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1

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

2

Tsunami earthquakes and subduction processes near deep-sea trenches  

Microsoft Academic Search

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

Yoshio Fukao

1979-01-01

3

Trench curvature and deformation of the subducting lithosphere  

NASA Astrophysics Data System (ADS)

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

Schettino, Antonio; Tassi, Luca

2012-01-01

4

Ocean bottom seismic and tsunami network along the Japan Trench  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

5

Paleoseismology off northern Japan: Sediments in the Japan Trench record earthquake activity  

NASA Astrophysics Data System (ADS)

The Japan Trench subduction zone has repeatedly been affected by large earthquakes as most recently in 2011 by the giant magnitude 9 Tohoku-Oki earthquake. Several studies indicate that the 2011 earthquake has induced large seafloor displacements and triggered submarine landslides and gravity flows. The depression of the Japan Trench floor acts as sediment trap, where earthquake triggered mass flows originating from the landward slope are deposited. Thus, the deep Trench floor (>7500 m water depth) is a suitable area to trace the paleoseismicity in the region. During the R/V SONNE cruise (SO219A) in 2012, sediment cores have been collected east of the 2011 earthquake epicenter in a 60 km north-south transect along the Japan Trench floor axis, as well as from a small basin on the upper mid slope. The sediment cores contain several turbidite sequences (few cm to m thick), mainly revealing a coarse sand layer on an erosive base and a gradually fining upward to hemipelagic diatomaceous mud. Tephrochronological analyses on intercalated ash layers within the records provide an age control and show that the cores cover the past ~15 ka. Detailed analyses of these records, by using their sedimentological and lithological characteristics, their physical properties (Multi Sensor Core Logging, MSCL) as well as their elemental composition (X-ray Fluorescence, XRF) allow to characterize and to identify specific turbidite units. We observe particular turbidite units with the same characteristics in different cores along the trench axis and on the mid slope. Besides the top-unit turbidite, being related to the 2011 Tohoku earthquake, we detect a widespread unique calcareous nanno fossil bearing turbidite mud as well as some sand turbidite units of similar elemental composition within the records. Their large spatial extent suggests earthquake related trigger mechanisms. Thus, these event deposits sampled from the deep Japan Trench provide important information on the paleoseismic activity of the off Tohoku.

Fink, H. G.; Ikehara, K.; Kanamatsu, T.; Nagahashi, Y.; Koelling, M.; Strasser, M.; Wefer, G.

2013-12-01

6

En echelon patterns of Calyptogena colonies in the Japan Trench  

NASA Astrophysics Data System (ADS)

The distribution of Calyptogena phaseoliformis colonies in right-stepping en echelon patterns was observed by the Japanese submersible Shinkai 6500 at the foot of the landward escarpment of the northern Japan Trench at around 6437 6274 m depth. The north-south trending Sanriku Escarpment has a thrust origin and is subparallel to the trench axis along which the Pacific plate is being subducted beneath the North America or Okhotsk plate at about 300° at a rate of about 7.8 to 8.3 cm/yr. The trends of colonies are concentrated at 250°, 300°, and 330°: each trend matches either an antithetic riedel shear, extension fracture, or synthetic riedel shear, respectively, within a left-lateral shear regime caused by the oblique subduction. Methane- and hydrogen sulfide bearing fluid advection from depth occurs essentially along the thrust fault, but finally seeps along the fractures at the sea floor. This supplies energy to the food chain through bacteria utilizing hydrogen sulfide, then eventually sustains the Calyptogena colonies. Because the clams select the best places to survive, the geometric arrangement of the clam colonies provides a kinematic indicator of relative plate motions.

Ogawa, Yujiro; Fujioka, Kantaro; Fujikura, Katsunori; Iwabuchi, Yo

1996-09-01

7

A mega-trench survey of huge tsunami traces in Nemuro lowland along the Kuril subduction zone  

Microsoft Academic Search

A mega-trench survey (100m length, 20m width and 5m depth) for huge tsunami traces were conducted using by civil engineering techniques in Nemuro lowland near Lake Nanbutou along the Kuril subduction zone, eastern Hokkaido, northern Japan. According to our stratigraphic methods such as tephrochronology and AMS14C dating, we identified 15 tsunami sands in marsh deposits since 5500 yrBP and their

F. Nanayama; K. Shigeno; M. Ishii; Y. Shitaoka; R. Furukawa; K. Nishio; K. Takano; S. Inokuma

2008-01-01

8

Seismicity and state of stress near the Japan Trench axis off Miyagi, northeast Japan, after the 2011 Tohoku-Oki earthquake  

NASA Astrophysics Data System (ADS)

The 2011 Tohoku-Oki earthquake ruptured roughly 200 km wide and 500 km long megathrust along the Japan Trench. The rupture propagated to the trench axis with a maximum slip about 50 m near the trench axis. As a consequence of this large near-trench slip, earthquakes have been activated near the axis of the Japan Trench off Miyagi, northeast Japan. We have conducted ocean bottom seismograph (OBS) experiments in the Japan Trench axis area, surrounding area of the IODP JFAST drilling site, since the occurrence of the 2011 Tohoku-Oki earthquake. Although conventionally used OBS cannot be deployed at seafloor deeper than 6000 m water depth, we used newly developed ultra-deep OBS using ceramic sphere, which can be deployed at a depth of 9000 m, for the observations in the trench axis. The ultra-deep OBS has almost equivalent dimensions and weight with the conventionally used OBS, thus we can handle it in the same manner with the conventionally OBS without any special operation. As a result of a series of the OBS observations, we obtained accurate hypocenter locations and focal mechanisms in both seaward and landward of the trench axis. Earthquakes near the trench axis area were located within the overriding and incoming/subducting plates with very few on the plate interface below the inner trench slope landward of the trench axis. Most of the earthquakes both in the overriding and incoming/subducting plates having normal or strike-slip faulting focal mechanisms with T-axis normal to the trench axis. This indicates that tensional stress is dominant in the trench axis area. However, most seaward part of the seismicity within the overriding plate is characterized by a localized cluster of trench-normal compressional earthquakes, which may relate to spatial variation of the frictional behavior of the shallowest part of the megathrust. On the other hand, trench-normal extensional earthquakes in the incoming/subducting Pacific plate were located at depths shallower than about 35 to 40 km. The deepest trench-normal extensional earthquake observed during OBS observations from December 2012 to January 2013 is slightly shallower than that observed from May to June in 2011 but further observations are required to investigate the temporal change. Continued and repeated earthquake observations could provide information on post-seismic behavior of the megathrust and recovery process of the stress state.

Obana, K.; Kodaira, S.; Takahashi, T.; Yamamoto, Y.; Nakamura, Y.; No, T.; Fujie, G.; Hino, R.; Shinohara, M.

2013-12-01

9

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

SciTech Connect

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

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

1981-10-10

10

Deep scientific dives in the Japan and Kuril Trenches  

Microsoft Academic Search

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

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

11

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

Microsoft Academic Search

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

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

2010-01-01

12

An ongoing large submarine landslide at the Japan trench  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

13

Development of Observatories for the Japan Trench Fast Drilling Project  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

14

Trench-parallel variations in Pacific and Indo-Australian crustal velocity structure due to Louisville Ridge seamount subduction  

NASA Astrophysics Data System (ADS)

Variations in trench and forearc morphology, and lithospheric velocity structure are observed where the Louisville Ridge seamount chain subducts at the Tonga-Kermadec Trench. Subduction of these seamounts has affected arc and back-arc processes along the trench for the last 5 Myr. High subduction rates (80 mm/yr in the north, 55 mm/yr in the south), a fast southwards migrating collision zone (~180 km/myr), and the obliquity of the subducting plate and the seamount chain to the trench, make this an ideal location to study the effects of seamount subduction on lithospheric structure. The "before and after" subduction regions have been targeted by several large-scale geophysical projects in recent years; the most recent being the R/V Sonne cruise SO215 in 2011. The crust and upper mantle velocity structure observed in profiles along strike of the seamount chain and perpendicular to the trench from this study, are compared to a similar profile from SO195, recorded ~100 km to the north. The affects of the passage of the seamounts through the subduction system are indicated by velocity anomalies in the crust and mantle of the overriding plate. Preliminary results indicate that in the present collision zone, mantle velocities (Pn) are reduced by ~5%. Around 100 km to the north, where seamounts are inferred to have subducted ~1 Myr ago, a reduction of 7% in mantle P-wave velocity is observed. The width of the trench slope and elevation of the forearc also vary along strike. At the collision zone a >100 km wide collapse region of kilometre-scale block faults comprise the trench slope, while the forearc is elevated. The elevated forearc has a 5 km think upper crust with a Vp of 2.5-5.5 km/s and the collapse zone also has upper crustal velocities as low as 2.5 km/s. To the east in the Pacific Plate, lower P-wave velocities are also observed and attributed to serpentinization due to deep fracturing in the outer trench high. Large bending faults permeate the crust and the Osbourn Seamount, currently on the verge of subduction, is fractured stepwise down into the trench. Pn velocities in the hinge zone of the Pacific Plate are as low as 7.3 km/s indicating that fracturing and serpentinization may also extend to sub-crustal depths. Finally, trench-parallel variations in subduction zone velocity structure are used to infer the degree to which seamount subduction has altered the physical state of the Pacific and Indo-Australian plates both pre- and post subduction.

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

2012-12-01

15

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

NASA Astrophysics Data System (ADS)

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

?ížková, Hana; Bina, Craig R.

2013-10-01

16

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

17

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

NASA Astrophysics Data System (ADS)

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

Horning, G.; Klein, F.

2012-12-01

18

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

Microsoft Academic Search

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

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

1987-01-01

19

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

20

Microseismicity survey of a seismic gap caused by the subduction of the Louisville seamount chain in the Tonga trench, 25°30'S to 28°S  

Microsoft Academic Search

The distribution of teleseismically recorded earthquakes in the Kermadec-Tonga subduction zone reveals a major seismic gap centered roughly at 26°S. The gap parallels the trench axis and stretches for approximately 250 km. The seismic gap coincides with the area, where the Louisville hotspot chain enters the Tonga trench. Subducting seamounts may therefore control seismic coupling and hence define seismogenic asperities

I. Grevemeyer; A. Dannowski; E. R. Flueh; S. Moeller

2009-01-01

21

Initiation and propagation of subduction along the Philippine Trench: evidence from the temporal and spatial distribution of volcanoes  

NASA Astrophysics Data System (ADS)

K-Ar ages of 37 samples collected from the Bicol peninsula, the Luzon island, Philippines, were determined by the unspiked sensitivity method in order to constrain the timing of initiation of subduction along the Philippine Trench. The measured K-Ar ages range from 0 to 7 Ma with two old outliers of 27 and 43 Ma. Together with K-Ar ages previously reported on volcanics in Leyte and eastern Mindanao, subduction volcanism has likely propagated from north to south: ˜6.6 Ma in Bicol and ˜3.5 Ma in Leyte and its vicinity. The temporal and spatial distribution suggests that the subduction volcanism started earlier in the north than in the south. This is consistent with the southern propagation of subduction along the Philippine Trench from ˜8 Ma.

Ozawa, Ayako; Tagami, Takahiro; Listanco, Eddie L.; Arpa, Carmencita B.; Sudo, Masafumi

2004-03-01

22

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

NASA Astrophysics Data System (ADS)

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.

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

1987-05-01

23

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

USGS Publications Warehouse

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.

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

24

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

NASA Astrophysics Data System (ADS)

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.

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

2013-04-01

25

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

26

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

27

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

PubMed Central

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

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

2013-01-01

28

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

PubMed

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

Peacock; Wang

1999-10-29

29

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

Microsoft Academic Search

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

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

2010-01-01

30

Provenance mixing in an intraoceanic subduction zone: Tonga Trench-Louisville Ridge collision zone, southwest Pacific  

NASA Astrophysics Data System (ADS)

Dredging on the lower slope of the Tonga forearc, at the intersection between the Louisville Ridge hotspot chain and the trench, yielded both Late Tertiary volcaniclastic sediments and Late Cretaceous slightly tuffaceous pelagic sediments. Petrography and phase chemistry of volcanic debris of Tertiary age samples indicates derivation from a low-K tholeiitic arc: volcanic glass has a low K 2O content and shows an Fe enrichment trend; plagioclase grains are high in An and low in Or components; pyroxene grains (calcic clinopyroxene, orthopyroxene and pigeonite) have low TiO 2 contents and show an Fe enrichment trend; alkali feldspar and biotite are absent. The composition of the Tertiary samples is similar to other Tonga forearc sediments and is consistent with their derivation from the adjacent magmatic arc. The character and composition of volcanogenic debris in Cretaceous age samples indicates derivation from an intraplate alkali igneous source: plagioclase compositions show an increasing Or component with increasing Ab; alkali feldspar is a rare additional component of these samples; calcic clinopyroxene has high TiO 2 contents and is titanaugite; amphibole is also rich in TiO 2 and is kaersutite; minor volcanic glass is rich in alkalis. Samples were dredged from the seaward slope of a small terrace on the lower trench slope. A planar reflector, located 2.6 km below the terrace is marked by an abrupt jump in seismic velocity and is interpreted as the top of an essentially undamaged Late Cretaceous guyot of the Louisville chain that was subducted at the Tonga trench about 0.5 Ma. This guyot is the likely source of the Late Cretaceous, intraplate igneous detritus collected at the dredge site. Some dredge samples yielded a mixed volcanic arc/intraplate provenance and/or Late Tertiary/Late Cretaceous ages. This probably represents the mixing of the Cretaceous seamount and Tertiary magmatic arc sources during post-collision slumping of the wedge of lower slope material as it re-establishes its critical taper.

Cawood, Peter A.

1990-04-01

31

Louisville Ridge subduction at the Tonga-Kermadec trench: preliminary velocity models from wide-angle seismics  

NASA Astrophysics Data System (ADS)

Subduction of the Louisville Ridge at the Tonga-Kermadec trench perturbs the structure of the forearc and changes the seismogenesis of the subduction zone. The oblique collision and high convergence rates between these two > 4000 km long features results in rapid along-trench variation from a post-collision zone north of the Louisville Ridge to a pre-collision zone to the south. Due to the obliqueness of collision there is a southward migration of the collision zone at ~200 km/Ma. At the collision zone, seafloor topography shows a zone of extensive faulting. Tectonic erosion of the overriding plate, collapse of the overriding plate into the trench and a seismic gap are inferred to be due to subduction of the Louisville Ridge. In addition, the consequences of subduction for the seamount are paramount to elucidating the seismogenic nature of the collision zone, with the mode of deformation of the down-going edifice inferred to have a significant effect on the earthquake potential of the system. Truncation of the seamount at shallow depths may result is no seismicity, while truncation at greater depth and pressure within the subduction system may be seismogenic. The pre-, post- and present-day collision zones are investigated with ~1800 km line length of new geophysical data collected along four profiles during April-June, 2011. This NERC-funded, multidisciplinary, multi-institutional project acquired wide-angle and multichannel seismic, gravity, magnetic and swath bathymetry data in a region centred at the point of Louisville Ridge subduction. Three of the new geophysical profiles strike perpendicular to the trench, one along-axis of the seamount chain, and one to the north and south. The profiles were designed to cross the main tectonic features of the subduction zone, explicitly to elucidate differences in crustal structure in the overriding plate. New results on the crustal velocity structure from wide-angle data are presented for the three deformation zones. Results are discussed in context of changes in structure of the overriding plate associated with changes in down-going plate topography and with possible seamount truncations.

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

2012-04-01

32

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

33

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

34

Microbial Diversity in Sediments Collected from the Deepest Cold-Seep Area, the Japan Trench  

Microsoft Academic Search

:   The Japan Trench land slope at a depth of 6,400 m is the deepest cold-seep environment with Calyptogena communities. Sediment samples from inside and beside the Calyptogena communities were collected, and the microbial diversity in the sediment samples was studied by molecular phylogenetic techniques.\\u000a From DNA extracted directly from the sediment samples, 16S rDNAs were amplified by the polymerase

Lina Li; Chiaki Kato; Koki Horikoshi

1999-01-01

35

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

36

Louisville Ridge subduction at the Tonga-Kermadec trench: preliminary models to compare pre- and post collision zone crustal velocity structure.  

NASA Astrophysics Data System (ADS)

New geophysical data were acquired from the collision zone of the Louisville Ridge and Tonga-Kermadec subduction system during April-June, 2011. Around 1800 km of data as four profiles were collected in this NERC-funded multidisciplinary, multi-institutional project. The subduction of the Louisville Ridge seamount chain is associated with changes in subduction zone seismogenesis, crustal structure, and trench strike and depth. Due to the oblique plate convergence, there is a southward migration of the Louisville Ridge of ~200 km/Ma. North of the Louisville Ridge the forearc has already undergone seamount subduction, whereas to the south of the forearc is not yet deformed by this process. Some of the highest convergence rates seen globally are observed along the Tonga-Kermadec trench and this, and the oblique nature of convergence, makes it an ideal locality to study the effects of seamount subduction. Three of the new geophysical profiles follow a perpendicular trend to the trench; one along-axis of the seamount chain, and one to both the north and south. Presented here are preliminary results for the southern pre-collision profile line across the subduction system. A background, pre-collisional crustal structure model of the system is required for comparison to those zones further north where seamount subduction has or is occurring. Variations in seismic velocity structure from the pre-to-post-collision zones may elucidate the degree of crustal deformation due to faulting, and alteration due to fluids/serpentinisation. The velocity model paves the way for a flexural study of the pre-collision line, which will provide information on seamount coupling during subduction and related seismicity.

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

2012-04-01

37

Changes in the crust and upper mantle near the Japan-Bonin trench  

SciTech Connect

Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multichannel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.005 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n=47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11--11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reduced T/sup 2//X/sup 2/ analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well-observed mantle reflection and its strong 8.2-km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3-km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor.

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

1980-01-10

38

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

NASA Astrophysics Data System (ADS)

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

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

1980-01-01

39

Incipient subduction of the Ontong Java Plateau along the North Solomon trench  

Microsoft Academic Search

The Ontong Java Plateau (OJP) in the western central Pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc (Solomon island arc—SIA). To better understand the evolution of the North Solomon trench (NST), active oblique convergence between the OJP and SIA, and late Neogene development of Malaita accretionary prism (MAP),

A. Taira; P. Mann; R. Rahardiawan

2004-01-01

40

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

41

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

PubMed

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

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

2014-05-01

42

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

PubMed Central

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

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

2014-01-01

43

Lateral variations in the structure of the overriding and down-going plates in response to subduction of the Louisville Ridge seamount chain at the Tonga-Kermadec Trench  

NASA Astrophysics Data System (ADS)

Seamount subduction at the Tonga-Kermadec trench is investigated using ~1800 km of new geophysical data collected along four profiles acquired during April-June, 2011 aboard the R/V Sonne. This NERC funded multidisciplinary, multi-institutional project obtained wide-angle and normal incidence seismic, gravity, magnetic and swath bathymetry data in the region of the Louisville Ridge subduction. Subduction of seamounts can be associated with changes in subduction zone seismogenesis, crustal structure, and trench strike and depth. Subduction of the Louisville ridge seamounts has already occurred at the trench and is postulated to have caused accelerated tectonic erosion of the overlying plate, collapse of the overriding plate into the trench and a gap in seismicity. The Tonga-Kermadec subduction system is of particular interest as it has some of the highest convergence rates seen globally, and has produced fatal earthquakes and Tsunami in recent years. Due to the oblique collision of the seamount chain at the trench, there is southward migration of the collision zone of ~200 km/Ma. Thus to the north of the Louisville Ridge, the forearc has already undergone seamount subduction, whereas to the south, it is yet to be perturbed by this process. We investigate collision at the trench with three new trench perpendicular profiles, one along axis of the seamount chain, one to the north and one to the south. The profiles were designed to cross the main tectonic features of the subduction zone explicitly to elucidate differences in crustal structure in the overlying plate. In addition, magmatic alteration of the crust of the down-going plate by the Louisville hot spot is investigated for flexural loading studies of the seamount chain and its intersection with the trench outer rise. New models for the crustal velocity structure from wide-angle data are presented and results are discussed in the context of changes in structure of the overriding plate associated with varying subducting plate topography and loading of the Pacific plate.

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

2011-12-01

44

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

USGS Publications Warehouse

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

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

1987-01-01

45

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)

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.

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

46

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

47

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

USGS Publications Warehouse

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

Takahashi, H.; Hirata, K.

2003-01-01

48

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

NASA Astrophysics Data System (ADS)

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

Raeesi, M.

2009-05-01

49

Seismicity and structural heterogeneities around the western Nankai Trough subduction zone, southwestern Japan  

NASA Astrophysics Data System (ADS)

The Nankai and Hyuga-nada seismogenic segments, in the western part of the Nankai subduction zone off southwestern Japan, have sometimes ruptured separately and sometimes simultaneously. To investigate the relationships among heterogeneities of seismic structure, spatial variation of the incoming plate, and the seismogenic segments, we carried out seismic observations in the western Nankai subduction zone and modeled the area with 3D seismic tomography using both onshore and offshore seismic data. Our seismic observations suggested that the pattern of seismicity is related to heterogeneities within the subducted plate rather than the seismogenic segments. The up-dip depth limit of seismicity along the plate boundary and in the oceanic crust is typically around 15 km, corresponding to the depth of dehydration of the oceanic crust. In addition, the seaward-extended seismicity observed where the subducted plate was considered to have rough internal structures. In the resulting velocity model, the up-dip limit of the area where the P-wave velocity just above the plate boundary exceeds 6 km/s corresponds to the up-dip limit of coseismic slip in the 1968 Hyuga-nada and 1946 Nankai earthquakes. Between the two coseismic rupture zones is an area of lower P-wave velocity about 40 km wide that is evidence of lateral heterogeneities in the upper plate along the trough-parallel direction. Structural heterogeneities in the upper plate may explain the variety of coseismic slip patterns in this region.

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

2014-06-01

50

Geology of the Gorny Altai subduction accretion complex, southern Siberia: Tectonic evolution of an Ediacaran Cambrian intra-oceanic arc-trench system  

NASA Astrophysics Data System (ADS)

The Gorny Altai region in southern Siberia is one of the key areas in reconstructing the tectonic evolution of the western segment of the Central Asian Orogenic Belt (CAOB). This region features various orogenic elements of Late Neoproterozoic-Early Paleozoic age, such as an accretionary complex (AC), high- P/ T metamorphic (HP) rocks, and ophiolite (OP), all formed by ancient subduction-accretion processes. This study investigated the detailed geology of the Upper Neoproterozoic to Lower Paleozoic rocks in a traverse between Gorno-Altaisk city and Lake Teletskoy in the northern part of the region, and in the Kurai to Chagan-Uzun area in the southern part. The tectonic units of the studied areas consist of (1) the Ediacaran (=Vendian)-Early Cambrian AC, (2) ca. 630 Ma HP complex, (3) the Ediacaran-Early Cambrian OP complex, (4) the Cryogenian-Cambrian island arc complex, and (5) the Middle Paleozoic fore-arc sedimentary rocks. The AC consists mostly of paleo-atoll limestone and underlying oceanic island basalt with minor amount of chert and serpentinite. The basaltic lavas show petrochemistry similar to modern oceanic plateau basalt. The 630 Ma HP complex records a maximum peak metamorphism at 660 °C and 2.0 GPa that corresponds to 60 km-deep burial in a subduction zone, and exhumation at ca. 570 Ma. The Cryogenian island arc complex includes boninitic rocks that suggest an incipient stage of arc development. The Upper Neoproterozoic-Lower Paleozoic complexes in the Gorno-Altaisk city to Lake Teletskoy and the Kurai to Chagan-Uzun areas are totally involved in a subhorizontal piled-nappe structure, and overprinted by Late Paleozoic strike-slip faulting. The HP complex occurs as a nappe tectonically sandwiched between the non- to weakly metamorphosed AC and the OP complex. These lithologic assemblages and geologic structure newly documented in the Gorny Altai region are essentially similar to those of the circum-Pacific (Miyashiro-type) orogenic belts, such as the Japan Islands in East Asia and the Cordillera in western North America. The Cryogenian boninite-bearing arc volcanism indicates that the initial stage of arc development occurred in a transient setting from a transform zone to an incipient subduction zone. The less abundant of terrigenous clastics from mature continental crust and thick deep-sea chert in the Ediacaran-Early Cambrian AC may suggest that the southern Gorny Altai region evolved in an intra-oceanic arc-trench setting like the modern Mariana arc, rather than along the continental arc of a major continental margin. Based on geological, petrochemical, and geochronological data, we synthesize the Late Neoproterozoic to Early Paleozoic tectonic history of the Gorny Altai region in the western CAOB.

Ota, Tsutomu; Utsunomiya, Atsushi; Uchio, Yuko; Isozaki, Yukio; Buslov, Mikhail M.; Ishikawa, Akira; Maruyama, Shigenori; Kitajima, Koki; Kaneko, Yoshiyuki; Yamamoto, Hiroshi; Katayama, Ikuo

2007-07-01

51

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

Microsoft Academic Search

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

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

1987-01-01

52

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

53

Ocean bottom seismographic study on the 2005 off-Sanriku intralate earthquake (M7.1) occurred in the outer rise of the Japan Trench  

Microsoft Academic Search

An earthquake with magnitude of 7.1 occurred on November 15, 2005 in the outer rise region of the middle part of the Japan Trench subudction system. This earthquake was the largest earthquake in this area since the occurrence of the 1933 Sanriku earthquake (M 8.1), which was accompanied with devastating tsunami along the Pacific coast of northeastern Japan. The normal

R. Hino; Y. Ito; Y. Yamamoto; K. Suzuki; H. Tsushima; S. Suzuki; M. Miyashita; T. Tomori; M. Arizono; G. Tange

2007-01-01

54

Great Earthquakes With and Without Large Slip to the Trench  

NASA Astrophysics Data System (ADS)

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 ?

Mori, J. J.

2013-12-01

55

Micro-seismicity survey of a seismic gap caused by the subduction of the Louisville seamount chain in the Tonga trench, 25°30’S to 28°S  

NASA Astrophysics Data System (ADS)

The distribution of teleseismically recorded earthquakes in the Kermadec-Tonga subduction zone reveals a major seismic gap centered roughly at 26°S. The gap parallels the trench axis and stretches for approximately 250 km. The seismic gap coincides with the area, where the Louisville hotspot chain enters the Tonga trench. Subducting seamounts may therefore control seismic coupling and hence define seismogenic asperities in subduction zones. Louisville seamounts rise 3 to 4 km above the regional seafloor. Seamounts and guyots are between 10 to 40 km in diameter and hence smaller than the width of the seismic gap, suggesting that other features - like the hotspot swell, crustal underplating or the flexural may contribute or control seismic locking. We deployed a network of 21 ocean-bottom-seismometers (OBS) and 2 ocean-bottom-hydrophones (OBH), including 9 broadband OBS with Guralp CMG-40T sensors. The network covered the southern portion of the seismic gap and the transition zone to “normal” seismic behavior. The ocean bottom seismic stations provided data from July 9, 2007 to December 31, 2007. For the earthquake location procedure we derived a minimum 1-D velocity model from active seismic wide-angle profiling in the uppermost 6 km of the fore-arc crust and earthquake arrival time data at greater depths. In total 1523 local and regional earthquake could be located. Within the network, 383 events have been recorded with a gap of <230 degree at 4 stations, and 160 events with a gap of <180 degree at 6 stations. It is interesting to note that local earthquakes (M < 4) did not mimic the teleseismic gap. Overall, seismicity seems to be randomly distributed within the network. Furthermore, in contrast to other subduction zones, where earthquakes occur predominantly along the subduction megathrust fault, we observed only a few events along the plate boundary. Thus, most local earthquakes occur in the uppermost mantle, perhaps caused by extension related to the slab-pull force of the down-going Pacific lithosphere. However, events that were related to the subduction thrust generally occur in small clusters with 10-20 sub-events, while earthquakes in the mantle of the lower plate were single events. One cluster has been observed in the outer rise seaward of the trench axis. Fife events of this cluster were also reported in the NEIC/PDE catalogue and the largest earthquake was reported as a tensional event in Lamont’s global CMT catalogue. During the time of network operation we recorded in total 15 events that have been reported in the NEIC/PDE catalogue. Earthquake in the global catalogue were biased with respect to the epicenters from the local network. In general, events were mislocated by 40-60 km in east-west direction. In latitude the error was generally in the order of <10 km.

Grevemeyer, I.; Dannowski, A.; Flueh, E. R.; Moeller, S.

2009-12-01

56

Unusually large earthquakes inferred from tsunami deposits along the Kuril trench  

Microsoft Academic Search

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

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

2003-01-01

57

Interseismic deformation at the Nankai subduction zone and the Median Tectonic Line, southwest Japan  

NASA Astrophysics Data System (ADS)

GPS velocities in the vicinity of the Nankai Trough, southwest Japan, were inverted into three components: interplate loading due to Nankai Trough subduction, deformation associated with deep slip on the Median Tectonic Line (MTL), and the residual rigid plate motion. The results show a gradual decrease of interplate seismic coupling between 25 and 40 km depth on the Nankai Trough interface. This is consistent with the deep slip model, in which the fault extends as a ductile shear zone, with a depth-variable interplate strain accumulation rate. The top 20 km of the interface may not be fully coupled, although the resolution is poor there. The low coupling at shallow depths is consistent with the accretionary prism detected by seismic surveys. The MTL slip rate is estimated to be between 0.00 and 5.50 mm/yr if we assume a vertical fault and between 0.00 and 3.88 mm/yr if we assume a north dipping fault. Combining our results with a geological estimate (4-9 mm/yr) and a geodetic estimate with a denser network (˜5 mm/yr) suggests that the MTL slip rate may be near the upper bound of our geodetic estimate, that is, ˜4-5 mm/yr. The rigid plate motion with respect to the stable Eurasian craton was estimated to be very small, indicating that southwestern Japan is on the Eurasian plate rather than a separate plate.

Aoki, Yosuke; Scholz, Christopher H.

2003-10-01

58

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

USGS Publications Warehouse

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

McCrory, P.A.

1995-01-01

59

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

Microsoft Academic Search

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

Kosuke Heki; Shin'ichi Miyazaki; Hiromichi Tsuji

1997-01-01

60

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

61

3D array observation of the low frequency earthquakes in Tokai subduction zone, central Japan  

NASA Astrophysics Data System (ADS)

Tokai area is the eastern side of Southwest Japan subduction where great earthquakes and deep low-frequency earthquakes (LFEs) occur along the convergent plate boundary. Researching the relationship between the great interplate earthquakes and activity of LFEs, Tono Research Institute of Earthquake Science (TRIES) installed two seismic arrays at Shimoyama in Tokai area. The first was a small-aperture array (six stations in the area of 120m diameter) with short-period velocity type seismographs. The second was a middle-aperture array (four stations in the area of 4 km diameter) with high-sensitive acceleration type seismographs. Geological Survey of Japan (AIST) also installed a seismic array of three borehole-type instruments with high-sensitive seismographs at three depths of 50m, 200m, and 600m at Shimoyama. We used seismic data of those three arrays and SMYH station of Hi-net array of National Research Institute of Earth Science and Disaster Prevention (NIED) as 3D array data for investigating LFEs. Using the 3D array (total 14 stations), we observed a remarkable activity of LFEs occurring in Tokai area in November 10-30, 2010. We analyzed the 3D array data to pick out direct P and S-waves propagating from LFE origins by using the semblance method (Neidel and Tarner, 1971). Assuming a homogeneous half space model with Vp=4.5 km/s and Vs=2.2 km/s, we obtained a semblance distribution for each component depending on the three factors of time, back-azimuth and incident angle of seismic waves. The maximum semblance point in each component shows a direct P-wave in UD, and S-wave in NS and EW, respectively. Incident angles and back-azimuths are compared with theoretical ones calculated by using JMA hypocenter data.

Suzuki, S.; Okubo, M.; Imanishi, K.; Kitagawa, Y.; Takeda, N.

2011-12-01

62

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

NASA Astrophysics Data System (ADS)

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

Wang, Kelin; Bilek, Susan L.

2014-01-01

63

Colwellia piezophila sp. nov., a novel piezophilic species from deep-sea sediments of the Japan Trench.  

PubMed

Two strains of obligately piezophilic bacteria were isolated from sediment collected from the bottom surface of a small canyon on the seaward slope of the Japan Trench at a depth of 6278 m. The isolated strains, Y223GT and Y251E, are closely affiliated with members of the genus Colwellia on the basis of 16S rRNA gene sequence analysis. The G + C contents of both strains were about 39 mol%. DNA-DNA hybridization values between these strains and Colwellia reference strains were significantly lower than those accepted as the phylogenetic definition of a species. The novel strains are Gram-negative, polarly flagellated and facultatively anaerobic. The optimal pressure for growth was 60 MPa at both 4 and 10 degrees C; the most rapid growth rate was observed at 10 degrees C and 60 MPa. No growth occurred at 15 degrees C under any pressure studied. The major isoprenoid quinone is Q-8. The predominant cellular fatty acids are C16 : 0 and C16 : 1. Based on the taxonomic differences observed, the isolated strains appear to represent a novel obligately piezophilic Colwellia species. The name Colwellia piezophila sp. nov. (type strain Y223GT = JCM 11831T = ATCC BAA-637T) is proposed. PMID:15388720

Nogi, Yuichi; Hosoya, Shoichi; Kato, Chiaki; Horikoshi, Koki

2004-09-01

64

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

65

Do Trenches Advance?  

NASA Astrophysics Data System (ADS)

In recent years, the work of W. Schellart and colleagues has shown that, in an 'absolute' frame of reference, some trenches at convergent plate boundaries advance, that is, they move toward the overriding plate instead of retreating seaward. Here we update and improve on prior analyses through use of recently estimated angular velocities and the propagation of errors. We apply the MORVEL global set of relative plate angular velocities [DeMets, Gordon, and Argus, 2010] and a new deep mantle reference frame derived from seismic anisotropy inferred from shear wave splitting data [Zheng, Gordon, and Kreemer, this meeting] to estimate absolute trench velocities and uncertainties. In contrast to prior studies, we find that trench retreat occurs along the entire South America plate subduction boundary. Consistent with prior studies, we find that the fastest trench advance occurs at locations along the Marianas-Izu-Bonin trench (43 × 8 mm/yr (95% c.l.)), especially along the Izu-Bonin segment, which appears to be the most clear-cut example of trench advance. We also find that the southern Kermadec trench (30 × 8 mm/yr) advances significantly, but tomographic imaging and depth of seismicity at the southern Kermadec trench suggest that only a short length of slab has been subducted there, so that not much--if any--slab is advancing there. Some indications of trench advance depend on poorly constrained estimates of the angular velocities of microplates overriding subducting lithosphere, such as the Kermadec microplate, and our work highlights the need for new data to better constrain such estimates.

Mathews, D. C.; Zheng, L.; Gordon, R. G.

2013-12-01

66

Cocos plate structure along the Middle America subduction zone off Oaxaca and Guerrero, Mexico: Influence of subducting plate morphology on tectonics and seismicity  

NASA Astrophysics Data System (ADS)

Two new bathymetric and magnetic surveys are presented from which the history and recent tectonics of the Cocos plate off the Middle America subduction zone are determined. The East O'Gorman fracture zone, a previously proposed outer rise feature, is not present along the Oaxaca trench outer rise near the trench axis. Several parallel ridges of seamounts are entering the subduction zone 15--20° from orthogonal to the trench axis. These ridges lie roughly parallel to the spreading direction and were created as off-axis volcanism. The southern Mexico trench outer rise exhibits reactivation of the inherited abyssal-hill and trench-parallel faults to accommodate extension from plate flexure. Plate boundary forces also produce a new unusual family of normal faults parallel to and flanking the seamounts. An anomalous increase in outer rise earthquakes accompany these faults indicating they are seismically active. Trench-parallel extension related to the geometry of the trench axis bend and increasing plate convergence angle may contribute to their genesis. Unusual outer-rise faulting off of the Japan trench (Kobayashi et al., 1998) which is coincident with a trench axis bend is used to argue that specific conditions at subduction zones may activate inherited fracture-zone-parallel weakness. This controls fault orientation at the Japan and Mexico outer rises. Seafloor morphology and seismicity evidence leads to a "slivered" ocean-crust model that is broken along the seamount-parallel faults at the subduction zone. This accounts for the consistent rupture geometry (40--100 km fault failure) and the uniformity in the character of the thrust waveforms. Limited magnitudes of shallow thrust earthquakes appear to be a consequence of the subducted slivering crust. A review of regions whose structures and forces are similar to those present in the subducting Cocos plate off Oaxaca indicates that crustal slivering is not unique. Forces other than a trench axis bend must be present to furnish the entirely unique seamount-parallel outer-rise faulting. Either/both convergence direction and spreading-parallel inherited weakness orientation may determine outer-rise fault strike. Methods for resolving ambiguous data are proposed.

Kanjorski, Nancy Marie

67

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

68

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

NASA Astrophysics Data System (ADS)

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

Hashimoto, Yoshitaka; Yamano, Natsuko

2014-12-01

69

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

70

Characteristic seismic activity in the subducting plate boundary zone off Kamaishi, northeastern Japan, revealed by precise hypocenter distribution analysis using ocean-bottom seismometers  

Microsoft Academic Search

High seismic activity prevails along the plate boundary to the east of northeastern Japan. To understand how this seismic activity is related to subduction process, hypocenter locations are re-determined using data obtained over 6 years by fiber-cabled permanent ocean bottom seismometers off Kamaishi. The double-difference method is adopted to obtain the relative location in more detail. As a result of

T. Okada; K. Sakoda; T. Matsuzawa; R. Hino; A. Hasegawa; S. Sakai; T. Kanazawa

2004-01-01

71

Gravity anomalies, forearc morphology and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

72

Multiscale seismic imaging of the Western-Pacific subduction zone  

NASA Astrophysics Data System (ADS)

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

Zhao, D.

2011-12-01

73

Smooth and rapid slip near the Japan Trench during the 2011 Tohoku-oki earthquake revealed by a hybrid back-projection method  

NASA Astrophysics Data System (ADS)

We developed a new back-projection method that uses teleseismic P-waveforms to integrate the direct P-phase with reflected phases from structural discontinuities near the source and used it to estimate the spatiotemporal distribution of the seismic energy release of the 2011 Tohoku-oki earthquake. We projected a normalized cross-correlation of observed waveforms with corresponding Green's functions onto the seismic source region to obtain a high-resolution image of the seismic energy release. Applying this method to teleseismic P-waveform data of the 2011 Tohoku-oki earthquake, we obtained spatiotemporal distributions of seismic energy release for two frequency bands, a low-frequency dataset and a high-frequency dataset. We showed that the energy radiated in the dip direction was strongly frequency dependent. The area of major high-frequency seismic radiation extended only downdip from the hypocenter, whereas the area of major low-frequency seismic radiation propagated both downdip and updip from the hypocenter. We detected a large release of seismic energy near the Japan Trench in the area of maximum slip, which was also the source area of the gigantic tsunami, when we used only the low-frequency dataset. The timing of this large seismic energy release corresponded to an episode of smooth and rapid slip near the Japan Trench, and reflects the strong dependence of the seismic energy distribution obtained on the frequency band of the input waveform dataset. The episode of smooth and rapid slip may have been the trigger for a release of roughly all of the accumulated elastic strain in the seismic source region of the 2011 Tohoku-oki earthquake.

Yagi, Yuji; Nakao, Atsushi; Kasahara, Amato

2012-11-01

74

A dynamic model of the curvature of the Mariana Trench  

Microsoft Academic Search

Oceanic trenches are prominent features of the Earth's surface. They represent the convergent plate boundaries and generally possess convex curvatures towards the subducting plates. Based on this observation, many static geometric models have been proposed for the origin of trench curvatures1-6. Unfortunately, many of these models have limited applicability7. Alternatively, a dynamic model has been proposed which suggests that trench

Albert T. Hsui; Sarah Youngquist

1985-01-01

75

Physical properties of the top of the subducting Philippine Sea plate beneath the SW Japan arc, derived from onshore - offshore integrated seismic survey  

NASA Astrophysics Data System (ADS)

The Nankai trough region, where the Philippine Sea Plate is subducting beneath the southwestern Japan arc, is a well-known seismogenic zone of interplate earthquakes (e.g. the 1944 Tonankai Earthquake (M=7.9) and the 1946 Nankai Earthquake (M=8.0)). A detailed crustal and upper mantle structure of the subducted Philippine Sea Plate and the overlying SW Japan arc is inevitably important to constrain the physical process of earthquake occurrence. In the summer of 1999, we conducted a highly dense onshore-offshore integrated seismic experiment in the eastern part of Shikoku Island and the adjacent Nankai trough, SW Japan. The most remarkable feature of the record sections is that extremely high amplitude reflections (bright reflections) can be recognized. This phase was interpreted as a reflected wave from the top of the subducting Philippine Sea plate at a depth of 18-30km (Kurashimo et al., 2002). Physical properties across the reflecting interface control amplitude versus offset (AVO) response. To obtain physical properties of the material between the subducting Philippine Sea plate and island arc crust, we investigated AVO response on this bright reflection. Analyzing this bright reflection, we could obtain the reflection coefficient (Rpp) as a function of the incident angle. Rpp tends to increase beyond about 50 degrees. To discuss about this characteristic, we calculated reflection coefficient for different velocity models. The single interface models (positive velocity contrast exists between the interface. negative velocity gradient exists upper side of the reflecter) can not explain the characteristic of the Rpp. Thin layer model (about 200 m) with a P-wave velocity of 4.0 km/s (a thin layer with a negative reflection coefficient at its upper boundary and a much larger, but positive reflection coefficient at its base exists) explains the characteristic of the Rpp. The P-wave velocity of the sediments shows 2.0-4.2km/s off Shikoku Island (Kodaira et al., 2002). These results suggest that sedimentary material subducts beneath the southern part of Sikoku Island with the oceanic crust.

Kurashimo, E.; Hirata, N.; Iwasaki, T.; Kodaira, S.; Kaneda, Y.

2004-12-01

76

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)

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.

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

2012-12-01

77

Non-volcanic seismic swarm and fluid transportation driven by subduction of the Philippine Sea slab beneath the Kii Peninsula, Japan  

NASA Astrophysics Data System (ADS)

To understand the mechanism of an intensive non-volcanic seismic swarm in the Kii Peninsula, Japan, we used a dense seismic linear array to measure fine-scale variations of seismic velocities and converted teleseismic waves. A low-velocity anomaly confined to just beneath the seismic swarm area is clearly imaged, which correlates spatially with an uplifted surface area and a highly conductive and strong attenuative body. These results suggest that fluids such as partial melt or water are present beneath this non-volcanic seismic swarm area. It is notable that the island arc Moho below the seismic swarm area is at a depth of approximately 32 km in the northern part of the seismic swarm area and shallows to approximately 20 km towards the south, due to the raised structure of the serpentinized mantle wedge. In addition, we show that the hydrated oceanic crust of the subducting Philippine Sea slab is characterized by low velocities with a high Poisson's ratio at depths of less than 40 km. In contrast, dehydration conversion from oceanic basalt to eclogite takes place at depths greater than 50 km. Water released from the subducting oceanic crust could cause serpentinization of the mantle wedge and infiltration into the forearc base of the overlying plate. The interaction between dehydration of the subducting oceanic crust and hydration of the mantle wedge and overlying plate exerts an important role in driving the non-volcanic seismic swarm activity in the Kii Peninsula.

Kato, Aitaro; Saiga, Atsushi; Takeda, Tetsuya; Iwasaki, Takaya; Matsuzawa, Toru

2014-12-01

78

Principal horizontal stress orientations from ODP Leg 186 sites on the deep-sea terrace of the Japan Trench prior to the 2011 Mw9.0 Tohoku-Oki, Japan, earthquake  

NASA Astrophysics Data System (ADS)

To obtain the principal horizontal stress orientations prior to the 2011 Mw9.0 Tohoku-Oki, Japan, earthquake, we analyzed wireline logging Formation MicroScanner (FMS) images and caliper data that were collected during Ocean Drilling Program Leg 186 and have not previously been analyzed for this purpose. We found stress-induced compressive failures (breakouts) and drilling-induced tensile fractures (DITFs) and obtained their azimuths. Therefore, we successfully determined principal horizontal stress orientations as of 1999 at sites in the source area of the 2011 Tohoku-Oki earthquake, on the deep-sea terrace of the Japan Trench. The maximum principal horizontal stress orientation at site 1151 of ODP Leg 186, located in an aseismic zone, was east-southeast, parallel with the plate convergence direction; and the stress orientation at site 1150 of Leg 186, in a seismically active zone, was south-southeast, suggestive of a regional stress feature due to the influence of seismic activity. Aftershocks with normal faulting mechanisms, observed in the hanging wall of the plate interface that ruptured during the Tohoku-Oki earthquake, indicate a normal faulting postseismic stress regime, which clearly differs from the stress state prior to the earthquake. Consequently, the stress state may have changed from a reverse to a normal faulting regime during the Tohoku-Oki earthquake sequence.

Lin, W.; Saito, S.; Sanada, Y.; Yamamoto, Y.; Hashimoto, Y.; Kanamatsu, T.

2011-12-01

79

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

NASA Astrophysics Data System (ADS)

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.

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

2015-02-01

80

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

81

Salty Trench  

NASA Technical Reports Server (NTRS)

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.

2004-01-01

82

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)

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).

Matsuzawa, T.; Iio, Y.

2011-12-01

83

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

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

84

Changes in paleostress state along a subduction zone preserved in an on-land accretionary complex, the Yokonami mélange in the Cretaceous Shimanto Belt, Kochi, southwest Japan  

NASA Astrophysics Data System (ADS)

change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on-land accretionary complex, the Yokonami mélange in southwest Japan, using the microfault inversion method. Microfaults were classified into two groups based on location: those occurring throughout the Yokonami mélange and those occurring in the Goshikinohama fault zone, which is considered to be a fossil seismogenic fault and is located at the northern end of the Yokonami mélange. Stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature: one is subhorizontal ?1 and subvertical ?3 with a smaller stress ratio, and the other is subvertical ?1 and subhorizontal ?3 with a larger stress ratio. The difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the Tohoku-Oki great earthquake.

Hashimoto, Yoshitaka; Eida, Mio; Ueda, Yodai

2014-10-01

85

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

NASA Astrophysics Data System (ADS)

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

Tanaka, Yoshiyuki

2014-02-01

86

Seismicity, crustal structure, and morphology of the Louisville Ridge Tonga-Kermadec Trench collisional system  

E-print Network

Seismicity, crustal structure, and morphology of the Louisville Ridge ­ Tonga-Kermadec Trench of the world's subduction zones. At ~26° S, the trench is intersected by the Louisville Ridge, a 4500 km long that make up the Louisville Ridge may act as either a barrier or asperity during large subduction zone

Watts, A. B. "Tony"

87

Evidence for retrograde lithospheric subduction on Venus  

NASA Technical Reports Server (NTRS)

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

Sandwell, David T.; Schubert, Gerald

1992-01-01

88

Metamorphic rocks of the Yap arc-trench system  

Microsoft Academic Search

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

J. Hawkins; R. Batiza

1977-01-01

89

Japan.  

ERIC Educational Resources Information Center

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…

Jones, Savannah C.

90

Trench Visualization  

NASA Technical Reports Server (NTRS)

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

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

2008-01-01

91

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

92

Triggering of tremor and inferred slow slip by small earthquakes at the Nankai subduction zone in southwest Japan  

NASA Astrophysics Data System (ADS)

correlation of earthquakes with tremor and slow slip has not been clearly quantified. We investigate 12 year earthquake and tremor catalogs for southwest Japan and find that nearby small intraslab earthquakes are weakly correlated with tremor. In particular, the intraslab earthquakes with magnitudes ?2.7 tend to be followed by tremor more often than expected at random by a factor of 2 to 6. The excess number of tremor before earthquakes is not as significant, although marginally more than expected. The underlying physical mechanism of the observed triggering of tremor and inferred slow slip by earthquakes is most likely to be the dynamic stress changes (several to several tens of kilopascals) rather than the much smaller static stress changes. The rate of triggering of tremor by earthquakes is similar to, although somewhat lower than, rates observed for similar amplitude stress changes due to the lower-frequency teleseismic surface waves and tidal stressing.

Han, Jiangang; Vidale, John E.; Houston, Heidi; Chao, Kevin; Obara, Kazushige

2014-11-01

93

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

USGS Publications Warehouse

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.

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

1999-01-01

94

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

NASA Astrophysics Data System (ADS)

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

Niu, Y.

2013-12-01

95

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones  

NASA Astrophysics Data System (ADS)

We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10-40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.

Satake, K.; Tanioka, Y.

96

Political representation Trench warfare  

E-print Network

Political representation Trench warfare Rational voting Candidate positioning Recap Mathematical vs science #12;Political representation Trench warfare Rational voting Candidate positioning Recap Themes;Political representation Trench warfare Rational voting Candidate positioning Recap Themes Mathematical

Gelman, Andrew

97

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

98

Subduction seismicity and tectonics in the lesser Antilles arc  

Microsoft Academic Search

We have studied the mechanisms of 17 earthquakes along the Lesser Antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate. No large thrust earthquakes occurred during the 1950-1978 study period; the three large (magnitude seven) events are all normal faults. One is a normal faulting event seaward of the trench. Its aftershock

Seth Stein; Joseph F. Engeln; Douglas A. Wiens; Kazuya Fujita; Robert C. Speed

1982-01-01

99

Subduction initiation: spontaneous and induced  

NASA Astrophysics Data System (ADS)

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

2004-10-01

100

Subduction initiation: spontaneous and induced  

NASA Astrophysics Data System (ADS)

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

Stern, Robert J.

2004-10-01

101

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

SciTech Connect

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)

Hilde, T.W.C.

1984-08-01

102

Accretionary processes at subduction zones in the eastern mediterranean  

NASA Astrophysics Data System (ADS)

The tectonic history of the eastern Mediterranean as well as the present tectonic style are affected by the collision and accretion of oceanic plateaus at subduction zones. Small but distinct oceanic plateaus exist today in the eastern Mediterranean away from the plate boundary. Many other oceanic plateaus crowd the plate boundary, particularly in the Hellenic arc area. Their presence results in the unusual complexity of this subduction zone, as is evidenced by the seismicity and bathymetry patterns. The present two arcs system in this region, the Hellenic and Cyprean arcs, is probably the consequence of the collision of a large continental plateau with an older arc. This plateau has been thrusted over the continental mass of Anatolia and pieces of it are to be found in the region between the two arcs. Active subduction in the Hellenic arc takes place in the outer trenches, particularly in the Ionian and Strabo trenches. Subduction of Mediterranean lithosphere in the Pliny and Ptolemy trenches is limited to their western parts. Inner segments of these trenches served as the main subduction trenches before the subduction zones migrated outward following the collision of oceanic plateaus.

Rotstein, Yair; Ben-Avraham, Zvi

1985-03-01

103

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

104

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

105

Geodynamic models of deep subduction  

NASA Astrophysics Data System (ADS)

Numerical and laboratory models that highlight the mechanisms leading to a complex morphology of subducted lithospheric slabs in the mantle transition zone are reviewed. An increase of intrinsic density with depth, an increase of viscosity, or phase transitions with negative Clapeyron slope have an inhibiting influence on deep subduction. The impingement of slabs on a viscosity and density interface has been studied in laboratory tanks using corn syrup. Slab interaction with equilibrium and non-equilibrium phase transitions has been modelled numerically in two dimensions. Both the laboratory and the numerical experiments can reproduce the variety of slab behaviour that is found in tomographic images of subduction zones, including cases of straight penetration into the lower mantle, flattening at the 660-km discontinuity, folding and thickening of slabs, and sinking of slabs into the lower mantle at the endpoint of a flat-lying segment. Aside from the material and phase transition properties, the tectonic conditions play an important role. In particular, the retrograde motion of the point of subduction (trench-rollback) has an influence on slab penetration into the lower mantle. A question that still needs to be clarified is the mutual interaction between plate kinematics and the subduction process through the transition zone.

Christensen, Ulrich

2001-12-01

106

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

NASA Astrophysics Data System (ADS)

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

Okamoto, A.; Takeshita, T.

2013-12-01

107

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.

108

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

NASA Astrophysics Data System (ADS)

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.

Jacob, J.; Dyment, J.

2013-12-01

109

Rheological effects on slab stagnation and trench rollback  

NASA Astrophysics Data System (ADS)

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.

Cizkova, Hana; Bina, Craig

2013-04-01

110

The mechanical properties of the deep portion of the subduction interface and of the mantle wedge revealed by postseismic motions after Tohoku and Maule earthquakes  

NASA Astrophysics Data System (ADS)

The interseismic and postseismic deformations preceding and following the two large subduction earthquakes of Maule (Chile, Mw8.8, 2010) and Tohoku (Japan, Mw9.1, 2011) have been closely monitored with modern geodetic techniques. We dispose of large datasets, (GEONET cGPS network in Japan and international collaboration networks in Chile, including survey mode GPS). In both cases, post-seismic deformations show similar behavior, with a vertical uplift on the oceanward side of the volcanic arcs, so called mid-field (between 300 and 500 km from the trench), and a large scale subsidence associated with non negligible horizontal deformations in the far-field (from 500 to 2000km from the trench). In addition, near-field data with complex patterns are available in Chile (thanks to the proximity between the trench and the coastline) and in Japan (thanks to sea bottom geodesy). We use a 3D finite element code (Zebulon Zset) to relate these deformations to the mechanical properties of the mantle in the subduction zone area. The meshes feature 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) a low viscosity wedge (LVW) above the subducting plate, extending potentially beneath the volcanic arc, b) a low viscosity channel (LVC) extending along the lower part of the subducting interface and just above it, potentially deeper. Burger rheologies have been adopted for all the viscoelastic regions. We invert for the mechanical properties and geometrical characteristics of the asthenosphere of the LVW and of the LVC. Our best fitting models feature, (i) an asthenosphere with a 'long-term' viscosity of the order of 2.1018 Pas, extending down to 300km; (ii) a LVC along the plate interface but not extending deeper in the mantle with viscosities of a few 1017 Pas, and (iii) a LVW restricted to the base of the lithosphere below the volcanic arc, with viscosities of a few 1017 Pas. Mid-field uplift is due to relaxation in both the LVW and the LVC. The viscoelastic mechanical properties deduced from the postseismic motions can also be used to model deformations through the whole seismic cycle. Predicted interseismic deformations seem to differ strongly from those predicted by purely elastic backslip models.

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

2014-05-01

111

Osmium Recycling in Subduction Zones  

PubMed

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

Brandon; Creaser; Shirey; Carlson

1996-05-10

112

Variations in oceanic plate bending along the Mariana trench  

NASA Astrophysics Data System (ADS)

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

Zhang, Fan; Lin, Jian; Zhan, Wenhuan

2014-09-01

113

Understanding the structure of subducting plates  

NASA Astrophysics Data System (ADS)

Seismic studies are helping scientists learn more about the structure of subducting oceanic plates. Using an air gun array and 80 ocean bottom seismometers spaced along a 500-kilometer profile, Fujie et al. conducted a seismic reflection and refraction survey at the Kuril trench in the northwestern Pacific margin, where part of the Pacific plate is subducting beneath the Okhotsk plate. They estimated the water content of the subducting plate by measuring the velocity of seismic waves—both P waves and S waves—through the plate. The ratio of seismic wave velocities (Vp/Vs) is an indicator of the lithology, porosity, and presence of fluid in the plate. Their findings showed that the water content in the plate increased toward the trench, along with greater bending and fracturing, suggesting that water enters the plate through the fractures. The authors conclude that the bending and fracturing of the plate as it subducts play an important role in the water cycle in subduction zones. (Geophysical Research Letters, doi:10.1029/2012GL054340, 2013)

Balcerak, Ernie

2013-04-01

114

Cookie Subduction  

NSDL National Science Digital Library

This is a quick activity that shows how large amounts of rock and sediment are added to the edge of continents during subduction. You may ask, how can such a huge phenomenon be demonstrated quickly and cheaply? The answer is simple: with a cookie!

Exploratorium

2012-06-26

115

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

116

Tomography of the subducting Cocos plate in central Mexico using data from the installation of a prototype wireless seismic network: Images of a truncated slab  

Microsoft Academic Search

The central Mexican subduction zone exhibits an oblique strike of the volcanic arc, the Trans-Mexican Volcanic Belt (TMVB), with respect to the trench, flat-slab subduction, and has no Wadati-Benioff zone. The oblique strike of the TMVB is explained by the changing rate of subduction at the trench. The shape of the slab beyond the flat slab section has been unknown

Allen Leroy Husker Jr.

2008-01-01

117

2D Numerical Models of Ridge-Trench Collision: Implications for Slab Detachment Beneath Baja California  

NASA Astrophysics Data System (ADS)

The approach of a buoyant spreading ridge to a subduction zone is a scenario that may lead to detachment of a subducted slab. Previous work has called upon the detachment process as a possible explanation for observed ridge abandonment and slab-window related magmatism in Baja CA/western Mexico, but such a scenario has not previously been tested using fully-dynamic numerical models. We use two-dimensional fully- dynamic models of ridge approach to a subduction zone to explore the dependence of detachment and resultant surface effects on subducted slab length, ridge-trench distance, spreading rate, and lithospheric yield strength. We find that our models, which include non-Newtonian rheology, demonstrate the following dynamics of ridge approach: (a) a decrease in subduction velocity as the ridge approaches the trench, (b) a shrinking surface plate that maintains a uniform subduction velocity, (c) rapid slab detachment at depths ranging from 55-95 km depth depending on the slab age (7-12 My) and (d) ridge abandonment distances of 125-225 km from the trench, and slab gap distances of 200-270 km from the trench. Slab gap distance is used as a proxy for the distance to a possible slab-window related magmatism. These results are consistent with observations in Baja CA, where detachment of the Cocos slab may explain abandonment of observed segments of the East Pacific Rise 50-200 km outboard of the trench and the presence of a non-arc magmatic pulse 100-250 km inboard of the trench, with geochemical signatures separate from that associated with the normal subduction history for the Farallon plate.

Burkett, E. R.; Billen, M. I.

2008-12-01

118

Nonvolcanic tremor along the Oaxaca segment of the Middle America subduction zone  

Microsoft Academic Search

The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench-to-coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests that they may represent motion in the transition zone,

Michael R. Brudzinski; Héctor R. Hinojosa-Prieto; Kristen M. Schlanser; Enrique Cabral-Cano; Alejandra Arciniega-Ceballos; Oscar Diaz-Molina; Charles DeMets

2010-01-01

119

Buoyancy control on continent subduction and implications for the dynamics of India-Asia convergence  

Microsoft Academic Search

At some point during their evolution, most trenches encounter continental lithosphere. Because average continental lithosphere is positively buoyant, collision is often the result. However, there is evidence that some continental crust and lithosphere does get subducted. For example, Indian plate kinematic reconstructions show that Meso-Cenozoic subduction consumed a highly heterogeneous lithosphere including small oceanic basins and substantial portions of the

S. Goes; F. A. Capitanio; G. Morra

2007-01-01

120

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

121

Cyclic stressing and seismicity at strongly coupled subduction zones  

Microsoft Academic Search

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

Mark A. J. Taylor; Gutuan Zheng; James R. Rice; William D. Stuart; Renata Dmowska

1996-01-01

122

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- ...

123

Sedimentation in the central segment of the Aleutian Trench: Sources, transport, and depositional style  

Microsoft Academic Search

The central segment of the Aleutian Trench (162°W to 175°E) is an intraoceanic subduction zone that contains an anomalously thick sedimentary fill (4 km maximum). The fill is an arcward-thickening and slightly tilted wedge of sediment characterized acoustically by laterally continuous, closely spaced, parallel reflectors. These relations are indicative of turbidite deposition. The trench floor and reflection horizons are planar,

A. J. Stevenson; D. W. Scholl; T. L. Vallier; M. B. Underwood

1990-01-01

124

Subduction of Oceanic Asthenosphere: A Critical Appraisal in Central Alaska  

NASA Astrophysics Data System (ADS)

Strong radial anisotropy, or transverse isotropy, observed in the oceanic asthenosphere has invited numerous discussions concerning its origin. Near subduction zones where the slab plunges into the mantle, shear wave birefringence (or splitting) measurements attributed to the sub-slab regime often reveal fast splitting direction sub-parallel to the trench, apparently at odds with predictions from a dominant sub-slab entrained flow. However, taking into account strong radial anisotropy observed in the oceanic asthenosphere beneath ocean basins, Song and Kawakatsu [2012, GRL in press] recently concluded that sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subducting oceanic asthenosphere, apparently manifested at several shallow subduction zones showing fast polarization direction sub-parallel to the absolute plate motion of the incoming plate. We refer to the term "subduction of oceanic asthenosphere" as a slightly different expression from slab entrainment and put emphasis on the fact that the entrained sub-slab mantle displays anisotropy property analogous to oceanic asthenosphere beneath ocean basins except the angle of symmetry axis changes with the slab dip. To further validate this scenario and distinguish it from more sophisticated hypothesis such as sub-slab trench-parallel flow, we examine complicated SKS splitting patterns observed across the fore-arc central Alaska [Christensen and Abers, 2010; Hanna and Long, 2012], where the rate of trench migration is very low regardless of the mantle reference frame. Observations of fast splitting direction vary from plate motion parallel near the trench to mostly trench-parallel beyond 100 km slab isodepth and there are strong back-azimuth variations in between these regions. After taking into account the rotation of anisotropy symmetry in the subducted oceanic asthenosphere with respect to the obliquity of plate motion and down-dip variations in slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as back azimuth dependent splitting direction across the entire fore-arc. Current validation further augments the idea that, apart from the source of anisotropy in the mantle wedge and subducting slab, subduction of oceanic asthenosphere is probably a dominant source of seismic anisotropy in central Alaska and potentially in most subduction zones. Furthermore, this result also provides an alternative view on the anisotropic symmetry in the mantle wedge and constrains the length scale in which 3D mantle flow may be important.

Song, T.; Kawakatsu, H.

2012-12-01

125

Geophysical signature of hydration-dehydration processes in active subduction zones  

NASA Astrophysics Data System (ADS)

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

Reynard, Bruno

2013-04-01

126

Evolution and diversity of subduction zones controlled by slab width.  

PubMed

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

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

2007-03-15

127

Seismic anisotropy and texture development during early stages of subduction  

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

128

Effects of subducting buoyant oceanic ridges on subduction zones: Area of influence and rotational effects  

NASA Astrophysics Data System (ADS)

The subduction of buoyant oceanic ridges into subduction zones is typically manifested by doming of arc rocks, shallowing of the trench, and diffuse or shallowly-dipping Benioff zones. Two important questions include: 1) what distance inboard of the overriding plate are effects observed; and 2) what is the degree that colliding ridges can induce large-scale rotations of forearc terranes and consequent "back-arc opening" behind rotated forearc blocks. I describe regional effects from five relatively narrow ridges actively entering subduction zones: 1) Carnegie; 2) Cocos; 3) Emperor seamount chain; 4) Louisville, and 5) D'Entrecasteaux. GPS from all areas shows a characteristic outward flow pattern in map view indicative of the strong landward push on the ridge along radial thrust systems within the overriding plate. This area of influence can extend 100s of kms. The pattern of outward flow from GPS vectors is consistent with the of bathymetry, gravity and earthquakes show some of these ridges act as strong indentors that push into the arc along strike-slip systems at their edges. In other cases likely related to thinner crust, no strong disruption of the outer forearc high or forearc basin is observed and adjacent to subducting ridges. Rotating forearc blocks are most expressed by examples where the direction of subduction is highly oblique and the least rotational effects are expressed where the direction of subduction is orthogonal. Wider ridges also appear to have fewer rotational effects.

Mann, W. P.

2012-12-01

129

Thermal and metamorphic environment of subduction zone episodic tremor and slip  

Microsoft Academic Search

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

Simon M. Peacock

2009-01-01

130

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

131

Propagation of the 2001-2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico  

NASA Astrophysics Data System (ADS)

The aseismic slow slip event of 2001-2002 in Guerrero, Mexico, with an equivalent magnitude MW ~ 7.5, is the largest silent earthquake (SQ) among many recently recorded by GPS in different subduction zones (i.e. Japan, Alaska, Cascadia, New Zealand). The sub-horizontal and shallow plate interface in Central Mexico is responsible for specific conditions for the ~100 km long extended transient zone where the SQs develop from ~80 to ~190 km inland from the trench. This wide transient zone and relatively large slow slips of 10 to 20 cm displacements on the subduction fault result in noticeable surface displacements of 5-6 cm during the SQs. Continuous GPS stations allow one to trace the propagation of SQs, and to estimate their arrival time, duration and geometric attenuation. These propagation parameters must be accounted in order to locate source of slow slips events and to understand the triggering effect that they have on large subduction earthquakes. We use long-baseline tiltmeter data to define new time limits (onset and duration) for the SQs and continuous records from 8 GPS stations to determine the propagation of the 2001-2002 SQ in Central Mexico. Data from the CAYA and IGUA GPS stations, separated by ~170 km and located along the profile perpendicular to the trench, are used to determine that the surface deformation from the 2001-2002 SQ started almost instantaneously. It propagated parallel to the coast at ~2 km/day with an exponential attenuation of the horizontal surface displacement and a linear decrease of its duration with distance. Campaign data obtained yearly from 2001 to 2005 at the Oaxaca GPS network have been modeled according to a propagation of the 2001-2002 SQ step-like displacement anomaly. This modeling shows that the SQ ceased gradually in the central part of the Oaxaca segment of the subduction zone (west of Puerto Angel, PUAN) and then it apparently triggered another SQ in SE Oaxaca (between PUAN and Salina Cruz, SACR). The estimated horizontal velocities for inter-event epochs at each GPS site are used to assess an average interplate coupling in the Central Oaxaca subduction zone.

Franco, S. I.; Kostoglodov, V.; Larson, K. M.; Manea, V. C.; Manea, M.; Santiago, J. A.

2005-10-01

132

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

133

Constraints on subducting plate strength within the Kermadec trench  

E-print Network

on Earth is divided into rigid tectonic plates separated by narrow zones of weakness, which include, and instantaneous deformation and weakening of the plates play an important role in determining how tectonic plates

Billen, Magali I.

134

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

NASA Astrophysics Data System (ADS)

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.

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

2014-09-01

135

Integrating plate coupling and transient slip events along the subduction zone interface in Nankai Trough, SW Japan using GEONET GPS time series  

NASA Astrophysics Data System (ADS)

Large earthquakes (M~=8) have occurred repeatedly along the Nankai trough, Southwest Japan, over the past 1000 years. Recently, time-varying slow earthquakes such as short-/long-term slow slip transients, non- volcanic tremor, low/very-low frequency earthquakes have been observed in the region. To understand the interseismic plate loading process and its relation to large earthquakes and slow transient events, we use recently reanalyzed GEONET GPS data to re-examine the plate loading rate and time variable deformation in space and time.In order to estimate interseismic velocities from the GPS time ser,es, we developed tools to identify earthquake and instrument related offsets. We use QOCA to fit an empirical model to the raw time series, estimate and remove seasonal variations and common mode errors across the network. As a result, we were able to determine the accurate horizontal and vertical interseismic velocities for the entire network using 10 years data (1996-2006), as opposed to the short-time velocity estimates in most previous studies. We inverted for the back slip (or slip deficit rate) along the Nankai trough using 3-D velocity field using a realistic interplate fault geometry. To allow for the nonuniform deformation source, we model interplate coupling beneath Shikoku and Kii Peninsula and Nankai-Suruga trough separately. The results show that strong coupling were found at 10-30 km beneath Shikoku and Kii Peninsula while Tokai region has maximum backslip rate of ~2-3cm/yr at the depth of 5-25 km. There is a good spatial correspondence between the inferred strong coupling region with the past large earthquake rupture zones. We applied the extended Network Inversion Filter [MacGuire and Segall, 2003] to model the spatial and temporal distribution of two major slow slip events: 2000-2004.5 Tokai slow slip event and 2002-2004 Bungo Channel event. The cleaned GPS time series provides much better resolution for transient slip analysis. The inverted slip histories show complex patterns with each event consisting of several subevents. Comparison of the plate coupling pattern and transient slip shows that part of strong coupling area beneath western Bungo Channel overlaps with the aseismic slip locus, indicating at least part of the accumulated slip deficit is released by episodic slow slip events in the region. The transient slip zones is located at the downdip portion of strong coupling region but updip of low frequency earthquakes. Excellent temporal coincidence between the resolved slow slip rate and LFE occurrence were found in both events, supporting the hypothesis that they are closely related and possibly reflect the modulation of slow slip on seismicity.

Liu, Z.; Owen, S.; Dong, D.; Lundgren, P.; Webb, F.; Fielding, E.; Simons, M.; Hetland, E. A.

2008-12-01

136

Trenching the Trough  

NASA Technical Reports Server (NTRS)

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

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

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

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

2004-01-01

137

Snow White 5 Trench  

NASA Technical Reports Server (NTRS)

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

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

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

This image has been enhanced to brighten shaded areas.

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

2008-01-01

138

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

E-print Network

Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench Keywords: subduction zones marine geophysics electromagnetics conductivity a b s t r a c t Water plays-source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile

Constable, Steve

139

Accretionary processes along the Middle America Trench off Costa Rica  

SciTech Connect

The geometry of large-scale structures within modern accretionary prisms is known entirely from seismic reflection studies using single or grids of two-dimensional profiles. Off Costa Rica the authors collected a three-dimensional reflection data set covering a 9 km wide {times} 22 km long {times} 6 km thick volume of the accretionary prism just arcward of the Middle America Trench. The three-dimensional processing and ability to examine the prism as a volume has provided the means to map structures from a few hundred meters to kilometers in size with confidence. Reflections from within the prism define the gross structural features and tectonic processes active along this particular portion of the Middle America Trench. So far in the analysis, these data illustrate the relationships between the basement, the prism shape, and overlying slope sedimentary deposits. For instance, the subducted basement relief (of several hundred meters amplitude) does seem to affect the larger scale through-going faults within the prism. Offscraping of the uppermost 45 m of sediments occurs within 4 km of the trench creating a small pile of sediments at the base of the trench. How this offscraped sediment is incorporated into the prism is still being investigated. Underplating of parts of the 400 m thick subducted section begin: at a very shallow structural level, 4 to 10 km arcward of the trench. Amplitude anomalies associated with some of the larger arcward dipping structures in the prism and surface mud volcanoes suggest that efficient fluid migration paths may extend from the top of the downgoing slab at the shelf edge out into the lower and middle slope region, a distance of 50 to 100 km.

Shipley, T.H.; Stoffa, P.L. (Univ. of Texas, Austin (USA)); McIntosh, K.; Silver, E.A. (Univ. of California, Santa Cruz (USA))

1990-06-01

140

Intrinsic and Extrinsic Factors in Subduction Dynamics  

NASA Astrophysics Data System (ADS)

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

Billen, Magali; Arredondo, Katrina

2014-05-01

141

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

USGS Publications Warehouse

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

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

1985-01-01

142

Interplate coupling and transient slip along the subduction interface beneath Oaxaca, Mexico  

Microsoft Academic Search

We describe and model GPS measurements of surface deformation from the Oaxaca segment of the Mexican subduction zone to characterize interseismic strain accumulation and episodic transient slip in this region and test seismologically-based models of strain accumulation and release along subduction interfaces. Deformation measured from 2001 to 2007 within our dense 31-station GPS array has consisted of (1) trench-normal horizontal

F. Correa-Mora; C. Demets; E. Cabral-Cano; O. Diaz-Molina

2008-01-01

143

Opening and closing slab windows in congested subduction zones  

NASA Astrophysics Data System (ADS)

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

Moresi, Louis

2013-04-01

144

Detrital garnet and chromian spinel chemistry of Permian clastics in the Renge area, central Japan: Implications for the paleogeography of the East Asian continental margin  

NASA Astrophysics Data System (ADS)

The Japanese Islands comprise a complex assembly of allochthonous continental shelf deposits and subduction-related accretionary complexes. The paleogeography of Permian clastic strata was restored based on chemical analysis of detrital garnets and chromian spinels, which are useful for provenance discrimination. Detrital garnet assemblages from the Permian clastic rocks in Japan are characterized by a grandite-rich assemblage and a grandite-poor assemblage. Most trench-fill deposits in Permian accretionary complexes contain the grandite-poor assemblage, which is composed of spessartine-rich almandine, grossular-rich almandine, and rarely pyrope-rich almandine, and is commonly associated with chromian spinels and volcanic rock fragments. It is suggested that the sediments were derived from a volcanic arc and associated non-calcareous contact-metamorphic rocks along the North China Block, where pelitic granulites including pyrope-rich almandine garnets are extensively exposed in the northern margin. Ophiolite and high-P/T schist also crop out along the continental margin. Most Permian shelf deposits and the trench-fill deposits of the central part of the accretionary complexes in Japan contain the grandite-rich assemblage. Analyses suggest provenance from a calc-alkaline volcanic arc and association with calcareous contact-metamorphic rocks including skarn deposits. Sediments lack ophiolite detritus. A part of the grandite-poor trench-fill deposits of the accretionary complexes is overlain by grandite-rich clastic rocks. This indicates that the grandite-rich detritus increased and the supply extended through the fore-arc basin to the trench. The trench-fill deposits in the northernmost part of the Permian accretionary complex in the Renge area yield a grandite-poor assemblage, particularly uvarovite-rich andradite garnets and chromian spinels derived from a back-arc basin basalt, which is very similar to the clastic composition of the Permian back-arc basin strata of Japan. Hence, we conclude that a back-arc basin existed in the northernmost region of the Permian subduction zone along the North China Block.

Takeuchi, Makoto; Kawai, Masaki; Matsuzawa, Nozomi

2008-12-01

145

Snow White Trench (Animation)  

NASA Technical Reports Server (NTRS)

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

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

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

2008-01-01

146

Trenching Martian Ground  

NASA Technical Reports Server (NTRS)

This image shows a three-dimensional model of the trench dug by the Mars Exploration Rover Spirit at a shallow depression dubbed 'Laguna Hollow' on the 47th martian day, or sol, of its mission. The rover dragged one of its wheels back and forth across the surface to create this 7-centimeter-deep (3-inch) hole. Afterwards, it investigated the freshly exposed soil with the scientific instruments located on its robotic arm. This model was created using images from the rover's panoramic camera.

2004-01-01

147

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

USGS Publications Warehouse

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.

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

2009-01-01

148

Complex mantle flow around heterogeneous subducting oceanic plates  

NASA Astrophysics Data System (ADS)

The foundering of oceanic lithospheres controls the circulation patterns of the mantle around subducting slabs. Here, we investigate the sensitivity of the mantle flow to slab buoyancy variations along convergent margins using three-dimensional numerical models of subduction in a viscous mantle. The models illustrate that in a buoyancy-driven system varying subduction velocity arising from negative buoyancy variations effectively drives pressure gradients and confers a general flow component sub-parallel to the margin's strike, allowing for material transport over large distances around the slabs. The along-slab velocity component introduces widespread horizontal simple shear in the mantle flow which is maximized beneath the slab between ?100 and ?350 km. Mantle flow complexities develop rapidly, although not instantaneously, upon subduction of heterogeneous plates. The resulting slab pull gradients are mostly accommodated by internal slab deformation, decoupling the mantle flow from motions at surface. Moderate slab pull variations have a minor impact on plate velocity, and might not result in trench motions, although effectively rearrange the flow. Slab buoyancy heterogeneities are firstly associated with age-dependent thickness variations, but also with slab break offs and windows, varying depth of subduction and entrainment of buoyant blocks. Because these are observed at all subduction zones, the process shown here should have a global relevance for the flow around slabs.

Capitanio, Fabio A.; Faccenda, Manuele

2012-11-01

149

Complex mantle flow around heterogeneous subducting oceanic plates  

NASA Astrophysics Data System (ADS)

The foundering of oceanic lithospheres controls the circulation patterns of the mantle around subducting slabs. Here, we investigate the sensitivity of the mantle flow to slab buoyancy variations along convergent margins using three-dimensional numerical models of subduction in a viscous mantle. The models illustrate that in a buoyancy-driven system varying subduction velocity arising from negative buoyancy variations, effectively drives pressure gradients and confers a general flow component sub-parallel to the margin's strike, allowing for material transport over large distances around the slabs. The along-slab velocity component introduces widespread horizontal simple shear in the mantle flow which is maximised beneath the slab between ~100 and ~350 km. Mantle flow complexities develop rapidly, although not instantaneously, upon subduction of heterogeneous plates. The resulting slab pull gradients are mostly accommodated by internal slab deformation, decoupling the mantle flow from motions at surface. Moderate slab pull variations have a minor impact on plate velocity, and might not result in trench motions, although effectively rearrange the flow. Slab buoyancy heterogeneities are firstly associated with age-dependent thickness variations, but also with slab break offs and windows, varying depth of subduction and entrainment of buoyant blocks. Because these are observed at all subduction zones, the process shown here should have a global relevance for the flow around slabs.

capitanio, F. A.; Faccenda, M.

2012-12-01

150

Downgoing plate controls on overriding plate deformation in subduction zones  

NASA Astrophysics Data System (ADS)

Although subduction zones are convergent margins, deformation in the upper plate can be extensional or compressional and tends to change through time, sometimes in repeated episodes of strong deformation, e.g, phases of back-arc extension. It is not well understood what factors control this upper plate deformation. We use the code Fluidity, which uses an adaptive mesh and a free-surface formulation, to model a two-plate subduction system in 2-D. The model includes a composite temperature- and stress-dependent rheology, and plates are decoupled by a weak layer, which allows for free trench motion. We investigate the evolution of the state of stress and topography of the overriding plate during the different phases of the subduction process: onset of subduction, free-fall sinking in the upper mantle and interaction of the slab with the transition zone, here represented by a viscosity contrast between upper and lower mantle. We focus on (i) how overriding plate deformation varies with subducting plate age; (ii) how spontaneous and episodic back-arc spreading develops for some subduction settings; (iii) the correlation between overriding plate deformation and slab interaction with the transition zone; (iv) whether these trends resemble observations on Earth.

Garel, Fanny; Davies, Rhodri; Goes, Saskia; Davies, Huw; Kramer, Stephan; Wilson, Cian

2014-05-01

151

Subduction: The Gatekeeper for Mantle Melting.  

NASA Astrophysics Data System (ADS)

Geodynamic models are used to show the importance of subduction in controlling vertical thermal and chemical fluxes from Earth's interior to surface. In our models subduction-induced circulation produces conditions favorable to both steady-state and episodic melt production and also plays the role of gatekeeper in thwarting large scale melt production from rising plumes. We use laboratory experiments to characterize three-dimensional (3D) flow fields in convergent margins in response to a range of subduction and back-arc deformation styles, and how these flows interact with upwellings. Models utilize a glucose working fluid with a temperature dependent viscosity to represent the upper 2000 km of the mantle. Subducting lithosphere is modeled with a descending Phenolic plate and back-arc extension is produced by moving Mylar sheets. Thermal plumes are generated from a pressurized, temperature controlled source. Our results show that naturally occurring transitions from downdip- to rollback-dominated subduction produce conditions that favor both widespread decompression melting in the mantle wedge and short-lived pulses of extensive slab melting. For cases of plume-subduction interaction, 3D slab-induced flow quickly converts the active upwelling to a passive thermal anomaly that bears little to no resemblance to traditional models for plume surface expressions. Instead of rising to make LIPs with age-progressive chains, the bulk of the original plume material is trapped below depths of melt production before being re-subducted by the slab. A limited volume of this passive, former plume material is capable of surfacing. Interestingly, this is seen to occur through a range of morphologies that are consistently offset from the original rise location (e.g., conduit). Surface expressions include anything from small circular patches to long, linear features with complex age trends (e.g., progressive or regressive) resulting from the competition between plate motions and deeper mantle flow. Spatial-temporal patterns in melt production, beyond those from background 3D flow, are extremely sensitive to the position of the deep plume relative to the trench. For example, the timing, volume and distribution of melt for upwellings originating beneath the back-arc plate (e.g. Yellowstone) are fundamentally different from those rising under the ocean side of the slab (e.g. Samoa).

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

2011-12-01

152

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

NASA Astrophysics Data System (ADS)

Geological and geophysical constraints to reconstruct the evolution of the Central Mediterranean subduction zone are presented. Geological observations such as upper plate stratigraphy, HP-LT metamorphic assemblages, foredeep/trench stratigraphy, arc volcanism and the back-arc extension process are used to define the infant stage of the subduction zone and its latest, back-arc phase. Based on this data set, the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross-sections from the northern Apennines and from Calabria to the Gulf of Lyon can be derived. Further, the reconstruction is used to unravel the main evolutionary trends of the subduction process. Results of this analysis indicate that (1) subduction in the Central Mediterranean is as old as 80Myr, (2) the slab descended slowly into the mantle during the first 20-30Myr (subduction speeds were probably less than 1cmyear-1), (3) subduction accelerated afterwards, producing arc volcanism and back-arc extension and (4) the slab reached the 660km transition zone after 60-70Myr. This time-dependent scenario, where a slow initiation is followed by a roughly exponential increase in the subduction speed, can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull. Finally, the third stage is controlled by the interaction between the slab and the 660km transition zone. In the southern region, this results in an important re-shaping of the slab and intermittent pulses of back-arc extension. In the northern region, the decrease in the trench retreat can be explained by the entrance of light continental material at the trench.

Faccenna, Claudio; Becker, Thorsten W.; Lucente, Francesco Pio; Jolivet, Laurent; Rossetti, Federico

2001-06-01

153

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides  

NASA Astrophysics Data System (ADS)

We designed three-dimensional dynamically self-consistent laboratory models of subduction to analyse the relationships between overriding plate deformation and subduction dynamics in the upper mantle. We investigated the effects of the subduction of a lithosphere of laterally variable buoyancy on the temporal evolution of trench kinematics and shape, horizontal flow at the top of the asthenosphere, dynamic topography and deformation of the overriding plate. Two subducting units, which correspond to a negatively buoyant oceanic plate and positively buoyant continental one, are juxtaposed via a trench-perpendicular interface (analogue to a tear fault) that is either fully-coupled or shear-stress free. Differential rates of trench retreat, in excess of 6 cm yr-1 between the two units, trigger a more vigorous mantle flow above the oceanic slab unit than above the continental slab unit. The resulting asymmetrical sublithospheric flow shears the overriding plate in front of the tear fault, and deformation gradually switches from extension to transtension through time. The consistency between our models results and geological observations suggests that the Late Cenozoic deformation of the Aegean domain, including the formation of the North Aegean Trough and Central Hellenic Shear zone, results from the spatial variations in the buoyancy of the subducting lithosphere. In particular, the lateral changes of the subduction regime caused by the Early Pliocene subduction of the old oceanic Ionian plate redesigned mantle flow and excited an increasingly vigorous dextral shear underneath the overriding plate. The models suggest that it is the inception of the Kefalonia Fault that caused the transition between an extension dominated tectonic regime to transtension, in the North Aegean, Mainland Greece and Peloponnese. The subduction of the tear fault may also have helped the propagation of the North Anatolian Fault into the Aegean domain.

Guillaume, B.; Husson, L.; Funiciello, F.; Faccenna, C.

2013-07-01

154

Interplate coupling at oblique subduction zones: influence on upper plate erosion.  

NASA Astrophysics Data System (ADS)

In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45° with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where the sedimentary infill of the trench is almost nonexistent, short-term shallow coupling occurs and friction between the frontal sector of the overriding plate and the downgoing plate triggers upper-plate bending. In this sector, after the early short-term coupling, the overriding plate is hereafter decoupled from the subducting slab. Moving along trench-strike, where sediments amount increases, the upper plate couples with the subducting plate and is dragged coherently downwards. If a large amount of sediments is stored in the trench the overriding plate is scraped off and incorporated as fragments along the plate interface. Our results suggest that a) one main parameter controlling coupling at convergent plate margins is the occurrence and the amount of sediment at the trench; b) the upper plate margin is dragged to depth or destroyed only where sediments thickness at the trench is large enough to promote interplate coupling, suggesting that a variation of sediment amount along the trench-axis influences the amount and style of transport of upper-plate material in the mantle.

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

2014-05-01

155

'Dodo' and 'Baby Bear' Trenches  

NASA Technical Reports Server (NTRS)

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.

2008-01-01

156

The coseismic slip distribution of a shallow subduction fault constrained by prior information: the example of 2011 Tohoku (Mw 9.0) megathrust earthquake  

NASA Astrophysics Data System (ADS)

In the literature, the inverted coseismic slip models from seismological and geodetic data for the 2011 Tohoku-Oki earthquake portray significant discrepancies, in particular regarding the intensity and the distribution of the rupture near the trench. For a megathrust earthquake, it is difficult to discern the slip along the shallow part of the fault from the geodetic data, which are often acquired on land. In this paper, we discuss the uncertainties in the slip distribution inversion using the geodetic data for the 2011 Tohoku earthquake and the Fully Bayesian Inversion method. These uncertainties are due to the prior information regarding the boundary conditions at the edges of the fault, the dip subduction angle and the smoothing operator. Using continuous GPS data from the Japan Island, the results for the rigid and free boundary conditions show that they produce remarkably different slip distributions at shallow depths, with the latter producing a large slip exceeding 30 m near the surface. These results indicate that the smoothing operator (gradient or Laplacian schemes) does not severely affect the slip pattern. To better invert the coseismic slip, we then introduce the ocean bottom GPS (OB-GPS) data, which improve the resolution of the shallow part of the fault. We obtain a near-trench slip greater than 40 m that reaches the Earth's surface, regardless of which boundary condition is used. Additionally, we show that using a mean dip angle for the fault as derived from subduction models is adequate if the goal is to invert for the general features of the slip pattern of this megathrust event.

Zhou, X.; Cambiotti, G.; Sun, W.; Sabadini, R.

2014-11-01

157

Stress states at the subduction input site, Nankai Subduction Zone, using anelastic strain recovery (ASR) data in the basement basalt and overlying sediments  

NASA Astrophysics Data System (ADS)

The three-dimensional orientations of stress and stress magnitudes in the basement basalt and overlying sediments at the subduction input site, IODP Site C0012, have been determined using anelastic strain recovery (ASR) analyses. The ASR results in the sedimentary sequence indicate that ?1 is nearly vertical. The magnitudes of ?2 and ?3 are very similar, indicating that the stress state in the sedimentary sequence is "at rest". On the other hand, ASR analyses in the basement basalt show that ?1 is nearly horizontal and oriented NE-SW, almost parallel (or slightly oblique) to the trench axis. ?3 plunges moderately to the NW. The stress state of the basement basalts suggests a strike-slip or thrust (reverse fault) regime, which is very different from a "state at rest" condition, which is the theoretical stress condition for the ocean floor far from a subduction zone. The basement basalt at the subduction input site (C0012) has experienced trench-parallel compression and trench-normal extension, consistent with the focal mechanisms of earthquakes in the vicinity. The estimated stress magnitudes show only small variations between the principal stresses, implying that the directions of principal stress could be easily rotated in association with any tectonically induced local stress variation. The stress orientation in the basement basalt seems to be the result of hinge extension during bending of the Philippine Sea Plate, either in association with subduction or with the formation of an anticline during intraoceanic thrusting.

Yamamoto, Yuzuru; Lin, Weiren; Oda, Hirokuni; Byrne, Timothy; Yamamoto, Yuhji

2013-07-01

158

Opportunity Trenches Martian Soil  

NASA Technical Reports Server (NTRS)

The Mars Exploration Rover Opportunity dragged one of its wheels back and forth across the sandy soil at Meridiani Planum to create a hole (bottom left corner) approximately 50 centimeters (19.7 inches) long by 20 centimeters (7.9 inches) wide by 9 centimeters (3.5 inches) deep. The rover's instrument deployment device, or arm, will begin studying the fresh soil at the bottom of this trench later today for clues to its mineral composition and history. Scientists chose this particular site for digging because previous data taken by the rover's miniature thermal emission spectrometer indicated that it contains crystalline hematite, a mineral that sometimes forms in the presence of water. The brightness of the newly-exposed soil is thought to be either intrinsic to the soil itself, or a reflection of the Sun. Opportunity's lander is in the center of the image, and to the left is the rock outcrop lining the inner edge of the small crater that encircles the rover and lander. This mosaic image is made up of data from the rover's navigation and hazard-avoidance cameras.

2004-01-01

159

Control of paleoshorelines by trench forebulge uplift, Loyalty Islands  

NASA Astrophysics Data System (ADS)

Unlike most tropical Pacific islands, which lie along island arcs or hotspot chains, the Loyalty Islands between New Caledonia and Vanuatu owe their existence and morphology to the uplift of pre-existing atolls on the flexural forebulge of the New Hebrides Trench. The configuration and topography of each island is a function of distance from the crest of the uplifted forebulge. Both Maré and Lifou are fully emergent paleoatolls upon which ancient barrier reefs form highstanding annular ridges that enclose interior plateaus representing paleolagoon floors, whereas the partially emergent Ouvea paleoatoll rim flanks a drowned remnant lagoon. Emergent paleoshoreline features exposed by island uplift include paleoreef flats constructed as ancient fringing reefs built to past low tide levels and emergent tidal notches incised at past high tide levels. Present paleoshoreline elevations record uplift rates of the islands since last-interglacial and mid-Holocene highstands in global and regional sea levels, respectively, and paleoreef stratigraphy reflects net Quaternary island emergence. The empirical uplift rates vary in harmony with theoretical uplift rates inferred from the different positions of the islands in transit across the trench forebulge at the trench subduction rate. The Loyalty Islands provide a case study of island environments controlled primarily by neotectonics.

Dickinson, William R.

2013-07-01

160

A multibeam reconnaissance of the Tonga Trench axis and its intersection with the Louisville guyot chain  

Microsoft Academic Search

A Seabeam reconnaissance of 1200 km of the deep sediment-starved axis of Tonga Trench delineated the fine-scale relief at the outcrop of a subduction zone generally characterized by tectonic erosion rather than accretion. The commonest axial cross-section has a steep (12°) irregular inner slope intersecting the thinly sedimented surface of Mesozoic ocean crust, which dips under it at 5–6°. There

Peter Lonsdale

1986-01-01

161

Genesis of jadeite-quartz rocks in the Yorii area of the Kanto Mountains, Japan  

NASA Astrophysics Data System (ADS)

This paper reports the results of U-Pb dating and REE (rare earth element) analysis of zircons separated from jadeite-quartz rocks within serpentinite mélanges in the Yorii area of the Kanto Mountains, Japan. These rocks contain jadeite, albite, and quartz, with minor aegirine-augite, zircon, monazite, thorite, allanite, and titanite. Mineral textures provide evidence of a jadeite + quartz = albite reaction during formation of these jadeite-quartz rocks. Zircon crystals separated from the jadeite-quartz rocks can be split into two distinct types, here named Types I and II, based on their morphology and REE concentrations. Type I zircons are prismatic and have fluid, jadeite, quartz, and albite inclusions. Those show positive Ce and negative Eu anomalies and HREE (heavy rare earth element) enriched chondrite normalized REE patterns and have higher REE concentrations than those generally found in magmatic zircons. Type I zircons would have precipitated from a fluid. Mineralogical observation provides that Type I zircon crystallized at the same timing of the formation of the jadeite-quartz rocks. Type II zircons are porous and have REE patterns indicative of a hydrothermal zircon. Both types of zircons are fluid-related. Type I zircons yield U-Pb ages of 162.2 ± 0.6 Ma, with an MSWD (mean square weighted deviation) of 1.4. At this time, Japan was still a part of the eastern margin of the Asian continent, with the subduction of the oceanic paleo-Pacific Plate leading to the formation of the Jurassic Mino-Tanba-Chichibu accretionary complex in Japan. The age data indicate that the jadeite-quartz rocks formed in a deep subduction zone environment at the same time as the formation of the Jurassic accretionary complex in a shallower near-trench subduction zone environment. The jadeite-quartz rocks contain high concentrations of Zr and Nb, with low LILE (large ion lithophile elements) concentrations, suggesting that the HFSE (high field strength elements) can be concentrated into jadeite-quartz rocks prior to a fluid moving up into the mantle wedge. Typical arc volcanic rocks are depleted in the HFSE, suggesting that the high HFSE concentrations within jadeite-quartz rocks are consistent with fluids being stripped of their HFSE prior to interaction with mantle material during the formation of arc magmas. Although these jadeite-bearing rocks are rare occurrences on the surface exposure, they could be abundant in or above subducted slabs.

Fukuyama, Mayuko; Ogasawara, Masatsugu; Horie, Kenji; Lee, Der-Chuen

2013-02-01

162

Twenty-two outsize tsunami deposits at Lake Harutori-ko along the southern Kuril Trench  

NASA Astrophysics Data System (ADS)

Along the southern Kuril Trench, which faces the Japanese island of Hokkaido, this fast subduction generated recurrent earthquakes up to magnitude ~8 in two centuries of written history. Eastern Hokkaido's largest well-documented interplate earthquake, the Tokachi-oki earthquake of moment magnitude (Mw) 8.1 in 1952, shook much of northeast Japan and generated tsunami waves 1-4 m high along the Hokkaido coast. Here we use deposits of prehistoric tsunamis in the lake bottom to infer the infrequent occurrence of larger earthquakes. The long stratigraphic record of lacustrine cores at Harutori-ko provides evidence for unusually large tsunamis in the past 9500 years. This lake or lagoon, which contains anoxic bottom water, is separated from the sea by a 5-m-high beach berm that the 1952 tsunami did not cross since 2500-3000 yrBP. Deposits beneath the lake bottom of Harutori-ko consist of diatomaceous laminated mud that alternates with sandy beds 0.1-1.0 m thick. A core 1 km from the modern beach contains 22 such beds. All of these beds grade upward from shell-bearing sand with gravel, through mud-clast breccia and laminated silt and sand, into organic mud. Stratigraphic series of these sand sheets, intercalated with volcanic-ash layers from Komagatake and Tarumai volcano, south western Hokkaido, and radiocarbon dating show that unusually large tsunamis occurred every ca. 430 years on average in the past 9500 years, most recently about 350 years ago in Edo era.

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

2004-12-01

163

Sedimentation and deformation in the Amlia Fracture Zone sector of the Aleutian Trench  

USGS Publications Warehouse

A wedge-shaped, landward thickening mass of sedimentary deposits composed chiefly of terrigenous turbidite beds underlies the west-south west-trending Amlia sector (172??20???-173??30???W) of the Aleutian Trench. Pacific oceanic crust dips northward beneath the sector's sedimentary wedge and obliquely underthrusts (30?? off normal) the adjacent Aleutian Ridge. The trench floor and subsurface strata dip gently northward toward the base of the inner trench slope. The dip of the trench deposits increases downsection from about 0.2?? at the trench floor to as much as 6-7?? just above basement. The wedge is typically 2-2.5 km thick, but it is thickest (3.7-4.0 km) near the base of the inner slope overlying the north-trending Amlia Fracture Zone and also east of this structure. Slight undulations and relatively abrupt offsets of the trench floor reflect subsurface and generally west-trending structures within the wedge that are superimposed above ridges and swales in the underlying oceanic basement. The southern or seaward side of some of these structures are bordered by high-angle faults or abrupt flexures. Across these offsets the northern side of the trench floor and underlying wedge is typically upthrown. West-flowing turbidity currents originating along the Alaskan segment of the trench (1200 km to the east) probably formed the greater part of the Amlia wedge during the past 0.5 m.y. The gentle northward or cross-trench inclination of the trench floor and underlying wedge probably reflects regional downbending of the oceanic lithosphere and trench-floor basement faulting and rotation. Much of the undulatory flexuring of the trench wedge can be attributed to differential compaction over buried basement relief. However, abrupt structural offsets attest to basement faulting. Faulting is associated with extensional earthquakes in the upper crust. The west-trending basement offsets are probably normal faults that dip steeply south or antithetic to the north dip of the subducting oceanic crust. Up-to-arc extensional faulting can be attributed to the downbending of the Pacific plate into the Aleutian subduction zone. The rupturing direction and dip is controlled by zones of crustal weakness that parallel north Pacific magnetic anomalies, which were formed south of a late Cretaceous-early Tertiary spreading center (Kula-Pacific Ridge). The strike of these anomalies is fortuitously nearly parallel to the Amlia sector. The up-to-arc fracturing style may locally assist in elevating blocks of trench deposits to form the toe of the trench's landward slope, which is in part underlain by a compressionally thickened accretionary mass of older trench deposits. Compressional structures that can be related to underthrusting are only indistinctly recorded in the turbidite wedge that underlies the trench floor. ?? 1982.

Scholl, D. W.; Vallier, T.L.; Stevenson, A.J.

1982-01-01

164

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides  

NASA Astrophysics Data System (ADS)

We design three-dimensional dynamically self-consistent laboratory models of subduction to analyze the relationships between overriding plate deformation and subduction dynamics in the upper mantle. We investigate the effects of the subduction of a lithosphere of laterally variable buoyancy on the temporal evolution of trench kinematics and shape, horizontal flow at the top of the asthenosphere, dynamic topography and deformation of the overriding plate. The interface between the two units, analogue to a trench-perpendicular tear fault between a negatively buoyant oceanic plate and positively buoyant continental one, is either fully-coupled or shear-stress free. Differential rates of trench retreat, in excess of 6 cm yr-1 between the two units, trigger a more vigorous mantle flow above the oceanic slab unit than above the continental slab unit. The resulting asymmetrical sublithospheric flow shears the overriding plate in front of the tear fault, and deformation gradually switches from extension to transtension through time. The consistency between our models results and geological observations suggests that the Late Cenozoic deformation of the Aegean domain, including the formation of the North Aegean Trough and Central Hellenic Shear zone, results from the spatial variations in the buoyancy of the subducting lithosphere. In particular, the lateral changes of the subduction regime caused by the Early Pliocene subduction of the old oceanic Ionian plate redesigned mantle flow and excited an increasingly vigorous dextral shear underneath the overriding plate. The models suggest that it is the inception of the Kefalonia Fault that caused the transition between an extension dominated tectonic regime to transtension, in the North Aegean, Mainland Greece and Peloponnese. The subduction of the tear fault may also have helped the propagation of the North Anatolian Fault into the Aegean domain.

Guillaume, B.; Husson, L.; Funiciello, F.; Faccenna, C.

2013-04-01

165

Contraction and extension in northern Borneo driven by subduction rollback  

NASA Astrophysics Data System (ADS)

During the Paleogene the Proto-South China Sea was subducted beneath northern Borneo. Subduction ended with Early Miocene collision of the Dangerous Grounds/Reed Bank/North Palawan block and the Sabah-Cagayan Arc. Much of northern Borneo then became emergent forming the Top Crocker Unconformity. Later in the Early Miocene subsidence resumed. It is proposed that northward subduction of the Celebes Sea initiated formation of the Sulu Sea backarc basin, followed by subduction rollback to the SE. This formed a volcanic arc, which emerged briefly above sea level and collapsed in the Middle Miocene. As rollback continued the Sulu Arc was active during Middle and Late Miocene between Sabah and the Philippines. Rollback drove extension in northern Borneo and Palawan, accompanied by elevation of mountains, crustal melting, and deformation offshore. There were two important extensional episodes. The first at about 16 Ma is marked by the Deep Regional Unconformity, and the second at about 10 Ma produced the Shallow Regional Unconformity. Both episodes caused exhumation of deep crust, probably on low angle detachments, and were followed by granite magmatism. The NW Borneo-Palawan Trough and offshore Sabah fold and thrust belt are often interpreted as features resulting from collision, regional compression or subduction. However, there is no seismicity, dipping slab or volcanicity indicating subduction, nor obvious causes of compression. The trough developed after the Middle Miocene and is not the position of the Paleogene trench nor the site of Neogene subduction. Inboard of the trough is a thick sediment wedge composed of an external fold and thrust belt and internal extensional zone with structures broadly parallel to the trough. The trough is interpreted as a flexural response to gravity-driven deformation of the sediment wedge, caused by uplift on land that resulted from extension, with a contribution of deep crustal flow.

Hall, Robert

2013-10-01

166

Nonvolcanic tremors in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

Nonvolcanic low frequency tremors (NVT) have been discovered and studied recently in Japan and Cascadia subduction zones and deep beneath the San Andreas Fault. The tremors activity is increasing during so-called silent earthquakes (SQ) in Japan and Cascadia. NVT clusters also migrate following the propagation of the SQ. The origin of the NVT is still unclear. The studies of NVT and SQ in different subduction zones are required to understand the cause for these phenomena. We discovered a number of NVT from daily spectrograms of continuous broad band records at seismic stations of Servicio Seismológico Nacional (SSN) an MASE project. The analyzed data cover a period of 2001-2004 (SSN) when in 2002 a large SQ has occurred in the Guerrero- Oaxaca region, and a steady-state interseismic epoch of 2005 and a new large SQ in 2006 (MASE). NVT occurred in the central part of the Mexican subduction zone (Guerrero) at approximately 200 km from the coast. We can not accurately localize the tremors because of sparse station coverage in 2001-2004. The MASE data of 2005-2006 show that NVT records in Mexico are very similar to those obtained in Cascadia subduction zone. The tremors duration is of 10-60 min, and they appear to travel at S-wave velocities. More than 100 strong NVT were recorded by most of the MASE stations with the epicenters clustered in the narrow band of ~40x150 km to the south of Iguala city and parallel to the coast line. NVT depths are poorly constrained but seem to be less than 40 km deep. We noticed a some increase of NVT activity during the 2001-2002 and 2006 SQs compared with an NVT activity for the "SQ quiet" period of 2003-2004 nevertheless. A lack of NVT for the period of 2-3 months after the SQ is apparent in 2002 and 2006.

Payero, J. S.; Kostoglodov, V.; Mikumo, T.; Perez-Campos, X.; Iglesias, A.; Clayton, R.

2007-05-01

167

Dynamics of 3-D thermo-mechanical subduction with an overriding plate  

NASA Astrophysics Data System (ADS)

Characterizing the role of the upper plate in dynamic subduction models is of paramount importance in understanding the subduction system as a whole. We investigate the effect of the overriding plate on subduction dynamics in a 3D, purely dynamic thermo-mechanical setup of the finite element code, CitcomCU (Moresi & Gurnis, 1996; Zhong, 2006). The reference models are purely Newtonian with a temperature dependent viscosity. Subduction is initiated by an asymmetric notch in the down going plate, and the two plates are decoupled by a weak, 15 km thick crustal layer, along shear is localized. As observed in previous 2D modeling studies the presence of an overriding plate decreases the vertical extent of the poloidal flow, resulting in reduced vertical subduction velocity, reduced trench retreat and a larger dip angle. This increased dip angle results in preferential slab folding, as opposed to flattening, at the 660-km discontinuity when the overriding plate is included. It has recently been suggested that toroidal flow, due to slab rollback, creates trench-perpendicular gradients in basal traction below the overriding lithosphere which can cause back-arc extension (Schellart & Moresi, 2013; Duarte et al., 2013). In our models, we also observe long wavelength back-arc extensional stresses, and more localized forearc compressive stresses. As in Piromallo et al. (2010), we decompose the velocity field into toroidal and poloidal components, in order to attempt to quantify the relationship between toroidal flow and overriding plate stress state for various model set-ups. We focus on how the presence of an overriding plate, and variable upper plate widths and strengths, modifies the partition between toroidal and poloidal flow, as well as overriding plate stress state and subducting plate dynamics (i.e. trench plate and penetration velocities). The presence of an overriding plate reduces the toroidal flow component of the velocity field and so significantly reduces the rate at which trench curvature, previously shown to occur in due to toroidal flow in 3D single plate models (e.g. Stegman et al., 2006), develops. Thus, dynamic modeling studies that neglect the presence of the overriding plate may be significantly overestimating the effect of toroidal flow on trench curvature. Finally, we examine how the presence of a piece of compositionally buoyant lithosphere, both in the subducting and overriding plates, modifies the dynamics of the subducting slab and the stress state of the overriding plate.

Holt, A.; Becker, T. W.

2013-12-01

168

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

NASA Astrophysics Data System (ADS)

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.

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

169

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

NASA Astrophysics Data System (ADS)

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.

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

2013-03-01

170

Constraining Cretaceous subduction polarity in eastern Pacific from seismic tomography and geodynamic modeling  

NASA Astrophysics Data System (ADS)

of recent mantle seismic images below the America ignited a debate on the Cretaceous subduction polarity in the eastern Pacific Ocean. The traditional view is that the massive vertical slab wall under eastern North America resulted from an eastward Farallon subduction. An alternative interpretation attributes this prominent seismic structure to a westward subduction of the North American Plate against a stationary intraoceanic trench. Here I design quantitative subduction models to test these two scenarios, using their implied plate kinematics as velocity boundary conditions. Modeling results suggest that the westward subduction scenario could not produce enough slab volume as seismic images reveal, as is due to the overall slow subduction rate (~2.5 cm/yr). The results favor the continuous eastward Farallon subduction scenario, which, with an average convergence rate of >10 cm/yr prior to the Eocene, can properly generate both the volume and the geometry of the imaged lower mantle slab pile. The eastward subduction model is also consistent with most Cretaceous geological records along the west coast of North America.

Liu, Lijun

2014-11-01

171

Continental response to active ridge subduction  

NASA Astrophysics Data System (ADS)

Apatite fission track ages from a ~2000 m elevation transect from the Patagonian fold and thrust belt (47.5°S) allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction. Accelerated cooling started at 17 Ma, predating the onset of ridge collision (14-10 Ma), and was followed by reheating between 10 and 6 Ma. Thermal modeling favors reheating on the order of 60°C at ~28°C/Ma due to east-migration of a slab window after the ridge-trench collision. Final rapid cooling since 4 Ma of ~18°C/Ma (geothermal gradient of 14°C/km) correlates with the presence of an orographic barrier and >1 km rock uplift in this region between 17.1 and 6.3 Ma. Increased precipitation and erosion since 4 Ma caused asymmetric exhumation, with 3-4 km on the leeside. Repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the Chile Triple Junction.

Haschke, M.; Sobel, E. R.; Blisniuk, P.; Strecker, M. R.; Warkus, F.

2006-08-01

172

Structure and deformation of the Tonga-Kermadec subduction system in the Louisville Ridge pre-collision zone  

NASA Astrophysics Data System (ADS)

New multichannel seismic data collected ~200 km south of the collision between the Tonga-Kermadec Trench and the Louisville Ridge show normal subduction zone structures indicating that the effects of the collision decay rapidly along-arc. Due to the obliqueness of plate convergence at the Tonga-Kermadec Trench, the collision zone is migrating rapidly southwards at 180 mm yr-1. Consequently, the along-arc transition from ridge-trench deformation to normal subduction zone deformation is rapid. To determine the 'normal' stratigraphy and structure of the subducting plate and forearc prior to Louisville Ridge collision, a 250 km-long multichannel seismic profile was shot perpendicular to the Kermadec Trench at ~28°S. Interpretation of coincident swath bathymetry and multichannel seismic data shows horst and graben structures on the down-going plate that offset the seafloor on large-scale, steeply-listric normal faults. These faults accommodate bending-induced extensional stresses generated as the trenchward plate dip increases. The graben have accumulated thin sedimentary deposits but are largely unfilled. Background sediment influx along the length of the 2700 km long subduction system is largely due to pelagic sedimentation and volcaniclastics from the arc, with regional sediment thicknesses in the trench of <400 m. However, a 50% increase in trench sediment thickness is observed, and is inferred to be due to increased sediment influx from an uplifting and eroding forearc. Trench and forearc structure are typical for a subduction system. However, the forearc mid-slope basin shows both arcward rotation of sedimentary sequences and extensional collapse on km-scale faults. A fresh fault scarp with ~400 m of throw leave exposed sedimentary sections indicating that extensional deformation of the forearc is both large-scale and recent. We propose that these faults accommodate the majority of forearc extension. Sediments on the upper trench slope document depositional hiatuses and ongoing deformation of the forearc, while turbidite flows and strong along-arc currents erode the forearc high and distribute sediments to the surrounding basins. Uplift and trenchward rotation of the forearc high are interpreted from changes in the tilt of upper forearc turbidite sequences and persistent extension of the backarc slope. This study of background crustal structure indicates that the forearc is deformed under 'normal' conditions where the trench is sediment starved. Erosion by subducting horst and graben of the down-going Pacific Plate alone is sufficient to modify the overlying plate, causing extension and uplift of the forearc. Nevertheless, the two-fold increase in width and the >2000 m shallowing of the trench in the collision zone highlight the large-scale effects that seamount collision has on trench and forearc structure. Comparisons between trench and forearc geomorphology in current and pre-seamount subduction locations are used to infer the degree to which seamount subduction has overprinted the background signature of normal Tonga-Kermadec subduction.

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

2013-12-01

173

Role of the overriding plate in the subduction process: Insights from numerical models  

NASA Astrophysics Data System (ADS)

Active convergent margins are primarily shaped by the interplay among the subducting plate, overriding plate, and mantle. The effect of important forces, like far-field mantle flow, overriding plate motion, and inter-plate coupling, however, remains partially ambiguous. In a preliminary attempt to clarify their role, a self-consistent, viscoelastic, plane-strain, mechanical finite element model, in which subducting plate, overriding plate and mantle interact dynamically, is developed. In this quasi-static framework with a freely moving slab, trench, and inter-plate fault, the role of a compressive overriding plate on subduction zone kinematics, morphology and stress-state is characterized. A slab interacting solely with a semi-analytical three-dimensional mantle flow formulation shows that local non-induced mantle flow influences slab geometry and kinematics, adding an important dynamic term to the system. The impact of an overriding plate on this system is determined completely by overriding plate trench-ward motions and is only pertinent if the overriding plate actively advances the trench. A trench-ward moving overriding plate indents the slab and thereby enforces trench retreat and decreases slab dip. It also stimulates over-thrusting of the overriding plate onto the slab, and thereby permits mountain building within the overriding plate. Frictional resistance is observed to have a dominant local effect within the overriding plate as it is increasingly dragged down, thereby inhibiting the growth of overriding plate topography. A distinguishable effect on large-scale trench motions and deep slab dip is, however, absent for re-normalized friction coefficients ranging up to about 0.2. Minor additional effects include a decrease in plate motions of about 15% and slab bending stresses of about 10%.

van Dinther, Ylona; Morra, Gabriele; Funiciello, Francesca; Faccenna, Claudio

2010-03-01

174

Role of the Overriding Plate in the Subduction Process: Insights from Numerical Models  

NASA Astrophysics Data System (ADS)

Active convergent margins are primarily shaped by the interplay among the subducting plate, overriding plate, and mantle. The effect of important forces, like far-field mantle flow, overriding plate motion, and inter-plate coupling, however, remains partially ambiguous. In a preliminary attempt to clarify their role, a self-consistent, visco-elastic, plane-strain, mechanical finite element model, in which subducting plate, overriding plate and mantle interact dynamically, is developed. In this quasi-static framework with a freely moving slab, trench, and inter-plate fault, the role of a compressive overriding plate on subduction zone kinematics, morphology and stress-state is characterized. A slab interacting solely with a semi-analytical three-dimensional mantle flow formulation shows that local non-induced mantle flow influences slab geometry and kinematics, adding an important dynamic term to the system. The impact of an overriding plate on this system is determined completely by overriding plate trench-ward motions and is only pertinent if the overriding plate actively advances the trench. A trench-ward moving overriding plate indents the slab and thereby enforces trench retreat and decreases slab dip. It also stimulates over-thrusting of the overriding plate onto the slab, and thereby permits mountain building within the overriding plate. Frictional resistance is observed to have a dominant local effect within the overriding plate as it is increasingly dragged down, thereby inhibiting the growth of overriding plate topography. A distinguishable effect on large-scale trench motions and deep slab dip is, however, absent for renormalized friction coefficients ranging up to about 0.2. Minor additional effects include a decrease in plate motions of about 15% and slab bending stresses of about 10%.

Funiciello, F.; Dinther, Y. V.; Morra, G.; Faccenna, C.

2009-12-01

175

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

NASA Astrophysics Data System (ADS)

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.

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

2014-11-01

176

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.

177

Trench Foot or Immersion Foot  

MedlinePLUS

... Children Safe From Drowning in Flooded Areas Avoiding Carbon Monoxide Poisoning During a Power Outage Driving Through Water After a Disaster Preventing Trench Foot or Immersion Foot Identification and Treatment of Hypothermia Related to Exposure While Working in Cold Water General Information about ...

178

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

NASA Astrophysics Data System (ADS)

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.

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

2012-03-01

179

Tectonics of the northernmost Tonga subduction zone and a comparative study of the tectonics of subduction zone terminations  

NASA Astrophysics Data System (ADS)

It is a geometric necessity for a plate to tear where a subduction plate boundary terminates at a transform boundary whose trends are at high angles to each other. The manifestation of these tears is a family of near-vertical faults that trend nearly parallel to the direction of convergence, known as hinge faults, which are characterized by near-vertical dip-slip earthquake focal mechanisms. To better understand the tectonic processes involved in hinge-fault zones, I used earthquake distribution patterns and inversion of long-period body waveforms of 56 moderate-to-large (5.0 ? Ms ? 7.5) events to analyze earthquakes in the highly active northern termination of the Tonga subduction zone at the "Samoa Corner". This inversion generated precise estimates of the focal mechanism parameters, including fault plane strike, dip, and slip, source-time function and focal depth for shallow (h ? 100 km) events in the northern region of the Tonga subduction zone. I find that this region is associated with distinct groups of earthquakes whose mechanisms document tearing of the Pacific Plate lithosphere, back-arc extension in the northern Lau Basin, strike-slip faulting along the Fiji Fracture Zone, and bending and underthrusting of the Pacific Plate at the Tonga trench. In addition, these mechanisms are associate with distinct bathymetric features, such as the Tonga Trench and the Tonga arc. In the second part of this thesis I consider other subduction zones that terminate or sharply change orientation in order to determine the conditions necessary for the existence of hinge-fault-type mechanisms and to compare the different regimes where these mechanisms occur. Using earthquake distribution patterns and Harvard Centroid moment tensor solutions I find that hinge-fault-type mechanisms are clearly evident in the northern subduction termination in Tonga as well as in the northern Scotia, southern Mariana, southern New Hebrides subduction zones. There is more limited evidence for hinge-fault-type mechanisms in the southern Caribbean and western Aleutian subduction zone terminations. In contrast, there is no evidence of hinge-faulting earthquakes in the southern Scotia, northern Caribbean, northern Mariana and eastern Alaska subduction zones. I find that the zones with clear evidence of hinge-fault-type mechanisms are always associated with (1) rapid convergence between the overriding and underthrusting plates, (2) subduction of relatively old (?60 m.y.) oceanic lithosphere, (3) subduction zones with active back-arc spreading, and (4) in areas where orthogonal subduction rapidly changes to orthogonal transform motion. Furthermore, there appears to be a correlation between the length of time since initiation of back-arc spreading and the depth at which hinge-fault-type mechanisms occur. These observations suggest that under normal circumstances oceanic lithosphere will bend and contort aseismically during subduction and that only under the conditions of very high strain rates combined with unusually strong lithosphere, will the plate tearing be manifested by hinge faulting earthquakes.

Millen, David Walter

2000-10-01

180

Silent Earthquakes, Structure, and Seismotectonics of the Mexican Subduction Zone  

NASA Astrophysics Data System (ADS)

Based on tide gauge, levelling and GPS data, we find evidence for a sequence of silent earthquakes in 1972, 1979, 1996, 1998, 2002, and 2003 in the central part of the Mexican subduction zone (Guerrero and Oaxaca states). Characteristic duration of these events was 4-6 months and the maximum equivalent magnitude exceeded Mw7.5. In all cases, with the exception of the event of 1996, the slow aseismic slips initiated in the Guerrero seismic gap and propagated laterally along the strike of the subduction zone. However, the propagation velocity of ~2 km/day could be estimated reliably only for the most recent 2002 event. The observations indicate that the total area affected by the 1972 and 2002 slow events may have been greater than ~300x700 km2. The shallow, subhorizontal configuration of the plate interface in Guerrero and partly in Oaxaca appears to be a controlling factor for the physical conditions favorable for such extensive slow slip. The entire partially coupled interplate zone in Guerrero is of ~160 km width (starting ~55 km inland from the trench) while the seismogenic, shallowest part of it is only ~40 km wide. The elastic half space dislocation models (EHSDM) applied to invert the observed slow aseismic slip displacements (2002 event) can not distinguish between the two main scenarios: (a) slow slip of ~10 cm occurring on the entire coupled interface, and (b) slip of 15-20 cm taking place only on the transition part of the plate interface from ~90 to 180 km. In the first case the anticipated large thrust earthquake in the Guerrero seismic gap should be somewhat delayed, while on the second case the seismic rupture may be advanced. Thermo-mechanical modeling of the Mexican subduction zone shows that the coupling cutoff of ~450oC on the plate interface at ~180-205 km from the trench is achievable only for the subhorizontal configuration of the subduction zone. In this case the predominant metamorphic facies on the surface of subducted crust should be blueschists. There are, however, several observations which can not be explained in the frame of the EHSDM, e.g., a considerable tilt observed in the coastal area and a relatively large displacement on the Popocatepetl volcano (~400 km from the trench) during the 2002 silent earthquake.

Kostoglodov, V.; Larson, K. M.; Singh, S.; Lowry, A. R.; Santiago, J.; Franco, S.; Bilham, R.

2004-05-01

181

Shear Wave Splitting and Seismic Anisotropy in the Chile Ridge Subduction Region  

NASA Astrophysics Data System (ADS)

We present new shear wave splitting measurements of SK(K)S and PKS phases recorded at 39 broadband seismic stations in the Chile triple junction region. The network, deployed December 2004-February 2005 and operated jointly by the University of Florida and the Universidad de Chile (Santiago), spans the region where the Chile Ridge subducts beneath South America, from the Pacific coast of the Taitao Peninsula to the Argentine border, and extends 250 km north and south of the actual triple junction. Given increasing temperature with depth, it has long been hypothesized that ridge subduction should result in creation of slab windows - asthenosphere-filled gaps between continually separating edges of oceanic lithosphere formed at the Earth's surface. The Chile Ridge Subduction Project was formulated in part to test this notion. The network was demobilized during January-February 2007. In conjunction with teleseismic travel time inversions and studies of seismic attenuation in the Chile Ridge subduction region, shear wave splitting as recorded at the Project network provides an excellent snapshot of upper mantle flow in the region: Shear wave splitting in the study area, which may be caused by mineral alignment during upper mantle flow or possibly by aligned pockets of partial melt, is strong (delay times up to 3 s) and highly variable, with a marked change from trench-parallel in the northern network to trench normal in the western Taitao Peninsula, near the most recently subducted Chile ridge segment.

Russo, R. M.; Mocanu, V. I.; Gallego, A.; Comte, D.; Murdie, R. E.; Vandecar, J. C.; van der Lee, S.

2007-12-01

182

Tectonic erosion along the Japan and Peru convergent margins  

E-print Network

Tectonic erosion along the Japan and Peru convergent margins R. VON HUENE U.S. Geological Survey, M erosion can be estimated quantita- tively if the position of the volcanic arc, the position the Japan and Peru Trenches indicate rates of erosion comparable to well-known rates of accretion. Proposed

Demouchy, Sylvie

183

KSC Launch Pad Flame Trench Environment Assessment  

NASA Technical Reports Server (NTRS)

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.

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

2010-01-01

184

Ridge subduction along the central Chilean margin: Insights from seismic and bathymetric studies  

NASA Astrophysics Data System (ADS)

The central Chilean margin was the target of RV SONNE cruise SO161. Here we present preliminary results of the seismic and swath surveys across the subduction zone and volcanic structures entering the trench. The subduction process is characterized by a change in dip angle of the downgoing plate, resulting in a segmentation of the margin. Segment boundaries commonly coincide with bathymetric elevations, ridges, or fracture zones on the oceanic plate. The 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. The Chile subduction zone is unique in that the growth and destruction of the margin may be observed in regions of subduction accretion and subduction erosion, respectively. The study area covers the eastern part of the aseismic Juan Fernandez Ridge and the continental slope off Valparaiso. Two coincident wide-angle and reflection seismic profiles are located across the subduction complex. Towards the trench, a wedge-shaped body of reduced velocities is observed and interpreted as accreted, compressed, and reworked trench material. A backstop probably consisting of Paleozoic basement is clearly defined by the high p-wave velocities ranging from 5.3 km/s to 6.0 km/s. In contrast to the landward part of the profile, the Moho is clearly visible on the seaward side of the trench, at a depth of about 11 km. The crustal structure of the O'Higgins seamount group is the target of tomographic investigations using two perpendicular cross lines of high ray coverage resulting from a dense instrument spacing of ca. 2.2 nm. The tomographic inversion scheme includes a first-arrival inversion process, which has been modified to also consider later arrivals. The aim of the study is to determine the rate of underplating and the ratio of extrusive vs. intrusive melt generated when the plate moved over the hotspot, which formed the Juan Fernandez Ridge.

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

2002-12-01

185

Slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1) along the Kuril Trench deduced from tsunami waveform inversion  

USGS Publications Warehouse

We inverted 13 tsunami waveforms recorded in Japan to estimate the slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1), which occurred southeast off Hokkaido along the southern Kuril subduction zone. The previously estimated source area determined from tsunami travel times [Hatori, 1973] did not coincide with the observed aftershock distribution. Our results show that a large amount of slip occurred in the aftershock area east of Hatori's tsunami source area, suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock. We also found more than 5 m of slip along the deeper part of the seismogenic interface, just below the central part of Hatori's tsunami source area. This region, which also has the largest stress drop during the main shock, had few aftershocks. Large tsunami heights on the eastern Hokkaido coast are better explained by the heterogeneous slip model than previous uniform-slip fault models. The total seismic moment is estimated to be 1.87 ?? 1021 N m, giving a moment magnitude of Mw = 8.1. The revised tsunami source area is estimated to be 25.2 ?? 103 km2, ???3 times larger than the previous tsunami source area. Out of four large earthquakes with M ??? 7 that subsequently occurred in and around the rupture area of the 1952 event, three were at the edges of regions with relatively small amount of slip. We also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface.

Hirata, K.; Geist, E.; Satake, K.; Tanioka, Y.; Yamaki, S.

2003-01-01

186

Compositional and Fluid Pressure Controls on the State of Stress on the Nankai Subduction Thrust  

NASA Astrophysics Data System (ADS)

We present the initial results of an ongoing experimental study that examines the role that clay minerals play in controlling the properties and state of stress on subduction plate boundary faults. We focus here on the Nankai Trough, SW Japan, because it is representative of a major class of sediment-rich subduction zones (including Alaska, Chile, and Cascadia) that are capable of generating very large earthquakes (M ~8 and greater) and are almost fully coupled in their interseismic periods. The plate boundary fault also seems to be extremely weak because the maximum principal stress currently has a trench parallel orientation in the backstop region. For this to occur the maximum shear stress on the subduction thrust cannot exceed approx. 18 MPa even at the down-dip end of the seismogenic zone (recent numerical simulations by Wang and He). Our preliminary results suggest that both fault mineralogy and regional excess fluid pressure contribute to the low resolved shear stresses on the subduction zone plate boundary. Ring and direct shear tests show that saturated clay phases in the fault possess intrinsically low residual friction coefficients (RFC) at stress levels between 1 and 30 MPa. The RFC values for smectite are 0.14 ñ 0.02, for illite 0.25 ñ 0.01, and for chlorite 0.26 ñ 0.02. We find that the relatively weak illite phase is mechanically dominant in the incoming Muroto section and smectite contributes to even lower RFC values (as low as 0.18) in the incoming strata off the Ashizuri peninsula. Off Muroto, the overall magnitude of the shear stress we predict in the frontal 30 km remains below 2 MPa and only rises towards and above 4 MPa at 50 km from the toe of the wedge because of the overpressuring (lambda* values of between 0.6 to 0.8) and weak clays (RFC values of aprox.0.32 for 60 % Illite + 40 % Qtz mudstone). This is consistent with the low critical wedge taper (4.1degrees) in this region and values are well below the maximum stress estimates derived by Wang and He. Off the Ashizuri region, where the critical taper is higher (7.9 degrees), the basal lambda* value for the decollement could be as low as 0.0 to 0.4 for a decollement lying in a clay-rich portion of the deeper underthrust section, which may have a RFC values as low as 0.24-0.26 even after clay diagenesis has removed the smectite. Even with the lower overpressures we estimate that the basal shear stresses should remain below approx. 12 to 18 MPa, consistent with the Wang and He maximum basal shear stress estimates, further suggesting there is no low stress paradox at this subduction zone. Overall, our data do not, support the proposal that the smectite to illite reaction is directly responsible for the onset of seismogenic behavior throughout the Nankai system because both smectite and illite have low RFC values, tend to velocity strengthen, and there is already a preexisting mechanical dominance of in much of the incoming section (particularly off Muroto).

Brown, K. M.; Kopf, A.; Underwood, M. B.

2002-12-01

187

A model for the termination of the Ryukyu subduction zone against Taiwan: A junction of collision, subduction/separation, and subduction boundaries  

USGS Publications Warehouse

The NW moving Philippine Sea plate (PSP) collides with the Eurasian plate (EUP) in the vicinity of Taiwan, and at the same time, it subducts toward the north along SW Ryukyu. The Ryukyu subduction zone terminates against eastern Taiwan. While the Ryukyu Trench is a linear bathym??trie low about 100 km east of Taiwan, closer to Taiwan, it cannot be clearly identified bathymetrically owing to the deformation related to the collision, making the location of the intersection of the Ryukyu with Taiwan difficult to decipher. We propose a model for this complex of boundaries on the basis of seismicity and 3-D velocity structures. In this model the intersection is placed at the latitude of about 23.7??N, placing the northern part of the Coastal Range on EUP. As PSP gets deeper along the subduction zone it collides with EUP on the Taiwan side only where they are in direct contact. Thus, the Eurasian plate on the Taiwan side is being pushed and compressed by the NW moving Philippine Sea plate, at increasing depth toward the north. Offshore of northeastern Taiwan the wedge-shaped EUP on top of the Ryukyu subducting plate is connected to the EUP on the Ryukyu side and coupled to the NW moving PSP by friction at the plate interface. The two sides of the EUP above the western end of the subduction zone are not subjected to the same forces, and a difference in motions can be expected. The deformation of Taiwan as revealed by continuous GPS measurements, geodetic movement along the east coast of Taiwan, and the formation of the Hoping Basin can be understood in terms of the proposed model. Copyright 2009 by the American Geophysical Union.

Wu, F.T.; Liang, W.-T.; Lee, J.-C.; Benz, H.; Villasenor, A.

2009-01-01

188

The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone  

NASA Astrophysics Data System (ADS)

In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the “plate boundary” as a characteristic “velocity-jump” at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

Piana Agostinetti, Nicola; Miller, Meghan S.

2014-12-01

189

Seismic evidence for a slab tear at the Puerto Rico Trench  

USGS Publications Warehouse

The fore-arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. Joint analyses of the ocean bottom seismographs and land-based seismic data reveal that the swarms are located at depths of 50–150 km. Focal mechanism solutions, found by jointly fitting P wave first-motion polarities and S/P amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strike- and dip-slip mechanisms at depths of 50–100 km, while events at depths of 100–150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes: The slab segment east of 65°W longitude is dominated by trench-normal tensile stresses at shallower depths (50–100 km) and by trench-parallel tensile stresses at deeper depths (100–150 km), whereas the slab segment west of 65°W longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50–100 km). The simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench-normal tension and deeper trench-normal compression, is consistent with flexure of the slab due to rollback. These results support the hypothesis that the subducting North American plate is tearing at or near these swarms. The 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating.

Meighan, Hallie E.; Pulliam, Jay; Brink, Uri ten; López-Venegas, Alberto M.

2013-01-01

190

Structure and seismic stratigraphy of the Bonin Trench-Arc system  

E-print Network

. An oblique collision between Japan and the Bonin Arc during the opening of the Japan Sea both compressed the arc and induced a clockwise torque along the northern margin of the arc which bent and fractured the arc, forming the en-echelon structures... al. (1973) states that "it is composed of late Miocene to Quaternary ashy clays which are nearly one kilometer thick on the east side of the Japan Trench off Tokyo, and which progressively thins eastward for 1000 to 1500 km. The source...

Bandy, William Lee

2012-06-07

191

Timing of subduction processes in Eastern Anatolia  

NASA Astrophysics Data System (ADS)

A new occurrence of eclogites was found in the Kesandere valley in the eastern most part of the Bitlis complex. These high pressure relics occur within the Bitlis basement rocks. These new findings complement eclogites from its central part at Mt. Gablor (Okay et al., 1985) south of Mu?. There, kyanite-eclogites occur within garnet mica schists and gneisses. P-T estimates have been reported with temperatures between 600° and 650°C at 1.0 to 2.0 GPa. A correlation to the Menderes Massif and a Pan African age was suggested (Okay et al., 1985). The eclogites from Kesandere differ from the one described by Okay et al. (1985) at Mt Gablor as far as their lithologic association and protolith are concerned. P-T conditions for these eclogites have been estimated by the use of DOMINO (De Capitani and Brown, 1987). Pressures range between 19 and 24 kb while temperatures range between 480 and 540 °C. These P-T conditions are somewhat colder that those estimated by Okay et al. (1985) for the Gablor mountains further to the west of the Bitlis complex. 39Ar/40Ar in situ laser ablation age determination of Fe,Mg-carpholite and blue amphibole bearing metasedimentary cover rocks of the Bitlis complex (Oberhänsli et al., 2010,2011) gave 79-74 Ma for the peak metamorphic assemblage Fe,Mg-carpholite-chlorite-phengite and 74- 71 Ma for the retrograde assemblage chloritoid-chlorite-phengite-kyanite. Leaving a short time span for the exhumation from ca. 35 to 20 km depth that can be interpreted to exhumation rates between 3 to 2 mm/a. New U/Pb age data on zircon from the eclogites, subducted to ca 60 km, gave ca. 85 Ma for the magmatic cores. No age relevant data could be retrieved from the small metamorphic rims on the zircon grains. This leaves only a very short time span of minimum 5 to 6 Ma for subduction of the mafic rocks. Thus the rate of subduction was in the range of 12 to 10 mm/a. Due to immediate and again rapid exhumation one can conlude that the mid ocean ridge where the basalts were produced was close to the subduction trench.

Oberhänsli, R.; Candan, O.; Koralay, E.; Bousquet, R.; Okay, A.

2012-04-01

192

Tomography and Dynamics of Western-Pacific Subduction Zones  

NASA Astrophysics Data System (ADS)

We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V/low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

Zhao, D.

2012-01-01

193

Generation of talc in the mantle wedge and its role in subduction dynamics in central Mexico  

NASA Astrophysics Data System (ADS)

Geophysical evidence shows the presence of low-seismic velocity material at the surface of slabs in subduction zones. In the central Mexican subduction zone this appears as a thin (˜4 km) low-velocity zone that absorbs nearly all of the strain. The P-to-S velocity ratio as a function of S wave velocity distinguishes among the various candidate hydrous (low-strength) minerals; the thin layer in the flat-slab region is most consistent with a layer showing enrichment in talc overlying normal MORB-like gabbro. Based on available thermodynamic data for equilibria for talc, its generation at the trench is nearly impossible, and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback. The evolution of this low-strength zone has important implications for the dynamics of the slab-flattening process as well as the geochemistry of the mantle wedge and arc in central Mexico.

Kim, YoungHee; Clayton, Robert W.; Asimow, Paul D.; Jackson, Jennifer M.

2013-12-01

194

Transition from strike-slip faulting to oblique subduction: active tectonics at the Puysegur Margin, South New Zealand  

NASA Astrophysics Data System (ADS)

South of New Zealand the Pacific-Australia (PAC-AUS) plate boundary runs along the intracontinental Alpine Fault, the Puysegur subduction front and the intraoceanic Puysegur Fault. The Puysegur Fault is located along Puysegur Ridge, which terminates at ca. 47°S against the continental Puysegur Bank in a complex zone of deformation called the Snares Zone. At Puysegur Trench, the Australian Plate subducts beneath Puysegur Bank and the Fiordland Massif. East of Fiordland and Puysegur Bank, the Moonlight Fault System (MFS) represents the Eocene strike-slip plate boundary. Interpretation of seafloor morphology and seismic reflection profiles acquired over Puysegur Bank and the Snares Zone allows study of the transition from intraoceanic strike-slip faulting along the Puysegur Ridge to oblique subduction at the Puysegur Trench and to better understand the genetic link between the Puysegur Fault and the MFS. Seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 Geodynz survey, and single beam echo soundings acquired by the NZ Royal Navy. The Snares Zone is the key transition zone from strike-slip faulting to subduction. It divides into three sectors, namely East, NW and SW sectors. A conspicuous 3600 m-deep trough (the Snares Trough) separates the NW and East sectors. The East sector is characterised by the NE termination of Puysegur Ridge into right-stepping en echelon ridges that accommodate a change of strike from the Puysegur Fault to the MFS. Between 48°S and 47°S, in the NW sector and the Snares Trough, a series of transpressional faults splay northwards from the Puysegur Fault. Between 49°50'S and 48°S, thrusts develop progressively at Puysegur Trench into a decollement. North of 48°S the Snares Trough develops between two splays of the Puysegur Fault, indicating superficial extension associated with the subsidence of Puysegur Ridge. Seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the Snares Fault, and terminate at the top of the Puysegur trench slope. Between ca. 48°S and 46°30'S, the relative plate motion appears to be distributed over the Puysegur subduction zone and the strike-slip faults located on the edge of the upper plate. Conversely, north of ca. 46°S, a lack of active strike-slip faulting along the MFS and across most of Puysegur Bank indicates that the subduction in the northern part of Puysegur Trench accounts for most of the oblique convergence. Hence, active transpression in the Snares fault zone indicates that the relative PAC-AUS plate motion is transferred from strike-slip faulting along the Puysegur Fault to subduction at Puysegur Trench. The progressive transition from thrusts at Puysegur Trench and strike-slip faulting at the Puysegur Fault to oblique subduction at Puysegur Trench suggests that the subduction interface progressively developed from a western shallow splay of the Puysegur Fault. It implies that the transfer fault links the subduction interface at depth. A tectonic sliver is identified between Puysegur Trench and the Puysegur Fault. Its northwards motion relative to the Pacific Plate implies that is might collide with Puysegur Bank.

Lamarche, Geoffroy; Lebrun, Jean-Frédéric

2000-01-01

195

The 2011 Tohoku-Oki earthquake: displacement reaching the trench axis.  

PubMed

We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (M(W)) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement. PMID:22144619

Fujiwara, Toshiya; Kodaira, Shuichi; No, Tetsuo; Kaiho, Yuka; Takahashi, Narumi; Kaneda, Yoshiyuki

2011-12-01

196

GPS Application : Theoretical Analysis of Coseismic Crustal Deformation of the Subduction Zone Colombia - Ecuador  

Microsoft Academic Search

The Colombia - Ecuador trench is located on the northwestern subduction zone along the Nazca and South America plate boundary. The zone is considered tectonically complex and it is continuously under large crustal deformation. The rapid convergence rate (58mm\\/a) in the zone caused a series of four large earthquakes (Mw>7.5) in the 20th century: 1906(Mw=8.8), 1942 (Mw=7.6), 1958 (Mw=7.7) and

A. L. Ramos Barreto; S. I. Franco; A. Iglesias

2010-01-01

197

Evolving force balance during incipient subduction  

Microsoft Academic Search

Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a

Michael Gurnis; Chad Hall; Luc Lavier

2004-01-01

198

Plate coupling along the Manila subduction zone between Taiwan and northern Luzon  

NASA Astrophysics Data System (ADS)

We use GPS data, trench parallel gravity anomaly (TPGA), and bathymetry to infer plate coupling patterns along the Manila subduction zone. Using a block model and a fault geometry constrained by seismicity, we simultaneously solve for the location of Euler pole and angular velocity between the Sunda and Luzon blocks as well as the slip-deficit rate on plate interface. Our estimates show that the Euler pole between the Sunda and Luzon blocks is situated at southern Palawan near 8.3°N and 119.4°E with the angular velocity of 4.6 Myr-1. The estimated convergence rate along the Manila Trench continuously decreases southward from 91 mm/yr at the northern tip of Luzon to 55 mm/yr north of Mindoro. The inversion of GPS data reveals partially locked fault patches extending from the West Luzon Trough to the east of Scarborough Seamount chain. The slip-deficit rate in this region is in the range of 20-30 mm/yr corresponding to a coupling ratio of 0.4. However, the fault slip behavior is not well resolved near the North Luzon Trough. Based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low, we investigate a variety of TPGA-based plate coupling models assuming different scaling between TPGA values and plate coupling ratios. The TPGA-based plate coupling models offer plausible rupture scenarios which are not constrained by current GPS data. The partially locked fault zone near 15-16.5°N may be associated with the subducted Scarborough Seamount wherein oceanic floor is highly fractured. The great subduction zone earthquake propagates beneath the Scarborough Seamount seems to be unlikely. The densification of GPS network in central Luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts.

Hsu, Ya-Ju; Yu, Shui-Beih; Song, Teh-Ru Alex; Bacolcol, Teresito

2012-06-01

199

Deflection of mantle flow beneath subducting slabs and the origin of subslab anisotropy  

NASA Astrophysics Data System (ADS)

compilations of subslab shear wave splitting parameters show a mix of trench-parallel and trench-perpendicular fast directions that often directly contradict predictions from two-dimensional models of slab-entrained flow. Here we show that subslab anisotropy is consistent with three-dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow. Each model represents a specific region for which high-quality source-side shear wave splitting data are available. We compare the distribution of finite strain in the models with shear wave splitting observations, showing that both trench-parallel and trench-perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs. Subslab maximum elongation directions calculated from our models depend on a combination of geometry factors (such as slab dip angle and maximum depth), mechanical parameters (such as decoupling between the slab and the subjacent mantle), and the orientation and magnitude of the regional mantle flow.

Paczkowski, Karen; Thissen, Christopher J.; Long, Maureen D.; Montési, Laurent G. J.

2014-10-01

200

An ocean bottom seismometer study of shallow seismicity near the Mid-America Trench offshore Guatemala  

NASA Astrophysics Data System (ADS)

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, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest 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. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake-generating stresses. The data permit construction of a magnitude-duration scale, calibrated with mb magnitudes for events located by the World-Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. 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 northwest of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, 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, but we suggest that it requires further investigation.

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

1985-11-01

201

Dynamics of free subduction from 3-D boundary element modeling  

NASA Astrophysics Data System (ADS)

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.

Li, Zhong-Hai; Ribe, Neil M.

2012-06-01

202

A new model of slab tear of the subducting Philippine Sea Plate associated with Kyushu-Palau Ridge subduction  

NASA Astrophysics Data System (ADS)

We suggest that the Kyushu-Palau Ridge (KPR) plays a key role in the subduction process of the Philippine Sea Plate (PSP) and the origin of the Abu volcano in the southwestern Japan. The 3-D P-wave velocity structure was imaged to approximately 300 km beneath the Abu volcano using a large number of P-wave arrivals from local earthquakes and teleseismic events. Our results indicate that a high-velocity anomaly beneath the Abu volcano is associated with the subducting PSP; however, the anomaly is not continuous, being interrupted apparently by a low-velocity anomaly zone extending northwestwards from 80 km to great depth. The PSP appears to be tearing and then forms a 'slab window' corresponding to KPR subduction at ca. 2 Ma. The low-velocity anomaly may indicate hot upper mantle material rising through the slab window and causing partial melting both of the lower crust of the overriding plate and the oceanic crust of the subducted KPR. A new model is presented for slab tearing of the PSP associated with the subduction of the buoyant, wide and thick KPR and directional change in the motion of the plate, contributing to better understanding of the Abu volcanism.

Cao, Lingmin; Wang, Zhi; Wu, Shiguo; Gao, Xiang

2014-12-01

203

Tectonics and mechanism of a spreading ridge subduction at the Chile Triple Junction based on new marine geophysical data  

NASA Astrophysics Data System (ADS)

The Chile Triple Junction (CTJ), an RTT-type triple junction located at 46°30'S off Taitao Peninsula, the western coast of Chile, is to be remarked in that the Chile Ridge, one of the typical mid-oceanic ridges that generate oceanic plates, is subducting underneath the South American continental plate. It is well known that earthquakes occur frequently off the Chilean coast. But earthquakes never occur on and around the CTJ and the fracture zones nearby (Tilmann et al., 2008). A thick crust is usually developed below an ordinary oceanic ridge. In this case the ridge never subducts due to the thick crust supported by the buoyancy of the mantle materials, even though the ridge reaches a trench. Or the subduction of the ridge may be delayed as compared with the case of the neighbouring sea floor. The purpose of this study is to solve the problem of the mechanism of a smooth subduction of the spreading ridge at the trench and the regional tectonics around the CTJ, mainly based on the marine geophysical data collected by the recent MR08-06 cruise by R/V MIRAI (Abe et al., AGU Fall 2009) and other cruise data from National Geophysical Data Center. Isostatic equilibrium in addition to the profiles of topography and gravity across the trench were examined on both sides of the CTJ. One of the principal results of this study is that ridge axis is associated with an axial deep covered with thick sediment unlike the case of typical ridge crests. The profiles of both topography and free air anomaly at Segment-I on Chile Ridge (just before subduction at Chile Trench) show quite different patterns from those at ordinary subduction zones. In addition, the topography of the seaward side is flat with poor relief and the abrupt landside slope is attached to the trench axis. To the south of the CTJ, the area of relatively negative free air anomaly reaches from the seaward side toward the fore-arc. This is due to the weight of the thick sediment accumulation apparently supplied from the coast. The single-channel seismic (SCS) profiles across the trench axis could not describe ridge subduction in detail, maybe due to the effect of the steep accretionary zone which interferes the penetration of acoustic signal into the sub-seafloor. Both ‘Outer swell’ and ‘outer gravity high’ which are regularly observed at the trench area in the western Pacific are missing in this study area. Both are derived from elastic bending of the rigid plate just before subduction. The observed topography and gravity data at the CTJ suggest that the subducting plate (both Nazca and Antarctic plates) is still hot and quick heat consumption may take place at the trench even at the spreading ridge, which may attain the smooth subduction at the CTJ.

Matsumoto, T.; Doi, A.; Kise, S.; Abe, N.

2010-12-01

204

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

USGS Publications Warehouse

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.

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

2004-01-01

205

Ophiolites of the deep-sea trenches of the western Pacific  

SciTech Connect

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.

Chudaev, O.

1990-06-01

206

Scientific brief on the March 2011 M9 Tohoku-oki Earthquake (Japan)  

E-print Network

of Japan. The earthquake is the 4th largest mea- sured since records began in the early 20th century.1 2004 4. Japan M9.0 2011 5. Kamchatka M9.0 1952 Table 1: Largest 5 Earthquakes on record. Source USGS Subduction regions such as the east coast of Japan gen- erate the world's largest earthquakes. The 5 largest

207

Very low frequency earthquakes along the Ryukyu subduction zone  

NASA Astrophysics Data System (ADS)

A total of 1314 very low frequency earthquakes (VLFEs) were identified along the Ryukyu trench from seismograms recorded at broadband networks in Japan (F-net) and Taiwan (BATS) in 2007. The spectra of typical VLFEs have peak frequencies between 0.02 to 0.1 Hz. Among those, waveforms from 120 VLFEs were inverted to obtain their centoroid moment tensor (CMT) solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11° × 14° in latitude and longitude and at depths of 0 to 60 km. Most of the VLFEs occur on shallow thrust faults that are distributed along the Ryukyu trench, which are similar to those earthquakes found in Honshu and Hokkaido, Japan. The locations and mechanisms of VLFEs may be indicative of coupled regions within the accretionary prism or at the plate interface; this study highlights the need for further investigation of the Ryukyu trench to identify coupled regions within it.

Ando, Masataka; Tu, Yoko; Kumagai, Hiroyuki; Yamanaka, Yoshiko; Lin, Cheng-Horng

2012-02-01

208

Middle Tertiary volcanism during ridge-trench interactions in western California  

SciTech Connect

Bimodal volcanism in the Santa Maria Province of west-central California occurred when segments of the East Pacific Rise interacted with a subduction zone along the California margin during the Early Miocene (about 17 million years ago). Isotopic compositions of neodymium and strontium as well as trace-element data indicate that these volcanic rocks were derived from a depleted-mantle (mid-ocean ridge basalt) source. After ridge-trench interactions, the depleted-mantle reservoir was juxtaposed beneath the continental margin and was erupted to form basalts. It also assimilated and partially melted local Jurassic-Cretaceous sedimentary and metasedimentary basement rocks to form rhyolites and dacites. 28 refs.

Cole, R.B.; Basu, A.R. (Rochester Univ., NY (United States))

1992-10-01

209

Insights from trace element geochemistry as to the roles of subduction zone geometry and subduction input on the chemistry of arc magmas  

NASA Astrophysics Data System (ADS)

Subduction zones of continental, transitional, and oceanic settings, relative to the nature of the overriding plate, are compared in terms of trace element compositions of mafic to intermediate arc rocks, in order to evaluate the relationship between subduction parameters and the presence of subduction fluids. The continental Chilean Southern Volcanic Zone (SVZ) and the transitional to oceanic Central American Volcanic Arc (CAVA) show increasing degrees of melting with increasing involvement of slab fluids, as is typical for hydrous flux melting beneath arc volcanoes. At the SVZ, the central segment with the thinnest continental crust/lithosphere erupted the highest-degree melts from the most depleted sources, similar to the oceanic-like Nicaraguan segment of the CAVA. The northern part of the SVZ, located on the thickest continental crust/lithosphere, exhibits features more similar to Costa Rica situated on the Caribbean Large Igneous Province, with lower degrees of melting from more enriched source materials. The composition of the slab fluids is characteristic for each arc system, with a particularly pronounced enrichment in Pb at the SVZ and in Ba at the CAVA. A direct compositional relationship between the arc rocks and the corresponding marine sediments that are subducted at the trenches clearly shows that the compositional signature of the lavas erupted in the different arcs carries an inherited signal from the subducted sediments.

Wehrmann, Heidi; Hoernle, Kaj; Garbe-Schönberg, Dieter; Jacques, Guillaume; Mahlke, Julia; Schumann, Kai

2014-10-01

210

SUBDUCTION ZONES Robert J. Stern  

E-print Network

to andesites [Rieder et al., 1997] in the ancient southern highlands of Mars suggest that plate tectonics of convergent plate mar- gins and are the geodynamic system that builds island arcs. Excess density Zone, subduction, tectonics Citation: Stern, R. J., Subduction zones, Rev. Geophys., 40(4), 1012, doi

Stern, Robert J.

211

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

NASA Astrophysics Data System (ADS)

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.

Morgan, J. P.; Vannucchi, P.

2012-12-01

212

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

NASA Astrophysics Data System (ADS)

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.

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

2003-04-01

213

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

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

214

Flexural characteristics of pre- and post-subduction lithosphere - the influence of large-scale plate loads  

NASA Astrophysics Data System (ADS)

Global studies show an association between seamount subduction and rapid changes in subduction zone characteristics. Observations of seismic and volcanic quiescence, uplift and deformation of the overriding plate, and variations in back-arc extension rate have all been documented near subducting seamounts. Modelling of plate interactions suggests that it is the rate and angle of convergence that determines the style and extent of deformation observed. However, these studies do not include along-strike variations in the structure of the overriding plate or density variations in the subducting plate. Nor do they address the question of whether deformation is a flexural response to bending of a loaded plate or a plastic response due to crustal thickening during convergence. The collision zone between the Louisville Ridge seamount chain and the Tonga-Kermadec trench at 26°S is associated with changes in subduction zone seismogenesis, crustal structure, and trench strike and depth. This makes it an ideal locality to determine not only the mechanical properties of both the down-going and overriding plates, but also the effect of large-scale plate topography on subduction zone processes. Approximately 1800 km of coincident multi-channel and wide-angle seismic data, coupled with gravity, magnetic and swath bathymetry data were acquired throughout the Tonga-Kermadec-Louisville Ridge region between April-June 2011 during a cruise aboard the R/V Sonne (SO215), the second leg of the TOTAL (Tonga Thrust earthquake asperity at Louisville Ridge) project. Profiles follow a trend both parallel and perpendicular to the trench, and oblique to and along the Louisville Ridge. The along and across ridge profiles were designed to determine the background structure of the down-going plate, the response to magmatic and topographic loading and the volume and extent of any crustal underplate. The across trench profiles were designed to determine the structure of both the pre- and post-collisional crust proximal to the trench, to the south and north of the collision zone respectively. Variations in seismic velocity structure from the pre-to-post collision zones elucidate the degree of crustal deformation due to faulting, and alteration due to fluid ingress and serpentinisation. The resulting crustal models, coupled with those for profiles from the first leg of the TOTAL project (cruise SO195), are used to provide the crustal structure reference for flexural modelling of the system, to determine how the seamounts are deformed on subduction, and what role they play in the region's seismogenesis, in effect testing the Kelleher and McCann hypothesis that bathymetric features control aspects of arc seismicity.

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

2012-12-01

215

Subduction in the Southern Caribbean  

NASA Astrophysics Data System (ADS)

The southern Caribbean is bounded at either end by subduction zones: In the east at the Lesser Antilles subduction zone the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west under the Southern Caribbean Deformed Belt accretionary prism, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system. A variety of seismic probes identify where the two plates tear as they begin to subduct (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Masy et al, 2009). The El Pilar system forms at the southeastern corner of the Antilles subduction zone by the Atlantic tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. In northwestern South America the Caribbean plate tears, its southernmost element subducting at shallow angles under northernmost Colombia and then rapidly descending to transition zone depths under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab produces the Merida Andes, the Perija, and the Santa Marta ranges. The southern edge of the nonsubducting Caribbean plate underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009). We infer that the underthrust Caribbean plate supports the coastal mountains, and controls continuing deformation.

Levander, A.; Schmitz, M.; Bezada, M.; Masy, J.; Niu, F.; Pindell, J.

2012-04-01

216

Plate deformation at the transition between collision and subduction: insights from 3D thermo-mechanical laboratory experiments  

NASA Astrophysics Data System (ADS)

3-D thermo-mechanical laboratory experiments of arc-continent collision investigate plate deformation at the transition between collision and subduction. Deformation in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. The largest along-strike gradient of trench-perpendicular compression produced by a passive margin turning by 90 degrees does not generate sufficiently localized shear strain in the transition zone to cause a strike-slip system. This is because of the fast propagation of lithosphere failure in the arc area. Deformation is thus continuous along-strike, but the deformation mechanism is three-dimensional and progressive structural variations arise because the coupling between neighboring segments induces either advanced or delayed failure of the arc lithosphere and passive margin. During the initial stage of collision, the accretionary wedge is partially subducted, the interplate zone is lubricated, and shear traction drops. Thus large convergence obliquity does not produce a migrating fore-arc sliver. Instead, the fore-arc motion is due to the pressure force generated by subduction of the buoyant continental crust. It follows that convergence obliquity does not yield trench-parallel deformation of the fore-arc and its influence on the collision process is limited. However, convergence obliquity may have shaped the active margin during the stage of oceanic subduction stage, prior to collision, and inherited structures may impact the propagation mechanism.

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

2012-04-01

217

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

NASA Astrophysics Data System (ADS)

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.

ten Brink, Uri S.; López-Venegas, Alberto M.

2012-05-01

218

Difference in the maximum magnitude of interplate earthquakes off Shikoku and in the Hyuganada region, southwest Japan, inferred from the temperature distribution obtained from numerical modeling. - The proposed Hyuganada triangle -  

NASA Astrophysics Data System (ADS)

To clarify differences in the maximum magnitude of interplate earthquakes offshore of Shikoku ( ? M 8) and in the western adjacent Hyuganada region ( ? M 7.5), southwest Japan, where the oceanic Philippine Sea (PHS) plate subducts beneath the continental Amuria plate along the Nankai Trough, we calculated the temperature distribution for the plate interface using a three-dimensional thermal convection model. We attribute the observed very low heat flow values (less than approximately 45 mW/m 2) in the southern part of the Hyuganada region to the subduction of the Kyushu-Palau Ridge, which is presumably older than the adjacent Shikoku Basin to the east. Trial and error methods suggest an age difference of about 20 Myr between these two areas. The heat flow calculated from the temperature distribution fits the observed heat flow in the Hyuganada region well. The results show that Hyuganada earthquakes may take place in the newly proposed "Hyuganada triangle," bounded by a southeastern isotherm of about 200 °C on the plate interface, a northwestern boundary between the upper surface of the subducting PHS plate and the continental Moho discontinuity, a northern isotherm of about 350 °C, and a northeastern margin-normal zone with a possible barrier or large friction parameter L. Interplate earthquakes in such a limited seismogenic zone may reach a maximum magnitude of M 7.5. Our results also suggest that a seismic gap seaward of the Hyuganada region may be caused by stable sliding arising from low temperature, resulting in a deeper updip limit than that offshore of Shikoku. The seismic gap offshore southwest of southern Kyushu along the Ryukyu Trench may result from the overlapping of two stable sliding regimes on the plate interface: one with low temperature at shallower depths caused by the subduction of the old PHS plate and the other a serpentinized mantle wedge at deeper depths.

Yoshioka, Shoichi

2007-11-01

219

Fluid processes in subduction zones.  

PubMed

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

Peacock, S A

1990-04-20

220

Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducting sediments  

Microsoft Academic Search

In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting

G. Shimoda; Y. Tatsumi; S. Nohda; K. Ishizaka; B. M. Jahn

1998-01-01

221

Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline.  

PubMed

The building of the Andes results from the subduction of the oceanic Nazca plate underneath the South American continent. However, how and why the Andes and their curvature, the Bolivian orocline, formed in the Cenozoic era (65.5?million years (Myr) ago to present), despite subduction continuing since the Mesozoic era (251.0-65.5?Myr ago), is still unknown. Three-dimensional numerical subduction models demonstrate that variations in slab thickness, arising from the Nazca plate's age at the trench, produce a cordilleran morphology consistent with that observed. The age-dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate, causing it to thicken. Thus, subducting older Nazca plate below the Central Andes can explain the locally thickened crust and higher elevations. Here we demonstrate that resultant thickening of the South American plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the Bolivian orocline. Additionally, the varying forcing along the margin allows stress belts to form in the upper-plate interior, explaining the widening of the Central Andes and the different tectonic styles found on their margins, the Eastern and Western Cordilleras. The rise of the Central Andes and orocline formation are directly related to the local increase of Nazca plate age and an age distribution along the margin similar to that found today; the onset of these conditions only occurred in the Eocene epoch. This may explain the enigmatic delay of the Andean orogeny, that is, the formation of the modern Andes. PMID:22113613

Capitanio, F A; Faccenna, C; Zlotnik, S; Stegman, D R

2011-12-01

222

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

NASA Astrophysics Data System (ADS)

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.

Reichert, Chr.; Spoc Scientific Shipboard Party, The

223

Seismic structure of subducted Philippine Sea plate beneath the southern Ryukyu arc by receiver function and local earthquakes tomography  

NASA Astrophysics Data System (ADS)

Seismic coupling of the Ryukyu subduction zone is assumed to be weak from the lack of historical interplate large earthquakes. However, recent investigation of repeating slow slip events (Heki & Kataoka, 2008), shallow low frequency earthquakes (Ando et al., 2012), and source of 1771 Yaeyama mega-tsunami (Nakamura, 2009), showed that the interplate coupling is not weak in the south of Ryukyu Trench. The biannually repeating SSEs (Mw=6.5) occur at the depth of 20-40 km on the upper interface of the subducted Philippine Sea plate beneath Yaeyama region, where earthquake swarm occurred on 1991 and 1992. To reveal the relation among the crustal structure, earthquake swarms, and occurrence of slow slip events (SSE), local earthquake tomography and receiver function (RF) analysis was computed in the southwestern Ryukyu arc. A tomographic inversion was used to determine P and S wave structures beneath Iriomote Island in the southwestern Ryukyu region for comparison with the locations of the SSE. The seismic tomography (Thurber & Eberhart-Phillips, 1999) was employed. The P- and S- wave arrival time data picked manually by Japan Meteorological Agency (JMA) are used. The 6750 earthquakes from January 2000 to July 2012 were used. For the calculation of the receiver function, the 212 earthquakes whose magnitudes are over 6.0 and epicentral distances are between 30 and 90 degrees were selected. The teleseicmic waveforms observed at two short-period seismometers of the JMA, and one broadband seismometer of F-net of National Research Institute for Earth Science and Disaster Prevention were used. The water level method (the water level is 0.01) is applied to original waveforms. Assuming that each later phase in a RF is the wave converted from P to S at a depth, I transformed the time domain RF into the depth domain one along each ray path in a reference velocity model. The JMA2001 velocity model is used in this study. The results of tomography show that the low Vp and high Vp/Vs anomalies are distributed along the hypocenters in the subducted slab. The plate interface is about 10 km above the slab earthquakes from the trace of negative RF amplitude. The slab earthquakes are distributed along the trace of positive RF amplitude. Therefore the slab earthquakes occur near the oceanic Moho of the PHS. The fault depth of the SSEs corresponds to the plate interface within 5 km. The fault-planes of the SSE are located above the low Vp and high Vp/Vs zone. Assuming that the difference between high Vp/Vs and low Vp/Vs originates to the fluid contents, this would be interpreted that the fluids from the subducted oceanic crust cannot be transported upward and is trapped at the plate interface. The observed strong S-wave reflector (Nakamura, 2001) in the upper interface of the subducted plate also supports the idea. The top of the faults of the SSEs connects to the cluster of earthquake swarms in the lower crust. This suggests that the trapped fluids are transported upward along the faults, accumulates in the lower crust, and induce the swarm of micro-earthquakes in the lower crust.

Nakamura, M.

2012-12-01

224

Evolution of a Subduction Zone  

NASA Astrophysics Data System (ADS)

The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences the subduction of the oceanic crust in terms of subduction velocity and subduction angle over time. We develop scaling laws combining the subduction velocity and angle depending on the mantle environment (and thus time). These laws can then be applied to continental growth simulations with 1D parameterized models (Höning et al., in press) or 2D/3D subduction zone simulations at specific geological times (using the correct subduction zone setting). References: Quinquis, M. et al. (in preparation). A comparison of thermo-mechanical subduction models. In preparation for G3. Noack, L., Van Hoolst, T., Dehant, V., and Breuer, D. (2013). Relevance of continents for habitability and self-consistent formation of continents on early Earth. XIII International Workshop on Modelling of Mantle and Lithosphere Dynamics, Hønefoss, Norway, 31. Aug. - 5. Sept. 2013. Höning, D., Hansen-Goos, H., Airo, A., and Spohn, T. (in press). Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage. Planetary and Space Science.

Noack, Lena; Van Hoolst, Tim; Dehant, Veronique

2014-05-01

225

SOURCE MODELING OF SUBDUCTION-ZONE EARTHQUAKES FOR LONG-PERIOD GROUND MOTION VALIDATION  

Microsoft Academic Search

The national seismic hazard map for Japan indicates 30-year probability in the Tokyo metropolitan area to be controlled by megathrust earthquakes along the Sagami and Nankai troughs of the Philippine Sea plate. This indicates that source modeling and realistic ground motion prediction for distant subduction-zone earthquakes are quite important. We have proposed two types of source modeling for four major

Hiroe Miyake; Kazuki Koketsu

226

Seismological constraints on the gabbro-eclogite transition in subducted oceanic crust  

Microsoft Academic Search

Dispersion of seismic waves travelling along the strike of the subducted Pacific plate beneath Tonga-Kermadec shows evidence for a high-velocity layer of crustal thickness extending to a depth of at least 450 km. The opposite dispersion, with low frequencies arriving first, is observed in Japan and has been taken, together with inferences from converted phases, as evidence of a thin,

David Gubbins; Andy Barnicoat; Joe Cann

1994-01-01

227

Long-term seismogenic process for major earthquakes in subduction zones  

Microsoft Academic Search

A qualitative physical process for the long-term seismogenesis of major earthquakes in subduction zones is proposed on the basis of quantitative empirical evidence that swarms, mainshocks and aftershocks are closely related phenomena. The relations, which have been identified in the comprehensive, long-term catalogues of New Zealand and Japan, represent swarms as predictors of mainshocks with respect to location, time and

F. F Evison; D. A Rhoades

1998-01-01

228

The seismic cycle at subduction thrusts: a parameter study  

NASA Astrophysics Data System (ADS)

In this study, we model long-term seismic cycles at subduction thrusts. The understanding of this process in nature is restricted due to three interrelated problems: (i) the limited spatiotemporal availability of direct observations of the (ii) mechanically and geometrically variable subduction zones, which (iii) are each at different stages of the seismic cycle, without completing one while direct observations took place. Numerical modeling has the potential to overcome these problems and, hence, contribute to improve long-term hazard assessment in subduction zones. We use the 2D continuum visco-elasto-plastic, seismo-mechanical numerical model of a laboratory-scaled subduction zone, consisting of a gelatin wedge underthrusted by a rigid plate. In a frictional boundary layer above the rigid plate, which is governed by strongly rate-dependent friction, the velocity-weakening seismogenic zone (SeZ) is surrounded at its up- and downdip limits by velocity-strengthening areas. This numerical modeling approach to model seismic cycles was validated (van Dinther et. al., 2013) against an innovated laboratory model (Corbi et. al., 2013). This follow-up parameter study investigates the role of subduction velocity and geometrical properties of the SeZ on the recurrence behavior and source properties of spontaneously evolving events with complex rupture patterns. Results are compared to natural observations and to laboratory models performed under comparable experimental conditions. In the numerical models, events nucleate predominantly near the limits of the SeZ. In a wider and shallower SeZ, a preference exists for nucleation near the downdip limit. The recurrence interval between the characteristic large events of each model setup decreases non-linearly with subduction velocity. Both the maximum source properties and the recurrence interval between the characteristic large events increases with the SeZ width. In contrast, the subduction velocity has only a minor impact on the maximum source properties. Especially in a SeZ close to the trench, we observe accelerations in the velocity-strengthening material for ruptures reaching the trench, leading to large peak slip velocities and coseismic slip. Detailed studies of specific models provide additional insights into the existing rupture styles. Sub-critical ruptures, and decaying, growing as well as back-propagating pulses of different sizes change the stress distribution inside the SeZ during a seismic cycle. Thereby, these smaller events influence the nucleation location and propagation of subsequent ruptures. When the stress in the SeZ reaches a critical state, the characteristic large events develop as crack-like ruptures or ruptures in the transitional regime between pulses and cracks. These kind of events lead to a significant stress drop across the entire SeZ and, hence, are followed by a period of relative quiescence and again smaller events. The wider the SeZ, the larger are the number and range of smaller events, which occur between the recurring characteristic large events.

Herrendoerfer, R.; van Dinther, Y.; Gerya, T.; Dalguer, L. A.; Corbi, F.; Funiciello, F.

2013-12-01

229

Back-arc strain in subduction zones: Statistical observations versus numerical modeling  

NASA Astrophysics Data System (ADS)

Recent statistical analysis by Lallemand et al. (2008) of subduction zone parameters revealed that the back-arc deformation mode depends on the combination between the subducting (vsub) and upper (vup) plate velocities. No significant strain is recorded in the arc area if plate kinematics verifies vup = 0.5 vsub - 2.3 (cm/a) in the HS3 reference frame. Arc spreading (shortening) occurs if vup is greater (lower) than the preceding relationship. We test this statistical law with numerical models of subduction, by applying constant plate velocities far away from the subduction zone. The subducting lithosphere is free to deform at all depths. We quantify the force applied on the two converging plates to sustain constant surface velocities. The simulated rheology combined viscous (non-Newtonian) and brittle behaviors, and depends on water content. The influence of subduction rate vs is first studied for a fixed upper plate. After 950 km of convergence (steady state slab pull), the transition from extensional to compressive stresses in the upper plate occurs for vs ˜ 1.4 cm/a. The effect of upper plate velocity is then tested at constant subduction rate. Upper plate retreat (advance) with respect to the trench increases extension (compression) in the arc lithosphere and increases (decreases) the subducting plate dip. Our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere, even if the modeled law is shifted toward higher rates of upper plate retreat, using our set of physical parameters (e.g., 100 km thick subducting oceanic plate) and short-term simulations. Our results make valid the choice of the HS3 reference frame for assessing plate velocity influence on arc tectonic regime. The subduction model suggests that friction along the interplate contact and the mantle Stokes reaction could be the two main forces competing against slab pull for upper mantle subductions. Besides, our simulations show that the arc deformation mode is strongly time dependent.

Arcay, D.; Lallemand, S.; Doin, M.-P.

2008-05-01

230

Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?  

NASA Astrophysics Data System (ADS)

A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.

Lin, Jing-Yi; Sibuet, Jean-Claude; Hsu, Shu-Kun; Wu, Wen-Nan

2014-12-01

231

Tears or thinning? Subduction structures in the Pacific plate beneath the Japanese Islands  

NASA Astrophysics Data System (ADS)

The nature of a subduction zone at depth is affected by the evolution of its tectonic system, and the geometry of the trench line can change over time due to slab roll-back or the arrival of a distinctive feature with the incoming oceanic lithosphere. The configuration of the plate has to accommodate such changes with buckling, thinning or the formation of tears depending on the rate of influx to the trench. Tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones. A good example is provided by Pacific plate subduction beneath the Japanese Islands. A horizontal tear in the plate below 300 km depth can be recognised at the southern end of the Izu-Bonin arc associated with the change in slab morphology to the much steeper Mariana arc. Beneath southern Honshu a break in the fast wavespeeds associated with the Pacific plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone. In the north, close to the Hokkaido bend in the subduction zone, the reduction in the shear wavespeed anomaly is just as dramatic, but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab. The thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly. The various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario. Not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone.

Kennett, B. L. N.; Furumura, T.

2010-05-01

232

Subduction Input Flux of Nitrogen in Altered Oceanic Basalt  

NASA Astrophysics Data System (ADS)

We have employed sealed-tube, carrier-gas-based methods newly developed in our laboratory to examine the nitrogen (N) concentration and isotopic composition of altered oceanic crust (AOC). Knowledge of the subduction input flux of N in AOC is critical in any attempt to mass-balance N across arc-trench systems, either globally or on an individual-margin basis. Uncertainties (expressed as 1? for >3 replicate analyses of both internal silicate standards and individual AOC samples) are less than 5% for N concentrations and on the order of 0.15‰ ?15Nair for samples with >5 ppm N. At current blank levels (overall system blank of 3.8±0.2 nmol N2 with a ?15N value of -7.3±0.4‰), uncertainty in ?15N increases to about 0.6 ‰ for samples with 1-2 ppm N. Preliminary analyses of 33 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Philippine Sea (with a larger number of samples from Site 801 outboard of the Mariana trench) indicate N concentrations of 1.3 to 17.8 ppm and ?15N of -5.1 to +8.3‰. Combined, the data for all cores document a crude direct relationship between N concentration and isotopic composition, with the lowest-N samples characterized by mantle-like ?15N values of approx. -5 ‰, and more N-rich samples yielding progressively increasing ?15N. This relationship presumably reflects high- to low-temperature alteration on the seafloor. For the Izu-Bonin-Mariana trench, AOC samples from Leg 129 Site 801 with 1.3 to 17.8 ppm N have ?15N values ranging from -5 to +1.2‰ but most fall between -1.5 and +1.2‰. For Site 801 (thus far 14 samples representing core depths of 450-600 mbsf, within the uppermost part of the basement section), there is no obvious correlation between N and K2O concentrations, indicating that the siting of N is more complicated than simple residence/sequestration in potassic alteration phases and likely reflects complex superposition of alteration effects produced over a large temperature range. Three samples from Site 417A/D contain 11 to 14 ppm N and show wide variation in ?15N (+1 to +8.3‰), whereas a small number of samples thus far analyzed for Site 1149 have extremely low N concentration (<3 ppm) and ?15N between -5 and 0‰. If altered basalt in the upper 1 km of the oceanic crust subducting along the Izu-Bonin-Mariana margin contains an average of 10 ppm N and the remaining 5 km of less-altered oceanic crust contains an average of 1 ppm N, subducting oceanic crust transfers 2.1×106 g/km N annually into that trench. This N input flux in AOC is comparable to that in seafloor sediments subducting into the same margin (for the sediments, 2.5×106 g/km N annually; see Sadofsky and Bebout, 2004, G-cubed), demonstrating that the input of N in AOC, and its isotopic consequences, must be considered in any assessment of convergent margin N flux. Our future work will include more complete coverage of the depth range sampled for Sites 801 and 1149 and will include analyses of various AOC composites (e.g., Site 801; see Plank et al., 2000, ODP Leg 185 Init. Rept.), in addition to work to further reduce the analytical blank.

Li, L.; Bebout, G. E.; Idleman, B. D.

2005-12-01

233

Interplate coupling along segments of the Central America Subduction zone  

NASA Astrophysics Data System (ADS)

We analyzed 5 major earthquakes that occurred during 1992 to 2012 in a segment of the Central America subduction zone along the coasts of Guatemala and El Salvador. These events include 1992/09/02 (Mw 7.7), 1993/09/10 (Mw 7.2), 2001/01/13 (Mw 7.7), 2012/08/27 (Mw 7.3) and 2012/11/07 (Mw 7.3). We derived the asperities of these earthquakes using two completely independent methods of body-waveform inversion and a gravity-derived measure, Trench Parallel Bouguer Anomaly (TPBA). Using TPBA we discuss the status of interplate coupling along the segment and interpret each of the major earthquakes as a piece of the governing rupture process. We delineate the critical unbroken asperities along the segment that will likely generate great earthquake(s) in the future.

Zarifi, Zoya; Raeesi, Mohammad; Atakan, Kuvvet

2013-04-01

234

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

Microsoft Academic Search

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

James L. Goedert; Richard L. Squires

1990-01-01

235

A global outer-rise/outer-trench-slope (OR/OTS) earthquake study  

NASA Astrophysics Data System (ADS)

Using improved seismic, bathymetric, satellite gravity and other geophysical data, we investigated the seismicity patterns and focal mechanisms of earthquakes in oceanic lithosphere off the trenches of the world that are large enough to be well recorded at teleseismic distances. A number of prominent trends are apparent, some of which have been previously recognized based on more limited data [1], and some of which are largely new [2-5]: (1) The largest events and the highest seismicity rates tend to occur where Mesozoic incoming plates are subducting at high rates (e.g., those in the western Pacific and the Banda segment of Indonesia). The largest events are predominantly shallow normal faulting (SNF) earthquakes. Less common are reverse-faulting (RF) events that tend to be deeper and to be present along with SNF events where nearby seamounts, seamount chains and other volcanic features are subducting [Seno and Yamanaka, 1996]. Blooms of SNF OR/OTS events usually occur just after and seaward of great interplate thrust (IPT) earthquakes but are far less common after smaller IPT events. (2) Plates subducting at slow rates (<20 mm/a) often show sparse OR/OTS seismicity. It is unclear if such low activity is a long-term feature of these systems or is a consequence of the long return times of great IPT earthquakes (e.g., the sparse OR/OTS seismicity before the 26 December 2004 M9.2 Sumatra earthquake and many subsequent OR/OTS events). (3) OR/OTS shocks are generally sparse or absent where incoming plates are very young (<20 Ma) (e.g., Cascadia, southern Mexico, Nankai, and South Shetlands). (4) Subducting plates of intermediate age (20 to about 65 Ma) display a diversity of focal mechanisms and seismicity patterns. In the Philippines, NE Indonesia, and Melanesia, bands of reverse faulting events occur at or near the trench and SNF earthquakes are restricted to OR/OTS sites further from the trench. (5) Clustering of OR/OTS events of all types commonly occurs where seamount chains, volcanic ridges, or volcanic plateaus enter OR/OTS regions (e.g., the Louisville Ridge in Tonga, the Juan Fernandez Ridge in Chile, the Ninety East Ridge in Sumatra, and the D’Entrecastaux Ridge in Vanuatu).

Wartman, J. M.; Kita, S.; Kirby, S. H.; Choy, G. L.

2009-12-01

236

Subduction of the Rivera Plate Beneath the Jalisco Block as Imaged by Magnetotelluric Data  

NASA Astrophysics Data System (ADS)

Two magnetotelluric profiles perpendicular to the trench provide information on the subduction of the Rivera plate under the Jalisco Block (JB). The geometry of the subducting slab is inferred by the anomalous conductor on the top of the profile in the central part of the JB. High-conductivity zones (< 50 ohm-m) at depths shallower than 10 km are associated to dewatering of the oceanic crust below the accretion prism in the SW portion of the profile. Away from the coast, observed upper crustal conductors (< 10 km) are interpreted as partial melt related to the Central Jalisco Volcanic Lineament. The source of the crustal conductor in the central part of the MT profile, ~150 km inland and down to depths of 40 km is interpreted as a region of interconnected fluids associated with the metamorphic dehydration of the oceanic plate. Contrasting resistivity at the mantle wedge at depths below 40 km suggest to us that hot mantle material may be migrating upwards, mixing with dehydration reaction products. Across Bahia de Banderas fault (BBF) zone subduction appears to stop or to occur closer to the trench at a steeper angle. The conductivity image at the NW edge of JB reveals no downwards dipping plate but an extended conductor apparently rising from depths > 40 km. Our results supports the mantle upwelling theory as an explanation to the reported 1.5 km uplift of the central part of the Jalisco Block, and the Rivera plate discontinuity across the BBF zone as suggested by seismicity data.

Alvarez, R.; Corbo, F.; Arzate, J.

2013-05-01

237

A seismological constraint on the age of a subducting slab: the Huatung basin offshore Taiwan  

NASA Astrophysics Data System (ADS)

At the northwestern corner of the Philippine basin, collision and subduction are taking place simultaneously as the Philippine Sea plate is obliquely subducting beneath the Ryukyu trench and NE Taiwan. What is engaging in these processes is the Huatung basin (HB) lithosphere, a small piece of oceanic lithosphere which, unlike the rest of the Philippine Sea plate, is controversial in its age and structure. Because certain ages of lithosphere correspond to certain overall velocity structures, we examine how old the subducting slab of the HB has to be to satisfy seismological observations. We select from broadband seismic networks on Taiwan a rough linear array that points to the events in the Kuril trench region, rendering a slab dipping towards the upcoming P wave field. The slab thus defocuses seismic energy and produces an amplitude low along the array with magnitude and spread controlled by the age of the slab. We employ a 2D finite-difference waveform technique and experimented with 2 types of slab models with various ages: a simplistic conduction model and a high-resolution slab-wedge convection model. The older and thicker the slab, the more widely the predicted amplitude low spreads. Comparison with the observations indicates that the best slab ages fall into 20-50 Ma. This is at odds with the 125 Ma Ar-Ar dating model. Now the issue is not how to make the chronologically old lithosphere seismologically young, but why those basaltic rock samples dated to be old are located on the HB.

Chang, Y.; Kuo, B.

2010-12-01

238

Subduction in the southern Caribbean: Images from finite-frequency P wave tomography  

NASA Astrophysics Data System (ADS)

The eastern boundary of the Caribbean plate is marked by subduction of the Atlantic under the Lesser Antilles. The southeastern plate boundary is characterized by a strike-slip margin, while different configurations of subduction of the southwest Caribbean under South America have been proposed. We investigate the slab geometry in the upper mantle using multiple-frequency, teleseismic P wave tomography. Waveforms from P and PKPdf phases from 285 (Mb > 5.0) events occurring at epicentral distances from 30° to 90° and greater than 150° were bandpass filtered and cross-correlated to obtain up to three sets of delay times for each event. The delay times were inverted using approximate first Fresnel zone sensitivity kernels. Our results show the subducting Atlantic slab, as well as a second slab in the west of the study area that we interpret as a subducting fragment of the Caribbean plate. Both slabs have steep dips where imaged and can be traced to depths greater than 600 km. These results are consistent with transition zone boundary topography as determined by receiver function analysis. The Atlantic slab extends continent-ward south of the plate bounding strike-slip margin. We interpret this extension as continental margin lithospheric mantle that is detaching from beneath South America and subducting along with the oceanic Atlantic slab. The steep subduction of the Caribbean occurs ˜500 km landward from the trench, implying an initial stage of shallow subduction as far to the east as the Lake Maracaibo-Mérida Andes region, as has been inferred from intermediate depth seismicity.

Bezada, M. J.; Levander, A.; Schmandt, B.

2010-12-01

239

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

NASA Astrophysics Data System (ADS)

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.

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

2012-12-01

240

Deep 'Stone Soup' Trenching by Phoenix  

NASA Technical Reports Server (NTRS)

Digging by NASA's Phoenix Mars Lander on Aug. 23, 2008, during the 88th sol (Martian day) since landing, reached a depth about three times greater than in any trench Phoenix has excavated. The deep trench, informally called 'Stone Soup' is at the borderline between two of the polygon-shaped hummocks that characterize the arctic plain where Phoenix landed.

The lander's Surface Stereo Imager took this picture of Stone Soup trench on Sol 88 after the day's digging. The trench is about 25 centimeters (10 inches) wide and about 18 centimeters (7 inches) deep.

When digging trenches near polygon centers, Phoenix has hit a layer of icy soil, as hard as concrete, about 5 centimeters or 2 inches beneath the ground surface. In the Stone Soup trench at a polygon margin, the digging has not yet hit an icy layer like that.

Stone Soup is toward the left, or west, end of the robotic arm's work area on the north side of the lander.

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.

2008-01-01

241

The fate of salt in the Cyprus subduction zone  

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

242

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

SciTech Connect

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.

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

1990-12-01

243

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

SciTech Connect

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.

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

1992-01-01

244

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

NASA Astrophysics Data System (ADS)

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).

Nishikawa, T.; Ide, S.

2013-12-01

245

Seismic Imaging of Flat Slab Subduction: Examples from the Pampean Flat Slab Region (Invited)  

NASA Astrophysics Data System (ADS)

The Nazca-South American subduction system extends along the western margin of South America for ~5500 km and exhibits significant variations in the dip of the descending plate, as defined by slab earthquakes and upper-plate deformation, despite minor differences in slab age and rate of subduction. At ~30°S, the Nazca plate subducts to ~100 km depth before assuming a sub-horizontal geometry for over 300 km inland beneath the western part of the Sierras Pampeanas of west-central Argentina. The flat-slab region in Argentina is characterized by an absence of modern arc volcanism and the presence of active basement cored uplifts of the Sierras Pampeanas up to 600 km from the trench. The subduction of the Juan Fernandez Ridge is often cited to explain the presence of this flat-slab geometry. We used data from temporary broadband seismic deployments to focus on this region in order to image the flat slab and overriding lithosphere. We combine body and surface-wave tomography, receiver functions, shear-wave splitting, and earthquake locations with published geologic data to improve our understanding of flat-slab subduction. The slab has a high rate of seismicity that correlates to the subducted Juan Fernandez Ridge suggesting that the slab is dehydrating and releasing water into the overlying plate. The shear-wave velocity model, determined from the joint inversion of ambient-noise and earthquake-generated surface wave data, indicates that velocities in the upper part of the slab increase to the east, consistent with initial hydration and gradual dewatering of the oceanic lithosphere. The mantle lithosphere above the flat slab has shear-wave velocities that decrease to the east, consistent with the gradual hydration of this region. Some of the lowest shear-wave velocities occur above where the slab begins to re-subduct into the mantle between 67-65°W. Shear-wave splitting results suggest relatively smoothly-varying strain across the entire flat-slab region, largely consistent with expected shear due to a combination of relative plate motion and pressure effects due to the varying slab geometry. The combination of different seismic techniques allows us to investigate the Nazca-South American subduction zones in greater detail than possible with any individual technique. Results highlight the complexity of subduction systems and show that hydration of the slab and overlying mantle wedge have a profound impact on the behavior of subduction systems

Porter, R. C.; Zandt, G.; Beck, S. L.; Anderson, M. L.; Gilbert, H. J.; Linkimer, L.; Alvarado, P. M.

2013-12-01

246

Subduction and interplate seismicity: new insights from statistical analysis of natural data and analog-numerical modeling  

NASA Astrophysics Data System (ADS)

Subduction zones are the site of the largest and most dangerous seismic events occurring on the Earth. The interface of converging plates is comprised of major fault zones where most of the total seismic moment is released due to the global seismicity. Despite first order similarities between convergent margins, the statistical distribution of interplate seismic activity on different subduction zones is considerably diverse: some regions are characterized by the occurrence of megathrust earthquakes while others show only minor seismic activity, with moderate-sized events. Determining the causes of this variability is challenging. While essential features characterizing the behavior of the subduction thrust faults are known, it is still difficult to merge them in a single, comprehensive picture. This is mainly related to the lack of direct observables (i.e. subduction thrust faults are not readily accessible developing in the deeper crust, in the offshore domain) and to a short (i.e. limited to the last century) instrumental seismic record. Here we present the main results of an interdisciplinary study realized in the framework of the ESF (European Science Foundation) - EURYI project 'Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modeling'. This project aims to improve the current understanding of the physics of subduction zone thrust fault earthquakes, analyzing and gathering a) a wide range of geometric, kinematic and seismological data on current subduction zones and related statistical analysis and b) results of innovative laboratory and numerical models. In particular, our results allow us to identify many interesting features characterizing interplate earthquakes, for example; 1) the key role played by the velocity of subduction in defining both the geometry and the mechanical behavior of the subduction thrust fault; 2) the competing role played by size and distribution of protrusions of the plate interface (which determine frictional contact properties of the subduction thrust fault and hence affect its seismogenic behavior; 3) that events with Mw ?8.5 preferentially occur in the vicinity of slab edges, where the upper plate is continental and the back-arc strain is neutral. Trench fill ticker than 1 km seems to have only a secondary effect; 4) the existence of different frequency-magnitude distributions across the trenches. This multidisciplinary research has a two-fold outcome. From a scientific point of view, it improves our current knowledge on the thrust fault earthquakes along subduction zones, highlighting the influencing factors. From a social point of view, our results contribute to improve the interplate earthquake forecasting capability.

Funiciello, F.; Corbi, F.; Heuret, A.; van Dinther, Y.; Presti, D.; Marzocchi, W.; Sandri, L.; Moroni, M.; Gerya, T.; Mai, P. M.; Dalguer, L. A.; Lallemand, S.; Piromallo, C.; Conrad, C. P.; Faccenna, C.; Seismological Section Of The Euryi (European Young Investigator Award) Team

2011-12-01

247

Plutonism, oblique subduction, and continental growth: An example from the Mesozoic of California  

SciTech Connect

Major episodes of Mesozoic plutonism in California correlate with periods of oblique subduction and trench-parallel transport of western California along intrabatholithic faults. Major episodes of plutonism occurred in the Late Jurassic, during left-oblique convergence, and in the mid-Cretaceous, during right-oblique convergence. In contrast, a conspicuous lull in plutonism (but continuation of volcanism) in the earliest Cretaceous coincides with a time when the North America-Farallon convergence vector, although large in magnitude, was oriented perpendicular to the trench. This correlation suggests that plutonism is facilitated by strike-slip faulting within the batholithic belt; one explanation, which helps to solve the plutonic room problem, is that plutons are passively emplaced at releasing bends in the strike-slip faults, and volume is conserved by thrusting at the trench. If this correlation is generally applicable, then it implies that mid-crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench. Continental growth in such areas may occur dominantly by volcanism.

Glazner, A.F. (Univ. of North Carolina, Chapel Hill (United States))

1991-08-01

248

Anelastic strain recovery reveals extension across SW Japan subduction zone  

E-print Network

of the decolle´ment. Citation: Byrne, T. B., W. Lin, A. Tsutsumi, Y. Yamamoto, J. C. Lewis, K. Kanagawa, Y Weiren Lin,2 Akito Tsutsumi,3 Yuhji Yamamoto,4 Jonathan C. Lewis,5 Kyuichi Kanagawa,6 Yujin Kitamura,7

Kanagawa, Kyuichi

249

Evolution of 3-D subduction-induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique  

NASA Astrophysics Data System (ADS)

We present analogue models of free subduction in which we investigate the three-dimensional (3-D) subduction-induced mantle flow focusing around the slab edges. We use a stereoscopic Particle Image Velocimetry (sPIV) technique to map the 3-D mantle flow on 4 vertical cross-sections for one experiment and on 3 horizontal depth-sections for another experiment. On each section the in-plane components are mapped as well as the out-of-plane component for several experimental times. The results indicate that four types of maximum upwelling are produced by the subduction-induced mantle flow. The first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub-slab domain. A third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity. The fourth type results from quasi-toroidal return flow around the lateral slab edges, which produces a maximum upwelling located slightly laterally away from the sub-slab domain and can have another maximum upwelling located laterally away from the mantle wedge. These upwellings occur during the whole subduction process. In contrast, the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady-state phase. The position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time. Their time-evolving magnitude is well correlated to the trench retreat rate. The maximum upwelling velocity located laterally away from the subducting plate is ?18-24% of the trench retreat rate during the steady-state subduction phase. It is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness, potentially promoting decompression melting. It could thereby provide a source for intraplate volcanism, such as Mount Etna in the Mediterranean, the Chiveluch group of volcanoes in Kamchatka and the Samoan hotspot near Tonga.

Strak, Vincent; Schellart, Wouter P.

2014-10-01

250

Seismic variability of subduction thrust faults: Insights from laboratory models  

NASA Astrophysics Data System (ADS)

Laboratory models are realized to investigate the role of interface roughness, driving rate, and pressure on friction dynamics. The setup consists of a gelatin block driven at constant velocity over sand paper. The interface roughness is quantified in terms of amplitude and wavelength of protrusions, jointly expressed by a reference roughness parameter obtained by their product. Frictional behavior shows a systematic dependence on system parameters. Both stick slip and stable sliding occur, depending on driving rate and interface roughness. Stress drop and frequency of slip episodes vary directly and inversely, respectively, with the reference roughness parameter, reflecting the fundamental role for the amplitude of protrusions. An increase in pressure tends to favor stick slip. Static friction is a steeply decreasing function of the reference roughness parameter. The velocity strengthening/weakening parameter in the state- and rate-dependent dynamic friction law becomes negative for specific values of the reference roughness parameter which are intermediate with respect to the explored range. Despite the simplifications of the adopted setup, which does not address the problem of off-fault fracturing, a comparison of the experimental results with the depth distribution of seismic energy release along subduction thrust faults leads to the hypothesis that their behavior is primarily controlled by the depth- and time-dependent distribution of protrusions. A rough subduction fault at shallow depths, unable to produce significant seismicity because of low lithostatic pressure, evolves into a moderately rough, velocity-weakening fault at intermediate depths. The magnitude of events in this range is calibrated by the interplay between surface roughness and subduction rate. At larger depths, the roughness further decreases and stable sliding becomes gradually more predominant. Thus, although interplate seismicity is ultimately controlled by tectonic parameters (velocity of the plates/trench and the thermal regime), the direct control is exercised by the resulting frictional properties of the plate interface.

Corbi, F.; Funiciello, F.; Faccenna, C.; Ranalli, G.; Heuret, A.

2011-06-01

251

Large earthquake processes in the northern Vanuatu subduction zone  

NASA Astrophysics Data System (ADS)

northern Vanuatu (formerly New Hebrides) subduction zone (11°S to 14°S) has experienced large shallow thrust earthquakes with Mw > 7 in 1966 (MS 7.9, 7.3), 1980 (Mw 7.5, 7.7), 1997 (Mw 7.7), 2009 (Mw 7.7, 7.8, 7.4), and 2013 (Mw 8.0). We analyze seismic data from the latter four earthquake sequences to quantify the rupture processes of these large earthquakes. The 7 October 2009 earthquakes occurred in close spatial proximity over about 1 h in the same region as the July 1980 doublet. Both sequences activated widespread seismicity along the northern Vanuatu subduction zone. The focal mechanisms indicate interplate thrusting, but there are differences in waveforms that establish that the events are not exact repeats. With an epicenter near the 1980 and 2009 events, the 1997 earthquake appears to have been a shallow intraslab rupture below the megathrust, with strong southward directivity favoring a steeply dipping plane. Some triggered interplate thrusting events occurred as part of this sequence. The 1966 doublet ruptured north of the 1980 and 2009 events and also produced widespread aftershock activity. The 2013 earthquake rupture propagated southward from the northern corner of the trench with shallow slip that generated a substantial tsunami. The repeated occurrence of large earthquake doublets along the northern Vanuatu subduction zone is remarkable considering the doublets likely involved overlapping, yet different combinations of asperities. The frequent occurrence of large doublet events and rapid aftershock expansion in this region indicate the presence of small, irregularly spaced asperities along the plate interface.

Cleveland, K. Michael; Ammon, Charles J.; Lay, Thorne

2014-12-01

252

Deep 'Stone Soup' Trenching by Phoenix (Stereo)  

NASA Technical Reports Server (NTRS)

Digging by NASA's Phoenix Mars Lander on Aug. 23, 2008, during the 88th sol (Martian day) since landing, reached a depth about three times greater than in any trench Phoenix has excavated. The deep trench, informally called 'Stone Soup' is at the borderline between two of the polygon-shaped hummocks that characterize the arctic plain where Phoenix landed.

Stone Soup is in the center foreground of this stereo view, which appears three dimensional when seen through red-blue glasses. The view combines left-eye and right-eye images taken by the lander's Surface Stereo Imager on Sol 88 after the day's digging. The trench is about 25 centimeters (10 inches) wide and about 18 centimeters (7 inches) deep.

When digging trenches near polygon centers, Phoenix has hit a layer of icy soil, as hard as concrete, about 5 centimeters or 2 inches beneath the ground surface. In the Stone Soup trench at a polygon margin, the digging has not yet hit an icy layer like that.

Stone Soup is toward the left, or west, end of the robotic arm's work area on the north side of the lander.

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.

2008-01-01

253

'Dodo-Goldilocks' Trench Elevation Map  

NASA Technical Reports Server (NTRS)

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.

2008-01-01

254

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

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.

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

2003-01-01

255

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

NASA Astrophysics Data System (ADS)

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.

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

2012-04-01

256

Tectonic evolution of subduction wedges at the Cordilleran margin  

NASA Astrophysics Data System (ADS)

We present here a comparison of the tectonic evolution of subduction wedges at the Cordilleran margin, as represented by the Franciscan wedge of northern California and the Cascadia wedge in the Olympic Mountains, Washington State. We use a geodynamic model to illustrate the growth and morphological evolution of these wedges with time. The Franciscan wedge was initiated at about 150 Ma and is locally active in Northern California, north of the Mendocino triple junction. The northern Cascadia wedge was initiated at about 35 Ma, at the same time that the Cascade volcanic arc started. Both wedges developed into broad bivergent wedges, some 250 km wide. The front of the wedge is marked by the trench and the rear of the wedge, by backfolding on the landward side of the forearc high. In both cases, the forearc high initiated in deep water and steadily grew by accretion and deformation. Emergence of the high above sea level marked the start of erosion. Continued erosion at rates of 0.5 to 1 km/m.y. accounts for much of the exhumation of the deep interior of the wedge. A common view is that a subduction wedge is made up entirely of materials accreted at the subduction thrust. However, the upper parts of the Cascadia and Franciscan wedges are actually made up of imbricated and backfolded rocks that originated as part of the overriding plate. In the Olympic Mountains, these units are the Crescent basalts and the Upper OSC (= Graywolf, Elwha, Grand Valley units of Tabor and Cady). In the Franciscan, these units are the Coast Range ophiolite and eastern belt of the Franciscan. There is clear evidence in both cases that the subduction zone was first initiated seaward of these units and that early wedge-related deformation was also located seaward as well. This history is expected given that when a subduction zone is first initiated, the upper plate is everywhere stronger than the subduction thrust. In other words, the taper of the upper plate is initially blunter than critical. If there were no accretion and the overriding plate remained cold and frictional, then it would be able to sustain subduction slip without deforming internally. Accretion changes this situation given that it will reduce the taper of the upper plate and cause it to deform as a critical wedge. Our numerical experiments indicate that the subduction wedge will start as a small accretionary complex. As it grows, it develops a high and the high will slow propagate rearward into the overriding plate. In this fashion, the region of active wedge deformation includes both accreted materials from the downgoing plate and captured materials from the overriding plate. A strong backstop can stop the rearward migration of the forearc high, but this does not seem to have happened at the Cascadia margin. Instead, it appears that the rearward migrate there is arrested when the wedge reaches a steady state size, where erosion is equal to accretion.

Brandon, M. T.; Fuller, C. W.; Willett, S. D.

2005-12-01

257

Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark of trench-parallel terrane migration?  

Microsoft Academic Search

We studied the back azimuth dependence of Ps converted phases at GSN station COR (Corvallis, Oregon) using broadband P receiver functions computed from 602 teleseismic earthquakes. The amplitudes and polarities of the transverse Ps phases are largely two-lobed, which indicates anisotropy with a tilted symmetry axis. A double-peaked Ps conversion at 4.5-6.5 s delay has the moveout of a dipping

Jeffrey Park; Huaiyu Yuan; Vadim Levin

2004-01-01

258

Subduction zones beneath Indonesia imaged by Rayleigh wave phase velocity tomography  

NASA Astrophysics Data System (ADS)

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.

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

2013-12-01

259

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

NASA Astrophysics Data System (ADS)

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.

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

2014-06-01

260

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

USGS Publications Warehouse

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.

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

2004-01-01

261

Gravity evidence for bend-faulting-related serpentinization of subducting lithosphere  

NASA Astrophysics Data System (ADS)

Here we explore the hypothesis that subducting lithospheric mantle may become extensively serpentinized during bend-faulting between the outer rise and the trench axis. Trenches are often associated with the generation of new normal faults and the reactivation of preexisting ones. Seismic reflection data shows that some of these faults cut through the crust deep into the lithospheric mantle — thus potentially acting as conduits for seawater to reach and react with lithospheric mantle rocks to make serpentine. If significant serpentinization occurs (in theory, all lithosphere cooler than ˜600oC and shallower than ˜180 km is capable of partial serpentinization should seawater reach it), then, because of the significant density reduction (~40%) between peridotite and serpentine there should be an associated gravity anomaly. Here we use the subduction zone gravity-topography compilation of Levitt and Sandwell (JGR, 1994) with additional more recent satellite derived seasurface gravity (Sandwell and Smith, 1998) and GEOMAR swath-bathymetry data to explore this hypothesis. To determine the gravity anomaly associated with lithospheric serpentinization we first create a forward model that assumes that the crustal thickness and density and the lithospheric density remain unchanged between the outer rise and trench axis. With this assumption we calculate the gravity anomaly predicted from the changing water depth (using Parker's high-order FFT-based algorithm), and subtract the observed seasurface gravity from it. The resulting residual anomaly would be zero if neither crustal thickness nor crustal nor lithosphere density changed between the outer rise and trench axis, and nonzero if changes did occur. We find that a persistent negative residual gravity anomaly is associated with plate-bending-related processes. Typical residual anomalies are zero at the outer rise, increasing roughly linearly to -50 to -100 mgal at the trench axis. Using the Bouguer formula to estimate the volume of the serpentinized region necessary to produce this signal, we find that a ˜25km-thick, ˜8% serpentinized lithospheric column will produce a negative residual anomaly of ˜100 mgal. Seismic data implies that neither sediment nor crustal variations are extreme enough to account for this large effect. Thus gravity evidence implies that significant lithospheric serpentinization is related to bend-faulting of subducting lithosphere.

Yamamoto, M.; Phipps Morgan, J.; Ranero, C.; Schneider, J.; Sandwell, D.

2003-04-01

262

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms  

NASA Astrophysics Data System (ADS)

Subduction earthquakes on plate-boundary megathrusts accommodate most of the global seismic moment release, frequently resulting in devastating damage by ground shaking and tsunamis. As many earthquakes occur in deep-sea regions, the dynamics of earthquake faulting in subduction zones is poorly understood. However, the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and fault rock studies in accretionary prisms exhumed from source depths of subduction earthquakes have greatly improved our understanding of earthquake faulting in subduction zones. Here, we review key advances that have been made over the last decade in the studies of fault rocks and in laboratory experiments using fault zone materials, with a particular focus on the Nankai Trough subduction zone and its on-land analog, the Shimanto accretionary complex in Japan. New insights into earthquake faulting in subduction zones are summarized in terms of the following: (1) the occurrence of seismic slip along velocity-strengthening materials both at shallow and deep depths; (2) dynamic weakening of faults by melt lubrication and fluidization, and possible factors controlling coseismic deformation mechanisms; (3) fluid-rock interactions and mineralogical and geochemical changes during earthquakes; and (4) geological and experimental aspects of slow earthquakes.

Ujiie, Kohtaro; Kimura, Gaku

2014-12-01

263

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

NASA Astrophysics Data System (ADS)

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.

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

2010-12-01

264

Puerto Rico Trench: Cruise Summary and Results  

NSDL National Science Digital Library

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.

Uri Brink

265

Flame trench analysis of NLS vehicles  

NASA Technical Reports Server (NTRS)

The present study takes the initial steps of establishing a better flame trench design criteria for future National Launch System vehicles. A three-dimensional finite element computer model for predicting the transient thermal and structural behavior of the flame trench walls was developed using both I-DEAS and MSC/NASTRAN software packages. The results of JANNAF Standardized Plume flowfield calculations of sea-level exhaust plume of the Space Shuttle Main Engine (SSME), Space Transportation Main Engine (STME), and Advanced Solid Rocket Motors (ASRM) were analyzed for different axial distances. The results of sample calculations, using the developed finite element model, are included. The further suggestions are also reported for enhancing the overall analysis of the flame trench model.

Zeytinoglu, Nuri

1993-01-01

266

Chicxulub Ejecta Impact Trenches And Terminal Impact  

NASA Astrophysics Data System (ADS)

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

Page, R. L.

2013-05-01

267

Chicxulub Ejecta Trenches and Terminal Impact  

NASA Astrophysics Data System (ADS)

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 3,519m 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 will 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 MAPPING BY JARED PHUMPHERY, MLA, KANSAS STATE UNIVERSITY

Page, R.

2012-12-01

268

Deepest Trenching at Phoenix Site on Mars  

NASA Technical Reports Server (NTRS)

NASA's Phoenix Mars Lander widened the deepest trench it has excavated, dubbed 'Stone Soup,' (in the lower half of this image) to collect a sample from about 18 centimeters (7 inches) below the surface for analysis by the lander's wet chemistry laboratory.

Phoenix's Surface Stereo Imager took this image on Sol 95 (Aug. 30, 2008), the 95th Martian day since landing. For scale, the rock to the right of the Stone Soup trench is about 15 centimeters (6 inches) across. The lander's robotic arm scooped up a sample from the left half of the trench for delivery the following sol to the wet chemistry laboratory.

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.

2008-01-01

269

Determining Earthquake Recurrence Intervals from Trench Logs  

NSDL National Science Digital Library

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.

Cashman, Patricia

270

300 Area process trench sediment analysis report  

SciTech Connect

This report describes the results of a sampling program for the sediments underlying the Process Trenches serving the 300 Area on the Hanford reservation. These Process Trenches were the subject of a Closure Plan submitted to the Washington State Department of Ecology and to the US Environmental Protection Agency in lieu of a Part B permit application on November 8, 1985. The closure plan described a proposed sampling plan for the underlying sediments and potential remedial actions to be determined by the sample analyses results. The results and proposed remedial action plan are presented and discussed in this report. 50 refs., 6 figs., 8 tabs.

Zimmerman, M.G.; Kossik, C.D.

1987-12-01

271

Subduction in Central Kermadec: Crustal Structures from the Incoming Plate and the Arc- Backarc Region From Wide-Angle Seismics  

NASA Astrophysics Data System (ADS)

The central part of the 2500-km long Tonga-Kermadec Trench is characterized by the subduction of the Louisville Ridge and unusually large seismicity approximately 200-300 km to the south of this ridge subduction. From this region we show preliminary results which have been derived from the recently acquired interpretation of seismic wide-angle reflection/refraction data. The data were collected along an almost 500-km long transect carried out in April 2007 using the R/V Sonne in order to determine the upper lithospheric structures of the incoming Pacific Plate and the overriding Australian Plate across the Colville and Kermadec Ridges. This transect lies immediately north of Raoul Island, the largest of the Kermadec Islands and which is presently a highly active volcano. This study is part of the MANGO project (Marine Geoscientific Inverstigations on the Input and Output of the Kermadec Subduction Zone) which comprises a 1000-km long working area north of New Zealand's North Island. It covers the transition from subduction of the Hikurangi Plateau in the south to erosive subduction of normal Pacific oceanic crust in the centre and thence accretionary subduction further north. Overall the subduction is accompanied by northward increasing seismicity. The aim of this project is to understand the transition throughout the different regimes, the variation of the structures to explain the accompanying seismicty, and the role and evolution of the stratovolcanoes. This will be achieved by analysing the structures of the sediment, crust and upper mantle and also material transfers from its input and output through subduction zone processes.

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.

2007-12-01

272

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

NASA Astrophysics Data System (ADS)

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.

Zhao, Shaorong; Takemoto, Shuzo

2000-08-01

273

Variations in Oxygen Fugacity among Forearc Peridotites from the Tonga Trench  

NASA Astrophysics Data System (ADS)

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).

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

2013-12-01

274

Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska subduction zone  

NASA Astrophysics Data System (ADS)

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (<6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North American plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio (Vp/Vs) exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-12 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.

2013-05-01

275

Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism  

NASA Astrophysics Data System (ADS)

We present the first detailed 2D seismic tomographic image of the trench-outer rise, fore- and back-arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo-Australian plate where ˜80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending-related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ˜7.3 km/s, ˜10% lower than typical for mantle peridotite (˜30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc-type igneous basement with uppermost and lowermost crustal velocities of ˜3.5 and ˜7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0-5.5 km/s) suggests that the outer fore-arc is probably fluid-saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore-arc. Extension in the outer fore-arc is evidenced by (1) trenchward-dipping normal faults and (2) the presence of a giant scarp (˜2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore-arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ˜7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.

Contreras-Reyes, Eduardo; Grevemeyer, Ingo; Watts, Anthony B.; Flueh, Ernst R.; Peirce, Christine; Moeller, Stefan; Papenberg, Cord

2011-10-01

276

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

NASA Astrophysics Data System (ADS)

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.

Anma, Ryo

2013-04-01

277

Versatile trench isolation technology for the fabrication of microactuators  

Microsoft Academic Search

A trench isolation technology employs trenches refilled with dielectric material to create, in a single layer, electrical isolation between mechanically joined components. This paper explores further use of this technology for MEMS fabrication, particularly the fabrication of electrostatic microactuators. Adding extra features to a two-mask trench isolation process new design opportunities, like isolation structures and isolation bumps, are created. The

E. Sarajlic; E. Berenschot; G. Krijnen; M. Elwenspoek

2003-01-01

278

Progressive accretion in the Middle America Trench, Southern Mexico  

Microsoft Academic Search

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.

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

279

Earthquakes and Bending of Plates at Trenches  

Microsoft Academic Search

The mechanisms, distribution, and total moment of earthquakes within the bending oceanic plate seaward of trenches constrain possible mechanical models of the lithosphere. The average annual horizontal slip in normal fault earthquakes, as estimated from the cumulative seismic moment, exceeds the total extension predicted by an elastic plate model. Thus bending is not predominantly an elastic process but must include

William M. Chapple; Donald W. Forsyth

1979-01-01

280

Chapter 8 -Trenching and Excavation Excavation Operations  

E-print Network

. Changing Weather Conditions: Hard packed soil can become soupy and unstable after rain. Trenches, which are safely sloped or shored in dry weather, can be very dangerous in wet weather. 5. Heavy loads in the area-energized. 11. Make sure that shoring material is the right kind, in good condition, and free of defects. 12

281

300 Area Process Trenches Closure Plan  

SciTech Connect

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.

Luke, S.N.

1994-08-15

282

Color View 'Dodo' and 'Baby Bear' Trenches  

NASA Technical Reports Server (NTRS)

NASA's Phoenix Mars Lander's Surface Stereo Imager took this image on Sol 14 (June 8, 2008), the 14th Martian day after landing. It shows two trenches dug by Phoenix's Robotic Arm.

Soil from the right trench, informally called 'Baby Bear,' was delivered to Phoenix's Thermal and Evolved-Gas Analyzer, or TEGA, on Sol 12 (June 6). The following several sols included repeated attempts to shake the screen over TEGA's oven number 4 to get fine soil particles through the screen and into the oven for analysis.

The trench on the left is informally called 'Dodo' and was dug as a test.

Each of the trenches is about 9 centimeters (3 inches) wide. This view is presented in approximately true color by combining separate exposures taken through different filters of the Surface Stereo Imager.

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.

2008-01-01

283

Japan Smoke  

... Smoke Plume from Industrial Fires in Miyagi Prefecture, Japan     View larger image ... a.m. The images are oriented with north at the left and east at the top, and cover an area measuring 85 kilometers by 115 kilometers ...

2013-04-16

284

Launch Pad Flame Trench Refractory Materials  

NASA Technical Reports Server (NTRS)

The launch complexes at NASA's John F. Kennedy Space Center (KSC) are critical support facilities for the successful launch of space-based vehicles. These facilities include a flame trench that bisects the pad at ground level. This trench includes a flame deflector system that consists of an inverted, V-shaped steel structure covered with a high temperature concrete material five inches thick that extends across the center of the flame trench. One side of the "V11 receives and deflects the flames from the orbiter main engines; the opposite side deflects the flames from the solid rocket boosters. There are also two movable deflectors at the top of the trench to provide additional protection to shuttle hardware from the solid rocket booster flames. These facilities are over 40 years old and are experiencing constant deterioration from launch heat/blast effects and environmental exposure. The refractory material currently used in launch pad flame deflectors has become susceptible to failure, resulting in large sections of the material breaking away from the steel base structure and creating high-speed projectiles during launch. These projectiles jeopardize the safety of the launch complex, crew, and vehicle. Post launch inspections have revealed that the number and frequency of repairs, as well as the area and size of the damage, is increasing with the number of launches. The Space Shuttle Program has accepted the extensive ground processing costs for post launch repair of damaged areas and investigations of future launch related failures for the remainder of the program. There currently are no long term solutions available for Constellation Program ground operations to address the poor performance and subsequent failures of the refractory materials. Over the last three years, significant liberation of refractory material in the flame trench and fire bricks along the adjacent trench walls following Space Shuttle launches have resulted in extensive investigations of failure mechanisms, load response, ejected material impact evaluation, and repair design analysis (environmental and structural assessment, induced environment from solid rocket booster plume, loads summary, and repair integrity), assessment of risk posture for flame trench debris, and justification of flight readiness rationale. Although the configuration of the launch pad, water and exhaust direction, and location of the Mobile Launcher Platform between the flame trench and the flight hardware should protect the Space Vehicle from debris exposure, loss of material could cause damage to a major element of the ground facility (resulting in temporary usage loss); and damage to other facility elements is possible. These are all significant risks that will impact ground operations for Constellation and development of new refractory material systems is necessary to reduce the likelihood of the foreign object debris hazard during launch. KSC is developing an alternate refractory material for the launch pad flame trench protection system, including flame deflector and flame trench walls, that will withstand launch conditions without the need for repair after every launch, as is currently the case. This paper will present a summary of the results from industry surveys, trade studies, life cycle cost analysis, and preliminary testing that have been performed to support and validate the development, testing, and qualification of new refractory materials.

Calle, Luz M.; Hintze, Paul E.; Parlier, Christopher R.; Bucherl, Cori; Sampson, Jeffrey W.; Curran, Jerome P.; Kolody, Mark; Perusich, Steve; Whitten, Mary

2010-01-01

285

A new subduction model in three-dimensional spherical geometry  

NASA Astrophysics Data System (ADS)

A variety of numerical models of subduction zone have been employed so far and they can be classified into two categories. One is the model that plate and slab movement are imposed kinematically. The advantage of this model is that it is easier to compare the results of numerical model with observations. The other is the model that plate and slab movements are determined more or less dynamically. This model is based more on physics than the first model is, which is the advantage of this type of models. Therefore, when we want to compare the results of numerical model with observations and at the same time keep the dynamic feature as much as we can, we need to develop a model that incorporates advantages of two kinds of models above. It is also desirable that they can easily extend to the real earth-like shape, namely, three-dimensional and spherical geometry in the future. Considering these, we have developed a new numerical subduction zone model in three-dimensional spherical shell. The model is an extension of previous study (Honda, 2008), which handles a rectangular geometry. The velocity is imposed on top surface and a small region around the plate boundary, so that, below that region, the slab is able to subduct under its own weight. Top surface plate velocity is determined based on the Euler’s theorem. The velocity imposed on the region around the plate boundary is separated into two subregions, the region above (Vc) or below (Vo) plate boundary (Figure 1). Vc is the expected velocity, if this subregion is assumed to move rigidly with the top surface velocity being that of the landward plate. Vo is determined so that the mass conservation inside the region is satisfied. Thus, this model can handle the trench migration. Our model is useful in considering the case in which we cannot neglect the effect of curvature of the earth such as large as long-length subduction zone. In this presentation, we show details of the model description and some examples of the application of our model. Figure 1. The velocity imposed on the region around the plate boundary.

Morishige, M.; Honda, S.; Tackley, P. J.

2009-12-01

286

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

NASA Astrophysics Data System (ADS)

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.

Stock, J. M.

2013-05-01

287

Flexural models of trench/outer rise topography of coronae on Venus with axisymmetric spherical shell elastic plates  

NASA Technical Reports Server (NTRS)

Magellan altimetry has revealed that many coronae on Venus have trenches or moats around their peripheries and rises outboard of the trenches. This trench/outer rise topographic signature is generally associated with the tectonic annulus of the corona. Sandwell and Schubert have interpreted the trench/outer rise topography and the associated tectonic annulus around coronae to be the result of elastic bending of the Venus lithosphere (though the tectonic structures are consequences of inelastic deformation of the lithosphere). They used two-dimensional elastic plate flexure theory to fit topographic profiles across a number of large coronae and inferred elastic lithosphere thicknesses between about 15 and 40 km, similar to inferred values of elastic thickness for the Earth's lithosphere at subduction zones around the Pacific Ocean. Here, we report the results of using axisymmetric elastic flexure theory for the deformation of thin spherical shell plates to interpret the trench/outer rise topography of the large coronae modeled by Sandwell and Schubert and of coronae as small as 250 km in diameter. In the case of a corona only a few hundred kilometers in diameter, the model accounts for the small planform radius of the moat and the nonradial orientation of altimetric traces across the corona. By fitting the flexural topography of coronae we determine the elastic thickness and loading necessary to account for the observed flexure. We calculate the associated bending moment and determine whether the corona interior topographic load can provide the required moment. We also calculate surface stresses and compare the stress distribution with the location of annular tectonic features.

Moore, W.; Schubert, Gerald; Sandwell, David T.

1992-01-01

288

Drainage Asperities on Subduction Megathrusts  

NASA Astrophysics Data System (ADS)

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.

Sibson, R. H.

2012-12-01

289

Patterns of seismogenesis for giant plate-boundary earthquakes in island-arc-type subduction systems  

NASA Astrophysics Data System (ADS)

The global record of giant earthquake occurrence in subduction zones during the instrumental and historical eras is woefully short; only about 16 events with magnitudes above 8.4 are reasonably well documented since 1700. We find no examples of giant (M > 8.4) interplate thrust events and/or wide-ranging tsunamis sourced in the classic island arcs with fast backarc spreading (Bonin, Marianas, Tonga-Kermadec, Vanuatu, and South Scotia). The Sumatra-Andaman Earthquake of 2004 (SAE) ruptured a sector of the INDIA-BURMA subduction boundary and evidently had no known historical antecedents, suggesting that the return time may be many centuries to millennia and consistent with low convergence rates. Moreover, the persistence of rupture to the north in the weakly volcanic Nicobar/Andaman sector gives one pause to reflect on the assumption that island arcs, especially those with active back-arc spreading such as the Marianas, do not produce great interplate- thrust earthquakes. The Andaman/Nicobar subduction segment is an unusual island arc. Only two arc volcanoes occur between the convergent plate boundary west of the Andamans and the backarc ridge/transform system to the east. Backarc spreading in the Andaman/ Nicobar segment is unusual because the NNW spreading directions are nearly parallel to the trench/deformation-front as do the INDIA-BURMA plate motions across it. This geometry suggests that arc-normal extension, trench migration and associated slab normal motions may not mechanically decouple this subduction system. The Nicobar sector of the rupture for the 2004 event is roughly 200 km wide judging from the aftershock distribution; a distribution that persists to the east under the Nicobar Islands, suggesting that the plate-boundary dip is very shallow in that latitude range. If this is correct, then the down-dip limitation on seismogenic slip set by serpentinized forearc mantle (Hyndman et al., 2003) may not control rupture width as it apparently does for many island arcs with steeper slab dips. Finally, this subduction system receives a large sediment influx from the giant Irrawadi and Ganges River delta systems to the north that drain the active collisional mountain belts further north. High sediment influx and trench sediment acumulation are factors that are present in 12 of the 14 subduction zones that have hosted the giant interplate thrust earthquakes mentioned earlier. This influx appears to be important in the development of a subduction channel of granular material in the plate boundary that may enable ruptures to run to great lengths by smoothing out barriers to earthquake slip. Several island-arc subduction systems (the Manus, SW Ryukyu, W Aleutians, and possibly the South Shetlands systems) have attributes similar to the Nicobar/Andaman segment of the SAE rupture. Moreover, arc volcanic activity is weak or absent and convergence also tends to be slow in all of these island-arc systems. Research done in collaboration with the USGS Tsunami Source Working Group (Steve Kirby and Eric Geist (Co-Chairs) David Scholl, Roland von Huene, Rick Blakely, Ray Wells, and Willie Lee (Secretary))

Kirby, S. H.

2006-12-01

290

Slab Geometry Control on Mantle Flow Regime: A case study from Central South America Subduction Zone  

NASA Astrophysics Data System (ADS)

The subduction of oceanic lithosphere along convergent plate margins plays an important role in the dynamics of the upper mantle beneath convergent margins and major orogenic belts. Many studies of mantle dynamics show that the flow pattern of the mantle varies greatly between different subduction zones as well as within the same subduction zone. The factors that control such variations are poorly understood and need to be investigated further in order to develop a better understanding of various subduction zone processes such as the deformation of mantle beneath convergent plate margins and transport of melts and volatiles in the mantle wedge above subducting slabs. Earlier studies of mantle flow inferred from seismic anisotropy via shear-wave splitting analysis indicated that the dynamics and deformation of subducting and overriding plates as well as the slab geometry have important roles on mantle flow regime. In an effort to test the significance of these factors in constraining the mantle dynamics along the central South America subduction zone, we carried out a shear-wave splitting analysis. Our study area covers southern Peru and northwestern Bolivia encompassing the northernmost Altiplano plateau where subduction of the Nazca plate begins to gradually flatten towards the north. The major part of the data for our analysis comes from the CAUGHT temporary seismic deployment (2010 - 2012) with 49 three-component broadband seismometers. In our study we used SKS, SKKS and PKS arrivals from over 80 teleseismic earthquakes, located between the distance-range of 60 to 140 degrees. We determined polarization direction and delay-time of shear-wave arrivals that are polarized into fast and slow components and split in time. The resultant fast polarization directions indicate the direction of mantle flow beneath the study area and the delay-times show the strength and depth extend of the associated seismic anisotropy. The results of our analysis revealed a geographically varying pattern of fast polarization directions with delay times ranging between 0.3 and 1.2 seconds. With the exception of few outlier measurements, larger delay times (over 1.0 second) are observed in the southwestern parts of the study area, closer to the trench. The fast polarization directions associated with these measurements are NE-SW and are nearly perpendicular to the strike of the trench and parallel to the absolute plate motion direction. Based on these observations and the shallow location of the slab, we infer that the location of the anisotropy in the southwest is below the slab and most probably due to the mantle flow entrained by the subducting Nazca plate. In the northwest the delay times reduce dramatically towards the Peruvian flat slab and we cannot detect significant azimuthal anisotropy. We attribute this to the change in flow pattern in the mantle related to the flattening slab. Based on these results we conclude that the geometry of the slab has a significant control on the mantle flow pattern in the central South America subduction zone.

Biryol, C. B.; Beck, S. L.; Zandt, G.; Wagner, L. S.

2013-12-01

291

GPS measurements and finite element modeling of the earthquake cycle along the Middle America subduction zone  

NASA Astrophysics Data System (ADS)

We model surface deformation recorded by GPS stations along the Pacific coasts of Mexico and Central America to estimate the magnitude of and variations in frictional locking (coupling) along the subduction interface, toward a better understanding of seismic hazard in these earthquake-prone regions. The first chapter describes my primary analysis technique, namely 3-dimensional finite element modeling to simulate subduction and bounded-variable inversions that optimize the fit to the GPS velocity field. This chapter focuses on and describes interseismic coupling of the Oaxaca segment of the Mexican subduction zone and introduces an analysis of transient slip events that occur in this region. Our results indicate that coupling is strong within the rupture zone of the 1978 Ms=7.8 Oaxaca earthquake, making this region a potential source of a future large earthquake. However, we also find evidence for significant variations in coupling on the subduction interface over distances of only tens of kilometers, decreasing toward the outer edges of the 1978 rupture zone. In the second chapter, we study in more detail some of the slow slip events that have been recorded over a broad area of southern Mexico, with emphasis on their space-time behavior. Our modeling indicates that transient deformation beneath southern Mexico is focused in two distinct slip patches mostly located downdip from seismogenic areas beneath Guerrero and Oaxaca. Contrary to conclusions reached in one previous study, we find no evidence for a spatial or temporal correlation between transient slip that occurs in these two widely separated source regions. Finally, chapter three extends the modeling techniques to new GPS data in Central America, where subduction coupling is weak or zero and the upper plate deformation is much more complex than in Mexico. Cocos-Caribbean plate convergence beneath El Salvador and Nicaragua is accompanied by subduction and trench-parallel motion of the forearc. Our GPS velocity field is best fit by a model with strongly locked faults in the volcanic arc and a weakly coupled subduction interface. In this region, seismic hazards associated with subduction are therefore low, but are high for crustal faults, in agreement with records of historic seismicity.

Correa Mora, Francisco

292

Imaging the megathrust zone and Yakutat/Pacific plate interface in the Alaska subduction zone  

NASA Astrophysics Data System (ADS)

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity, and (2) the interplate coupled zone where the great 1964 earthquake (magnitude 9.3) exhibited the largest amount of rupture. The joint teleseismic migration of two array datasets based on teleseismic receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin (3-5 km) low-velocity layer (shear wave velocity of ~2.0-2.5 km/s) that is ~20-40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio of 1.9-2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of ~15 km. Both velocities and thickness of the low-velocity channel abruptly increase downdip in central Alaska, which agrees with previously published results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Rondenay, S.

2013-12-01

293

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

NASA Astrophysics Data System (ADS)

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.

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

2015-01-01

294

Millennial coastal uplift rates and the seismic cycle in the 2011 Mw 9.0 Tohoku-oki earthquake area, Japan  

NASA Astrophysics Data System (ADS)

The Mw 9.0 Tohoku-oki earthquake and the record of major historical seismic events including the AD 869 Jogan earthquake on the Japan trench illustrate the active tectonic capability on the subduction zone. The recent coastal deformation is revealed by the 2011 coseismic subsidence (up to 1.2 m) and postseismic uplift (up to 20 cm within 1.5 years) obtained from leveling and GPS measurements. 100 years before the earthquake, subsidence at a rate of ~1 mm/a was documented. The correlation between these short-term geodetic results and long-term geologic data is decisive for understanding of the tectonic process and the related earthquake cycle on the subducting Pacific slab. Thus, we study the millennial vertical deformation along the coastline of northern Honshu Island (38.2°N to 41.2°N) in the frame of the PALET project (ANR-JST Flash program). The exposure of emerged marine terraces, wave-cut platforms and notches allow us to determine the deformation rate during the Late Pleistocene and Holocene. Coastline terraces of marine isotope stages MIS5e (124 ka) to MIS19 (~780 ka) indicate uplift rates of 0.2-0.4 mm/a and 0.1-0.2 mm/a in the northern and southern study area, respectively. Numerous younger notches and wave-cut platforms are identified at several height levels between 1 and 10 m above sea level. Two radiocarbon samples of wood remnants yielded an age of ~2.8 cal ka BP for a 3.2 m high terrace in the north (40.7°N), and a shell fragments on a notch in resistant conglomerates (39.7°N) revealed an age of 47.1 ± 2.2 cal ka BP. After correction for sea level change, both data points yield uplift rates of ~1 mm/a, which denotes clear acceleration in uplift during the Late Quaternary. An elastic dislocation model of the co-, post- and interseismic slip distribution shows how the successive coastal subsidence during M9-class earthquakes is concealed by the long-term uplift due to deep creeping deformation. The distribution of lower uplift rates in the southern area coincides with the region of the strongest 2011 coseismic subsidence and this implies the repetition of M~9 earthquake deformation with several cycles of coseismic subsidence on this section of the subduction zone. Furthermore, a different seismic behavior with recent large earthquakes (Mw 7.4-8.2) exists in the north, whereas a pre-2011 seismic gap of M>7.5 earthquakes prevailed in the seismically coupled southern part of the Japan trench. All factors attest for a segmentation of the subduction zone that prevailed in its actual position since at least 780 ka, and that the southern segment presumably generated several M~9 earthquakes in the past.

Mechernich, Silke; Meghraoui, Mustapha; Cetin, Esra; Toda, Shinji; Okumura, Koji

2013-04-01

295

Seismic investigation of the transition from continental to oceanic subduction along the western Hellenic Subduction Zone  

E-print Network

The western Hellenic subduction zone (WHSZ) exhibits well-documented along-strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab ...

Pearce, Frederick Douglas

296

Aseismic deep subduction of the Philippine Sea plate and slab window  

NASA Astrophysics Data System (ADS)

We have made great efforts to collect and combine a large number of high-quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to ˜460 km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many teleseismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese stations alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone.

Huang, Zhouchuan; Zhao, Dapeng; Hasegawa, Akira; Umino, Norihito; Park, Jung-Ho; Kang, Ik-Bum

2013-10-01

297

Analog Modeling of the Juan Fernández Ridge, Central Chile, and Implications for Flat-Slab Subduction Dynamics  

NASA Astrophysics Data System (ADS)

This study compares the strain experienced by the subducting lithosphere in analog models to the strain recorded by earthquakes in the subduction zone that includes the Juan Fernández Ridge (JFR), near 33 S, 73 W, off the coast of central Chile. The JFR is an aseismic hot spot ridge that has a thickened oceanic crust. The overthickened crust reduces the total density of the slab when compared to the surrounding slab areas, and thus increases the buoyancy of the subducting Nazca plate at this particular location. It is hypothesized that the Nazca plate experiences “flat-slab” subduction at the JFR subduction zone due to this buoyancy. Brudzinski and Chen (2005) argue that, due to the poorly aligned direction of maximum extension (T axes) for earthquakes in the subducting slab in flat-slab subduction zones, the theory of “slab pull” may not be valid for flat-slab subduction zones, and there must be other forces at work. However, Anderson et al. (2007) develop new, more precise slab contours from newly determined earthquake locations and use these contours to qualitatively compare the earthquake data to slab dip directions and thus expected slab-pull directions. They conclude that T axes are parallel to slab dip, and thus slab pull is the only force necessary for explaining the T axis direction. In this study, we quantitatively compare extension produced in analog "flat-slab" models in the laboratory to T axes from the Anderson et al. (2007) study, extending and further testing their idea. Several materials comprise the analog models. Light corn syrup represents the asthenosphere, while silicon putty represents the lithosphere. Recreating the dynamics of the buoyant JFR necessitates two different densities of silly putty: a denser one for the bulk of the slab, and a less dense one for the buoyant ridge. Shallow circular indentations (strain ellipses) on the slab facilitate recording of the strain in the subducting slab. Video and still pictures record each experiment. The results show that the modeled slab effectively reproduces the shape of the subducting Nazca plate at the JFR subduction zone. The average direction of maximum extension for each side of the modeled slab is compared to the average T axis trend for equivalent areas of the Nazca slab. The averaged directions of maximum extension match up fairly well with the averaged trends of the T axes; since slab pull is the only force acting upon the modeled slab, this shows that slab pull is an adequate method of explanation for the stresses and resulting strains found in subducting slabs, including flat-slab subduction zones. However, due to heterogeneous stresses, the Nazca plate does not subduct completely symmetrically, and in fact the model that best fits the actual slab contours is an asymmetrically subducting slab. In addition, the T axes show that at the third inflection point of the slab, the place where the slab steepens and subducts into the deeper mantle, the direction of slab pull is still to the sides of the aseismic ridge, not perpendicular to the oceanic trench. This means that even as the ridge is subducting, its buoyancy still exerts some control over the strain.

Woodell, D.; Anderson, M. L.

2009-12-01

298

Fluids escape in subduction zones: new constraints from 3-D microtomography data  

NASA Astrophysics Data System (ADS)

Large amounts of H2O are carried into trenches via subduction of the sediments, basaltic crust and uppermost mantle that make up the oceanic lithosphere. A major question is how much of this subducted H2O is released into the overlying mantle wedge, promoting melting, and how much is carried deeper into the mantle. This depends, at least in part, on whether H2O is able to form an interconnected network among the mineral grains that make up the rock down to very low fluid fractions. In order to achieve connectivity and allow the fluid phase to escape, a minimum amount of fluid (critical porosity) is required when dihedral angles are more than 60 degrees. We investigated the distribution of seawater in simplified sediment analogs (i.e. quartz for siliceous sediments; calcite for carbonate sediments), in natural clays (kaolinite and montmorillonite) and in bulk eclogite. Experiments were performed in a piston-cylinder apparatus at 2 GPa and 650°C. Fluid fractions ranged from ~10% to ~1% to determine the porosity at which connectivity of the seawater network is lost for each rock type. We used synchrotron X-ray microtomographic techniques (at Argonne National Laboratory, IL) to obtain 3-D images of the pore space network in order to constrain the grain scale distribution of fluids in a subducted slab. This nondestructive 3-D imaging technique has a spatial resolution of 0.7 ?m and provides quantitative information on geometrical parameters of fluid topology, such as porosity, dihedral angle distribution, fluid channel sizes and connectivity. The geometrical parameters were extracted using the VSG Avizo software. This study lays the groundwork for determining the 3-D grain scale distribution of fluids in a range of subducted lithologies. Results from this study provide important new insights into the amount of fluid that can be transported into the deep mantle by subduction.

Le Roux, V.; Gaetani, G. A.; Slaugenwhite, J.; Miller, K.

2013-12-01

299

New thermo-kinetic models of olivine metastability in subducting lithosphere: implications for deep-focus earthquakes  

NASA Astrophysics Data System (ADS)

A shear instability postulated to occur during the transformation of metastable olivine to its high-pressure polymorphs, wadsleyite and ringwoodite, has been proposed as the mechanism for deep-focus earthquakes. In order to evaluate this possibility we have formulated updated thermo-kinetic models to predict the amount of metastable olivine in a variety of subduction zones. Our models use newly derived activation energy parameters based on recent experiments in the (Mg,Fe)2SiO4 system, and we incorporate latent heat feedback due to the transformation into both the kinetics and the thermal model. We also consider the effects of transformation stress on growth kinetics and intracrystalline transformation, previously thought to be important only at high shear stresses. Our modeling predicts substantially smaller metastable olivine wedges than previous models that did not properly account for latent heat (Kirby et al., 1996) or used older kinetic parameters (e.g. Daessler et al., 1996; Devaux et al., 1997). Results of models considering only grain boundary nucleation and growth include the following: 1) In subduction zones with a thermal parameter (? = vertical convergence rate ¥ age of the lithosphere at the trench) less than ~5000 km, no significant metastable olivine wedge develops; this includes the Nazca subduction zone, in which the 1994 Bolivian earthquake occurred at a depth of 630 km. 2) For subduction zones such as Izu-Bonin, the Marianas, and Eastern Indonesia, with ? in the range 6000-10000 km, our models predict a maximum depth of metastability of 450-500 km. The maximum depths of earthquakes in these subduction zones are, respectively, 550, 670 and 670 km (e.g., Kirby et al, 1996). 3) In Tonga, the subduction zone with the most rapid convergence rates on Earth, an olivine wedge may persist to depths >660 km only if trench rollback (due to back-arc spreading) is taken into account. This depth is reduced by 80-120 km if trench rollback is not considered in the overall convergence rate. When intracrystalline transformation is taken into account, the maximum depth of metastability is further reduced by as much as 100 km, due to the large increase in the density of nucleation sites. Inhibition of growth by transformation stress can increase the depth interval over which transformation takes place, but is unlikely to be a dominant factor, especially if the intracrystalline mechanism operates. These results indicate that other mechanisms besides transformational faulting in metastable olivine must operate to cause the deepest earthquakes. References: Daessler et al, 1996, PEPI 94, 217-239\\Devaux et al, 1997, JGR 102, 24627-24637\\Kirby et al, 1996, Rev. Geophys., 34, 261-306\\

Mosenfelder, J. L.; Marton, F. C.; Rubie, D. C.

2001-12-01

300

Conductivity Probe after Trench-Bottom Placement  

NASA Technical Reports Server (NTRS)

Needles of the thermal and conductivity probe on NASA's Phoenix Mars Lander were positioned into the bottom of a trench called 'Upper Cupboard' during Sol 86 (Aug. 21, 2008), or 86th Martian day after landing. This image of the conductivity probe after it was raised back out of the trench was taken by Phoenix's Robotic Arm Camera. The conductivity probe is at the wrist of the robotic arm's scoop.

The probe measures how fast heat and electricity move from one needle to an adjacent one through the soil or air between the needles. Conductivity readings can be indicators about water vapor, water ice and liquid water.

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.

2008-01-01

301

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

NASA Astrophysics Data System (ADS)

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.

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

2006-12-01

302

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

NASA Astrophysics Data System (ADS)

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.

Font, Yvonne; Segovia, Monica; Theunissen, Thomas

2010-05-01

303

Lithospheric normal faulting beneath the Aleutian trench  

Microsoft Academic Search

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

Katsuyuki Abe

1972-01-01

304

Shaped charges blast sea-line trenches  

Microsoft Academic Search

Novel methods are now used to build sea-line trenches in rock quicker and easier with explosives. The object of this technique is to reduce offshore heavy equipment and manpower requirements. The job is simpler, safer, and cheaper to perform. The new 2-component chemical explosives are self-sterilizing mixtures. If lost during rough seas or not used within 12 hr, they become

Ridgeway

1975-01-01

305

Reappraising elastic thickness variation at oceanic trenches  

Microsoft Academic Search

We reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free-air gravity profiles which are perpendicular to oceanic trenches. As in many previous studies, our starting point is the well-known floating elastic plate model. In order to remove the influence of short-wavelength features not associated with lithospheric bending, adjacent

Madeleine Bry; Nicky White

2007-01-01

306

The lateral variation of the shear-wave splitting values just above the subducting ridge  

NASA Astrophysics Data System (ADS)

In Tokai region, central part of Japan, the Philippine Sea plate is descending to the NW direction. The configuration of the subducting slab has been revealed by the seismic tomography and refraction/reflection studies. Those studies suggested that the top of the slab was not smooth. A clear image of the subducting ridges was figured out. The characteristic of the asperity is one of the important topics to know the mechanism of the plate boundary earthquakes. There are many discussions for the relationship between the subducting sea mount and asperity. The Tokai region is one of the good fields to know the relationship. The location and configurations of the subducting ridges have been revealed by the refraction and reflection studies. The subducting ridge is located just beneath the land area. If the subducting ridge causes the stress concentration, the spatial pattern of the stress distribution will be changed. It will be detectable by the shear wave splitting analysis at the seismic stations above the ridge. In this area, slow-slip was also reported. The Tokai area is a very good field to know the whole image of the plate coupling. We did temporal seismic observation with about 70 seismic stations in Tokai region. The seismic stations are located as a linear array trending to NW direction which is consistent with the direction of subduction of the Philippine Sea plate. The length of the array was about 100 km. It was operated from April, 2008 to September, 2008. We studied shear wave splitting in this area using the array. The array is located just above the subducting ridge. The spatial variation of the shear-wave splitting values was obtained. The maximum stress direction of the area is NW-SE. The polarization direction of the area with the direction of NW-SE was obtained. That is very consistent with the stress field of the area. The tame lag values at the south are little bit larger than that of north. But, we could not see large lateral variation of the polarization direction and time-lag values along the profile line. We could not find out any close relationship between the topography of the subducting ridge and spatial variation of shear-wave splitting. It is expected that the effect of the subducting ridge to the stress pattern in the crust seems to be small.

Iidaka, T.; Kato, A.; Ikuta, R.; Yoshida, Y.; Katsumata, K.; Iwasaki, T.; Sakai, S.; Tsumura, N.; Yamaoka, K.; Watanabe, T.; Kunitomo, T.; Yamazaki, F.; Okubo, M.; Suzuki, S.; Hirata, N.

2011-12-01

307

Difference in acoustic properties at seismogenic fault along a subduction interface: Application to estimation of effective pressure and fluid pressure ratio  

NASA Astrophysics Data System (ADS)

Fluid pressure along subduction plate boundaries plays a role in seismogenesis and tsunami genesis because it is strongly related to the physical properties of faults. In this study, we conducted P-wave velocity (Vp) and S-wave velocity (Vs) measurements for the hanging wall and footwall of a fossil subduction interface with pseudotachylyte located at the northern edge of the Mugi mélange in the Cretaceous Shimanto Belt, Shikoku, southwest Japan. This area corresponds to the depth of the shallow seismogenic zone in the transition zone between the inner and outer wedges. By combining the acoustic properties with parameters obtained from Amplitude Variation with Offset (AVO) analysis on the Nankai seismic profile, we estimated the fluid pressure at the seismogenic fault. The Mugi mélanges are composed of shale matrices and are juxtaposed with the coherent unit of the Hiwasa formation in the north, which is composed mainly of sandstone. We collected 5 sandstone samples from the hanging wall (Hiwasa formation) and 4 mudstone samples from the footwall (Mugi mélange). We conducted velocity measurements while controlling both the fluid pressure and confining pressure by using two pumps. The effective pressure in each measurement ranged from 5 to 65 MPa with intervals of 5 MPa. The Vp and Vs of sandstone increase exponentially with effective pressure from ~ 4500 to ~ 5000 m/s and ~ 2500 to ~ 3000 m/s, respectively. The Vp and Vs of the mudstone also increased exponentially from ~ 4100 to ~ 4500 m/s and ~ 1900 to ~ 2200 m/s, respectively. We used AVO parameters along the décollement based upon a seismic profile of the Nankai trough, which is off Muroto 40-45 km landward from the trench axis, corresponding to approximately 66 MPa of effective pressure under a hydrostatic condition. By combining the velocities obtained from this study and the AVO parameters derived from the Muroto seismic data, we estimate the mean effective pressures for the hanging wall and the footwall as approximately 10-20 MPa and 8-10 MPa, respectively. The normalized fluid pressure ratios for the hanging wall and the footwall correspond to approximately 0.82-0.91 and 0.91-0.93, respectively. This high fluid pressure indicates a very low effective friction coefficient along the décollement in the transition zone, possibly causing a rupture to propagate to the shallower outer wedge and thus generating a large tsunami.

Hashimoto, Yoshitaka; Doi, Noriaki; Tsuji, Takeshi

2013-07-01

308

Comparative Thermal Structures of Circum-Pacific Subduction Zones  

NASA Astrophysics Data System (ADS)

Subduction zone thermal structure influences Benioff Zone (BZ) seismicity, slab dehydration/metamorphism, geochemical fluxes, and arc magma production. To evaluate effects of relative differences in slab-surface temperature (SST), we have simulated thermal structures of ten circum-Pacific cross-arc transects (Andean, Central American, Cascades, E Aleutian, Kurile, NE Japan, and Mariana arcs) showing large variations in slab age (9-134 Ma), convergence rate (2-10 cm/yr), duration (40-226 Ma), slab length (200-1200 km), and slab geometry. A finite-element method was used to simulate mantle convection and a finite-difference solver used to compute heat transfer. A staggered grid for discretization enhanced accurate modeling of mantle convection. All models assume realistic curved slab geometries as constrained by BZ earthquake hypocenters. Shear-heating is evaluated using rheology of wet quartzite to simulate the presence of a veneer of subducted sediment near upper slab surfaces. Error propagation analysis indicates that the major uncertainty in SST profile is associated with (1) uncertainty in thickness of the overriding lithospheric plate, which determines the size of the convecting wedge domain, and (2) shear-heating contributions. Assuming constant lithosphere thickness (100 km), we estimate a nearly two-fold range in SST (~300 to 600 oC) at BZ depths below the respective volcanic fronts. Moreover, for most transects similar SSTs (~800+/-50 oC) are predicted at depths of the deepest BZ earthquakes. Even where slow subduction of young slabs occur, Ts approaching the wet solidus of oceanic crust or sediments are predicted only if the lithospheric lid is relatively thin or there is very strong shear-heating. Thus, we consider direct melting of subducted materials unlikely in any of the model transects. Assuming that highly fluid-mobile B is derived largely from subducted slabs, B-enrichment data help constrain our thermal models. We show that B-enrichment (e.g., B/Zr ratios) in arc volcanic rocks is anti-correlated with SSTs at 100 km depth. This relation is consistent with progressive loss of fluids and fluid-mobile elements as descending slabs warm, and with variation in slab thermal structure from arc to arc.

Huang, S.; Leeman, W. P.; Sisson, V. B.

2001-12-01

309

A Quantitative Analysis of Flexural Faulting in the Cocos Plate at the Middle America Trench from Nicaragua to Costa Rica  

NASA Astrophysics Data System (ADS)

Flexural faulting in the Cocos plate offshore Nicaragua and Costa Rica seaward of the Middle America Trench was quantitatively analyzed by correlating a stochastic statistical analysis of Cocos plate hydrosweep bathymetry data with a measurement of basement fault relief in MCS data. This work was conducted in order to examine the potential relationship between oceanic basement structure and the efficiency of sediment subduction as signified by geochemical anomalies in the Central American Volcanic Arc. RMS height, characteristic width, and the azimuth of the hydrosweep bathymetry surface topography are shown to be an accurate indicator of basement fault size and distribution as identified in the MCS data. Using hydrosweep bathymetry as a proxy for basement fault structure provides a broader data coverage of the Cocos plate than MCS data alone. The characteristic width and azimuth of the surface topography demonstrate that where the Cocos plate is East Pacific Rise crust, the flexural faults appear to be reactivations of abyssal hill fabric. When the Cocos plate becomes Cocos-Nazca Spreading Center crust off the Nicoya Peninsula, the flexure of the subducting plate breaks new faults. The MCS data image a fairly similar sediment thickness as the Cocos plate approaches the trench (400-500 m) but a decrease in basement fault throw from Nicaragua to Costa Rica. High basement fault relief ( ~500 m) is present in the Cocos plate off Nicaragua but low relief basement faults (<300 m) exist off Costa Rica. The regional scale over which this transition is observed in the MCS data is narrowed in focus by the stochastic modeling of the bathymetry. This transition in fault size, as illustrated by a sharp decrease in the RMS height of the surface relief, is spatially coincident with the boundary between Nicaragua and Costa Rica. During subduction, the flexural response of the downgoing slab can result in large scale extensional faulting in the oceanic crust that generates horsts and grabens, or half-grabens as in the case of Costa Rica and Nicaragua, at the trench. If the thickness of the subducting sediment column is on the same order of magnitude or less, the grabens can act as traps for the downgoing sediment, preserving the entire sediment column for subduction, including the uppermost units. Offscraping or underplating may be more prevalent when graben formation on the downgoing plate is minimal or small in comparison with the overlying sediment thickness. The Central American Volcanic Arc exhibits a wide range of 10Be concentrations, from background to highly enriched. As 10Be is only present in the shallowest sediments on a subducting plate, the high 10Be concentrations observed in the Central American arc volcanics are a clear indication that the youngest, uppermost sediments on the downgoing plate are transported to the volcanic roots. Young sediment incorporation into the volcanics seems to be strongest for Nicaragua but minimal to nonexistent for Costa Rica. It is possible that variation in graben relief along strike of the trench is partially if not largely responsible for this change in arc volcanic 10Be signatures.

Kelly, R. K.; McIntosh, K. D.; Silver, E. A.; Goff, J. A.; Ranero, C. R.; von Huene, R.

2001-12-01

310

The influence of slab-length variations on mantle circulations in the southern Chilean subduction zone  

NASA Astrophysics Data System (ADS)

The depth of the slab base may vary from less than 100 kilometers to several hundred kilometers along the strike of the southern Chilean subduction zone. The variation in slab length would play an important role on shaping the flow pattern. In this study dynamical subduction zone models were constructed to examine the combined effects of slab geometry, plate age, mantle rheology and heterogeneities of plate thickness on the regional mantle circulations. In the models various degrees of slab rollback occur along strike, significantly influenced by the slab length. The differential slab rollback leads to the along-arc pressure gradient causing trench-parallel flow components, while the lower portion of the deep segments acts as a slab edge in part to induce three-dimensional edge flow. These factors together give rise to complex flow patterns, and lead to a great extent of trench-parallel components in the mantle wedge and sub-slab mantle and significant upwelling in the back arc. The length scales of both the poloidal and toroidal components are variable and generally smaller, but the 3D edge flow affects a significantly wider region, compared with flow associated with a slab with a constant length. The model results may help to provide explanations for the complicated anisotropy patterns in the mantle wedge and the sub-slab mantle, the heat sources of the northern Patagonian basalts and the subduction-related components of the MORB from the Chile Ridge. The results may also help to constrain the detailed flow structure in the vicinity of the slab window and the Antarctic slab geometry.

Lin, Shu-Chuan

2014-05-01

311

The Geometry of the Subducting Slabs Beneath the PRVI Microplate Based on 3D Tomography  

NASA Astrophysics Data System (ADS)

The Puerto Rico and the Virgin Islands (PRVI) microplate is located between two subduction zones, with the Puerto Rico trench to the north and the Muertos trough to the south. The Puerto Rico trench is caused by southward-directed subduction of the North American Plate, and the Muertos trough is the northern boundary of the Caribbean Plate. There is no active volcanism on Puerto Rico; however, earthquake depths and seismic tomography imply that the slab of Caribbean plate continues northward beneath Puerto Rico. Puerto Rico overlies these two slabs with extension to both the west (Mona Passage) and southeast (Anacapa Passage). The cause of the extension is unknown, but GPS measurements show that Puerto Rico is experiencing anti-clockwise rotation, and the extension associated with the Anacapa Passage may be produced by this rotation. To the west, it is debated whether the Mona Passage is a boundary between two micro-plates or simple a local rift basin. To address the sources of the extension and the cause of the rotation, we are investigating if the deep structures can be the dynamic source for the observed kinematic movements. We collected data on earthquakes occurring between 2009-2011 in the PRVI region and relocated them using the SEISAN code provided by the Institute of Solid Earth Physics, University of Bergen. The FMTOMO code from Australian National University was used for 3D tomography from P and S wave arrival times. By comparing the relocated epicenters and the 3D tomography results, the subducting slabs were identified. When integrated with the results of previous studies, the geometric model of the slabs is a critical key to understanding the evolution of the PRVI microplate in the past and the future.

Xu, X.; Keller, G. R.; Holland, A. A.; Keranen, K. M.; Li, H.

2011-12-01

312

Paleogeodetic records of seismic and aseismic subduction from central Sumatran microatolls, Indonesia  

USGS Publications Warehouse

We utilize coral microatolls in western Sumatra to document vertical deformation associated with subduction. Microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track the long-term aseismic deformation that occurs during the intervals between earthquakes (paleogeodetic data). This paper focuses on the twentieth century paleogeodetic history of the equatorial region. Our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface. The site closest to the trench rose 90 cm, whereas sites further east sank by as much as 35 cm. Our model reproduces these paleogeodetic data with a 2.3 m slip event on the interface 88 to 125 km from the trench axis. Our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence. Likewise, interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence. Among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or "silent even" in 1962, 27 years after the 1935 event. Paleogeodetic deformation rates in the decades before, after, and between the 1935 and 1962 events have varied both temporally and spatially. During the 25 years following the 1935 event, submergence rates were dramatically greater than in prior decades. During the past four decades, however, rates have been lower than in the preceding decades, but are still higher than they were prior to 1935. These paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century. Copyright 2004 by the American Geophysical Union.

Natawidjaja, D.H.; Sieh, K.; Ward, S.N.; Cheng, H.; Edwards, R.L.; Galetzka, J.; Suwargadi, B.W.

2004-01-01

313

Slab stagnation and buckling in the mantle transition zone: Rheology, phase transition, trench migration, and seismic structure  

NASA Astrophysics Data System (ADS)

Subducting slabs may exhibit buckling instabilities and consequent folding behavior in the mantle transition zone for various combinations of dynamical parameters, accompanied by temporal variations in dip angle, plate velocity, and trench retreat. Parameters governing such behavior include both viscous forces (slab and mantle rheology) and buoyancy forces (slab thermal structure and mineral phase relations). 2D numerical experiments show that many parameter sets lead to slab deflection at the base of the transition zone, typically accompanied by quasi-periodic oscillations (consistent with previous scaling analyses) in largely anticorrelated plate and rollback velocities, resulting in undulating stagnant slabs as buckle folds accumulate subhorizontally atop the lower mantle. Slab interactions with mantle phase transitions are important components of this process (Bina and Kawakatsu, 2010; ?ížková and Bina, 2013). For terrestrial parameter sets, trench retreat is found to be nearly ubiquitous, and trench advance is quite rare - due to both rheological structure and ridge-push effects (?ížková and Bina, 2013). Recent analyses of global plate motions indicate that significant trench advance is also rare on Earth, being largely restricted to the Izu-Bonin arc (Matthews et al., 2013). Consequently, we explore the conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry associated with the Philippine Sea region. Detailed images of buckled stagnant slabs are difficult to resolve due to smoothing effects inherent in seismic tomography, but velocity structures computed for compositionally layered slabs, using laboratory data on relevant mineral assemblages, can be spatially low-pass filtered for comparison with tomographic images of corresponding resolution. When applied to P-wave velocity anomalies from stagnant slab material beneath northeast China, model slabs which undulate due to compound buckling fit observations better than a flat-lying slab (Zhang et al., 2013). Earthquake hypocentral distributions and focal mechanisms may provide clearer insights into slab buckling, as they appear to vary systematically across regions of slab stagnation (Fukao and Obayashi, 2013). Stress fields computed from our dynamical models may help to illuminate such observations. References: Bina, C.R., and H. Kawakatsu, Buoyancy, bending, and seismic visibility in deep slab stagnation, Phys. Earth Planet. Inter., 183, 330-340, 2010. ?ížková, H., and C.R. Bina, Effects of mantle and subduction-interface rheologies on slab stagnation and trench rollback, Earth Planet. Sci. Lett., 379, 95-103, 2013. Fukao, Y., and M. Obayashi, Deepest hypocentral distributions associated with stagnant slabs and penetrated slabs, Fall Meeting Abstracts, AGU, DI14A-01, 2013. Li, Z.-H., and N.M. Ribe, Dynamics of free subduction from 3-D boundary element modeling, J. Geophys. Res., 117, B06408. Matthews, D.C., L. Zheng, and R.G. Gordon, Do trenches advance? Fall Meeting Abstracts, AGU, T43D-2682, 2013. Zhang, Y., Y. Wang, Y. Wu, C. Bina, Z. Jin, and S. Dong, Phase transitions of harzburgite and buckled slab under eastern China, Geochem. Geophys. Geosys., 14, 1182-1199, 2013.

Bina, Craig; Cizkova, Hana

2014-05-01

314

Japan: Tsunami  

... tsunami triggered by the March 11, 2011, magnitude 8.9 earthquake centered off Japan's northeastern coast about 130 kilometers (82 ... inland from the eastern shoreline is visible in the post-earthquake image. The white sand beaches visible in the pre-earthquake view are ...

2013-04-16

315

Areas of slip of recent earthquakes in the Mexican subduction zone  

NASA Astrophysics Data System (ADS)

The Mexican subduction zone is unusual: the width of the seismogenic zone is relatively narrow and a large portion of the co-seismic slip generally occurs below the coast, ~ 45 to 80 km from the trench. The earthquake recurrence interval is relatively short and almost the entire length of the zone has experienced a large (Mw?7.4) earthquake in the last 100 years (Singh et al., 1981). In this study we present detailed analysis of the areas of significant slip during several recent (last 20 years) large earthquakes in the Mexican subduction zone. The most recent earthquake of 20 March 2012 (Mw7.4) occurred near the Guerrero/Oaxaca border. The slip was concentrated on the plate interface below land and the epicentral PGAs ranged between 0.2 and 0.7g. The updip portion of the plate interface had previously broken during the 25 Feb 1996 earthquake (Mw7.1), which was a slow earthquake and produced anomalously low PGAs (Iglesias et al., 2003). This indicates that in this region the area close to the trench is at least partially locked, with some earthquakes breaking the down-dip portion of the interface and others rupturing the up-dip portion. The Jalisco/Colima segment of the subduction zone seems to behave in a similar fashion. The 9 October 1995 (Mw 8.0) earthquake generated small accelerations relative to its size. The energy to moment ratio, E0/M0, is 4.2e-6 (Pérez-Campos, Singh and Beroza, 2003), a value similar to the Feb, 1996 earthquake. This value is low compared to other thrust events in the region. The earthquake also had the largest (Ms-Mw) disparity along the Mexican subduction zone, 7.4 vs 8.0. The event produced relatively large tsunami. On the contrary, the 3 June 1932 earthquake (Ms8.2, Mw8.0), that is believed to have broken the same segment of the subduction zone, appears to be "normal." Based on the available evidence, it may be concluded that the 1932 event broke a deeper patch of the plate interface relative to the 1995 event. The mode of rupture in the subduction zone between the two areas mentioned above is not known. This part of the subduction zone includes the rupture area of the 1985 Michoacán earthquake (Mw8.0) and the "Guerrero Gap" which is a section of the subduction zone that has not had a large earthquake in the last 100 years. The downdip and updip patches on the plate interface, which, generally, rupture independently may slip during one great earthquake. This possibility must be accounted for in the estimation of maximum-magnitude earthquake along the subduction zone.

Hjorleifsdottir, V.; Sánchez-Reyes, H. S.; Singh, S.; Ji, C.; Iglesias, A.; Perez-Campos, X.

2012-12-01

316

Uplift and subduction erosion in southwestern Mexico since the Oligocene: pluton geobarometry constraints  

NASA Astrophysics Data System (ADS)

Details of the late Oligocene to Middle Miocene uplift and tectonic erosion episodes of the southwestern continental margin of Mexico can be inferred using Al tot geobarometry of igneous hornblendes, geochronology, and field relations. On the basis of such analyses carried out between Acapulco and Huatulco we find the following: (1) Calc-alkaline batholiths exposed along the coast from Acapulco to Huatulco, mostly in the 35-25 Ma age range, were emplaced at depths between 13 and 20 km. (2) The contact relationships between these plutons and their host rocks, and the exposure of volcanic counterparts, 70 km from the coastline, indicate a landward decrease in the amount of uplift. (3) A comparison of the time differences between intrusion and cooling ages of batholiths along the coast suggest that cooling rates were, in general, higher between Acapulco and Huatulco than those along the margin between Puerto Vallarta and Manzanillo, 700 km northwest of Acapulco. (4) The uplift of this coastal belt occurred during the late stages of magmatism and after its cessation, triggering intensive subaerial erosion of supracrustal rocks and the exposure of midcrustal rocks such as granitic batholiths and amphibolite facies metamorphic assemblages of the Xolapa Complex. These findings, in conjunction with the geometry of the present continental margin, as well as the offshore tectonic and stratigraphic features, support previous interpretations of very active late Oligocene to Middle Miocene subduction erosion after the onset of strike-slip tectonics related to the detachment and subsequent eastward displacement of the Chortis block. Subduction erosion involved both trench sediments and crystalline (continental framework) rocks. Different rates of continental framework erosion are assessed on the basis of the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism. Subsidence of the offshore continental basement suggests intense episodes of basal erosion of lower continental crust, whereas the construction of the present day accretionary prism and the uplift of the upper slope indicate a decline in the frontal and basal erosion of the continental framework. Comparing the calculated depths of pluton crystallization with the present depth of the continental crust-subducted slab boundary, interpreted using previously published seismic refraction and gravity models, we conclude that onshore basal erosion played a subordinate role during Miocene episodes of subduction erosion. Major removal of lower crustal sections was probably restricted to offshore regions. Plate reconstructions of the Cocos plate and its predecessors with respect to North America indicate that the uplift and probably the offshore subduction erosion in this region coincided with the initial stages of the subhorizontal trajectory of the Guadalupe plate beneath southwestern Mexico.

Morán-Zenteno, Dante J.; Corona-Chavez, Pedro; Tolson, Gustavo

1996-06-01

317

Chronology of historical tsunamis in Mexico and its relation to large earthquakes along the subduction zone  

NASA Astrophysics Data System (ADS)

The chronology of historical earthquakes along the subduction zone in Mexico spans a time period of approximately 400 years. Although the population density along the coast of Mexico has always been low, relative to that of central Mexico, several of the large subduction earthquakes reports include references to the presence of tsunamis invading the southern coast of Mexico. Here we present a chronology of historical tsunamis affecting the Pacific coast of Mexico and compare this with the historical record of subduction events and to the existing Mexican and worldwide catalogs of tsunamis in the Pacific basin. Due to the geographical orientation of the Pacific coat of Mexico, tsunamis generated on the other subduction zones of the Pacific have not had damaging effects in the country. Among the tsunamis generated by local earthquakes, the largest one by far is the one produced by the earthquake of 28 March 1787. The reported tsunami has an inundation area that reaches for over 6 km inland. The length of the coast where the tsunami was reported extends for over 450 km. In the last 100 years two large tsunamis have been reported along the Pacific coast of Mexico. On 22 June 1932 a tsunami with reported wave heights of up to 11 m hit the coast of Jalisco and Colima. The town of Cuyutlan was heavily damaged and approximately 50 people lost their lives do to the impact of the tsunami. This unusual tsunami was generated by an aftershock (M 6.9) of the large 3 June 1932 event (M 8.1). The main shock of 3 June did not produce a perceptible tsunami. It has been proposed that the 22 June event is a tsunami earthquake generated on the shallow part of the subduction zone. On 16 November 1925 an unusual tsunami was reported in the town of Zihuatanejo in the state of Guerrero, Mexico. No earthquake on the Pacific rim occurs at the same time as this tsunami and the historical record of hurricanes and tropical storms do not list the presence of a meteorological disturbance that could explain a surge wave of the height reported. Here we investigate the morphology of the trench in this region to analyze whether a local landslide on the trench slope or in the continental shelf could be the cause of this tsunami.

Suarez, G.; Mortera, C.

2013-05-01

318

Differences in Vp and Vs at a seismogenic subduction interface: application to fluid pressure estimation by AVO analysis with Nankai seismic profile  

NASA Astrophysics Data System (ADS)

Fluid pressure along subduction plate boundaries is thought to play a role in seismogenesis and tsunami genesis because it is strongly related to physical properties of faults. The fluid pressure at on-set of "seismogenic zone" (in transition zone) along plate boundary can be a clue to understand the mechanisms of rupture propagation to shallow outer wedge due to fluid pressure. Many studies have been done to estimate fluid pressure along subduction interface combining physical properties of sediments and seismic profiles. Those studies focused on shallower part of decollement or megaspray fault, not deeper seismogenic subduction interface. In this study, we have conducted Vp and Vs measurements for hanging wall and footwall of a fossil subduction interface with pseudotachylyte located at the northern edge of the Mugi mélange, Cretaceous Shiamnto Belt, Shikoku, SW Japan. Differences in the velocities between hanging wall and footwall were applied to estimate fluid pressure at the seismogenic fault by Amplitude Variation with Offset (AVO) analysis on Nankai seismic profiles. Mugi mélange is composed of tectonic mélanges comprised mainly by shale matrices and sandstone blocks. Temperature and pressure are ~180°C and ~135MPa, respectively, on the basis of fluid inclusion analysis. A fossil seismogenic fault with pseudotachylyte is observed at the northern most boundary of Mugi mélange, juxtaposing with a coherent unit including mainly sandstones in the north. We have collected 5 sandstones from hanging wall, 4 mudstones from footwall; total number of samples is 14. Samples were formed into cylindrical shape; 1.5 inches in diameter and ~6cm long. Direction of measurements is perpendicular to foliations. We controlled both fluid and confining pressures using two pumps. Effective pressure in each experiment ranges form 5MPa to 65MPa with 5 MPa intervals. We used 500kHz of S-wave transducer (PZT) as a source-receiver pair. Because it also generates weak p-wave, Vp and Vs are obtained from the single measurement. Vp and Vs of sandstones increase exponentially with effective pressure from about 4500m/s to about 5000m/s and from about 2500m/s to about 3000m/s, respectively. Vp and Vs of mudstones also increase exponentially from about 4100m/s to about 4500m/s and from about 1900m/s to about 2200m/s, respectively. We used a seismic profile of Nankai trough off Muroto (Park et al., 2001) for AVO analysis. The reflector of decollement from 40-45km landward from trench axis was taken for the analysis where is at the transition zone with about 4.5km in depth corresponding to about 64MPa of effective pressure under hydrostatic condition. Because sandstone in the hanging wall is lack of mineral veins and Mugi mélanges in footwall includes much amount of mineral veins along minor faults, we assumed hydrostatic conditions for hanging wall and gave variations in fluid pressure only in footwall. Combining the velocities obtained from this study and AVO analysis on the Moroto profile, about 17MPa of effective pressure was estimated as a maximum effective pressure. The result indicates that abnormal fluid pressure ratio is about 0.74 along decollement at the depth.

Hashimoto, Y.; Doi, N.; Tsuji, T.

2011-12-01

319

Motion of continental slivers and creeping subduction in the northern Andes  

NASA Astrophysics Data System (ADS)

Along the western margin of South America, plate convergence is accommodated by slip on the subduction interface and deformation of the overriding continent. In Chile, Bolivia, Ecuador and Colombia, continental deformation occurs mostly through the motion of discrete domains, hundreds to thousands of kilometres in scale. These continental slivers are wedged between the Nazca and stable South American plates. Here we use geodetic data to identify another large continental sliver in Peru that is about 300-400 km wide and 1,500 km long, which we call the Inca Sliver. We show that movement of the slivers parallel to the subduction trench is controlled by the obliquity of plate convergence and is linked to prominent features of the Andes Mountains. For example, the Altiplano is located at the boundary of converging slivers at the concave bend of the central Andes, and the extending Gulf of Guayaquil is located at the boundary of diverging slivers at the convex bend of the northern Andes. Motion of a few large continental slivers therefore controls the present-day deformation of nearly the entire Andes mountain range. We also show that a 1,000-km-long section of the plate interface in northern Peru and southern Ecuador slips predominantly aseismically, a behaviour that contrasts with the highly seismic neighbouring segments. The primary characteristics of this low-coupled segment are shared by ~20% of the subduction zones in the eastern Pacific Rim.

Nocquet, J.-M.; Villegas-Lanza, J. C.; Chlieh, M.; Mothes, P. A.; Rolandone, F.; Jarrin, P.; Cisneros, D.; Alvarado, A.; Audin, L.; Bondoux, F.; Martin, X.; Font, Y.; Régnier, M.; Vallée, M.; Tran, T.; Beauval, C.; Maguiña Mendoza, J. M.; Martinez, W.; Tavera, H.; Yepes, H.

2014-04-01

320

Structure of the subducted Cocos Plate from locations of intermediate-depth earthquakes  

NASA Astrophysics Data System (ADS)

Locations of 3,000 earthquakes of 40 to 300 km depth are used to define the 3-D structure of the subducted Cocos Plate under central and southern Mexico. Discrepancies between deep-seated lineaments and surface tectonics are described. Features of particular interest include: (1) a belt of moderate activity at 40 to 80 km depth that parallels the southern boundary of the Mexican Volcanic Plateau; (2) an offset of 150 km across the Isthmus of Tehuantepec where all seismic activity is displaced toward the northeast; (3) three nests of frequent, deep-seated events (80 to 300 km depth) under southern Veracruz, Chiapas and the coast of Mexico-Guatemala. The active subduction process is sharply delimited along a NW-SE lineament from the Yucatan Peninsula, of insignificant earthquake activity. The focal distribution of intermediate-depth earthquakes in south-central Mexico provides evidence of stepwise deepening of the subduction angle along the Trench, starting at 15 degrees under Michoacan-Guerrero to 45 degrees under NW Guatemala. Historical evidence suggests that the hazard to Mexico City from large intermediate-depth earthquakes may have been underestimated.

Lomnitz, C.; Rodríguez-Padilla, L. D.; Castaños, H.

2013-05-01

321

Marine Gravity Measurements at the Subduction Zone offshore Central Chile  

NASA Astrophysics Data System (ADS)

Gravity measurements were carried out during RV SONNE cruise SO-161 (SPOC) in late 2001 between 28° S and 44° S offshore Central Chile along a total length of about 17500 km. The mean accuracy of the data measured with the seagravimeter system KSS31M of BGR is better than 1 mGal. Further foreign marine gravity data were not included due to their considerable lower accuracy. Additional marine gravity data derived from satellite altimetry are needed to augment our data from the survey area. The SPOC data set was compared with 3 different satellite gravity data compilations. The data set with the best statistical results for the gravity differences was used for further gravity map compilations. The map of the freeair gravity is dominated by the anomalies of the main topographic features in the survey area. In the W the oceanic crust of the subducting Nazca Plate is characterized by weak positve gravity anomalies. Landward the anomalies decrease rapidly to less than minus 150 mGal in the Chilean trench. Further towards the coast extends a broad zone of alternating positve and negative freeair gravity anomalies. These could be interpreted either in terms of morphology of the continental slope or heterogeneous density distribution in the upper crust. Additionally Bouguer gravity anomalies were calculated. The anomalies on the Nazca Plate are strongly positive with a clear south - north trending increase of values, which reflect the increasing age of the oceanic crust. The effect of isostatic compensation was calculated assuming Vening-Meinesz models with different parameters. The gravity effect of the isostatic compensation root was eliminated from the Bouguer gravity anomalies and serves as a residual field. The interpretation of isostatic residual fields in this complicated tectonic environment leads to the detection of a series of offshore basins. In the N and the centre of the survey area the distribution of the profiles is rather uniform. For these areas 3D density models were developed. The models consider the results of the multi channel reflection seismic and especially the refraction seismic lines. The models show the density structure of the accretionary wedge which expands from N to S, the continental slope and various basins near to the coast. The possibility of a subduction channel in the southern area is discussed. The effect and trace of fracture zones before and after subduction were investigated.

Heyde, I.; Kopp, H.; Reichert, C.

2003-12-01

322

Seismotectonics of Central Mexico Subduction Zone From Crustal Deformation Studies  

NASA Astrophysics Data System (ADS)

The Pacific coast of Central Mexico is one of the most tectonically active plate margins. Large subduction thrust earthquakes up to Mw 8.0 repeat every 40-60 years. Crustal deformation measurements reveal seismotectonic processes resulting from this plate convergence. Crustal deformation studies in the Mexican subduction zone began in 1992 with GPS campaigns in Jalisco. High precision leveling and tilt meter observations commenced in 1995, and the first continuous GPS site was installed in 1997. Based on these data we estimate a variation of interseismic steady state coupling (?) along most of the subduction zone. On the shallow (less than 25 km depth) seismogenic zone beneath the coast ? = 0.9. This highly coupled zone begins 20 - 25 km from the trench and extends downdip for about 50 km. It coincides with the rupture zones of large thrust earthquakes. Deeper inland, the plate interface is not freely slipping; instead the continental North American and oceanic Cocos plates are partially coupled, ? = 0.4 - 0.7. In Guerrero state, a partially coupled zone extends for more than 100 km. This anomalous width can be attributed to the specific subhorizontal geometry of the plate interface. Continuous GPS observations and available tide gauge data have elucidated a sequence of slow transient slips or silent quakes (SQ) on the partially coupled portion of plate interface in Guerrero and Oaxaca in 1972, 1979, 1996, 1998, 2001, and 2006. The duration of these slow events ranges between 6-12 months. The maximum equivalent magnitude is Mw 7.5 for 2001-2002 SQ and probably significantly surpasses this value in the 1972 SQ. A modeled slip on the interplate fault for 2001-2002 SQ is 10-20 cm. We estimate the SQ propagation of more than 600 km SE from central Guerrero (101° W) during 1972 and 2001-2002 SQ. The propagation rate is about 1-2 km/day. A relation between large earthquakes and SQ is not yet clear. It seems that the 1996 SQ in Guerrero was triggered by the 14 September 1995, Mw 7.3 thrust Copala earthquake. The 2001-2002 SQ is linked to the unusual 8 October 2002, Mw 5.9, shallow normal-faulting Coyuca earthquake. Recently we have observed a number of low frequency non volcanic tremors (NVT) occurring at the edge of the partially coupled zone and the subsequent free slipping plate interface. The NVT high activity and it relation with the SQ is still uncertain.

Kostoglodov, V.; Larson, K. M.; Franco-Sánchez, S. I.

2006-12-01

323

Subduction Zone Concepts and the 2010 Chile Earthqake (Arthur Holmes Medal Lecture)  

NASA Astrophysics Data System (ADS)

Knowledge of convergent margin systems evolved from hypothesis testing with marine geophysical technology that improved over decades. Wegener's drift hypothesis, Holmes mantle convection, and marine magnetic anomaly patterns were integrated into an ocean spreading concept that won wide acceptance after ocean drilling confirmed the crustal younging trend toward the Mid-Atlantic ridge. In contrast, the necessary disposal of oceanic and trench sediment at convergent margins remained largely hypothetical. Fresh interpretations of some coastal mountains as exposing ancient convergent margin rock assemblages and the seismologist's "Wadati-Benioff" zone were combined into a widely-accepted hypothesis. A convergent margin upper plate was pictured as an imbricate fan of ocean sediment thrust slices detached from the lower plate. During the 1980s ocean drilling to test the hypothesis revealed what then were counter-intuitive processes of sediment subduction and subduction erosion. Rather than the proposed seaward growth by accretion, many margins had lost material from erosion. In current concepts, individual margins are shaped by the net consequences of subduction accretion, sediment subduction, and subduction erosion. Similarly, recently acquired age data from ancient subduction complexes reveal periods dominated by accretion separated by periods dominated by tectonic erosion. Globally, the recycling of continental crustal material at subduction zones appears largely balanced by magmatic addition at volcanic arcs. The longevity of the original imbricate fan model in text books confirms its pictorial simplicity, because geophysical images and drill core evidence show that it commonly applies to only a relatively small frontal prism. A better understanding of convergent margin dynamics is of urgent societal importance as coastal populations increase rapidly and as recent disastrous earthquakes and tsunamis verify. The shift in convergent margin concepts has developed through 50 years of improved acquisition techniques, analysis capabilities, and imaging that greatly improves resolution in observational data. The first observatories in boreholes that place instruments closer to earthquake rupture zones have been deployed. Technological improvement should be strongly pursued to meet future challenges. Advanced seismic imaging and the new riser drilling vessel Chikyu are tools to significantly advance understanding of earthquake mechanics but availability is restricted by current global science budgets. The present scientific knowledge leaves great earthquakes and tsunamis an unpredictable "stealth" natural hazard of great proportions.

von Huene, Roland

2010-05-01

324

Numerical modeling of outer rise deformation in the Tonga subduction system: Coupling between outer rise deformation, slab weakening and plate velocities  

NASA Astrophysics Data System (ADS)

During subduction, bending and flexure of oceanic lithosphere generates a topographic bulge seaward of the trench known as the outer rise, which commonly exhibits extensional deformation attributed to slab pull forces and bending stresses. The resulting brittle and viscous deformation may play a significant role in long-term geodynamic processes by limiting the ability of subducted oceanic lithosphere to act as a stress guide driving surface plate motions through slab pull. Recent numerical studies provide varying estimates of slab pull's contribution to surface plate motions [e.g., van Summeren et al. 2012, Ghosh & Holt 2012], while observational constraints suggest old oceanic lithosphere may weaken by 3-4 orders of magnitude as it bends and descends beneath the overriding plate [Arredondo & Billen, 2012]. Preliminary numerical models of outer rise deformation during oceanic-continental convergence (40 Myr oceanic lithosphere) exhibit 10x-150x viscous weakening in the upper plate near the trench, with time-dependent variations related to both changes in slab depth and downgoing-overriding plate coupling (Naliboff et al., in prep). In order to more closely examine the relationship between outer rise deformation, geophysical observations and plate velocities, we consider 2D subduction models of the Tonga subduction system where flow will be strictly driven by upper mantle slab buoyancy as defined by the Slab1.0 model [Hayes et al. 2012]. The resulting subducting plate deformation patterns are compared to observations of outer rise faulting, elastic thickness measurements and outer rise seismicity. While keeping buoyancy forces fixed, we will examine the role of brittle rheology, hydration and downgoing-overriding plate coupling in patterns of subducting plate deformation. These results will provide improved physical understanding of the relationship between slab pull, plate velocities and downgoing plate weakening, and a basis for future work examining the role of additional slab weakening mechanism such as grain size reduction.

Naliboff, J. B.; Billen, M. I.; Gerya, T.; Saunders, J. K.

2012-12-01

325

Permeabilities of Subduction Zone Sediments  

NASA Astrophysics Data System (ADS)

Permeabilities of subseafloor sediments control fluid expulsion from sediments as they are subducted or accreted and thus, compaction state, fluid overpressures, and deformation. We compare results from Integrated Ocean Drilling Program samples to compare to previously-developed permeability-porosity relationships for subduction zone sediments. Hemipelagic clay samples obtained from the incoming plate Kumano transect of the Nankai Trough (NanTroSEIZE) yield slightly lower permeability for a given porosity than previously reported results from Nankai Trough's Muroto transect and are lower than clay-rich sediments from the upper plate of CRISP offshore the Osa Peninsula of Costa Rica (CRISP). Samples from the Pacific Equatorial Transect (PEAT) and the South Pacific Gyre provide further insight to permeability behavior of sediments deposited in the Pacific basin. South Pacific Gyre sediments consist of slowly deposited pelagic clay with little biogenic or coarse clastic input. Measured permeabilities for given porosities are consistently lower than values reported for clay-rich sediments of Nankai and Costa Rica. PEAT samples comprise biogenic oozes and yield inconsistent results, with some of the highest permeabilities (10-14 m2) as well as some results similar to clay-rich sediments.

Screaton, E.; Gamage, K. R.; Daigle, H.; Harris, R. N.

2013-12-01

326

Y sign: clinical indicator to stop trenching and start cracking.  

PubMed

Phacoemulsification is challenging in soft cataracts due to the difficulty in cracking the nucleus by a divide-and-conquer technique or by chopping. We describe another technique, the Y sign for trenching endpoint: a clinical sign that indicates the surgeon should stop trenching and start cracking during classical divide-and-conquer nucleofractis emulsification using proximal downslope trenching. This technique exploits the morphological structure of the lens to precisely gauge the depth of the trench at which the surgeon can split even a soft nucleus into small wedges without instrument cheese-wiring and thus perform safer phacoemulsification while reducing the risk for posterior capsule rupture. PMID:23522581

Kurian, Mathew; Das, Sudeep; Umarani, B; Nagappa, Somshekar; Shetty, Rohit; Shetty, Bhujang K

2013-04-01

327

Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?  

USGS Publications Warehouse

Published areas of high coseismic slip, or asperities, for 29 of the largest Circum-Pacific megathrust earthquakes are compared to forearc structure revealed by satellite free-air gravity, bathymetry, and seismic profiling. On average, 71% of an earthquake's seismic moment and 79% of its asperity area occur beneath the prominent gravity low outlining the deep-sea terrace; 57% of an earthquake's asperity area, on average, occurs beneath the forearc basins that lie within the deep-sea terrace. In SW Japan, slip in the 1923, 1944, 1946, and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350??C isotherm on the plate boundary, the inferred downdip limit of the locked zone. Basin-centered coseismic slip also occurred along the Aleutian, Mexico, Peru, and Chile subduction zones but was ambiguous for the great 1964 Alaska earthquake. Beneath intrabasin structural highs, seismic slip tends to be lower, possibly due to higher temperatures and fluid pressures. Kilometers of late Cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion. The deep-sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes. Basin-centered asperities could indicate a link between subsidence, subduction erosion, and seismogenesis. Whatever the cause, forearc basins may be useful indicators of long-term seismic moment release. The source zone for Cascadia's 1700 A.D. earthquake contains five large, basin-centered gravity lows that may indicate potential asperities at depth. The gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore, except in northwestern Washington, where the low extends landward beneath the coast. Transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes. Published in 2003 by the American Geophysical Union.

Wells, R.E.; Blakely, R.J.; Sugiyama, Y.; Scholl, D. W.; Dinterman, P.A.

2003-01-01

328

Evidence of large prehistoric megathrust earthquakes along the Manila and Philippine trenches deduced from emergent Holocene coral reefs  

NASA Astrophysics Data System (ADS)

Uplifted coral reefs along the coasts of western Pangasinan (Luzon Island) and eastern Davao Oriental (Mindanao Island) in the Philippines are studied in relation to past megathrust earthquakes along the Manila and Philippine trenches. West of Luzon I