilities, chemical heterogeneities, and slab windows) may be required to explain this compositional variety.

  15. Structural architecture of oceanic plateau subduction offshore Eastern Java and the potential implications for geohazards

    NASA Astrophysics Data System (ADS)

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

    2011-01-01

    The region offshore Eastern Java represents one of the few places where the early stage of oceanic plateau subduction is occurring. We study the little investigated Roo Rise oceanic plateau on the Indian plate, subducting beneath Eurasia. The presence of the abnormal bathymetric features entering the trench has a strong effect on the evolution of the subduction system, and causes additional challenges on the assessment of geohazard risks. We present integrated results of a refraction/wide-angle reflection tomography, gravity modelling, and multichannel reflection seismic imaging using data acquired in 2006 south of Java near 113°E. The composite structural model reveals the previously unresolved deep geometry of the oceanic plateau and the subduction zone. The oceanic plateau crust is on average 15 km thick and covers an area of about 100 000 km2. Within our profile the Roo Rise crustal thickness ranges between 18 and 12 km. The upper oceanic crust shows high degree of fracturing, suggesting heavy faulting. The forearc crust has an average thickness of 14 km, with a sharp increase to 33 km towards Java, as revealed by gravity modelling. The complex geometry of the backstop suggests two possible models for the structural formation within this segment of the margin: either accumulation of the Roo Rise crustal fragments above the backstop or alternatively uplift of the backstop caused by basal accumulation of crustal fragments. The subducting plateau is affecting the stress field within the accretionary complex and the backstop edge, which favours the initiation of large, potentially tsunamogenic earthquakes such as the 1994 Mw= 7.8 tsunamogenic event.

  16. 3D absolute hypocentral determination - 13 years of seismicity in Ecuadorian subduction zone

    NASA Astrophysics Data System (ADS)

    Font, Yvonne; Segovia, Monica; Theunissen, Thomas

    2010-05-01

    In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunately, the local network shows great density discrepancy between the Coastal and Andean regions where numerous stations were installed to survey volcanic activity. Consequently, seismicity in and around the interplate seismogenic zone - producer of the most destructive earthquakes and tsunamis - is not well constrained. This study aims to improve the location of 13 years seismicity occurred during an interseismic period in order to better localize the seismic deformation and gaps. The first step consists in the construction of a 3D "georealistic" velocity model. Because local tomography cannot provide satisfactory model, we combined all local crustal/lithospheric information on the geometry and velocity properties of different geological units. Those information cover the oceanic Nazca plate and sedimentary coverture the subducting plate dip angle; the North Andean Block margin composed of accreted oceanic plateaus (the Moho depth is approximated using gravity modeling); the metamorphic volcanic chain (oceanic nature for the occidental cordillera and inter-andean valley, continental one for the oriental cordillera); The continental Guyana shield and sedimentary basins. The resulting 3D velocity model extends from 2°N to 6.5°S and 277°E to 283°E and reaches a depth of 300 km. It is discretized in constant velocity blocks of 12 x 12 x 3 km in x, y and z, respectively. The second step consists in selecting an adequate sub-set of seismic stations in order to correct the effect of station density disequilibrium between coastal and volcanic regions. Consequently, we only keep the most representative volcanic stations in terms of azimuthal coverage, record frequency and signal quality. Then, we define 5 domains: Offshore/coast, North-Andean margin, Volcanic chain, Southern Ecuador, and a domain deeper than 50 km. We process earthquake location only if at least 3 proximal stations exist in the event's domain. This data selection allows providing consistent quality location. The third step consists in improving the 3D MAXI technique that is well adapted to perform absolute earthquake location in velocity model presenting strong lateral Vp heterogeneities. The resulting catalogue allows specifying the deformation in the subduction system. All seismicity previously detected before trench occurs indeed between the trench and the coastal range. South of 0°, facing the subducting Carnegie Ridge, the seismicity aligns along the interplate seismogenic zone between an updip limit shallower than ~8 km and a downdip limit that reaches up to 50 km depth. The active seismogenic zone is interrupted by a gap that extends right beneath the coastal range. At these latitudes, a diffuse intraplate deformation also affects the subducting plate, probably induced by the locally thickened lithosphere flexure. Between the trench and the coast, earthquake distribution clearly defines a gap, which size is comparable to the 1942 M7.9 asperity (ellipse of axes ~55/35 km). A slab is clearly defines and dips around 25 to 30°. The slab seismicity is systematically interrupted between 100-170 km, approximately beneath the volcanic chain. North of 0°, i.e. in the megathrust earthquake domain, the interseismic activity is clearly reduced. The interplate distribution seems to gather along alignments perpendicular to the trench attesting probably of the margin segmentation. The North Andean overriding margin is undergoing active deformation, especially at the location where the Andean Chain strike changes of direction. At these latitudes, no earthquake occurs deeper than 100 km depth.

  17. Seismic velocity discontinuities in the crust and uppermost mantle beneath the Kanto district, central Japan, identified from receiver function imaging and repeating earthquake activity

    Microsoft Academic Search

    Toshihiro Igarashi

    2009-01-01

    We constructed vertical cross-sections of depth-converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the Kanto district, central Japan. Repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting Philippine Sea plate and the subducting Pacific plate. As a result, we present images of some major

  18. Viability of longitudinal trenches for capturing contaminated groundwater.

    PubMed

    Hudak, Paul F

    2010-04-01

    Using a groundwater flow and mass transport model, this study compared the capability of trenches with permeable backfill for capturing hypothetical contaminant plumes in homogeneous and heterogeneous unconfined aquifers. Longitudinal (parallel to groundwater flow), as well as conventional transverse (perpendicular to groundwater flow) trench configurations were considered. Alternate trench configurations intercepted the leading tip of an initial contaminant plume and had identical length, equal to the cross-gradient width of the plume. A longitudinal trench required 31% less time than its transverse counterpart to remediate a homogeneous aquifer. By contrast, in simulated heterogeneous aquifers, longitudinal remediation timeframes ranged from 41% less to 33% more than transverse trenches. Results suggest that longitudinal trenches may be a viable alternative for narrow contaminant plumes under low-groundwater velocity conditions, but may be impractical for plumes with wide leading tips, or in complex heterogeneous aquifers with divergent flow. PMID:20237910

  19. Detrital zircon U-Pb reconnaissance of the Franciscan subduction complex in northwestern California

    USGS Publications Warehouse

    Dimitru, Trevor; Ernst, W. Gary; Hourigan, Jeremy K.; McLaughlin, Robert J.

    2015-01-01

    In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ?2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit's interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151-137 Ma, but it is mostly much younger. (2) Central Belt: SND; ca. 103 too 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager's detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An eocene Princeton River-Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16-14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by positive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37-32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.

  20. The Relationships of Plate Triple-junction Evolution, Trench-Arc Lengthening, Boninite Generation, and SSZ Spreading Centers to Ophiolite Formation, High-Temperature Soles, and Obduction

    NASA Astrophysics Data System (ADS)

    Casey, J.; Dewey, J. F.

    2014-12-01

    A review of modern-day island arcs, the locations of boninite eruptions, the conditions necessary for hot upper plate spreading, potential regions of shallow SSZ flux melting, and formation of high-temperature metamorphic soles along the subduction channels indicates that many future, recent and ancient large slab ophiolite obduction events can be related to triple junctions that link SSZ spreading centers with trenches. These subduction systems leading to large slab ophiolite obduction events typically face stable continental margins. Boninitic melt generation requires hydrous melting of refractory mantle peridotite under an extremely high-temperature and low-pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic mantle asthenosphere and lithosphere, which previously likely experienced melt extraction. Metamorphic conditions associated with metamorphic soles formation likewise require a hot upper plate lithosphere that, based on sole protolith, geochronologic and thermochronologic data, rapidly heats and then refrigerates and decompresses MORB-OIB type subcreted lithosphere. Numerous examples of present-day and recent SSZ spreading centers that link with two trenches or a trench and transform are considered ideal sites for ophiolite and boninite generation. The SZZ fore-arc spreading centers that link to the trench lines and triple junctions at the front of the arc may also continue towards the arc and back arc, creating no distinction between fore-arc and back-arc spreading episodes or to the transform-linked spreading centers from fore-arc to back arc. These SSZ spreading centers, which may be transiently produced during arc evolution over short or protracted time periods, act to open gaps in the arc massif and lengthen the trench, fore-arc and the arc crustal massif. They lead to an evolving arc magmatic front that begins in the infant fore-arc where ophiolite generation occurs at, near or in the trench, followed then by migration to sites distal from the trench line where a new arc massif is established on newly created oceanic lithosphere. Modern and ancient analogues of these setting are reviewed.

  1. Is the Chrystalls Beach Accretionary Melange a Fossil Subduction Channel Shear Zone?

    NASA Astrophysics Data System (ADS)

    Fagereng, A.; Sibson, R. H.

    2007-12-01

    In the northeast of the Hikurangi subduction margin, a 1-2 km thick layer of high Vp/Vs, low Qp and distributed microseismicity is present along the subduction megathrust interface (Eberhart-Phillips & Chadwick, 2002). This zone is interpreted as a 'subduction channel' consisting of a fluid-saturated, highly sheared mixture of trench-fill sediments, which have been subducted below (or eroded from) the accretionary prism (von Huene & Scholl, 1991). Seismic style within subduction channels may vary from large megathrust ruptures to aseismic slip associated with microseismic activity. The factors controlling these variations in style are not well understood due to the inaccessible nature of active subduction thrust interfaces. The Chrystalls Beach Complex, SE Otago, New Zealand, is a possible analogue for the