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We investigate the 3-D P and S wave velocity structure of the East Japan forearc using a large number of high-quality arrival-time data of local earthquakes recorded by a permanent seafloor seismic network (S-net) and the land-based Kiban seismic network. Geometries of the subducting slab boundary and the Conrad and Moho discontinuities under the forearc are refined by referring to results of previous studies including active-source seismic soundings and seismicity located by the S-net. The obtained P and S wave velocity images are quite similar to each other, suggesting that they reflect robust features of the 3-D seismic structure. The subducting Pacific slab exhibits high velocities, whereas significant low-velocity (low-V) anomalies are revealed in the overlying Okhotsk plate beneath the forearc, which may reflect accretionary materials containing abundant fluids. The subducting oceanic crust atop the Pacific slab is revealed clearly as a thin low-V layer extending from the trench axis down to ~100 km depth under the volcanic front. Our results of earthquake relocation using the 3-D velocity model indicate that the double seismic zone in the subducting Pacific slab occurs from the trench axis down to ~180 km depth. The occurrence and focal mechanisms of the double seismic zone are caused by slab deformation during its subduction, such as slab bending or unbending, as well as other processes including hydration, dehydration, compositional variations, and phase changes in the subducting slab. [Display omitted] • Seafloor S-net and Kiban network data are used to study the 3-D forearc structure. • The Conrad, Moho and Pacific slab geometries in the East Japan forearc are refined. • P and S wave velocity images are quite similar, reflecting robust structural features. • The subducting oceanic crust is revealed down to ~100 km depth under the volcanic front. • The intraslab double seismic zone occurs from the trench axis down to ~180 km depth. [ABSTRACT FROM AUTHOR]

The motivation for this work was to model the crustal thickness, and the geometry of the Mohorovičić (Moho) discontinuity, the crust-upper mantle boundary around the Japanese archipelago. The western Pacific Ocean is one of the most eminent subduction zones in the world. The subduction zone complexity is due to alteration within the geometry of the stagnant slab. We compiled a three-dimensional density distribution model for subduction plates around the Japan Islands, and estimated the depth to Moho using the constrained non-linear inversion of gravity data derived from the latest Gravity field and Ocean Circulation Explorer, or GOCE, model known as GOCO06s. The study had three steps, starting with the data corrections followed by the inversion, and ending with the modeling and comparing of the estimated results with previous models and studies. In this study, we used available receiver function results, seismic tomography models, global crustal thickness, and geological and tectonic information as constraints. The model estimated the crustal thickness, surface geometry of the Moho, and geometry of the stagnant slab in the Kuril Trench, Japan Trench, Nankai Trough, and Izu-Bonin Trough. In gravity inversion, we used 34 km and 600 kg m−3 as the Moho depth reference and density contrast, respectively. Results showed that the crustal thickness ranged from 14 to 43 km with a − 0.78 mean misfit for Moho depths derived from seismic P-wave analysis. The calculated gravity data had a 0.16 mGal misfit with the observed data. The normalized analytical signal helped to sharply delineate the different plates and their edges. Three cross-sections were conducted to model the stagnant slab along the subduction zone of the Eastern Eurasian plate, western Pacific plate, Okhotsk plate, and Philippine Sea plate. The eastern area of the entire region (Pacific plate) had a low crust thickness and high density compared to the western area (Eurasian plate), which had a high crust thickness and low density. • 3D gravity inversion of the Northwestern Pacific Plate at the Japan Islands. • Estimating the Moho depths around the Japan Islands. • 3D density modeling of the Stagnant Slabs and Subduction plates. [ABSTRACT FROM AUTHOR]

Abstract: The petrological consequences of deep subhorizontal deflection (“stagnation”) of subducting slabs should affect both apparent seismic velocity structures and slab morphology. We construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high-pressure phase transitions in mantle minerals, focusing upon Japan as our study area. We calculate associated thermo-petrological buoyancy forces and bending moments which (along with other factors such as viscosity variations and rollback dynamics) may contribute to slab deformation. We consider effects of variations in depth of stagnation, post-stagnation dip angle, phase transition sharpness, transition triplication due to multiple intersection of geotherms with phase boundaries, and potential persistence of metastable phases due to kinetic hindrance. We also estimate seismic velocity anomalies, as might be imaged by seismic tomography, and corresponding seismic velocity gradients, as might be imaged by receiver-function analysis. We find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation. Such buoyancy forces vary with the depth at which stagnation occurs, so that slabs may seek an equilibrium slab stagnation depth. Metastable phase bending moment gradients further enhance slab stagnation, but they thermally decay after ∼600–700km of horizontal travel, potentially allowing stagnant slabs to descend into the lower mantle. Stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660-km seismic discontinuity, affecting the seismological visibility of such features. Seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique (travel-time tomography vs. boundary-interaction phases). While seismic tomography appears to yield images of stagnant slabs, discontinuity topography beneath Japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone. However, model slabs which bottom around ∼780–810km and then bend upwards by a few degrees can match both the tomographic and receiver-function images. [Copyright &y& Elsevier]

Abstract: Hypocenter relocations of earthquakes in the Pacific slab have shown an anomalous deepening of a seismic belt in the upper-plane of the double seismic zone at depths of 80–120km beneath the Hokkaido corner, while it is located at depths of 70–90km in the surrounding Tohoku and eastern Hokkaido areas. Seismic tomographic inversions performed beneath the Hokkaido corner have shown that a low-velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the Pacific slab and makes direct contact with the upper surface of the Pacific slab. These observations suggest that: 1) the low-velocity zone just above the Pacific slab is part of the subducted Kuril forearc sliver. 2) The contact with the subducted, and so relatively cold, sliver materials prevents the mantle wedge from heating the Pacific slab and causes a lower temperature condition in the Pacific slab crust beneath the Hokkaido corner than in the surrounding areas. 3) As a result, a delay of eclogite-forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there. [Copyright &y& Elsevier]

Abstract: We estimate detailed three-dimensional seismic velocity structures in the subducting Pacific slab beneath Hokkaido, Japan, using a large number of arrival-time data from 6902 local earthquakes. A remarkable low-velocity layer with a thickness of ~10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust. The layer gradually disappears at depths of 70–80 km, suggesting the breakdown of hydrous minerals there. We find prominent low-velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 Kushiro-oki earthquake (M7.8). Since seismic velocities of unmetamorphosed peridotite are much higher than the observations, hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake. On the other hand, regions between the upper and lower planes, where seismic activity is not so high compared to the both planes, show relatively high velocities comparable to those of unmetamorphosed peridotite. Our observations suggest that intermediate-depth earthquakes occur mainly in regions with hydrous minerals, which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab. [Copyright &y& Elsevier]

Abstract: We examine the upper mantle and transition zone beneath the western Pacific using multiple ScS reverberations. A low-velocity zone (LVZ) is found above the 410-km discontinuity oceanward of the subducting Honshu slab at an average depth of 356km, with a thickness that ranges from 50 to 75km assuming the LVZ continues to the 410-km discontinuity, which is locally elevated. The low-velocity region is evident in previous tomographic studies, and our results suggest that the anomaly is best explained by a layer of partial melt. The layer may be entrained from above by subduction or produced in situ by the combined effects of water and temperature. A self-consistent model that explains local P-wave velocities (Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters 243, 149–158) and our observations calls for a maximum temperature anomaly of ∼150°C and a resulting maximum olivine water content after melting of 0.100wt. %. [Copyright &y& Elsevier]

Abstract: Both the global and regional P wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions. In particular, low-velocity anomalies have been observed beneath the descending high-velocity slabs in a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, make it difficult to substantiate this feature and evaluate the vertical extent of the low-velocity structure. From broadband waveform modeling of triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeast Asia. For the two events beneath the Southern Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH velocity anomalies are shown in our preferred models for the Japan subduction zone region, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan subduction zone. The respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under-slab areas. Based on mineral physics data, the depth-averaged ∼1.5% P and ∼2.5% SH velocity differences in the depth range of 560–760km between the two regions could be primarily explained by a 350–450K temperature variation, although the presence of about 0.5–1wt.% water might also contribute to the subtle velocity variations near the base of the transition zone in the Southern Kurile. From our modeling results, we speculate that the slow structure in the Southern Kurile may be correlated to the low-velocity zone observed previously around the 410-km discontinuity under Northern Honshu. If this is the case, both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast Asia. [Copyright &y& Elsevier]