Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/s40645-017-0126-9

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Other Link： http://link.springer.com/article/10.1186/s40645-017-0126-9/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-017-0619-1

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2016GL071798

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/ncomms13863

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/gji/ggw313

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.epsl.2015.06.021

Web of Science

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Publishing type：Research paper (scientific journal)   Publisher：Mineralogical Society of America  

DOI： 10.2138/am-2015-5240

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Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.tecto.2015.05.021

Scopus

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

We estimate the three‐dimensional (3‐D) P wave attenuation structure beneath Kyushu, Japan, using a large number of high‐quality waveform data. Our results show that the mantle wedge is characterized by high‐attenuation regions in the fore‐arc corner and in the back‐arc beneath volcanoes, with the two regions separated by a low‐attenuation area. The volcanic gap in central Kyushu is underlain by low attenuation below the Moho. High attenuation in the fore arc is probably associated with serpentinized peridotite, while that in the back arc is interpreted as an upwelling flow that is the source of arc magmas. The presence of low‐attenuation mantle that separates the high‐attenuation hydrated, fore‐arc, and back‐arc mantle regions suggests that fluids are supplied from two depth levels of the slab by different mechanisms. Low attenuation beneath the volcanic gap probably results from intricate 3‐D mantle flow that is caused by tectonic processes such as back‐arc extension and ridge collision.

DOI： 10.1002/2015gl063084

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/2015GL063084

Language：English  

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gca.2014.04.019

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

In this study, we imaged a detailed seismic attenuation structure (frequency‐independent Q⁻¹) beneath Hokkaido, Japan, by merging waveform data from a dense permanent seismic network with those from a very dense temporary network. Corner frequency of each event used for t* estimation was determined by the S coda wave spectral ratio method. The seismic attenuation (Q_p⁻¹) structure is clearly imaged at depths down to about 120 km. For the fore‐arc side of Hokkaido, high‐Q_p zones are imaged at depths of 10 to 80 km in the crust and mantle wedge above the Pacific slab. Low‐Q_p zones are clearly imaged in the mantle wedge beneath the back‐arc areas of eastern and southern Hokkaido. These low‐Q_p zones, extending from deeper regions, extend to the Moho beneath volcanoes, the locations of which are consistent with those of seismic low‐velocity regions. These results suggest that the mantle wedge upwelling flow occurs beneath Hokkaido, except in the area where there is a gap in the volcano chain. In contrast, an inhomogeneous seismic attenuation structure is clearly imaged beneath the Hokkaido corner. A broad low‐Q_p zone is located at depths of 0–60 km to the west of the Hidaka main thrust. The location almost corresponds to that of the seismic low‐velocity zone in the collision zone. The fault planes of the 1970 M6.7 and 1982 M7.1 earthquakes are located at the edges of this broad low‐Q_p zone. Observations in this study indicate that our findings contribute to understanding the detailed arc‐arc collision process, magmatism, and seismotectonics beneath Hokkaido.

DOI： 10.1002/2014jb011099

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/2014JB011099

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/1880-5981-66-69

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Other Link： http://link.springer.com/article/10.1186/1880-5981-66-69/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/bf03352481

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Other Link： https://link.springer.com/article/10.1186/BF03352481/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/1880-5981-66-12

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Other Link： http://link.springer.com/article/10.1186/1880-5981-66-12/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

In response to the subduction of the young Shikoku Basin of the Philippine Sea Plate, arc magmas erupted in SW Japan throughout the late Cenozoic. Many magma types are present including ocean island basalt (OIB), shoshonite (SHO), arc‐type alkali basalt (AB), typical subalkalic arc basalt (SAB), high‐Mg andesite (HMA), and adakite (ADK). OIB erupted since the Japan Sea back‐arc basin opened, whereas subsequent arc magmas accompanied subduction of the Shikoku Basin. However, there the origin of the magmas in relation to hot subduction is debated. Using new major and trace element and Sr‐Nd‐Pb‐Hf isotope analyses of 324 lava samples from seven Quaternary volcanoes, we investigated the genetic conditions of the magma suites using a geochemical mass balance model, Arc Basalt Simulator version 4 (ABS4), that uses these data to solve for the parameters such as pressure/temperature of slab dehydration/melting and slab flux fraction, pressure, and temperature of mantle melting. The calculations suggest that those magmas originated from slab melts that induced flux melting of mantle peridotite. The suites differ mostly in the mass fraction of slab‐melt flux, increasing from SHO through AB, SAB, HMA, to ADK. The pressure and temperature of mantle melting decreases in the same order. The suites differ secondarily in the ratio of altered oceanic crust to sediment in the source of the slab melt. The atypical suites associated with hot subduction result from unusually large mass fractions of slab melt and unusually cool mantle temperatures.

DOI： 10.1002/2013gc005132

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/2013GC005132

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/1880-5981-66-114

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jseaes.2012.12.031

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

We apply a three‐step approach to estimate three‐dimensional (3‐D) P wave attenuation (Q_p⁻¹) structure beneath northeastern Japan. First, corner frequencies of earthquakes are determined using the spectral‐ratio method for S‐coda waves. Then, whole‐path attenuation terms, t*, and site‐amplification factors are simultaneously estimated by a joint inversion. The set of t* is finally inverted for 3‐D attenuation structure. The results show that the mantle wedge has low attenuation in the fore arc and high attenuation in the back arc. A depth profile of Q_p⁻¹ in the back‐arc mantle is explained by attenuation expected for a two‐dimensional (2‐D) thermal model with Q_p/Q_s = 2 and grain sizes of 1 and 3 cm. However, an inclined high‐attenuation zone observed in the back‐arc mantle wedge, which is interpreted as an upwelling flow, shows higher attenuation than that calculated from the 2‐D thermal model. The higher seismic attenuation is probably caused by the concentration of partial melt in the upwelling flow. A combined interpretation of seismic attenuation and velocity structures further suggests that the degree of partial melt in the upwelling flow varies along the arc and that volcanoes are clustered transverse to the arc, below which the upwelling flow contains a higher degree of melt. These observations indicate that magmatism is controlled by a mantle‐wedge process that depends strongly on spatial variations in the degree of partial melt in the upwelling flow. Our results further imply the breakdown of hydrous minerals in a hydrous layer above the Pacific plate at a depth of ~120 km.

DOI： 10.1002/2013jb010388

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tecto.2013.07.003

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Publishing type：Research paper (scientific journal)   Publisher：Geological Society of London  

Abstract

We review the evolution of late Cenozoic magmatism in the NE Japan arc, and examine the relationship between the magmatism and the crust–mantle structure. Recent studies reveal secular changes in the mode of magmatic activity, the magma plumbing system, erupted volumes and magmatic composition associated with the evolution of crust–mantle structures related to the tectonic evolution of the arc. The evolution of Cenozoic magmatism in the arc can be divided into three periods: the continental margin (66–21 Ma), the back-arc basin (21–13.5 Ma) and the island-arc period (13.5–0 Ma). Magmatic evolution in the back-arc basin and the island-arc periods appears to be related to the 2D to 3D change in the convection pattern of the mantle wedge related to the asthenosphere upwelling and subsequent cooling of the mantle. Geodynamic changes in the mantle caused back-arc basin basalt eruptions during the back-arc basin opening (basalt phase) followed by crustal heating and re-melting, which generated many felsic plutons and calderas (rhyolite/granite phase) in the early stage of the island-arc period. This was followed by crustal cooling and strong compression, which ensured vent connections and mixing between deeper mafic and shallower felsic magmas, erupting large volumes of Quaternary andesites (andesite phase).

DOI： 10.1144/sp385.15

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jseaes.2013.04.016

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

Using quantitative analysis of waveforms, we investigate a seismic cluster that occurred at a depth of 67 km in the Philippine Sea slab 8 months after the 11 March 2011 megathrust Tohoku‐oki earthquake (M_w 9.0). The sequence started with an M 4.1 normal‐fault event on 14 November in the deepest part of the cluster, and subsequent earthquakes migrated upward by 6 km along a narrow conduit‐like zone. The earthquakes have stress drops of 0.5–40 MPa, and groups of earthquakes with coherent waveforms are observed. We explain these observations in terms of fluid‐related embrittlement and the migration of overpressured fluids. The low‐permeability plate interface of the Pacific plate may have been broken by the coseismic or postseismic slips of the Tohoku‐oki earthquake, with the fluids subsequently being liberated from the underlying crust of the Pacific slab and migrating into the Philippine Sea slab due to the pore pressure gradient. The tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area. The heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress. The lag of 8 months may represent the time needed for the unsealing of the plate interface, the upward migration of fluids, and the increase in pore pressure to become sufficient to overcome the lithostatic pressure.

DOI： 10.1002/jgrb.50246

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Publishing type：Research paper (scientific journal)   Publisher：Geological Society of America  

DOI： 10.1130/g33796.1

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

We estimated the P wave velocity structure of the crust of the subducting Pacific plate beneath northeast Japan using arrival time data of P‐to‐S‐converted waves. The results show that the P wave velocity of the subducting crust varies along the arc and increases abruptly at a depth of ~100 km, from 6.5–7.0 km/s in the fore arc to 7.5–8.5 km/s in the back arc. The P wave velocity in the fore arc is ~10% lower than theoretically expected values for the metamorphosed mid‐ocean ridge basalt material. Seismicity in the subducting crust is most active at depths of 70–80 km where P wave velocities are lowest. The marked reduction of P wave velocity suggests the coexistence of aqueous fluids with hydrous minerals. Abundant fluids elevate pore fluid pressures and reduce effective normal stress, promoting intensive seismic activity in the low‐velocity crust. Our observations provide seismic evidence that earthquakes in the subducting crust are facilitated by fluid‐related embrittlement.

DOI： 10.1002/grl.50468

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2013.03.022

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Publishing type：Research paper (scientific journal)   Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.122.398

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Using travel time data from both a nationwide dense seismic network and a dense temporary seismic network, we obtain a high‐resolution three‐dimensional seismic velocity structure beneath the Hokkaido corner. Considerable inhomogeneity in the seismic velocity structure is clearly imaged above the subducting Pacific slab. Our results indicate that a broad low‐velocity zone of P and S waves, with velocities consistent for crustal rocks, is observed west of the Hidaka main thrust at depths of 35–90 km. The images also indicate that several smaller‐scale high‐velocity zones are located at depths of 0–35 km, striking approximately north‐south and inclined to the east‐northeastward at 40°–60°. All of these anomalous high‐velocity zones are located at the deeper extension of Neogene thrust faults. The clearest high‐velocity zone is located beneath the Hidaka metamorphic belt and is in contact with the eastern edge of the broad low‐velocity zone. Moreover, the boundary between the clearest high‐velocity and the broad low‐velocity zones corresponds to the fault plane of the 1970 Mj (magnitude determined by the Japan Meteorological Agency) 6.7 Hidaka earthquake. The western boundary of another small high‐velocity zone, at depths of 20 to 30 km within the broad low‐velocity zone, corresponds to the fault plane of the 1982 Mj 7.1 Urakawa‐oki earthquake. These observations suggest that these two large and anomalously deep inland earthquakes occurred at sharp material boundaries under a northeast‐southwest compressional stress field caused by ongoing arc‐arc collision process.

DOI： 10.1029/2012jb009356

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Recent three‐dimensional (3‐D) travel‐time tomographic models of southwestern (SW) Japan image the subducting Philippine Sea plate (PHSP) and low velocity anomalies with long‐wavelength spatial resolution. However, the agreement between the synthetics calculated with the existing 3‐D model and data is not satisfactory forsP and S waves in the higher frequency range (≥0.2 Hz). The unsatisfactory agreement can be attributed to the parameters of the initial wave speed model for the tomographic inversion, as well as the difficulty in detecting S‐wave travel times accurately. In order to improve the wave speed model, we first constructed a 3‐D reference model by forward modeling using waveform data from an intraslab earthquake and then performed repeated travel‐time inversions using the 3‐D reference model as the initial model. The newly derived high‐resolution model 3DM_SWJ can produce synthetics that are substantially in better agreement with the data than previous models. The distribution of highP‐wave toS‐wave speed ratios (V_P/V_S) calculated from model 3DM_SWJ shows short‐wavelength heterogeneities that could have been formed in relation to the subduction of the Philippine Sea Plate beneath the Eurasian plate. This combined use of forward waveform modeling for slab and crust configuration and repeated tomographic inversion of travel‐times provides a better 3‐D structural model of seismic properties for understanding tectonic features related to the subduction process, in addition to better estimating ground motions.

DOI： 10.1029/2012jb009345

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Publishing type：Research paper (scientific journal)   Publisher：Copernicus GmbH  

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

DOI： 10.5194/se-3-355-2012

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tecto.2011.09.019

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

A magnitude 7.3 foreshock occurred at the subducting Pacific plate interface on March 9, 2011, 51 h before the magnitude 9.0 Tohoku earthquake off the Pacific coast of Japan. We propose a coseismic and postseismic afterslip model of the magnitude 7.3 event based on a global positioning system network and ocean bottom pressure gauge sites. The estimated coseismic slip and afterslip areas show complementary spatial distributions; the afterslip distribution is located up‐dip of the coseismic slip for the foreshock and northward of hypocenter of the Tohoku earthquake. The slip amount for the afterslip is roughly consistent with that determined by repeating earthquake analysis carried out in a previous study. The estimated moment release for the afterslip reached magnitude 6.8, even within a short time period of 51h. A volumetric strainmeter time series also suggests that this event advanced with a rapid decay time constant compared with other typical large earthquakes.

DOI： 10.1029/2012gl052430

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.5026/jgeography.121.128

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tecto.2011.11.013

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2011gl048754

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.5047/eps.2011.06.027

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2011gl048432

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2010gl046413

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Double Wadati‐Benioff seismic zones (DSZ) with two parallel planes of seismicity separated by 15–30 km are a global feature of subduction zones in the 50–200 km depth range. Upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated. Serpentine or hydrous‐phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration. High‐resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization. Earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs. The contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2–3 kilometers beneath oceanic crust.

DOI： 10.1029/2010gl045494

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tecto.2010.10.010

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We estimate the configuration of the Philippine Sea plate (PHS) subducted beneath Kanto to understand the seismotectonics in the Tokyo metropolitan area. In addition to small repeating earthquakes that delineate both the upper and lower boundaries of the PHS, previously unrecognized converted waves at its upper boundary from lower‐boundary repeater sources provide a new constraint on the position of the boundary. The results from 794 S‐to‐P and 212 P‐to‐S converted waves together with ∼200 repeater locations show that (1) the PHS has a wedge‐like shape, decreasing in thickness from 50 km beneath Tokyo to zero at its northeastern limit, and (2) the PHS is relatively flat in the eastern part of Kanto and bends upward near its northeastern limit. From the fact that the PHS in the Izu‐Bonin forearc also has a similar wedge‐like shape, we infer that its shape was formed before its subduction. On the other hand, the relatively flat PHS at its northeastern limit and its upward bend are probably a consequence of ongoing deformation from an interaction with the underlying thick Pacific plate (PAC). The history of the PHS as a forearc before subduction and its contact with the PAC after subduction suggests that the temperature of this part of the PHS is colder than other regions, which probably causes anomalously deep seismicity on the PHS in Kanto. Colder PHS temperatures imply that the source regions of large earthquakes may extend deeper than expected, an important fact when assessing the locations of disastrous earthquakes for Tokyo.

DOI： 10.1029/2009jb006962

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.5047/eps.2010.06.007

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2009JB006962

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We perform travel‐time tomography beneath Kanto, Japan, to obtain detailed structures of the wedge‐shaped serpentinized mantle previously observed in the Philippine Sea slab. The results suggest that the western boundary of the serpentinized mantle (serpentine boundary) is subvertical and P‐ and S‐wave velocities vary by 15%–20% across it over the short distance of ∼10 km. Two intraslab earthquakes, in 1921 (M7.0) and 1987 (east off Chiba earthquake; M6.7), are inferred to have occurred along the serpentine boundary, accompanied by right‐lateral movement, based on analyses of focal mechanisms and aftershock distribution. A subvertical earthquake cluster penetrating the entire Philippine Sea slab is observed along the serpentine boundary, and four earthquakes in the cluster have strike‐slip focal mechanisms similar to that of the 1987 earthquake. Focal mechanisms obtained for past large earthquakes and present‐day microearthquakes suggest a concentration of right‐lateral deformation along the mechanically weak serpentine boundary. The Philippine Sea slab may have been torn in two along this boundary, with the eastern portion being left behind relative to subduction of the western portion. Assuming that one of the large aftershocks (M7.1) of the 1923 Kanto earthquake (M7.9), which occurred off the Boso Peninsula, ruptured the same asperity as did the 1987 earthquake, the slip deficit accumulated along the serpentine boundary during the 64 year interval is consistent with the fault slip of the 1987 earthquake. Interaction between the seismic slip along the plate interface and the serpentine boundary can explain the series of M7‐class earthquakes before and after the 1923 Kanto earthquake.

DOI： 10.1029/2009jb006863

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2009.12.038

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Publishing type：Research paper (scientific journal)   Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.119.205

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Publishing type：Research paper (scientific journal)   Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.119.190

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gr.2009.03.007

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gr.2009.05.014

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gr.2008.12.010

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1‐D velocity model. Then, traveltime tomography is carried out to estimate three‐dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS‐PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust‐type) earthquakes on the PAC slab is deepened by ∼30 km locally under the slab contact zone. This deepening is probably caused by a lower‐temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low‐velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS‐PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.

DOI： 10.1029/2008jb006101

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Publishing type：Part of collection (book)   Publisher：Cambridge University Press  

DOI： 10.1017/cbo9780511635380.008

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.chemgeo.2008.10.020

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2009.04.003

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2009jb006449

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We perform travel‐time tomography to estimate detailed seismic velocity structures in the crust of the Pacific slab from northeastern (NE) Japan to the Kanto district, Japan, and reveal that the depth extent of the low‐velocity (hydrated) oceanic crust varies along the arc. The low‐velocity oceanic crust is subducting to depths of 120–150 km beneath Kanto, which is 40–70 km deeper compared to NE Japan. Such deeper preservation of the low‐velocity oceanic crust beneath Kanto can be explained by lower‐temperature conditions in the Pacific slab as a result of the subduction of the Philippine Sea slab immediately above it. These observations suggest that dehydration reactions accompanied by large velocity changes are controlled principally by temperatures, not by pressures. We also find spatial correspondence between intensive seismicity in the oceanic crust and the disappearance depth of the low‐velocity oceanic crust, suggesting that breakdown of hydrous minerals triggers earthquakes in the oceanic crust.

DOI： 10.1029/2008gl036865

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2008.09.032

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Simultaneous measurements of ultrasonic compressional (V_p) and shear wave velocities (V_s) were conducted under pressures and temperatures representative of the lower crust on five mafic xenoliths derived from the northeast Japan arc crust. For comparison with the observed seismic heterogeneity of the lower crust of the northeast Japan arc, the velocity deviations (dV_p and dV_s) of the xenoliths were calculated using the average reference V_p (6.61 ± 0.1 km/s) and V_s (3.76 ± 0.1 km/s). The dV_p and dV_s values of the xenoliths at 700°C and 0.8 GPa are 0.8% and −1.2% for hornblende gabbro, 4.8% and 4.0% for hornblende‐pyroxene gabbro, 0.9% and −1.5% for amphibolite, 2.5% and 1.7% for pyroxene amphibolite, and 0.9% and −5.6% for hornblendite, respectively. We compared the velocity perturbations in the lower crust determined by seismic tomography with dV_p and dV_s of the xenoliths using “V_p−V_s−V_p/V_s deviation diagrams.” On the basis of our measurements, we suggest that (1) the seismically high‐V_p and V_s regions beneath the Tobishima Basin consist of hornblende‐pyroxene gabbro, (2) hornblende gabbro is a predominant rock type beneath the Dewa Hills and Ou Backbone Range, (3) the low‐velocity anomalies beneath the active volcano areas may be caused by the existence of partial melts of hornblende gabbro, and (4) the low‐V_p and high‐V_s regions beneath the Kitakami Mountains consist of quartz‐plagioclase‐bearing rocks. Our data demonstrate that the seismic heterogeneity in the lower crust of the northeast Japan arc reflects variations in rock composition and temperature that are related to the regional geological history.

DOI： 10.1029/2008jb005587

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Three‐dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern (SW) Japan is determined by applying double‐difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in Japan. A region of low S wave velocity and high Vp/Vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from Tokai to Kyushu. This characteristic layer dips shallowly in the direction of slab subduction. Compared with the upper surface of the Philippine Sea slab based on seismic reflection and refraction surveys on seven survey lines, we interpret that the low‐Vs and high‐Vp/Vs layer corresponds to the oceanic crust of the Philippine Sea slab. On the basis of the position of the low‐Vs and high‐Vp/Vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes, the upper surface of the Philippine Sea slab is reliably determined for the entire area of SW Japan. Nonvolcanic deep low‐frequency earthquakes that occurred associated with the subduction of the Philippine Sea slab are distributed along the isodepth contour of 30 km in SW Japan, except for the Tokai district where the depth of deep low‐frequency earthquakes becomes gradually deeper toward northeast.

DOI： 10.1029/2007jb005274

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We estimate detailed seismic‐velocity structure around the Pacific slab beneath northeastern Japan by double‐difference tomography. A remarkable low‐velocity zone with a thickness of ∼10 km, which corresponds to much hydrated oceanic crust, is imaged coherently along the arc at the uppermost part of the slab. The zone gradually disappears at depths of 70–90 km, suggesting the occurrence of intensive dehydration reactions there. The concentration of intraslab earthquakes at these depths supports dehydration‐embrittlement hypothesis as a mechanism for generating intraslab earthquakes. A low‐velocity zone imaged immediately above the slab at depths >70 km probably reflects a hydrous layer that absorbs water expelled from the slab and carries it to deeper depths along the slab. Our observations suggest that an along‐arc variation in arc volcanism might be related to that in the development of the hydrous layer above the slab.

DOI： 10.1029/2008gl034461

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2008.04.039

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Language：English  

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Language：Japanese  

J-GLOBAL

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2007.10.028

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin.60.123

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Publishing type：Research paper (scientific journal)   Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.117.76

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Publishing type：Research paper (scientific journal)   Publisher：Geochemical Society of Japan  

DOI： 10.2343/geochemj.42.51

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Publishing type：Research paper (scientific journal)   Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.117.59

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We carry out high‐resolution three‐dimensional seismic tomography of the crust and upper mantle beneath southwestern (SW) Japan using arrival‐time data obtained from the nationwide seismograph network. The tomographic images provide new insights into the configuration of the Philippine Sea slab and arc magmatism. The results confirm the existence of an aseismic portion of the Philippine Sea slab at greater depths beyond the seismic portion. The Philippine Sea slab is subducting aseismically down to at least a depth of 200 km in Chubu, 60–80 km in Kinki, and 60 km in Chugoku and is subducting seismically to depths of 150–200 km in Kyushu. In the Chubu district, it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area, whereas in the Kinki district, it is subducting subhorizontally as far as the Japan Sea. The differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity, suggesting the importance of the geometry of the Philippine Sea slab on arc magmatism. Our tomographic images further imply subduction of the Philippine Sea slab down to a depth of around 70 km northwest of the Izu Peninsula. The Philippine Sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the Pacific slab at depths of 150–200 km.

DOI： 10.1029/2006jb004770

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We present high‐resolution tomographic images of the crust under the active arc volcanoes in the central part of Northeast Japan (Tohoku) determined by using arrival times of first P waves and post‐critically reflected waves from the Moho discontinuity (PmP). Results of detailed resolution analyses show that the addition of PmP data can improve significantly the resolution of the lower crustal structure. After the PmP data are included in the tomographic inversion, the low‐velocity (low‐V) anomalies in the lower crust under the active volcanoes can be better imaged. The low‐V zones are clearly visible in the entire crust beneath the volcanoes extending from the surface down to the Moho discontinuity. Arc‐magma related, deep, low‐frequency microearthquakes are located around the low‐V zones in the lower crust under the volcanoes, which occurrence is probably associated with the movement of fluid magma under the volcanoes.

DOI： 10.1029/2007gl030026

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

A belt of intraslab seismicity in the Pacific slab crust parallel to iso‐depth contours of the plate interface has been found beneath Hokkaido and Tohoku. Hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso‐depth contours beneath Kanto, deepening toward the north from ∼100 km to ∼140 km depth. The depth limit of the contact zone with the overlying Philippine Sea slab is located close to and parallel to this obliquely oriented seismic belt, suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab. The contact with this cold slab hinders the heating of the Pacific slab crust by hot mantle wedge, which would cause delay of eclogite‐forming phase transformations and hence deepening of the seismic belt there. The depth limit of the subducting low‐velocity crust also deepens toward the north, supporting this idea.

DOI： 10.1029/2007gl029616

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2006.11.024

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Language：English  

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin.59.325

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin.60.1

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the Pacific slab beneath NE Japan. We found a seismic “belt” parallel to the iso‐depth contour of the plate interface that is beneath the forearc area at depths of 70–100 km, indicating that the distribution of the upper plane earthquakes is non‐uniform. The location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where H₂O contents change in the slab crust. Events in the upper seismic plane have mainly down‐dip compression‐type focal mechanisms but several events have normal fault‐type (NF‐type) versions, whose spatial distribution appears to correspond to these boundaries. These NF events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions.

DOI： 10.1029/2006gl028239

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Hypocenters of main shocks and aftershocks of the 1933 M=7.1, 1936 M=7.4, 1937 M=7.1 and 1978 M=7.4 Miyagi-oki earthquakes are relocated using S-P times reported in the Seismological Bulletin of the Japan Meteorological Agency (JMA) and those re-read from original smoked-paper seismograms observed at the Mizusawa station of the National Astronomical Observatory of Japan (NAOJ) and the Mukaiyama station of Tohoku University. In order to reduce the error caused by inaccuracies of the arrival times and the small number of seismic observation stations, we determined the hypocenters by using a grid search method that assumed that the events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of these four earthquakes were determined to be close to each other, while the distributions of their aftershocks seem to disperse on the upper boundary of the Pacific plate. These distributions show that aftershock areas of the 1933, 1936 and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central and westernmost portions, respectively. This result suggests that the 1933, 1936 and 1937 events possibly ruptured a part of the source area of the 1978 event, i.e., its eastern, central and western portions, respectively.

DOI： 10.1186/bf03352666

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Other Link： https://link.springer.com/article/10.1186/BF03352666/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

A detailed investigation of the hypocenter distribution beneath Kanto, Japan, reveals a NW‐SE‐trending linear alignment of seismicity within the subducted Pacific slab. We estimate the 3D seismic velocity structure in the Pacific slab to understand the factors controlling the genesis of such intraslab earthquakes. A narrow low‐velocity zone is imaged within the subducted slab over a length of ∼150 km, which partly penetrates into the mantle portion of the slab. The low‐velocity zone correlates in space with the NW‐SE‐trending earthquake cluster. A reactivation of hydrated fracture zone formed prior to subduction is probably related to the low‐velocity anomaly. Dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone, accompanied by intraslab earthquakes. These observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes.

DOI： 10.1029/2006gl026773

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

An M=5 earthquake occurred on September 15, 1998, in the Nagamachi-Rifu fault (NRF), northeastern Japan. In the aftermath of this event, many seismograph stations were constructed temporarily around the fault, forming a dense network of stations with a spatial separation of 5 km. We report here our estimation of the three-dimensional velocity structures of the P and S waves using arrival-time data recorded at these stations with the aim of understanding the heterogeneous structure around the NRF. Low-Velocity and high Poisson’s ratio anomalies are imaged in the lower crust beneath the volcanic area, which are probably associated with the partially molten materials conveyed through the upwelling flow in the mantle wedge. A distinct low-velocity anomaly, which is explainable by the existence of H₂O-filled pores, is observed in the mid crust at the deeper extension of the NRF. Two low-velocity anomalies that are probably associated with the remnants of magmatic activity that formed the Shirasawa caldera and with the existence of thick late-Cenozoic sedimentary layers are observed at depths shallower than 10 km in the hanging wall of the NRF. Our results successfully characterize the major features of the complex velocity structure around the NRF, with implications for the existence of fluid-rich regions in the mid to lower crust.

DOI： 10.1186/bf03351989

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Other Link： https://link.springer.com/article/10.1186/BF03351989/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Shear‐wave splitting in the southwestern part of the Kurile arc and the northeastern (NE) Japan arc is investigated using the waveforms from local earthquakes. For both arcs observed shear‐wave splitting shows clear evidence for a striking rotation of fast direction across the arc, suggesting the different feature of anisotropy between the fore‐arc and back‐arc sides. Trench‐parallel fast directions are observed in the fore‐arc side, which is consistent with the anisotropy expected from the deformation of the B‐type olivine fabric as well as the anisotropy in the slab. Fast directions observed in the back‐arc side show approximately E‐W and ESE‐WNW in the NE Japan arc and N‐S in the southwestern Kurile arc, which are characterized by the local maximum‐dip direction of the subducted Pacific plate not by the direction of plate convergence. This relationship suggests that the direction of the mantle return flow is governed by the local slab geometry.

DOI： 10.1029/2005gl025053

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

We applied a 3-D seismic tomography inversion algorithm to arrival-time data obtained, during 18 years from 1985 to 2002, from local seismic networks and aftershock studies in the Marmara region, in order to better understand the upper crustal structure of the complex tectonic region. We integrated all the available data set into a common data set and relocated the events, using a 1-D velocity model. We then selected 3,949 earthquakes and obtained 92355 arrival times, in total, consisting of 59,313P-wave and 33,042S-wave arrival times. In this paper we present detailed crustal structures forV p,V sandV p/V sratios from the surface down to 15 km depth with good resolution in terms of the hit count analysis of seismic rays, the checkerboard and restoring resolution tests for the studied region. The results obtained from the inversion suggest that the western part of the North Anatolian Fault Zone shows strong lateral heterogeneity. We concluded that no clear pattern exists between the distribution of microearthquakes and aftershocks and the velocity perturbations presented in this paper. This is probably due to complex tectonic and geological structures. Large coseismic slip associated with the two recent strong earthquakes was found to correspond to higher velocity anomalies, as was often found recently. On the other hand, low velocity values correspond to the sedimentary units or the alluvium regions, as supported by the low resistivity and gravity values. Thus the results presented in this paper are quite consistent with the other data such as gravity, resistivity and magnetic anomalies, indicating that our model is reliable and efficient and it should be useful for further interpretation of tectonic and geological problems in this region. It should be pointed out, however, that the results ofS-wave perturbations andV p/V sratio could not be discussed in detail because of insufficient quality of theS-wave data and the reliability of the results is not very high. Nonetheless, the fact that recent large Izmit earthquake and moderate aftershocks that occurred in or around the high velocity zones near the low velocity region suggests that high velocity regions found in the area close to Istanbul in the Marmara Sea and also at the Iznik-Mekece fault are potential sites for strain energy accumulation and release.

DOI： 10.1186/bf03351882

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Other Link： https://link.springer.com/article/10.1186/BF03351882/fulltext.html

Language：Japanese   Publisher：Japan Association for Quaternary Research  

DOI： 10.4116/jaqua.44.i

CiNii Books

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.tecto.2005.03.018

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

DOI： 10.1186/bf03351830

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Other Link： https://link.springer.com/article/10.1186/BF03351830/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2005.02.033

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Publishing type：Research paper (scientific journal)  

DOI： 10.1186/BF03351830

Scopus

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Publishing type：Research paper (scientific journal)   Publisher：Japan Association for Quaternary Research  

DOI： 10.4116/jaqua.44.195

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin1948.58.3_153

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin1948.58.3_283

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Three‐dimensional (3‐D) distribution of scattering coefficients in the crust beneath northern Miyagi prefecture of northeastern Japan is estimated using an envelope inversion method in order to investigate the short‐wavelength inhomogeneous structure that cannot be imaged by conventional travel‐time tomography. The inversion revealed two large scattering coefficient zones (LSZs) which are distributed beneath active volcanoes and around the mainshock fault of the 1962 M 6.5 Northern Miyagi earthquake. These LSZs merge in the lower crust, and the characteristic distribution correlates well with the previously reported seismic low‐velocity zones, which are perhaps caused by fluids. We inferred that fluids are supplied from the lower crust to the upper crust along the LSZs, resulting in inhomogeneous structures such as localized distribution of fluid‐filled cracks. It is expected that this distribution of fluids will cause strain and stress accumulation, affecting seismic activity in this region.

DOI： 10.1029/2004gl021261

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.epsl.2004.06.011

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin1948.56.4_425

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Publishing type：Part of collection (book)   Publisher：American Geophysical Union  

DOI： 10.1029/150gm08

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Publishing type：Research paper (scientific journal)   Publisher：Seismological Society of Japan  

DOI： 10.4294/zisin1948.56.4_413

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/s0377-0273(03)00155-0

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We estimated thermal structure in the mantle wedge of NE Japan by applying the experimental results of olivine‐dominated rocks to seismic attenuation data. Obtained temperatures at a 40 km depth are 1000–1130°C and 960–1090°C beneath the volcanic front and the back‐arc side, respectively. We cannot obtain the areas with a temperature higher than 1400°C in the mantle wedge, which have been inferred from petrological studies. This is perhaps due to the localization of such anomalous areas. The obtained temperature is higher than that of the wet solidus of peridotite in the greater part of the back‐arc side, which suggests that the addition of aqueous fluids plays an important role in generating melts in the mantle wedge.

DOI： 10.1029/2003gl017185

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/s0031-9201(01)00307-7

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Language：Japanese  

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/s0040-1951(01)00181-0

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

We estimated both P (V_p) and S wave velocity (V_s) structures beneath northeastern Japan by applying a tomographic method to 169,712 P and 103,993 S wave arrival time data from 4338 local events. The average value of V_p/V_s ratio is ∼1.69 in the upper crust, ∼1.75 in the lower crust, and ∼1.77 in the uppermost mantle. These differences in V_p/V_s ratio may mainly reflect the lithological variations with depth. Low‐V_p, low‐V_s, and high V_p/V_s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge. Here the low‐V_p, low‐V_s, and high V_p/V_s values are interpreted because of partial melting materials, which imply the presence of a vast amount of melt in the uppermost mantle. In the lower crust, low‐V_p, low‐V_s, and high V_p/V_s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas, which suggests that melting of lower crustal materials occurs just beneath each active volcano. In contrast, the upper crust beneath active volcanoes exhibits low V_p, low V_s, and low V_p/V_s. These results suggest that the partial melting zones under northeastern Japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes. The low‐V_p, low‐V_s, and low V_p/V_s features in the upper crust suggest the existence of H₂O (rather than melt) right beneath active volcanoes.

DOI： 10.1029/2000jb000008

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

The assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab, at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern Japan. The combination of evidence from velocity tomography in the mantle wedge above the slab and mapping of earthquake size distribution within it strongly points to a source of fluids at the top of the slab at 140 to 150 km depth, from where material rises along an inclined path to the volcanoes.

DOI： 10.1029/2000gl012558

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.tecto.2005.03.018

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Language：Japanese   Publisher：Japanese Society for Active Fault Studies  

An earthquake of magnitude 6.1 occurred in Shizukuishi town, Iwate prefecture, northeastern Japan, on September 3,1998. The focal area of this event is located near Iwate volcano, which has been active since February,1998. The focal mechanism of the main shock is a reverse fault, whose pressure axis is nearly horizontal and almost east-west-a typical focal mechanism for shallow earthquakes beneath the northeastern Japan arc. Precise hypocenter locations of the main shock and its aftershocks delineate a fault plane that strikes northeast and dips northwest in the depth range 3-8km. The rupture probably began about 7km deep at the northern edge of the fault plane, then propagated southward along a deep part of the Nishine active fault.

DOI： 10.11462/afr1985.1998.17_1

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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J-GLOBAL

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Language：Japanese  

CiNii Books

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Language：Japanese   Publisher：The University of Tokyo  

CiNii Books

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Other Link： http://hdl.handle.net/2261/13251

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Language：Japanese  

CiNii Books

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Language：Japanese  

CiNii Books

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