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Anisotropic structures within the crust are frequently perceived to originate from stress-induced cracks, which have been mainly estimated on land through different wave speeds of orthogonally polarized S waves propagating in the anisotropic media. However, such estimations of crustal anisotropic structures in ocean areas, particularly for subduction zones around trenches, have not been investigated in detail due to the lack of long-term ocean bottom observations. In this study, we used ocean bottom seismometers of a permanent network deployed across the Japan Trench and the southern part of the Kuril Trench and applied the shear-wave splitting analysis to P-to-s converted waves extracted by receiver function analyses using teleseismic events. We estimated the anisotropic structures in marine sediments and oceanic crust for the incoming Pacific Plate and marine sediments for the overriding North American Plate. The obtained fast polarization directions for the incoming plate are mainly oriented to be parallel to the trench axis for the marine sediment and oceanic crust, which are formed by normal faults and cracks due to the upward plate bending in the outer-rise region, whereas results for marine sediments at the northern part of the Japan Trench are obliquely aligned to the trench axis. The oblique direction is consistent with the magnetic lineations of the incoming plate, indicating that ancient faults within the plate, which were formed in the shallow part of the crust during the creation of the oceanic plate at the ridge, are reactivated by the plate flexure. For the overriding plate, the fast polarization directions in the northern and southern parts of the study area are nearly normal to the trench axis. The central part shows two distinct features: the fast polarization directions parallel to the trench axis and small degrees of anisotropy. These patterns may reflect crack alignments associated with the lateral variation in postseismic crustal deformation after the 2011 Tohoku-Oki earthquake. Our results suggest substantial lateral variations in the stress field at the tip of the overriding plate along the strike direction. [ABSTRACT FROM AUTHOR]

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido. [ABSTRACT FROM AUTHOR]

A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous. [ABSTRACT FROM AUTHOR]

Devastating megathrust earthquakes and slow earthquakes both occur along subducting plate interfaces. These interplate seismic activities are strongly dependent on the nature of the plate interface, such as the shape of the plate interface and the materials and physical conditions along the plate interface. The oceanic plate, which is the input to the subduction zone, is the first order control on the nature of the plate interface. To reveal the nature of the subduction inputs to the northeastern Japan arc, we have conducted large-scale controlled-source seismic surveys of the northwestern part of the oceanic Pacific plate. The obtained seismic data have revealed (1) oceanic plate structural evolution caused by plate bending prior to subduction, suggesting the promotion of the oceanic plate hydration; (2) spatial variation of the oceanic plate structure, such as variations in the thickness of sediment and crust; (3) that the spatial variations are caused by both ancient plate formation processes and more recent volcanic activities; and (4) that spatial variations of the nature of the subduction inputs show good correlation with the along-strike variations in the seismic structure and seismic activities after subduction, including the coseismic slip distribution of the 2011 Tohoku earthquakes and the structural differences between the northern and the southern Japan Trench. These observations indicate that the incoming oceanic plate structure is much more spatially variable than previously thought and also imply that the spatial variation of the subduction inputs is a key controlling factor of the spatial variation of various processes in subduction zones, including interplate seismic activities and evolution of the forearc structure. [ABSTRACT FROM AUTHOR]

Inversion for seismological structures of the oceanic lithosphere–asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties in fitting phases for shorter-period components in the previous global tomography have yielded resultant models that possess poor resolutions above |$\sim \, 50$| km depth. Recent developments of broad-band ocean-bottom seismometer (BBOBS) arrays provide larger amount of seismic data with epicentral distances of <20°. In this study, we develop an appropriate method to fully utilize the information contained in the shorter-period components of BBOBS arrays. We first fit the envelopes without phase information to analyse the shorter-period components (8–60 s) which are generally unavailable in the conventional phase fitting. We then use the resultant model as our initial model for waveform inversion of the longer periods (12.5–200 s) to fit the phase, which allows us to infer a continuous structure model from the crust to the asthenosphere. We demonstrate the validity of this combined envelope-fitting and waveform inversion method by analysing the waveform data from a BBOBS array that was deployed in the Northwestern Pacific and has recorded events in the vicinity of the Japan Trench to obtain the average velocity structure between the event and station arrays. We independently resolve the crustal compressional and shear wave velocities, and thickness by analysing the envelopes, which minimizes biases in the subsequent waveform inversion. We also find that the waveform inversion improves the resolution in the asthenosphere. Our results suggest that further extension of this method should improve our knowledge of the oceanic lithosphere–asthenosphere system. [ABSTRACT FROM AUTHOR]

Dehydration of subducting oceanic lithosphere (slab) induces Li‐isotope fractionation between the fluid and the slab, suggested by the δ7Li variation (∼10‰) in exhumed subduction complexes. Given that arc magmas represent melt of the supraslab mantle, a large δ7Li variation is anticipated for arc volcanic rocks. However, the δ7Li values in these rocks are mostly homogeneous within the range of mid‐ocean ridge basalts (+1.6 to +5.6‰). The lack of a subduction‐related δ7Li signature has been explained by (a) homogenization by mixing of different magma sources, (b) loss of Li from the slab via dehydration, or (c) homogenization by diffusive exchange of slab‐derived Li and the mantle. The Chugoku district in SW Japan is an ideal place to study the process responsible for Li‐isotope variation in arc magmas, since the Chugoku volcanic rocks show large δ7Li variation (−1.9 to +7.4‰). High δ7Li values (+6.3 to +7.4‰) are found in some high‐Sr andesites and dacites (adakites) whereas low δ7Li values (−1.0 to −0.1‰) are found in high‐Mg andesites. The parental magmas of these rocks have been sourced from subducted oceanic crust and sediments, respectively, with various extents of the interaction with wedge mantle. The limited extents of Li isotope modification are indicated by the similarity of the δ7Li values of these rocks and their supposed sources. The models for a slab dehydration and a diffusive exchange between slab‐derived melt and mantle demonstrate that the δ7Li signatures of the sources can be preserved in the adakites if they ascent rapidly in mantle. Plain Language Summary: Many of Earth's volcanoes occur near the trenches, where one plate subducts beneath another. These volcanoes are formed as a result of the melting of mantle that contains materials of subducted plates and the transportation of melts through the mantle. A quantitative understanding of these important processes has been hampered by the lack of geochemical tools to examine consecutive chemical reactions beneath the volcanoes. Here we conducted a lithium isotope investigation of the volcanic rocks from the Chugoku district in Southwest Japan. This district hosts the volcanoes of andesites and dacites which have been derived from the melting of materials in the subducted plate. We found that these volcanic rocks preserve Li‐isotope compositions of the original materials when they melted. This in turn suggests that these magmas ascent rapidly (<300,000 years) in the mantle. Key Points: Volcanic rocks in the Chugoku district in Southwest Japan show large δ7Li variation (−1.9‰ to +7.4‰)Li isotopes fractionate to the limited extent during arc magma productionSlab‐derived δ7Li features can be preserved if slab‐derived fluids/melts have distinct δ7Li values from mantle and ascend rapidly [ABSTRACT FROM AUTHOR]

Long‐term and short‐term temporal variations in seismic velocity have been investigated using seismic interferometry, and significant short‐term changes are typically recognized as velocity reductions caused by large earthquakes. In this study, using a dense network of ocean bottom seismometers (Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench: S‐net), we measured temporal variations in seismic velocity at the top of the overriding plate in the region between the Japanese Islands and the Japan Trench. We detected abrupt velocity reductions due to the nearby Mw 7‐class earthquakes in 2016 and 2021, and gradual velocity increases mainly observed near the trench. The sudden velocity reductions due to the shallow earthquake in 2016 within the overriding plate are large above the hypocenter, while those due to the three deep earthquakes in 2021 are large near the trench, although these earthquakes occurred far from the trench. The three earthquakes are located at the top of and within the subducting oceanic crust of the Pacific Plate. The spatial distributions of the velocity reductions for the four earthquakes are consistent with those of the peak ground velocities (PGVs). This indicates that for the three deep earthquakes, seismic waves trapped within the descending Pacific Plate reach to the trench, and the trapped waves induce large PGVs, and hence substantially reducing the velocity. Long‐term variations observed near the trench are possibly caused by the recovery process of coseismic velocity reductions due to large earthquakes and/or fluid migration induced by the tectonic stress field in the region. Plain Language Summary: Seismic interferometry is a powerful tool for estimating temporal variations in seismic velocity. We applied this technique to the continuous records observed at a dense network of permanent ocean bottom seismometers deployed around the Japan Trench and examined temporal variations in seismic velocity at the shallow depths of the overriding plate. As a result, we observed sudden velocity reductions associated with the nearby Mw 7‐class earthquakes in 2016 and 2021. The region with large velocity reductions in the 2016 earthquake is above its hypocenter, while large reductions in the cases of the three 2021 earthquakes are observed near the trench, although their epicenters are distant from the trench. The depth of the 2016 earthquake is 25 km and those of the three 2021 earthquakes are 50–60 km. These results indicate that seismic waves trapped within the subducting Pacific Plate in the case of deep earthquakes reach the trench and induce large sudden reductions due to strong seafloor motions. Long‐term variations in seismic velocity were also observed near the trench, which are caused by the recovery process of earthquakes and/or fluid migration induced by the tectonic stress field. Key Points: We examined temporal variations in seismic velocity at shallow depths of the overriding plate around the Japan TrenchSeismic waves trapped within the subducting plate cause velocity reductions far from the epicentersLong‐term variations are caused by the recovery processes of coseismic velocity reductions and/or fluid migration due to the tectonic stress field [ABSTRACT FROM AUTHOR]

Geophysical observations indicate that patches of localized fracturing occur within otherwise viscous regions of subduction plate boundaries. These observations place uncertainty on the possible down‐dip extent of the seismogenic zone, and as a result the maximum magnitude of subduction thrust earthquakes. However, the processes controlling where and how localized fracturing occurs within otherwise viscous shear zones are unclear. We examined three exposures of exhumed plate boundary on Kyushu, Japan, which contain subducted sediments and hydrated oceanic crust deformed at ∼300 to ∼500°C. These exposures preserve subduction‐related viscous deformation, which in two of the studied exposures has a mutually overprinting relationship with quartz veins, indicating localized cyclical embrittlement. Where observed, fractures are commonly near lithological contacts that form viscosity contrasts. Mineral equilibrium calculations for a metabasalt composition indicate that exposures showing cyclical embrittlement deformed at pressure‐temperature conditions near dehydration reactions that consume prehnite and chlorite. In contrast, dominantly viscous deformation occurred at intervening pressure‐temperature conditions. We infer that at conditions close to metamorphic dehydration reactions, only small stress perturbations are required for transient embrittlement, driven by localized dehydration reactions reducing effective stress, and/or locally increased shear stresses along rheological contrasts. Our results show that the protolith composition of the subducting oceanic lithosphere controls the locations and magnitudes of dehydration reactions, and the viscosity of metamorphosed oceanic crust. Therefore, compositional variations might drive substantial variations in slip style. Plain Language Summary: Along tectonic plate boundaries, with increasing depth, pressure and temperature, plate movement by brittle fracturing is suppressed, and mechanisms allowing steady slip become more efficient. Along subduction plate boundaries, where one plate sinks beneath another, observations indicate that a proportion of slip sometimes occurs by fracturing at pressure‐temperature conditions where steady slip typically dominates. We use outcrops of an ancient, inactive plate boundary exposed on Kyushu, Japan to investigate the reason for this behavior. We found that chemical reactions, triggered by increasing temperature in sinking oceanic crust, produce water, with the effect of locally raising the fluid pressure within the plate boundary. We suggest that locally raised fluid pressures assist fracturing at pressure‐temperature conditions where steady slip typically dominates. In some outcrops, fractures are especially common along contacts between different rock types, suggesting that mixing of different materials along the plate boundary might also favor fracturing. Key Points: Brittle fracturing occurred locally within viscously deforming hydrated oceanic crust and subducted sedimentsFracturing is localized at viscosity contrasts and P‐T conditions of metamorphic dehydration reactionsRocks that deformed at P‐T conditions away from dehydration reactions record dominantly viscous behavior [ABSTRACT FROM AUTHOR]

The extension of back‐arc basins and formation of marginal seas following the subduction of oceanic lithosphere are usually attributed to the rollback of subducting slabs and distorted mantle convection. However, for the Japan Sea, the largest marginal sea in the northwestern Pacific, its opening is unlikely only resulted from the subduction of the Pacific plate because of the coeval Philippine plate subduction and the arcuate arc volcanic zone. Therefore, the present‐day thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. Here, we observe two belts of strong Pn attenuation beneath the Japan Sea; their strikes are generally consistent with local Pn anisotropy and the retreat directions of the Pacific and Philippine trenches. Hence, there seem to be two divergent mantle flows in the uppermost mantle, pushing a "double door" for the Japan Sea opening. Plain Language Summary: The western Pacific is one of the most active regions of global tectonics. For the Japan Sea, the largest marginal sea in the northwest Pacific, its formation mechanism is still controversial. The Japan Sea may have experienced a complex evolution process driven by the superposition of multiple mechanisms, and finally produced a diamond‐shaped ocean. The present thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. In this study, we construct a high‐resolution broadband Pn attenuation model for the uppermost mantle beneath the Japan Sea. Two strong Pn attenuation belts are observed in this region, with their strikes generally consistent with the local Pn velocity anisotropy and the opening directions of the Japan Sea. Therefore, two divergent mantle flows likely exist in the uppermost mantle, pushing the opening of the Japan Sea like a "double door." These mantle flows could be part of mantle convection in a big mantle wedge, where ascending hot materials from the deep mantle not only feed volcanoes in northeastern Asia but also thicken the back‐arc oceanic crust. Key Points: A high‐resolution broadband Pn attenuation model is obtained beneath the Japan SeaHot mantle materials ascend to feed the volcanoes in NE Asia, intruding into and thickening the oceanic crust in the Japan SeaDivergent mantle flows likely push the fan‐shaped rotational opening of the Japan Sea [ABSTRACT FROM AUTHOR]

Lateral spatial variations of weak portions at the plate boundary in subduction zones have been estimated primarily by the distribution of slow earthquakes mainly occurring around seismogenic zones. However, the detailed depth profile of weak faults remains elusive. Here, we deployed six ocean bottom seismometers in the Nankai subduction zone, Japan, to observe reflections originated from drilling vessel Chikyu ship noise (hydroacoustic P wave) that was persistently radiated from a fixed position at the sea surface, and retrieved P-to-s (Ps) reflections from multiple dipping faults near the plate boundary. The Ps amplitudes were stacked and compared according to the degrees of tidal stresses, and high amplitudes were observed at high tide (compression). A migration technique shows that the locations where velocity contrasts fluctuate were estimated at both the megasplay fault and another fault between the megasplay fault and the top of the oceanic crust. This indicates that the physical properties of these faults are altered by tidal stress. The physical-property changes are attributed to fluid connections and isolations within the faults due to tidal stress fluctuations, inducing the variation of seismic anisotropy. Such a variation was confirmed by a three-dimensional numerical simulation for wave propagation with anisotropic medium. Our observation suggests that multiple weak faults are present around the plate boundary, and the obtained changes of fault physical properties may have implications for in-depth understanding of tidal triggering of earthquakes. [ABSTRACT FROM AUTHOR]

Deep tectonic tremors occur at the downdip extent of the seismogenic zone due to fluid processes. Beneath the northeastern Kii Peninsula, southwestern Japan, there is an along‐dip bimodal distribution of tremor. However, no constraint exists on the structures controlling that distribution. We extract detailed seismological structures from multi‐band receiver functions and evaluate conditional differences in the distribution. To achieve high resolution images along the plate interface, we utilize records of regional deep‐focus earthquakes from the Pacific slab. Cross‐section images show the subducting oceanic plate with depth‐dependent phases along the bimodal distribution, revealing a conspicuous plate interface at the updip portion and an inconspicuous interface below the mantle wedge at the downdip portion. This indicates that episodic tremors occur in the high pore‐fluid plate interface below the impermeable forearc crust, and that continual tremors occur at the permeable mantle wedge corner, owing to continuous fluid supply from the oceanic crust. Plain Language Summary: Deep slow earthquakes have mainly been detected at the deeper extent of estimated large‐slip regions of large‐scale regular earthquakes in the Nankai subduction zone, southwestern Japan. Epicenters of tectonic tremors are also downdip‐aligned. However, some clusters of continual tremor with frequent small bursts were found at further downdip portions beneath the northeastern Kii Peninsula. The complexity of the bimodal tremor distribution poses a structural question regarding whether the tectonic tremor occurs below a mantle wedge or below the continental crust. We utilize a receiver function method that surveys subsurface velocity boundaries and evaluate detailed seismological structures around the plate interface using a multi‐band analysis. Furthermore, regional deep‐focus earthquake records are effectively utilized for receiver function mapping. The high‐frequency cross section exhibits depth dependence of plate‐interface phases, which demarcates active regions of updip events and downdip continual tremor, thus revealing that episodic tremor occurs below the continental crust and continual tremor occurs at the mantle wedge corner. The high‐contrast updip interface reveals that a large amount of fluid is confined at the plate interface below the impermeable forearc crust, which may lead to active episodic slow earthquakes at updip portions. Key Points: Multi‐band receiver functions from global and regional deep events provide detailed plate‐interface structures in the Kii PeninsulaHigh‐frequency images show a clear depth dependence for the subducting plate interfaceAlong‐dip difference in overriding plate materials and permeability explains bimodal distribution of tectonic tremor [ABSTRACT FROM AUTHOR]

We conducted centroid moment tensor (CMT) inversions of moderate (Mw 4.5–6.5) earthquakes in the Kanto region, Japan, using a local three-dimensional (3D) model. We then investigated the effects of our 3D CMT solutions on long-period ground motion simulations. Grid search CMT inversions were conducted using displacement seismograms for periods of 25–100 s. By comparing our 3D CMT solutions with those from the local one-dimensional (1D) catalog, we found that our 3D CMT inversion systematically provides magnitudes smaller than those in the 1D catalog. The Mw differences between 3D and 1D catalogs tend to be significant for earthquakes within the oceanic slab. By comparing ground motion simulations between 1D and 3D velocity models, we confirmed that observed Mw differences could be explained by differences in the rigidity structures around the source regions between 3D and 1D velocity models. The 3D velocity structures (especially oceanic crust and mantle) are important for estimating seismic moments in intraslab earthquakes, which are related to fault size estimation. A detailed discussion for intraslab seismicity can be conducted using the 3D CMT catalog. The seismic moments also directly affect the amplitudes of ground motions. The 3D CMT catalog allows us to directly conduct the precise forward and inverse modeling of long-period ground motion without adjusting source models, which have been typically applied in the cases using the 1D CMT catalog. We also conducted long-period ground motion simulations using our 3D CMT solutions to evaluate the reproducibility of long-period ground motions at stations within the Kanto Basin. The simulations of our 3D CMT solutions well-reproduced observed ground motions for periods longer than 10 s, even at stations within the Kanto Basin. The reproducibility of simulations was improved from those using solutions in the 1D catalog. [ABSTRACT FROM AUTHOR]

The Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P -to- S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab. [ABSTRACT FROM AUTHOR]

Surface heat flow has been widely used to constrain the thermal structure of subduction zones. However, the forward modelling approaches in previous geodynamic studies have only provided limited information on the model parameters controlling the thermal structure, which makes model validation difficult. Here we apply a probabilistic inversion technique based on Bayes' theorem to surface heat flow data from Tohoku in Japan and Cascadia to simultaneously infer five model parameters that appear to have the greatest influence on the thermal structure of subduction zones. The surface heat flow is predicted via 2-D steady-state thermomechanical modelling. The Metropolis algorithm is used to obtain the posterior probability distributions. A comparison of our results with previous estimates indicates that our activation energy for the shear viscosity of dislocation creep is lower in both regions, and our radiogenic heat production rate in the upper continental crust is lower in Cascadia. These findings suggest that our geodynamic models cannot explain the surface heat flow observations with the acceptable ranges of model parameter values. We therefore need to refine the models by including, for example, the effects of recent backarc extension, vigorous thermal convection beneath the overriding plate and fluid circulation in the uppermost part of the oceanic crust. The approach presented here also allows us to determine trade-offs between the parameters. This study provides a framework to validate and refine geodynamic models based on various types of observations. [ABSTRACT FROM AUTHOR]

To explore a local relationship between geological structures and the occurrence of very-low-frequency earthquakes (VLFEs), a particular class of slow earthquakes with characteristic periods of 10–100 s, we investigated three-dimensional (3D) structural features using reprocessed 3D seismic data from the Nankai Trough off Kumano, southwestern Japan. In this region, VLFEs have been observed along the subducting Philippine Sea Plate. Although the detailed source distribution of VLFEs was estimated by means of recent land-based and offshore seismic networks, the relation with geological features is not well understood. First, we reprocessed the 3D seismic data with advanced techniques and reinterpreted the fault distribution in the sediment layer of the accretionary prism and tracked two key horizons: a décollement and the oceanic crust surface. In the accretionary prism sediments, multiple continuous reflectors of basal detachments in the underthrust sequence and conjugate faults cutting the shallow imbricated thrust sequence were identified. In contrast to the gentle variation in the décollement surface, the topographic relief of the oceanic crust was prominent, with ridges and surface displacement due to faults in the oceanic crust. Then, we compared the structural features with the VLFE source locations. Most VLFEs were located deep in the underthrust sediments where the sediments may consist of underconsolidated muds. Furthermore, a high spatial correlation was observed between the VLFE distribution and the oceanic crust topographic relief. The maximum stress direction, which was inferred from the conjugate faults in the imbricated thrust zone, was consistent with the spatial relation between the VLFE localization and the oceanic crust central ridge. Oceanic crust ridges may cause strain accumulation in the underthrust sediments on the landward sides of the ridges, and low-angle slow thrust movements might be caused using weak slip planes in the underthrust muddy sediments. That is, the topographic relief of the oceanic crust may control the occurrence of shallow VLFEs in the Nankai Trough. [ABSTRACT FROM AUTHOR]

Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decay within the first 10 million years as a result of pervasive hydrothermal circulation and alteration. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder heat advection. Here we show, however, that a systematic loss of ~20% in the amplitude of the anomalies arises between the outer rise and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decay as reflecting the opening of normal faults and fissures caused by extension on the outer flexural rise and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench. Plain Language Summary: Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decrease within the first 10 million years as a result of the widespread circulation of hot seawater within the oceanic crust and the resulting alteration of its magnetic minerals. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder the free exchange of seawater between the crustal aquifer and overlying ocean. Here we show, however, that a systematic loss of ~20% in the amplitude of the anomalies appear between the outer rise, an elevation caused by the flexure of the plate entering subduction, and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decrease as reflecting the opening of faults and cracks caused by extension at the top of the bent oceanic plate and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench. Key Points: Decaying seafloor spreading magnetic anomalies are observed before subduction in Japan‐Kuril subduction zoneThe damping shape decaying pattern corresponds to increasing heat flow and the hydration of the oceanic crust approaching the trenchesThe extra alteration of magnetic minerals by renewed water circulation from the flexure‐related opening of normal faults cause the decay [ABSTRACT FROM AUTHOR]

Volcanism following the initiation of subduction is vital to our understanding of this specific magma-generation environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collection of volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10 Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8 Ma. Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45 Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMU ocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45 Ma tholeiite and calcalkaline volcanic rocks. Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspond to the increasing depth of the leading edge of the slab through this 5 Myr period. [ABSTRACT FROM AUTHOR]

Early magnetic studies of the Japan Trench showed that seafloor spreading magnetic anomalies progressively fade away and disappear during subduction, reflecting the increasing distance to magnetized sources and the removal of their remanent magnetization with alteration and increasing temperature. An improved magnetic anomaly map derived from both scalar and vector magnetic anomaly data, coupled with a better knowledge of the slab geometry in one hand, of the magnetic structure of the oceanic crust on the other hand, allow us to constrain the thermal structure of the subducting slab. We, for the first time, identify two steps in the anomaly disappearance: first the magnetization of extrusive basalt is rapidly erased between 9 and 12 km, where titanomagnetite reaches its blocking temperature between 150 °C and 350 °C, then the magnetization of deeper crustal layers slowly decreases down to ~20 km, reflecting the progressive slab heating toward the Curie temperature of magnetite, 580 °C. The resulting slab temperatures are higher than predicted by most thermal models. Recent observations and models suggest rejuvenated hydrothermal activity triggered by lithospheric flexure before subduction that may significantly heat up the subducting oceanic crust through thermal blanketing and possibly serpentinization, with consequences on the depth of the seismogenic zone. [ABSTRACT FROM AUTHOR]

Although seafloor massive sulfide (SMS) deposits are crucially important metal resources that contain high‐grade metals such as copper, lead, and zinc, their internal structures and generation mechanisms remain unclear. This study obtained detailed near‐seafloor images of electrical resistivity in a hydrothermal field off Okinawa, southwestern Japan, using deep‐towed marine electrical resistivity tomography. The image clarified a semi‐layered resistivity structure, interpreted as SMS deposits exposed on the seafloor, and another deep‐seated SMS layer at about 40‐m depth below the seafloor. The images reinforce our inference of a new mechanism of SMS evolution: Upwelling hydrothermal fluid is trapped under less‐permeable cap rock. The deeper embedded SMS accumulates there. Then hydrothermal fluids expelled on the seafloor form exposed SMS deposits. Plain Language Summary: Hydrothermal circulation of seawater through the permeable ocean crust engenders formation of seafloor massive sulfide (SMS) deposits, which present high potential for metal mining. Geophysical surveys using modes such as electrical and electromagnetic methods have revealed that SMS deposits exhibit lower resistivity than the surrounding host rock. However, because detailed geophysical images of internal structures of SMS deposits are lacking, the spatial distribution of SMS deposits and the evolutionary processes of SMS deposits remain unclear. For this study, we applied a deep‐towed marine electrical resistivity tomography (ERT) system to capture detailed images of electrical resistivity structures of SMS deposits in the Iheya North hydrothermal field, Okinawa Trough, southwestern Japan. An optimal sub‐seafloor resistivity section reveals a semi‐layered structure consisting of double low‐resistivity SMS layers: exposed and deep‐seated ones. Between the SMS layers, a cap rock layer is recognized as a moderately resistive zone. This detailed structure offers an explanation of how the SMS deposits accumulate: Hydrothermal fluids upwelling from the deep crust are trapped by less‐permeable cap rock, which results in the precipitation and accumulation of SMS deposits below the cap rock. Then fluids passing through the cap rock to the seafloor produce SMS deposits on the seafloor. Key Points: Deep‐towed marine electrical resistivity tomography revealed massive sulfide deposits on the seafloor as low‐resistivity zonesLow‐resistivity zones are semi‐layered with deposits exposed on the seafloor and other deposits at 40‐m depth below the seafloorDetailed near‐seafloor images of electrical resistivity offer an explanation of how the SMS deposits accumulate [ABSTRACT FROM AUTHOR]

The Oeyama ophiolite is considered as one of the major ophiolites in the Japanese Islands that forms a part of the "circum-Pacific Phanerozoic multiple ophiolite Belts". In this study, zircon U-Pb age and their REE geochemistry, and Nd-Sr isotope geochemistry are presented for the gabbroic rocks of the Oeyama ophiolite to determine the precise crystallization age and characterize the proper tectonic setting of this ancient oceanic crust. The zircon grains of the two studied gabbroic samples yield 206Pb/238U ages of 545.4 ± 2.6 Ma and 532.4 ± 3.1 Ma. From five gabbroic rocks, a comparable 566 ± 95 Ma Sm-Nd whole rock isochron age is obtained. The time-corrected εtNd values (t = 545 Ma) for each sample are ranging from +7.2 to +7.7 that suggest their normal mid-oceanic basalt (N-MORB) affinity. Discrimination diagrams using zircon U/Yb versus Hf and Y also indicate their oceanic crustal origin. This is so far known the oldest date of ophiolite in the Japanese accretionary system. On the basis of the present dataset, we have offered a scenario of the potential tectonic development highlighting the journey of this group of oceanic crustal rocks from the mid-oceanic region till their final accretion to the Japanese Islands. The present tectonic model envisages an initial oceanward subduction at ca. 530 Ma switched over to the present continentward subduction at ca. 400 Ma. Moreover, the three different units of the Oeyama ophiolite have been re-classified based on the present age dataset, and the formation and evolution of these units constituting the typical lower part of the ocean plate are explained. Unlabelled Image • Zircon U-Pb age of 545 ± 3 Ma for Oeyama ophiolite is reported for the first time. • This is so far known the oldest date of Japanese ophiolite. • Nd isotopic signature and zircon chemistry suggest a depleted mantle origin. • Latest Neoproterozoic-Carboniferous arc-backarc-forearc crusts are characterized. • Model of ocean formation and spreading, 150 myr after Rodinia breakup is suggested. [ABSTRACT FROM AUTHOR]

Recently, a M w 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the M w 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the M w 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence. • Double-difference tomography of 2021/02/13 M w 7.1 earthquake near northeastern Japan. • The earthquake nucleates at the boundary between low and high velocity anomalies. • The rupture plane shows similar dimension as the outer-rise normal faults. • Mantle serpentine dehydration could increase the pore pressure at the mainshock. • The earthquake sequence is likely cause by reactivation of a pre-existing fault. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Polished sections of ferromanganese crusts on underwater rises in the Sea of Japan were studied with a JXA8100 microprobe analyser. Mineral phases of REE have been detected. They have Ln2O3 chemical composition, where Ln is La–Ce–Pr–Nd, La–Ce–Nd, or, much more rarely, La–Ce and La–Ce–Pr. With regard to the same chemical composition of REE grains in the ore crusts and basalts from Medvedev Volcano, it has been concluded that these REE were supplied during postvolcanic gas–hydrothermal processes. [Copyright &y& Elsevier]

SUMMARY High-sensitivity, dense seismic observations and numerical simulations revealed a distinct guided SP converted wave of oceanic mantle earthquakes that occurred within the subducting Philippine Sea (PHS) plate and a waveguide effect of the PHS Plate beneath southwest Japan. Based on seismograms, we identify the oceanic mantle earthquakes of the double seismic zone and find a new type of later phase for these earthquakes. The primary features of these later phases are as follows: (1) these phases are observed at the northeastern inland stations of the convex slab axis, (2) the vertical and radial components are dominant and (3) the apparent velocity is approximately 8.0 km s-1. To examine the origin of these later phases, we simulate a 2-D P- SV seismic wavefield considering the PHS slab geometry. Based on the numerical models, we conclude that the later phases are guided SP converted waves trapped within the oceanic crust, which acts as low-velocity layer. A series of numerical simulations indicates that two factors are necessary to reproduce the observed SP guided waves: (i) a low-velocity channel must exist between stations and sources and (ii) the oceanic crust must only be in contact with the lower crust near the stations. [ABSTRACT FROM AUTHOR]

Abstract: Mt. Ishimaki is the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan. The Ishimaki limestone is thought to be seamount-type limestone. The P1 elements of the conodonts Norigondolella navicula and Ancyrogondolella quadrata found in the limestone indicate it is of Norian age. The Sr isotopic compositions of 45 Ishimaki limestone samples ranged from 0.7055 to 0.7077. Eighteen of these samples had lower Sr isotopic compositions than the lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. The Sr isotopic compositions in the limestone block are generally lower at the base of the block and higher at the top. The present Sr isotopic compositions of the Ishimaki limestone are unlikely to have been reduced by post-depositional alteration because most of the limestone samples had a low amount of Mn (<300ppm) or high Sr/Mn ratios (>2) and the conodont elements had low (1–2) CAI (conodont alteration index) values. Additionally, there was little acid-insoluble residue. Thus, the low Sr isotopic compositions are thought to represent the strontium of the past ambient seawater. The low Sr isotopic compositions are in complete disagreement with the generally recognised range of seawater Sr isotopic compositions in the Norian stage of the Late Triassic (0.7075–0.7078) because the depositional environment of the Ishimaki limestone was closed or semi-closed from the Panthalassic Ocean. Therefore, the Sr isotopic composition of the limestone differs from that of the Panthalassic seawater. The low Sr isotopic compositions were probably affected by Sr inflows from mafic oceanic crust by hydrothermal fluid circulation or from hinterlands surrounded by mafic rocks by river water circulation. [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: The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C. [Copyright &y& Elsevier]

Seafloor spreading magnetic anomalies of oceanic plates progressively decay and disappear when approaching and entering subduction. Previous studies focused on the Japan Trench showed that before subduction, the flexure of oceanic lithosphere opens normal faults and rejuvenates hydrothermal circulation which results in the alteration of magnetic minerals. After subduction, thermal demagnetization affects the magnetic minerals of the oceanic crust when the increasing temperature of the subducting slab passes their Curie temperature. No other subduction zone was investigated so far. Here we study the evolution of magnetic anomalies in five subduction zones. The major observed differences are controlled by the age of the lithosphere and related physical properties. Before subduction, magnetic anomalies of old oceanic lithosphere show a significant decay (20-30%) related to alteration whereas those of young lithosphere at the Cascadia subduction zone displays no such decay as a consequence of negligible flexure on the young Juan de Fuca plate. After subduction, the magnetic anomalies of the subducting plate show a rapid decay reflecting the thermal demagnetization of extrusive basalt, amounting for 30-40% in old oceanic lithosphere and more than 70% in the young lithosphere of Cascadia. Beyond complete thermal demagnetization of extrusive basalt, about 40% of the magnetization remains in the old lithosphere and less than 30% in the young lithosphere of Cascadia. • Among eleven investigated subduction zones, nine display decaying magnetic anomalies and five are suitable for detailed analysis. • Before subduction, flexure, fluid circulation and the alteration of magnetic minerals play a major role in the magnetic anomaly decay. • Young subducting lithosphere shows no flexure, and therefore no magnetic anomaly decay, before subduction. • After subduction, thermal demagnetization of the extrusive basalt layer plays a major role in the magnetic anomaly decay. [ABSTRACT FROM AUTHOR]

• Mafic schists from subducted oceanic crust deform by semi-brittle processes at the conditions of experiments. • Strength of the maciffic schists increases with increasing degree of prograde metamorphism. • All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values). • Our experimental results are consistent with that the general absence of crustal seismicity in hot subduction zones. • The semi-brittle deformation could control the slab-mantle decoupling and the exhumation process of these rocks. We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1 GPa, 400 °C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1 GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500 °C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust. [ABSTRACT FROM AUTHOR]

We applied interface-controlled kinetics, including interface-controlled Ostwald ripening and non-hydrostatic dissolution–precipitation, to metamorphic zircon and phengite growth in a high-pressure (high-P) metamorphic complex. This kinetic modeling yields the growth duration of metamorphic zircon, based on the assumption that dissolution and precipitation of zircon is much slower than that of phengite. The model was applied to zircon and phengite growth in the Nishisonogi unit of the high-P Nagasaki metamorphic complex, western Kyushu, Japan. Given that detrital zircons that are tens of microns in size remain after metamorphism, our model assumption is justified. Our results show that the duration of metamorphic zircon growth in an individual rock ranges from 10 to 20 Myr. In general, the duration of metamorphic zircon growth in the whole Nishisonogi unit is ca. 30 Myr. This prolonged duration implies that high-P metamorphism is sustained by continuous subduction of hydrated oceanic crust and overlying trench-fill sediments. The continuous subduction and accretion of these materials may supply the metamorphic fluid into the high-P metamorphic domain in the deeper part of the accretionary prism, thereby contributing to interface-controlled kinetics in the metamorphic complex. [ABSTRACT FROM AUTHOR]