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1

Seismic velocity structure of subducting Pacific Ocean slab near Japan trench deduced by airgun-OBS surveys  

Microsoft Academic Search

Recent seismic studies around the Japan trench subduction zone have revealed that both the interplate and intraplate earthquakes occur under strong influence of fluid, in particular the water. Although it is believed that the subducted slab is the most significant carrier of the water into the earth interior, it is not fully understood how the water is transported into the

R. Azuma; R. Hino; Y. Ito; T. Takanami; R. Miura; K. Ichijo; K. Mochizuki; T. Igarashi; K. Uehira; T. Sato; M. Shinohara; T. Kanazawa

2008-01-01

2

Structure and lithology of the Japan Trench subduction plate boundary fault  

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

3

Constraints on interseismic deformation at Japan trench from VLBI data  

NASA Technical Reports Server (NTRS)

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

Argus, Donald F.; Lyzenga, Gregory A.

1993-01-01

4

Diverse Seismicity and Anomalous Boron Fluid Geochemistry in the Japan Trench  

Microsoft Academic Search

During Ocean Drilling Program (ODP) Leg 186, the forearc to the Japan Trench offshore Japan was investigated. The Pacific Plate, subducted at a rate of 8-10 cm\\/y beneath Eurasia, is more than 100 Ma old. Little accretion has occurred during the Neogene and subduction erosion has caused a decrease and subsidence of 6 km of the continental margin off Japan.

A. Deyhle; A. Kopf

2001-01-01

5

Trench motion, slab geometry and viscous stresses in subduction systems  

NASA Astrophysics Data System (ADS)

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

Royden, Leigh H.; Husson, Laurent

2006-11-01

6

Trench-parallel crustal anisotropy along the trench in the fore-arc region of Japan  

NASA Astrophysics Data System (ADS)

In northeastern Japan, the Pacific plate is descending beneath the North American plate. It is generally understood that trench-normal principal stress is dominant in the crust along the Japanese island arc, because the stress field is controlled by the force of the subducting slab. Observations of shear wave splitting using crustal earthquakes reveal a marked lateral variation in fast-polarization direction with distance from the trench. Trench-normal and trench-parallel fast-polarization directions are observed on the back-arc and fore-arc sides, respectively. In this study, two-dimensional finite element modeling with subducting slab was conducted to investigate the interseismic stress field during earthquake cycles, taking into account linear viscoelasticity. In the model, trench-normal compression is found to dominate in the island arc region. However, an extensional field appears in the shallow upper crust of the fore-arc region during earthquake cycles. The trench-parallel crustal anisotropy can be explained by this extensional field.

Iidaka, Takashi; Muto, Jun; Obara, Kazushige; Igarashi, Toshihiro; Shibazaki, Bunichiro

2014-03-01

7

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

8

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

9

Trench migration and overriding plate stress in dynamic subduction models  

NASA Astrophysics Data System (ADS)

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

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

2015-04-01

10

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

11

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

NASA Astrophysics Data System (ADS)

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

Rodríguez-González, Juan; Billen, Magali I.; Negredo, Ana M.

2014-09-01

12

Weak interplate coupling beneath the subduction zone off Fukushima, NE Japan, inferred from GPS\\/acoustic seafloor geodetic observation  

Microsoft Academic Search

We have been carrying out GPS\\/acoustic seafloor geodetic observations at several reference points situated along the Japan Trench, a major plate boundary of subduction. A time series of horizontal coordinates of one of the seafloor reference points, located off Fukushima, obtained from seven campaign observations for the period 2002 2008, exhibits a linear trend with a scattering root mean square

Yoshihiro Matsumoto; Tadashi Ishikawa; Masayuki Fujita; Mariko Sato; Hiroaki Saito; Masashi Mochizuki; Tetsuichiro Yabuki; Akira Asada

2008-01-01

13

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

14

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

NASA Astrophysics Data System (ADS)

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

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

2015-03-01

15

Marine incursions of the past 1500 years and evidence of tsunamis at Suijin-numa, a coastal lake facing the Japan Trench  

Microsoft Academic Search

Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit

Yuki Sawai; Yushiro Fujii; Osamu Fujiwara; Takanobu Kamataki; Junko Komatsubara; Yukinobu Okamura; Kenji Satake; Masanobu Shishikura

2008-01-01

16

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

17

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

18

Heat flow anomaly on the seaward side of the Japan Trench associated with deformation of the incoming Pacific plate  

NASA Astrophysics Data System (ADS)

Extensive heat flow measurements were conducted on the seaward side of the Japan Trench for investigation of the extent and the origin of high heat flow previously found on the incoming Pacific plate. The obtained data combined with the existing data showed that high and variable heat flow values are pervasively distributed seawards of the northern half of the trench and within about 150 km of the trench axis. In this anomalous zone, the average heat flow is 60 to 70mW/m2, appreciably higher than the value typical for the seafloor age of about 135 m.y. The occurrence of the anomalous heat flow along the trench indicates that it results from processes closely related to deformation of the incoming plate. Heating by intra-plate "petit-spot" volcanism and/or fluid flow along normal faults developed on the trench slope may yield local heat flow peaks but cannot raise regional average heat flow. The most probable cause of the observed widespread anomalous heat flow is efficient vertical heat transport by hydrothermal circulation in a permeable layer in the oceanic crust, which is gradually developed by fracturing due to plate bending. Similar heat flow and temperature structure anomalies on the seaward side of the trench may exist in other subduction zones.

Yamano, Makoto; Hamamoto, Hideki; Kawada, Yoshifumi; Goto, Shusaku

2014-12-01

19

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

20

Spatial variation in seismotectonics and stress conditions across the Kurile and Japan trenches inferred from the analysis of focal mechanism data in Hokkaido, northern Japan  

NASA Astrophysics Data System (ADS)

We analyzed seismotectonic characteristics of focal mechanism data across the Kurile and Japan trenches in the Hokkaido region, north Japan, to check for spatial variation in seismotectonics. For this purpose we selected highly constrained focal mechanism data from the Harvard CMT catalog and Japanese National Research Institute for Earth Science and Disaster Prevention (NIED) for the period 1976 to July 2007. Further, we selected data with high double couple components within a depth range of ±7.5 km from the inferred boundary of the subducting Pacific slab. We found a systematic variation in seismotectonic affinity of the data exhibiting eight distinct zones in the region. Among them, the outer rise region exhibits a normal fault regime with biaxial extension along and across the trenches. In the regions hosting the Pacific slab at shallow depths, most of the focal mechanism data lie in the thrust regime and the odd-event regime. This is due to a complicated stress condition with a major component of seismic deformation being accommodated by extensional and strike-slip movements in a thrust regime. Dominantly uniaxial compression acting downdip of the subducted Pacific slab is found in the regions hosting the rupture zones of large subduction earthquakes such that during an interseismic period tectonic stress is loaded by subduction. In the proximity of the Hidaka collision zone the stress conditions suggest that the impact of collision on ongoing seismotectonics is inevitable. In such regions we found thrust regimes with biaxial compression along the collision and subduction directions that favor bulging and/or contortion of the subducted slab.

Ghimire, Subesh; Kasahara, Minoru

2009-03-01

21

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

22

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

23

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

24

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

25

Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan  

Microsoft Academic Search

Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan. Epicenters of the tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 kilometers. The depth of the tremors averaged about 30 kilometers, near the Mohorovic discontinuity. Each tremor lasted for at most a few weeks. The location of

Kazushige Obara

2002-01-01

26

Nonvolcanic deep tremor associated with subduction in southwest Japan.  

PubMed

Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan. Epicenters of the tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 kilometers. The depth of the tremors averaged about 30 kilometers, near the Mohorovic discontinuity. Each tremor lasted for at most a few weeks. The location of the tremors within the subduction zone indicates that the tremors may have been caused by fluid generated by dehydration processes from the slab. PMID:12040191

Obara, Kazushige

2002-05-31

27

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

28

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

29

The December 7, 2012 Japan Trench intraplate doublet (Mw 7.2, 7.1) and interactions between near-trench intraplate thrust and normal faulting  

NASA Astrophysics Data System (ADS)

A pair of large earthquakes ruptured within the Pacific plate below the Japan Trench about 14 s apart on December 7, 2012. The doublet began with an Mw 7.2 thrust event 50-70 km deep, followed by an Mw 7.1-7.2 normal-faulting event in the range 10-30 km deep about 27 km to the south-southwest. The deep lithosphere thrust earthquake is the largest such event to be recorded seaward of the rupture zone of the great March 11, 2011 Tohoku Mw 9.0 earthquake. It follows an extensive intraplate normal-faulting aftershock sequence since 2011 extending up to 100 km east of the trench. Many small normal faulting aftershocks of the doublet occurred along a 60 km-long trench-parallel-trend beneath the inner trench slope. The complex overlapping signals produced by the doublet present challenges for routine long-period moment tensor inversion procedures, but the inadequacy of any single point-source inversion was readily evident from comparisons of different data sets and solutions using different frequency bands. We use a two double-couple inversion of W-phase signals to quantify the doublet characteristics, along with an iterative deconvolution of P-wave signals that extracts a compatible three sub-event sequence. The occurrence of a large deep compressional event near the trench several years subsequent to a great megathrust event is similar to a sequence that occurred in the central Kuril Islands between 2006 and 2009, and appears to be associated with stress changes caused by the preceding interplate thrusting and intraplate normal faulting. Recent large deep compressional events in the Philippine Trench and northern Kermadec Trench regions may be influenced by strain accumulation on adjacent locked interplate megathrusts. Regions having more pronounced curvature of the subducting plate may have unrelaxed bending stresses, facilitating occurrence of large deep thrust faulting in advance of future megathrust failures, as was observed in 1963 in the central Kuril Islands region but not in the gently curving Pacific plate offshore of the great Tohoku event.

Lay, Thorne; Duputel, Zacharie; Ye, Lingling; Kanamori, Hiroo

2013-07-01

30

Mapping P-wave anisotropy of the Honshu arc from Japan Trench to the back-arc  

NASA Astrophysics Data System (ADS)

We determined a 3-D P-wave anisotropic tomography beneath the entire Honshu arc using about 448,000 high-quality P-wave arrival times from 18,335 local earthquakes that occurred beneath the Northeast (NE) Japan land area and the fore-arc area under the Pacific Ocean. Our results show that low-velocity (low-V) zones exist beneath the active arc volcanoes in the crust and in the central portion of the mantle wedge above the subducting Pacific slab. The low-V anomalies are related to the arc magmatism. Low-V zones are also revealed in the fore-arc area, which are probably caused by large volumes of water releasing upwards from dehydration of the subducting oceanic crust and sediments. The anisotropic amplitude in the upper crust is weaker than that in other portions under NE Japan. In the mantle wedge, the fast velocity direction (FVD) is generally trench-normal in back-arc area, which may reflect that the olivine a axis aligns with the transport direction induced by the slab-driven corner flow. The FVD becomes trench-parallel in the central portion of the fore-arc mantle wedge, which is possibly induced by the olivine of B-type fabric in the slab-driven corner flow. The FVD shows trench-parallel in the low-V zones in the fore-arc mantle wedge close to the upper boundary of the Pacific slab, which may reflect the B-type olivine fabric dominating in those areas, or it may be induced by the dextral shearing of the overlying crust. The trench-parallel FVD is also revealed beneath the volcanic front, indicating complex 3-D mantle flows in the mantle wedge. The FVD in the subducting Pacific slab is mostly trench-parallel, which may reflect one or more of the following possibilities: (1) the original fossil anisotropy of the Pacific plate formed at the mid-ocean ridge, (2) cracks within the slab, and (3) the olivine fabric transition due to the changes in water content, stress, and temperature.

Wang, Jian; Zhao, Dapeng

2010-10-01

31

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

32

Alteration of the Subducting Oceanic Lithosphere at the Southern Chile Trench-Outer Rise  

NASA Astrophysics Data System (ADS)

Hydrothermal circulation and brittle faulting process within the oceanic lithosphere is usually confined to the upper crust for oceanic lithosphere created at intermediate-to-fast spreading rates. Lower crust and mantle, however, are relatively dry and undeformed. Recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench-outer rise where extensional bending-faulting affects the hydrogeology of the oceanic crust and mantle. To understand the degree of hydration, we studied the seismic velocity structure of the incoming Nazca plate offshore Southern Chile (~ 43° S) as part of the TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes) initiative. Seismic wide-angle and multichannel data are used to derive 2D velocity model using joint refraction and reflection travel time tomography. The ~ 250 km wide-angle seismic profile starts seaward of the trench axis on the 14.5 Ma old oceanic Nazca Plate, offshore of the rupture area of the Great 1960 Chile earthquake (Mw=9.5). The profile runs perpendicular to the Chile Ridge and parallel to the Chiloe and Guafo Fracture Zones. The velocity model derived from the tomography inversion consists of a ~ 5.5-km thick oceanic crust and shows P-wave velocities typical for mature fast-spreading structures in the seaward section of the profile, with uppermost mantle velocities as fast as ~ 8.3 km/s. Approaching the Chile trench, however, seismic velocities appear to be lower, suggesting a certain degree of hydration and alteration either in the oceanic crust and the uppermost mantle. The reduction of the velocities roughly starts at the outer rise and continues up to the trench, defining a clear low velocity zone. In addition, anomalous low heat flow values at the outer rise indicate an efficient inflow of cold seawater into the oceanic crust through a high basement outcrop.

Contreras-Reyes, E.; Grevemeyer, I.; Flueh, E.; Scherwath, M.; Heesemann, M.

2006-12-01

33

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

34

Timing and magnitude of shortening within the inner fore arc of the Japan Trench  

NASA Astrophysics Data System (ADS)

New structural data and kinematic modeling provide evidence for Plio-Quaternary, inner fore-arc shortening inboard of the Japan Trench, northeastern Honshu, accommodated by the Futaba fault, a high-angle, basement-involved fault that bounds the Abukuma massif on the east. Significant throw along the Futaba fault associated with exhumation of the massif is implied by a regionally extensive footwall syncline, the absence of Neogene sediments in the hanging wall, and high relief in the hanging wall adjacent to the fault. Kinematic fault-related fold modeling best reproduces fold geometry with 2.0-3.1 km of dip slip along a 40°-55° west dipping reverse fault. At the southern tip of the fault, tephra horizons of known age within units that predate and postdate deformation bracket the onset of deformation to 3.95-5.6 Ma and are used to calculate an average slip rate of 0.5-0.7 mm/yr, a throw rate of 0.3-0.5 mm/yr, and a shortening rate of 0.3-0.5 mm/yr. The northeastern Japan subduction zone is viewed as a classic example of an erosive margin, where offshore subsidence records have been used to argue for Neogene basal erosion of the upper plate. Tectonic erosion rates have been estimated from reconstructions of the paleomargin that assume no upper plate deformation and temporally constant fore-arc taper. Evidence presented here for Neogene fore-arc shortening, however, suggests that the upper plate is deformable and implies that that offshore subsidence records may reflect a combination of tectonic erosion and upper plate shortening.

Regalla, Christine; Fisher, Donald; Kirby, Eric

2010-03-01

35

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

36

Characteristics of Plate Boundary Reflected Phases at the Japan Trench Obtained by OBS-Airgun Study in 2001. -Preliminary Results-  

NASA Astrophysics Data System (ADS)

Recent GPS and rapture process studies for large earthquakes suggest that earthquake generation is controlled by the distribution of asperities. Among the subduction zones around the Japanese islands, the subduction zone along the Japan Trench is one of the well studied areas in viewpoint of asperity distribution. The seismic experiment using controlled source-OBS study in 1996 revealed strong correlation between seismic reflection intensity and the seismic activity in 38o40'N and 39o00'N on the forearc slope of the Japan Trench (Fujie et al., 2002). P-wave velocity structure under the NS-line can be represented by horizontal layering structure, and the plate boundary is located at slightly deeper depth than 10 km (Suyehiro and Nishizawa, 1994 and Fujie et al., 2000). In order to study 3-D distribution of seismic reflection intensity at the subdction plate boundary for this region, we carried out a seismic experiment in 2001 in a part of the area of the 1996 experiment, with a denser OBS array. Thirty-nine OBSs were placed on 30 km x 50 km along seven seismic lines. Airguns were used as artificial seismic sources, and their average total chamber volume was 57 liters. While making record sections, each trace was plotted at the mid point between the source and receiver locations so that reflected arrivals were plotted at their approximated reflection points. The distribution of seismic reflection intensity observed in this experiment agreed well with the result of Fujie, 2000. In their results, strong seismic reflections from plate boundary occur at aseismic region. The seismic reflection intensities were found to be largely heterogeneous over the surveyed area. We identified these reflected arrivals as reflections from plate boundary.

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

2002-12-01

37

Draft du 6 avril-2009, submitted (and rejected) to Advances in Space Research Chilean-French cGPS Operations along the Subduction Trench in Chile, South-1  

E-print Network

GPS Operations along the Subduction Trench in Chile, South-1 America2 3 Christophe Vigny1 , Anne Socquet2 , Alain Division Technique de l'INSU, Meudon, France11 4 Departamento de Geofísica (DGF), Universidad de Chile, Santiago, Chile.12 13 Abstract:14 The subduction in Chile is very active with in average a Mw 8 event every

Vigny, Christophe

38

Interaction of subducted slabs with the mantle transition-zone: A regime diagram from 2-D thermo-mechanical models with a mobile trench and an overriding plate  

NASA Astrophysics Data System (ADS)

zone slab deformation influences Earth's thermal, chemical, and tectonic evolution. However, the mechanisms responsible for the wide range of imaged slab morphologies remain debated. Here we use 2-D thermo-mechanical models with a mobile trench, an overriding plate, a temperature and stress-dependent rheology, and a 10, 30, or 100-fold increase in lower mantle viscosity, to investigate the effect of initial subducting and overriding-plate ages on slab-transition zone interaction. Four subduction styles emerge: (i) a "vertical folding" mode, with a quasi-stationary trench, near-vertical subduction, and buckling/folding at depth (VF); (ii) slabs that induce mild trench retreat, which are flattened/"horizontally deflected" and stagnate at the upper-lower mantle interface (HD); (iii) inclined slabs, which result from rapid sinking and strong trench retreat (ISR); (iv) a two-stage mode, displaying backward-bent and subsequently inclined slabs, with late trench retreat (BIR). Transitions from regime (i) to (iii) occur with increasing subducting plate age (i.e., buoyancy and strength). Regime (iv) develops for old (strong) subducting and overriding plates. We find that the interplay between trench motion and slab deformation at depth dictates the subduction style, both being controlled by slab strength, which is consistent with predictions from previous compositional subduction models. However, due to feedbacks between deformation, sinking rate, temperature, and slab strength, the subducting plate buoyancy, overriding plate strength, and upper-lower mantle viscosity jump are also important controls in thermo-mechanical subduction. For intermediate upper-lower mantle viscosity jumps (×30), our regimes reproduce the diverse range of seismically imaged slab morphologies.

Garel, F.; Goes, S.; Davies, D. R.; Davies, J. H.; Kramer, S. C.; Wilson, C. R.

2014-05-01

39

An Andean model of interplate coupling and strain partitioning applied to the flat subduction zone of SW Japan (Nankai Trough)  

NASA Astrophysics Data System (ADS)

Transcurrent motion is commonly observed in cases of oblique subduction, yet the causes remain controversial. A model of increased interplate coupling linked to flat subduction is proposed as a mechanism for producing strain partitioning at a great distance (several hundred km) from the trench. The model, originally developed for the North Andean margin, is applied to the geodynamics of SW Japan. Here, along the Nankai Trough and below adjacent SW Honshu, flat subduction occurs due to the relatively young age (<20 Ma) of the oceanic lithosphere and the buoyancy of the flanking Palau-Kyushu Ridge and the Izu Bonin arc. Although dextral strike-slip motion occurs along the median tectonic line (MTL) in Shikoku as shown by GPS studies, it has remained seismically inactive for the last 1000 years. Evidence is presented supporting a second transcurrent fault system to the north of the MTL, the North Chugoku Shear Zone, associated with four M?7 and nine additional M?6 earthquakes in the past 300 years. Dextral strike-slip focal mechanisms for 15 modern and historical events indicate a significant degree of subduction-related strain partitioning manifested by right-lateral strike-slip faulting along the adakitic volcanic arc, some 400 km from the Nankai Trough.

Gutscher, Marc-André

2001-04-01

40

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

41

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

NASA Astrophysics Data System (ADS)

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

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

2010-07-01

42

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

NASA Astrophysics Data System (ADS)

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

Nishimura, T.

2013-12-01

43

Dense GPS Array Observations Across the Nankai Subduction Zone, Southwest Japan  

Microsoft Academic Search

Interseismic deformation in an oblique subduction zone is a mixture of short-term crustal shortening in the direction of plate convergence and permanent margin-parallel movement of a forearc block. We have deployed two dense GPS traverse arrays across the southwest Japan arc to better illustrate strain partitioning in the Nankai subduction zone. In 1998 we constructed the first array that composed

T. Tabei; S. Miyazaki; M. Hashimoto; T. Matsushima; T. Kato; S. Kato

2004-01-01

44

Interplate seismogenic zones along the Kuril-Japan trench inferred from GPS data inversion  

Microsoft Academic Search

In the subduction zones around Japan, where four plates interact with one another, large earthquakes have occurred repeatedly. These interplate earthquakes are part of the process of tectonic stress accumulation and release that is driven by relative plate motion. Stress accumulation between earthquakes results from slip deficit (slip that is insufficient to fully accommodate plate movement). For the prediction of

Chihiro Hashimoto; Akemi Noda; Takeshi Sagiya; Mitsuhiro Matsu'Ura

2009-01-01

45

Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback  

Microsoft Academic Search

Subduction of tectonic plates limited in lateral extent and with a free-trailing tail, i.e., ``free subduction,'' is modeled in a three-dimensional (3-D) geometry. The models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength, width, and thickness. We investigate the time evolution of this progressive rollback

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

2006-01-01

46

Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback  

Microsoft Academic Search

Subduction of tectonic plates limited in lateral extent and with a free-trailing tail, i.e., “free subduction,” is modeled in a three-dimensional (3-D) geometry. The models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength, width, and thickness. We investigate the time evolution of this progressive rollback

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

2006-01-01

47

Earthquake occurrence along the Java trench in front of the onset of the Wadati-Benioff zone: Beginning of a new subduction cycle?  

Microsoft Academic Search

Spatial distribution of earthquake foci in the central part of the Sunda Arc was analyzed using global seismological data. The analysis revealed the existence of a distinct strip of earthquakes distributed along the Java trench, separated by a trench-parallel, 50-150 km wide aseismic link from seismicity belonging to the Wadati-Benioff zone of the recently subducting slab. The seismicity pattern at

Ales Spicák; Václav Hanus; Jirí Vanek

2007-01-01

48

Earthquake occurrence along the Java trench in front of the onset of the Wadati-Benioff zone: Beginning of a new subduction cycle?  

Microsoft Academic Search

Spatial distribution of earthquake foci in the central part of the Sunda Arc was analyzed using global seismological data. The analysis revealed the existence of a distinct strip of earthquakes distributed along the Java trench, separated by a trench-parallel, 50–150 km wide aseismic link from seismicity belonging to the Wadati-Benioff zone of the recently subducting slab. The seismicity pattern at

Aleš Špi?ák; Václav Hanuš; Ji?í Van?k

2007-01-01

49

Strong B enrichment and anomalous ? 11B in pore fluids from the Japan Trench forearc  

Microsoft Academic Search

Interstitial pore waters from Ocean Drilling Program (ODP) Sites 1150 and 1151, both penetrating ?1200 m of sub-seafloor sediment from the upper Japan Trench forearc, were analyzed for B concentrations and ?11B isotopes. B concentrations cover a wide range from 329 to 3920 ?M (0.8–9.3× seawater) and vary isotopically from +20 to +46‰ (relative to SRM 951). In general, B

Annette Deyhle; Achim Kopf

2002-01-01

50

Numerical modeling of subduction beneath non-uniform overriding plates: Time-dependent evolution of slab geometry and trench-parallel flow  

NASA Astrophysics Data System (ADS)

Seismic anisotropy measurements show that the fast spreading direction below the slab is aligned parallel to the trench in the central region and perpendicular near the edges. Above the slab it has a complex pattern, often showing abrupt transitions between trench-parallel and trench-perpendicular directions and sharp changes in intensity. The origin of this complex pattern is poorly understood, however, previous models have shown that variations in slab geometry can cause trench-parallel flow above the slab. In turn, overriding plate thermal state influences the slab dip, which suggests a causal link between overriding plate structure, slab geometry and mantle flow in subduction zones. We study the effect of along-strike variations in thermal thickness of the overriding plate on the evolution of slab geometry and induced mantle flow. To perform the study we implement generic 3D time dependent thermo mechanical numerical models of buoyancy driven subduction using CitcomS. We find that increased hydrodynamic suction beneath the colder portion of the overriding plate causes shallower slab dip. The variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. The mantle flow pattern responds to the changing geometry of the slab, which makes the process strongly time-dependent. The location and strength of trench-parallel flow vary throughout the simulations, which suggests that the global variability in seismic anisotropy in present-day observations is in part due to the non-steady-state behavior of subduction systems. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones, where both slab dip and overriding plate thermal state are strongly variable and correlated.

Rodriguez-Gonzalez, Juan; Billen, Magali; Negredo, Ana

2014-05-01

51

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

52

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

53

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

54

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

55

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

NASA Astrophysics Data System (ADS)

The Tonga-Kermadec plate boundary is the most linear, fastest converging and most seismically active subduction zone on Earth. The margin is intersected at ~26° S by the Louisville Ridge seamount chain. Crustal structure of both the overthrusting Indo-Australian and subducting Pacific plate are sufficiently uniform north and south of the contemporary collision zone to make this an ideal location to study the mechanics and seismological consequences of seamount subduction. We present here a synthesis and interpretation of structural observations from the Louisville collision zone made during three marine geophysical surveys onboard R/V Sonne in 2004, 2007-2008 and 2011. The Louisville collision zone is characterized by a 3000 m reduction in trench depth and a 15° anticlockwise rotation of the trench axis. Swath bathymetry data reveal a pronounced forearc high (~ 2000 m relative to adjacent regions), which is correlated with a free-air gravity and magnetic anomaly high (50 mGal and 200 nT peaks respectively). Morphological characteristics are accompanied by a 40 % reduction in seismicity compared to regions immediately to the north and south. Forward modeling of active source seismic travel-times constrain the subducting Pacific plate to ~30 km depth and suggests that it is ~6 km thick and has Vp 6.2-6.8 km/sec. The overthrusting Indo-Australian plate has Vp 4.5-6.8 km/sec and a Moho depth of 15 km. The mantle wedge has Vp ~8.0 km/sec. Beneath the forearc high, seismic wave-speeds within the upper-plate are 0.3-0.5 km/sec slower than regions to the north and south and a up to 3 km thick volume of anomalously low Vp (<4.5 km/sec at > 10 km depth) is inferred to overlie the subduction interface. This latter observation is interpreted as subducting and underplated volcaniclastic sediments, which reach up to 1-2 km in thickness within the flanking flexural moats of the Louisville Ridge. The projected width of the ridge and flanking moats are well correlated with the seismic gap suggesting a gap causality related to subducting sediments and possibly high pore-fluid pressure. In addition to the development of fracture networks in the overthrusting plate, which likely result in lower Vp, we suggest the presence of subducting sediments likely contribute to forming conditions favoring intact and aseismic seamount subduction. Moreover, subducting seamounts and aseismic ridges uplift the overlying wedge resulting in a translation of anomalous plate-interface to anomalous trench-slope topography. We note a strong correlation between the sharp up-dip limit of anomalous trench-slope topography and the intersection of the subducting slab with the forearc Moho. Hence, we extend existing mechanical models of seamount subduction in suggesting the influence of subducting relief on the structure of the overthrusting plate is limited by the slab - forearc Moho intersection. Global observations of subducting relief are presented in support of this extension.

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

2013-12-01

56

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

57

Seismic activity of very low-frequency earthquake on the subducting Philippine Sea plate near the Nankai Trough, southwest Japan  

Microsoft Academic Search

The Nankai trough subduction zone in southwest Japan is characterized by some kinds of _gslow earthquake_h. Around the deep side of the seismogenic zone on the subducting Philippine Sea plate, non-volcanic tremor is distributed in a narrow belt along the strike of the plate (Obara, 2002). On the other hand, on shallower parts of the seismogenic zone, an anomalous seismic

K. Obara; Y. Ito

2004-01-01

58

Interseismic deformation of the Nankai subduction zone, southwest Japan, inferred from three-dimensional crustal velocity fields  

Microsoft Academic Search

We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the

T. Tabei; M. Adachi; S. Miyazaki; T. Watanabe; S. Kato

2007-01-01

59

Repeated GPS\\/Acoustic seafloor positioning off Fukushima Prefecture in central Japan  

Microsoft Academic Search

Our group in Tohoku University has been trying to measure horizontal seafloor displacement in the subduction zone along the Japan trench using a GPS\\/Acoustic (GPS\\/A) positioning system. Sanriku region is the area that the Pacific plate is subducted under the North American plate. The plate coupling varies along the subduction zone: the coupling is strong off Miyagi, weak off Iwate,

T. Mizukami; H. Fujimoto; Y. Osada; M. Kido; M. Nishino; S. Miura

2006-01-01

60

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

61

A detailed receiver function image of the upper mantle discontinuities in the Japan subduction zone  

Microsoft Academic Search

We have imaged the upper mantle discontinuities in a 30×20° large region at the active continental margin of the Japan subduction zone and neighboring areas, using P-to-S converted phases from teleseismic records of permanent broadband stations. The 410 km discontinuity is detected within ±10 km of its global average position. An interesting exception in its observation is a gap near

X. Li; S. V. Sobolev; R. Kind; X. Yuan; Ch. Estabrook

2000-01-01

62

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

NASA Astrophysics Data System (ADS)

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

Padhy, Simanchal; Furumura, Takashi; Maeda, Takuto

2014-11-01

63

High-Resolution Subduction Zone Seismicity and Velocity Structure in Ibaraki, Japan  

Microsoft Academic Search

We use double-difference tomography (tomoDD) [Zhang and Thurber, 2003] and waveform-derived cross-correlation differential arrival times to invert for the earthquake locations and P- and S-wave velocity distributions in the subduction zone under Ibaraki Prefecture of north-central Honshu, Japan. The Ibaraki region is attractive for its high rate of slab seismicity and for the presence of an intermediate-depth double seismic zone.

D. R. Shelly; G. C. Beroza; H. Zhang; C. H. Thurber; S. Ide

2004-01-01

64

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

65

Stresses in the subducting slab beneath southwest Japan and relation with plate geometry, tectonic forces, slab dehydration, and damaging earthquakes  

Microsoft Academic Search

We infer stresses in the subducting slab beneath southwest Japan from focal mechanisms and discuss their geodynamical implications. The majority of the 824 fault plane solutions used in this study were obtained by the Japan Meteorological Agency for earthquakes that occurred during 1970–2002 and were previously unpublished. The focal mechanisms are believed to reflect stresses in both the untransformed slab

Kelin Wang; Ikuko Wada; Yuzo Ishikawa

2004-01-01

66

The Japan Trench Fast Drilling Project (JFAST): Success in logging, sampling and instrumenting the megathrust in the region of large slip during the 2011 Tohoku-Oki earthquake  

NASA Astrophysics Data System (ADS)

The very large fault slip during the 11 March 2011 Tohoku-Oki earthquake, reaching a maximum of >50 m near the Japan Trench, is the largest ever observed for an earthquake and responsible for the peak tsunami heights of 20 to 40 meters that devastated a large portion of the coast of northeast Honshu. Although the cause of significant seismic slip at shallow depths is not entirely understood, a number of possible contributing factors have been identified. Key questions include how displacement was accommodated near the trench, and whether coseismic weakening of the shallow megathrust had a role in the mechanics of such large displacement. These and other questions are being addressed with data from the recently completed rapid-response expedition undertaken by the Integrated Ocean Drilling Program (IODP). Drilling the plate boundary interface was technically challenging because of the 6.9 km water depth and the need to penetrate > 800 m through the prism to reach the subducting plate. Nonetheless, three successful holes were drilled to the target depth. Logging and spot coring data from the first two boreholes indicate that the location of the plate-boundary décollement is tens of meters above bedded chert on the basaltic crust of the subducting plate. Notably, the décollement is considerably thinner than subduction thrusts drilled elsewhere. Distinguishing characteristics of the décollement that are compatible with coseismic weakening include the pronounced localization of shear to a meters-thick layer of scaly clay and to mesoscale slip surfaces within the layer. A one meter section of the scaly clay was retrieved, which provides ample material for characterization of structural, physical, chemical and mechanical properties of the plate interface, and post-cruise analyses of samples are already producing new results. The third hole was completed during the second leg of the expedition, and a temperature measurement string was successfully installed across the plate boundary to quantify heat generated in this shallow region of the fault zone and allow determination of the fault shear strength during the earthquake.

Chester, F. M.; Mori, J. J.; Eguchi, N.; Toczko, S.; Fulton, P. M.; Brodsky, E. E.

2012-12-01

67

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

68

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

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

69

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

70

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

71

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

NASA Astrophysics Data System (ADS)

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

Schellart, W. P.; Rawlinson, N.

2013-12-01

72

A triple-planed structure of seismicity and earthquake mechanisms at the subduction zone off Miyagi Prefecture, northern Honshu, Japan  

NASA Astrophysics Data System (ADS)

A detailed cross-section of seismicity combined with types of earthquake mechanisms was constructed for the subduction zone off Miyagi Prefecture, northern Honshu, Japan, where a large Ms = 7.5 earthquake occurred on June 12, 1978. Nodal-plane solutions were determined for fourteen earthquakes with body wave magnitudes ?5.4 using data recorded at WWSSN and Japanese stations. For smaller-magnitude events, the types of focal mechanism were analysed using P-wave first-motion data recorded at regional stations. The results obtained are: (1) For the area 150-200 km landward from the trench axis, the cross-section suggests a triple-planed seismic zone; that is, the zone of thrusting overlaps the so-called double-planed seismic zone. The upper and lower planes of the double-planed seismic zone are characterized by down-dip compression and down-dip tension, respectively. (2) Thrusting terminates at a depth of 60 km, where the aseismic front in the area is located, and it merges into the zone of down-dip compression in the deeper part, in a down-dip direction. In contrast, the depth of the zone of down-dip compression appears to shift by 10 km at the aseismic front. (3) The extension of the zone of down-dip compression seaward beyond the aseismic front and the shift of the depth of the zone may be caused by loading within the descending slab through a mechanical coupling between the plates at the focal area of the 1978 earthquake prior to its occurrence.

Seno, Tetsuzo; Pongsawat, Bubpha

1981-09-01

73

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

74

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

75

Japan trench studies on earthquake, mass-wasting deposits and related tsunami based on most recent submarine survey  

NASA Astrophysics Data System (ADS)

On 11 March 2011, Tohoku, northeast Japan, experienced a great earthquake (Mw 9.0, Mt 9.1). Seismic and tsunami inversion analyses have shown that tsunami waves with a maximum run-up height of 38 m were generated after the mainshock by topographic changes on the seafloor in the toe region of the Japan Trench slope off Sendai. These inversion analyses (Maeda et al., 2011) and bathymetric surveys (Fujiwara, JAMSTEC press release, 2011) indicate that the toe region slipped about 50 m along the thrust. If the thrust fault rapidly deformed the seafloor, as suggested by Ide et al. (2011), the basic theory of tsunamigenesis would predict the generation of tsunamis all along the axis of the Japan trench. The Japan Trench slope can be divided into an upper slope, a midslope terrace, and a lower slope. The average slope angle of the upper and lower slopes is 5°, but the angle of the midslope terrace is only a few degrees (von Huene and Lallemand, 1990). Some residual convex parts of the upper and lower slopes have slope angles of ~10°, particularly in the region from 39°10'N to 40°30'N. These include many large, convex upward, arcuate topographic features that indicate submarine sliding of sediment masses with widths and lengths of several kilometers (Sasaki, 2004). These submarine slides have been attributed to tectonic erosion (von Huene and Lallemand, 1990). Most of the slides on the upper slope are characterized by many normal faults (Tsuru et al., 2002; von Huene and Lallemand, 1990). In our study area, an active normal fault may have ruptured during the 2011 Tohoku earthquake as shown by Tsuji et al. (2011). Our deep-sea camera observations suggest that the sliding is still occurring, and also that sliding might have been triggered by the 2011 Tohoku earthquake. We demonstrate a new scenario to excite the tsunami that the toe of the slope moves by the double effect of seismic slip and submarine sliding. This scenario has been proposed to have occurred in the Nankai accretionary prism, southwestern Japan (Kawamura et al., 2011). If this scenario is correct, the mechanisms of both the tsunami generation and the overshoot at the toe of the trench slope can be explained by this double movement. Authors: Kiichiro Kawamura, Takafumi Kasaya, Tomoyuki Sasaki, Toshiya Kanamatsu, Arito Sakaguchi, Takeshi Tsuji, Yujiro Ogawa, YK11-E04 Leg1 and YK11-R06 Leg1 Shipboard Scientists

Kawamura, K.; Kasaya, T.; Sasaki, T.; Kanamatsu, T.; Sakaguchi, A.; Tsuji, T.

2011-12-01

76

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

E-print Network

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

Kelly, Robyn K

2003-01-01

77

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

78

On-land active thrust faults of the Nankai-Suruga subduction zone: The Fujikawa-kako Fault Zone, central Japan  

NASA Astrophysics Data System (ADS)

This paper describes the tectonic topography that characterizes recent thrusting along, the on-land active fault zone of the Nankai-Suruga subduction zone, called the Fujikawa-kako Fault Zone, located near the triple junction of the Eurasian (EUR), Philippine Sea (PHS), and North American (NA) plates, in the western side of Mt. Fuji, central Japan. The analysis was based on interpretations of aerial photographs and 3D perspective images made with Digital Elevation Model (DEM) data, field investigations, and trench excavations. Our study shows the following new observations: 1) distinct east-facing fault scarps are developed on the west-facing slopes, alluvial fans, and terraces of western Mt. Fuji; ii) the total length of the fault zone is ~ 40 km; iii) the Older stage (ca. 8000-14,000 yr) Fuji lavas have been displaced by as much as 70 m; and iv) the 864-865 AD Jogan lava flow is displaced by 2-4 m vertically along the scarp at the northeastern end of the fault zone. Based on the offsets of lavas and mudflow deposits, as well as historical records, it is found that i) the vertical slip rate for the fault zone is up to 5-8 mm/yr, ii) the recurrence interval of morphogenic earthquakes is estimated to be 150-500 yr, and iii) the most recent seismic faulting event along the Fujikawa-kako Fault Zone is inferred to be related to the 1854 AD (M 8.0-8.5) Ansei-Tokai earthquake. When compared with the active intraplate faults of Honshu, Japan, the relatively high slip rates and short recurrence intervals for morphogenic earthquakes in the Fujikawa-kako Fault Zone indicate that the activity of this fault zone is closely related to subduction-zone earthquakes and plate convergence near the triple plate junction of the EUR, PHS, and NA plates.

Lin, Aiming; Iida, Kenta; Tanaka, Hideto

2013-08-01

79

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

80

Upper-mantle seismic discontinuities in a subduction zone (Japan) investigated from P to S converted waves  

Microsoft Academic Search

The lateral variations of the upper mantle discontinuities in a subduction context are investigated by the use of broadband wave data provided by the GEOSCOPE network at the Inuyama station (Japan). Differential arrival times of P and P-to-S conversions on the discontinuities are used. These converted phases, though of weak amplitude, can be observed by stacking many seismograms. A stack

Jean-Louis Thirot; Jean-Paul Montagner; Lev Vinnik

1998-01-01

81

Time-dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan  

Microsoft Academic Search

We estimate time variation of interplate coupling between the subducting Philippine Sea and the overriding continental plates along the western part of the Nankai trough, Japan, during the period between 1996 and 1999. After subtracting annual components and average annual velocities, deviation from steady crustal deformation in the Shikoku and Kyushu islands is examined. Around 15 GPS stations facing the

Shinzaburo Ozawa; Makoto Murakami; Takashi Tada

2001-01-01

82

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

NASA Astrophysics Data System (ADS)

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

Murotani, S.; Satake, K.

2012-12-01

83

Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (Mw 9.0) and subduction dynamics  

NASA Astrophysics Data System (ADS)

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate- to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Liu, Xin; Zhao, Dapeng; Li, Sanzhong

2014-02-01

84

A New View on the Space-Time Pattern of M>7 Earthquakes in the Northern Japan to Kurile Subduction Zones  

NASA Astrophysics Data System (ADS)

The northern Japan to southern Kurile trenches have been regarded as a typical subduction zone with spatially and temporally regular recurrence of great (M>8) interplate earthquakes. The source regions were grouped into six segments by Utsu (1972; 1984). The Headquarters for Earthquake Research Promotion of the Japanese government (2004) divided the southern Kurile subduction zone into four regions and evaluated future probabilities of great interplate earthquakes. Besides great interplate events, however, many large (M>7) interplate, intraslab, outer-rise and tsunami earthquakes have also occurred in this region. First, we depicted the space-time pattern of M>7 earthquakes along the northern Japan to Kuril trench, based on the relocated mainshock-aftershock distributions of all types of earthquakes occurred since 1913. We relocated the hypocenters reported in the ISC, ISS, and BCIS bulletins by using the HYPOSAT (Schweitzer, 2003) and the Modified JHD method (Hurukawa, 1995). Then, in order to examine more detail space pattern, or rupture areas, of M>7 earthquakes since 1963, we estimated coseismic slip distributions by the Kikuchi and Kanamori's (2003) teleseismic body wave inversion method. The WWSSN waveform data were used for earthquakes between 1963 and 1990, and digital teleseismic waveform data compiled by the IRIS were used for events after 1990. Mainshock hypocenters relocated by us were used as initial rupture points. As a result, we found that the more complex feature of earthquake occurrence. Each region has been ruptured by a M8-class interplate earthquake or by multiple M7-class events. Offshore Urup Is. is source region of the 1963 Urup earthquake (M 8.5). Large interplate earthquakes occurred in the eastern and western part of the 1963 source region in 1991 (M 7.6) and 1995 (M 7.9), respectively. Their aftershock areas almost re-occupied the 1963 aftershock area. The 1963, 1991, and 1995 coseismic slip distributions show that the southwestern asperity of the 1963 event seems to be re-ruptured by the 1995 earthquake. Giant (the 2011 Tohoku earthquake of M9.0 which occurred just southern region of our study area), great and large interplate earthquakes occurred in the Kurile to Japan subduction zone after 1990s successively. The aftershock areas and coseismic slip distributions clearly show that only three seismic gaps (offshore northern part of Aomori pref., offshore eastern Hokkaido to Etorofu Is., and offshore between Urup and Simushir Is.) have remained in this region. Great intraslab earthquakes occurred in 1958 and 1994. The 1915 and 1918 great earthquakes may have been intraslab events. Many outer-rise earthquakes and the 1963 and 1975 tsunami earthquakes occurred near the trench axis. The 2009 Simushir earthquake (M 7.4) with reverse faulting occurred within the aftershock area of the 2007 great outer-rise event (M 8.1). The 2007 and 2009 coseismic slip distributions show that the 2007 normal faulting occurred in the shallower part of the Pacific plate and the 2009 reverse intraplate faulting occurred in the deeper part.

Harada, T.; Satake, K.

2012-04-01

85

Cross-Correlation-Based Relocation of Intermediate-Depth Subduction Seismicity in Japan  

NASA Astrophysics Data System (ADS)

We present very high precision hypocenter relocations of intermediate-depth subduction earthquakes off the Ibaraki Prefecture of north-central Honshu, Japan and analyze their implications for subduction mechanics and the mechanism of intermediate-depth earthquakes. Our relocations are obtained using cross-correlation-derived differential arrival times and the double-difference algorithm of Waldhauser and Ellsworth (2000). Cross-correlation-based relocation of subduction seismicity substantially increases the location precision over relocations using only catalog phase picks and dramatically improves routine catalog locations. Our previous relocations in neighboring regions using the double difference method on catalog arrival times have shown a substantial increase in organization of seismicity, compared with the catalog locations. In particular, we observed a narrowing of the seismogenic zone at depth and a narrowing of each limb of the double seismic zone when viewed in cross-section. We have also observed unusual events that appear to be within the mantle wedge above the slab and substantial seismicity within the slab between the two planes of the double seismic zone. We chose the Ibaraki region for its high rate of slab seismicity, its relatively simple geometry, and for the presence of an intermediate-depth double seismic zone. We relocate nearly 5000 mostly intermediate-depth events occurring in this region between June 2002 and May 2003 using more than 100,000 waveforms from the JMA unified catalog. Many of these waveforms are from stations in Hinet, a recently installed national borehole network that provides particularly high-quality data.

Shelly, D. R.; Beroza, G. C.; Ide, S.

2003-12-01

86

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

87

High-Resolution Subduction Zone Seismicity and Velocity Structure in Ibaraki, Japan  

NASA Astrophysics Data System (ADS)

We use double-difference tomography (tomoDD) [Zhang and Thurber, 2003] and waveform-derived cross-correlation differential arrival times to invert for the earthquake locations and P- and S-wave velocity distributions in the subduction zone under Ibaraki Prefecture of north-central Honshu, Japan. The Ibaraki region is attractive for its high rate of slab seismicity and for the presence of an intermediate-depth double seismic zone. We relocate ~8000 events occurring in this region between June 2002 and June 2004. We use a combination of ~200,000 absolute travel times, ~5 million catalog-derived differential times, and ~5 million cross-correlation differential times derived from more than 150,000 waveforms, with roughly equal numbers of P- and S-wave data. Many of the waveforms are from HiNet borehole stations that provide particularly high-quality data. We also use data from JMA, the University of Tokyo, and Tohoku University. Since it is natural to expect sharp velocity contrasts in a subduction zone, we regularize the inversion using the total variation (TV) approach implemented through iteratively reweighted least squares. Because TV is an L1-norm regularization, sharp changes in velocity are penalized no more than gradual ones, but undulations in the velocity model remain damped. We will compare the TV results with those determined by standard least-squares, L2-norm regularization. Our results show increasingly organized seismicity including narrowing by up to 50% of the upper and lower limbs of the double seismic zone as viewed in cross-section. We find a zone of interplate events extending as deep as 60 km, forming a very distinct lineation in cross-section. Focal mechanisms support the interpretation that these are low angle, subduction interface events. These earthquakes are accompanied by a zone of very high Vp/Vs ratio within the downgoing plate, just beneath the seismicity, suggesting that high pore-pressures may enable seismic slip on the subduction interface at depths where aseismic slip would otherwise predominate. These events represent significantly deeper seismic coupling than the 37-43 km maximum depth observed in this area previously by Tichelaar and Ruff [1993], but are consistent with the maximum depth of 50-70 km for low-angle thrust events found by Igarashi et al. [2001] farther to the north.

Shelly, D. R.; Beroza, G. C.; Zhang, H.; Thurber, C. H.; Ide, S.

2004-12-01

88

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

89

Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons  

NASA Astrophysics Data System (ADS)

The mechanisms of growth and destruction of continental lithosphere have been long debated. We define and test a unifying plate tectonic driving mechanism that explains the numerous petrological, geophysical, and geological features that characterize the destruction of cratonic lithospheric roots. Data from three Archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs, some along the mantle transition zone, beneath the cratons. During flat slab subduction dehydration reactions add water to the overlying mantle wedge. As the subducting slabs roll back, they suck in mantle material to infill the void space created by the slab roll back, and this fertile mantle becomes hydrated. The roll-back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting, generating new magmas that gradually destroy the roots of the overlying craton through melt-peridotite reactions. Calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through this mechanism, warranting a re-evaluation of continental growth rates with time.

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

2014-09-01

90

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

NASA Astrophysics Data System (ADS)

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

Liu, Xin; Zhao, Dapeng

2015-04-01

91

Downdip velocity changes in subducted oceanic crust beneath Northern Japan—insights from guided waves  

NASA Astrophysics Data System (ADS)

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

Garth, Tom; Rietbrock, Andreas

2014-09-01

92

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

93

Rupture extent of the 1978 Miyagi-Oki, Japan, earthquake and seismic coupling in the northern Honshu Subduction Zone  

NASA Astrophysics Data System (ADS)

Underthrusting at subduction zones can cause large earthquakes at shallow depths, but is always accommodated by aseismic creep below a certain depth. This transition depth is referred to as the depth of seismic coupling and can be directly observed in a subduction zone as the lower depth extent of significant moment release of the deepest large underthrusting earthquakes. In 1978, a large (Ms=7.5) earthquake occurred off the coast of Miyagi Prefecture in northern Honshu. Its focal mechanism represents underthrusting of the Pacific plate beneath Honshu. Since the hypocenter is located 150 km landward from the trench and there are no other large interplate earthquakes further landward from the trench axis, this event defines the maximum depth of the coupled zone. The lower limit of significant moment release of the Miyagi-Oki earthquake is obtained by analysis of the long-period P waves. The deconvolved source time function consists of a dominant single pulse with peak moment release at 12 s and a total duration of 18 s. The rupture extent of this dominant pulse does not extend deeper than 40 km, thus the transition from coupled to uncoupled in northern Honshu occurs at or above 40 km depth.

Tichelaar, Bart W.; Ruff, Larry J.

1988-10-01

94

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

95

Subduction Structure beneath the Eastern Part of the Kii Peninsula, SW Japan, from Wide-Angle Reflection Experiment  

Microsoft Academic Search

The Nankai trough region, off SW Japan, is a famous seismogenic zone of M8 class interplate earthquakes associated with the northwestward subduction of the Philippine Sea (PHS) plate. In 2006, an intensive onshore-offshore seismic refraction\\/wide-angle reflection survey was conducted in the eastern part of the Nankai trough area. This experiment, funded by JAMSTEC, was aimed to elucidate the geometry of

T. Iwasaki; E. Kurashimo; A. Nakanishi; T. Iidaka; H. Katao; Y. Kaneda

2009-01-01

96

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan  

Microsoft Academic Search

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the

Takao Eguchi; Yukio Fujinawa; Eisuke Fujita; Sin-Iti Iwasaki; Isao Watabe; Hiroyuki Fujiwara

1998-01-01

97

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

NASA Astrophysics Data System (ADS)

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

Iwaki, A.; Iwata, T.

2008-12-01

98

Deformation of Japan as measured by improved analysis of GEONET data  

Microsoft Academic Search

The Japan subduction zone represents a complex set of plate interfaces with significant trench-parallel variability in great earthquakes and transient deep slip events. Within the Japan arc the Nankai segment of the Eurasian-Philippine plate boundary is one of the classic subduction zone segments that last produced a set of temporally linked great earthquakes in the 1940's. Recently, down-dip of the

S. E. Owen; D. Dong; F. H. Webb; B. J. Newport; M. Simons

2006-01-01

99

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

100

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

101

Past "earthquake/tsunami" event deposits found in the Japan Trench: Results from the Sonne SO219A and Mirai MR12-E01 cruises  

NASA Astrophysics Data System (ADS)

Epicenter of the 2011 off the Pacific Coast of Tohoku earthquake located under the sea floor. The large sea-floor displacement is inferred from the repeated bathymetric surveys. Because the Japan Trench is a remarkable depression near the epicenter, the gravity flows generated by the earthquake and its related phenomena might be focused in the depression (more than 7500 m in water depth) and might form the gravity flow deposits. Therefore, the Japan Trench is a target area to detect the past earthquake event deposits. To obtain the past earthquake records, we conducted two cruises; Sonne SO219A and Mirai MR12-E01 cruises. All of the cores obtained from the Japan Trench floor by two cruises showed the same lithostratigraphy. The 2011 event deposits, which were composed of thin sand at base and diatomaceous mud/ooze with multistoried upward fining grading structure, occurred at the uppermost part of the cores. Below the 2011 event deposit, at least three thick (several tens cm to a few meter thick) turbidite units were recognized. Third turbidite unit was very unique and was characterized by the calcareous nanno fossil bearing turbidite muds suggesting the transportation from upper-mid slope. A volcanic ash from the Towada volcano intercalated in hemipelagic mud between second and third turbidite units. Preliminary results on our tephra correlation using geochemical and petrographic properties suggest that the ash might be correlative to Towada-a ash, which occurred just above the Jogan tsunami deposits on the Sendai Plain. Exact correlation of the ash layer is very important to connect the deep-sea event deposits in the Japan Trench and on-shore tsunami deposits on the Sendai Plain.

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

2012-12-01

102

Seismological Features of the Subducting Slab Beneath the Kii Peninsula, Central Japan, Revealed by Receiver Functions  

NASA Astrophysics Data System (ADS)

We report seismological evidence that the subducting Philippine Sea slab (PHS) beneath the Kii Peninsula, central Japan, can be divided into three segments. Offshore the Kii Peninsula, the "Tonankai" and "Nankai" fault segments suffer mega-thrust earthquakes that repeat every 100 to 150 years. The structure of the young, thin, contorted PHS is important to the seismo-tectonics in this region. We apply the receiver function (RF) analysis to 26 Hi-net short-period and 4 F-net broad-band seismographic stations. In the case that dipping velocity discontinuities and/or anisotropic media exist beneath seismometer, both radial RFs and transverse RFs contain useful information to estimate underground structure. For isotropic media with a dipping-slab interface, back- azimuthal variation in RFs depends largely on three parameters, the downdip azimuth, dip angle and the depth of the interface. We stack both radial and transverse RFs with allowance a time-shift caused by the dipping interface, searching for optimal parameters based on the grid-search technique at each station. At some stations located near the eastern coastline of the Kii Peninsula, the dip angle of the interface inferred from RF stacking is much steeper than that estimated by the local seismicity. This discrepancy arises from the interference of two slab-converted phases, suggesting a layer atop the slab. In these cases we refine the stack to distinguish two slab phases and estimate three parameters of each dipping interface separately. Two interfaces with the same dip direction and low dip angle are estimated at these stations, with depth difference near 6 km. Thus, the shallower interface may be related to the layer within the oceanic crust and the deeper one is the slab Moho. These double-layered interfaces are detected only at stations located up-dip of a belt-like distribution of non- volcanic low-frequency tremor. Comparing the interface dips estimated in this study with the direction of slab motion determined by the GPS observation, we can classify the slab beneath the Kii Peninsula into three segments: eastern, central and southern. Within the eastern part, the RF-estimated dip direction is shifted about 30° clockwise from the GPS- estimated slab motion, and the dip angle is small. The difference of RF-estimated slab dip and GPS-estimated slab motion is small in the central part. Discrepancy between slab dip and motion estimates increases to 40°--90° in the western part. At the some stations located in the western and eastern part of the Kii Peninsula, the dip angle of the interface is much steeper than another model estimated by the local seismicity. Since the local seismicity distribution reflects well the larger-scale geometry of the slab, the RF-estimated dip angle may imply anisotropic wavespeeds within strongly sheared media above the slab. The boundary between the eastern and central regions of the Kii Peninsula corresponds to the region where (1) a topographic high in the slab interface is subducting and (2) the intraslab seismicity shows double-layered activity. The boundary between central and southern regions is coincident with the segment boundary of megathrust earthquakes in the Nankai region. The structural features revealed by RF-stacking may be an important key to the seismotectonics around the Kii Peninsula.

Shiomi, K.; Park, J.

2007-12-01

103

Upper boundary of the Pacific plate subducting beneath Hokkaido, Japan, estimated from ScSp phase  

NASA Astrophysics Data System (ADS)

Three-dimensional geometry of the upper boundary of the Pacific plate subducting beneath Hokkaido, Japan, was obtained using the ScSp phase: the phase converted from ScS (S wave reflected at the core-mantle boundary) to P wave at the plate boundary. Taking the advantage of a dense seismic network, "Hi-net", recently deployed across the Japanese islands, we applied several seismic array analyses to the recorded waveform data for a large nearby deep earthquake, in order to enhance very weak ScSp signals in the original records. At first, we set up five blocks for the region in plate dip directions. After aligning the travel times of ScS and stacking seismograms among stations in a given sub-block perpendicular to each dip direction, we searched for the optimal plate model (i.e., two-dimensional geometry of the upper boundary) for each block. The model was parameterized by seven depth grids, and seismograms were stacked based on the travel time of ScSp as a time lag of each sub-block, so that the optimal model would yield the maximum spectral energy of ScSp after stacking. This model parameter search was conducted, using ray tracings of ScSp with a reference velocity model and a non-linear inversion scheme (Neighbourhood Algorithm). The optimal model of each block was combined each other by cubic spline interpolation, in order to construct an overall three-dimensional geometry of the upper boundary of the plate. Next, we performed the frequency-wavenumber ( f- k) spectral analysis to refine the above result. Assuming each station as a reference point, we made beam output from records of its adjacent stations as a function of wavenumber vector ( kx, ky) and frequency. The peak of its power spectrum was considered to represent the wavenumber vector of ScSp, that is, azimuth of arrival and slowness, so that we can estimate the position and depth of the corresponding ScS- ScSp conversion. In the frequency range from 0.5 to 1.5 Hz, we could estimate the conversion points for 21 stations or hypothetical arrays, and revised the geometry of the upper boundary obtained by the non-linear stacking approach in the previous step. The final plate model was compared with the distribution of intraplate earthquakes in the Pacific plate. This comparison clearly reveals that the upper seismic zone merges with the lower from 150 to 200 km in depth, deviating systematically away from the upper boundary where the boundary is slightly bumped in a convex manner.

Osada, Kinue; Yoshizawa, Kazunori; Yomogida, Kiyoshi; Suetsugu, Daisuke; Bina, Craig; Inoue, Toru; Wiens, Douglas; Jellinek, Mark

2010-11-01

104

Comment on "The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle" by W.P. Schellart  

E-print Network

Discussion Comment on "The potential influence of subduction zone polarity on overriding plate online xxxx The Schellart's [Schellart, W.P., 2007, The potential influence of subduction zone polarity are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones

Doglioni, Carlo

105

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

106

Seismic activity of very low-frequency earthquake on the subducting Philippine Sea plate near the Nankai Trough, southwest Japan  

NASA Astrophysics Data System (ADS)

The Nankai trough subduction zone in southwest Japan is characterized by some kinds of _gslow earthquake_h. Around the deep side of the seismogenic zone on the subducting Philippine Sea plate, non-volcanic tremor is distributed in a narrow belt along the strike of the plate (Obara, 2002). On the other hand, on shallower parts of the seismogenic zone, an anomalous seismic activity has been detected. The waveform of the earthquake is characterized by a band-limited low-frequency content of between 10 and 20 seconds. We call the earthquake _gvery low-frequency (VLF) earthquake_h. The filtered seismogram with a passband of 10 to 100 seconds periods operated on the output from high-sensitivity accelerometer (tiltmeter) installed in every NIED Hi-net station is used for detection and location analysis. The waveform of the VLF earthquake looks like to that of teleseismic event, however the spatial pattern of the amplitude and the apparent velocity of the VLF earthquake are quite different from those of teleseismic events. Because waveforms of the VLF earthquake are quite similar in neighbor stations, epicenters are estimated by using a cross correlation analysis. The Hi-net stations in southwest Japan are divided into some groups with a diameter of about 100km. The cross correlation is calculated for every pair of stations in each group in order to measure the time lag which gives the highest cross correlation coefficient. Then, the set of time lags obtained in each group are used to estimate the propagation direction and the apparent velocity. Finally, the epicenter of the VLF earthquake is estimated by focusing the back projection of the ray propagation calculated with good resolution in each group. In the year of 2003, there are two active clusters of the VLF event near the Nankai trough; off Cape Muroto and the Hyuga-nada region. The VLF seismic activity usually lasts for a month in each cluster. Both activities are located on the seaward updip portion of the seismogenic zone on the subducting Philippine Sea plate. Just after the occurrence of 2003 Tokachi Earthquake in northeast Japan on September 26, 2003, the VLF seismic activity in the eastern part of Hyuga-nada region became very active and continued for a month. Considering the reverse fault type mechanism and shallow depth estimated by the centroid moment tensor (CMT) analysis, the VLF earthquakes might occur in the accretionary prism or on the decollement. Moreover, both active clusters correspond to the extension of the chain-like sea mounts on the ocean floor of the Philippine Sea. Therefore, the occurrence of the VLF earthquake might be related to the existence of the subducting sea mount. At present, there is no clear relationship between the episodic tremor and the VLF earthquake activity in their time histories. However, both seismic phenomena are located on transition zones between the locked zone and decoupled aseismic zones at the deeper part and shallower part on the subducting plate boundary. Therefore, these low-frequency families must be representing the subduction process of the young oceanic plate.

Obara, K.; Ito, Y.

2004-12-01

107

High-resolution subduction zone seismicity and velocity structure beneath Ibaraki Prefecture, Japan  

NASA Astrophysics Data System (ADS)

We use double-difference tomography and waveform-derived cross-correlation differential times to estimate earthquake locations and P and S wave velocity structure in the subduction zone under Ibaraki and neighboring prefectures of north central Honshu. We find evidence in both earthquake hypocenters and the velocity structure that the Philippine Sea plate, or perhaps a microplate fragment, may be caught between the subducting Pacific plate and overriding Okhotsk plate in this region. In the southern part of the study area, we find a zone of interplate events extending as deep as 60 km, forming a distinct lineation in cross section. Focal mechanisms support the interpretation that these are low-angle, subduction interface events. We infer that these events probably occur on the interface between the Pacific and Philippine Sea plates rather than between the Pacific and Okhotsk plates. In the upper part of the downgoing Pacific plate, we find a zone of high Vp/Vs ratio (˜1.95), which may be explained by high pore fluid pressures within the subducting crust. At a depth of 60-80 km, this region of high Vp/Vs appears to diffuse into the overlying mantle wedge, possibly indicating the upward release of fluids from the slab.

Shelly, David R.; Beroza, Gregory C.; Zhang, Haijiang; Thurber, Clifford H.; Ide, Satoshi

2006-06-01

108

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

109

Tidal triggering of earthquakes in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, Japan  

Microsoft Academic Search

We found a characteristic space–time pattern of the tidal triggering effect on earthquake occurrence in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. We measured the correlation between the Earth tide and earthquake occurrence using microearthquakes that took place in the

Sachiko Tanaka; Haruo Sato; Shozo Matsumura; Masakazu Ohtake

2006-01-01

110

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

111

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

Microsoft Academic Search

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

A. Iwaki; T. Iwata

2008-01-01

112

Preliminary rock-magnetic studies of core samples from the IODP Japan Trench Fast Drilling Project (JFAST)  

NASA Astrophysics Data System (ADS)

During IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST), boreholes were drilled through the prism and across the fault that is thought to have slipped during the 11 March 2011 Tohoku-Oki Earthquake. 74 subsamples of the core recovered from hole C0019E were subjected to rock magnetic analyses to identify magnetic minerals, determine the magnetic-grain size distribution and investigate rock magnetic changes related to fault zone processes. Magnetic susceptibility was measured with a MFK1-FA Kappabridge instrument at three different frequencies: 976 Hz (F1), 3904 Hz (F2) and 15616 Hz (F3), and frequency-dependent susceptibility (?fd%) was calculated using the equation ?fd%=(?@F2-?@F3)/?@F2×100%. Magnetic hysteresis curves and backfield DC demagnetization curves of isothermal remanent magnetization were measured using an alternating gradient field magnetometer. Bulk coercivity parameters (Hc, Hcr) show a significant down-core variation. Samples from the lower part of the frontal prism sediment (688-820 mbsf) showed high Hc and Hcr (15-52 mT and 45-85 mT). Samples from hemipelagic sediment we interpret was deposited on the oceanic basement (824-832 mbsf) showed slightly lower Hc and Hcr (13-26 mT and 34-65 mT). A sheared scaly clay zone between two sediment types (820-824 mbsf), inferred to be the plate boundary decollement, clearly has low Hc and Hcr (10-13 and 22-24 mT). The difference in Hc and Hcr suggests a difference in magnetic mineralogy. Saturation magnetization (Ms) and saturation remanence (Mr) show a down-core increasing trends with no obvious changes at the geologically-indicated plate boundary.Magnetic susceptibility shows different down-core variation with coercivity parameters. Generally, the scaly clay has the highest susceptibility and susceptibility of the hemipelagic sediment is relatively lower. For example, the mean values of low frequency (F1) magnetic susceptibility are 74.21, 387.83 and 30.58×10-8 m3 kg-1 for the lower part of frontal prism sediment, sheared scaly clay zone and hemipelagic sediment, respectively. The variation may be related to the difference in magnetic mineralogy and relative concentrations. In addition, magnetic susceptibility shows a strong dependence on measured frequency, and frequency-dependent susceptibility (?fd%) generally increases with depth, which may reflect an increasing contribution of the superparamagnetic components with depth. Further measurements, such as low-temperature magnetic properties, and thermomagnetic measurements of magnetic susceptibility and induced magnetization are intended.

Mishima, T.; Yang, T.; Mori, J. J.; Chester, F. M.; Eguchi, N.; Toczko, S.

2012-12-01

113

Internal structure of a thrust associated with subduction underplating from the Okitsu melange in the Shimanto accretionary complex, Japan  

NASA Astrophysics Data System (ADS)

The structure of large-displacement, plate-boundary faults in subduction zones is poorly defined relative to other tectonic settings. The thrust faults in the Okitsu melange constitute a duplex, which juxtapose oceanic pillow basalt (hanging walls) and trench-fill-sedimentary rocks (footwalls), and may represent a paleo-underplating zone of a plate-boundary subduction thrust at seismogenic depth. One well exposed duplex-fault, with displacement greater than several kilometers, is examined through structural mapping at scales of 1:100 to 1:1, mesoscale fabric analysis, and geochemical analysis. The duplex-fault zone displays a unique, asymmetric internal structure: brittle deformation dominates in the hanging wall and brittle and ductile deformation is evident in the footwall. The internal structure and surrounding host rocks reflect three accretionary processes: underthrusting, underplating, and uplifting along Out-of-Sequence-Thrusts (OOSTs). On the basis of style of deformation and displacement field, two distinct deformation episodes are identified, which likely correspond to underthrusting and underplating. The elongation of basalt pillows and the boudinage structure of sandstone blocks in shale indicate an elongation of 1.1 to 1.5 parallel to the oceanic crust layer throughout the host rocks of both the hanging wall and footwall, consistent with uniform extension of the crust during underthrusting. Pillow basalt and sedimentary rocks are juxtaposed by a single cataclasite layer composed of decimeter thick ultracataclasite derived from both rock types along the duplex-fault. Mesoscale implosion breccia and micro textures indicative of pressure solution are observed along the master duplex-fault and thought to be products of alternating fast- and slow-rate slip, respectively. Mesoscale fault fabric indicates a paleostress with the maximum principal compression at 15-20° to the master duplex-fault reflecting layer parallel contraction, which is consistent with the displacement field expected for underplating and a high frictional strength of the duplex-fault. The asymmetric structure may reflect dissimilar tectonic paths of the hanging wall and footwall. The basalt in the hanging wall displays only a localized, brittle structure because it is faulted only during underplating. An increase in fracture density and decrease in the size of comminuted pillows towards the fault correspond to mapped zonation of protocataclasite, cataclasite, and ultracataclasite. In contrast, the footwall consists of the ultracataclasite layer flanked by a 20-meter-thick zone of ductile shear, reflecting deformation during both underthrusting and underplating. A significant post-underplating overprint is lacking in both the duplex-fault and host rocks, consistent with translation of the entire melange along OOSTs, possibly represented by the tectonic contacts of the Okitsu melange.

Kanaya, T.; Chester, F.; Sakaguchi, A.

2005-12-01

114

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

115

Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone  

Microsoft Academic Search

Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non-volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the

Kazushige Obara; Hitoshi Hirose; Fumio Yamamizu; Keiji Kasahara

2004-01-01

116

Structure of a paleo subduction décollement, Suzume Fault, Okitsu Mélange, Shimanto Accretionary Complex, Japan  

NASA Astrophysics Data System (ADS)

The Suzume fault is an internal thrust of an exhumed duplex of underplated rock in the Shimanto accretionary complex. On the basis of structure, age, and paleo-temperature of rock in the duplex, the thrust is a subduction décollement exhumed from the shallow region of the seismogenic zone. Mesoscale structure of the thrust was characterized to investigate slip processes and structural evolution of the décollement. The thrust exhibits an asymmetric structure characterized by a decimeter-thick ultracataclasite bounded by a several-meter-thick zone of fractured basalt in the hanging wall and a 20-meter-thick zone of ductily sheared sedimentary rock in the footwall. The structure of the thrust is similar to that of active décollements drilled at the frontal portions of prisms, but differs in the greater intensity of ductile deformation and the occurrence of ultracataclasite. Hence, the footwall of the Suzume fault records aseismic, distributed shear in poorly consolidated sediment during shallow underthrusting followed by coseismic, localized-slip in lithified sedimentary rock during underplating at depth. The hanging wall, in contrast, records only the later stage of coseismic, localized-slip associated with underplating. The ultracataclasite layers in the Suzume fault exhibit shear localization onto a through-going fracture surface and fabrics indicative of distributed flow, which are likely associated with the seismic cycle, i.e., alternating co- and inter-seismic slip. Off-fault fracture fabric in the hanging wall, related to thrusting at depth, records predominantly up-dip propagation of fault slip events. /// A synthesis of observations of ~20 subduction megathrusts representing the 0-15 km depth range, combined with the findings from the Suzume fault and modern Nankai megathrusts, documents a systematic decrease in the thickness of the slip zone, changes in deformation mechanisms and fluid-rock reactions, and an increase in shear heating, with depth. These changes may correlate with a decrease in friction rate-dependence and the progression in the mode of slip with depth that occurs in the up-dip regions of subduction megathrusts.

Kanaya, T.; Chester, F. M.; Sakaguchi, A.

2011-12-01

117

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

NASA Astrophysics Data System (ADS)

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

Lynner, Colton; Long, Maureen D.

2014-06-01

118

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

119

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

NASA Astrophysics Data System (ADS)

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

Hashimoto, Y.; Eida, M.

2013-12-01

120

Trench Fever  

MedlinePLUS

... adults A A A Trench fever is a bacterial disease transmitted by body lice. The symptoms include relapsing ... in the shins. Overview Trench fever is a bacterial infection caused by the bacterium Bartonella quintana, which is ...

121

From subduction to collision: Control of deep processes on the evolution of convergent plate boundary  

E-print Network

of oceanic subduction, (3) an episode of continental subduction, during which the trench absorbs all: Plate boundary-- general (3040); KEYWORDS: subduction, collision, continental subduction, oceanic of an oceanic basin. However, several orders of evidence indicate that arrival at the trench of a continental

122

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

NASA Astrophysics Data System (ADS)

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

Nakajima, Junichi

2014-12-01

123

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

124

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

NASA Astrophysics Data System (ADS)

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

Saito, Y.; Yamada, T.

2013-12-01

125

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

126

Paleomagnetic records of core samples of the plate-boundary thrust drilled during the IODP Japan Trench Fast Drilling Project (JFAST)  

NASA Astrophysics Data System (ADS)

IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST), drilled across the plate-boundary décollement zone near the Japan Trench where large slip occurred during the March 2011 Tohoku-oki earthquake. We conducted paleomagnetic measurements of the core sample retrieved from the highly-deformed sediments comprising the plate-boundary décollement zone. Whole-round samples for structural analyses from five depth intervals of the core (0-12 cm, 12-30 cm, 43-48 cm, 48-58 cm, and 87.5-105 cm), were trimmed into oriented slabs with typical dimensions of 3x3x5 cm that are now being used to make petrographic sections for microstructural and chemical study. The remainder of the core sample was split into working and archive halves. We measured remanent magnetization of 16 trimmed slabs and the archive half of the core sample. The slabs were subjected to natural remanent magnetization (NRM) measurements in 0.5-1 cm intervals and progressive alternating field demagnetization (AFD) up to 80 mT with a 2G755 pass-through superconducting rock magnetometer at Kochi University. The archive half of the core sample was subjected to NRM measurement and AFD up to 20 mT with a 2G760 superconducting rock magnetometer installed on R/V Chikyu. Typically, two or three paleomagnetic components were isolated during the AFD of slab samples up to 80 mT. One ';soft' component was demagnetized below 20-30 mT, and another ';hard' component was not demagnetized even with AFD in 80 mT. A third component may be separated during AFD at the intermediate demagnetizing field, and may overlap the soft and hard components. The multiple slab samples cut from an identical whole-round sample have generally consistent paleomagnetic direction of the hard component. Contrastingly, the direction of the soft component is less consistent between adjacent slabs, and even varies within a single slab. The direction variation of the soft component possibly reflects the cm-scale strain and rotation of the highly-deformed sediments within the plate-boundary décollement zone. Studies of the relationship of the direction variation to the microstructure are ongoing, and will be reported at the meeting. The consistency of the hard component direction within highly deformed sediment implies it was recently acquired. Further studies of the acquisition mechanism of the hard component are also intended.

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

2013-12-01

127

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

128

Evidence for Hydrothermal Circulation and Alteration on 20-25 Ma Crust About to be Subducted in the Middle American Trench  

NASA Astrophysics Data System (ADS)

The TicoFlux I expedition was the first of two expeditions to focus on the hydrogeologic properties of the incoming Cocos plate off the Nicoya Peninsula. The overarching goals for the project are to gain a better understanding of the effects of hydrothermal circulation on the chemical and thermal state of the subducting plate, and how these factors impact the mechanics and fluxes in the subduction zone. The expedition was an integrated program that included the collection of seismic, heat flow, pore water and sedimentological data. Our focus was to use systematic variations in pore water chemical, sedimentological, and mineralogical depth profiles to map the pattern of fluid flow, define the basement fluid composition, determine sediment characteristics, and constrain the extent of crustal alteration resulting from hydrothermal flow and diagentic reactions. We collected 43 gravity and piston cores from 11 sites. Each site was selected on the basis of bathymetry and the underlying basement topography. In some cases heat flow data aided in site selection. Most sites are located near exposed basaltic basement, with thin sediment cover, as these are the most likely locations for fluid discharge from oceanic crust. The remaining sites were chosen because of high basement relief coupled with sediment that is less than 100 m thick. Most of the cores consist of dark olive gray hemipelagic mud with abundant diatoms and radiolarians and distinct layers of volcanic ash. The remaining cores contain nannofossil chalk and/or smectite-rich clay with Mn nodules, basalt fragments, or both. Four of the 11 sites have a clay-rich layer. Pore waters from 5 of the 11 sites sampled are consistent with present day alteration by hydrothermal fluids. Pore water evidence for flow includes low Mg, K, Li, Rb, phosphate and carbonate concentrations and high Ca concentrations, consistent with observations of other ridge-flank environments. None of the cores have calculated upwelling seepage speeds greater than 1 cm/yr. The TicoFlux II expedition, which will take place in Summer of 2002, will concentrate on several of these sites, as well as other locations of likely fluid seepage. We can then better constrain the pattern of fluid flow and the composition of the basement fluids.

Friedmann, P. K.; Wheat, C. G.; Underwood, M.; Hoke, K.; Fisher, A. T.; Silver, E.; Hutnak, M.; Harris, R. N.; Stein, C.

2001-12-01

129

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

USGS Publications Warehouse

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

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

1991-01-01

130

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

PubMed Central

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

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

2012-01-01

131

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

132

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

NASA Astrophysics Data System (ADS)

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

Yoshioka, Shoichi; Takagi, Rumi; Matsumoto, Takumi

2015-05-01

133

Evaluation of seismic reflection characteristics from non-asperities along the subduction zone to actively monitor subduction zone behavior  

NASA Astrophysics Data System (ADS)

It is widely accepted that shallow-intermediate depth major earthquakes along subdution zones occur in asperities surrounding by non-asperities. Non-asperities might comprise liquid-like layer which can generate strong seismic reflections. Rapid change of physical state in the non-asperities might trigger seismic slip along asperities because of high strain accumulation there. Strong seismic reflections were observed along the Japan Trench (Fujie et al., 2002) and the Nankai subduction zones (Iidaka et al, 2003). The observation of strong reflections along the subduction zone suggests the presence of liquid-like layer which comprises non-asperities. We showed the possibility to detect change of physical state in the strong seismic reflection zone in the Tokai subduction zone using ACROSS seismic system (Tsuruga et al., 2006). To perform a better observation, it is necessary to know seismic characteristics assuming presence of liquid-like phase along the subduction zone. For this purpose, we performed waveform simulation. We examined the seismic refraction and wide-angle reflection phases in the subduction zone. In a structural model, the trench axis is located at x =100km at the center of the model, and an oceanic plate subducts beneath the forearc basin between 0 and 100 km. The zone between 100 and 200 km is pure oceanic region. The oceanic crust has 7 km in thick. The thickness of forearc basin is thinning toward the trench axis. Above the subducting plate, 500-m thick decollement with Vp=1.6-2.2 km/s is placed. Vp at just top of the oceanic mantle is 8.0 km/s. The seismograms and travel times were calculated by 2D-FDM (Larsen, 2000) and graph method (Kubota et al., 2005), respectively. Assuming appropriate Vp, Vs, density and Q-values structural models, we computed shot-gather records using 4-Hz Ricker wavelet explosive sources placed at the ocean bottom and receivers aligned at 30-m below the sea surface. Grid space in space is 30 m, and time step is 2 ms. At particular locations, we can recognize the strong reflection from the decollement with negative polarity due to the negative impedance contrast as follows: 1) At 100 km from the trench axis, clear reflection from the decollement at 6-km below the ocean bottom and PmP from the subducting oceanic Moho are identified. Reflection from the decollement has large amplitude between offset distance of 0 and 30"@km, but PmP does large amplitude between 30-50 km. Pn traveling in the oceanic mantle has clear appearance. 2) At 50 km, characteristics of shot-gather records have similar characteristics of ones at 100km case. 3) At 100 km (trench axis), synthetic waveforms at the offset distance > 100km are similar to the seismic records at typical oceanic crust. Large Pg and PmP are identified. Pn is seen for both sides of trench axis. Reflection from PmP around 0-offset is weak, but it has very large amplitude at 20-40 km by wide-angle reflection. The thickness of decollement used is a little thick (0.5km) in our numerical test. If 16-Hz wavelet is used as a source similar to real observation, such layer has about 125 m in thick.. Although we assume a pure liquid layer, we can expect strong seismic reflection from the target layer even for not-pure liquid layer. Through the above simulation, we can choose appropriate observation method for active monitoring of the subduction zone dynamics.

Murase, K.; Tsuruga, K.; Kasahara, J.

2008-12-01

134

Trench mouth  

MedlinePLUS

Bad breath Crater-like ulcers between the teeth Fever Foul taste in the mouth Gums appear reddened and swollen Grayish film ... your mouth for signs of trench mouth, including: Crater-like ulcers filled with plaque and food debris ...

135

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

136

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

137

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)

A 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

138

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

139

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

140

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

141

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

142

Buoyant subduction on Venus: Implications for subduction around coronae  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

143

Japan.  

ERIC Educational Resources Information Center

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

Geiger, Rita; And Others

144

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

145

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

146

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

147

Phoenix Trenches  

NASA Technical Reports Server (NTRS)

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

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

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

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

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

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

2008-01-01

148

Japan.  

PubMed

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

1989-02-01

149

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

NASA Astrophysics Data System (ADS)

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

Itaba, S.; Kimura, T.

2013-12-01

150

Plumes, plateaux and congestion in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

151

Detailed structure of the upper mantle discontinuities around the Japan subduction zone imaged by receiver function analyses  

Microsoft Academic Search

High-resolution receiver function (RF) images of the upper mantle structure beneath the Japan Islands are obtained by RF analysis of the P-wave coda portions of 389 teleseismic events observed at 138 high-density broadband stations. We construct RFs through frequency-domain division using a water level of 0.01 and Gaussian low-pass filters of 1.0, 0.5, 0.3, and 0.1 Hz, and produce depth-migrated

Takashi Tonegawa; Kazuro Hirahara; Takuo Shibutani

2005-01-01

152

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone  

Microsoft Academic Search

Non-volcanic deep low-frequency tremors in southwest Japan exhibit a strong temporal and spatial correlation with slow slip detected by the dense seismic network. The tremor signal is characterized by a low-frequency vibration with a predominant frequency of 0.5–5 Hz without distinct P- or S-wave onset. The tremors are located using the coherent pattern of envelopes over many stations, and are

Kazushige Obara; Hitoshi Hirose

2006-01-01

153

Japan.  

PubMed

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

1987-02-01

154

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

155

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

156

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

157

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

NASA Astrophysics Data System (ADS)

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

Sawai, Michiyo; Hirose, Takehiro; Kameda, Jun

2014-12-01

158

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

NASA Astrophysics Data System (ADS)

The 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

159

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

NASA Astrophysics Data System (ADS)

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

Loveless, John P.; Meade, Brendan J.

2010-02-01

160

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

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

161

High rates of arc consumption by subduction processes: Some consequences  

E-print Network

of the volcanic arc through time. Lallemand et al. con- cluded that the long-term subcrustally eroded sediment.y. period is long enough to erode the initial width of the volcanic-arc and fore-arc massif of any subduction zone, assuming a mean trench­volcanic-arc landward migration of 5 km/m.y. and a mean trench

Demouchy, Sylvie

162

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

163

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

164

JAPAN  

NSDL National Science Digital Library

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

Mr. Hughes

2006-03-04

165

Strong Quake Strikes Japan  

NASA Astrophysics Data System (ADS)

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

Showstack, Randy

2011-03-01

166

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

167

The earliest mantle fabrics formed during subduction zone infancy  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

168

Post-seismic deformation of the 2011 Tohoku earthquake, Japan  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

169

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

170

Triple junctions as a cause for anomalously near-trench igneous activity between the trench and volcanic arc  

Microsoft Academic Search

In the eastern Aleutian arc, western Sumatra, and southwestern Japan, the distribution of igneous rocks cannot be explained by direct analogy with the igneous activity in modern island arcs. In these places, episodes of intrusion occurred within the accretionary prism, anomalously close to an active trench. For the eastern Aleutian arc and Sumatra, we speculate that the near-trench igneous rocks

R. Stephen Marshak; Daniel E. Karig

1977-01-01

171

Evidence for retrograde lithospheric subduction on Venus  

NASA Technical Reports Server (NTRS)

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

Sandwell, David T.; Schubert, Gerald

1992-01-01

172

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

173

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

174

Digging a Paleoseismic Trench  

USGS Multimedia Gallery

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

175

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

Microsoft Academic Search

The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure 1). The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S. Virgin Islands, where 4 million U.S. citizens reside. Widespread damage in Puerto Rico and Hispaniola from an

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

2004-01-01

176

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

E-print Network

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

Müller, Dietmar

177

Shallow very-low-frequency earthquakes around Japan: Recent studies and observation  

NASA Astrophysics Data System (ADS)

Very-low-frequency (VLF) earthquakes have been observed in three regions around Japan. (1) Deep VLF earthquakes have occurred in the down-dip part of the Nankai subduction zone [Ito et al., 2007]. (2) Shallow VLF earthquakes have occurred within the accretionary prism in the up-dip portion of the Nankai subduction zone [Obara and Ito, 2005; Ito and Obara, 2006]. The stress drops of these shallow VLF events were very low, in the range 0.1--10kPa; this corresponds to 0.1--1% of the range for ordinary earthquakes [Ito and Obara, 2006]. Ito and Obara [2006] suggested that the largest shallow VLF earthquake (MW 4.0) occurred on a circular fault of radius ~5--10 km. They proposed that the shallow VLF events were related to numerous reverse fault systems located in areas of high fluid pressure within the accretionary prism. (3) Shallow VLF earthquakes have occurred in the region off Tokachi, northern Japan, along the Japan Trench [Asano et al., 2008], where the Pacific Plate subducts beneath the Japanese land area. The occurrence of these shallow VLF earthquakes suggests that VLF events can occur on the plate boundary at depths shallower than that of the main seismogenic zone [Asano et al., EPS, 2008]. The megasplay faults in the Nankai subduction zone are observed to generate a reverse-polarity reflection on seismic reflection profiles [Park et al.,2002]; this may indicate the existence of an elevated fluid process in the fault zones [Shipley et al., 1994]. Hydrodynamics phenomena responsible for the seismic signals detected by ocean bottom seismometers were first reported by Brown et al. (2005) using osmotically-driven fluid flow meters (CAT meters); these meters were used to detect temporal changes in the rate of cold seepage of a shallow subduction system in the regions of the Costa Rica subduction zone. The Pacific plate is subducting beneath Tohoku, northeastern Japan, at the Japan Trench. An aseismic slip has been observed to occur as a post- seismic slip following the occurrence of large earthquakes. Tsunami earthquakes, a type of slow earthquakes, have also occurred in the vicinity of the Japan Trench; thus far, there have been no observations of non-volcanic tremors, VLF earthquakes, and short-term slow slips in NE Japan, and the lack of observations can be attributed to the difficulty in detecting slow earthquakes near the trench. Recently, we have deployed ocean bottom seismographs and have set up geodetic and hydraulic stations in northeastern Japan to detect and observe shallow slow earthquakes and the corresponding transient hydrotectonic processes. The CAT meters were installed on cold seeps that were discovered by diving surveys of the manned submersible SHINKAI 6500. We found Calyptogena, which suggested the existence of a cold seep, during the diving survey. The seeps are distributed near a splay fault that was detected from a seismic reflection image [Tsuji et al., 2008], suggesting that the fluid in the cold seeps migrates from the splay fault to the seafloor. The Calyptogena colonies are distributed along the strike of the landward slope. We have also developed a simplified ocean-bottom benchmark (SOBB) that comprises three types of sensors; short-period seismometers, broadband seismometers, and pressure gauges.

Ito, Y.; Obara, K.; Asano, Y.; Fujimoto, H.; Hino, R.; Ashi, J.; Tsuji, T.

2008-12-01

178

Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance  

NASA Astrophysics Data System (ADS)

Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread, but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

Olive, Jean-Arthur; Pearce, Frederick; Rondenay, Stéphane; Behn, Mark

2014-05-01

179

Anomalous crustal movement detected through seafloor geodetic observation at accretionary wedge in the central Ryukyu Trench  

Microsoft Academic Search

The Ryukyu trench is a major convergent plate boundary where the Philippine Sea plate is subducting beneath the Eurasian plate at a rate of 8 cm\\/yr. The interplate coupling is assumed to be weak. However, numerical simulation of the tsunami showed that the 1771 Yaeyama tsunami was caused by a thrust faulting in the subducted sediment beneath the accretionary wedge

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

2009-01-01

180

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

181

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

182

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

Microsoft Academic Search

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,

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

2010-01-01

183

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

184

Repeating earthquake activities associated with the Philippine Sea plate subduction in the Kanto district, central Japan: A new plate configuration revealed by interplate aseismic slips  

Microsoft Academic Search

We detect repeating earthquakes associated with the Philippine Sea plate subduction to reveal the plate configuration. In the Kanto district, we find 140 repeating earthquake groups with 428 events by waveform similarity analysis. Most repeating earthquakes in the eastern part of the Kanto district occur with a regular time interval. They have thrust-type focal mechanisms and are distributed near the

Hisanori Kimura; Keiji Kasahara; Toshihiro Igarashi; Naoshi Hirata

2006-01-01

185

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

186

The earthquake cycle in subduction zones  

NASA Technical Reports Server (NTRS)

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

Melosh, H. J.; Fleitout, L.

1982-01-01

187

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

188

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

189

Diatom-based elevation transfer function along the Pacific coast of eastern Hokkadio, northern Japan - an aid in paleo-seismic study along the coasts near Kurile subduction zone  

NASA Astrophysics Data System (ADS)

This paper provides a training data set for diatom-based elevation transfer functions, which are applicable to paleoseismic studies at southwestern Kurile subduction zones, northern Japan. Contemporary diatom samples were collected from four transects at Akkeshi and Onnetoh salt marshes along the Pacific coast of eastern Hokkaido. The relationships between diatom species and environmental variables were elucidated by canonical correspondence analysis (CCA) and partial CCAs. Partial CCAs associated with Monte Carlo permutation tests show that elevation accounts for a significant portion of the total variance in the diatom data. Therefore, statistically significant transfer functions quantifying the relationship between contemporary diatom assemblages and elevation can be developed. Diatom-based transfer functions (DBTFs) were developed using weighted averaging and partial least squares (WA-PLS) and applied to fossil diatom assemblages from Onnetoh estuary. The reconstructed curve of elevations contains at least three obvious emergence and DBTFs evaluated the magnitude of the three emergences as approximately 80 cm. The results are consistent with paleoecological data produced by previous studies. If these cycles represent subsidence during an interseismic period and uplift associated with interplate earthquake along the Kurile subduction zones, DBTFs at salt marshes of eastern Hokkaido can contribute to reconstruction of the recurrence intervals and the magnitudes of earthquakes.

Horton, B. P.; Sawai, Y.

2003-12-01

190

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

191

An asperity model for fault creep and interseismic deformation in northeastern Japan  

NASA Astrophysics Data System (ADS)

We explore the potential geodetic signature of mechanical stress shadows surrounding inferred major seismic asperities along the Japan-Kurile subduction megathrust. Such stress shadows result from a decrease in creep rates late in the interseismic period. We simplify the rupture history along this megathrust as the repeated rupture of several asperities, each with its own fixed recurrence interval. In our models, megathrust creep throughout the interseismic period evolves according to velocity strengthening friction, as opposed to common kinematic back-slip models of locked or partially locked (i.e. coupled) regions of the megathrust. Such backslip models are usually constrained by onshore geodetic data and typically find spatially extensive and smooth estimates of plate coupling, a likely consequence of model regularization necessitated by poor model resolution. Of course, these large coupled regions could also correspond to seismogenic asperities, some of which have not experienced a significant earthquake historically. A subset of existing kinematic models of coupling along the Japan Trench, particularly those that use both horizontal and vertical geodetic data, have inferred a surprisingly deep (˜100 km) locked zone along the megathrust or have called upon complex, poorly constrained megathrust processes, such as subduction erosion, to explain the geodetic observations. Here, we posit two scenarios for distributions of asperities on a realistic 3-D megathrust interface along the Japan-Kurile Trench off NE Japan. These scenarios reflect common assumptions made before and after the 2011 Mw 9 Tohoku-oki earthquake. We find that models that include two shallow M9-class asperities (one corresponding to the 2011 Tohoku-Oki earthquake and one offshore of Hokkaido) and associated stress-shadows can explain geodetic observations of interseismic strain along the eastern halves of Honshu and Hokkaido. Specifically, models including localized fault creep can explain most of the observed long-term vertical subsidence in this region during the past century and thus appealing to processes such as deep locking or subduction erosion may not be required.

Kanda, Ravi V. S.; Hetland, Eric A.; Simons, Mark

2013-01-01

192

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

193

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

194

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

195

The Puerto Rico Trench,the deepest part of the Atlantic Ocean,is located where the North  

E-print Network

tectonics of the plate boundary and to provide constraints for hazard assessment,the morphology (NOAM) plate is subducting under the Caribbean plate (Figure 1).The trench region may pose significant free-air gravity anomaly on Earth,-380 mGal,located 50 km south of the trench,where water depth is 7950

ten Brink, Uri S.

196

Control of seafloor aging on the migration of the IzuBoninMariana trench Claudio Faccenna a,  

E-print Network

Control of seafloor aging on the migration of the Izu­Bonin­Mariana trench Claudio Faccenna a) numerical models, reproducing the effects of the lithospheric aging during subduction. The result of our the increasing stiffness of the slab prevents the slab to unbend once it has subducted. We adapt this physical

Demouchy, Sylvie

197

Tracing trace elements from sediment input to volcanic output at subduction zones  

Microsoft Academic Search

AT ocean trenches, sea-floor sediments may either be scraped off the subducting plate, or accompany it into the mantle. Some of the subducted sediment may then be recycled to the arc crust by magmatism1; the rest may be recycled into the mantle, and contribute to mantle heterogeneity2. Strong evidence for sediment contributions to arc volcanism has come from isotope tracers,

Terry Plank; Charles H. Langmuir

1993-01-01

198

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

Microsoft Academic Search

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

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

2002-01-01

199

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

200

Receiver functions in northeast China – implications for slab penetration into the lower mantle in northwest Pacific subduction zone  

Microsoft Academic Search

Seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest Pacific subduction zone beneath Japan and northeast China have suggested contrary subduction scenarios. There was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660-km discontinuity over several hundred kilometers.

Xueqing Li; Xiaohui Yuan

2003-01-01

201

Relationship between outer forearc subsidence and plate boundary kinematics along the Northeast Japan convergent margin  

NASA Astrophysics Data System (ADS)

Tectonic erosion along convergent plate boundaries, whereby removal of upper plate material along the subduction zone interface drives kilometer-scale outer forearc subsidence, has been purported to explain the evolution of nearly half the world's subduction margins, including part of the history of northeast Japan. Here, we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern Japan. A synthesis of newly updated analyses of outer forearc subsidence, the timing and kinematics of upper plate deformation, and the history of plate convergence along the Japan trench demonstrate that the onset of rapid fore-arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the Sea of Japan and with an acceleration in convergence rate at the trench. In Plio-Quaternary time, relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate. The coincidence of these changes across the forearc, arc, backarc system appears to require an explanation at the scale of the entire plate boundary. Similar observations along other western Pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule, rather than the exception. We suggest that a significant component of forearc subsidence at the northeast Japan margin is not the consequence of basal tectonic erosion, but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics. If correct, this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins.

Regalla, Christine; Fisher, Donald M.; Kirby, Eric; Furlong, Kevin P.

2013-12-01

202

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- ...

203

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

204

Interplate coupling along the Nankai Trough, southwest Japan, inferred from inversion analyses of GPS data: Effects of subducting plate geometry and spacing of hypothetical ocean-bottom GPS stations  

NASA Astrophysics Data System (ADS)

We estimated the slip-deficit rate distribution on the plate boundary between the subducting Philippine Sea plate and the continental Amurian plate along the Nankai Trough, southwest Japan. Horizontal and vertical displacement rates were calculated from land-based Global Positioning System (GPS) data during the 5-year period from 1 January 2005 to 31 December 2009. We employed an inversion analysis of geodetic data using Akaike's Bayesian information criterion (ABIC), including an indirect prior constraint that slip distribution is smooth to some extent and a direct prior constraint that slip is mainly oriented in the plate-convergent direction. The results show that a large slip deficit exists at depths ranging from 15 to 20 km on the plate boundary in a belt-like form. The maximum slip-deficit rate was identified off Shikoku and reached 6 cm/year. The slip-deficit rate differed by as much as 1 cm/year when using a different geometric model of the subducting plate. On the basis of the spatial distribution of estimation errors and the resolution of the obtained slip-deficit rate on the plate boundary, we also found that the offshore slip-deficit rate cannot be estimated with sufficient accuracy using only land-based GPS data. Therefore, we tested the improvement in results when introducing hypothetical ocean-bottom GPS stations. The stations were arranged in four along-arc and across-arc spacings of 80 km and 40 km. The ocean-bottom data improved the estimation errors and resolutions, and successful results were obtained for a checkerboard with each square 75 km × 76 km. Our results indicate that 40-km along-arc and across-arc two-dimensional spacing of ocean-bottom GPS stations is required to obtain reliable slip-deficit distributions near the trough axis, assuming the current estimation accuracy for ocean-bottom horizontal displacement rates.

Yoshioka, Shoichi; Matsuoka, Yoshiko

2013-07-01

205

Metamorphic Perspectives of Subduction Zone Volatiles Cycling  

NASA Astrophysics Data System (ADS)

Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and ?15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like ?15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites indicates retention of seafloor N signatures and, in some cases, enrichments in sedimentary N due to forearc metamorphic fluid-rock interactions (Halama et al., this session). A global estimate of C cycling, using seafloor inputs (carbonate and organic matter) and estimates of volcanic CO2 outputs, indicates ~40% return (with large uncertainty) of the subducting C in volcanic gases. This imbalance appears plausible, given the evidence for deep carbonate subduction, in UHP marbles, and the preservation of graphite in UHP metasediments, together seemingly indicating that large fractions of subducting C survive forearc-to-subarc metamorphism. Estimates of return efficiency in the Central America arc, based on data for volcanic gases, are lower and variable along strike (12-29%), quite reasonably explained by de Leeuw et al. (2007, EPSL) as resulting from incomplete decarbonation of subducting sediment and AOC, fluid flow patterns expected given sediment section thickness, and varying degrees of forearc underplating. The attempts to mass-balance C and N across individual arc-trench systems demonstrate valuable integration of information from geophysical, field, petrologic, and geochemical observations. Studies of subduction-zone metamorphic suites can yield constraints on the evolution of deeply subducting rocks and the physicochemical characteristics of fluids released in forearcs and contributing to return flux in arc volcanic gases.

Bebout, G. E.

2008-12-01

206

Ancient subduction zone in Sakhalin Island  

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

207

Variable Rupture Mode at Subduction Zones Around the Pacific  

NASA Astrophysics Data System (ADS)

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

Satake, K.

2005-12-01

208

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

Microsoft Academic Search

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

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

2008-01-01

209

Dynamic interaction between tectonic plates, subducting slabs, and the mantle  

Microsoft Academic Search

Mantle convection models have been formulated to investi- gate the relation between plate kinematics and mantle dynamics. The cylin- drical geometry models incorporate mobile, faulted plate margins, a phase change at 670 km depth, non-Newtonian rheology, and tectonic plates. Models with a variety of parameters indicate that a relatively stationary trench is more likely to be associated with a subducted

Shijie Zhong; Michael Gurnis

1997-01-01

210

Deformation across the Alaska-Aleutian Subduction Zone near Kodiak  

USGS Publications Warehouse

The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending north-northwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak Island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.

Savage, J.C.; Svarc, J.L.; Prescott, W.H.

1999-01-01

211

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

212

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

213

'Snow White' Trench  

NASA Technical Reports Server (NTRS)

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

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

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

2008-01-01

214

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

215

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

NASA Astrophysics Data System (ADS)

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

Lynner, Colton; Long, Maureen D.

2015-06-01

216

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

217

Phoenix's Snow White Trench  

NASA Technical Reports Server (NTRS)

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

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

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

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

2008-01-01

218

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

219

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

220

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

NASA Astrophysics Data System (ADS)

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

Morozov, I. B.; Zheng, H.

2005-12-01

221

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.

222

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

E-print Network

Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling three-dimensional models of static Coulomb stress change. Model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the Coulomb stress on strike-slip faults

ten Brink, Uri S.

223

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

224

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

225

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

226

ELSEVIER Tectonophysics 241 (1995) 259-277 Tectonic regime of the southern Kurile Trench  

E-print Network

of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic unit that constitutes an efficient buttress. A piggy back basin, possibly filled by sediments that have islands) consist of Late Cretaceous to Early Paleogene volcanic rocks (Yamada et al., 1990). The volcanic

Demouchy, Sylvie

227

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

228

Evolution of texture and seismic anisotropy during subduction (Invited)  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

229

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

230

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

NASA Astrophysics Data System (ADS)

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

Rajendran, C.

2013-05-01

231

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

NASA Astrophysics Data System (ADS)

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

Bandy, W. L.; Mortera-Gutierrez, C. A.; Ortiz-Zamora, G.; Ortega-Ramirez, J.; Galindo Dominguez, R. E.; Ponce-Núñez, F.; Pérez-Calderón, D.; Rufino-Contreras, I.; Valle-Hernández, S.; Pérez-González, E.

2010-12-01

232

Snow White Trenches  

NASA Technical Reports Server (NTRS)

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

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

This image has been enhanced to brighten shaded areas.

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

2008-01-01

233

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

234

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

235

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

236

The influence of subduction processes on the geochemistry of Japanese alkaline basalts  

Microsoft Academic Search

The composition of volcanic rocks erupted in complex plate tectonic settings can provide information on the nature of the underlying mantle. We show here that the geochemistry of alkali basalts from Japan and eastern Asia varies systematically with distance from the Japanese island-arc. Samples from northeastern Japan, relatively close to the Japan Trench, are enriched in K, Sr, Ba and

Eizo Nakamura; Ian H. Campbell; Shen-Su Sun

1985-01-01

237

Dehydration of serpentinized slab mantle: Seismic evidence from southwest Japan  

Microsoft Academic Search

The seismicity in the subducting Philippine Sea slab (PHS) beneath southwest Japan shows a variety of modes of occurrence. We try to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and\\/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehydration

Tetsuzo Seno; Dapeng Zhao; Yoji Kobayashi; Masao Nakamura

2001-01-01

238

Trench Left By Sampler Scoop  

NASA Technical Reports Server (NTRS)

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

1976-01-01

239

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

USGS Publications Warehouse

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

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

1984-01-01

240

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

241

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

NASA Astrophysics Data System (ADS)

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

Satake, K.; Nanayama, F.

2003-12-01

242

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

USGS Publications Warehouse

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

Bakun, W.H.

2005-01-01

243

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

NASA Astrophysics Data System (ADS)

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

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

2015-03-01

244

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

245

Characterization of Subduction Seismicity at the Global Scale  

NASA Astrophysics Data System (ADS)

In the framework of the EURYI Project 'Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modelling', a global collection of recent subduction seismicity - partitioned between interplate, intraslab and upper plate events - has been performed. Based on EHB hypocenter and CMT Harvard catalogues, the hypocenters, nodal planes and seismic moments of worldwide subduction-related earthquakes were extracted for the period 1976 - 2007. Data were collected for centroid depths between sea level and 700 km and for magnitude Mw ? 5.5. For each subduction zone, a set of trench-normal transects were constructed choosing a 120km width of the cross-section on each side of a vertical plane and a spacing of 1 degree along the trench. For each of the 505 resulting transects, the whole subduction seismogenic zone was mapped as focal mechanisms projected on to a vertical plane after their faulting type classification according to the Aki-Richards convention. Transect by transect, was defined a "subduction box" identifying the spatial limits of the seismicity strictly related to the subduction zone under investigation, then the seismicity considered not related to the subduction process under investigation was removed. For each transect, the events previously identified by Heuret et al (2011) as subduction interface earthquakes were removed from the "subduction box". Upper plate seismicity (i.e. earthquakes generated within the upper plate as a result of the subduction process) was then extracted from the remaining events. We can be reasonably confident that the remaining seismicity can be related to the subducting plate. In this way the subduction seismicity has been properly classified in three categories: interpolate-, intraslab- and upper plate-seismicity. Following Heuret et al. (2011), the 505 transects have been merged into 62 larger segments that were ideally homogeneous in terms of their seismogenic zone characteristics. Several checks were performed, in order both to remove duplicate events, often occurring in these catalogues due to their marginal location relative to the boundary of adjacent transects and to verify if, due to the trench curvature and/or to the huge event clustering typical of some subduction zones, the same event is classified as belonging both to the upper plate and the intraslab category. Comparisons between main seismic parameters (e.g. maximum magnitude, number of events, cumulated seismic moment, recurrence time) with relation to both the different categories selected and the different segments have been performed and the correlation with a wide range of subduction-related parameters taken from the literature (e.g. plates/slab kinematics, thermal parameters) have been evaluated. This allowed to highlight possible cause-effect relationships and to obtain a snapshot on the general behavior of global subduction-related seismicity.

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

2012-12-01

246

Subduction and break-off controls on Indentation tectonics during India-Asia convergence  

NASA Astrophysics Data System (ADS)

Large deformations of the Asian continent and drastic change in the geometry of the plate boundary are the result of the interaction between the subduction of Tethys followed by the subduction of India continent and the Asian upper plates during convergence. The link between Asian tectonics, oceanic and continental subductions and the breakoff episodes has been explored by comparing global tomographic images showing remnants of slabs and Asian tectonics reconstruction. It allows formulating hypotheses on how deep subduction and indentation tectonics are coupled, and constrains self-consistent three-dimensional numerical models of coupled subducting - upper plates in an ambient mantle that we perform. We find that the subduction of the buoyant continent progressively decreases the driving force available and is followed by a convergence velocity drop, similar to the observed, however deformation is mostly accommodated along the upper plate margin. When slab detaches during subduction, as evidenced for India using global tomography, similar convergence rates are sustained, yet transient stresses propagate far into the upper plate interiors, localising along a belt at a high angle with the trench. Breakoff at the ocean-continent boundary confers long-lived complex slab morphology, with shallow upper-plate underthrusting laterally stepping to steeper dip along the trench. Following breakoff, the trench curvature progressively increases as convergence proceeds. It is similar to the geometry of the indian lithosphere, underthrusting to the west the Asian lithosphere early in the collision time, then subducts lately far north of the front beneath the Hindu-Kush, while more to the east India subducts much earlier and more to the south. The breakoff episodes evidenced using global tomography likely provided the conditions for large stress surge in the Asian lithosphere that resulted in the formation of the long-lived major intra-continental faulting systems of the Red River, Altyn Tagh and Tien Shan.

Replumaz, A.; capitanio, F. A.

2013-12-01

247

Reactivation of an old plate interface as a strike-slip fault in a slip-partitioned system: Median Tectonic Line, SW Japan  

NASA Astrophysics Data System (ADS)

In models for strain-partitioning at obliquely-convergent plate boundaries, trench-parallel slip occurs on a vertical fault. Trench-parallel slip at the Nankai subduction zone, SW Japan, is mapped along the Median Tectonic Line (MTL) which dips approximately 40°N. To understand its structural context and how the MTL functions in this slip-partitioned system, we collected a set of three seismic profiles in the Kii peninsula south of Osaka, using a multi-scale acquisition strategy that provides increasingly fine resolution. To understand its fault kinematics, we analyzed microseismic activity in two locations on the fault, using source data from Japan's Hi-net monitoring network. Structural details suggest that the MTL functioned as a megathrust during subduction of the Cretaceous Sanbagawa HP metamorphic belt. Its current pattern of microseismicity shows that it behaves as a strike-slip fault with no indication of a vertical fault at or around its surface trace. Thus, trench-parallel slip at the Nankai is now accommodated on an inclined fault plane in an unusual form of partitioning. This system appears to have developed out of a two-phase tectonic history in which a thrust structure that formed under initial-phase compressive stresses has been reactivated as a strike-slip fault under subsequent-phase shear stresses. Its unusual kinematics show that shear failure can occur on an existing non-vertical fault plane at a regional scale in preference to the rupture of a new ideal (vertical) fault plane.

Sato, Hiroshi; Kato, Naoko; Abe, Susumu; Van Horne, Anne; Takeda, Tetsuya

2015-03-01

248

Velocities of Subducted Sediments and Continents  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

249

Lithospheric Scale Deformation in Mega-thrust Subduction Zones  

NASA Astrophysics Data System (ADS)

Although the general plate tectonic model of subduction zone deformation and its relationship to the earthquake cycle for mega-thrust earthquakes is well known, there is neither consistency in such descriptions nor compatibility among seismological, geodetic, and geologic frameworks for such events. In particular in most seismologic studies of mega-thrust earthquakes there is an implicit assumption that the co-seismic slip is essentially symmetric across the fault surface - that is both the upper and lower plates moved equal amounts (but in opposite directions) during the rupture. Implicit in many geologic studies along convergent margins is the assumption that most permanent deformation is within the upper plate and the subducting slab basically transits the seismogenic zone with little permanent deformation. This perspective serves as the framework for many animations of subduction zone tectonics. Two subduction zone locales, the Kuriles and Solomon Islands, that have hosted recent Mw 8+ earthquakes demonstrate two end-member styles of subduction zone processes neither consistent with the conventional view. The November 2006 (thrust) and January 2007 (normal) earthquake pair in the Kuriles provide an opportunity to quantify the deformation within the subducting Pacific slab during the interseismic period. Based on the correspondence in slip during these events, we are able to both estimate the deformation (dominantly in the subducting slab and not in the overriding plate) and place a constraint on the static frictional strength of the megathrust interface of approximately 2-5 MPa. The 2007 Solomon Island Mw 8+ earthquake shows a distinctly different pattern of interseismic deformation. During this event, the propagating rupture traversed an active transform plate boundary between the separately subducting Australia and Solomon Sea plates. We interpret this to represent a situation in which interseismic deformation is primarily in the upper (Pacific) plate allowing the rupture to jump the fundamental barrier of a plate boundary. This is also compatible with limited GPS data available for the Australia plate near the trench indicating unimpeded subduction of Australia and thus little internal deformation of the subducting slab. These two subduction regimes indicate that there is likely a full continuum in how deformation is accommodated during subduction, and implies that attempts to determine the megathrust (and associated tsunami) potential of subduction zones using observations of upper-plate deformation is problematic.

Furlong, K. P.; Ammon, C.; Lay, T.

2008-12-01

250

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

E-print Network

TOKYO YOKOHAMA Pacific PlateEurasian Plate Japan JapanTrench 2011Eq 1896 1933 0 125 250 km Figure 1: Map. Earthquakes are caused by the movement of the Earth's tec- tonic plates; huge rigid areas of the Earth

251

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

252

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

USGS Publications Warehouse

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

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

2009-01-01

253

Effect of time-evolving age and convergence rate of the subducting plate on the Cenozoic adakites and boninites  

NASA Astrophysics Data System (ADS)

Partial melting of subducting oceanic crust expressed as high-Mg volcanic rocks such as adakites and boninites has been actively studied for decades, and Lee and King (2010) reported that time-evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust (e.g., Aleutians). However, few subduction model experiments have considered time-evolving subduction parameters, posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones. Therefore, we constructed two-dimensional kinematic-dynamic subduction models for the Izu-Bonin, Mariana, Northeast Japan, Kuril, Tonga, Java-Sunda, and Aleutian subduction zones that account for the last 50 Myr of their evolution. The models include the time-evolving age and convergence rate of the incoming oceanic plate, so the effect of time-evolving subduction parameters on transient partial melting of oceanic crust can be evaluated. Our model calculations revealed that adakites and boninites in the Izu-Bonin and Aleutian subduction zones resulted from transient partial melting of oceanic crust. However, the steady-state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones, indicating that time-evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites. Our model calculations confirm that other geological processes such as forearc extension, back-arc opening, mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the Mariana, Northeast Japan, Tonga, and southeastern Java-Sunda subduction zones.

Kim, Yoon-Mi; Lee, Changyeol

2014-12-01

254

Subduction Processes Along The Peruvian Margin From Wide Angle Seismic Data  

Microsoft Academic Search

Seismic wide angle data were acquired during RV SONNE cruise SO146 within the GEOPECO project (Geophysical Experiments at the Peruvian Continental Margin - investigations of tectonics, mechanics, gas hydrates and fluid transport) along the Pe- ruvian continental margin. The oceanic Nazca Plate, which is approximately 28 to 38 million years new at the Peruvian trench, is subducting under the South

A. Broser; J. Bialas; A. Hampel; N. Kukowski

2002-01-01

255

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

256

K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene-Oligocene thermo-tectonic reactivation  

NASA Astrophysics Data System (ADS)

Systematic K-Ar dating and geochemical analyses of Paleogene cauldrons in the Sanin Belt of SW Japan have been made to explore the relationship between the timing of their formation and the Paleogene subduction history of SW Japan documented in the Shimanto accretionary complex. We also examine the magma sources and tectonics beneath the backarc region of SW Japan at the eastern plate boundary of Eurasia. Fifty-eight new K-Ar ages and 19 previously reported radiometric age data show that the cauldrons formed during Middle Eocene to Early Oligocene time (43-30 Ma), following a period of magmatic hiatus from 52 to 43 Ma. The hiatus coincides with absence of an accretionary prism in the Shimanto Belt. Resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench, as a common tectono-thermal event within a subduction system. The cauldrons are composed of medium-K calc-alkaline basalts to rhyolites and their plutonic equivalents. These rocks are characterized by lower concentrations of large ion lithophile elements (LILE) including K 2O, Ba, Rb, Th, U and Li, lower (La/Yb) n ratios, lower initial Sr isotopic ratios (0.7037-0.7052) and higher ?Nd( T) values (-0.5 to +3.5) relative to Late Cretaceous to Early Paleogene equivalents. There are clear trends from enriched to depleted signatures with decreasing age, from the Late Cretaceous to the Paleogene. The same isotopic shift is also confirmed in lower crust-derived xenoliths, and is interpreted as mobilization of pre-existing enriched lithospheric mantle by upwelling depleted asthenosphere. Relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the SW Japan arc-trench system during the Eocene to Oligocene. Newly identified Late Eocene low silica adakites and high-Mg andesites in the Sanin Belt and Early Eocene A-type granites in the SW Korea Peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate. The backarc region was an extensional tectonic setting, and some Paleogene rift basins and Sanin Belt cauldrons occur in linear arrays. The Eocene-Oligocene Sanin-SE Korea continental arc lies on the NE extension of the East China Sea Basin, the initial stage of which probably formed by continental arc rifting. This rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab, in response to decreased convergence rate between the Pacific and Eurasian plates.

Imaoka, T.; Kiminami, K.; Nishida, K.; Takemoto, M.; Ikawa, T.; Itaya, T.; Kagami, H.; Iizumi, S.

2011-01-01

257

Design of temporary trench supports  

E-print Network

DESIGN OF TEMPORARY TRENCH SUPPORTS A Thesis by MARC YOUSSEF BALLOUZ Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May 1991 Major Subject...: Civil Engineering DESIGN OF TEMPORARY TRENCH SUPPORTS A Thesis by MARC YOUSSEF BALLOUZ Approved as to style and content by: uis J. Tho son (Chair of Commit tee) yne A. Dunlap (Member) Ja R. Walton (Member) Jame T. . Yao (Head of partment...

Ballouz, Marc Youssef

1991-01-01

258

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

259

Scientists Examine Challenges and Lessons From Japan's Earthquake and Tsunami  

NASA Astrophysics Data System (ADS)

A week after the magnitude 9.0 great Tohoku earthquake and the resulting tragic and damaging tsunami of 11 March struck Japan, the ramifications continued, with a series of major aftershocks (as Eos went to press, there had been about 4 dozen with magnitudes greater than 6); the grim search for missing people—the death toll was expected to approximate 10,000; the urgent assistance needed for the more than 400,000 homeless and the 1 million people without water; and the frantic efforts to avert an environmental catastrophe at Japan's damaged Fukushima Daiichi Nuclear Power Station, about 225 kilometers northeast of Tokyo, where radiation was leaking. The earthquake offshore of Honshu in northeastern Japan (see Figure 1) was a plate boundary rupture along the Japan Trench subduction zone, with the source area of the earthquake estimated at 400-500 kilometers long with a maximum slip of 20 meters, determined through various means including Global Positioning System (GPS) and seismographic data, according to Kenji Satake, professor at the Earthquake Research Institute of the University of Tokyo. In some places the tsunami may have topped 7 meters—the maximum instrumental measurement at many coastal tide gauges—and some parts of the coastline may have been inundated more than 5 kilometers inland, Satake indicated. The International Tsunami Information Center (ITIC) noted that eyewitnesses reported that the highest tsunami waves were 13 meters high. Satake also noted that continuous GPS stations indicate that the coast near Sendai—which is 130 kilometers west of the earthquake and is the largest city in the Tohoku region of Honshu—moved more than 4 meters horizontally and subsided about 0.8 meter.

Showstack, Randy

2011-03-01

260

Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge  

NASA Astrophysics Data System (ADS)

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0 < Te < 3 km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te < Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

Ratheesh Kumar, R. T.; Windley, B. F.; Rajesh, V. J.; Santosh, M.

2013-12-01

261

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

262

Deformation of Japan as measured by improved analysis of GEONET data  

NASA Astrophysics Data System (ADS)

The Japan subduction zone represents a complex set of plate interfaces with significant trench-parallel variability in great earthquakes and transient deep slip events. Within the Japan arc the Nankai segment of the Eurasian-Philippine plate boundary is one of the classic subduction zone segments that last produced a set of temporally linked great earthquakes in the 1940's. Recently, down-dip of the Nankai seismogenic portion of the plate interface, transient slip events and seismic tremor events were observed. Through analysis of the GEONET GPS data, the spatial and higher frequency temporal characteristics of transient slip events can be captured. We describe our analysis methods, the spatial filtering technique that has been developed for use on large networks, a periodic signal filtering method that improves on commonly-used sinusoidal function models, and the resultant velocities and time series. Our newly developed analysis method, the GPS Network Processor, gives us the ability to process large volumes of data extremely fast. The basis of the GPS Network Processor is the JPL-developed GIPSY-OASIS GPS analysis software and the JPL-developed precise point positioning technique. The Network Processor was designed and developed to efficiently implement precise point positioning and bias fixing on a 1000-node (2000 cpu) Beowulf cluster. The entire 10 year ~1000-station GEONET data set can be reanalyzed using the Network Processor in a matter of days. This permits us to test different processing strategies, each with potentially large influence on our ability to detect strain transients from the subduction zones. For example, we can test different ocean loading models, which can effect the diurnal positions of coastal GPS sites by up to 2 cm. We can also test other potentially important factors such as using reprocessed satellite orbits and clocks, the parameterization of the tropospheric delay, or the implementation of refined solid body tide estimates. We will present the results of tests that we have run to date at the meeting.

Owen, S. E.; Dong, D.; Webb, F. H.; Newport, B. J.; Simons, M.

2006-12-01

263

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

264

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

265

B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models  

Microsoft Academic Search

Several hypotheses have been proposed to explain trench-parallel shear wave splitting in the mantle wedge of subduction zones. These include 3-D flow effects, parallel melt filled cracks, and B-type olivine fabric. We predict the distribution of B-type and other fabrics with high-resolution thermal and stress models of subduction zones. A composite viscous rheology is used that incorporates wet diffusion creep,

Erik A. Kneller; Peter E. van Keken; Shun-ichiro Karato; Jeffrey Park

2005-01-01

266

Structural and thermal control of seismic activity and megathrust rupture dynamics in subduction zones: Lessons from the Mw 9.0, 2011 Tohoku earthquake  

NASA Astrophysics Data System (ADS)

The 2011 Tohoku megathrust earthquake ruptured a vast region of the northeast Japan Trench subduction zone in a way that had not been enough anticipated by earthquake and tsunami risk scenarios. We analyzed the Tohoku rupture combining high-frequency back-projection analysis with low frequency kinematic inversion of the co-seismic slip. Results support the to-day well-accepted broadband characteristics of this earthquake. Most of the seismic moment is released during the first 100 s, with large co-seismic slip (up to 55 m) offshore Miyagi in a compact region on the landward side of the trench. Coherent high-frequency radiation areas and relatively low co-seismic slip are a distinctive signature of the slab-mantle interface. The broadband characteristics of the Tohoku rupture are interpreted, integrating the seismic activity and structure information on the NE Japan forearc region, as a signature of along-dip segmentation and segment interactions, that result from thermal structure, plate geometry, material composition and fracture heterogeneities along the plate boundary interface. Deep mantle corner flow and low dehydration rates along the cold subduction slab interface lead to an extended seismogenic slab-mantle interface, with strong bi-material contrast controlling larger propagation distance in the downdip preferred rupture direction. Off Miyagi, plate bending below the mantle wedge, ?142.3°E at ?25 km depth, is associated with the eastern limit of the deep M7-8-class thrust-earthquakes, and of the strongest coherent high-frequency generation areas. The region of the slab-crust interface between the mantle wedge limit, ?142.7°E at ?20 km depth, and a trenchward plate bending, ?143.2°E at ?15 km, acted as an effective barrier resisting for many centuries to stress-loading gradient induced by deep stable sliding and large earthquakes along the slab-mantle interface. The 2011 Tohoku earthquake, whose hypocenter is located on the east side of the mantle wedge limit, released the accumulated stress in this region and succeeded to overcome the plate bending, driving the upper plate boundary interface to slip co-seismically, regardless its frictional property, thanks to a combination of dynamic effects associated with bi-material rupture directivity and stress changes induced by reflection from the surface of waves released by the unstable slip. This conceptual framework provides elements for reappraisal of long-term seismic activity and occurrence of rare and extreme tsunamigenic megathrust in other subduction zones, like those of North-Central Lesser Antilles, Central and Northern Chile.

Satriano, Claudio; Dionicio, Viviana; Miyake, Hiroe; Uchida, Naoki; Vilotte, Jean-Pierre; Bernard, Pascal

2014-10-01

267

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

268

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

269

Central Andean Giant Ore Deposits: Links to Forearc Subduction Erosion, Shallowing Subduction and Thickening Crust  

NASA Astrophysics Data System (ADS)

An outstanding question on the Central Andean margin is the relationship between tectonic processes like ebbing arc volcanism, shallowing of the subducting slab and crustal thickening, and the origin of giant porphyry and epithermal Cu, Au and Ag deposits. Another potentially important factor in forming these major mineral deposits is forearc subduction erosion, which is postulated to have removed up to ~250 km of Central Andean forearc crust since the Jurassic. Geochemical and geophysical studies provide insights into possible links. Evidence for partial melts of removed and subducted forearc crust reaching the arc magma source and thus the magmas that host the ore deposits comes from the chemistry of late Neogene volcanic rocks on both the northern and southern margin of the Chilean-Pampean flat-slab (28°-33°S), where the frontal arc was displaced ~50 km into the foreland between ~10 and 3 Ma. This chemical evidence consists of transient ultra-steep REE patterns, elevated Mg, Cr and Ni contents and steps in isotopic ratios that are particularly notable in the glassy adakitic 8-3 Ma (Pircas Negras) andesites on the northern flat-slab margin at 27°-28°S. Well constrained reconstructions of the margin near 26-28°S that assume a sustained 300 km wide arc-trench gap and ~50 km of forearc removal suggest an accelerated average forearc subduction erosion rate over 150 km3/my/km between 8 and 3 Ma. Noting that the late Miocene arc is now at least ~ 260 km from the trench from 26°S to 34°S and that the active arc extrapolates through the amagmatic flat-slab region (28°-33°S) at 300 km from the trench, accelerated forearc removal could be inferred from ~34°S to 26°S at ~10 to 3 Ma. Geophysical evidence for forearc crust entering the mantle wedge as the flatslab shallowed could come from low Vp/Vs seismic ratios in the mantle wedge under the flatslab, which Wagner et al. (2010) attribute to orthopyroxene. Formation of this orthopyroxene could be explained by forearc crust reacting with the mantle wedge. Thus, the slab shallowing, crustal thickening and forearc subduction erosion in the flatslab region, which began at ca 20-18 Ma and accelerated after 11-10 Ma could have set the stage for the formation of the Los Pelambres, Rio Blanco and El Teniente giant Cu porphyries between ~ 11-4 Ma. The backarc 8-6 Ma Bajo de la Alumbrera Cu-Au district near 27°S, also formed east of the migrating volcanic arc on the northern flatslab margin at this time. This deposit is notable for now being above a high Qp mantle seismic anomaly overlying the slab, which is at a depth of ~150 km. Elsewhere, Ag-Zn mineralization in the ~14-12 Ma Potosi district near 19.5°S in the Altiplano backarc, which has been suggested to have occurred in the early stages of steepening of a shallow slab, would potentially predate flushing of eroded forearc material from an expanding mantle wedge. In the same vein, a lack of known big Cu-Au-Ag deposits associated with the late Neogene giant plateau ignimbrite complexes, considered to be fomed over steepening subduction zones characterized by low Vp and Vs and high Qp tomographic seismic anomalies, could also partially reflect loss of forearc subducted components from an expanding wedge.

Kay, S. M.; Mpodozis, C.

2013-05-01

270

[Adrian Stokes and 'trench jaundice'].  

PubMed

On the day that Great Britain declared war on Germany in 1914, the Irish physician and bacteriologist Adrian Stokes travelled to London to volunteer. One week later he left for France with the first British troops as an officer with the Royal Army Medical Corps. He spent most of the First World War attached to No. 1 Mobile Bacteriological Laboratory at the Remy Siding British-Canadian field hospital in Flanders. In April 1916, he was confronted with an outbreak of trench jaundice, also known as epidemic jaundice (Weil's disease). Conditions in the trenches contributed to the hundred cases identified by Stokes in a short period. In 1917, he was the first to publish (in The Lancet) the finding that the bacterium Spirochaeta icterohaemorrhagiae, the causative agent of epidemic jaundice, could be isolated from the kidneys of rats. A subsequent rat control campaign in the trenches successfully curbed the disease. PMID:25804113

Wever, P C

2015-01-01

271

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

272

Geoid anomalies in the vicinity of subduction zones  

NASA Technical Reports Server (NTRS)

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

Mcadoo, D. C.

1980-01-01

273

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

274

A close link between serpentinization and seismogenesis in the Philippine Sea slab beneath Kanto, Japan  

NASA Astrophysics Data System (ADS)

The Kanto district in central Japan, which surrounds the Tokyo metropolitan area, is known as a unique region in the world in terms of plate tectonics. This region is located behind a trench-trench-trench (TTT) triple junction with two obliquely subducting oceanic plates, the Philippine Sea (PHS) and Pacific (PAC) plates. Recent studies have shown that the bottom of the PHS slab is in contact with the upper surface of the Pacific slab beneath Kanto (Uchida et al., 2009, EPSL; Nakajima et al., 2009, JGR). As a result of the subduction of the two slabs, many disastrous M~8 and numerous M~7 earthquakes have struck the Tokyo metropolitan area. For example, the 1923 Kanto earthquake (M7.9), one of the most destructive earthquakes in the 20th century, caused severe damage to the Tokyo metropolitan area, along with 105,000 fatalities. We perform travel-time tomography beneath Kanto, Japan, to obtain detailed structures of the subducting PHS slab and to discuss the seismogenesis of interplate and intraplate earthquakes in terms of the contact of the two slabs. We detected a wedge-shaped prominent low-velocity zone with high Vp/Vs at the easternmost portion of the PHS slab. The western boundary of the low-velocity zone is sub-vertical, and seismic velocities vary by 15-20% across it over a short distance of ~10 km. This low-velocity zone is interpreted as serpentinized mantle of the PHS slab because serpentinization of the mantle of the PHS plate is observed along the Izu-Bonin trench before its subduction (Kamimura et al., 2002, PEPI). We found two clear relation between serpentinization and seismicity around the PHS slab. First, thrust earthquakes between the bottom of the PHS slab and the upper surface of the PAC slab occur inhomogenously in space and are almost absent along the PHS-PAC interface overlain by the serpentinized PHS-slab mantle. This observation strongly suggests that low viscosity of the serpentine prevents the plate interface from slipping seismically, and hence the serpentine can be a barrier of seismic rapture along the plate interface. Second, two intraslab earthquakes in 1921 (M7.0) and 1987 (east off Chiba earthquake, M6.7) appear to have occurred along the western boundary of the serpentinized mantle (serpentinized boundary) accompanied by right-lateral movement, based on analyses of focal mechanisms and aftershock distribution. A sub-vertical earthquake cluster penetrating the entire Philippine Sea slab is also observed along the serpentine boundary, and four earthquakes in the cluster have strike-slip focal mechanisms, similar to that of the 1987 earthquake. Focal mechanisms obtained for past large earthquakes and present-day microearthquakes suggest the concentration of right-lateral deformation along the mechanically weak serpentinized boundary.

Nakajima, J.; Hasegawa, A.; Umino, N.; Demachi, T.

2010-12-01

275

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

276

Shallow subduction, ridge subduction, and the evolution of continental lithosphere  

SciTech Connect

Subduction of oceanic lithosphere beneath continental crust at a shallow angle has occurred throughout the Phanerozoic Eon. Ridge subduction often follows shallow subduction and causes bimodal volcanism and crustal rifting, forming back-arc basins. Recent models for Archean plate tectonics propose very fast rates of spreading (400-800 km/Ma) and convergence, and sinking rates comparable to or slower (<10 km/Ma) than those of today. As faster convergence and slower sinking correspond to subduction at shallower angles, shallow subduction and ridge subduction must have been ubiquitous during the Archean permobile regime. This is compatible with a back-arc-basin origin for Archean greenstone belts. The common coexistence of tholeiitic and calc-alkaline igneous rocks in Archean greenstone belts, also implies ridge subduction. The authors envisage a transition, between 2.4 and 1.8 Ga., from a regime dominated by shallow subduction and repeated ridge subduction to one of normal plate tectonics with steeper subduction. Spreading rates decreased; continental plates became larger and stable shelves could develop at trailing margins. Shallow subduction became the exception, restricted to episodes of abnormally fast convergence; nevertheless, the long span of post-Archean time makes it unlikely that any part of the continental crust has escaped shallow subduction and ridge subduction. These processes recycle much volatile-rich oceanic crust into the sub-continental upper mantle, thereby underplating the crust, effecting upper-mantle metasomatism and affecting intraplate magmatism.

Helmstaedt, H.; Dixon, J.M.; Farrar, E.; Carmichael, D.M.

1985-01-01

277

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

278

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

NASA Astrophysics Data System (ADS)

Interpretation 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

279

Vertical slab sinking and westward subduction offshore of Mesozoic North America  

NASA Astrophysics Data System (ADS)

Subducted slabs in the mantle, as imaged by seismic tomography, preserve a record of ancient subduction zones. Ongoing debate concerns how direct this link is. How long ago did each parcel of slab subduct, and where was the trench located relative to the imaged slab position? Resolving these questions will benefit paleogeographic reconstructions, and restrict the range of plausible rheologies for mantle convection simulations. We investigate one of the largest and best-constrained Mesozoic slab complexes, the "Farallon" in the transition zone and lower mantle beneath North America. We quantitatively integrate observations from whole-mantle P-wave tomography, global plate reconstructions, and land geological evidence from the North American Cordillera. These three data sets permit us to test the simplest conceivable hypothesis for linking slabs to paleo-trenches: that each parcel of slab sank only vertically shortly after entering the trench That is, we test whether within the limits of tomographic resolution, all slab material lies directly below the location where it subducted beneath its corresponding arc. Crucially and in contrast to previous studies, we do not accept or impose an Andean-style west coast trench (Farallon-beneath-continent subduction) since Jurassic times, as this scenario is inconsistent with many geological observations. Slab geometry alone suggests that trenches started out as intra-oceanic because tomography images massive, linear slab "walls" in the lower mantle, extending almost vertically from about 800 km to 2000+ km depth. Such steep geometries would be expected from slabs sinking vertically beneath trenches that were quasi-stationary over many tens of millions of years. Intra-oceanic trenches west of Mesozoic North America could have been stationary, whereas a coastal Farallon trench could not, because the continent moved westward continuously as the Atlantic opened. Overlap of North American west-coast positions, as reconstructed in a hotspot reference frame, with elongate slab walls predicts where and when the intra-oceanic trenches would have been overridden by the westward-moving continent. Land geology plays the role of a validating data set: trench override is predicted to coincide with accretion of buoyant arc terranes, deformation of the continental margin and slab window volcanism. We find excellent agreement between predicted and observed accretion episodes, validating both vertical sinking (within observational uncertainties of a few hundred kilometers laterally), and westward subduction beneath an archipelago of island arcs west of Jura-Cretaceous North America. Amalgamation of the arcs with North America occurred as the intervening ocean crust was consumed. Implied slab sinking rates are of 10±2 mm/a, uniformly for three different slab walls. We conclude that the hypothesis of essentially vertical slab sinking produces a self-consistent model that explains first-order observations of 200 Ma - 50 Ma Cordilleran geology. By contrast, the standard scenario of a continental Farallon trench requires massive amounts of slab to be laterally displaced by 1000+ km after subduction, and offers no explanation for a long series of Cretaceous terrane accretions.

Sigloch, Karin; Mihalynuk, Mitchell G.

2013-04-01

280

Planarization And Integration Of Shallow Trench Isolation  

Microsoft Academic Search

STI process flow and planarization requirements are reviewed. An STI planarization mask was designedand utilized for test wafer patterning to investigate STI CMP planarization. Test wafers were processedthrough a typical STI process sequence, including trench etch, trench liner oxidation, trench-fill, and CMP.Two different CVD techniques, ozone TEOS thermal CVD and HDPCVD, were investigated for trench-fill.CMP experiments were carried out with

Dennis Ouma; Duane Boning; Fritz Redeker; J. Tony Pan; James Chung; Jason Whitby; Ping Li

1998-01-01

281

Trench Gate Power MOSFET: Recent Advances  

E-print Network

Trench Gate Power MOSFET: Recent Advances and Innovations Raghvendra Sahai Saxena and M. Jagadesh Kumar Raghavendra S. Saxena and M. Jagadesh Kumar, "Trench Gate Power MOSFET: Recent Advances Power MOSFET: Recent Advances and Innovations Raghvendra Sahai Saxena and M. Jagadesh Kumar The trench

Kumar, M. Jagadesh

282

Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.  

PubMed

We combine spatial variations of P- and S-wave speeds, 1000 fault plane solutions, and 6600 well-determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the Tonga back-arc. We show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere. The detached slab may be from a previous episode of subduction along the fossil Vitiaz trench about 5 to 8 million years ago. The juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle. Such a configuration, if common in the past, would allow the preservation of a primordial component in the lower mantle. PMID:11431564

Chen, W P; Brudzinski, M R

2001-06-29

283

Development of texture and seismic anisotropy during the onset of subduction  

NASA Astrophysics Data System (ADS)

How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strain-history-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. (2012a). The setting lends itself as a case study, because it is fairly young and, therefore, 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 steady state flow. Second, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel subslab mantle flow. We demonstrate that although a barrier amplifies trench-parallel subslab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se. In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the subslab mantle.

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

2014-01-01

284

Morphology of the Central Chilean Continental Margin Between 33°S and 37°S: Transition From Subduction Erosion to Subduction Accretion  

NASA Astrophysics Data System (ADS)

Along much of southern Chile, the trench is filled with about 2-2.5 km thick turbidites, supplied from the erosion of the Andes. Elongated anticline ridges at the front of the continental slope indicate that sediment accretion is prevalent in this area. The gentle topography of the trench gradually deepens from south to north, causing along-trench northward sediment transport. A continuous meandering channel that distributes turbidites can be followed along the trench from about 41°S until about 33°S. Here, the Juan Fernández Ridge on the ocean plate enters the subduction zone and changes margin tectonics dramatically. The ridge forms a barrier inhibiting transport of sediments further to the north. It separates a flooded trench to the south from a sediment starved segment to the north, and marks the abrupt boundary between tectonic erosion and sediment accretion. The area just south of this boundary between 33°S and 37°S was recently mapped by a high-resolution multibeam survey with German RV METEOR. The morphology of the surveyed area shows an irregular slope toe facing the trench, surprisingly, the turbidite-flooded trench is not faced by a well-developed system of accretionary ridges. Generally, the morphology of the continental slope displays three different regions: The lower slope typically displays a rugged terrain, including collapse structures, whereas the middle slope is characterized by a series of smooth terraces probably representing mid-slope basins. The transition from the lower to the middle slope occurs across a roughly margin-parallel 150-km-long distinct lineament of alternating narrow highs and troughs. These structures indicate local uplift and subsidence along the same lineament, suggesting strike-slip deformation along a fairly continuos array of faults. Their sharp relief and the transition from a rugged to a smooth morphology across them suggest that faulting is currently active. Across a moderate change in slope dip, the middle slope grades into the upper slope, which displays a smooth morphology and gentle dips. The entire slope structure is cut by several large canyons that zigzag from near the coast to the trench. The canyons head is typically located at the mouth of the largest rivers in the area and possibly transport most of the sediment reaching the trench.

Weinrebe, W.; Ranero, C. R.; Diaz Naveas, J.; Meteor67 Scientific Party

2006-12-01

285

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

286

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.

287

Possible evidence for subducted sedimentary materials beneath central California.  

USGS Publications Warehouse

Seismic reflection and refraction data from the Coast Ranges of central California W of the San Andreas fault suggest the presence of a low-velocity (4.5-5.0 km/s) wedge at a depth of about 15-22 km that dips E beneath rocks of both the Franciscan assemblage and the Salinian block. We propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 Ma. The refraction data also suggest the presence of a second low-velocity zone that extends at least to mid-crustal depths beneath the San Andreas fault zone.-Authors

Trehu, A.M.; Wheeler, W.H., IV

1987-01-01

288

Subduction initiation: spontaneous and induced  

Microsoft Academic Search

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

Robert J. Stern

2004-01-01

289

Are subduction zones inherently weak?  

NASA Astrophysics Data System (ADS)

An outstanding question in geodynamics is how weak are plate boundaries when compared to their interiors? Particularly, how weak are subduction zone interfaces? Because subduction is believed to be the major driver of plate tectonics, this question is of fundamental importance for geodynamics. Several lines of evidence suggest that subduction zones are weak and that the unique availability of water on Earth plays a key role. We have evaluated the strength of subduction zone interfaces using two approaches: i) an empirical relationship between shear stress at the interface and subduction velocity, deduced from laboratory experiments of subduction; and ii) a parametric study of natural subduction zones that provides new insights on subduction zone interface strength. Our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low (< 33 MPa). To account for this global requirement, we propose that there is a feedback mechanism between subduction velocity, water release rate from the subducting plate and serpentinization and weakening of the forearc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally.

Duarte, Joao; Schellart, Wouter; Cruden, Alexander; Gutscher, Marc-André

2014-05-01

290

Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: finding the NVT sweet spot  

E-print Network

Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: finding, México Abstract Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone and are therefore triggered by the SSE. 1. Introduction Non-volcanic tremor (NVT) in the Japan, Cascadia, and Costa

Paris-Sud XI, Université de

291

Crustal deformation at the southernmost part of the Ryukyu subduction (East Taiwan) as revealed by new marine seismic experiments  

E-print Network

by new marine seismic experiments Thomas Theunissen a, ,1 , Serge Lallemand a,f , Yvonne Font b Elsevier B.V. All rights reserved. 1. Introduction The Ryukyu Subduction zone between Kyushu Island (Japan

Demouchy, Sylvie

292

Plate Tectonics: From Initiation of Subduction to Global Plate Motions (Augustus Love Medal Lecture)  

NASA Astrophysics Data System (ADS)

Plates are driven by buoyancy forces distributed in the mantle, within cooling oceanic plates (ridge push) and within subducted slabs. Although the case is often made that subducted slabs provide the principle driving force on plate motion, consensus has not been achieved. This is at least partially due to the great difficulty in realistically capturing the role of slabs in observationally-constrained models as slabs act to drive and resist plate motions through their high effective viscosity. Slab buoyancy acts directly on the edge of the plate (slab pull), while inducing mantle flow that tends to drag both subducting and overriding plates toward the trench. While plates bend during subduction they undergo a form of 'plastic failure' (as evident through faulting, seismicity and reduction of flexural parameters at the outer trench wall). The birth of a new subduction zone, subduction initiation, provides important insight into plate motions and subduction dynamics. About half of all subduction zones initiated over the Cenozoic and the geophysical and geological observations of them provide first order constraints on the mechanics of how these margins evolved from their preexisting tectonic state to self-sustaining subduction. We have examples of subduction initiation at different phases of the initiation process (e.g. early versus late) as well as how margins have responded to different tectonic forcings. The consequences of subduction initiation are variable: intense trench roll back and extensive boninitic volcanism followed initiation of the Izu-Bonin-Mariana arc while both were absent during Aleutian arc initiation. Such differences may be related to the character of the preexisting plates, the size of and forces on the plates, and how the lithosphere was initially bending during initiation. I will address issues associated with the forces driving plate tectonics and initiating new subduction zones from two perspectives. A common thread is the origin and evolution of intense back arc spreading and rapid roll back associated with some ocean-ocean subduction zones. I will look at the dynamics driving global plate motions and the time-dependence of trench rollback regionally. Capitalizing on advances in adaptive mesh refinement algorithms on parallel computers with individual plate margins resolved down to a scale of 1 kilometer, observationally constrained, high-resolution models of global mantle flow now capture the role of slabs and show how plate tectonics is regulated by the rheology of slabs. Back-arc extension and slab rollback are emergent consequences of slab descent in the upper mantle. I will then describe regional, time-dependent models, address the causes and consequences of subduction initiation, and show that most back arc extension follows subduction initiation. Returning to the global models, inverse models using the full adjoint of the variable viscosity, Stokes equation are now possible and allow an even greater link between present-day geophysical observations and the dynamics from local to global scales.

Gurnis, Michael

2013-04-01

293

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

294

'Snow White' Trench After Scraping  

NASA Technical Reports Server (NTRS)

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

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

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

2008-01-01

295

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

296

Aftershock Statistics of Large Subduction Earthquakes  

NASA Astrophysics Data System (ADS)

Large mega-thrust subduction earthquakes generate prolific aftershock sequences which can last an extended period of time and affect wide spatial areas. Among those aftershocks the largest ones can cause additional damage and pose significant risks to infrastructure and population. Therefore, the modeling of aftershock sequences of large subduction earthquakes is of tremendous importance for seismic hazard assessment and earthquake risk mitigation. It can also play a prominent role in the stochastic ground shaking modeling of major mainshock-aftershock sequences. In this work, we analyze statistical properties of aftershock sequences of large subduction earthquakes world-wide which occurred from 1973 to present, including recent catastrophic events in Sumatra, Chile, and Japan. We use the information provided in the NEIC catalog to extract 60 aftershock sequences generated by the mainshocks of magnitude 7.0 and above. We construct their temporal decay rates and magnitude-frequency statistics. To model their temporal behaviour we apply a well-established modified Omori's law in order to infer its characteristics. In the magnitude domain we model the magnitude-frequency statistics using a scaling relation of Gutenberg and Richter and estimate the corresponding parameters. We also analyze statistically the difference between the magnitudes of the mainshocks and the corresponding largest aftershocks in the sequences and discuss this in terms of Bath's law. One of the main goals of this work is to investigate the variation in parameter values of the above empirical laws with respect to the magnitude of the mainshock. Our main finding states that the most of the parameters do not depend on the magnitude of the mainshock. However, they show a broad variation in values across different subduction settings. The obtained results can be used to constrain the parameters for the stochastic ground shaking models of mainshock-aftershock sequences aimed at describing the effects of mega-thrust earthquakes.

Goda, K.; Ivanian, A.; Shcherbakov, R.; Atkinson, G. M.

2012-12-01

297

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

298

Crustal origin of trench-parallel shear-wave fast polarizations in the Central Andes  

NASA Astrophysics Data System (ADS)

In this study, SKS and local S phases are analyzed to investigate variations of shear-wave splitting parameters along two dense seismic profiles across the central Andean Altiplano and Puna plateaus. In contrast to previous observations, the vast majority of the measurements reveal fast polarizations sub-parallel to the subduction direction of the Nazca plate with delay times between 0.3 and 1.2 s. Local phases show larger variations of fast polarizations and exhibit delay times ranging between 0.1 and 1.1 s. Two 70 km and 100 km wide sections along the Altiplano profile exhibit larger delay times and are characterized by fast polarizations oriented sub-parallel to major fault zones. Based on finite-difference wavefield calculations for anisotropic subduction zone models we demonstrate that the observations are best explained by fossil slab anisotropy with fast symmetry axes oriented sub-parallel to the slab movement in combination with a significant component of crustal anisotropy of nearly trench-parallel fast-axis orientation. From the modeling we exclude a sub-lithospheric origin of the observed strong anomalies due to the short-scale variations of the fast polarizations. Instead, our results indicate that anisotropy in the Central Andes generally reflects the direction of plate motion while the observed trench-parallel fast polarizations likely originate in the continental crust above the subducting slab.

Wölbern, I.; Löbl, U.; Rümpker, G.

2014-04-01

299

Double saloon door tectonics in the Japan Sea, Fossa Magna, and the Japanese Island Arc  

NASA Astrophysics Data System (ADS)

A number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the Japanese Arc. These include: a pair of terranes, SW and NE Honshu, which rotated in opposite directions from 22-21 Ma to 14-11 Ma; rotated terranes which comprise a retro-arc fold/thrust belt attached to an accretionary wedge intruded by a magmatic arc; contemporaneous backarc extension from 24 to 21 Ma which is brought to a halt by progressive collision of the Izu-Bonin and Japan Arcs from 15 to 5 Ma; isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust, during backarc rifting; such isolation may be due to simultaneous rifting or to progressively seaward rifts, associated with ridge jumps towards the subduction zone; opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30-26 Ma to 16-15 Ma; concurrent development of a major arc-orthogonal rift, the Fossa Magna, from 23-18 Ma to 14 Ma, which was thereafter inverted from 15 Ma to the Recent; a northeast propagating rift in the northern Japan Basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary. Driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest Pacific. Opposite rotational torques, leading to opposite terrane rotations, may be caused by rollback of a curved trench hingeline, or by the divergent slab sinking forces of the Pacific and Philippine Sea Plates.

Martin, A. K.

2011-01-01

300

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust  

NASA Astrophysics Data System (ADS)

Understanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate-induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench-parallel variations in exhumation have a direct impact on exploration paradigms for porphyry-related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.

Spikings, Richard; Simpson, Guy

2014-05-01

301

Seismic Imaging of the Middle America Subduction Zone Beneath Mexico  

NASA Astrophysics Data System (ADS)

P-wave coda from teleseismic events were used to compute receiver functions followed by formal inversions for discontinuous variations in elastic properties beneath a dense seismic array that crosses Mexico from Acapulco on the Pacific coast, through Mexico City, almost to Tempico on the Gulf of Mexico. Broadband data from the Meso-America Subduction Experiment (MASE) line were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere. We observe nearly horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continent for a distance of approximately 300 km from the Middle America Trench, with a clear image of a very thin low-velocity oceanic crust (7-8 km) which dips at 15-20 degrees then flattens and slightly thickens (~10 km). At approximately 250 km inland the inferred subducting crust undergoes a change in seismic character, specifically a disruption in the crustal velocity signature, which may reflect the initiation of partial eclogitization of the subducting crust or release of fluids via dehydration that would result in a reduced velocity contrast at the Moho. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). The image of the steeply subducted Cocos slab underneath the TMVB is enhanced by using the P-to-S converted phases, following the method used in southern Central America to image a steeply dipping subducted slab (> 60 degrees) for the TUCAN experiment (MacKenzie et al, 2010), however is complicated by the wide active volcanic arc and deep sedimentary basins in the middle of the array. The continental Moho is clearly imaged at ~40 km deep beneath the TMVB and shallows (~25 km) towards the Gulf of Mexico. The deeper seismic structure underneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ~70-80 km within the upper mantle. This feature, which spans horizontally beneath the entire arc (~150 km), may delineate the top of a layer of ponded partial melt, which is consistent with previous geodynamic modeling of melt migration, and also evidenced from P-wave velocity tomographic images of this and other subduction zones although images of this layer was not sharply resolved.

Miller, M. S.; Kim, Y.; Pearce, F. D.; Clayton, R. W.

2011-12-01

302

Digging in 'Snow White' Trench  

NASA Technical Reports Server (NTRS)

This image was acquired by NASA's Phoenix Mars Lander's Surface Stereo Imager on the 44th Martian day of the mission, or Sol 43 (July 7, 2008), after the May 25, 2008, landing, showing the current sample scraping area in the trench informally called 'Snow White.'

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

2008-01-01

303

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

304

Advancing Subduction Zone Science After a Big Quake  

NASA Astrophysics Data System (ADS)

After a long quiet period for earthquake activity with magnitude greater than 8.5, several great subduction megathrust earthquakes occurred during the past decade: Sumatra in 2004 and 2005, Chile in 2010, and Japan in 2011. Each of these events caused loss of life and damage to critical infrastructure on an enormous scale. And, in April, a Mw 8.2 earthquake occurred off the Chilean coast.

Beck, Susan; Rietbrock, Andreas; Tilmann, Frederik; Barrientos, Sergio; Meltzer, Anne; Oncken, Onno; Bataille, Klause; Roecker, Steven; Vilotte, Jean-Pierre; Russo, Raymond M.

2014-06-01

305

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 D.; Llanes Estrada, Pilar; Smith, Shepard; Parker, Eugene; Uozumi, Toshihiko

2004-01-01

306

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

307

Gravity and Flexure Modelling of Subducting Plates  

NASA Astrophysics Data System (ADS)

The long-term strength of the lithosphere is determined by its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. Flexure studies have revealed a dependence of Te on thermal age. In the oceans, loads formed on young (70 Ma) seafloor. In the continents, loads on young (1000 Ma) lithosphere. Recent studies have questioned the relationship of Te with age, especially at subduction zones, where oceanic and continental lithosphere are flexed downwards by up to ~6 km over horizontal distances of up to ~350 km. We have therefore used free-air gravity anomaly and topography profile data, combined with forward and inverse modelling techniques, to re-assess Te in these settings. Preliminary inverse modelling results from the Tonga-Kermadec Trench - Outer Rise system, where the Pacific plate is subducting beneath the Indo-Australian plate, show large spatial variations in Te that are unrelated to age. In contrast to the southern end of the system, where Te is determined by the depth to the 600° C and 900° C isotherms, the northern end of the system shows a reduction in strength. Results also suggest a reduction in Te trenchward of the outer rise that is coincident with a region of pervasive extensional faulting visible in swath bathymetry data. In a continental setting, the Ganges foreland basin has formed by flexure of the Indo-Australian plate in front of the migrating loads of the Himalaya. Preliminary forward modelling results, using the Himalaya as a known surface topographic load, suggest that Te is high - consistent with the great age of Indian cratonic lithosphere. However, results from inverse modelling that solves for unknown loads (vertical shear force and bending moment) show significant scatter and display trade-offs between Te and these driving loads.

Hunter, J. A.; Watts, A. B.; SO 215 Shipboard Scientific Party

2012-04-01

308

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

309

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

SciTech Connect

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

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

1983-01-01

310

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

311

Structure and seismic stratigraphy of the Bonin Trench-Arc system  

E-print Network

that of the Journal of Geophysical Research. the Eastern Pacific along the coasts of North and South America. The Bonin Trench-Arc System is of the oceanic-oceanic type with an associ- ated back arc basin; however, it is anomalous in many respects when compared... is noted to consist of highly faulted and deformed material. Karig and Sherman (1975) concluded that this first element is a ridge composed of imbricate thrust packets consisting of sediments offscraped from the subducting plate. As more material...

Bandy, William Lee

1982-01-01

312

Structure of the subduction zone south of Taiwan constrained by OBS data  

NASA Astrophysics Data System (ADS)

Along the Manila trench the oceanic part of the Eurasian plate, i.e., the South China Sea (SCS) basin, is subducting eastward under the Philippine Sea plate. The Manila trench terminates in the north offshore southern Taiwan, where the subduction is impeded by the collision between the Luzon arc and the Eurasian continental margin which have built the Taiwan orogeny. The subduction zone structure at the ocean-continent junction south of Taiwan is so far unknown because of the lack of in-situ sampling. We have conducted OBS experiments in this region during 2009 - 2011. Events with M > 2 were relocated to better delineate the crustal and slab structure. We carried out tomographic inversion for Vp, Vs, and Vp/Vs and examine several topics including (1) whether the mantle wedge is warmer because it is exposed laterally to the asthenosphere to the north, (2) whether the forearc wedge is serpentinized, and (3) whether the impact of the collision on the subduction zone is significant.

Lin, Yi-Wei; Peng, Cheng-Chien; Kuo, Ban-Yuan

2012-04-01

313

Spontaneous development of arcuate single-sided subduction in global 3-D mantle convection models with a free surface  

NASA Astrophysics Data System (ADS)

We present temporally evolving 3-D global mantle convection models with single-sided subduction and a free surface in both 3-D Cartesian and fully spherical geometry. Special focus is given to the spontaneous development of three-dimensional structures at the surface and in the upper mantle. We find that an arcuate shape is the natural form for trenches and slabs. Cartesian models are used first to study the dynamic evolution of subduction zones, spreading ridges, and interconnected transform features. These experiments highlight the strong variation of spontaneously developing, arcuate slab curvature and subduction polarity along the trench strike. The spontaneous development of spreading ridges leads to lateral offsets between separated segments that are characterized by normal transform motion. Spherical models then allow insights into the evolution of plate tectonics on a sphere. Investigated are the spontaneous evolution of slab geometry, trench motion, and subduction-induced mantle flow. Two new dynamical features are discovered: "back-slab spiral flow" and "slab tunneling." 2014. American Geophysical Union. All Rights Reserved.

Crameri, Fabio; Tackley, Paul J.

2014-07-01

314

Seismological evidence for the high-velocity zone in the Kuril Trench area from ocean bottom seismometer observations  

Microsoft Academic Search

An ocean bottom seismograph was deployed on the seaward side of the Kuril Trench off Hokkaido, Japan, in 5460 m of water in August during the 1975 Joint Soviet-American Tsunami Expedition. During the 7-day record, S-P times were distributed in three groups: 19-24 s, corresponding to aftershocks of the June 10 and June 13, 1975, earthquakes southeast of Nemuro, Japan,

Junzo Kasahara; Robert R. Harvey

1977-01-01

315

Subduction Controls of Hf and Nd Isotopes in Lavas of the Aleutian Island Arc  

SciTech Connect

The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with {var_epsilon}{sub Hf} of +12.0 to +15.5 and {var_epsilon}{sub Nd} of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment + peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the trench east of the Amlia Fracture Zone, which is being subducting beneath the arc at Seguam Island. Mixing trends between mantle wedge and sediment end members become flatter in Hf-Nd isotope space at locations further west along the arc, indicating that the sediment end member in the west has either higher Nd/Hf or is more radiogenic in Hf compared to Nd. This pattern is interpreted to reflect an increase in pelagic clay relative to the terrigenous subducted sedimentary component westward along the arc. Results of this study imply that Hf does not behave as a conservative element in the Aleutian subduction system, as has been proposed for some other arcs.

Yogodzinski, Gene; Vervoort, Jeffery; Brown, Shaun Tyler; Gerseny, Megan

2010-08-29

316

Terrane Stations: intra-oceanic subduction assembled western North America  

NASA Astrophysics Data System (ADS)

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

Sigloch, K.; Mihalynuk, M. G.

2012-12-01

317

Dynamic topography in subduction zones: insights from laboratory models  

NASA Astrophysics Data System (ADS)

The topography in subduction zones can exhibit very complex patterns due to the variety of forces operating this setting. If we can deduce the theoretical isostatic value from density structure of the lithosphere, the effect of flexural bending and the dynamic component of topography are difficult to quantify. In this work, we attempt to measure and analyze the topography of the overriding plate during subduction compared to a pure shortening setting. We use analog models where the lithospheres are modeled by thin-sheet layers of silicone putty lying on low-viscosity syrup (asthenosphere). The model is shorten by a piston pushing an oceanic plate while a continental plate including a weak zone to localize the deformation is fixed. In one type of experiments, the oceanic plate bends and subducts underneath the continental one; in a second type the two plates are in contact without any trench, and thus simply shorten. The topography evolution is monitored with a laser-scanner. In the shortening model, the elevation increases progressively, especially in the weak zone, and is consistent with expected isostatic values. In the subduction model, the topography is characterized, from the piston to the back-wall, by a low elevation of the dense oceanic plate, a flexural bulge, the trench forming a deep depression, the highly elevated weak zone, and the continental upper plate of intermediate elevation. The topography of the upper plate is consistent with isostatic values for very early stages, but exhibits lower elevations than expected for later stages. For a same amount of shortening of the continental plate, the thickening is the same and the plate should have the same elevation in both types of models. However, comparing the topography at 20, 29 and 39% of shortening, we found that the weak zone is 0.4 to 0.6 mm lower when there is an active subduction. Theses values correspond to 2.6 to 4 km in nature. Although theses values are high, there are of the same order as dynamic topography and could represent the dynamic effect of the slab sinking into the asthenosphere and lowering the elevation of the upper plate.

Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

2014-05-01

318

Kinematic modeling of crustal deformation in the Central Japan  

Microsoft Academic Search

Central Japan is situated on a complex plate boundary region where 3 or 4 tectonic plates or microplates converging one another. Crustal deformation along the southern coast is dominated by locking effects on the subducting plate boundary, while East-West compression caused by collision between northeast and southwest Japan is prominent further inland. In order to understand plate interaction and seismotectonics

K. Ozawa; T. Sagiya

2008-01-01

319

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

NASA Astrophysics Data System (ADS)

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

Li, Tao; Hampel, Andrea

2013-04-01

320

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

321

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

322

Incoming plate faulting in the Northern and Western Pacific and implications for subduction zone water budgets  

NASA Astrophysics Data System (ADS)

The greatest uncertainty in the amount of water input into the Earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate. Recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench. We explore the maximum depth of extension within the incoming plate at Northwestern Pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments. We relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988-2011 by inverting teleseismic broadband P and SH waveforms. We observe that the top 10-15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes; 60% of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle, 80% occur above 10 km within the mantle, and 95% occur above 15 km. There is evidence for variation throughout the different regions of study, for example extensional earthquakes occur down to 20 km below the crust in the western Aleutians and Izu-Bonin. We propose that the incoming plate mantle is most strongly hydrated in the upper 5 km, and that partial serpentinization exists regionally within the incoming plate mantle to ?15 km. Making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water, we calculate that the total water carried into the Northern and Western Pacific subduction zones is generally higher than previous estimates, and is approximately 4- 6 ×108 Tg /Myr, or ? 45- 70 ×103 Tg /Myr per kilometer of subduction zone.

Emry, Erica L.; Wiens, Douglas A.

2015-03-01

323

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

NASA Astrophysics Data System (ADS)

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

Fryer, P. B.

2013-12-01

324

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

325

Constraining the boundary between the Sunda and Andaman subduction systems: Evidence from the 2002 Mw 7.3 Northern Sumatra earthquake and aftershock relocations of the 2004 and 2005 great earthquakes  

Microsoft Academic Search

The 2004 Mw 9.0 Sumatra-Andaman earthquake initiated along the Andaman subduction zone, north of the last great Sumatra earthquake along the Sunda Trench in 1861. During the 2005 Mw 8.7 Banyak Islands earthquake, a portion of the 1861 rupture subsequently failed. The boundary between the 2004 and 2005 ruptures broadly coincides with local trench rotation and the southern edge of

Heather R. DeShon; E. Robert Engdahl; Clifford H. Thurber; Michael Brudzinski

2005-01-01

326

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

327

Crust and subduction zone structure of Southwestern Mexico  

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

328

Subduction zones: observations and geodynamic models  

Microsoft Academic Search

This review of subduction and geodynamic models is organized around three central questions: (1) Why is subduction asymmetric? (2) Are subducted slabs strong or weak? (3) How do subducted slabs interact with phase transformations, changes in mantle rheology, and possibly chemical boundaries in the mantle? Based on laboratory measurements of the temperature dependence of olivine, one would conclude that the

Scott D. King

2001-01-01

329

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

330

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

331

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

332

Yarditecture : new walls for trench town  

E-print Network

"The yard" -- the typical housing typology of low-income downtown, Jamaica where multiple households are contained within a surrounding wall. This thesis envelops itself in Trench town, Jamaica, the epicenter of what is ...

Malcolm, Christopher J., Jr

2014-01-01

333

Subduction zone guided waves: 3D modelling and attenuation effects  

NASA Astrophysics Data System (ADS)

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

Garth, T.; Rietbrock, A.

2013-12-01

334

The southern termination of the Philippine Trench  

NASA Astrophysics Data System (ADS)

Recent studies of the southern end of the Philippine Trench using GLORIA sidescan sonar and single channel seismic data and geological studies on the island of Halmahera suggest that the trench is in the process of propagating south and that some of the ESE-WNW convergence is transferred via a broad NE-SW zone of dextral strike-slip across northern Halmahera into the Molucca Sea Collision Zone. NE-SW ridges and lineaments on the seafloor can be traced into major faults and structural trends on land. The bathymetric expression of the Philippine Trench is lost where it meets an oceanic plateau on the Philippine Sea Plate. This elevated and probably thickened plateau appears to have inhibited any further propagation of the trench southwards. To the north of the plateau there is a well-developed accretionary prism, but to the south deformation of sediments on the seafloor is less intense. A prominent ridge, with sediments passively banked up against it marks the southern limit of deformation associated with the Philippine Trench. To the south lies the eastern Halmahera-Waigeo Ophiolite Terrane, an area of shallow water and islands underlain by ophiolitic basement between Halmahera and the Sorong Fault Zone. There are no bathymetric or structural features to indicate any form of active link between the Philippine Trench and the Sorong Fault through or along the northern side of this ophiolite terrane. There is no earthquake evidence for any form of fault linking the Philippine Trench to either the Palau Trench or the Sorong Fault, but there are numerous earthquake epicentres reported between the southern end of the trench and the Molucca Sea. The island of Halmahera is considered to lie in a diffuse boundary zone at the margin of the Philippine Sea Plate. Continued collision of the opposing arcs in the Molucca Sea will ultimately prevent further E-W convergence between Halmahera and Sulawesi. It is predicted that either the Philippine Trench will propagate south along its present line to meet the Sorong Fault or that another trench will develop further to the east. In either case the arc and ophiolite terrane of Halmahera might then be accreted onto the Eurasian margin.

Nichols, Gary; Hall, Robert; Milsom, John; Masson, Doug; Parson, Lindsay; Sikumbang, Nafrizal; Dwiyanto, Bambang; Kallagher, Helen

1990-11-01

335

Quaternary strain rates distribution and crust-mantle structure of the southern Northeast Japan  

NASA Astrophysics Data System (ADS)

Driving mechanisms of intracontinental deformation at subduction margins have been a interesting research target to understand dynamic interaction between subducting oceanic slabs, overriding continental crust and mantle structures. Driven by this motivation, we estimated spatial distributions of intraplate permanent strain rates accommodated by active faults and fault-related folds in southern Northeast Japan during the late Cenozoic time, based on combinations of recently obtained deep to shallow seismic re?ection data, and rates of fault slip determined by offsets of geomorphic features or stratigraphic horizons identified of drilled shallow boreholes across fault and/or fold scarps. Tectonic setting of the northeastern Japan in late Cenozoic times, underlain by westward subducting old and cold Pacific plate, is characterized by north to northeast trending active thrust sheets that deform Neogene deposits. Although previous studies indicated that active reverse faults are predominant in this region, revised active fault mapping after the 2011 Tohoku-oki earthquake (M9.0) and its normal-fault aftershock sequence indicate that active normal faults are widely distributed on the southeastern flank of the coastal mountains along the Pacific coast and continental shelf off the southern Northeast Japan. Estimated strain rates accommodated by active faults and folds are an order of 10-8/yr for each structures, that are in general 10 to 100 times higher than previous estimates only from surficial Quaternary active fault data and historical seismicity. Contrastingly, geodetic strain rates observed the 2011 Tohoku-oki earthquake shows 10 times higher than those estimates in this study. Most of these active thrusts are reactivated normal faults originally formed during Miocene in extensional stress regimes. Trench-normal, spatial distributions of the longer-term permanent strain rates is characterized by a distinctive trend that strain rates in back-arc are apparently 10 times higher than in fore-arc region, quite similar to those estimated based on late Cenozoic folded/faulted strata. Most of these active thrusts are reactivated normal faults originally formed during Miocene in extensional stress regimes. Longer-wavelength, late Quaternary uplift and subsidence overprinting these short wavelength strains, estimated by fluvial incision rates based on terhrostratigraphy, and borehole stratigraphy in alluvial plains, indicate relatively uniform, moderate uplift rates in fore-arc and west of the volcanic front, and very fast subsidence rates in back-arc. Late Cenozoic major tectonic records in southern Northeastern Japan after Miocene Japan Sea opening are, in summary, mainly characterized by Quaternary strong compression and coeval fast subsidence in back-arc region. Crust-mantle structures of the southern Northeast Japan based on seismic tomography, seismic reflection and refraction profiles indicates crustal thickening beneath the Ou backbone Range probably due to magmatic underplating. In addition, back-arc subsidence is underlain by low Vp in the upper mantle, suggesting that downwelling of the mantle lithosphere may be driving present-day surface fast subsidence.

Ishiyama, Tatsuya; Sato, Hiroshi; Kato, Naoko; Imaizumi, Toshifumi; Matsubara, Makoto; Takeda, Tetsuya

2013-04-01

336

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

337

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

338

Investigation of upper mantle discontinuities near northwestern Pacific subduction zones using precursors to sSH  

Microsoft Academic Search

Tangential component recordings of deep and intermediate depth earthquakes are analyzed for the presence of sS precursors. It is established that these precursors can be used to determine mantle stratification above subduction zones. For the northwestern Pacific area under study, oceanic crust beneath Izu Japan, continental crust under North Korea with 36 km crustal thickness and 18 percent shear wave

Zhi Zhang; Thorne Lay

1993-01-01

339

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

340

Monkey Wrench in the Central American Subduction Factory: Improved Age Estimates for the Suduction History of the Cocos Ridge  

Microsoft Academic Search

Subduction of the Cocos and Carnegie ridges at the Middle America and Colombian trenches, respectively, have been shown by a variety of marine and land-based studies to have profound and disruptive effects on subaerial and submarine geomorphology, the spacing and composition of arc volcanism, cross-arc traverse faulting, forearc deformation and erosion, back-arc thrust faulting, and anomalously large forearc and backarc

L. Gahagan; P. Mann

2002-01-01