Existence of a seismic belt in the upper plane of the double seismic zone extending in the along‐arc direction at depths of 70–100 km beneath NE Japan

Kita, S.; Okada, Tomomi; Nakajima, Jun; Matsuzawa, Toru; Hasegawa, Akira

doi:10.1029/2006gl028239

Cited by 139 publications

(125 citation statements)

References 19 publications

(34 reference statements)

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“…Eclogitization results in the disappearance of the seismic signature of the oceanic crust at depths ranging from about 60 km in hot subduction zones down to about 150-200 km in the coldest subduction zones (Abers 2000;Helffrich and Abers 1997) where hydrated silicate lawsonite with low seismic velocities may be preserved (Chantel, et al 2012;Reynard and Bass 2014;Schmidt and Poli 1998). Extensive dehydration of the oceanic crust is also associated with seismicity along the main Wadati-Benioff zone (Kita, et al 2006). …”

Section: Rock Hydration and Pore Fluids From Seismic Observationsmentioning

confidence: 99%

“…These are associated with portions of the oceanic crust whose seismic properties suggest fluid overpressure (Audet, et al 2009;Audet and Schwartz 2013). Deeper (>30 and down to 60-90 km depths), the plate interface is decoupled and seismically silent (Wada, et al 2008), and diffuse seismicity in the oceanic crust is linked to eclogitization reactions (Hacker, et al 2003;Kita, et al 2006). The region above the decoupled plate interface is tectonically isolated and its nature can only be probed by geophysical means.…”

Section: Introductionmentioning

confidence: 99%

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Mantle hydration and Cl-rich fluids in the subduction forearc

Reynard

2016

Prog. in Earth and Planet. Sci.

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In the forearc region, aqueous fluids are released from the subducting slab at a rate depending on its thermal state. Escaping fluids tend to rise vertically unless they meet permeability barriers such as the deformed plate interface or the Moho of the overriding plate. Channeling of fluids along the plate interface and Moho may result in fluid overpressure in the oceanic crust, precipitation of quartz from fluids, and low Poisson ratio areas associated with tremors. Above the subducting plate, the forearc mantle wedge is the place of intense reactions between dehydration fluids from the subducting slab and ultramafic rocks leading to extensive serpentinization. The plate interface is mechanically decoupled, most likely in relation to serpentinization, thereby isolating the forearc mantle wedge from convection as a cold, potentially serpentinized and buoyant, body. Geophysical studies are unique probes to the interactions between fluids and rocks in the forearc mantle, and experimental constrains on rock properties allow inferring fluid migration and fluid-rock reactions from geophysical data. Seismic velocities reveal a high degree of serpentinization of the forearc mantle in hot subduction zones, and little serpentinization in the coldest subduction zones because the warmer the subduction zone, the higher the amount of water released by dehydration of hydrothermally altered oceanic lithosphere. Interpretation of seismic data from petrophysical constrain is limited by complex effects due to anisotropy that needs to be assessed both in the analysis and interpretation of seismic data. Electrical conductivity increases with increasing fluid content and temperature of the subduction. However, the forearc mantle of Northern Cascadia, the hottest subduction zone where extensive serpentinization was first demonstrated, shows only modest electrical conductivity. Electrical conductivity may vary not only with the thermal state of the subduction zone, but also with time for a given thermal state through variations of fluid salinity. High-Cl fluids produced by serpentinization can mix with the source rocks of the volcanic arc and explain geochemical signatures of primitive magma inclusions. Signature of deep high-Cl fluids was also identified in forearc hot springs. These observations suggest the existence of fluid circulations between the forearc mantle and the hot spring hydrothermal system or the volcanic arc. Such circulations are also evidenced by recent magnetotelluric profiles.

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Section: Rock Hydration and Pore Fluids From Seismic Observationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Mantle hydration and Cl-rich fluids in the subduction forearc

Reynard

2016

Prog. in Earth and Planet. Sci.

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“…Intriguingly, the upper plane of seismicity seems to be confined to the oceanic crust ( Fig. 2; Kita et al, 2006Kita et al, , 2010a, with a marked reduction in number and deepening of the earthquakes with respect to the top of the crust at greater depth. A similar pattern, and correlation with low seismic velocities in the crust, is seen in the central Alaska subduction zone (Abers et al, 2006).…”

Section: Introductionmentioning

confidence: 99%

“…1; Tanaka et al, 2004). The correlation between the predicted blueschistout boundary and the cessation of seismicity is even more intriguing since the eclogite that forms still contains signifi- Kita et al (2006) and Abers et al (2012). The earthquakes from 120 km wide box shown in the insert are projected on the center cross section, normal to the trench.…”

Section: Introductionmentioning

confidence: 99%

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

2012

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Abstract. The cause of intermediate-depth (> 40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The wellinstrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediatedepth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths > 80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by timedependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediatedepth seismicity.

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“…The increase in velocity in the crust appears to be correlated with an abrupt decrease in seismic activity beyond a depth range of the upper plane seismic belt that is defined as a concentrated crustal seismicity at depths of 70 to 90 km (Kita et al 2006). This phenomenon suggests that earthquakes in the crust are facilitated as a result of substantial pore fluid generated by dehydration reactions to eclogite from hydrous minerals (e.g., Abers et al 2013).…”

Section: Introductionmentioning

confidence: 93%

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

et al. 2014

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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.

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Existence of a seismic belt in the upper plane of the double seismic zone extending in the along‐arc direction at depths of 70–100 km beneath NE Japan

Cited by 139 publications

References 19 publications

Mantle hydration and Cl-rich fluids in the subduction forearc

Mantle hydration and Cl-rich fluids in the subduction forearc

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

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

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