Western Pacific Subduction Zones Research Articles

Terrane Collision‐Induced Subduction Initiation: Mode Selection and Implications for Western Pacific Subduction System

AbstractIn the regime of plate tectonics, the subduction of an oceanic plate generally terminates with the collision and accretion of continental terranes. Then, a new subduction zone may form in the neighboring oceanic plates, which is defined as the terrane collision‐induced subduction initiation (SI). Based on the analyses of the western Pacific subduction system in the Cenozoic, three types of collision‐induced SI have been observed: subduction polarity reversal, subduction transference and far‐field subduction. However, the dynamics and controlling factors of SI mode selection after terrane collision are not clear. In this study, a multi‐terrane collision model has been conducted with variable rheological strength of continental terranes and different convergence velocities. The model results indicate that the relative strength of the terranes controls the SI mode selection, with the new subduction zone tending to form beneath weaker terranes. In addition, the higher convergence velocity can facilitate the collision‐induced SI. An analytical study of force balance has been further conducted, which provides a mechanical explanation for the numerical prediction of a weak overriding terrane as a favorable SI site. The numerical models and force balance analyses are further compared with the natural cases in the western Pacific subduction system. This indicates that subduction polarity reversal is the most favorable mode after terrane collision in the western Pacific, possibly due to the weakness of overriding plate during the preceding subduction‐induced fluid/melt activity. This comprehensive study provides systematic constraints for the dynamics of collision‐induced subduction jump, especially for the western Pacific subduction zones in the Cenozoic.

Multichannel seismic evidence for tectonic migration in the Mariana subduction system

AbstractThe Mariana subduction system is an ideal place for ocean–ocean convergence boundary research. As a typical example of an extensional subduction model, the subduction process, subduction mechanism and subduction effect of this system have long been popular research topics. The seismic reflection information of oceanic sedimentary layers shown on multichannel seismic profiles is an intuitive record of small‐ to medium‐scale structural activity and sedimentary sequence evolution and is an important source for studying plate‐coupling processes and tectonic dynamic action. This paper is based on the raw multichannel seismic data of MGL1204 in the Mariana arc forearc region; these data were obtained by using linear interference filter technique noise attenuation technology and multiple velocity iterations to accurately image the oceanic sedimentary layer, and the imagery is used to study the Cenozoic fracture activity and residual strata thickness distribution characteristics of the middle part of the Mariana forearc region and to explore the tectonic migration characteristics of the Cenozoic sedimentary basins. We believe that due to the influence of regional stress field changes caused by the movement directions of the Pacific Plate and Philippine Sea Plate towards the Eurasian Plate, the development of secondary structures in the western Pacific subduction zone shows a characteristic west‐to‐east migration, and the structural characteristics of the Mariana forearc Cenozoic sedimentary basin are consistent with the regional tectonic features. The faults, stratigraphy and depocentres of the oceanic sedimentary layer show a migration pattern from west to east and from north to south overall; 25 and 13 Ma are identified as two key tectonic migration periods, and the tectonic migration characteristics and time records of the basin also provide direct evidence for the plate reconstruction model of trench retreat and Philippine Sea plate rotation.

Deep Geophysical Anomalies Beneath the Changbaishan Volcano

AbstractSubsurface imaging is key to understanding the origin of intraplate volcanoes. The Changbaishan volcano, located about 2,000 km away from the western Pacific subduction zone, has several debated origins. To investigate this, we compared regional seismic tomography with the electrical resistivity results and obtained high‐resolution 1D and quasi‐2D velocity‐depth profiles. We show that the upper mantle is characterized by two anomalies exhibiting distinct features which cannot be explained by the same mechanism. We document a localized low‐velocity anomaly atop the 410‐km discontinuity, where the P‐wave velocity is reduced more than that of the S‐wave (i.e., lower Vp/Vs). We propose that this anomaly is caused by the reduction of the effective moduli during the phase transformation of olivine. The other anomaly, located between 300 and 370 km depth, reveals a significant reduction of the S‐wave velocity (i.e., higher Vp/Vs), associated with a reduction of the electrical resistivity, altogether consistent with partial melting.

Reworking of ancient tectonic amalgamation belt beneath the central north of North China Craton revealed by dense seismic observations

The North China Craton (NCC) is one of the oldest cratons in the world, and its internal tectonic belt is often used to investigate the earth’s tectonic evolution events. During the late Mesozoic and Cenozoic, the western Pacific subduction zone caused the restructuring of NCC by damaging the craton beneath eastern NCC, resulting in the distinct lateral differences between western and eastern NCC, which ultimately formed the current NCC. Furthermore, the subsequent tectonic events activated the ancient tectonic weak zones, and their traces are imprinted in the deep earth. Here, we investigated the crust structures with a high-density seismic array beneath the splice position of the eastern margin of the Khondalite Belt and the northern part of the central orogenic belt in NCC. The array included 140 short-period seismographs spaced at 2–3 km intervals, which recorded teleseismic three-component waveforms over a one-month period. P-wave receiver functions calculated from 25 teleseismic events provided an image of the crustal structure. The weak Moho and Moho offset under the study area are visible in the migration image of receiver functions. The geological investigations and the rock outcrops were combined to establish the strong coupling relationship between the present surface fault-depression system and deep structures. The deep material circulation, which governs the surface extension of the basin-range structure, is controlled by the deep material circulation which is ultimately derived from the continuous subduction of the western Pacific. The study’s findings indicate that the ancient amalgamative belt might have transformed into a weak zone easily susceptible to modification by plate tectonic movements.

Global water distribution in the mantle transition zone from a seismic isotropic velocity model and mineral physics modeling

Although the discoveries of hydrous ringwoodite inclusions and ice-VII inclusions in natural diamonds suggest a hydrous mantle transition zone (MTZ), water content and distribution in the MTZ remain unclear. Here combining a global P- and S-wave isotropic velocity tomography and mineral physics modeling, we image the water distribution in the MTZ. Our results indicate that the MTZ is a main water reservoir inside the Earth, and the total water content of the MTZ is about 0.64–1 seawater. The upper MTZ (410–520 km) and the lower MTZ (520–660 km) contain 0.3–0.5 wt% and 0.15–0.2 wt% water, respectively, implying water contents of the MTZ decrease with increasing depths. The most hydrous regions are mainly located near subduction zones, where the upper MTZ and the lower MTZ can contain water up to 0.5–1 wt% and 0.2–0.5 wt%, respectively, indicating water is transported into the MTZ by hydrous slabs. In addition, old subducted slabs in the western Pacific subduction zone are more hydrous than young subducted slabs in the eastern Pacific subduction zone. We also propose a water circulation model which integrates our results of the water content and distribution in the MTZ.

Effect of the Western Pacific plate subduction on upper mantle in the eastern segment of the Central Asian Orogenic Belt: Revealed by long-period magnetotelluric data

Ever since the Mesozoic era, the eastern segment of the Central Asian Orogenic Belt (CAOB) has been significantly affected by the subduction of the Pacific plate. In order to better understand the subduction effect of the Pacific plate at different distances, we set up 65 magnetotelluric stations to acquire a three-dimensional inversion resistivity structure. The results illustrate that: there are three large-scale high-conductivity anomalies C1, C2 and C3 affecting the crust and mantle of the eastern segment of the Central Asian Orogenic Belt. The high-conductivity anomaly C1 extends from the surface to the asthenosphere and is connected with C2, which exists in the asthenosphere below the Xing'an block. The high-conductivity anomaly C3 under the Songliao block extends downward from the surface to the asthenosphere. We interpret that these anomalies are caused by aqueous fluid and partial melting based on petrophysical analysis. In the light of the high-temperature high-pressure theory, the estimated water content and melting fraction of C3 is considerably higher than that of C2. We infer that the lateral heterogeneity of the fluid-related properties is caused by the variable distance to the Western Pacific subduction zone. The C3 anomaly is located above the stagnant plate and controlled by the medium-distance effects of Pacific plate subduction, which have been influenced by intense dehydration and seriously transforming. The C2 anomaly is located at the end of the stagnant plate and controlled by the long-distance effects of Pacific plate subduction with slight dehydration and transformation.

Stress Transfer and Migration of Earthquakes from the Western Pacific Subduction Zone Toward the Asian Continent

Stress Transfer and Migration of Earthquakes from the Western Pacific Subduction Zone Toward the Asian Continent

Stress Transfer and the Impact of the India-Eurasia Collision and the Western Pacific Subduction on the Geodynamics of the Asian Continent

The interaction between the India-Eurasia collision and the Western Pacific subduction and their contribution to recent geodynamics of the Asian continent are discussed. We perform a comparative analysis of the data available from world literature and new data on the slow strain and earthquake migration from the India-Eurasia collision and the Western Pacific subduction zones. Based on the concepts of wave dynamics of the deformation processes, a localization scheme is constructed illustrating the migration of slow strain fronts in central and eastern Asia, and the wave geodynamic impact of collision and subduction on the Asian continent is shown.

Eastward mantle flow field underneath East Asia quantified by combining shear-wave splitting orientations and absolute plate motion observations

Although Earth's surface motion is well known, the flow field in the underlying mantle is not. Mantle flow is typically calculated on the basis of inferred density variations, and flow directions can also be reflected in seismically observed anisotropy, but those observations leave ambiguity on the depth and direction of the deformation. Anisotropy orientations in East Asia, outside Tibet, have been interpreted in various ways and have often been linked to deformation in the asthenosphere related to absolute plate motion and/or mantle wedge deformation. Here, we re-analyze published seismic anisotropy data and find that orientations outside Tibet can be much better explained when considering absolute plate motion (APM) of the Earth's surface in addition to coherent sub-asthenospheric mantle flow, than when comparing orientations to APM alone. The direction and magnitude of the required sub-asthenospheric flow depend on the absolute reference frame used for the surface velocities, but when considering an APM frame with an intermediate global net-rotation we find an eastward flow of 1-2 cm/yr. This flow is faster than the surface motion, and generally in the same direction, from which we conclude that the mantle leads the plate motion. Our inferred flow is similar to those independently calculated based on buoyancy forces driven by density variations, most notably the high density anomalies associated with the western Pacific subduction zones, but possibly also the upwelling underneath Africa. Additionally, based on our predicted sub-asthenospheric flow and absolute motion of the lithosphere, we predict asthenospheric-based XKS orientations underneath all of east Asia and find it to differ significantly with observed XKS orientations where either the lithosphere is thick and/or strain rate is high, which suggests that at those places observed XKS orientations reflect the integrated deformation in both asthenosphere and lithosphere.

ВОЛНОВОЕ ГЕОДИНАМИЧЕСКОЕ ВОЗДЕЙСТВИЕ ТЕКТОНИЧЕСКИХ ПРОЦЕССОВ НА АМУРСКУЮ ПЛИТУ

The analysis of data on the migration of earthquakes and slow deformations from the Indo-Eurasian collision and the Western Pacific subduction zones is given, and the wave “geodynamic impact” of these tectonic processes on the Amurian plate and surrounding structures is shown. The interaction and a relative contribution of collision and subduction to the recent geodynamics of the Amurian plate are discussed. A scheme is constructed showing localizations of the slow strain wave manifestation in the areas of central and eastern Asia. The calculations are performed aimed at revealing a transverse migration of earthquakes (M ≥ 6.5) directed from the Japan and the Kuril-Kamchatka trenches toward the Asian continent during the time period from 1960 to 2015. The migration of earthquakes along the profile crossing Hokkaido Island occurs at velocities of 15 and 23 km/yr, whereas the migration velocity from the Kuril-Kamchatka Trench via Sakhalin Island is evaluated from 20 to 40 km/yr at different depths. We focus on an insufficient study of the influence of the Western Pacific subduction on the formation of the deformation field in continental Asia.

Distinct slab interfaces imaged within the mantle transition zone

Oceanic lithosphere descends into Earth’s mantle at subduction zones and drives material exchange between Earth’s surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab. Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest Pacific subduction zone: Implications for rheology and geodynamics

Regional triplication waveforms of five intermediate-depth events are modeled to simultaneously obtain the compressional (P) and shear (SH) wave velocity structure beneath northwestern Pacific subduction zone. Both the P- and SH-wave velocity models for three different sub-regions show a low-velocity layer (LVL) with a thickness of ∼55-80 km lying above the 410-km discontinuity with a ∼900 km lateral extent from the Japan Sea to the northeastern Asian continental margin. With the dihedral angle approaching to zero around 400 km, a minute amount of melt atop the 410-km discontinuity caused by the hydrous slab might completely wet olivine grain boundaries and result in a low seismic velocity layer in this specific subduction zone. This mechanism suggests that the 410-LVL is a low viscosity zone that would partially decouple the upper mantle from the transition zone. We infer that the widespread 410-LVL provides evidence for a water-bearing mantle transition zone beneath the western Pacific subduction zone.

Shallow Lower Mantle Viscosity Modulates the Pattern of Mantle Structure

AbstractTomographic images of Earth's mantle reveal that structure, even at the largest spatial scales, does not remain correlated across all mantle depths. This is a surprising result given the relationship between a dominantly spherical harmonic degree‐2 pattern in the lower mantle and the pattern of plate motions at the surface, that is, the fact that the two large, low‐shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean are located near regions of surface divergence and away from regions of recent and ongoing subduction. Here, we combine analysis of seismic tomography models and numerical simulations of global‐scale mantle circulation to show that the change in correlation in mantle structure across the extended transition zone (400–1,000‐km depth), as revealed by the mantle radial correlation function (RCF), can be attributed primarily to the influence of seismically fast regions in the transition zone beneath the western Pacific subduction zones. Similar changes in the correlation of structure in global models of mantle circulation can be explained by a relatively large increase in viscosity (of at least a hundred times) between the upper mantle and lower mantle. We also demonstrate that a stronger endothermic phase transition at a depth of 660 km helps explain the observed pattern of the mantle RCF and the correlation between geodynamic models and seismic tomography. We account for the differences in resolution and parameterization of geodynamic versus tomographic models through the application of a tomographic resolution operator.

Subduction Dynamics and Mantle Pressure: 2. Towards a Global Understanding of Slab Dip and Upper Mantle Circulation

AbstractWe investigate the relationship between the global distribution of deep slab dips (at 250‐ to 300‐km depth) and pressure and circulation in the upper mantle. Using an analytic method to compute dynamic pressure in a 3‐D global upper mantle domain, and a force balance between slab dip, slab buoyancy, and pressure, we model dips for all major subduction zones. Overall, our models suggest that global‐scale mantle flow, as dictated by the shapes and velocities of Earth's plates and slabs, plays a fundamental role in creating the global pattern of slab dips. The dip trends of the South American and western Pacific subduction zones are controlled, in our models, by spatial variations in the dynamic pressure associated with flow. Our best fitting models produce global root mean square dip misfits of less than 10° for asthenospheric viscosities of 2.5–4.0 × 1020 Pas. This result is only obtained with a large flux of asthenosphere from upper to lower mantle at subduction boundaries, occurring on the overriding plate side of slabs, without which dips are significantly steeper than observed. This effect cannot be resolved by processes that affect only certain subduction systems and requires flux of asthenosphere into the lower mantle at subduction systems globally (or an alternative mechanism that produces more negative pressures on the overriding plate side of slabs). Upper mantle pressure fields that fit global slab dips yield negative dynamic pressure on the upper plate side of slabs, positive pressure on the subducting plate side, and an east‐to‐west pressure increase beneath the Pacific Plate.

Slab interactions in 3-D subduction settings: The Philippine Sea Plate region

The importance of slab–slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific–Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu–Bonin–Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu–Bonin–Mariana slabs. When weak back-arc regions are included, slab–slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab–slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations.

Dynamics of the Ryukyu/Izu-Bonin-Marianas double subduction system

Trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle, but the inherent dynamics are not fully understood. One interesting case is the migration of the Izu-Bonin-Marianas trench (IBM) that accommodates the subduction of the Pacific beneath the Philippine Sea Plate (PSP), which is in turn subducting beneath the Eurasian plate along the Ryukyu trench. The history of the IBM trench is dominated by fast, episodic retreat from 40 to 15Ma. However, around 10–5Ma, the IBM trench reversed its motion from retreat to advance. The switch in trench motion occurred soon after the breakoff of the PSP slab along the Ryukyu trench and the onset of new subduction, and represents a fundamental change in the dynamics of the western Pacific subduction zones. Here, Following the modelling study of Čížková and Bina (2015), which suggested a link between IBM trench advance and Ryukyu subduction, we run 2-D numerical experiments to test the influence of a newly formed Ryukyu slab on the established IBM subduction zone, we run two-dimensional numerical experiments to test the influence of this newly formed Ryukyu slab on the IBM subduction zone. The results from our geodynamic model compare favourably with the reconstructed trend, indicating that the switch in trench motion along the IBM trench may indeed be related to the onset of a new subduction zone along the Ryukyu trench. Our analysis substantiates the idea that advancing trench motions in the western Pacific are due to the establishment of a double subduction system. Further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission.

Volatile element isotopes of submarine hydrothermal mineral deposits in the Western Pacific

AbstractThe abundance and isotopic compositions of volatile elements trapped in fluid inclusions of submarine hydrothermal mineral deposits in Western Pacific subduction zones (Okinawa Trough, Izu‐Bonin arc, Mariana Trough, and Lau Basin) and in Kuroko ores in northeastern Japan are presented. The helium isotopic compositions corrected for air contribution of the Okinawa and Mariana troughs, ranging 4.49–7.68 Ra are lower than those of the Izu‐Bonin and Lau Basin, 7.62–8.91 Ra. This characteristic might reflect the differences in regional tectonic setting. The Okinawa and Mariana troughs are related to back‐arc spreading with strong graben sedimentary signature, whereas the Izu‐Bonin arc is associated with island arc magmatism. The arc contribution to the Lau Basin volcanism is significantly strong, even though it is assigned to back‐arc spreading. Nitrogen isotopes can also be explained by a similar hypothesis, whereas argon and carbon isotopes cannot be used to discriminate tectonic setting. δ13C–CO2/3He and δ15N–N2/36Ar diagrams elucidate the source of carbon and nitrogen. The MOR‐type mantle contributions to carbon are mostly smaller in the Okinawa and Mariana troughs (ranging 0.06–8.9% with the average of 2.4%) than in the Izu‐Bonin and Lau Basin (2.1–25% with the average of 7.7%). The sedimentary contributions to nitrogen are larger in the Okinawa and Mariana troughs (11–65% with the average of 35%) than in the Izu‐Bonin and Lau Basin (4–24% with the average of 15%), and the Kuroko samples agree well with the latter. Carbon and nitrogen fluxes are again higher in Okinawa trough than in Izu‐Bonin arc.

Numerical modelling of the relationship between the present tectonic stress field and the earthquakes in the Western Pacific Subduction Zone

AbstractThe tectonic stress field of the Western Pacific Subduction Zone (WPSZ) has been affected by the interaction among the Euro‐Asian, Philippine and Pacific Plates, and the effect is manifested by the occurrence of huge earthquakes in this region. Here, using numerical simulation of the finite element method, attempts were made to evaluate the NW‐directed subduction of the Western Pacific Plate on the three secondary subduction zones, namely the Japan Subduction Zone, the Izu‐Bonin Subduction Zone and the Mariana Subduction Zone. In these zones, the structural geometry and viscoelastic property of the model were determined, as well as two main megathrust fault planes identified, by means of a comprehensive structural analysis of tomography and Crustal 2.0 data. This study shows that the tectonic stress field and earthquakes in the three secondary subduction zones have different distribution features under continuous NW‐directed subduction of the Western Pacific Plate; i.e. (1) the strong coupling area in the Japan Subduction Zone reflected subducted seamounts, ocean ridges or other topographic highs that control the frequent occurrence of historical large earthquakes and stress concentration; (2) in Izu‐Bonin Subduction Zone, there were fewer earthquakes compared with the Japan Subduction Zone, the reason for this kind of distribution is that the contact area and properties have been changed between the subducting Pacific Oceanic Plate and the overlying Philippine Oceanic Plate; and (3) earthquakes that happened in the Mariana Subduction Zone have complicated types, including thrust‐type, normal‐type and strike‐slip‐type earthquakes, which are triggered by the subducting and retreating of the Pacific Oceanic Plate, and an angular difference between the subducting direction of the Pacific Oceanic Plate and the strike of the Mariana Trench. In summary, formation of differences among the tectonic stress field and earthquake distribution of the three subduction zones are correlated not only to the geometrical structures of the major faults but also to the subducted seamounts and the strike directions of the trenches. Copyright © 2016 John Wiley & Sons, Ltd.

On the influence of the asthenospheric flow on the tectonics and topography at a collision-subduction transition zones: Comparison with the eastern Tibetan margin

The tectonic and topographic evolution of southeast Asia is attributed to the indentation of India into Eurasia, gravitational collapse of the uplifted terrains and the dynamics of the Sunda and other western Pacific subduction zones, but their relative contributions remain elusive. Here, we analyse 3D numerical geodynamic modelling results involving a collision-subduction system and show that vigorous asthenospheric flow due to differential along-strike slab kinematics may contribute to the surface strain and elevations at collision-subduction transition zones. We argue that protracted northward migration of the collisional front and Indian slab during south to south-westward rollback subduction along the Sunda margin might have produced a similar asthenospheric flow. This flow could have contributed to the southeast Asia extrusion tectonics and uplift of the terrains around the eastern Himalayan syntaxis and protruding from southeast Tibet. Therefore, we suggest that the tectonics and topographic growth east and southeast of Tibet are controlled not only by crustal and lithospheric deformation but also by asthenospheric dynamics.

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

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