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1

FDM Simulation of an Anomalous Later Phase from the Japan Trench Subduction Zone Earthquakes  

Microsoft Academic Search

We investigated the development of a distinct later phase observed at stations near the Japan Trench associated with shallow, outer-rise earthquakes off the coast of Sanriku, northern Japan based on the analysis of three-component broadband seismograms and FDM simulations of seismic wave propagation using a heterogeneous structural model of the Japan Trench subduction zone. Snapshots of seismic wave propagation obtained

Shinako Noguchi; Takuto Maeda; Takashi Furumura

2011-01-01

2

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

3

Permeability and consolidation behavior of sediments subducting at the Japan Trench: Implications for physical properties in the high slip region of the 2011 Tohuko-Oki earthquake  

NASA Astrophysics Data System (ADS)

In subduction zones, high porosity sediments on the oceanic plate undergo rapid consolidation due to tectonically-driven loading. The evolution of sediment permeability during consolidation mediates drainage and fluid pressure distribution. In particular, sediments subducted at the trench are in direct contact with the overlying plate boundary megathrust; thus their in situ conditions and physical properties hold important implications for fault behavior. Excess fluid pressures, likely facilitated by low sediment permeability, have been implicated as a potential mechanism for slow-slip and low-frequency earthquakes at many convergent margins, and in the 2011 Tohoku earthquake, elevated fluid pressures may have promoted the extraordinarily high slip that propagated to the trench. Here, we report on experiments designed to measure the deformation behavior and permeability of sediment samples obtained from Deep Sea Drilling Project (DSDP) leg 56, Site 436, a reference site located in the Pacific Basin, ~100-km seaward of the Japan trench and 200-km north of the high slip area in the 2011 Tohoku earthquake. We conducted deformation and permeability tests under a range of stress paths, including isotropic loading, uniaxial strain, and triaxial compression, at mean effective stresses ranging from ~350 kPa - 65 MPa. We determine permeability by Constant Rate of Strain (CRS) testing in the uniaxial strain case, and by steady-state flow-through in triaxial tests. We determine sample porosity during deformation by continuously monitoring specimen pore fluid volume and axial strain. Our results demonstrate that porosity decreases systematically with effective mean stress, but depends on stress path. Permeability follows a log-linear decrease as a function of porosity for both uniaxial and isotropic loading. Values of permeability decrease from 10-16 m2 at a mean effective stress of ~4 MPa (corresponding to a porosity of 72%), to 10-19 m2 at a mean effective stress of 65 MPa (porosity of 35%). These data, together with ongoing experiments under additional stress paths, will provide constraints to extrapolate porosity and permeability, and for numerical models that estimate pore fluid pressure that can be expected farther down-dip in the case of non-uniaxial strain.

Lauer, R. M.; Kitajima, H.; Saffer, D. M.

2012-12-01

4

Oblique convergence and deformation along the Kuril and Japan trenches  

Microsoft Academic Search

The hypothesis that present-day deformation within the southern Kuril forearc is driven by oblique subduction of the Pacific plate is tested using 397 horizontal slip directions derived from shallow-thrust earthquakes from the Kuril and Japan trenches for the period 1963-1991. A simple two-plate model fits the 397 slip vectors significantly worse than a model that permits strike-slip motion of the

Charles Demets

1992-01-01

5

A subduction zone reference frame based on slab geometry and subduction partitioning of plate motion and trench migration  

NASA Astrophysics Data System (ADS)

The geometry of subducted slabs that interact with the transition zone depends critically on the partitioning of the subduction velocity (vS$\\perp$) at the surface into its subducting plate motion component (vSP$\\perp$) and trench migration component (vT$\\perp$). Geodynamic models of progressive subduction demonstrate such dependence with five distinct slab geometries and corresponding partitioning ratios (vSP$\\perp$/vS$\\perp$): slab draping (vSP$\\perp$/vS$\\perp$ ? 0.5), slab draping with recumbent folds (0.5 < vSP$\\perp$/vS$\\perp$ < ˜0.8), slab piling (˜0.8 ? vSP$\\perp$/vS$\\perp$ ? ˜1.2), slab roll-over with recumbent folds (˜1.2 < vSP$\\perp$/vS$\\perp$ < ˜1.5) and slab roll-over (vSP$\\perp$/vS$\\perp$ ? ˜1.5). The model findings have been applied to subduction zones in nature with well-resolved slab geometries, for which subduction partitioning ratios have been calculated during the last 20 million years in two global reference frames: the Indo-Atlantic and Pacific hotspot reference frames. The model-nature comparison determines in which reference frame subduction partitioning ratios are most in agreement with observed slab geometries. In the Indo-Atlantic frame, five (out of five) selected subduction zone segments with well-resolved slab geometries, plate velocities and trench velocities (Japan, Izu-Bonin, Mariana, Tonga, Kermadec) agree with the geodynamic model predictions, as calculated subduction partitioning ratios match the observed slab geometries. In the Pacific frame the partitioning ratio of only one subduction zone segment (Izu-Bonin) matches observations. It is thus concluded that the Indo-Atlantic hotspot reference frame is preferred over the Pacific one as a subduction zone reference frame in which to describe plate motions, subduction kinematics and mantle flow.

Schellart, W. P.

2011-08-01

6

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

USGS Publications Warehouse

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

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

1987-01-01

7

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

NASA Astrophysics Data System (ADS)

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

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

1987-05-01

8

Dehydration of incoming sediments at the Japan Trench  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

9

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

NASA Astrophysics Data System (ADS)

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

Arredondo, Katrina M.; Billen, Magali I.

2012-04-01

10

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

11

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

E-print Network

Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark of trench: Rheology--crust and lithosphere; KEYWORDS: anisotropy, Cascadia, subduction Citation: Park, J., H. Yuan, and V. Levin (2004), Subduction zone anisotropy beneath Corvallis, Oregon: A serpentinite skid mark

12

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

13

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

14

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

NASA Astrophysics Data System (ADS)

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

Arredondo, K.; Billen, M. I.

2011-12-01

15

Modelling lithospheric ageing during subduction: Implications for the Izu-Bonin-Mariana trench migration  

NASA Astrophysics Data System (ADS)

Lithosphere bends and subducts into the mantle at trenches. Subducting plate cools, thickens, and contracts in time, inducing changes in the strength of the litosphere and its buoyancy as it grows older. Recent worldwide databases reveal that most of the trenches roll back, but a significant amount of them migrates in advancing toward the upper plate. On Earth this dichotomy is exhibited in the Pacific area: most of the advancing slabs are located in the Western Parific, whereas the Eastern Pacific trenches retreat. It seems that the age of the slab, and consequently the slab strength, to a certain extent controls the subduction style. In this study we select a region where geological studies illustate a variability in trench migration in time. The Izu-Bonin-Mariana (IBM) system, the oldest subducting lithosphere on Earth, represents a significant esample. The reconstruction of the trench migration of the IBM subducting system shows a long episode of asymmetric trench rollback, followed by a recent phase of trench advance. We suggest that this evolution of trench migration from retreating to advancing mode would be the consequence of the subduction of progressively older and stiffer lithospheric material. We tested this speculation by performing 2-D numerical, fully dynamic models, implemented in order to mimic the influence of the progressive lithospheric ageing on trench kinematics. The arrival of older and stiffer lithosphere at the trench has, as consequence, the gradual slow down of the trench migration, and the continuous increase of the resisting force for bending. Hence, the plate is forced to move in advancing, and the change in trench migration occurs. Despite the simplified assumptions of the model, we are confident in the robustness of our results. This model can be applied to the IBM subduction system reconciling down-dip slab shape with the recent transition of trench kinematics from retreating to advancing style.

di Giuseppe, E.; Faccenna, C.; Funiciello, F.; van Hunen, J.; Lallemand, S.

2010-12-01

16

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

Microsoft Academic Search

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

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

2008-01-01

17

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

PubMed

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

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

2013-01-01

18

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

NASA Astrophysics Data System (ADS)

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

Casey, J.; Dewey, J. F.

2013-12-01

19

Plate boundary forces at subduction zones and trench-arc compression  

NASA Astrophysics Data System (ADS)

Viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone. The subduction fault is included using the dual node technique. It is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench-arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates. This may be masked at the trench by bending stress. It is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces. Slab pull and trench suction were found to be of comparable magnitude within the range 1.0 to 4.0 × 10 12 N/m in models studied with vertical subduction, and there are indications that this may also apply when the slab dips at 45° as a result of viscous flow induced by rollback. When the slab dips beneath the arc-backarc region, it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction. This suggests that backarc tension associated with Marianas type trench-arch systems may be related to the nearly vertical slab whereas backarc compression in the Chilean type may result from the low dip and small downpull of the slab. It is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors.

Bott, M. H. P.; Waghorn, G. D.; Whittaker, A.

1989-12-01

20

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

21

Structure and Kinematic History of the Japan Trench toe off Tohoku  

NASA Astrophysics Data System (ADS)

Recent slip models show that the 2011 Tohoku-Oki earthquake (Mw 9.0) ruptured to the trench. Differential bathymetric analysis from before and after the event confirms ~ 50 m of seaward displacement near the toe, which is predominantly responsible for the destructive tsunami. Interpretation of high-resolution depth migrated seismic reflection data across the Japan Trench near the 2011 earthquake epicentral region provide the structural context for subduction-related deformation. To constrain possible structural interpretations, we conducted fault restorations using Landmark's LithoTect software. The subducting Pacific plate is covered by ~400 m of pelagic and hemipelagic sediment. The outer trench slope margin is characterized largely by basement-displacing flexural related normal faults that generate horsts and graben. The morphologic trench in this area is a graben, conjugate to a subducted landward horst and an incoming seaward horst block. Seismic correlation across the seaward margin horst reveals displacement of ~140 m and shows that a small amount, less than ~50 m, of trench fill, likely deposited by slumping. The incoming sediment section in the trench is cut by landward-dipping thrusts that sole into a basal décollement, which ascends over the landward horst block, appearing to connect with the surface identified as the décollement at the J-FAST drill site. This décollement step down of ~200 m into the graben sediments is clearly imaged as a strong reflector with up to a ~20 degree seaward dip. Restoration along this fault plane step down creates an original steep inclination for the local seafloor, a potential driving force for slumping. Furthermore, this rupture pathway of down stepping into a graben may be a mechanism for the farther landward slumps and wedge morphology. Uplift at the toe is recorded by deformed offset horizons that coincide with local topographic highs of the seafloor. These compressional structures are interpreted to consist of three main thrusts branching from the décollement at less than 400 m below the seafloor. Although our method cannot distinguish co-seismic slip, it indicates that the rupture path for the Tohoku earthquake likely followed the basal décollement and propagated to the toe thrust faults in the trench , then breaching the surface. Kinematic restoration of this low angle prism toe shows thrust faults dipping ~10-40 degrees landward with over 1,000 m of displacement along the basal décollement in the graben. These results provide geologic constraints on slumping and uplift geometries and reveal deformational histories near the trench.

Boston, B.; Moore, G. F.; Nakamura, Y.; Kodaira, S.

2012-12-01

22

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

23

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

NASA Technical Reports Server (NTRS)

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

King, Scott D.; Hager, Bradford H.

1990-01-01

24

Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise  

Microsoft Academic Search

Hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates. Lower crust and mantle rocks are therefore relatively dry and undeformed. However, recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench-outer rise, where extensional

Eduardo Contreras-Reyes; Ingo Grevemeyer; Ernst R. Flueh; Martin Scherwath; Martin Heesemann

2007-01-01

25

Alteration of the subducting oceanic lithosphere at the southern central Chile trench–outer rise  

Microsoft Academic Search

Hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates. Lower crust and mantle rocks are therefore relatively dry and undeformed. However, recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench–outer rise, where extensional

Eduardo Contreras-Reyes; Ingo Grevemeyer; Ernst R. Flueh; Martin Scherwath; Martin Heesemann

2007-01-01

26

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

PubMed

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

Kneller, Erik A; van Keken, Peter E

2007-12-20

27

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

28

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

29

Computer simulation of trench trapped Rayleigh wave  

Microsoft Academic Search

We study development of a trench trapped Rayleigh wave which is often observed during shallow subduction zone earthquake occurring in the Japan Trench by analyzing broadband waveforms and corresponding FDM simulation of seismic wave propagation using heterogeneous structure model of the subduction zone. The seismic waves propagating along ocean are influenced by strong lateral heterogeneous structure which is constructed by

S. Noguchi; T. Maeda; T. Furumura

2010-01-01

30

Chemical composition of sediments subducting at the Izu-Bonin trench  

NASA Astrophysics Data System (ADS)

In this characterization brief, report here comprehensive major and trace element analyses of over 45 sediment samples from Ocean Drilling Program Site 1149, located seaward of the Izu-Bonin trench. The combination of these core analyses with a complete set of geochemical logging data enables us to calculate the bulk composition of the sedimentary column subducting at the Izu trench with high accuracy (uncertainties ?13%). Izu sediment has lower concentrations than global subducting sediment for most elements, due to ˜50% dilution by biogenic material (˜45% opal and ˜10% carbonate), but is relatively enriched in Ba, Pb, and rare earth elements (REE), except Ce, due to nonterrigenous inputs. Sediments subducting into the Izu and Mariana trenches differ compositionally due to ocean island-sourced volcaniclastics in Marianas sediments, enriching them in Nb, Ta, Ti, and LREE, and continentally derived eolian material in Izu sediments, enriching them in Cs, Rb, Th, and U. These differences predict along-strike variations in sediment input that should be manifested in the composition of volcanic output from the Honshu-Izu-Bonin-Mariana arc systems. Such variations are observed as an increase in Th/La in both sediments and arc volcanics from the Marianas in the south to Honshu in the north.

Plank, Terry; Kelley, Katherine A.; Murray, Richard W.; Stern, Lacie Quintin

2007-04-01

31

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

32

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

33

A regime diagram for subduction dynamics from thermo-mechanical models with a mobile trench and an overriding plate  

NASA Astrophysics Data System (ADS)

The penetration or stagnation of subducted slabs in mantle transition zone and lower mantle influences Earth's thermal, chemical and tectonic evolution. Yet, the mechanisms responsible for the wide range of observed slab morphologies within the transition zone remain debated. Here, we investigate how downgoing and overriding plate ages controls the interaction between subducted slabs and mantle transition zone. We use 2-D thermo-mechanical models of a two-plate subduction system, modeled with the finite-element, adaptative-mesh code Fluidity. We implement a temperature- and stress-dependent rheology, and viscosity increases 30-fold from upper to lower mantle. Trench position evolves freely in response to plate dynamics. Such an approach self-consistently captures feedbacks between temperature, density, flow, strength and deformation. Our results indicate that key controls on subduction dynamics and slab morphology are: (i) the slab's ability to induce trench motion; and (ii) the evolution of slab strength during sinking. We build a regime diagram that distinguishes four subduction styles: (1) a "vertical folding" mode with stationary trench (young subducting plates, comparatively old overriding plates); (2) slabs that are "horizontally deflected" along the 660-km deep viscosity jump (initially young subducting and overriding plates); (3) an inclined slab morphology, resulting from strong trench retreat (old subducting plates, young overriding plates); and (4) a two-stage mode, displaying bent (rolled-over) slabs at the end of upper-mantle descent, that subsequently unbend and achieve inclined morphologies, with late trench retreat (old subducting and overriding plates). We show that all seismically observed slab morphologies can arise from changes in the initial plates ages at the onset of subduction.

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

2014-05-01

34

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

35

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

NASA Astrophysics Data System (ADS)

A mega-trench survey (100m length, 20m width and 5m depth) for huge tsunami traces were conducted using by civil engineering techniques in Nemuro lowland near Lake Nanbutou along the Kuril subduction zone, eastern Hokkaido, northern Japan. According to our stratigraphic methods such as tephrochronology and AMS14C dating, we identified 15 tsunami sands in marsh deposits since 5500 yrBP and their recurrence interval was estimated almost 300-350 years. It has been concluded that it is necessary to estimate other tsunami sources of the southern part of Kuril Islands as well, other than tsunami sources caused by multi-segment of interplate earthquake (probably M 8.6) such as off Tokachi and Nemuro areas. Furthermore, we found strange large tsunami scores in past foreshore and backshore sediments. These were clearly traced to each tsunami deposits in marsh deposits. According to our sedimentological methods on the mega-trench wall, these tsunami sands only derived from the beach area, and although they do not show clear graded bedding because of a single source. Also they commonly have convolution structures and erosion bases and include peat clasts and internal bed forms such as antidune, plane bed, dune, and current ripple, reflecting bedload transportation on the beach and marsh.

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

2008-12-01

36

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

37

The role of trench retreat on the geometry and stress regime in the subduction complexes of the Mediterranean  

NASA Astrophysics Data System (ADS)

Numerical modelling of the Aegean and Calabrian subductions using geological reconstructions starting 75-80 Myr before the present reproduces the present-day geometries of the slabs and mantle wedges imaged by seismic tomography. The change from a shallow subduction dip of about 15° from the surface to a depth of 200 km into a dip of 60° below this depth in the Aegean subduction is caused by the south-west migration of the trench at the velocity of 3 cm/yr, obtained from GPS and geological data. The largest dip of 70° Calabrian subduction is caused by the termination of the active phase of opening of the Tyrrhenian back-arc basin and by the essentially purely gravitational sinking of the slab, in agreement with the null trench-migration velocity obtained from GPS data in Calabria. These two different tectonic settings are the ultimate causes of the different styles of seismicity within the two subduction complexes.

Rizzetto, Catia; Marotta, Anna Maria; Sabadini, Roberto

2004-06-01

38

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

39

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

40

Constraints of subducted slab geometries on trench migration and subduction velocities: flat slabs and slab curtains in the mantle under Asia  

NASA Astrophysics Data System (ADS)

The past locations, shapes and polarity of subduction trenches provide first-order constraints for plate tectonic reconstructions. Analogue and numerical models of subduction zones suggest that relative subducting (Vs) and overriding (Vor) plate velocities may strongly influence final subducted slab geometries. Here we have mapped the 3D geometries of subducted slabs in the upper and lower mantle of Asia from global seismic tomography. We have incorporated these slabs into plate tectonic models, which allows us to infer the subducting and overriding plate velocities. We describe two distinct slab geometry styles, ';flat slabs' and ';slab curtains', and show their implications for paleo-trench positions and subduction geometries in plate tectonic reconstructions. When compared to analogue and numerical models, the mapped slab styles show similarities to modeled slabs that occupy very different locations within Vs:Vor parameter space. ';Flat slabs' include large swaths of sub-horizontal slabs in the lower mantle that underlie the well-known northward paths of India and Australia from Eastern Gondwana, viewed in a moving hotspot reference. At India the flat slabs account for a significant proportion of the predicted lost Ceno-Tethys Ocean since ~100 Ma, whereas at Australia they record the existence of a major 8000km by 2500-3000km ocean that existed at ~43 Ma between East Asia, the Pacific and Australia. Plate reconstructions incorporating the slab constraints imply these flat slab geometries were generated when continent overran oceanic lithosphere to produce rapid trench retreat, or in other words, when subducting and overriding velocities were equal (i.e. Vs ~ Vor). ';Slab curtains' include subvertical Pacific slabs near the Izu-Bonin and Marianas trenches that extend from the surface down to 1500 km in the lower mantle and are 400 to 500 km thick. Reconstructed slab lengths were assessed from tomographic volumes calculated at serial cross-sections. The ';slab curtain' geometry and restored slab lengths indicate a nearly stationary Pacific trench since ~43 Ma. In contrast to the flat slabs, here the reconstructed subduction zone had large subducting plate velocities relative to very small overriding plate velocities (i.e. Vs >> Vor). In addition to flat slabs and slab curtains, we also find other less widespread local subduction settings that lie at other locations in Vs:Vor parameter space or involved other processes. Slabs were mapped using Gocad software. Mapped slabs were restored to a spherical model Earth surface by two approaches: unfolding (i.e. piecewise flattening) to minimize shape and area distortions, and by evaluated mapped slab volumes. Gplates software was used to integrate the mapped slabs with plate tectonic reconstructions.

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

2013-12-01

41

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

42

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

43

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

44

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

45

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

NASA Astrophysics Data System (ADS)

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

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

1987-05-01

46

Seabeam survey at the southern end of the Manila trench. Transition between subduction and collision processes, offshore Mindoro Island, Philippines  

NASA Astrophysics Data System (ADS)

The morphological and structural study conducted at the southern tip of the Manila trench, reveals that convergence between the South China Sea basin and Luzon is accommodated differently depending on the nature of the subducted slab. When the oceanic crust is subducted, a simple accretionary prism-fore arc basin pattern is developed. Conversely, where the continental margin of this basin is subducted, intraplate deformation is randomly distributed across the major part of the fore arc area which is fragmented into various crustal microblocks. Results of seabeam mapping and detailed geophysical surveying conducted at this subduction-collision transition zone, during the POP2 cruise, with R.V. "Jean Charcot" are presented and discussed here and allow us a new insight into the mechanism of such a subduction-collision transition zone.

Rangin, Claude; Stephan, Jean Francois; Blanchet, Rene; Baladad, David; Bouysee, Phillipe; Min Pen Chen; Chotin, Pierre; Collot, Jean Yves; Daniel, Jacques; Drouhot, Jean Marcel; Marchadier, Yves; Marsset, Bruno; Pelletier, Bernard; Richard, Mary Annick; Tardy, Marc

1988-01-01

47

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

48

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

49

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

Microsoft Academic Search

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

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

1995-01-01

50

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

PubMed

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

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

2013-01-01

51

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

52

Seamount subduction changes hydrogeology to form chemosynthetic community-Finding of a huge community at the Cadet seamount, Kuril Trench  

Microsoft Academic Search

Huge chemosynthetic animal communities with carbonate chimneys and bacterial mats were found on the southern slope of the Cadet Seamount, a subducted seamount which was found in 1984 by the SeaBeam mapping at the southern end of the Kuril Trench forearc. We had three dives at depth range from 5340m to 4050m by submersible Shinkai 6500 in June 2002. Multi-channel

K. Fujioka; T. Sato; T. Miwa; T. Tsuru; Y. Kido; A. Nakanishi; C. Kato

2002-01-01

53

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

Microsoft Academic Search

Pore fluid pressures in subduction zones are a primary control on fault strength and slip dynamics. Numerous studies document elevated pore pressures in the outer wedge along several margins. Seismic observations and the occurrence of non-volcanic tremor provide additional evidence for the presence of near-lithostatic pore pressures at the plate interface far down-dip from the trench (~35 km depth). Here

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

2010-01-01

54

Trench-parallel shortening in the forearc caused by subduction along a seaward-concave plate boundary: Insights from analogue modelling experiments  

NASA Astrophysics Data System (ADS)

Three-dimensional thermo-mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward-concave plate boundary can generate trench-parallel shortening in the forearc near the axis of curvature. The model deformation is analyzed with a Particle Imaging Velocimetry (PIV) system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature. Moreover, PIV allows for separation of the trench-parallel and trench-perpendicular components of strain, regardless of trench orientation. The resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature. Trench-perpendicular profiles show that along the oblique limbs of the plate boundary, the forearc is submitted to trench-normal shortening and trench-parallel shearing but not trench-parallel shortening or extension. This contrasts with the situation near the axis of symmetry where the forearc is submitted to trench-parallel and trench-perpendicular normal shortening, but is not sheared. The experimental results confirm that trench-normal thrusts observed in the fore-arc of the Central-Andes can be a mechanical consequence of subduction along a seaward-concave plate boundary if the degree of interplate coupling is large.

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

2014-01-01

55

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

56

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

57

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

58

The middle america trench as an example of a subduction zone  

NASA Astrophysics Data System (ADS)

Short Sea-Beam survey cruises were conducted by the R.V. "Jean Charcot" in march 1980 along the Middle America Trench between Panama and Acapulco, with particular emphasis on the IPOD Legs 66 and 67 areas. These detailed mappings have revealed that the Cocos horst- and graben pattern, related to the bending of the oceanic plate, strikes generally 20°-30° oblique to the trench direction. This en-échelon pattern, closely parallel to the magnetic anomalies, is here proposed as being due to the reactivation of inherited fractures generated at the East Pacific Rise. Sea-Beam data, together with UTMSI multichannel seismic reflection profiles and IPOD drilling results, clearly show that at least two distinct geodynamic processes are presently working along the same trench: accretion and no accretion. They also show a relative structural homogeneity of the oceanic side of the trench in contrast to the dual aspect of the continental side.

Aubouin, Jean; Stephan, Jean-François; Roump, Jacqueline; Renard, Vincent

1982-06-01

59

Kinematic links between subduction along the Hellenic trench and extension in the Gulf of Corinth, Greece: A multidisciplinary analysis  

NASA Astrophysics Data System (ADS)

Rapid northeast-vergent subduction along the Hellenic trench, at ~ 35 mm/yr, exists in concert with widespread extensional and strike-slip faulting within the upper plate lithosphere of western Greece. Integration of regional geomorphic, geologic, seismic, GPS, remote sensing and field data demonstrates that young and active deformation in the area, extending from the Hellenic subduction boundary near Kephalonia to the Gulf of Corinth, consists of an interconnected network containing highly localized zones of deformation. These bound a series of crustal fragments with relatively little internal deformation. These deformation zones merge to form triple junction-like features at the western end of the Gulf of Corinth and in the Amvrakikos Gulf. At the western end of the Corinth Gulf, most of its 14 ± 2 mm/yr of extension is relayed to the northwest along a prominent zone of left-slip and extension through Lake Trichonis and the Amphilochia fault zone (11 ± 2 mm/yr). The remaining displacement across the western Gulf of Corinth is relayed into 7 ± 2 mm/yr of right-slip on the southwest-striking Achaia fault zone, which traverses the northwestern margin of the Peloponnesus. A second triple-junction like feature occurs in the Amvrakikos Basin, where the left-slip Amphilochia fault zone, the right-slip Kephalonia transform fault (15 ± 2 mm/yr) and the convergent thrust front of northern Hellenides (4 ± 2 mm/yr) are joined. Thus the extensional deformation in the Gulf of Corinth can be shown to be connected to convergence and subduction along the Hellenic trench through a series of discrete deformation zones that are not dissimilar from those observed in the global plate tectonic system.

Vassilakis, E.; Royden, L.; Papanikolaou, D.

2011-02-01

60

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

61

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

NASA Astrophysics Data System (ADS)

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

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

2010-09-01

62

The Japan Trench Fast Drilling Project (IODP Exp. 343&343T JFAST): Making Scientific Drilling History in the Japan Trench  

NASA Astrophysics Data System (ADS)

The international scientific community began planning the Japan Trench Fast Drilling Project (JFAST) soon after the 11 March Tohoku Earthquake occurred. Predicted rapid decay of any thermal anomaly resulting from shear heating, which may allow the frictional strength of the main slip zone to be calculated, dictated that temperature measurements needed to be made within 18 months of the initial earthquake. Based on the drilling and observatory request from the science team, the Center for Deep Earth Exploration (CDEX) began scoping activities for this project, and rapidly became aware of some of the daunting technical challenges involved in drilling in approximately 7 km of water. The deepest water depth drilling in scientific ocean drilling history was in the Marianas Trench in 1978 in water depth of 7,049.5 m with 15.5 m penetration. The original plan of JFAST required logging-while-drilling (LWD) and sample collection from 1,000 m below the seafloor in 7,000 m water depth in the Japan Trench. Beyond this, temperature observatories needed to be deployed into the borehole. A scientific drilling proposal was submitted to the Integrated Ocean Drilling Program (IODP) by 1 August 2011 and our preparation for the operation began in parallel. To reach the plate boundary target, and to install an observatory, we had to develop several new tools (e.g., a casing running tool). The strength and performance of the drill string was also a major technical and engineering issue. Taking the limitations of the operational time window into account, our original strategy was, in about 7,000 m of water depth near the axis of the Japan Trench, to 1) drill a 8-1/2" hole with LWD and install an observatory 900 m below the seafloor, 2) drill a 10-5/8" hole with coring assembly and collect core samples from the deeper part of the hole then install another observatory 900 m below the seafloor. IODP Expedition 343 (JFAST) started on 1 April 2012 (less than 13 months after the earthquake). Several mechanical and weather issues prohibited completion of the above planned operations but we had reached the following operational targets by the end of this expedition of 24 May 2012: 1. Penetrated 850.5 m below seafloor and obtained geophysical data by LWD which allowed the plate boundary interface to be located. 2. Collected core samples from 648 m to 844.5 m below the seafloor, including samples of the plate boundary fault zone. Both operations were completed in water depth of 6889.5 m. However, due to a lack of operational time, the installation of temperature observatories was not performed. Consequently, IODP and CDEX/JAMSTEC decided to return to the site, in a follow-up expedition (IODP Expedition 343T), to install a temperature observatory into a borehole started by the original part of the expedition. IODP Expedition 343T began on 5 July 2012 and successfully installed a temperature observatory into an 854.8 m borehole in 6,897.5 m of water, operations completed on 19 July 2012.

Eguchi, N.; Toczko, S.; Maeda, L.; Sawada, I.; Saruhashi, T.; Kyo, N.; Namba, Y.; Chester, F.; Mori, J. J.; Science Party, I.

2012-12-01

63

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

64

Thermal Studies at the Middle America Trench Offshore Costa Rica and Nankai Trough, Japan  

NASA Astrophysics Data System (ADS)

Knowledge of the temperature distribution at convergent margins is important to understanding physical and chemical processes such as fluid flow, diagenesis, and faulting mechanics in the forearc region. Seafloor probe measurements offer an economical method for obtaining transects of heat flow across the forearc and along strike. Because these measurements only prick the seafloor they are sensitive to near seafloor processes such as bottom water temperature variations, deformation, and shallow fluid circulation and, although important in their own right, can obfuscate thermal inferences at depth. Ocean drilling provides access to deeper environments where downhole tools, acoustic measurements, and logging technologies can provide important scientific insight. We review recent heat flow results from the Costa Rica and Nankai convergent margins emphasizing ocean drilling transects where measurements of heat flow are available from seafloor probe and ocean drilling. Heat flow measurements offshore the erosive Costa Rican margin show strong along strike variations that reflect different styles of fluid flow and have important impacts on forearc processes. Along both the Nicoya and CRISP drilling transects, heat flow from seafloor probes and ocean drilling are consistent and indicate hydrothermal circulation prior to and after subduction. Fluid flow advects heat from deeper along the subduction thrust and deposits it near the seafloor cooling and warming these regions, respectively. The accretionary Nankai trough also shows important along strike changes in heat flow related to the age of oceanic crust at the trench. Heat flow and geochemical results are consistent with basement fluid flow at the Muroto transect but are more ambiguous at the NanTroSEIZE transect.

Harris, R. N.; Solomon, E. A.; Spinelli, G. A.; Scientific Team of IODP Drilling Expedition 334

2011-12-01

65

Moritella japonica sp. nov., a novel barophilic bacterium isolated from a Japan Trench sediment.  

PubMed

Strain DSK1 is a novel moderately barophilic bacterium isolated from the Japan Trench at a depth of 6,356 m. Phylogenetic analysis based on 16S ribosomal DNA sequences showed that strain DSK1 represents a separate lineage with the Shewanella barophiles branch and is closely related to Moritella marina. Comparisons of the temperature and pressure range for growth and some biochemical characteristics indicate that strain DSK1 differs from M. marina and Shewanella barophilic species. Furthermore, strain DSK1 displays a low level of DNA similarity to the Moritella and Shewanella type strains; however this isolate characteristically produces DHA (22:6) as a membrane fatty acids, and the fatty acid profile of this strain is similar to that of M. marina. Because of these differences, strain DSK1 appears to represent a novel deep-sea Moritella species. The name Moritella japonica is proposed. The type strain is JCM 10249. PMID:12501424

Nogi, Yuichi; Kato, Chiaki; Horikoshi, Koki

1998-08-01

66

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

67

Collision and subduction structure of the Izu–Bonin arc, central Japan, revealed by refraction\\/wide-angle reflection analysis  

Microsoft Academic Search

Since the middle Miocene, the Izu–Bonin arc (IBA) has been colliding from the south with the Honshu arc in central Japan associated with subduction of the Philippine Sea plate (PSP). This process is responsible for forming a complex crustal structure called the Izu Collision Zone (ICZ). To obtain direct evidence of the deep structure dominated by collision and subduction, an

Ryuta Arai; Takaya Iwasaki; Hiroshi Sato; Susumu Abe; Naoshi Hirata

2009-01-01

68

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

NASA Astrophysics Data System (ADS)

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

Raeesi, M.

2009-05-01

69

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

70

Configuration of the subducting Philippine Sea slab in the eastern part of southwestern Japan  

NASA Astrophysics Data System (ADS)

It is important to know the shape of a subducting slab in order to understand the mechanisms of interplate earthquakes and the process of subduction. Seismicity data and converted phases have been used to detect plate boundaries. Many seismic studies have been done to declare the configuration of the subducting Philippine Sea plate in the southwestern Japan. The configuration models of the subducting Philippine Sea slab estimated from many studies are consistent at the western part of southwestern Japan. The configuration of the Philippine Sea slab at the region was confirmed. However, the configuration of the subducting Philippine Sea plate estimated by the Receiver Function analyses was not consistent with that of seismic tomographic studies at the eastern part of southwestern Japan. At the region, the configuration of the Philippine Sea slab from Receiver Function analyses seems to be bent. On the other hand, the configuration estimated by seismic tomography studies suggested that the shape of Philippine Sea slab was smooth laterally. The configuration of the Philippine Sea slab has not yet been confirmed at the eastern part of southwestern Japan. A spatially high-density seismic network is helpful to detect the boundaries of the Philippine Sea slab. We used a spatially high-density temporal seismic array in the area. The seismic images obtained from receiver function analysis were researched comparing the seismic structure obtained from the results of the refraction surveys. The configuration of the Philippine Sea plate is obtained at the eastern part of southwestern Japan. Our model of the Philippine Sea slab is consistent with that estimated from tomography studies. The Philippine Sea plate does not bend sharply at the eastern part of the southwestern Japan. We can trace the Philippine Sea slab to a depth of 45 km. The configuration of the slab deeper than 45 km was not revealed. The very weak image of the boundary, which corresponds to the upper mantle reflector beneath the source area of the 2000 Western Tottori earthquake, was detected using the spatially dense array.

Iidaka, T.; Igarashi, T.; Iwasaki, T.

2009-12-01

71

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

72

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

73

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

74

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

75

Statistical forecasts and tests for small interplate repeating earthquakes along the Japan Trench  

NASA Astrophysics Data System (ADS)

Earthquake predictability is a fundamental problem of seismology. Using a sophisticated model, a Bayesian approach with lognormal distribution on the renewal process, we theoretically formulated a method to calculate the conditional probability of a forthcoming recurrent event and forecast the probabilities of small interplate repeating earthquakes along the Japan Trench. The numbers of forecast sequences for 12 months were 93 for July 2006 to June 2007, 127 for 2008, 145 for 2009, and 163 for 2010. Forecasts except for 2006-07 were posted on a web site for impartial testing. Consistencies of the probabilities with catalog data of two early experiments were so good that they were statistically accepted. However, the 2009 forecasts were rejected by the statistical tests, mainly due to a large slow slip event on the plate boundary triggered by two events with M 7.0 and M 6.9. All 365 forecasts of the three experiments were statistically accepted by consistency tests. Comparison tests and the relative/receiver operating characteristic confirm that our model has significantly higher performance in probabilistic forecast than the exponential distribution model on the Poisson process. Therefore, we conclude that the occurrence of microrepeaters is statistically dependent on elapsed time since the last event and is not random in time.

Okada, M.; Uchida, N.; Aoki, S.

2012-08-01

76

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

77

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea  

NASA Astrophysics Data System (ADS)

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.

Jiang, Guoming; Zhao, Dapeng; Zhang, Guibin

2008-06-01

78

High seismic attenuation in the reflective layers of the Philippine Sea Subduction Zone, Japan  

NASA Astrophysics Data System (ADS)

Intrinsic seismic attenuation gives additional constraints on the physical properties of the deep medium. However, in many cases attenuation is masked by scattering loss. In this work, the high-frequency Q-value, i.e. parameter of seismic attenuation, is studied in the Kii Peninsula segment of the Philippine Sea subduction zone of Japan. The geometrical spreading factor, which is necessary to exclude before inversion of the Q-value, is calculated numerically using a realistic 3-D velocity model and ray approximation. Generally, estimated "total" Q-values agree well with results of other studies and with common expectations based on tectonic structure, except for one striking result: Q-values for the lower crust and the subducting oceanic crust become extremely low, Qtotal˜20-30ƒ . In order to interpret this result we compiled attenuation-related phenomena that were observed in the studied region: (1) the seismogenic upper crust; (2) aseismic lower crust; (3) reflective lower crust (RLC); (4) belt-like zone of the deep low-frequency tremor generation (LFT), that is parallel to the slab; (5) low-frequency earthquakes (LFE); and (6) reflective subducting oceanic crust (SOC). Analysis of ray coverage reveals that anomalously low Q-value in RLC and SOC can be explained mostly by high scattering attenuation (i.e., low Qsc value) in the reflective layers.

Petukhin, Anatoly; Kagawa, Takao

79

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

80

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

81

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

82

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

83

Investigation of P- and S-wave Anisotropy beneath the Japan Subduction Zone  

NASA Astrophysics Data System (ADS)

To understand the dynamics of the subduction zone, we investigated P- and S-wave seismic anisotropy beneath the Japan islands through travel-time and receiver function analyses. Assuming weakly anisotropic media with horizontal symmetry axes, we resolved the three-dimensional P-wave anisotropic structure (with heterogeneity and azimuthal anisotropy described by the fast propagation direction and the strength of anisotropy) beneath the Japan islands. The P-wave tomography manifested the crust anisotropy with the fast direction parallel to the trend of large-scale geological structures, the mantle anisotropy parallel/sub-parallel to the direction of the present-day absolute plate motions, and the frozen slab anisotropy within the old Pacific slab. Furthermore, the young Philippine Sea slab showed anisotropy that reflects the present-day tectonic condition. In addition, we measured splitting for some clear Ps phases identified on receiver functions to investigate the S-wave anisotropy. The preliminary measurements revealed a crust anisotropy that polarizes the fast S-wave in the same direction as the P-wave crust anisotropy. Unfortunately, however, S-wave anisotropy in the mantle region including slabs remains unsolved, because anisotropy in the uppermost layer has a significant effect on all Ps phases. To understand the subduction system, consideration of the characteristics of both P- and S-anisotropy is required. In particular, mantle anisotropy is an essential factor. Accordingly, to resolve the three-dimensional S-wave anisotropic velocity structure of the Japan islands, we are working on a tomographic study using S-wave travel time under the same conditions as described for the P-wave.

Ishise, M.; Watanabe, M.; Oda, H.

2011-12-01

84

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

85

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

86

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

87

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

88

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

89

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

90

Centrifuge modeling of initial subduction and back-arc extension: suggested tectonics of seawards migration of the Hellenic Trenches  

Microsoft Academic Search

We report the results of centrifuge experiments simulating the early stages of the co-occurrence of subduction, of initial development arcuate thrust faulting at convergent ocean-continent boundaries, and of growth of back-arc basins, where the principal force affecting the model is enhanced gravity in a centrifuge spinning at 500g. Our basic series of models consisted of 3 units simulating the asthenosphere

Y. Mart; E. Aharonov; G. Mulugeta; T. Tentler

2003-01-01

91

The Mariana Trench: A new view based on multibeam echosounding  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

92

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

93

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

94

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

95

Psychromonas kaikoae sp. nov., a novel from the deepest piezophilic bacterium cold-seep sediments in the Japan Trench.  

PubMed

Two strains of obligately piezophilic bacteria were isolated from sediment collected from the deepest cold-seep environment with chemosynthesis-based animal communities within the Japan Trench, at a depth of 7434 m. The isolated strains, JT7301 and JT7304T, were closely affiliated with members of the genus Psychromonas on the basis of 16S rDNA sequence analysis. Hybridization values for DNA-DNA relatedness between these strains and the Psychromonas antarctica reference strain were significantly lower than that accepted as the phylogenetic definition of a species. The optimal temperature and pressure for growth of the isolates were 10 degrees C and 50 MPa and they produced both eicosapentaenoic acid (C20:5omega3) and docosahexaenoic acid (C22:6) in the membrane layer. Based on the taxonomic differences observed, the isolated strains appear to represent a novel obligately piezophilic Psychromonas species. The name Psychromonas kaikoae sp. nov. (type strain JT7304T = JCM 11054T = ATCC BAA-363T) is proposed. This is the first proposed obligately piezophilic species of the genus Psychromonas. PMID:12361254

Nogi, Yuichi; Kato, Chiaki; Horikoshi, Koki

2002-09-01

96

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

97

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

98

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

99

Long-term and short-term vertical deformation rates of the forearc along the NE Japan subduction zone  

NASA Astrophysics Data System (ADS)

We estimated the long-term vertical deformation rate of the northeastern (NE) Japan forearc along the Japan Trench by using the height distribution of MIS 5.5 marine terraces as determined from tephra and cryptotephra stratigraphy. The uplift rate at the north Pacific coast of NE Japan was estimated from the relative heights between the MIS 5.5 marine terrace surface and eustatic sea levels to be 0.10-0.31 (mostly 0.19-0.31) m ka-1, which is faster than the uplift rate of the south Pacific coast of NE Japan (0.11-0.19 m ka-1). The short-term vertical velocity profile, obtained from GPS observations, showed that the north Pacific coast of NE Japan is today being uplifted at a maximum rate of 3.7 +/- 0.4 mm yr-1 as a result of after-slip related to the 1994 inter-plate moment release, whereas tidal-gauge records show that it was subsiding over several decades preceding the 1970s. The south Pacific coast of NE Japan has also subsided for those several decades and is currently subsiding at a maximum rate of 1.9 +/- 0.4 mm yr-1. Thus, the current observed short-term (geodetically determined) vertical velocities do not reflect long-term (geological) vertical tectonic movement. Short-term vertical movement is probably driven by elastic deformation caused by interplate coupling. However, long-term uplift is probably the result of crustal thickening rather than mega-thrusting.

Matsu'Ura, T.; Furusawa, A.; Saomoto, H.

2008-12-01

100

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

101

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

102

On the initiation of subduction  

NASA Technical Reports Server (NTRS)

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

Mueller, Steve; Phillips, Roger J.

1991-01-01

103

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

104

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple-planed deep seismic zone  

Microsoft Academic Search

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M<=5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a ``source region

Toshihiro Igarashi; Toru Matsuzawa; Norihito Umino; Akira Hasegawa

2001-01-01

105

Advancing trenches: tectonic significance and dynamic implications  

NASA Astrophysics Data System (ADS)

Recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate. Those advancing trenches are mostly located in the Western Pacific and correspond to the subduction of very old, Mesozoic oceanic lithosphere. While retreating trenches are commonly explained by the slab pull action of the descending lithosphere, the origin of advancing trenches is still debated. Since this relationships is dependent upon the adopted reference frame, we select region where geological studies show the variability of trench migration style with time. The Izu-Bonin-Mariana (IBM) region represents a key example. The detailed reconstruction of the trench migration of the IBM subducting system reveals that after a long episode of asymmetric rollback, the IBM trench recently started advancing. We propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material. We test this hypothesis by means of two-dimensional (2-D) numerical models, reproducing the effects of the lithospheric aging during subduction. The result of our numerical tests show that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted.

Faccenna, Claudio; di Giuseppe, Erika; Funiciello, Francesca; van Hunen, Jeroen; Lallemand, Serge

2010-05-01

106

Mega-thrust and Intra-slab Earthquakes beneath Tokyo Metropolitan Area around subduction and collision zones in JAPAN  

NASA Astrophysics Data System (ADS)

In central Japan the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9). The vertical proximity of this down going lithospheric plate is of concern because the greater Tokyo urban region has a population of 42 million and is the center of approximately 40 % of the nation's economic activities. A M7+ earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions.The M7+ earthquake is evaluated to occur with a probability of 70 % in 30 years by the Earthquake Research Committee of Japan.We started the Special Project for Earthquake Disaster Mitigation in Tokyo metropolitan areas, a project to improve information needed for seismic hazards analyses of the largest urban centers. Under the project we will deploy a 400-sation dense seismic array in metropolitan Tokyo and Kanto, referred to as the Metropolitan Seismic Observation network (MeSO-net) in next 4 years. The target area of the present project is unique in tectonic setting because two oceanic plates, Philippine Sea plate (PSP) and Pacific plate (PAC), are subducting beneath the Kanto and also a volcanic arc, Izu-Bonin arc, is colliding with Honshu arc. The situation makes the tectonics complicated: there are both zones of smooth subduction and collision of the oceanic plate with the landward plate, either the Eurasian plate or the North American plate. Furthermore, the PSP encounters the PAC at shallow depth in the eastern Kanto region. The newly developing MeSO-net will contribute to understand the generation mechanism associated with the plate subduction and collision. Assessment in Kanto of the seismic hazard requires identification of all significant faults and possible earthquake scenarios and rupture behavior, regional characterizations of the PSP geometry and the overlying Honshu arc physical properties. Our study addresses (1) improved regional characterization of the PSP geometry based on new deep seismic reflection profiles (Sato etal.,2005), reprocessed off-shore profiles (Kimura et al.,2005), and a dense seismic array in the Boso peninsula (Hagiwara et al., 2006) and (2) identification of collision of internal heterogeneity (Wu et al., 2007). We compile these results and present a new model which will be verified by data from the planned MeSO-net. We present a relatively high resolution tomographic image from so far obtained data to show a low velocity zone which suggests a possible internal failure of the slab; a source region of the M7+ intra-slab earthquake. Our study contributes a new assessment of the seismic hazard in the Tokyo metropolitan area. tokyo.ac.jp/shuto/EN/index.html

Hirata, N.; Kasahara, K.; Hagiwara, H.; Satow, H.; Shimazaki, K.; Koketsu, K.; Wu, F.; Okaya, D.

2007-12-01

107

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

108

Modeling of slow slip events along the deep subduction zone in the Kii Peninsula and Tokai regions, southwest Japan  

NASA Astrophysics Data System (ADS)

In the subduction zone of southwest Japan, short-term slow slip events (SSEs) occur with low frequency tremors (LFTs) at intervals of several months. Recently LFTs have been located with high resolution and their activities have been examined in detail. By setting the generation zones of SSEs such that these zones contain the LFT hypocenters, we simulate SSEs on a 3D plate interface beneath the Kii Peninsula and Tokai regions by using a rate- and state-dependent friction law with a small cut-off velocity for the evolution effect. Our numerical results show that recurrence intervals of SSEs in the southern and central Kii Peninsula, in the northern Kii Peninsula, and in the Tokai region are 2.5-3.0, 4.8-5.6, and 3.5-4.5 months, respectively, which are consistent with observed SSE activity. Our simulation also produces a multisegment event that propagates from the Kii to Tokai segments at a speed of 10 km/day, which is consistent with observations. The results suggest that generation zones of LFTs coincide with SSE regions and that these two events are different manifestations of the same slip process along the subduction zone. We also perform 3D modeling of faster events accompanied by short-term SSEs by considering local circular patches with a smaller critical displacement, surrounded by SSE zones with a larger critical displacement. Local circular patches are set at the observed LFT locations. The simulation reproduces fast events that propagate at a speed of 100-200 km/day during a single SSE event.

Shibazaki, Bunichiro; Obara, Kazushige; Matsuzawa, Takanori; Hirose, Hitoshi

2012-06-01

109

A global view of shear wave splitting and mantle flow in subduction systems  

NASA Astrophysics Data System (ADS)

The character of the mantle flow field in subduction zone regions remains poorly understood, despite its importance for our understanding of subduction dynamics. Observations of seismic anisotropy, which manifests itself in shear wave splitting, can shed light on the geometry of mantle flow in subduction zones, but placing constraints on anisotropy in various parts of the subduction system (including the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle) remains challenging from an observational point of view. In order to identify dynamic processes that make first-order contributions to the pattern of mantle flow in subduction zones, we analyze a global compilation of shear wave splitting measurements for a variety of ray paths, including SK(K)S and teleseismic S phases as well as local S and source-side splitting from slab earthquakes. Key challenges associated with assembling such a compilation include correctly assessing and accounting for any dependence of local S splitting parameters on frequency and correctly characterizing any contribution to SKS splitting measurements from anisotropy within the subducting slab that is unrelated to active mantle flow. We present local case studies from the Japan and Izu-Bonin-Marianas subduction zones that explore frequency-dependent splitting due to heterogeneous anisotropy in the mantle wedge and that use a variety of raypath combinations to isolate the contribution from anisotropy within the slab. Keeping these results in mind, we have compiled shear wave splitting measurements from subduction zones globally from the literature and from our own work to produce estimates of average shear wave splitting parameters - and their spatial variation - for the mantle wedge and the sub-wedge region for individual subduction segments. These estimates are then compared to other parameters that describe subduction. The sub-wedge splitting signal is relatively simple and is dominated by trench-parallel fast directions in most subduction zones worldwide (with a few notable exceptions). Average sub-wedge delay times correlate with the absolute value of trench migration velocities in a Pacific hotspot reference frame, which supports a model in which sub-slab flow is usually trench-parallel and is induced by trench migration. Shear wave splitting patterns in the mantle wedge are substantially more complicated, with large variations in local S delay times and complicated spatial patterns that often feature sharp transitions between trench-parallel and trench-perpendicular fast directions. Using our global compilation of local S splitting measurements and other subduction-related parameters (such as convergence and trench migration velocities, the age, dip, and morphology of the subducting slab, thickness and stress state of the overriding plate, volcanic production, and depth to volcanism), we carry out hypothesis testing of the wide variety of models that have been proposed to explain mantle wedge anisotropy. These include models that invoke corner flow, transpression due to oblique subduction, trench-parallel flow, crustal foundering, B-type olivine fabric, the LPO of serpentinite, melt-controlled anisotropy, or a combination of these mechanisms. We are currently working to integrate our seismological results with the results from geodynamical modeling (both numerical and analog) to test the geodynamic plausibility of our proposed model for mantle flow in subduction systems and to explore the implications of the model for subduction zone processes and, more generally, for mantle dynamics.

Long, Maureen; Silver, Paul; Hanna, Jenny; Wirth, Erin; Kincaid, Chris; Montesi, Laurent

2010-05-01

110

Abstract The December 26, 2004 SumatraAndaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes  

E-print Network

Ã? Indonesia Ã? Sumatra Ã? Tsunami Ã? Andaman Islands 1 Introduction The Mw = 9.3 December 26, 2004 zone megathrust plate boundary on the Sunda Trench (Bird 2003). The December earthquake and its tsunami,127,000 people displaced (United Nations Office of the Special Envoy for Tsunami Recovery 2006). The shaking

Levin, Vadim

111

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

112

Gravity anomalies, forearc morphology and seismicity in subduction zones  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

113

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

114

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

115

Earthquake hazards on the cascadia subduction zone  

Microsoft Academic Search

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also

T. H. Heaton; S. H. Hartzell

1987-01-01

116

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

117

Izanagi-Pacific Ridge Subduction and its Geodynamic Consequences  

NASA Astrophysics Data System (ADS)

As part of a global plate tectonic model for 140 Ma to the present we present a revised plate reconstruction for the western Pacific and investigate its geodynamic consequences. In our plate model, mid-ocean ridge subduction beneath southern Japan occurs at 60-55 Ma, 20 million years later than proposed for Kula-Pacific or Farallon- Izanagi ridge subduction. The difference arises because Izanagi-Pacific (I-P) spreading ceases in previous models after 110 Ma while our model incorporates continued spreading until the I-P ridge subducts beneath eastern Asia at 60-55 Ma. We regard cessation of spreading at the I-P ridge between 110 and 80 Ma as unlikely as the Izanagi plate was undergoing rapid motion, driven by net slab-pull force, from the north-northwest, immediately prior to the proposed spreading cessation. Metamorphism of the Ryoke Belt in southern Japan has previously been attributed to Kula-Pacific ridge subduction at 85 Ma, but the high-T/low-P Ryoke Belt cannot be uniquely linked to a ridge subduction event. We propose that sub-parallel subduction of the I-P mid-ocean ridge beneath Japan at 60-55 Ma resulted in nearly simultaneous slab break-off along the length of the Japanese trench (approximately 2700 km). Geological evidence for this model includes cessation of a major accretion phase in the late Cretaceous, emplacement of the Okitsu Melange due to subduction of hot, buoyant material at 55 Ma, and cross-cutting fault fabrics that indicate a counter-clockwise rotation in relative plate motions between Eurasia and the I-P plate, consistent with palaeothermal and palaeopressure data, some time between 55 and 34 Ma. Rapid subduction of the I-P ridge, over a vast distance, may have triggered a chain reaction of tectonic plate reorganizations. With complete subduction of the I-P ridge at 55 Ma, forces acting on the western edge of the Pacific Plate would have changed from ridge-push to slab pull, changing Pacific absolute plate motions from northwest to west. A combination of Australian and Pacific plate motion changes between 53 Ma and 50 Ma then initiated both the Tonga-Kermadec subduction system and the Izu-Bonin-Marianas subduction systems around 50Ma, likely due to convergence across a fracture zone caused by the Pacific plate motion change. We suggest that the observed slowdown of sub-Pacific mantle flow at 47 Ma was due to progressive impediment of lateral sub-Pacific mantle flow by the descending slabs of the Izu-Bonin-Marianas and Tonga-Kermadec subduction zones.

Müller, R. D.; Whittaker, J. M.; Sdrolias, M.

2007-12-01

118

Seismogenic Mega-splay Fault in Subduction Zone-Modern and Ancient Examples from Southwest Japan  

NASA Astrophysics Data System (ADS)

In the Nankai Trough, SW Japan, 1944 Tonankai earthquake took place along a mega splay fault branching from the plate boundary between the upper Eurasian and lower Phillipine Sea Plate (Park et al., 2002). The splay fault is recognized as a strong reflector showing partly negative polarity with a wavelength of ca. 200 m (Park et al., 2002). The fault appears to make a boundary between the underthrusted sediments in the deep portion (> 6km), cut the Nankai accretionary prism in a shallow part, and appears a cold seep spot on the sea bottom surface (Ashi, person. Com.). A major out-of-sequence thrust (the Nobeoka Thrust) cutting the ancient accretionary prism of the Shimanto Belt into two subbelts is observed in the Kyushu Island, SW Japan (Murata, 1986; Kondo et al., in press). The fault was located under the thermal condition ca. 250¡OC in terms of vitrinite reflectance and fluid inclusion geo-thermo-barometry, that is enough within the seismogenic zone by thermal model (Hyndman et al., 1997) and is a good analog for the modern splay fault in the Nankai Trough mentioned above. Structural analysis of the thrust shows that the damage zone around the thrust is a few hundred meters thick (Kondo, et al., in press) and is characterized by strong brittle deformation with ubiquitous development of crack-seal and shear-parallel veins. Hanging-wall side of the Nobeoka Thrust is composed of shale/sandstone dominated phyllites, of which deformation is characterized primarily by plastic flow and pressure solution overprinted by brittle cataclastic shear and cracking. Several thin cataclasitc shear zones represent extremely fine fragmental components or re-crystalized clays. Their chemical aspect indicates preferential Fe and Mg concentration, which is suggestive of preferential melting of micaceous and chrolite of host rocks due to frictional heating. Asymmetric development of cracks surrounding the thin shear zones is also characteristic. The asymmetric cracks are quite resemble to that developed in the process zone of fault (Vermilye and Sholz, 1998), that indicates upward propagation of rupture. The footwall side of the Nobeoka Thrust is completely brittle, and crack seal or shear zone filling quartz or calcite veins are developed. Geometric relationship of these veins with the fault is quite systematic, therefore indicates quite active fluid deposite and flow in the damage zone. Fluid inclusion analysis indicates that fluid passing through strongly deformed part of damage zone was hotter than that in less deformed part. Such a passage may be possible only during dynamic dilatant rupture associated with seismic event.

Kimura, G.; Okamoto, S.

2004-12-01

119

Limits on great earthquake size at subduction zones  

NASA Astrophysics Data System (ADS)

Subduction zones are where the world's greatest earthquakes occur due to the large fault area available to slip. Yet some subduction zones are thought to be immune from these massive events, where quake size is limited by some physical processes or properties. Accordingly, the size of the 2011 Tohoku-oki Mw 9.0 earthquake caught some in the earthquake research community by surprise. The expectations of these massive quakes have been driven in the past by reliance on our short, incomplete history of earthquakes and causal relationships derived from it. The logic applied is that if a great earthquake has not happened in the past, that we know of, one cannot happen in the future. Using the ~100-year global earthquake seismological history, and in some cases extended with geologic observations, relationships between maximum earthquake sizes and other properties of subduction zones are suggested, leading to the notion that some subduction zones, like the Japan Trench, would never produce a magnitude ~9 event. Empirical correlations of earthquake behavior with other subduction parameters can give false positive results when the data are incomplete or incorrect, of small numbers and numerous attributes are examined. Given multi-century return times of the greatest earthquakes, ignorance of those return times and our relatively limited temporal observation span (in most places), I suggest that we cannot yet rule out great earthquakes at any subduction zones. Alternatively, using the length of a subduction zone that is available for slip as the predominant factor in determining maximum earthquake size, we cannot rule out that any subduction zone of a few hundred kilometers or more in length may be capable of producing a magnitude 9 or larger earthquake. Based on this method, the expected maximum size for the Japan Trench was 9.0 (McCaffrey, Geology, p. 263, 2008). The same approach indicates that a M > 9 off Java, with twice the population density as Honshu and much lower building standards, is possible. The Java Trench, and others that are considered of the low-coupling type (i.e., Hikurangi, Marianas, Tonga, Kermadec), require increased awareness of the possibility for a great earthquake and tsunami.

McCaffrey, R.

2012-12-01

120

Modeling the activity of short-term slow slip events along deep subduction interfaces beneath Shikoku, southwest Japan  

NASA Astrophysics Data System (ADS)

We developed a model of short-term slow slip events (SSEs) on the 3-D subduction interface beneath Shikoku, southwest Japan, considering a rate- and state-dependent friction law with a small cutoff velocity for the evolution effect. We assume low effective normal stress and small critical displacement at the SSE zone. On the basis of the hypocentral distribution of low-frequency tremors, we set three SSE generation segments: a large segment beneath western Shikoku and two smaller segments beneath central and eastern Shikoku. Using this model, we reproduce events beneath western Shikoku with longer lengths in the along-strike direction and with longer recurrence times compared with events beneath central and eastern Shikoku. The numerical results are consistent with observations in that the events at longer segments have longer recurrence intervals. The activity of SSEs is determined by nonuniform frictional properties at the transition zone. We also attempt to model the very low frequency (VLF) earthquakes that accompany short-term SSEs, on a 2-D thrust fault. We consider a local patch in which the friction parameters are varied. In the case that critical displacement is very small at the patch, fast multiple slips occur at the patch. In the case that the effective normal stress is high at the patch, the patch acts as a barrier to SSEs; when it ruptures, however, rapid slip occurs. Because the source time functions of these cases are somewhat different, it would be possible in the future to assess if either case is an appropriate model for VLF earthquakes.

Shibazaki, Bunichiro; Bu, Shuhui; Matsuzawa, Takanori; Hirose, Hitoshi

2010-04-01

121

Behaviour of subducted water and its role in magma genesis in the NE Japan arc: A combined geophysical and geochemical approach  

NASA Astrophysics Data System (ADS)

Water at subduction zones is carried to mantle depths by the subducting oceanic plate and then released by dehydration. It then migrates upwards and contributes to melting of the mantle wedge to form primary arc magma. The magma thus captures and transfers water to the crust, or outgasses water to the atmosphere. Water, either in fluids or melts in both the slab and the mantle, promotes the dissolution and mobilization of elements and affects the physical properties of the sub-arc slab, mantle, and seismicity. In this paper, we present a coherent model to explain the geophysical and geochemical role of water beneath NE Japan. We first investigate the seismic structures of the downgoing slab and sub-arc mantle and examine the role of subducted water in forming these structures. We then use the Arc Basalt Simulator version 4, a petrological-geochemical model developed to describe the geochemical behaviours of water and elements in the slab, mantle, and arc basalt. Parameters governing these petrogenetic processes are also estimated by the model and compared to geophysical observations. The combined approach shows that (1) subducted sediment and igneous oceanic crust are almost fully hydrated, whereas only partial hydration occurs in the oceanic mantle; (2) this high slab water content leads to melting of the slab sediment and the uppermost basalt layer beneath the arc; (3) the released water via slab liquid promotes 3-25% melting of the mantle wedge at a depth of 50-30 km at a mantle temperature of 1250-1350 °C; (4) virtually 89% of slab water is released, 22% of the water returns to the forearc, and 38% enters the arc crust with the magma; and (5) 11% of the subducted water retained beyond a depth of 180 km is held in the slab, and 29% in nominally anhydrous minerals in the wedge mantle.

Kimura, Jun-Ichi; Nakajima, Junichi

2014-10-01

122

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

123

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

124

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

NASA Astrophysics Data System (ADS)

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

Hashimoto, M.

2013-12-01

125

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

126

Structure of the Sunda Trench lower slope off sumatra from multichannel seismic reflection data  

Microsoft Academic Search

Multichannel seismic reflection profiles across the Sunda Trench slope off central Sumatra reveal details of subduction zone structure. Normal faults formed on the outer ridge of the trench offset deep strate and the oceanic crust, but die out upsection under the trench sediments. At the base of the inner trench slope, shallow reflectors are tilted seaward, while deeper reflectors dip

Gregory F. Moore; Joseph R. Curray

1980-01-01

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

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

129

Subduction Tectonic Erosion, Sediment Accretion and Arc Collisions in maintaining the Continental Crust  

Microsoft Academic Search

Estimates of modern continental crustal recycling in subduction zones can be made from plate convergence velocities, the thicknesses of trench sediments, volumes and ages of accretionary complexes together with rates of trench retreat. Plate convergence rates appear to be the primary control on crustal subduction, with convergence >7.5 cm\\/yr associated with tectonic erosion. Collision of aseismic ridges with trenches drives

P. Clift; P. Vannucchi; H. Schouten

2007-01-01

130

Subduction Tectonic Erosion, Sediment Accretion and Arc Collisions in maintaining the Continental Crust  

Microsoft Academic Search

Estimates of modern continental crustal recycling in subduction zones can be made from plate convergence velocities, the thicknesses of trench sediments, volumes and ages of accretionary complexes together with rates of trench retreat. Plate convergence rates appear to be the primary control on crustal subduction, with convergence >7.5 cm\\/yr associated with tectonic erosion. Collision of aseismic ridges with trenches drives

P. Clift; P. Vannucchi; H. Schouten

2004-01-01

131

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

132

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

133

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

134

Shortening deformation of the back-arc rift basin in the central northern Honshu, Japan  

Microsoft Academic Search

The Pacific plate is being subducted beneath northern Honshu, Japan, forms a classical example of trench-arc-back arc system. The compressional stress, perpendicular to the northern Honshu arc, has produced the shortening deformation in the Miocene back arc rift basins since the Pliocene. Two narrow up-rift zones run parallel to the arc: Dewa hills on the west and Ou Backbone range

N. Kato; H. Sato

2003-01-01

135

Predicting trench and plate motion from the dynamics of a strong slab  

Microsoft Academic Search

The motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle. These models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation. Here, using a recent compilation of a global data set, we found that the motion of trenches (either advancing or retreating

Claudio Faccenna; Arnauld Heuret; Francesca Funiciello; Serge Lallemand; Thorsten W. Becker

2007-01-01

136

Metamorphic rocks of the Yap arc-trench system  

Microsoft Academic Search

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

J. Hawkins; R. Batiza

1977-01-01

137

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

138

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

139

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

140

Subduction and the mantle flow field (Invited)  

NASA Astrophysics Data System (ADS)

The character of the mantle flow field in subduction zone regions remains poorly understood, despite it importance for our understanding of subduction dynamics. Observations of seismic anisotropy, which manifests itself in shear wave splitting, can shed light on the geometry of mantle flow in subduction zones, but placing constraints on anisotropy in various parts of the subduction system (including the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle) remains challenging from an observational point of view. In order to identify dynamic processes that make first-order contributions to the pattern of mantle flow in subduction zones, we analyze a global compilation of shear wave splitting measurements for a variety of ray paths, including SK(K)S and teleseismic S phases as well as local S and source-side splitting from slab earthquakes. Average shear wave splitting parameters - and their spatial variation - for the mantle wedge and the sub-wedge region are obtained and compared to other parameters that describe subduction. The sub-wedge splitting signal is relatively simple and is dominated by trench-parallel fast directions in most subduction zones worldwide (with a few notable exceptions). Average sub-wedge delay times correlate with the absolute value of trench migration velocities in a Pacific hotspot reference frame, which supports a model in which sub-slab flow is usually trench-parallel and is induced by trench migration. Shear wave splitting patterns in the mantle wedge are substantially more complicated, with large variations in local S delay times and complicated spatial patterns that often feature sharp transitions between trench-parallel and trench-perpendicular fast directions. Using our global compilation of local S splitting measurements and other subduction-related parameters (such as convergence and trench migration velocities, the age, dip, and morphology of the subducting slab, thickness and stress state of the overriding plate, volcanic production, and depth to volcanism), we carry out hypothesis testing of the wide variety of models that have been proposed to explain mantle wedge anisotropy. These include models that invoke corner flow, transpression due to oblique subduction, trench-parallel flow, crustal foundering, B-type olivine fabric, the LPO of serpentinite, melt-controlled anisotropy, or a combination of these mechanisms.

Long, M. D.; Silver, P. G.

2009-12-01

141

From normal to oblique subduction: Tectonic relationships between Java and Sumatra  

Microsoft Academic Search

The convergent motion of the Indian-Australian and the Eurasian Plates results in subduction at the Sunda Arc. Obliquity of subduction beneath Sumatra induces large strike-slip faults in Sumatra and its margin, whereas the subduction is almost perpendicular to the trench southwest of Java. The nature of the transition between these two subduction regimes is of major interest. New data collected

J. A. Malod; Komar Karta; M. O. Beslier; M. T. Zen

1995-01-01

142

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

NASA Astrophysics Data System (ADS)

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

143

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

NASA Astrophysics Data System (ADS)

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

Lee, C.; King, S. D.

2011-12-01

144

Overriding plate controls on subduction evolution  

NASA Astrophysics Data System (ADS)

Geologic and geophysical observations indicate that the thickness, density, and strength of the lithosphere vary on the Earth. However, the role of the overriding plate lithosphere properties on the evolution and morphology of subduction is not well understood. This paper presents time-dependent numerical models of subduction that vary the overriding plate thickness, strength, and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology. We examine the effect of these parameters on subduction evolution, in particular, the emergence of (a) asymmetric versus symmetric subduction, (b) trench retreat versus advance, (c) subduction zone geometry, (d) slab stagnation versus penetration into the lower mantle, and (e) flat slab subduction. Almost all of the models presented result in sustained asymmetric subduction from initiation. Trench advance occurs in models with a thick and or strong overriding plate. Slab dip, measured at a depth below the plate boundary interface, has a negative correlation with an increase in overriding plate thickness. Overriding plate thickness exerts a first-order control over slab penetration into the lower mantle, with penetration most commonly occurring in models with a thick overriding plate. Periods of flat slab subduction occur with thick, strong overriding plates producing strong plate boundary interface coupling. The results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction as well as providing an explanation for the variation of slab geometry across subduction zones on Earth, where similar patterns of evolution are observed.

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

2014-08-01

145

Trench Connection  

PubMed Central

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

Jamieson, Alan J.; Fujii, Toyonobu

2011-01-01

146

Japan.  

ERIC Educational Resources Information Center

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

Jones, Savannah C.

147

Japan  

ERIC Educational Resources Information Center

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

Hawkins, John N.

1986-01-01

148

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

149

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

150

Seismic behavior and geodetic locking in areas of rough seafloor subduction  

NASA Astrophysics Data System (ADS)

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

Bilek, S. L.; Wang, K.

2012-12-01

151

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

152

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

Jacob, Jensen; Dyment, Jerome

2014-05-01

153

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

NASA Astrophysics Data System (ADS)

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

Niu, Y.

2013-12-01

154

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones  

NASA Astrophysics Data System (ADS)

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

Satake, K.; Tanioka, Y.

155

A Possible Precursor of an Anticipated Subduction Zone Thrust Earthquake in the Tokai Region, Central Japan, Detected by a Continuous GPS Network Measurements  

NASA Astrophysics Data System (ADS)

The Tokai region is located along the Pacific coast of central Japan and about 200 km to the south west of Tokyo. The Suruga trough, a subduction plate boundary between a continental plate and the Philippine Sea Plate, runs just off shore of this area. This region is a well known seismic gap along the Suruga-Nankai trough. Continuous GPS data since 1994 and historical geodetic survey data for about 100 years consistently suggest a steady strain accumulation in this region until recently. Since the beginning of 2001 we detected a change in crustal deformation rates around the Tokai region using continuous GPS data. This change is preceded by transient phenomena associated with a large seismo-volcanic event in the Izu islands during July-October 2000. This 2000 Izu islands event caused a significant crustal deformation of up to 3 cm in the Tokai and Kanto regions, central Japan. After October 2000, the effect of the Izu islands activity faded out and the Tokai and Kanto regions returned approximately to a previous normal state of crustal deformation. However, from March 2001, another stage of ground displacements started in the Tokai region. Detrended GPS coordinate time series data show southeastward motion of about 1 cm for the period between March and August 2001. Taking into account the effect of the 2000 Izu islands event, which still continues with much less activity than in summer 2000, we estimate slip motion on the plate boundary in the Tokai region. The inversion result shows that a slow thrust slip occurs on the plate boundary around Lake Hamana in the western Tokai region. Estimated seismic moment due to the slow slip has been increasing linearly from March to August 2001, while the slip area seems to be expanding to the east to a more tightly coupled area for the same period. The slip area cannot be well resolved in the eastern Tokai region because the effect of the Izu islands activity also explains a part of the observed southeastward motion there. However we cannot rule out a existence of a slow slip in the eastern Tokai region, a presumed central source region of the forthcoming Tokai earthquake. In any case the existence of change of deformation rate is now significant considering the accuracy of GPS measurements. Intensive monitoring on the temporal evolution of this phenomenon and timely update of a physical model are necessary.

Ozawa, S.; Murakami, T.; Kaidzu, M.; Tada, T.; Sagiya, T.; Yarai, H.; Nishimura, T.

2001-12-01

156

A Geophysical Study of the Manila Trench, Luzon, Philippines 1. Crustal Structure, Gravity, and Regional Tectonic Evolution  

Microsoft Academic Search

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The trench trends northerly and is associated with a volcanic arc, an east dipping Benioff zone beneath Luzon, and a well-developed fore arc basin system. The Luzon Trough fore arc basins lie landward of the Manila Trench and contain up

Dennis E. Hayes; Stephen D. Lewis

1984-01-01

157

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 ...

Hughes, Mr.

2006-03-04

158

Unusually large earthquakes inferred from tsunami deposits along the Kuril trench  

USGS Publications Warehouse

The Pacific plate converges with northeastern Eurasia at a rate of 8-9 m 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 100-200 km long, have been considered characteristic of Hokkaido's plate-boundary earthquakes. But here we use deposits of prehistoric tsunamis to infer the infrequent occurrence of larger earthquakes generated from longer ruptures. Many of these tsunami deposits form sheets of sand that extend kilometres inland from the deposits of historical tsunamis. Stratigraphic series of extensive sand sheets, intercalated with dated volcanic-ash layers, show that such unusually large tsunamis occurred about every 500 years on average over the past 2,000-7,000 years, most recently ???350 years ago. Numerical simulations of these tsunamis are best explained by earthquakes that individually rupture multiple segments along the southern Kuril trench. We infer that such multi-segment earthquakes persistently recur among a larger number of single-segment events.

Nanayama, F.; Satake, K.; Furukawa, R.; Shimokawa, K.; Atwater, B.F.; Shigeno, K.; Yamaki, S.

2003-01-01

159

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

160

Physical characteristics of subduction interface type seismogenic zones revisited  

Microsoft Academic Search

Based on global earthquake catalogs, the hypocenters, nodal planes, and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007. Assuming that the seismogenic zone coincides with the distribution of 5.5 ? M < 7 earthquakes, the subduction interface seismogenic zones were mapped for 80% of the trench systems and characterized with geometrical

Arnauld Heuret; Serge Lallemand; Francesca Funiciello; Claudia Piromallo; Claudio Faccenna

2011-01-01

161

Subduction erosion processes with application to southern Mexico  

NASA Astrophysics Data System (ADS)

Finite-element numerical models of ocean-continent subduction are used to investigate the roles of crustal frictional strength, subduction angle, and convergence rate in subduction erosion processes. These models exhibit two distinct modes of subduction erosion: (1) slow and steady, removing small blocks of material continually, and (2) fast and non-steady, removing a large forearc block in a single event. The slow mode, called edge-weakening subduction erosion, is enhanced by steeper subduction angles but acts to shallow the subduction angle at crustal depths. The fast mode, called internal-weakening subduction erosion, is enhanced by shallow subduction angles but acts to steepen the subduction angle at crustal depths. The two modes may alternate cyclically in nature and may account, in part, for the variation in subduction angle observed at the modern western American subduction zones. The slow, edge-weakening subduction erosion mode correlates well to subduction erosion processes widely reported for natural subduction zones. The fast, internal-weakening subduction erosion mode has previously been described only for subduction zones involving continental lithosphere on the lower plate. The removal of a 150--250 km wide forearc block from southern Mexico between 27--25 Ma and 21--19 Ma may be a first type example of internal-weakening subduction erosion at an ocean-continent subduction zone. The numerical models showing internal-weakening subduction erosion and the geological record of southern Mexico share the following geological features synchronous with forearc removal: (1) rapid trench migration rates approaching orthogonal plate convergence rates, (2) a step-wise shift in the locus of arc magmatism towards the upper plate, (3) forearc subsidence at the new margin of the upper plate, (4) a zone of crustal unroofing within the upper plate's new forearc region, and (5) a zone of subduction-antithetic thrust-sense shearing inboard of the crustal unroofing. These features can be used to test other subduction zones for suspected internal-weakening subduction erosion events. An internal-weakening subduction erosion event results in a rapid drop in the shear stress sustained across a subduction zone. For this reason, the timing of an internal-weakening subduction erosion event on the Acapulco Trench at the end of the Oligocene may account, in part, for the timing of initial Cocos-Nazca rifting at ca. 23 Ma.

Keppie, Duncan Fraser

162

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

163

Subduction initiation: spontaneous and induced  

NASA Astrophysics Data System (ADS)

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

2004-10-01

164

Subduction initiation: spontaneous and induced  

NASA Astrophysics Data System (ADS)

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

Stern, Robert J.

2004-10-01

165

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

NASA Astrophysics Data System (ADS)

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

Ujiie, K.; Yamaguchi, H.

2004-12-01

166

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....

2009-01-26

167

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

168

Seamarc II Studies of Subducting Seamounts.  

National Technical Information Service (NTIS)

Effects of subduction related tectonic processes on the oceanic plate are expressed in the morphology of some of these seamounts located in or very near the axes of the Mariana and Izu/Bonin submarine trenches. Fractures, presumably caused by bending of t...

P. Fryer, D. M. Hussong

1985-01-01

169

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

170

Seismic Anisotropy Beneath the Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

The Sumatra subduction zone is located on the eastern side of the Sunda Arc between the Sunda Strait and the Andaman Islands, where the Indo-Australian plate is subducting beneath the Eurasian plate. An important tool in understanding the style and geometry of deformation within a subduction zone is the measurement of seismic anisotropy, through observations of shear wave splitting, which provides information about the mantle flow. In Sumatra two temporary seismic networks were deployed within the Mentawai and Northern region, between December 2007 and October 2008 and April 2008 and February 2009, respectively. Here we use new splitting measurements from SKS and local S phases from these networks, to characterize the type and amount of anisotropy within the region. High quality SKS splitting results obtained from 16 stations show a coherent fast direction of NNW (350°) to SSE (170°) with delay times of 1.0 to 3.0s. The observations do not conform to the standard classification of trench parallel and trench perpendicular, instead they are trench oblique, approximately parallel to the direction of motion of the subducting Indo-Australian plate. The magnitude and direction of splitting suggests that the anisotropy is due too entrained flow in the asthenosphere beneath the subducting lithosphere. Local S wave splitting measurements were obtained from 85 rays, generated by earthquakes at focal depths of 15 km-200 km and recorded across 39 stations. The polarization of the fast shear wave is trench parallel along the islands and can be contributed to shape preferred orientation of cracks in the top of the subducting slab and overriding crust. In the forearc a rotation in fast direction to trench perpendicular is observed, with a clear positive correlation between the distance the ray has traveled in the mantle and delay time, suggesting anisotropy originates from entrained flow within the mantle wedge. In the Sumatra fault region and the back arc, events originating at the plate interface show both trench perpendicular and trench parallel fast directions with delay times of up to 0.4 s, while shallow events on the Sumatra fault indicate a clear fault parallel direction with delay times of 0.1 s to 0.18 s. This pattern of shear wave splitting suggests layers of different anisotropy, one deeper layer due to entrained flow within the Sumatran mantle wedge causing trench perpendicular direction, and the second shallower one within the overriding crust due to shape preferred orientation possibly caused by the Sumatran Fault.

Collings, R.; Rietbrock, A.; Nippress, S.; Lange, D.; Tilmann, F. J.; Natawidjaja, D.; Suwargadi, B.

2011-12-01

171

The Mariana Trench  

NSDL National Science Digital Library

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

2011-02-01

172

Fluid flow in ocean crust cools the Cascadia subduction zone  

Microsoft Academic Search

Temperatures along subduction zone plate boundary faults have been used to estimate the area and extent of the seismogenic zone. Recent studies of the well-constrained Nankai margin of Japan show that hydrothermal circulation in the subducting crust cools the subduction zone and widens the area of the plate boundary fault that is between the key temperatures of 150 and 350

B. D. Cozzens; G. A. Spinelli

2010-01-01

173

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

NASA Astrophysics Data System (ADS)

Shear stresses ? on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone. In this study, the temperatures and shear stresses on megathrusts in 11 subduction zones around the Pacific rim (Hikurangi, Tonga, Izu-Ogasawara, western Nankai, northeastern Japan, Aleutians, western Alaska, Cascadia, northern Chile, southern Chile) and SE Asia (northern Sumatra) have been determined. The main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses, in the plane of a trench- or arc-normal section. For a typical brittle and ductile megathrust rheology, frictional shear stress ? = ??gz, for depth z, and ductile shear stress ? = A exp (B/RT) at temperature T, where ?, A, B are rheological parameters treated as constants. Rheological constants common to all the megathrusts (?crust, ?mantle, B) are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure, using a simplex minimization algorithm, taking account of the induced mantle corner flow at depth (65 ± 15 km (2?)) and constant radiogenic heating (0.65 ± 0.3 ?W m-3 (2?)) throughout the crust. The A constants are solved individually for each subduction zone, assuming that the maximum depth of interplate slip earthquakes marks the brittle-ductile transition. The best fit solution shows two groupings of megathrusts, with most subduction zones having a low mean shear stress in the range 7-15 MPa (?crust = 0.032 ± 0.006, ?mantle = 0.019 ± 0.004) and unable to support elevations >2.5 km. For a typical frictional sliding coefficient ˜0.5, the low effective coefficients of friction suggest high pore fluid pressures at ˜95% lithostatic pressure. Tonga and northern Chile require higher shear stresses with ?crust = 0.095 ± 0.024, ?mantle = 0.026 ± 0.007, suggesting slightly lower pore fluid pressures, at ˜81% lithostatic. Ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency, with B = 36 ± 18 kJ mol-1. Amantle values are sensitive to the precise value of B but are in the range 1-20 kPa. The power law exponent n for mantle flow is poorly constrained but is likely to be large (n > 4). The brittle-ductile transition in the crust occurs at temperatures in the range 370°C-512°C, usually close to the base of the crust and in the mantle at much lower temperatures (180°C-300°C), possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties. In subduction zones where the subducted slab is older than 50 Ma, a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are ?300°C. In much younger subduction zones, the stress transmission is confined mainly to the crust. The shear stresses, particularly in the crust, may be kept low by some sort of lubricant such as abundant water-rich trench fill, which lowers the frictional sliding coefficient or effective viscosity and/or raises pore fluid pressure. The unusual high stress subduction zone in northern Chile lacks significant trench fill and may be poorly lubricated, with a mean shear stress ˜37 MPa required to support elevations >4 km in the high Andes. However, where the crust is thin in sediment-starved and poorly lubricated subduction zones, such as Tonga, the mean shear stress will still be low. Sediment may lubricate megathrusts accommodating underthrusting of continental crust, such as in the Himalayas or eastern central Andes, which have a low mean shear stress ˜15 MPa.

Lamb, Simon

2006-07-01

174

Geodynamic insights into patterns of shear wave anisotropy in subduction zones  

NASA Astrophysics Data System (ADS)

Shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that have trench-parallel fast directions and abrupt rotations. Several hypotheses have been proposed to explain these unexpected patterns. This work shows tests of the olivine fabric transition and three-dimensional flow hypotheses. The tool used to carry out this investigation is geodynamic modeling with seismological constraints on subduction zone geometry and rheological parameters based on rock deformation experiments. The geodynamic models presented in this work show that a transition to B-type olivine fabric can produce trench-parallel shear wave splitting with delay times greater than 1 s above the cold fore-arc mantle of subduction zones. The olivine fabric transition model adequately reproduces local-S splitting parameters associated with phases that sample the deep fore-arc mantle of the Ryukyu subduction system. The three-dimensional subduction zone models presented in this work show that both a transition to flat-slab subduction and strong trench curvature generate trench-parallel stretching in the warm are and back arc mantle. These models may explain trench-parallel shear wave splitting and abrupt rotations in fast direction in the are and back arc-mantle of the Andean and Marianas subduction systems. This thesis demonstrates that a variety of mechanisms may be necessary to account for trench-parallel anisotropy in subduction zones.

Kneller, Erik Arthur

175

Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust  

NASA Astrophysics Data System (ADS)

Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (1 0 0)[0 0 1] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (0 0 1)[0 1 0], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP = 20.4%, AVS = 11.5%) and epidote (AVP = 9.0%, AVS = 8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [0 0 1] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP = 9.6%, AVS = 19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [0 0 1] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6-9.2%) is weak compared with that of epidote blueschist (8.4-11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones.

Kim, Daeyeong; Katayama, Ikuo; Michibayashi, Katsuyoshi; Tsujimori, Tatsuki

2013-09-01

176

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

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

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

2011-12-01

177

Earthquake hazards on the cascadia subduction zone.  

PubMed

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

Heaton, T H; Hartzell, S H

1987-04-10

178

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

Microsoft Academic Search

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

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

2008-01-01

179

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

NASA Astrophysics Data System (ADS)

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

Gvirtzman, Z.; Stern, R. J.

2004-12-01

180

Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones  

NASA Astrophysics Data System (ADS)

We have investigated potential effects on a subduction zone during oceanic plateau-trench collision, using three-dimensional numerical models. An oceanic plateau of varied density was embedded in the slab, which was pinned at one end. The plateaus strongly influenced the shape of the trench. For a plateau with a higher density, the trench retreat rate was reduced in the region surrounding the plateau, and the plateau subducted along with the slab. For lower density plateaus, the trench in the region of the plateau advanced, and the plateau compressed and resisted subduction, spreading laterally along the trench. With a weaker slab rheology, the arcuate shape of the trench towards the free end of the trench was enhanced. Beneath the most buoyant plateaus, a tear formed in the subducted portion of the slab, soon after the slab tip reached the top of the lower mantle. We compare the model results with a region in the northwest Pacific, where the Ogasawara Plateau meets the trench of the Izu-Bonin-Mariana subduction zone.

Mason, W. G.; Moresi, L.; Betts, P. G.; Miller, M. S.

2010-03-01

181

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

Microsoft Academic Search

Correlations among subduction zone seismicity, convergence rate and subducting plate age are reassessed considering the possible roles of both recurrence times and fault zone temperatures. Distributions of earthquakes with respect to subducting lithosphere age and convergence rate are grossly explained by a recurrence relation when ages and rates at the world's trenches are taken into account. Correlations between maximum earthquake

Robert McCaffrey

1997-01-01

182

Deep mantle subduction flux  

NASA Astrophysics Data System (ADS)

We assess the flux of incompatible trace elements into the deep mantle in the Aleutian, Central America, Izu-Bonin, Kurile, Lesser Antilles, Mariana, Sunda, and Tonga subduction zones. We use a simple mass balance approach in which we assume that all of the material lost from the subducting crust and sediment (the "slab") is incorporated into the magmas erupted above the subduction zone, and we use these assumptions to calculate a residual slab composition. The calculated residual slabs are enriched in incompatible elements compared to mid-ocean ridge basalts and highly enriched compared to primitive or depleted mantle. Almost all of the subducted Nb, Ta, and intermediate and heavy rare earths survive into the deep mantle, as do most of the light rare earths. On average, 73% of Th and Pb, 74% of K, 79% of U, 80% of Rb, 80% of Sr, and 82% of Ba survive into the deep mantle. Pb/Ce ratios are systematically lower, and Nb/U ratios are systematically higher, in the deep mantle flux than they are in the flux of material into the trench. Nevertheless, most residual slabs have Pb/Ce and Nb/U ratios outside the typical mantle range. Changes to U/Pb and Th/U ratios tend to be small and are not systematic. Rb/Sr ratios significantly decrease in some subduction zones but increase in others. In contrast, Sm/Nd ratios increase by small but significant amounts in most arcs. Based on these results, we attempt to predict the Sr, Nd, and Pb composition of anciently recycled material now in the mantle. We find that such material would most resemble enriched mantle II-type oceanic island basalts (OIB). None of our calculated residual slabs would evolve to Sr-Nd-Pb isotopic compositions similar to either high 238U/204Pb or enriched mantle I. The range of Sr and Pb isotope ratios in anciently recycled material is similar to that seen in modern OIB, but Nd isotopic compositions do not range to ?Nd values as low as those in some modern OIB. Neither radiogenic nor unradiogenic Pb isotope compositions can be exclusively associated with recycled material.

Porter, Katherine A.; White, William M.

2009-12-01

183

Nonvolcanic tremors in the Mexican subduction zone  

Microsoft Academic Search

Nonvolcanic low frequency tremors (NVT) have been discovered and studied recently in Japan and Cascadia subduction zones and deep beneath the San Andreas Fault. The tremors activity is increasing during so-called silent earthquakes (SQ) in Japan and Cascadia. NVT clusters also migrate following the propagation of the SQ. The origin of the NVT is still unclear. The studies of NVT

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

2007-01-01

184

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

185

Metastable olivine wedge and deep dry cold slab beneath southwest Japan  

NASA Astrophysics Data System (ADS)

Oceanic plates subducted at trenches penetrate into the deep mantle, and encounter a structural boundary at a depth of 410 km where olivine, the dominant element of mantle rocks, transforms into a higher density form wadsleyite. This transformation may be delayed within the coldest core of subducting plates (slabs) due to kinetic effects, and it has been suggested that metastable olivine may persist deeper than 410 km. Using high density seismic array data in Japan, we show the direct image of the structure corresponding to this metastable olivine wedge (MOW) beneath southwest Japan. Numerical simulation of a subducting slab, including the kinetic effect of water (H 2O) on the olivine-wadsleyite transformation, indicates that the presence of the imaged MOW requires an insignificant amount of water (less than 100 wt. ppm) be present in the slab mantle, thus a deep dry cold slab. We infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab, but no significant amount within the slab itself. We also demonstrate that a numerical simulation including the kinetics of 660-km phase transformation can reconcile the observed deep depression of the 660-km discontinuity with a gentle Clapeyron slope.

Kawakatsu, Hitoshi; Yoshioka, Shoichi

2011-02-01

186

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

187

The influence of small stresses on the dynamics of glaciers and subduction zones  

E-print Network

in the northern Cascadia subduction zone, J. Geophys. Res. ,and slip on the Cascadia subduction zone: the chatter ofsubduction zones and intraplate faults has increased significantly, including southwest Japan (Obara, 2002), Cascadia (

Walter, Jacob Ineman

2012-01-01

188

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

NASA Astrophysics Data System (ADS)

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

Gvirtzman, Zohar; Stern, Robert J.

2004-04-01

189

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

190

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

191

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

192

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

193

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

194

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

E-print Network

2012; published 19 May 2012. [1] Kinematic similarities between the Sumatra and Puerto Rico Trenches (10 ) to the NA-CA plate boundary, similar to that along Sumatra subduction zone. Following the 2004 Sumatra earthquake and tsunami, concern grew that the Puerto Rico Trench might be capable of producing

ten Brink, Uri S.

195

A new insight on the geometry of subducting slabs in northern Luzon, Philippines  

Microsoft Academic Search

We used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern Luzon and propose a model for describing the geometry of the subducted slab of the Eurasian plate beneath the northern segment of the Manila Trench. We took into consideration some of the observed bathymetric features (i.e. the bend in the trench line of the

Bartolome C. Bautista; Maria Leonila P. Bautista; Kazuo Oike; Francis T. Wu; Raymundo S. Punongbayan

2001-01-01

196

Constraints on backstop geometry of the southwest Ryukyu subduction based on reection seismic data  

E-print Network

the Manila Trench, the Eurasia plate subducts below the Philippine Sea plate thus originating the Luzon the Ryukyu Trench. The Philippine Sea plate, carrying the Luzon Arc, is converging toward Taiwan at a rate.elsevier.com/locate/tecto * Corresponding author. #12;Y.Fontetal./Tectonophysics333(2001)135±158136 PHILIPPINE SEA PLATE Ryukyu A rc Luzon

Demouchy, Sylvie

197

Common Observables of Trench Migration and Plate Motion in Different Global Reference Frames  

NASA Astrophysics Data System (ADS)

Plate velocities and trench migration velocities are commonly described in some sort of global "absolute" reference frame. From calculating such motions for all plates and subduction zones on Earth, one might obtain insight into the importance of various driving and resistive forces of plate tectonics and plate boundary migration. Trench migration velocities and plate velocities have been calculated for all subduction zones on Earth in eight global reference frames. The calculations show that such velocities can differ substantially between different global reference frames (up to 4 cm/yr), in particular between one Pacific hotspot reference frame (HS3- NUVEL1A) and all the others. In addition, this reference frame shows a bimodal distribution of trench velocities, while all the others show a Gaussian distribution. Nevertheless, some common features are observed irrespective of the reference frame. First, trench retreat always dominates over trench advance, with 62-78% of the trench segments retreating, while the mean and median trench velocities are always positive (retreating). Second, trench retreat is always slow in the middle of wide subduction zones, i.e. far (>2000 km) from lateral slab edges (<2 cm/yr in seven reference frames). Third, fast trench retreat (>6 cm/yr) is only found close (<1500 km) to lateral slab edges. Fourth, plates with a substantial percentage of their circumference attached to a subducting slab (Pacific, Nazca, Cocos, Philippine, Australia) move trenchward. These calculations are predicted by three-dimensional geodynamic models of free subduction with a variable slab width (300-7000 km), in which the slab to upper mantle viscosity ratio is low (100-200). This suggests that trench velocities and plate velocities are indeed primarily controlled by the negative buoyancy and width of subducting slabs. It further suggests that slab/upper mantle viscosity ratios in nature are 100-200, as the models show trench motion dominated by retreat, and forward dipping "slab-draping" geometries (e.g. Tonga) or steep slab geometries (e.g. Kermadec) as observed in nature. Overturned "roll-over" slab geometries are not observed.

Schellart, W. P.; Stegman, D. A.; Freeman, J.; Moresi, L.

2007-12-01

198

Deformation of Serpentinite and its Implications for the Aseismicity and Seismic Low Velocity Anomaly in Subduction Zones  

NASA Astrophysics Data System (ADS)

Recent seismic data suggest that a low velocity layer up to ˜ 7 km thick exists at the top of the subducting slab in many subduction zones (Abers, 2000). Both P and S-wave velocities are consistently 5-7 % lower than the surroundings. There are also observations of a distinct low seismicity zone in the Japan Trench (Fujie et al., 2002) and even an aseismic zone in central Oregon (Brocher et al., 2003). We have investigated a possible cause of these observations by deforming a natural serpentinite at the pressures of 1.0 - 3.4 GPa and the temperatures of 923 - 1023 K. We conducted triaxial deformation experiments at a constant strain rate using a Griggs-type apparatus. Oxygen fugacity was buffered by Ni/NiO. We find that under conditions where serpentine is dehydrating during deformation, Mode-I cracks are commonly observed in serpentine as well as in relict olivine. A similar phenomenon (Mode-I cracks) may occur at the top of the subducting slab due to the deformation of a hydrated mantle wedge above a dehydrating subducting slab. We propose that such aligned Mode-I cracks, which fill with water as they open, may result in the low seismic velocity in subduction zones (i.e. Alaska, the Aleutians, Kuriles, Honshu, the Marianas, and Nicaragua). We also find that deformation of the serpentinite under a differential stress display faults and localized shear zones, delineated by ultra fine-grained solid reaction products, formed as byproducts of antigorite dehydration. We observe this phenomenon under all conditions tested. A distinct low seismicity zone in the Japan Trench region, aseismicity beneath central Oregon, and other aseismic zones in mantle wedges may be attributable to "superplastic" flow under low stresses within the ultra-fine-grained dehydration products in dehydration-induced fault zones. Abers, G. A. Earth Planet. Sci. Lett., 176, 323-330 (2000). Fujie, G. et al., Geophys. Res. Lett., 29, 7 (2002). Brocher, T. M. et al., Geology, 31, 267-270 (2003).

Jung, H.; Green, H. W.

2003-12-01

199

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

200

'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

201

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

202

Sediment trench fill effect on trench age-depth relation  

E-print Network

SEDIMENT TRENCH FILL EFFECT ON TRENCH AGE-DEPTH RELATION A Thesis by BRIAN LYNN RADER Submitted to the Graduate Coliege of Texas ARM University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August f985... Major Subject: Geophysics SEDIMENT TRENCH FILL EFFECT ON TRENCH AGE-DEPTH RELATION A Thesis by BRIAN LYNN RADER Approved as to style and content by: Dr. T. W. C. Hilde (Chairman) Dr. D. Fahlquist (Member) Dr. . Bryant (Member) Dr. E. Hoskins...

Rader, Brian Lynn

2012-06-07

203

Origin of the Palau and Yap trench-arc systems  

NASA Astrophysics Data System (ADS)

This paper attempts to account for several unique morphological and geological characteristics of the Palau and Yap trench-arc systems in the northwestern Pacific Ocean using a new hydrodynamic model postulated herein. The model is a simple mathematical treatment of a stationary viscous flow pattern in the asthenosphere that causes an appreciable amount of mechanical resistance against a vertically subducted rigid slab moving horizontally together with the flat plate. A couple of thrusting forces comprising uplifting pressure generated in the frontal edge of the overriding lid and a new type of tectonic erosion along the surface of the vertical slab seem to reasonably explain apparently contradictory features of the Palau and Yap trench arcs. Long-term GPS measurements show that the Palau and Yap trench arcs are moving over the viscous asthenosphere toward the NWW with velocities as great as 10 cm yr-1. Stresses along the trench arcs predicted by the model would be sufficiently large to maintain their present morphology: (1) water depths greater than 8000 m in spite of the extremely small plate convergence rates and relatively young ages of subducted plate; (2) V-shaped cross-section of the axial bottom of these trenches, implying little cover of slumped deposits in spite of very steep landward slopes; (3) serpentinized peridotites, gabbros, basalts and limestones are exposed on the landward slope with neither sediment cover nor ferromanganese hydro-oxide coatings; (4) abnormally close proximity of the Palau and Yap islands to the trench axis; (5) predominant distribution of barrier reefs in their western periphery apparently opposite to the easterly to northeasterly prevailing wind.

Kobayashi, Kazuo

2004-06-01

204

The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone  

E-print Network

and shortterm SSE in the subduction zones of Japan and Cascadia supported mainly the hypothesisThe 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone Vladimir Kostoglodov) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE

Clayton, Robert W.

205

The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone  

E-print Network

and short term SSE in the subduction zones of Japan and Cascadia supported mainly the hypothesisThe 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone Vladimir Kostoglodov) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE

Shapiro, Nikolai

206

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

207

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

208

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.

209

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

210

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

211

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

212

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.

213

Crustal deformation and block kinematics in transition from collision to1 subduction: GPS measurements in northern Taiwan, 1995-20052  

E-print Network

-south trending Luzon island arc of the Philippine Sea plate and the71 northeast-southwest trending Chinese-continent (Luzon-Chinese) collision to the converging Ryukyu trench41 subduction and back-arc opening along separated by the surface projection of the NW-trending46 boundary of the subducting Philippine Sea plate

Lin, Andrew Tien-Shun

214

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

215

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

216

Trench migration and upper plate strain over a convecting mantle  

NASA Astrophysics Data System (ADS)

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

Husson, Laurent

2012-12-01

217

Trench migration and upper plate strain over a convecting mantle  

NASA Astrophysics Data System (ADS)

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

Husson, Laurent

2013-04-01

218

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

219

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

220

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

221

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

222

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

223

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

224

The subduction zone flow field from seismic anisotropy: a global view.  

PubMed

Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |V(t)|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |V(t)|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity V(c); their relative influence is governed by the relative magnitude of |V(t)| and V(c). PMID:18202286

Long, Maureen D; Silver, Paul G

2008-01-18

225

Present-day chaotic formations around the Japanese trenches: Comparison to the on land examples from the Shimanto and Miura-Boso, and from the Franciscan, Mineoka and Ankara  

NASA Astrophysics Data System (ADS)

Four different types of chaotic formations were recognized by the submersible observation around the Japanese trenches, including the Nankai and Sagami troughs, Boso triple junction, Japan trench, and Izu-Bonin arc, and each type is summarized and discussed in view of comparison to the on land examples, such as from the Franciscan, Shimanto and Miura-Boso belts in the circum Pacifc margins, and the Ankara. The submarine geologies are present actual examples to give us a critical key to understanding the formation processes and emplacement mechanisms for the so-called mélange bodies, either sedimentary, tectonic or diapiric. Some are made of alternated beds of sandstone and mudstone that show broken or block-in-matrix fashion, in most cases in muddy matrix. These are commonly developed on the trench landward slope toe of the Nankai and Sagami troughs and Boso triple junction area as well as the Japan trench slope. One type is from the landward slope, but another type is from the oceanward slopes. The former type is in places calcareous cemented, probably caused by hydraulic fracturing by high pore pressure along the thrust fault and oxidized methane-made carbonate precipitation. They are seen on the feet of the thrust-dominated slope and to be compared to the so-called sedimentary mélanges due to the gravitational sliding, which occur because of tectonically induced steep slopes. Most of such thrusts are related to large subduction type earthquakes, and await for further critical consideration on to the relation to the asperity problem. Some of large scale gravitational collapses may be related to the seamount or ridge subduction to the trench, both in case of accretionary and non-accretionary type margins, the former is for the examples from the Nankai and Sagami troughs and the Boso triple junction, latter for the Japan trench. In all cases on land and under the sea in the trench landward slope, some calcareous breccias are associated with methane-fluid supported animals within injection or diapiric intrusion. On the other hand, in the Nankai prism and the on land Miura-Boso Peninsulas, many examples of sandy matrix supported mudstone breccia are a result of liquefaction and injection of such coarse-grained clastic fragments during the earthquake shake and subsequent landsliding. Those deposits are faulted, folded and injected in various stages, some before accretionary prism incorporation, some after. Some are of sedimentary origin by gravitational process, others tectonic or diapiric, but in most cases thrust duplexes and complex folds are common. The third and fourth are mélanges including igneous, metamorphic and/or ophiolitic rock blocks. They look similar to the on land examples in the Franciscan, Mineoka (Boso, central Japan) and the Ankara, and used to be attributable to the diapiric origin, as those that have been already known as serpenitine mud volcanoes with metamorphic block at the foot of the Izu-Bonin-Mariana forearc. However, such analogue need careful consideration how the rock association would form to the final emplacement. As the fourth new type, we found an example of deep (1.5 to 2 GPa) metamorphic rock blocks of eclogitic conditions from the fault line in the schistose serpentinite (antigorite-dominated) in the middle part of the Izu arc near the Ohmachi seamount. This implies for the incorporation and exhumation of igneous and metamorphic rocks in the island arc setting, and may give an adequate analogue to the specific mélange formation of the Franciscan, Mineoka and Ankara.

Ogawa, Yujiro; Kawamura, Kiichiro; Tsunogae, Toshiaki; Mori, Ryota; Chiba, Tae; Sasaki, Tomoyuki

2010-05-01

226

Friction and stress coupling on the subduction interfaces  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

227

Trench shoring assembly with corner guard accessory. [trench box  

Microsoft Academic Search

This invention relates to a trench shoring assembly which includes a pair of spaced-apart side walls for vertical disposition within a trench. Spreader pipes and spreader collars interconnect the side walls and allow limited pivotal movement between the side walls. Each side wall includes outer and inner metal plates connected to a horizontally extending hollow top beam which extends longitudinally

1977-01-01

228

2D numerical modelling of intra-oceanic arc extension and trench migration  

NASA Astrophysics Data System (ADS)

Modern intra-oceanic subduction zones often develop series of magmatic chains and basins (Stern et al., 2002), (Takahashi, 2008/09), (Larter 2003) and it is not yet fully understood how and why these structures develop. We performed systematic numerical experiments with a 2D coupled petrological-thermo-mechanical numerical model of an intra-oceanic subduction process. Our model includes different slab push velocities, different weakening effects (dehydration of the subducted crust, aqueous fluid transport, partial melting of both crustal and mantle rocks and melt extraction processes), different ages of the subduction slab and the overriding plate as well as spontaneous slab bending. With a long-term model of subduction dynamics, we tested the effects of geometry, rheology, composition, dehydration and melting processes and their influences to the development of extension or compression in intra-oceanic arcs. Based on numerical results we established three different types of extension regimes: i) extension in the fore-arc, ii) extension within the magmatic arc, and iii) no extension, but compression. In general, in all our experiments, the first spreading episode mostly occurs in the fore-arc. Such fore-arc spreading can be observed in the very eastern part of the Aleutian arc system, where decompression melt pierces the thin crust and/or pushes the magmatic arc away from the trench and it becomes a paleoarc, similar to the Kyushu-Palau Ridge (Stern et al., 2003). At the same time a new magmatic arc grows between the trench and the spreading centre. Some experiments show an intra-arc-spreading, such as the active Mariana arc and the inactive West Mariana Ridge (split an initially homogeneous arc into two distinct parts), (Stern et al., 2003). We also found four different trench migration patterns depending on the degree of coupling between the plates: i) trench retreating, ii) episodic retreating and advancing with a total retreat of the trench, iii) trench advancing, iv) stable trench position over time. Our numerical results on episodic trench movements match well with natural observations (Clark et al., 2008) concerning the periodicity in the back-arc tectonic regimes.

Baitsch Ghirardello, B.; Gerya, T.

2010-12-01

229

Opening and closing slab windows in congested subduction zones  

NASA Astrophysics Data System (ADS)

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

Moresi, Louis

2013-04-01

230

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

231

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

232

Ablative subduction - A two-sided alternative to the conventional subduction model  

NASA Technical Reports Server (NTRS)

The plausibility of a two-sided fluid-based model of lithospheric subduction that is based upon current views of lithospheric structure is examined. In this model the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double-sided, since it is found that even a buoyant plate can be dragged downward by a dense descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. This process, called 'ablative subduction,' makes it possible to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. When experiments modeling the evolution of simple fluid 'slabs' are performed, slab profile is found to be strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow Andean-style profiles.

Tao, Winston C.; O'Connell, Richard J.

1992-01-01

233

Chapter 8 -Trenching and Excavation Excavation Operations  

E-print Network

140 Chapter 8 - Trenching and Excavation Excavation Operations The most serious hazard of trenches: If the trench is 5 feet deep or more, it must be shored or sloped. If there is a possibility of soil movement stress. 7. If a trench is 5 feet deep or more, work should be supervised by an individual knowledgeable

234

Kinematics and flow patterns in deep mantle and upper mantle subduction models: Influence of the mantle depth and slab to mantle viscosity ratio  

Microsoft Academic Search

Three-dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models, an upper mantle model and a deep mantle model, and for various subducting plate\\/mantle viscosity ratios (?SP\\/?M = 59–1375). The models investigate the mantle flow field, geometrical evolution of the slab, sinking kinematics, and relative contributions of subducting plate motion and trench migration to

W. P. Schellart

2008-01-01

235

High-Velocity Frictional Behavior of Incoming Pelagic Sediments to the Tohoku Subduction zone  

NASA Astrophysics Data System (ADS)

The 2011 Tohoku earthquake (Mw 9.0) off the Pacific coast of Japan produced huge slip (~50 m) on the shallow part of the fault close to the toe of the megathrust (e.g., Fujiwara et al., 2011), resulting in destructive tsunamis. Although the multiple causes of such large slip at shallow depths is expected, the frictional property of sediments around the fault, especially at coseismic slip velocities, may significantly contribute to large slip along the fault. We thus investigate the frictional properties of pelagic sediments to be subducting beneath the Tohoku region at high velocities and large displacement toward understanding the rupture processes to cause large slip at the shallow portion of subduction zone. We have conducted friction experiments on incoming pelagic sediment on the Pacific plate (DSDP, Site 436, Leg56, 378 mbsf) that consider as simulated fault gouge. The site locates about 100 km northeast from the Hole C0019E drilled during the IODP Expedition 343 (J-FAST). The sediment contains mainly montmorillonite and its blackish color is quite similar to the sheared sediments in the plate boundary fault recovered during the Expedition 343. Experiments are performed at slip velocities of 2.5 x 10-4 to 1.3 m/s, normal stresses of 0.5 to 2.0 MPa and slip displacement of about 16 m under brine saturated conditions, using a rotary-shear friction apparatus. One gram of gouge was placed between rock cylinders of sandstone or gabbro of 25 mm diameter with Teflon sleeve outside to contain gouge. Both gouge sample and host rock were saturated with brine. At slip velocity of 1.3 m/s and normal stresses of 0.5 to 2.0 MPa, a typical slip weakening behavior is observed; friction coefficient of the sediment rapidly increases 0.1 - 0.3 at the onset of sliding and subsequently decreases to 0.06 - 0.15 over the displacement of > 1 m. However, peak friction and frictional work during slip-weakening (fracture energy) are markedly lower as compared to similar studies conducted on other fault gouge (e.g., Mizoguchi et al., 2007; Ujiie and Tsutsumi, 2010). In addition, steady-state friction coefficient at normal stress of 1 MPa is less than 0.2 over a wide range of slip velocity from 0.25 mm/s to 1.3 m/s. These results suggest that the incoming pelagic sediments to the Japan Trench are energetically very easy for earthquake ruptures to propagate at shallow portion of the Tohoku subduction zone.

Sawai, M.; Hirose, T.

2012-12-01

236

Ophiolite Generation and Emplacement in Subduction Rollback Systems  

NASA Astrophysics Data System (ADS)

Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Oceanic lithosphere created at mid-ocean ridges is generally recycled back into the mantle via subduction, and only rarely do fragments of this MOR lithosphere become accreted into continental margins (ridge-trench collision, accretion margin tectonics). Mantle flow and slab rollback may result in one or more episodes of arc splitting and basin opening, producing a collage of ‘proto-arc and forearc oceanic lithosphere' in suprasubduction zone settings. SSZ Tethyan ophiolites generally have Penrose-type oceanic crust and contain well-developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT (island arc tholeiite) to boninitic (extremely refractory) proto-arc assemblages. The IAT and boninitic magmas were derived from depleted peridotites that had already experienced previous MORB-type melt extraction during the early stages of ophiolite formation in the Tethyan subduction rollback systems. Rapid slab rollback and associated extension in the arc-forearc region caused increasing asthenospheric diapirism and corner flow toward the forearc mantle, resulting in shallow partial melting of the highly refractory harzburgites producing boninitic magmas. Differences in the geochemical evolution of Tethyan ophiolites resulted from variations in their subduction zone geodynamics. The production of increasingly more calc-alkaline rocks in the later stages of the generation of some Tethyan ophiolites suggests higher sediment input into the melting regimes via subduction and hence a longer period of subduction and arc maturity during their SSZ evolution. Emplacement of SSZ ophiolites onto rifted continental margins, particularly in the Tethyan realm, was a result of trench-continent collisions, followed by continental underplating, blueschist metamorphism, and slab breakoff in the downgoing plate.

Furnes, Harald; Dilek, Yildirim

2010-05-01

237

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

238

Early Cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the Franciscan subduction complex  

Microsoft Academic Search

During subduction at the Franciscan trench beginning at 170–160 Ma and continuing to the present, marine sedimentary and lesser volcanic rocks have been underthrust, accreted, and metamorphosed to form the Franciscan accretionary wedge. The South Fork Mountain Schist (SFMS) forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to

Trevor A. Dumitru; John Wakabayashi; James E. Wright; Joseph L. Wooden

2010-01-01

239

Early Cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the Franciscan subduction complex  

Microsoft Academic Search

During subduction at the Franciscan trench beginning at 170-160 Ma and continuing to the present, marine sedimentary and lesser volcanic rocks have been underthrust, accreted, and metamorphosed to form the Franciscan accretionary wedge. The South Fork Mountain Schist (SFMS) forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to

Trevor A. Dumitru; John Wakabayashi; James E. Wright; Joseph L. Wooden

2010-01-01

240

Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system  

E-print Network

of the Ryukyu Arc/trench system, the Luzon Arc, carried by the Philippine Sea Plate (PSP), impacts the south by the Philippine Sea Plate is subducting obliquely beneath the southernmost Ryukyu Margin. Bathymetric swath of the Chinese passive margin by the Luzon Arc indentor has produced a westward bend of the southern Ryukyu Arc

Demouchy, Sylvie

241

Subduction of the South China Sea axial ridge below Luzon (Philippines)  

Microsoft Academic Search

The Scarborough Seamount chain, present at the axis of the extinct South China Sea spreading center, is being subducted obliquely along the Manila Trench. A detailed Seabeam survey of this convergent zone reveals that the fabric of the ridge is characterized by N60°E trending normal faults and N130°E transform faults. This ridge can be traced into the forearc area. This

G. Pautot; C. Rangin

1989-01-01

242

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

243

Control of paleoshorelines by trench forebulge uplift, Loyalty Islands  

NASA Astrophysics Data System (ADS)

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

Dickinson, William R.

2013-07-01

244

Seismic anisotropy, lithospheric deformation, and mantle flow in subduction zones, continental keels, and the core-mantle boundary  

NASA Astrophysics Data System (ADS)

This thesis consists of four chapters that evaluate the location, orientation, and strength of seismic anisotropy in the lithosphere and mantle in several tectonic settings, including two western Pacific subduction zones, the tectonically stable region of eastern North America, and two regions of the core-mantle boundary beneath the Pacific Ocean. The analyses in these chapters not only utilize existing methods, but also develop new, innovative techniques to determine and investigate patterns of shear wave splitting. Beneath northwest Pacific subduction zones, we found evidence for seismic anisotropy to depths as great as 410 km in some regions. In addition, we determined that fast directions are roughly parallel to the direction of absolute Pacific plate motion beneath Izu-Bonin, roughly parallel to the strike of the trench near Japan, and roughly parallel to the direction of transpressional shear in the southern Kurils near Sakhalin Island. In the Marianas subduction zone, we found strong evidence of frequency dependence in fast directions from phases that sample the northwestern portion of the subducting slab, but did not find evidence for frequency dependence in splitting times. All of the data can be explained by models containing anisotropy in the subducting slab and mantle wedge, and possibly anisotropy in the overriding Philippine Sea plate. Beneath eastern North America, we performed measurements of shear wave splitting and combined them with the results of a simple finite-difference model to examine mantle flow around a realistic continental keel. Using this model, we calculated predicted shear wave splitting produced in the mantle resulting from flow around and beneath the keel. We found that splitting produced by modified mantle flow can explain most, but not all, of the shear wave splitting observations in this region. Beneath the central and northern Pacific Oceans, we found evidence for seismic anisotropy within the lowermost mantle (D? ). Beneath the northern Pacific, this anisotropy may be due to lateral flow along the core-mantle boundary induced by lateral spreading of downwelling paleoslab material. Beneath the central Pacific, anisotropy may be due to lateral flow of lower mantle material toward the Hawai'ian plume source.

Fouch, Matthew James

245

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

246

Evidence for crustal thickening and shortening of the overriding plate during incipient plate/plate subduction  

NASA Astrophysics Data System (ADS)

Evidence from a study of the Macquarie Ridge Complex and comparison with other plate boundaries displaying varying development of intraplate compressional structures, indicate that thickening and shorting of the overriding plate occur during incipient subduction. The Macquarie Ridge Complex has been produced by a tectonic history oblique compression between the Indian/Australian and Pacific plates at their common boundary south of New Zealand. In the central Macquarie Ridge Complex intraplate transpressive forces have resulted in the uplift of oceanic crust of the Indian/Australian plate to form the Macquarie Ridge and incipient subduction of the Pacific oceanic plate at the Macquarie Trench. The uplift has been effected by thickening and shortening of the oceanic crust of the overriding Indian/Australian plate accompanied by shallow seismicity at the ridge, whereas the Pacific plate at the trench has not been thickened. Post-1964 earthquakes of the Macquarie Ridge Complex form a single narrow band some tens of kilometres wide. In the central Macquarie Ridge Complex the band of seismicity is confined to the ridge and not the trench, suggesting that crustal thickening rather than subduction is the preferred style of tectonism for incipient subduction zones. Fault plane solutions indicate dextral motion that is consistent with anticlockwise rotation of the Pacific plate relative to the Australian plate, and with transpression and incipient subduction in the central segment of the Complex.

Williamson, P. E.; Jones, T. D.; McCue, K. F.

1989-04-01

247

Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia  

NASA Astrophysics Data System (ADS)

the subduction processes along the Sunda Trench requires detailed constraints on the subducting lithosphere. We build a detailed tectonic map of the Wharton Basin based on reinterpretation of satellite-derived gravity anomalies and marine magnetic anomalies. The Wharton Basin is characterized by a fossil ridge, dated ~36.5 Ma, offset by N-S fracture zones. Magnetic anomalies 18 to 34 (38-84 Ma) are identified on both flanks, although a large part of the basin has been subducted. We analyze the past plate kinematic evolution of the Wharton Basin by two-plate (India-Australia) and three-plate (India-Australia-Antarctica) reconstructions. Despite the diffuse plate boundaries within the Indo-Australian plate for the last 20 Ma, we obtain finite rotation parameters that we apply to reconstruct the subducted Wharton Basin and constrain the thickness, buoyancy, and rheology of the subducting plate. The lower subductability of younger lithosphere off Sumatra has important consequences on the morphology, with a shallower trench, forearc islands, and a significant inward deviation of the subduction system. This deviation decreases in the youngest area, where the Wharton fossil spreading center enters subduction: The discontinuous magmatic crust and serpentinized upper mantle, consequences of the slow spreading rates at which this area was formed, weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism. Deeper effects include the possible creation of asthenospheric windows beneath the Andaman Sea, in relation to the long-offset fracture zones, and east of 105°E, as a result of subduction of the spreading center.

Jacob, Jensen; Dyment, Jérôme; Yatheesh, V.

2014-01-01

248

Bulletin of the Seismological Society of America, Vol. 76, No. 3, pp. 675-708, June 1986 SOURCE CHARACTERISTICS OF HYPOTHETICAL SUBDUCTION  

E-print Network

to the Cascadia subduction zone are used to hypothesize the nature of shallow sub- duction earthquakes that might to the Cascadia subduction zone are those in southern Chile, southwestern Japan, and Colombia. These zones have all experienced very large earthquake sequences, and if the Cascadia subduction zone is also capable

Greer, Julia R.

249

Exploring a Link Between Great and Giant Megathrust Earthquakes and Relative Thickness of Sediment and Eroded Debris in the Subduction Channel to Roughness of Subducted Relief  

NASA Astrophysics Data System (ADS)

SEDIMENT SUBDUCTION AND SEISMICITY: At sediment-nourished (>1-2 km) subduction zones (SZs), instrumentally recorded great and giant earthquakes (Eqs) plus the geologically well-vetted Cascadia's Mw9.0 of 1700, corresponds to the occurrence of: 52 percent of Mw8.0 and larger (12 of 23), 57 percent of Mw8.3 and larger (8 of 14), 67 percent of Mw8.5 and larger (8 of 12), 71 percent of Mw8.8 and larger (5 of 7), 67 percent of Mw9.0 and larger (4 of 6), and 100 percent of Eqs larger than Mw9.0 (3 of 3). The implications of this observation were first explored by Ruff (1989, Pure and Applied Geophysics, v. 129, p. 263-282). SIGNIFICANT EXCEPTIONS: A significant percentage of powerful megathrust Eqs have nucleated at poorly sedimented trenches (<1.0-0.5 km). For example, the 1952 Kamchatka Mw9.0 event, and the horrendous March 11, 2011 Tohoku-Oki Mw9.0 megathrust. Other great megathrust ruptures have also torn the Peru-Chile SZ opposite the sediment-starved trench sectors of southern Peru (2001 Mw8.4) and northern Chile (1922 Mw8.3), and probably at higher magnitudes in the pre-instrumental 19th century. RELATIVE THICKNESS AND THE SMOOTHNESS CONJECTURE: Ruff (1989) surmised that excess sediment (i.e., at least 1-2 km thick) entering a subduction zone constructs a laterally homogenous layer within the plate-separating subduction channel where seismogenic rupturing occurs. The layer of subducted sediment works to even the trench-parallel distribution of coupling strength (asperities) by smothering the roughness or rugosity of subducted sea-floor relief. In doing so sediment subduction contributes to the lengthy (>300-500 km) rupturing of great and giant megathrust Eqs. Larger Mw Eqs do not correlate with increasing thickness of subducted sediment. We add here the observation that great and giant Eqs occurring along sediment-poor SZs are typically underthrust by wide sectors of seafloor of low average bathymetric relief, in particular that caused by seamounts and fracture zones. These SZs also exhibit evidence of active basal subduction erosion and the progressive tilting of the margin downward toward the trench and the consequent shedding of avalanche and slide debris to the trench floor. Pilling of material along the landward side of trench buries underthrusting relief, including that of horst and graben bathymetry. Frontal subduction erosion and sediment subduction convey this material into the subduction channel. As documented by von Huene et al (1994, JGR v. 99, n. B11, p. 22, 349), these inferred megathrust-conditioning processes and an exceptionally smooth underthrusting oceanic plate are applicable to the Tohoku-Oki rupture area. We conjecture that at sediment-poor SZs tectonic erosion and mass wasting can thicken the subduction channel adequately to overwhelm and smother low underthrusting bathymetric roughness and thus contribute favorably to lengthy megathrusting rupturing.

Scholl, D. W.; Kirby, S. H.; von Huene, R.

2011-12-01

250

Subducted sedimentary serpentinite mélanges: Record of multiple burial-exhumation cycles and subduction erosion  

NASA Astrophysics Data System (ADS)

Serpentinite matrix mélanges give insight into large-scale convergent plate margin processes, particularly because of the derivation of the serpentinite from oceanic mantle. Similar to shale-matrix mélanges, a field geologist may easily recognize the sedimentary origins of little-deformed serpentinite matrix mélanges, but mélanges within accretionary prisms have undergone significant deformation and recrystallization of matrix. Serpentinite mélanges of the Franciscan subduction complex of California have a seemingly intact and foliated matrix. Such exposures contrast sharply with the granular undeformed sedimentary serpentinite mélanges of the coeval Great Valley Group (GVG) forearc basin deposits that depositionally overlie Coast Range Ophiolite (that structurally overlies the Franciscan). Nonetheless, Franciscan serpentinite mélanges display evidence of sedimentary origins, including sedimentary breccia composed of exotic block material (Tolay Ridge), sedimentary serpentinite breccia (Panoche Pass Road), basal serpentinite conglomerate with exotic clasts (Sunol Regional Wilderness), and serpentinite sandstones and conglomerates, including a basal conglomerate overlying coherent metagraywacke (Tiburon Peninsula). These examples record two burial-exhumation cycles to blueschist facies depths. In addition, a mélange/breccia in the Panoche Pass area may have components that record three burial-exhumation cycles to blueschist (or greater) depth. Exhumation rates for various cycles ranged from about 1.2 to 10 mm/year. The Tiburon Peninsula serpentinite mélange occupies the structurally highest horizon in the Franciscan of the San Francisco Bay area, and regional field relationships indicate deposition at ca. 100 Ma. Apparently, about 65 Ma of subduction erosion/non accretion followed initiation of Franciscan subduction in this region. The oldest Franciscan serpentinite mélanges are at least 35 Ma younger than sedimentary serpentinites of the GVG. Subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench. Evidence of multiple-burial exhumation cycles indicates reworking of subduction complex material, consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages.

Wakabayashi, John

2012-09-01

251

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

NASA Astrophysics Data System (ADS)

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

Nakanishi, Masao; Hashimoto, Jun

2011-12-01

252

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

253

Tectonics of the hellenic trench: A synthesis of sea-beam and submersible observations  

NASA Astrophysics Data System (ADS)

Detailed bathymetric investigations were performed in four zones of the Hellenic trench system in 1978, using the multi-narrow beam echo-sounder "Sea-Beam" on R.V. "Jean Charcot". In 1979, a field-study using the 3000 m submersible "Cyana" was conducted in three of the four sites covered by Sea-Beam. We summarize here the principal tectonic results of these studies and publish the corresponding Sea-Beam maps. It is shown that compressional structures affect the sedimentary cover of the outer wall and trench whereas extensional tectonics is present on the inner wall. In addition, there is evidence that most of the upper sedimentary cover is not subducted but rather piles up in front of the trench to form the Mediterranean ridge. We suggest that the surface limit of the zone of mechanical decoupling occurs near the base of the inner wall. We show that the gross structure of the trench as well as the strain pattern observed is controlled by the kinematics of subduction and we discuss its evolution since its initiation about 13 m.y. ago.

Huchon, Philippe; Lybéris, Nicolas; Angelier, Jacques; Le Pichon, Xavier; Renard, Vincent

1982-06-01

254

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

255

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

256

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

257

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

258

Deflection of Mantle Flow Around Subducting Slabs: The Effect of the Viscous Lower Mantle on Subslab Seismic Anisotropy  

NASA Astrophysics Data System (ADS)

Global compilations of subslab shear wave splitting parameters indicate that seismic anisotropy beneath slabs is dominated by trench-parallel fast splitting directions in the vast majority of subduction zones worldwide. These observations suggest that flow fields within the subslab mantle often have strong components out of the plane of the classic two-dimensional corner flow subduction zone models. Three-dimensional regional geodynamics models demonstrate that the subslab mantle flow field is determined primarily by interactions among background mantle flow, trench migration, and slab geometry. We studied this interaction numerically in a trench-fixed reference frame by calculating a net background flow field that combines the ambient mantle flow due to large-scale mantle circulation with trench migration velocities. When the slab is decoupled from the mantle, trench-parallel subslab flow is produced not only in systems with a net trench-parallel background flow but also in many systems with trench-perpendicular net background mantle flow. In this case trench-parallel subslab flow occurs when the flow is redirected around the edges of the slab, which acts as an obstruction to background mantle flow. The redirection of the flow path depends on the dip and length of the slab, as well as the viscosity of the lower mantle. Subduction zones with steeper dip angles and deeper slabs promote trench-parallel flow by causing a more efficient obstruction to flow. The amount of trench-parallel flow in the subslab decreases if alternative escape routes exist for the flow deflected by the slab, such as in the case of shorter slabs where the mantle can easily flow under the slab. Allowing mantle flow to proceed into the lower mantle may provide an alternative escape route and decrease trench-parallel subslab flow. However, this system depends on the viscosity of the lower mantle. Our numerical models show that a lower mantle viscosity of thirty times that of the upper mantle, in agreement with global mantle convection simulation and analyses of geoid and postglacial rebound, is sufficient to inhibit the redirected flow under the slab and produce the trench-parallel subslab flow directions observed for the majority of subduction zones globally.

Paczkowski, K. A.; Montesi, L. G.; Long, M. D.; Thissen, C.

2013-12-01

259

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

260

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

261

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

262

Subduction erosion along the North Chile margin  

NASA Astrophysics Data System (ADS)

Swathbathymetry off Antofagasta reveals many tectonic features of the north ChileTrench. Since sediment on the oceanic plate is only ˜100 m thick, and since the continental slopeis essentially eroded crystalline crust, basement rock is commonly near the sea floor. Therefore,high resolution bathymetry is a good indicator of basement structure. The narrow shelf and upperslope are characterized by canyons crossed locally by extensional faults. Mejillones Ridge ispositioned anomalously seaward of the coastal Cordillera and its emergent crest forms theMejillones Peninsula. The flanks of this 100 km long uplifted feature are characterized by manynormal faults. Normal faults partition the middle slope into blocks obscured by a downslopemobile blanket of debris thick enough to mute all but the major basement structure. In the middleslope, coherent detached blocks disintegrate rapidly by mass wasting. The lower slope is slidedebris pushed into low ridges and troughs by the well developed subducting horst and grabentopography of the oceanic plate. Detritus and sediment moves downslope into graben, levels thesubducting plate, and is pushed up over small ridges. Sediment and debris piled against the base ofthe slope as the plates converge is underthrust with the subducting plate. Compressional featuresare confined to the lower quarter of the slope and are isolated from extension of the middle andupper slope. 1999 Elsevier Science Ltd. All rights reserved

Huene, R. von; Weinrebe, W.; Heeren, F.

1999-02-01

263

Three-dimensional thermal structure of subduction zones: effects of obliquity and curvature  

NASA Astrophysics Data System (ADS)

Quantifying the precise thermal structure of subduction zones is essential for understanding the nature of metamorphic dehydration reactions, arc volcanism, and intermediate depth seismicity. High resolution two-dimensional (2-D) models have shown that the rheology of the mantle wedge plays a critical role and establishes strong temperature gradients in the slab. The influence of three-dimensional (3-D) subduction zone geometry on thermal structure is however not yet well characterized. A common assumption for 2-D models is that the cross-section is taken normal to the strike of the trench with a corresponding velocity reduction in the case of oblique subduction, rather than taken parallel to velocity. A comparison between a full 3-D Cartesian model with oblique subduction and selected 2-D cross-sections demonstrates that the trench-normal cross-section provides a better reproduction of the slab thermal structure than the velocity-parallel cross-section. An exception is found in the case of a strongly curved trench, such as in the Marianas, where strong 3-D flow in the mantle wedge is generated. In this case it is shown that the full 3-D model should be evaluated for an accurate prediction of the slab thermal structure. The models demonstrate that the use of a dynamic slab and wedge, separated by a kinematic boundary, yields good results for describing slab velocities in 3-D.

Bengtson, A. K.; van Keken, P. E.

2012-11-01

264

Numerical modeling on the source parameter scaling relations and synthetic surface deformation of episodic slow slip events in subduction zones  

NASA Astrophysics Data System (ADS)

A spectrum of slow slip phenomena in subduction zones, ranging from low-frequency earthquakes to long-term slow slip events (SSE), demonstrates a linear relation between event moment M0 and duration T spanning 7 to 8 orders of magnitude in each dimension [Ide et al., Nature, 2007; Gao et al., BSSA, 2012]. However, such a linear relation is not clearly established for each type of slow phenomena. For example, slow earthquakes beneath the Kii Peninsula in western Japan show M0 ~ T1.5 [Ide et al., GRL, 2008]. Except for the episodic SSEs in Cascadia, SSEs in other subduction zones have more scattered M0 -T relation. Understanding this scaling relation is thus key to the study of the physical mechanism of SSEs and why they are different from regular earthquakes. Here we set up a 3D planar Cascadia subduction fault model using gabbro rate-state friction parameters and incorporating near-lithostatic pore pressure around the velocity-weakening to strengthening friction stability transition at depths between 30 and 40 km. SSEs appear at the transitional depths and repeat roughly every year. Complex along-strike migration patterns are also modeled by introducing small (<5%) along-strike perturbations in frictional parameters. By identifying individual episodes of SSEs using a slip velocity threshold, we quantify the following SSE source properties, along-strike propagation length, total duration, equivalent moment, stress drop, migration speed, derive their scaling relations and compare to the observations summarized by Gao et al. [2012]. Our modeled SSEs have nearly constant stress drops between 0.001 and 0.01 MPa, due to the small effective normal stress on the order of 1 MPa in the SSE region. They have moment M0 between 1017.5 to 1019.5 Nm and duration T between 0.1 to 1 year, and the relation is best described by M0 ~ T1.8 for about 70 episodes recorded in 20 years along the 1000 km long margin. More simulation cases are conducted to systematically investigate how the moment-duration relation may be dependent on friction parameters and pore pressure condition. We also compute the resulted surface deformation (synthetic GPS time series) due to the spatiotemporal evolution of slip velocity on the subduction fault. Preliminary modeling results show that the along-strike segmentation in Cascadia SSEs may result from the slip velocity variation as SSEs propagate along the trench. Comparison between the spectra of modeled synthetic displacement and the Plate Boundary Observatory GPS time series along Cascadia will be conducted, to determine the detection threshold of the synthetic signal.

Liu, Y.

2013-12-01

265

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

266

Tonga Trench gabbros and peridotites: A suit of temporal and spatial forearc materials  

NASA Astrophysics Data System (ADS)

The Tonga trench is one of the deepest oceanic regions in the world (10,866 m). Various types of rocks have been dredged and drilled at several localities on the landward slopes of the trench during Boomerang Leg8 in 1996. In particular, very pristine peridotites outcrop at the most deep landward trench slope. We show that the trench can be divided into two regions: southern region and northern region. The peridotites in the southern region have high-Cr# (0.46-0.83) which were typical of forearc peridotites, whereas the peridotites in the northern region have evidences of the reaction with magma during partial melting. Olivine fabrics are characterized by E-type and D-type. Although E-type and D-type are no clear relationship of mineral composition, grain size and equilibrium temperature, the only difference between E-type and D-type were fabric intensities: D-type has higher fabric intensity than that for E-type. Geochronological study revealed that the gabbros in the southern region have the oldest ages of ~52 Ma that are as old as the oldest rocks in the Izu-Bonin-Mariana arc recording the subduction infancy (e.g., Ishizuka et al., 2011 EPSL). We argue that the southern region preserves the oldest mantle fabrics that took place during the subduction infancy, where pristine and serpentinized peridotites have been deformed in the region where high strain field occurred due to the dragged flow. Eventually, they expose in a very neat condition (i.e. active tectonic erosion and fast ascent rate) resulting from an unique tectonic setting including fast subducting plate (24 cm/yr), fast spreading plate (15 cm/yr) and slab rollback.

Michibayashi, K.; Shinkai, Y.; Tani, K.; Uehara, S.; Harigane, Y.; Ishii, T.; Bloomer, S. H.

2012-12-01

267

The analyses of P-wave velocity structures in the Manila subduction system  

NASA Astrophysics Data System (ADS)

During the TAIGER project in 2009, we deployed 260 OBSs around offshore Taiwan, and recorded the refracted wave from the deep crust. The seismic source was provided by a 6000 cu-in airgun array from R/V Langseth. During leg 1 and 4 of TAIGER experiment, 3 OBS lines, T1 (consists of MGL0908_05 and MGL0908_07), T2 (consists of MGL0908_01 and MGL0908_09), and MGL0905_27, were shot in the Manila subduction system. We picked OBS first arrivals and inversed the P-wave velocity model to demonstrate the major feactures such as the Manila subduction zone, Luzon arc and part of West Philippine Basin. In the results, the oceanic crust thickness of the northeast part of the South China Sea was identified which is about 10-12 km thick in west of the LRTPB (Luzon-Ryukyu Transform Plate Boundary). But the thickness of the crust thins across the LRTPB to Manila Trench (6-8km), and start to subduct into the Philippine Sea Plate beneath the Manila Trench. Both sides of the Gagua Ridge are the oceanic crust, and their thickness is about 8 km. Even the P-wave velocity model cannot demonstrates the subducting angle very well along Manila Trench, but the depth of the Moho can be showed about 16 km at west of the Manila Trench, and about 25 km deep beneath the accretionary prism. In addition, we also used the earthquake data from CWB, IRIS, and Philippine. The subduction zone of the Benioff zone can be extended up to 100 km. Several splay faults and out-of-sequence thrusts (OOST) are also detected from multi-beam echo sounder and multi-channel seismic data. The stress of the splay faults and the OOST increases from collision and compression. If the stress accumulates for a long time, the mega earthquake and tsunami probably may be occurred.

Liang, C.; Chen, H.; Wu, H.; Lee, C.

2011-12-01

268

'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

269

New insights on the tectonics along the New Hebrides subduction zone based on GPS results  

NASA Astrophysics Data System (ADS)

At the New Hebrides (NH) subduction zone, ridges born by the subducting Australia plate enter the trench and collide with the overriding margin. Results from GPS surveys conducted on both sides of the trench and new bathymetry maps of the NH archipelago bring new light on the complex tectonics of this area. Convergence vectors present large variations that are not explained by Australia/Pacific (A/P) poles and that define four segments. Vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors. Slow convergence (i.e., 30-40 mm/yr) is found at the central segment facing the D'Entrecasteaux Ridge. The southern segment moves faster than A/P motion predicts (89 to 124 mm/yr). Relatively to a western North Fiji basin (WNFB) reference, the northern and southern segments rotate in opposite directions, consistently with the extension observed in the troughs east of both segments. Both rotations combine in Central Vanuatu into an eastward translation that "bulldozes" the central segment into the WNFB at ˜55 mm/yr. That model suggests that the motion of the central segment, forced by the subduction/collision of the D'Entrecasteaux ridge, influences the motion of the adjoining segments. The New Caledonia archipelago is motionless with respect to the rest of the Australia plate despite the incipient interaction between the Loyalty ridge and the NH margin. Southeast of the interaction area, convergence is partitioned into a ˜50 mm/yr trench-normal component accommodated at the trench and a ˜90 mm/yr trench-parallel component, close to the A/P convergence, and presumably accommodated by a transform boundary at the rear of the NH arc.

Calmant, StéPhane; Pelletier, Bernard; Lebellegard, Pierre; Bevis, Michael; Taylor, Frederick W.; Phillips, David A.

2003-06-01

270

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

271

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

272

The distinct hydrogeological system of the forearc of the Middle America Trench: significance for long-term tectonics and updip limit of the interplate seismogenic zone  

Microsoft Academic Search

The distribution and flow of fluid has been widely studied at accretionary prisms, but at convergent margins where tectonic erosion affects overriding plates fluid distribution and tectonics are far less understood. Observations along the erosional subduction zone of Middle America Trench indicates a hydrogeological system distinctly different from those that have been described at accretionary prisms. The hydrogeological system has

C. R. Ranero; I. Grevemeyer; H. Sahling; U. Barckhausen; C. Hensen; K. Wallmann; W. Weinrebe

2008-01-01

273

Pacific trench motions controlled by the asymmetric plate configuration  

NASA Astrophysics Data System (ADS)

We present a novel explanation for absolute trench-normal motions of slabs surrounding the Pacific. Rapid subduction-zone retreat on the eastern side of the Pacific and slow advance in the west can result from the large-scale asymmetric plate configuration. We use simple fluid dynamics to explain the mechanical background of this hypothesis, and we use the results of a simple finite difference scheme to estimate the effect on trench motion velocities. The hypothesis is based on two key assumptions. First, we follow the concept of plate-scale horizontal counterflow in the asthenosphere driven by accretion of asthenosphere into lithosphere and by plate motion. Second, we assume that horizontally wide slabs without large slab windows drift passively in the mantle flow field and do not retreat as a result of flow around the slab. If the asthenosphere transfers flow-related horizontal shear stress into deeper levels of the mantle, an asymmetry in the plate configuration leads to different net pressure forces on the two slabs and thus affects the retreat behavior. In an ocean with an asymmetric ridge position, the slab of the smaller plate should retreat faster than the slab of the large plate, which may even advance. Also, the domain of a slower moving plate should collapse faster than the domain of the faster plate. Our model explains the counterintuitive negative correlation between slab age and retreat velocity observed in the Pacific. It also accords with the topographic asymmetry of the ridge flanks along the Pacific rise.

Nagel, Thorsten J.; Ryan, William B. F.; Malinverno, Alberto; Buck, W. Roger

2008-06-01

274

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

275

The 1994 Java tsunami earthquake: Slip over a subducting seamount  

NASA Astrophysics Data System (ADS)

On June 2, 1994, a large subduction thrust earthquake (Ms 7.2) produced a devastating tsunami on the island of Java. This earthquake had a number of unusual characteristics. It was the first recorded large thrust earthquake on the Java subduction zone. All of the aftershock mechanisms exhibit normal faulting; no mechanisms are similar to the main shock. Also, the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the main shock and the aftershocks. We then invert long-period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip. A dip of 12°, hypocenter depth of 16 km and moment of 3.5×l020 N m (Mw 7.6) give the best fit to the combined seismic data and are consistent with the plate interface geometry. The source spectrum obtained from both body and surface waves has a single corner frequency (between 10 and 20 mHz) implying a stress drop of ˜0.3 MPa. The main energy release was preceded by a small subevent lasting ˜12 s. The main slip occurred at ˜20 km depth, downdip and to the NW of the hypocenter. This area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount. We interpret the Java earthquake as slip over this subducting seamount, which is a locked patch in an otherwise decoupled subduction zone. We find no evidence for slow, shallow rupture. No thrust aftershocks are expected if the entire locked zone slipped during the main shock, but extension of the subducting plate behind the seamount would promote normal faulting as observed. It seems probable that such a source model could also explain the size and timing of the observed tsunami.

Abercrombie, Rachel E.; Antolik, Michael; Felzer, Karen; EkströM, GöRan

2001-04-01

276

Subduction of the Bougainville seamount (Vanuatu): mechanical and geodynamic implications  

NASA Astrophysics Data System (ADS)

New bathymetric data gathered during a Seabeam survey (SEAPSO cruise, leg 1) enable us to re-examine the flexural response of the oceanic lithosphere subducting under the New Hebrides (Vanuatu) island arc. The Bougainville seamount and Sabine bank are interpreted as immerged fossil atolls, the recent subsidence of which is related to the subduction of the oceanic lithosphere. The position and altitude of the different fossil atolls which belong to the d'Entrecasteaux or Loyalty ridges are in good agreement with predictions of elastic flexure of the lithosphere. We deduce an average effective elastic thickness of the lithosphere of about 22 km for the area under study. This value is slightly smaller than the one corresponding to the lithospheric age as given by the magnetic anomalies, but is in good agreement with the age after a correction for thermal rejuvenation. This assumption of a thermal rejuvenation of the North Loyalty basin is also supported by previously reported high heat-flow values and attenuation of Sn seismic waves. However, the location and depth of the trench in front of the North Loyalty basin do not agree with the model which fits the other data. This discrepancy is interpreted as the result of variations of the value of Pb (vertical force per unit length of trench applied at the edge of the plate) along the subduction zone. Such variations may be related to the length of the subducted slab, which is shorter in front of the d'Entrecasteaux and Loyalty ridges than in front of the North Loyalty basin, according to the hypocentral distribution of earthquakes and a tomographic image of the slab.

Dubois, J.; Deplus, C.; Diament, M.; Daniel, J.; Collot, J.-Y.

1988-06-01

277

New constraints on Aegean subduction from SEASAT altimetry  

NASA Astrophysics Data System (ADS)

Profiles of crustal structure for the Aegean fore arc area are modeled according to geoid undulations and free air gravity anomalies, taking into account the subducted slab. The first profile, drawn across the eastern (convergent) branch of the arc, exhibits a large amount of subducted crustal material and a local trough in the Moho under the so-called Mediterranean Ridge superimposed on a long wavelength flexure increasing toward the trench. The Moho topography suggests a lithosphere with no rigidity below the Mediterranean Ridge, while the flexure associated with the subduction implies an elastic thickness of 80 km. This discrepancy leads the authors to assume the presence of an additional load acting downward below the Mediterranean Ridge. On the second profile, drawn across the eastern (strike-slip) branch, there is a deep trough in the Moho, far from the topographic trench. This Moho deepening represents a large isostatic unbalance and requires an active compensation mechanism. Thus, both profiles (and especially the eastern one, where the strike-slip dominant motion does not allow a proper subduction) require a downward force acting seaward of the arc. On the latter branch, it is proposed that gravitational instability induces the growth of a cold bulb at the base of the old and dense African lithosphere. It is noted that, through viscous coupling with the top of the lithosphere, a 115-km bulb can produce an important collapse of the Moho and that the resulting gravitational anomalies are comparable to the observations. The growth of such a cold lithospheric bulb may be the first stage of a southward jump of the Aegean arc.

Genthon, P.; Souriau, M.

1987-01-01

278

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

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

279

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

280

Effect of Sediments on Rupture Dynamics of Shallow Subduction Zone Earthquakes and Tsunami Generation  

NASA Astrophysics Data System (ADS)

Low-velocity fault zones have long been recognized for crustal earthquakes by using fault-zone trapped waves and geodetic observations on land. However, the most pronounced low-velocity fault zones are probably in the subduction zones where sediments on the seafloor are being continuously subducted. In this study I focus on shallow subduction zone earthquakes; these earthquakes pose a serious threat to human society in their ability in generating large tsunamis. Numerous observations indicate that these earthquakes have unusually long rupture durations, low rupture velocities, and/or small stress drops near the trench. However, the underlying physics is unclear. I will use dynamic rupture simulations with a finite-element method to investigate the dynamic stress evolution on faults induced by both sediments and free surface, and its relations with rupture velocity and slip. I will also explore the effect of off-fault yielding of sediments on the rupture characteristics and seafloor deformation. As shown in Ma and Beroza (2008), the more compliant hanging wall combined with free surface greatly increases the strength drop and slip near the trench. Sediments in the subduction zone likely have a significant role in the rupture dynamics of shallow subduction zone earthquakes and tsunami generation.

Ma, S.

2011-12-01

281

Three-dimensional thermal structure of subduction zones: effects of obliquity and curvature  

NASA Astrophysics Data System (ADS)

Quantifying the precise thermal structure of subduction zones is essential for understanding the nature of metamorphic dehydration reactions, arc volcanism, and intermediate depth seismicity. High resolution two-dimensional (2-D) models have shown that the rheology of the mantle wedge plays a critical role and establishes strong temperature gradients in the slab. The influence of three-dimensional (3-D) subduction zone geometry on thermal structure is however not yet well characterized. A common assumption for 2-D models is that the cross-section is taken normal to the strike of the trench with a corresponding velocity reduction in the case of oblique subduction, rather than taken parallel to velocity. A comparison between a full 3-D Cartesian model with oblique subduction and selected 2-D cross-sections demonstrates that the trench-normal cross-section provides a better reproduction of the slab thermal structure than the velocity-parallel cross-section. An exception is found in the case of strongly curved subduction, such as in the Marianas, where strong 3-D flow in the mantle wedge is generated. In this case it is shown that the full 3-D model should be evaluated for an accurate prediction of the slab thermal structure.

Bengtson, A. K.; van Keken, P. E.

2012-07-01

282

Archean Subduction: Fact or Fiction?  

NASA Astrophysics Data System (ADS)

Subduction drives plate tectonics and builds continental crust, and as such is one of the most important processes for shaping the present-day Earth. Here we review both theory and observations for the viability and style of Archean subduction. High Archean mantle temperature gave low mantle viscosity and affected plate strength and plate buoyancy. This resulted in slower or intermittent subduction, either of which resulted in Earth cooling profiles that fit available data. Some geological observations are interpreted as subduction related, including an “arc” signature in various igneous rocks (suggesting burial of surface material to depths of 50-100 km), structural thrust belts and dipping seismic reflectors, and high-pressure-low-temperature and low-pressure-high-temperature paired metamorphic belts. Combined geodynamical and geochemical evidence suggests that subduction operated in the Archean, although not, as often assumed, as shallow flat subduction. Instead, subduction was more episodic in nature, with more intermittent plate motion than in the Phanerozoic.

van Hunen, Jeroen; Moyen, Jean-François

2012-05-01

283

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  

Microsoft Academic Search

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

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

2010-01-01

284

Development of the accretionary prism along Peru and material flux after subduction of Nazca Ridge  

NASA Astrophysics Data System (ADS)

Results from Ocean Drilling Program Leg 112 indicated tectonic erosion of the Peruvian convergent margin during subduction of the Nazca Ridge followed by renewed accretion against the erosional scar. Seismic images and swath mapping morphology show a regional geology shaped as subduction of Nazca Ridge migrated 800 km along the Peru Trench. The accretionary prism is imaged at two stages of development since upper Miocene time with depth-migrated seismic records in which the ``back stop'' is well defined. From these data, material flux can be quantified. After the ridge crest subducted and accretion dominated over erosion, the prism grew rapidly to 10- to 15-km width. During rapid growth, the margin taper was large and about 60 percent of the sediment supply accreted. As the prism growth slowed, only 30 percent accreted despite an increased trench sediment supply. Since the convergence rate changed little, the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted. The decreased prism growth may be partially controlled by varying the structure of the back stop. The force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary. Thus the shear stress transmitted across the plate boundary is insufficient to drive ``out-of-sequence'' thrust faults that would thicken the back stop or raise it beyond a limited height. A constant back top height limits accretion, and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop. Nazca Ridge subduction affected the convergent margin mass flux for about 8 Ma. Mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest. When growth of the accretionary prism slowed, a greater terrigenous input through the trench was accommodated by sediment subduction. The resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling.

von Huene, R.; Pecher, I. A.; Gutscher, M.-A.

285

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

NASA Astrophysics Data System (ADS)

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

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

2002-12-01

286

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

USGS Publications Warehouse

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

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

2009-01-01

287

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

PubMed

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

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

2011-12-01

288

Jurassic accretion tectonics of Japan  

Microsoft Academic Search

The Jurassic accretionary complex and coeval granites in Japan represent remnants of the Jurassic arc-trench system developed between the Asian continent and Pacific Ocean. The Jurassic accretionary complex occurs as a large-scale nappe that is tectonically sandwiched be- tween the overlying pre-Jurassic nappes and underlying post-Jurassic nappes. By virtue of new research styles (microfossil mapping and chronometric mapping) the following

Yukio Isozaki

1997-01-01

289

Trenching in the New Madrid seismic zone  

SciTech Connect

Trenching studies of the San Andreas fault have been of great value to geologists in California for determining not only the prehistoric occurrences of earthquakes on the fault but also the age of these movements. In the New Madrid seismic zone, US Geological Survey scientists have been trenching across suspected faults to try to assess earthquake frequency in the Central US. The following photographs document these trenching studies.

Not Available

1990-01-01

290

Tectonic erosion along the Japan and Peru convergent margins  

E-print Network

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

Demouchy, Sylvie

291

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

292

How much, where and why? On the diversity of large earthquakes across subduction zones, including "silent"ones  

NASA Astrophysics Data System (ADS)

The December 2004 Sumatra earthquake, the largest in 40 years, gives new insights into the largest subduction zone earthquakes. Earlier ideas about where such earthquakes can occur were based on convergence rate and age of the subducting lithosphere, and to some extent, trench sediment thickness, with mega-events occurring only where young plates (preferably bearing large sedimentary loads) subduct fast. We analyze critically an improved dataset reflecting better known convergence rates (for example, Nazca-South America significantly slower than previously assumed), and reassessed earthquake moments (including recently "discovered" events such as the 1700 Juan de Fuca mega-earthquake, and excluding some large trench earthquakes which turn out to be normal faults), with much of the correlation between event size and age/speed vanishing. In addition, we find that the size of the largest trench earthquakes and seismic coupling fractions depend at best weakly on trench sediment thickness. Taking further into account the concept of rupture variability pioneered in Nankai by Ando (1975), and more recently documented in Chile by Cisternas et al. (2005), this suggests that subduction zones perceived as "silent" or rupturing at relatively moderate levels (M.le.8) could occasionally feature much larger events on the mega earthquake scale, a scenario examplified by the Sumatra event, but potentially applicable to the Makran and Tonga systems, and raising questions in environements such as Java, the Lesser Antilles or the Marianas. In apparently silent subduction systems, the recent 2006 Java "tsunami earthquake" also reopens the debate on the regional distribution of these events characterized by extremely slow rupture, and of their supplementary -- or complementary -- role in the subduction process.

Stein, S.; Okal, E. A.

2006-12-01

293

Slab-Dip Variability and Trench-Parallel Flow beneath Non-Uniform Overriding Plates: Insights form 3D Numerical Models  

NASA Astrophysics Data System (ADS)

Forces driving plate tectonics are reasonably well known but some factors controlling the dynamics and the geometry of subduction processes are still poorly understood. The effect of the thermal state of the subducting and overriding plates on the slab dip have been systematically studied in previous works by means of 2D and 3D numerical modeling. These models showed that kinematically-driven slabs subducting under a cold overriding plate are affected by an increased hydrodynamic suction, due to the lower temperature of the mantle wedge, which leads to a lower subduction angle, and eventually to the formation of flat slab segments. In these models the subduction is achieved by imposing a constant velocity at the top of the overriding plate, which may lead to unrealistic results. Here we present the results of 3D non-Newtonian thermo-mechanical numerical models, considering a dynamically-driven self-sustained subduction, to test the influence of a non-uniform overriding plate. Variations of the thermal state of the overriding plate along the trench cause variation in the hydrodynamic suction, which lead to variations of the slab dip along strike (Fig. 1) and a significant trench-parallel flow. When the material can flow around the edges of the slab, through the addition of lateral plates, the trench parallel flow is enhanced (Fig. 2), whereas the variations on the slab dip are diminished.; Effect of a non-uniform overriding plate on slab-dip. 3D view of the 1000 C isosurface. ; Effect of a non-uniform overriding plate on trench-parallel flow. Map view of the slab at different depths and times, showing the viscosity (colormap) and the velocity (arrows).

Rodríguez-González, J.; Billen, M. I.; Negredo, A. M.

2012-12-01

294

A Comparison of Flat Subduction in Central Mexico and Southern Peru  

NASA Astrophysics Data System (ADS)

In this talk we compare the subduction zone images obtained from dense seismic arrays in two flat-slab subduction systems. In Mexico, the slab is horizontal with no traditional wedge between the slab and overriding plate. Instead there is a very thin, very low-velocity layer that decouples the slab. There is no apparent impactor feature (plateau or ridge) on the oceanic plate or its conjugate that can explain the flattening of the slab. The slab is apparently tearing as the flat slab portion rolls back. In Peru, the Nasca Ridge is an obvious impactor, but it lacks sufficient buoyancy to be the sole cause that flattens the slab. The slab also starts flattening some 200 km in the along trench direction before the point where the ridge intersects the trench. In Peru there is a wedge of approximately 50 km in thickness. There is no apparent tear or rollback associated with the slab.

Clayton, R. W.; Kim, Y.; Phillips, K. E.; Dougherty, S. L.; Perez-Campos, X.; Chen, T.

2013-05-01

295

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

296

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

297

A WIDER SEISMOGENIC ZONE AT CASCADIA DUE TO HYDROTHERMAL CIRCULATION IN SUBDUCTING OCEAN CRUST  

E-print Network

June 2011To Juliana Joy, for all the early mornings and late nights associated with our stay in Temperatures along subduction zone plate boundary faults have been used to estimate the area and extent of the seismogenic zone. Recent studies of the wellconstrained Nankai margin of Japan show that hydrothermal circulation in the subducting crust cools the subduction zone and widens the area of the plate boundary fault that is between the key temperatures of 150 and 350 °C. Here, I present new thermal models for the Cascadia subduction zone that include the effects of fluid flow in the subducting crust. This fluid circulation cools the subduction zone and widens the thermally-defined seismogenic zone by shifting the intersection of the 350 °C isotherm within the plate boundary fault ~30 – 55 km landward. In contrast to the Nankai margin, the observed surface heat flux pattern for the thickly sedimented Cascadia margin provides only a weak constraint on subduction zone temperature. The tomographically-determined basalt-to-eclogite transition in the subducting slab is an additional constraint on the Cascadia subduction zone thermal models. The models

Brian Dodd Cozzens

298

Cascadia Subduction Zone  

USGS Publications Warehouse

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

Frankel, Arthur D.; Petersen, Mark D.

2008-01-01

299

Possible Morphologic Indicators for the Location of Large Slow Earthquakes in Subduction Zones  

NASA Astrophysics Data System (ADS)

Global observations of convergent margin morphology may identify regions more likely to produce tsunami earthquakes. Earthquake observations and laboratory modeling show that subduction of seafloor relief influences the accretionary prism (AP), interplate coupling and the occurrence of large interplate earthquakes. The amplitude, wavelength, and direction of convergence of seafloor roughness appear to relate to the scale of the disruption of the subduction process. Sandbox models of AP deformation caused by subduction of relatively small seafloor relief causes a displacement of the active decollement into the AP, rather than along its base. If the relief has a "wake" (e.g. seamount), passive entrance of the toe of the AP, in the wake of the descending seafloor relief, down into the subduction channel occurs, leaving an indented toe. More importantly, when seafloor relief is sufficiently deep in the subduction zone, the decollement descends to the base of the AP. However, this new base has been little faulted if at all, as it was carried passively down the subduction channel. Therefore, conditions for normal earthquake rupture may not exist, and a tsunami earthquake may be more likely. The morphology of an indented AP can be easily identified globally in ETOPO-2 bathymetry. As most convergent margins define small circles and to first order, the bathymetry of the inner wall of the trench is nearly smooth, trench inner wall bathymetry should define conic sections. Residual bathymetry would be the difference between a regionally predicted conic section and observed bathymetry. Anomalously deep residual bathymetry successfully identifies the location of the Tonga, Nicaragua, 1963 Kuril Islands, and eastern Java slow earthquakes. Both slow earthquakes in Peru (1960, 1996) occur along non-accreting margin segments presently subducting rough seafloor with sediment filled troughs. The 1968 and 1994 "hybrid" earthquakes off Honshu are anomalous in that their ruptures began updip as slow earthquakes and later became regular earthquakes. Their initial ruptures lie in the source region of the 1896 slow earthquake.

McCann, W. R.

2007-05-01

300

Along-arc segmentation and interaction of subducting ridges with the Lesser Antilles Subduction forearc crust revealed by MCS imaging  

NASA Astrophysics Data System (ADS)

We present the results from a new grid of deep penetration multichannel seismic (MCS) profiles over the 280-km-long north-central segment of the Lesser Antilles subduction zone. The 14 dip-lines and 7 strike-lines image the topographical variations of (i) the subduction interplate décollement, (ii) the top of the arcward subducting Atlantic oceanic crust (TOC) under the huge accretionary wedge up to 7 km thick, and (iii) the trenchward dipping basement of the deeply buried forearc backstop of the Caribbean upper plate. The four northernmost long dip-lines of this new MCS grid reveal several-kilometre-high topographic variations of the TOC beneath the accretionary wedge offshore Guadeloupe and Antigua islands. They are located in the prolongation of those mapped on the Atlantic seafloor entering subduction, such as the Barracuda Ridge. This MCS grid also provides evidences on unexpected huge along-strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments. Their mapping clearly indicates two principal areas of active deformation in the prolongation of the major Barracuda and Tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the Atlantic seafloor. Although different in detail, the two main deforming forearc domains share similarities in style. The imaged deformation of the sedimentary stratification reveals a time- and space-dependent faulting by successive warping and unwarping, which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely-oriented elongated and localized topographical ridges. The induced faulting producing vertical scarps in this transport does not require a regional arc-parallel extensional regime as proposed for the inner forearc domain, and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver. A contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone. Its mapping documents along-arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench. With the exception of the southernmost part, the newly mapped updip limit reaches 25 km closer to the trench, thus indicating a possible wider seismogenic zone over almost the whole length of the study area.

Laigle, Mireille; Becel, Anne; de Voogd, Béatrice; Sachpazi, Maria; Bayrakci, Gaye; Lebrun, Jean-Frédéric; Evain, Mikael

2013-09-01

301

Seismicity Within the West Sumatra Subduction Zone  

NASA Astrophysics Data System (ADS)

Oblique subduction along the western Sunda arc results in strain partitioning along the Sumatra margin. The oblique 65 mm/yr convergence is partitioned into 45 mm/yr of thrust motion accommodated along the megathrust and 11-28 mm/yr of dextral strike slip motion along the Sumatra Fault. Past seismicity along the Sumatra-Andaman margin indicates that previous large earthquakes (1797, 1833, 1861, 2004, 2005) along Sumatra margin, resulted from shallow thrust faulting below the fore-arc. The Sumatra Fault extends for 1900km from the Sunda Strait to the Andaman sea across Sumatra, creating a plate sliver known as the Sunda fore-arc. The obliquity of the subduction zone and increase in slip rate as you move northwest wards along the fault (6mm/yr at the Sunda strait to 25 mm/yr at the equator, increasing to to 50 mm/yr in the Andaman sea) results in stretching of the Sunda fore-arc. As only two thirds of the trench parallel motion is taken up by the Sumatran Fault in southern Sumatra, one third of the motion must be taken up elsewhere. Within the Sunda fore-arc, east of the Mentawai Islands at the boundary between the fore-arc ridge and the fore-arc basin lies the 600km long Mentawai fault. Originally it was suggested that the Mentawai fault was a large strike slip fault but more recent studies indicate that this may not be the case. Here, data is presented from a seismic network located within the Mentawai region of Sumatra between November 2007 and October 2008, recording the the post seismic activity of the Mw 8.5 Bengkulu 2007 earthquake including the Mw 7.2 event in February 2008. From the continuous data, arrival times of compressional (P) and shear waves (S) for 1037 events (12 600 P picks and 6 285 S picks) were manually picked, with a high quality subset of 386 events being selected for a simultaneous inversion to determine accurate hypocentres, a 1D velocity model for the region and station corrections. Hypocentre locations in the first half of the survey (November 2007 to April 2008) exhibit a high level of aftershock activity from the 2007 and 2008 earthquakes focused between the North and South Pagai islands, 100km from the trench at depths of 10-30km. Two large clusters of activity are seen 175km and 200km from the trench, within the offshore fore-arc between the mainland and the islands, which coincide with the SW and NE boundaries of the fore-arc basin. The first cluster probably corresponds to the Mentawai fault but the second cluster that extends from the slab into the overriding crust does not appear to correspond with any previously mapped faults. In the second half of the survey (April 2008 to October 2008) as the aftershock rate declines significantly, seismic activity is more evenly spread on the subduction interface.

Collings, R.; Rietbrock, A.; Lange, D.; Tilmann, F. J.; Natawidjaja, D. H.; Suwargadi, B. W.

2009-12-01

302

Return to Petropavlovsk-Kamchatsky: Subduction in the Far North  

NASA Astrophysics Data System (ADS)

For intensity of volcanic and seismic activity and the excitement of geophysical exploration, few places on Earth can match the Kamchatka-Aleutian junction, where the torn Pacific slab dives into the asthenosphere, carrying with it the distal end of the Emperor-Hawaiian Seamount Chain. Thus, it was fitting that researchers with a focus on North Pacific subduction began, in Petropavlovsk-Kamchatsky, Russia, their second round of biennial meetings that have cycled through that city, Sapporo, Japan, and Fairbanks, Alaska. The 4th Biennial International Workshop on Japan Kamchatka Aleutian Subduction Processes )JKASP( convened at the Institute of Volcanology and Seismology )IVS( of the Russian Academy of Sciences, recently formed through unification of the Institute of Volcanology and Institute of Volcanic Geology and Geochemistry.

Gordeev, Evgenii; Kasahara, Minoru; Eichelberger, John

2004-11-01

303

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone, Pacific Northwest  

Microsoft Academic Search

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes ofMs=7.0 are used to estimate the response spectra that may result from earthquakesMwMw 9.5) is the largest event that

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

304

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone, Pacific Northwest  

Microsoft Academic Search

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes of M s>=7.0 are used to estimate the response spectra that may result from earthquakes M w<81\\/4. Large variations

Thomas H. Heaton; Stephen H. Hartzell

1989-01-01

305

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

E-print Network

). On 11th March 2011, a magnitude 9.0 earthquake (a one in 20 year event, globally) struck the east coastScientific brief on the March 2011 M9 Tohoku-oki Earthquake (Japan) Richard Walters and John TOKYO YOKOHAMA Pacific PlateEurasian Plate Japan JapanTrench 2011Eq 1896 1933 0 125 250 km Figure 1: Map

306

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)

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

307

Bending-related faulting and mantle serpentinization at the Middle America trench.  

PubMed

The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes. PMID:14508480

Ranero, C R; Morgan, J Phipps; McIntosh, K; Reichert, C

2003-09-25

308

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

309

Linking Ridge Subduction to Paleomagnetic Rotations in the Andes  

NASA Astrophysics Data System (ADS)

Paleomagnetic data from 104 sites of upper Oligocene to Pliocene rocks from the coastal and Western Cordillera regions of central and northern Peru record a coherent pattern of counterclockwise rotations generated in the last 10 Ma. Based on the time-space relationship of deformation, magmatism and Nazca-South America plate convergence, the pattern of rotations can best be explained by a punctual and widespread tectonic event linked to subduction of the Nazca Ridge. If correct, slab coupling and concomitant mountain building are most pronounced at the beginning stages of ridge subduction, when down dip motion is inhibited. The system likely then evolves to a more normal subduction regime, probably when it becomes more energetically favorable to renew significant down dip motion rather than transfer convergent motion into building topography through tectonic shortening. As the ridge continues to subduct, significant changes in ridge-trench convergence angle, or possibly major changes in ridge topography, can increase or decrease coupling, and thus modulate compressive deformation. Ridge collisions may have lasting effects on the continental margin by weakening the crust to a level where deformation can proceed with lower levels of differential stress. This may for instance explain why the Subandean Zone (in this model initiated in times of increased coupling between the slab and overriding continental plate) remains tectonically active. It is possible that a succession of buoyant ridge subduction events played an important role in producing the paleomagnetic rotations recorded along the entire Andean chain. In this way, the modern Andes would be composed of distinct parts, with the Juan Fernandez ridge being partly responsible for shaping the Bolivian Orocline at circa 25 Ma, the Nazca ridge for the Peruvian Andes north of the Abancay deflection (15.5 S) until about 7 S at circa 8 to 4 Ma, and the Carnegie ridge for the recent deformation seen in the Northern Andes.

Torres, V.; Rousse, S.; Gilder, S.; Farber, D.; McNulty, B.; Sempere, T.; Patriat, P.

2004-12-01

310

Numerical Modeling of Free Subduction: a Boundary-Element Approach  

NASA Astrophysics Data System (ADS)

Many important aspects of subduction can be understood by studying an idealized model of purely buoyancy-driven ("free") subduction of an isolated thin fluid sheet with constant viscosity in an ambiant fluid with a lower viscosity. The problem can be efficiently solved using the boundary-element method, wherein a Stokes flow in a given region of space is represented in terms of the velocities and tractions at its boundaries. Essential advantages of the method include reduction of the dimensionality of the problem (from 3D to 2D or 2D to 1D); the capacity to implement a true free surface; accurate interface tracking; and the complete absence of wall effects (unless they are desired.) Moreover, the two-fluid configuration implemented by the method closely resembles that of many recent laboratory experiments on subduction. The study I will describe proceeds hierarchically from simple models to more complex ones. The simplest reference case is free subduction of a 2D sheet in an infinite half-space, with surface topography included to ensure the resolution of Stokes's paradox. Next, an impermeable boundary at 660 km depth is included, and then a viscosity jump. Finally, the same sequence is followed for a 3D sheet of finite width, for which toroidal flow around the sides of the slab is important. I will present a number of quantitative scaling laws for key subduction parameters such as the surface plate speed, the rate of trench rollback, and the slab geometry, and compare their predictions with relevant geophysical observations and with laboratory experiments (e.g., those of the group at the University of Rome-III.)

Ribe, N. M.

2007-12-01

311

Shallow subduction zone earthquakes and their tsunamigenic potential  

NASA Astrophysics Data System (ADS)

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

Polet, J.; Kanamori, H.

2000-09-01

312

Mantle melting and crustal recycling in subduction zones  

NASA Astrophysics Data System (ADS)

Major element data for basalts from approximately 100 arc volcanoes are examined in order to test a model whereby the mantle melts to varying extents beneath different arcs. Because the downgoing plate is at a fairly constant depth (approximately 120 km) beneath arc volcanoes worldwide, the total length of the mantle column that is available for adiabatic melting is largely dependent on the thickness of the arc crust. Chemical parameters that reflect the degree of mantle melting might then correlate with crustal thickness. Major element data for arc basalts are corrected for the effects of differentiation by calculating values at 6 percent MgO. Na(6.0) and Ca(6.0) correlate strongly with the thickness of the arc crust: basalts erupted onto thick crust are rich in Na and poor in Ca. These characteristics are typical of low degree mantle melts. Thus, where the crust is thick, the mantle melting column is short and the mantle melts to a small extent, producing high Na and low Ca melts. Mantle melting variations can also explain the variations in some trace elements (e.g., Sc, Ce, Zr) but others (e.g., Ba, K, Sr) may reflect subducted sediment instead. In order to test the sediment subduction model, sediment fluxes into trenches are estimated. Over 250 new chemical analyses are reported for marine sediments from DSDP/ODP Sites 765, 595, 596, and 183. Based on evaluation of these reference sites, relationships are found between the geochemical and lithologic variations in sediments. This lithologic approach is used to calculate the bulk composition for eight trench sections (Java, Tonga, Aleutians, Antilles, Guatemala, Mexico, Vanuatu, and Marianas). The sediment input fluxes correlate well with the associated arc enrichments in K, Rb, Cs, Sr, Ba, U, and Th for the eight arc/trenches pairs examined. The subducted input and volcanic output fluxes can be balanced if the entire oceanic crust (sediment + basalt) loses elements to the arc. The results of this flux balance are that the sediment loses roughly 10-30 percent of all the elements to the arc, while the basalt loses more variable amounts (0 percent, 2.5 percent, 7-15 percent, and 30 percent of its Th, U, alkalis, and Ba, respectively). The relatively constant proportions from the sediment suggest that the transport phase to the mantle wedge is a sediment melt, while the variable proportions from the basalt suggest an aqueous fluid. In this model, elements are recycled in subduction zones via sediment melting and basalt dehydration.

Plank, Terry Ann

313

Mantle Flow in the Rivera-Cocos Subduction Zone  

NASA Astrophysics Data System (ADS)

Western Mexico, where the young and small Rivera plate and the adjacent large Cocos plate are subducting beneath the North American plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the MARS (Mapping Rivera Subduction zone) array that was deployed from January, 2006, through June, 2007, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ~0.2 sec for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera-Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a down-dip direction but also have recently rolled back toward the trench and that the Colima rift is intimately related to the tearing between the Rivera and Cocos plates.

Leon Soto, G.; Ni, J. F.; Grand, S. P.; Sandvol, E. A.; Valenzuela Wong, R.; Guzman-Speziale, M.; Gomez Gonzalez, J. M.; Dominguez Reyes, T.

2009-12-01

314

Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system  

NASA Astrophysics Data System (ADS)

Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These "fore-arc basalts" (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.

Reagan, Mark K.; Ishizuka, Osamu; Stern, Robert J.; Kelley, Katherine A.; Ohara, Yasuhiko; Blichert-Toft, Janne; Bloomer, Sherman H.; Cash, Jennifer; Fryer, Patricia; Hanan, Barry B.; Hickey-Vargas, Rosemary; Ishii, Teruaki; Kimura, Jun-Ichi; Peate, David W.; Rowe, Michael C.; Woods, Melinda

2010-03-01

315

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

316

The nature of the northern terminus of the Tonga Trench, southwest Pacific  

E-print Network

used for the study of the northern closure of the Tonga subduction zone by Hanus and Vanek (1979b). The Niua Fo'ou fracture zone is the left-lateral strike-slip referred to in the text. The Tonga Trench is denoted by a dotted line, active volcanoes... that the basalts and ultramafics that they recovered were fault-exposed components of conduits associated with the 11 aligned volcanoes of the Tonga Ridge, rather than samples of the underlying mantled They added, however, that extensive portions of the upper...

Wright, Dawn Jeannine

2012-06-07

317

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

318

Influence of the thermal state of the overriding plate on subduction dynamics and slab geometry  

NASA Astrophysics Data System (ADS)

Significant variability in slab dip can be found between different subduction zones, or along a single subduction zone. The effects on subduction dynamics and slab geometry of a number of factors such as trench motion, the presence of a low viscosity mantle wedge and slab rheology have been previously investigated. In contrast, the influence of the thermal thickness of the overriding plate on slab dip has not been studied before, even though the thermal thickness is expected to significantly influence the suction force acting on the slab due to corner flow in the mantle wedge due to both increased viscosity of a cooler mantle wedge and narrowing of the wedge corner. The main purpose of this study is to perform a systematic analysis of the influence of the thermal state of both the overriding and subducting plates on the balance between gravitational and hydrodynamic torques, and therefore on slab dip. We present the results of non-Newtonian numerical thermo-mechanical modeling of subduction, where we vary the age of both the overriding and subducting plates in order to test its effect on the slab dip at different depth ranges. We find that colder overriding plates result in shallower slab dips, as a result of the increased suction force in the mantle wedge. The influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere. Modeling results are qualitatively compared to the large dip variability of the Cocos slab including a flat-slab segment. We suggest that this variability is likely related to the change of the thermal state of the overriding plates, with flat subduction occurring under cold lithosphere in southwestern Mexico and steep subduction under the warmer lithosphere of the northwestern Caribbean plate.

Rodriguez-Gonzalez, J.; Negredo, A. M.; Billen, M. I.

2010-12-01

319

Distinguishing wedge, slab, and sub-slab anisotropy in the Izu-Bonin-Mariana subduction zone  

NASA Astrophysics Data System (ADS)

Observations of shear wave splitting in subduction zones are numerous and robust, but the source and geodynamical implications of the anisotropy remain imperfectly constrained. This results in no small part from the difficulty distinguishing anisotropy in the overriding plate, the mantle wedge above the subducting slab, the slab itself, and the sub-slab mantle. Observations of subduction systems around the world provide strong evidence that trench-parallel fast directions dominate beneath the mantle wedge, but different models have been proposed to explain these observations, invoking trench-parallel sub-slab mantle flow, anisotropy due to aligned cracks within the slab, or a combination of the two. In order to distinguish contributions from anisotropy in different parts of the Izu-Bonin-Marianas subduction system, we examine shear wave splitting for a variety of raypath combinations that sample the wedge, slab, and sub-slab region in different ways. We measure the splitting of both local and teleseismic S phases originating from slab earthquakes, estimating source-side splitting by correcting teleseismic S phases for the effect of upper mantle anisotropy beneath the receiver. In the relatively shallow part of the slab, we distinguish between events in the upper and lower planes of seismicity from the well-defined double Wadati-Benioff zone. This approach allows us to assemble a dataset of splitting measurements that sample the wedge, the slab, and the sub-slab mantle in different ways and thus isolate the contribution from anisotropy in different parts of the system.

Hanna, J. E.; Long, M. D.; Brudzinski, M. R.

2009-12-01

320

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

321

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

322

The termination of the southern New Hebrides subduction zone (southwestern Pacific)  

NASA Astrophysics Data System (ADS)

Recent data (bathymetric map and petrological investigations) as well as reappraisal of local seismological events for more than 20 years allow us to present a new geodynamic interpretation of the southern New Hebrides subduction zone. A multidisciplinary approach to this complex area, where transform movements relay subduction process, clearly shows the interrelationships between intermediate seismicity, lithospheric plates tectonics and volcanic activity. According to our model, two hinge zones, tearing the downgoing slab along directions which parallel the convergence vector (N70°E), reasonably account for most seismological and petrological data from the region. The approach of the Loyalty islands ridge towards the trench and its increasing influence on the subduction regime is beginning to complicate this scheme.

Monzier, M.; Maillet, P.; Herrera, J. Foyo; Louat, R.; Missegue, F.; Pontoise, B.

1984-01-01

323

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

324

Modeling of Mantle Convection in 3D Subduction Zones  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

325

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

326

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

327

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

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

328

Post subduction thermal regime of the western North America and effects on the Great Valley, Sierra Nevada and northern Baja California provinces  

Microsoft Academic Search

Tectonic evolution of the transform margin of western North America has attracted great interest. This boundary formed as the fault-fault-trench type Mendocino triple junction has moved north. The tectonic transition is expected to have a significant thermal signature because the subduction zones are quite distinct from any other plate boundary zones with their depressed heat flow in the outer arc

Kamil Erkan

2007-01-01

329

Hough Transform Used to Detect Body to Surface Wave Converted phases from Bathymetric Topography Associated with a Trench  

NASA Astrophysics Data System (ADS)

We detected surface waves that are generated by body wave scattering at the trench off the Pacific coast of Mexico. We examined seismic sections from the MASE (Meso American Seismic Experiment) network, a 100 station, broadband seismic network that crossed Mexico. When incident S waves from teleseismic events reach the trench, part of the energy gets converted into surface waves that are trapped in the crust. The scattered surface waves are recognized because they have low apparent velocity and branch off from the main generating phase which, in contrast, h