seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

waveguide effects within the philippine sea slab beneath southwest japan inferred from guided sp converted waves 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 a2 dp 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

tomographic evidence for hydrated oceanic crust of the pacific slab beneath northeastern japan implications for water transportation in subduction zones 1 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

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

seismic evidence for high pore pressures in the oceanic crust implications for fluid related embrittlement 1 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

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

mantle wedge flow pattern and thermal structure in northeast japan effects of oblique subduction and 3 d slab geometry abstract we develop a 3 d thermal model for the northeast japan subduction margin using a realistic slab geometry for the subducting pacific plate and investigate the effects of oblique subduction and 3 d slab geometry on the mantle wedge flow pattern and the thermal structure in the tohoku region the mantle wedge flow pattern is nearly two dimensional resulting in a thermal structure similar to those obtained by a 2 d model owing to the simple slab geometry and subduction nearly perpendicular to the margin however in hokkaido oblique subduction leads to 3 d mantle wedge flow with northerly inflow and west northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in tohoku due to lower sinking rate of the slab between hokkaido and tohoku the slab has a hinge like shape due to a relatively sharp change in the dip direction in this hinge zone northerly mantle inflow from hokkaido and westerly mantle inflow from tohoku converge discouraging inflow from northwest and resulting in a cooler mantle wedge the model predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross chains the predicted 3 d thermal structure correlates well with the along arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down dip extent of interplate earthquakes

variable spatio temporal clustering of microseismicity in the hellenic subduction zone as possible indicator for fluid migration abstract this study investigates the variability of spatio temporal microseismicity clustering and the occurrence of mutual triggering of events along the subduction interface in south eastern aegean as indication for fluid flow along and above the plate interface we quantify spatial temporal and spatio temporal microseismicity clustering from the outer to the inner forearc and at intermediate depths waveform similarity indicates a decreasing of spatially clustered events from the outer and central towards the inner forearc and at intermediate depths highly similar events cross correlation 0 9 used as proxy for spatial clustering decrease from the outer 30 2 and central forearc 34 9 towards the inner forearc 20 5 and at intermediate depth 6 9 such highly similar events show increasing median inter event times from the outer and central towards the inner forearc and at intermediate depth 0 35 0 34 16 45 and 70 days respectively the epidemic type aftershock sequences etas model employed to investigate microseismicity temporal clustering indicates an increase of the percentage of independent events from the outer 32 and central 46 forearc to the inner forearc 93 and at intermediate depth 93 hence etas results suggest that mutual triggering of events is significant in the outer and central forearc and it is almost absent in the inner forearc and at intermediate depths autocorrelation analysis investigating spatio temporal clustering shows the tendency of earthquakes to occur close in space and time in the outer and central forearc while in the inner forearc and especially at intermediate depth earthquakes are more homogeneously and randomly distributed combining the results from spatial temporal and spatio temporal analysis we suggest that the different spatio temporal patterns hint at systematic variations in the presence of migrating fluids on active faults close to failure triggering of seismicity is significant in the outer and central forearc indicating fluid flow from the subduction interface and it is diminishing towards the inner forearc at intermediate depths the nearly complete absence of mutual triggering of earthquakes indicates that there is little evidence for migration of fluids on active faults close to failure because intermediate depth seismicity in the hellenic subduction zone occurs at p t conditions where dehydration reactions are expected fluids released by dehydration reactions within the slab are very likely migrating directly into the overlying mantle without triggering earthquakes

anomalous deepening of a seismic belt in the upper plane of the double seismic zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials abstract hypocenter relocations of earthquakes in the pacific slab have shown an anomalous deepening of a seismic belt in the upper plane of the double seismic zone at depths of 80 120 km beneath the hokkaido corner while it is located at depths of 70 90 km in the surrounding tohoku and eastern hokkaido areas seismic tomographic inversions performed beneath the hokkaido corner have shown that a low velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the pacific slab and makes direct contact with the upper surface of the pacific slab these observations suggest that 1 the low velocity zone just above the pacific slab is part of the subducted kuril forearc sliver 2 the contact with the subducted and so relatively cold sliver materials prevents the mantle wedge from heating the pacific slab and causes a lower temperature condition in the pacific slab crust beneath the hokkaido corner than in the surrounding areas 3 as a result a delay of eclogite forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there

anomalously low aftershock productivity of the 2019 mw 8 0 energetic intermediate depth faulting beneath peru abstract the 26 may 2019 peru m w 8 0 earthquake struck within the nearly horizontal underthrust nazca plate at depths from 110 to 150 km below the upper amazon near a steep bend in the plate where it plunges down to a deep earthquake zone little prior seismicity occurred in this region but large intraslab events with similar normal faulting mechanisms have occurred to the west the event is situated in a similar slab position to the 2017 puebla morelos mexico earthquake but the remote location resulted in limited loss of life and damage back projection imaging and finite fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170 km long zone at a rupture speed v r 3 km s with three normal faulting patches of up to 4 5 m slip despite the mainshock size it produced only three m4 0 aftershocks within 300 km one nearby the aftershock productivity of the 2019 peru earthquake is very low even among all large intermediate depth earthquakes but similar to that for other large peruvian intraslab events comparison of aftershock productivity of the peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat peru slab

existence of interplane earthquakes and neutral stress boundary between the upper and lower planes of the double seismic zone beneath tohoku and hokkaido northeastern japan abstract using data from the recently constructed nationwide dense seismic network we determined the hypocenters and focal mechanisms of many intermediate depth intraslab earthquakes within the pacific slab beneath northeastern japan the results show that in addition to the upper and lower planes of the double seismic zone a considerable number of intraslab earthquakes also occur between the two planes this interplane earthquake activity is not homogeneously distributed in space being high beneath eastern hokkaido and the fore arc regions of southeast and central tohoku the focal mechanisms of the interplane earthquakes tend to be of the down dip compressional type dc in tohoku and the hokkaido corner but down dip tensional type de in eastern hokkaido upper plane earthquakes are characterized by dc type stress while lower plane earthquakes are de type in both tohoku and eastern hokkaido the existence of interplane earthquakes enables estimation of the position of the neutral plane between the upper plane dc stress and the lower plane de stress we did so by applying stress tensor inversions to focal mechanism data obtained in the present study and japan meteorological agency jma data the results show that the neutral plane is located about 22 km beneath the upper plate interface beneath tohoku but only about 10 km beneath the upper surface beneath eastern hokkaido this difference in the location of the neutral plane may be due to the difference in buoyancy force exerted by the less dense metastable olivine wedge that is a result of the oblique plate subduction beneath hokkaido comparison of large intraslab earthquakes beneath the two regions shows that their aftershock areas are limited by the neutral plane suggesting that large earthquake ruptures are confined to either the dc or de stress field and do not go beyond the neutral plane

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

seismic waves guided by untransformed oceanic crust subducting into the mantle the case of the kanto district central japan a model of the subduction of oceanic crust is established through an analysis of the upper mantle earthquakes occurring beneath the kanto district central japan a distinct pair of later p and s phases xp and xs commonly observed at specific stations for these events in a depth range of 40 60 km and is considered to take place in association with the subducting motion of the philippine sea plate the xp phase is found from its particle motions to be a longitudinal seismic wave a principal component analysis of wavelets including the initial p and the xp phases shows that the latter arrives at the earth s surface with a slightly shallower angle relative to the former close agreement between the relative arrival time ratio of xs s xp p and the vp vs ratio of crustal or mantle rocks suggests that both xp and xs travel along the same path as longitudinal and tangential waves respectively one can clearly observe these later phases at stations located where the upper surface of the philippine sea plate is apparently in direct contact with the bottom of the crust of the honshu island apparent velocities of the xp and the xs phases are comparable to the seismic velocities of the main layer of the oceanic crust or the lower continental crust which are never expected for an upper mantle earthquake if the velocity structure is laterally homogeneous travel time curves for several earthquakes indicate that these apparent velocities can be directly related to the seismic velocity in the source region the hypocentral distribution of the events related to the subducting motion of the philippine sea plate does not form such a thin seismic zone as observed in southwest japan but extends over the entire thickness of the philippine sea plate the earthquakes from which the later phases are observed occur only in a shallower portion of this thick and contorted seismic zone the deeper portion does not generate such events these characteristics are best explained if these later phases are regarded as the seismic waves guided by the subducted portion of the gabbroic oceanic crust at the top of the philippine sea plate taking the result of our earlier work into account we conclude that the subducting oceanic crust remains in a gabbroic phase without transformation to eclogitic rocks at depths down to about 60 km along the entire northern boundary of the philippine sea plate

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

p wave velocity and anisotropy of lawsonite and epidote blueschists constraints on water transportation along subducting oceanic crust p wave velocity vp and the anisotropy of lawsonite and epidote blueschists were measured up to 1 0 gpa and 400 c using the ultrasonic pulse transmission technique the slowest vp in the direction normal to foliation is similar between lawsonite and epidote blueschists 7 0 7 2 km s at 1 0 gpa and room temperature while the fastest vp in the direction parallel to lineation markedly differs between lawsonite blueschists 7 4 7 6 km s at 1 0 gpa and room temperature and epidote blueschist 7 9 km s at 1 0 gpa and room temperature crystallographic orientation measurements for main constituent min erals revealed that both epidote 0 1 0 axes fastest vp direction in epidote single crystal and amphibole 0 0 1 axes fastest vp direction in amphibole single crystal are preferentially oriented parallel to lin eation to enhance vp anisotropy of the epidote blueschist in contrast lawsonite 0 0 1 axes fastest vp direction in lawsonite single crystal are oriented subnormal to foliation whereas amphibole 0 0 1 axes are oriented subparallel to lineation so that relatively weak vp anisotropy was observed in the lawsonite blueschist our experimental results in conjunction with recent seismological observations suggest that the vp of the subducting oceanic crust at 50 km has been observed in several subducting slabs however the high vp values at 50 km depth are difficult to be explained by blueschists this indicates that the blueschist would be at least partially transformed to hydrous mineral bearing eclogite at 50 km depth in subducting oceanic crusts

guided waves propagating in subducted oceanic crust 1 we use guided waves traveling updip along the surface of the nazca slab to image subducted oceanic crust at the chile peru subduction zone observed p onsets of intermediate depth events near 21 s in northern chile reveal waveguide behavior with growing focal depth low frequency energy 2 hz becomes more and more dominant and higher frequencies arrive delayed sometimes resembling two distinct phases to explain the observations we employ two dimensional finite difference fd simulations of complete p sv wave propagation along an updip profile of the subduction zone the fd calculations shed some light on several basic issues regarding crustal waveguides the development of guided waves dependent on event focal depth is simulated further we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle simulations also yield constraints on source locations relative to the low velocity structure finally the frequency content of p onsets is used to constrain the thickness of the waveguide the results indicate that a structure of 4 5 km average width and 7 low velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km the layer is interpreted as the unaltered lower part of the subducted oceanic crust suggesting that complete eclogite transformation in the chile peru subduction zone is unlikely to take place until beyond the volcanic front

thermal petrological controls on the location of earthquakes within subducting plates we find that in young and warm subducting plates earthquakes occur just below the moho in older plates earthquakes occur throughout the subducting oceanic crust as well as the subducting mantle we document this behavior in several subduction zones where there are independent constraints on earthquake locations and slab structure specifically for northern and southern japan alaska and cascadia the differences in earthquake depth relative to subducting crust may reflect large differences in temperature and thus locations of major dehydration reactions in colder slabs the crust passes through blueschist facies dehydration reactions while in cascadia and nankai the major dehydration reactions in crust may be due to zoisite and amphibole breakdown or associated melting the cold paths allow more mineral bound h2o to be retained within the crust at shallow depths eventually released upon dehydration over shorter time intervals than warm paths the cold path dehydration reactions also result in net positive volume changes of solid fluid with solid volume decreasing less than the volume of h2o produced on hot paths the net volume changes are negative with solid volumes decreasing more than the volume of h2o produced the difference in behavior could drive a net increase in pore pressure upon dehydration for the cold but not the hot crustal paths the difference in rate of release in h2o and difference in sign of net system volume change may promote seismogenesis in cold subduction zones but inhibit it in the crust of warm slabs within the mantle of the downgoing plate earthquakes mostly occur where serpentine is stable or breaks down in both settings

tomographic imaging of hydrated crust and mantle in the subducting pacific slab beneath hokkaido japan evidence for dehydration embrittlement as a cause of intraslab earthquakes 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

oscillating quaternary water levels of the marmara sea and vigorous outflow into the aegean sea from the marmara sea black sea drainage corridor abstract detailed interpretation of single channel air gun and deep tow boomer profiles demonstrates that the marmara sea turkey experienced small amplitude 70 m fluctuations in sea level during the later quaternary limited in magnitude by the sill depth of the strait of dardanelles moderate subsidence along the southern shelf and quaternary glacio eustatic sea level variations created several stacked deltaic successions separated by major shelf crossing unconformities which developed during the transitions from global glacial to interglacial periods near the strait of dardanelles a series of sand prone deposits are identified beneath an uppermost holocene transparent mud drape the sandy deposits thicken into mounds with the morphology and cross sectional geometries of barrier islands sand waves and current generated marine bars all cross stratification indicates unidirectional flow towards the dardanelles prior to the deposition of the transparent drape which began 7000 years bp in strong support of the notion that the marmara sea flowed westwards into the aegean sea through the dardanelles at times of deglaciation in northern europe the global sea level curve shows that at 11 000 and 9500 years bp sea level rose to the sill depths of the straits of dardanelles and bosphorus respectively the effect from 11 000 to 9500 years bp was seawater incursion into the marmara sea drowning and formation of algal serpulid bioherms atop lowstand barrier islands and transgression of shelves and lowstand deltas at 9500 years bp glacial meltwater temporarily stored in the black sea lake developed into a vigorous southward flow toward the aegean sea forming west directed sandy bedforms in the western marmara sea and initiating deposition of sapropel s1 in the aegean sea this strong outflow persisted until 7000 years bp after which a mud drape began to accumulate in the marmara sea and euryhaline mediterranean mollusks successfully migrated into a progressively more saline black sea where sapropel deposition began most eastern mediterranean sapropels from s1 to s11 appear to correlate with periods of rising sea level and breaching or near breaching of the bosphorus sill these events are believed to coincide with times of vigorous outflow of low salinity fresh surface waters transiting the black sea marmara sea corridor and ultimately derived from melting of northern european ice sheets

high resolution seismic velocity structure beneath the hokkaido corner northern japan arc arc collision and origins of the 1970 m 6 7 hidaka and 1982 m 7 1 urakawa oki earthquakes 1 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

depth variations in seismic velocity in the subducting crust evidence for fluid related embrittlement for intermediate depth earthquakes we investigated seismic wave velocity in the subducting crust of the pacific slab beneath eastern hokkaido northern japan to detect depth dependent properties of the seismic velocities in the crust we analyzed guided waves that propagate in the crust and estimated p wave velocity vp of 6 5 7 5 km s and s wave velocity vs of 3 6 4 2 km s at depths of 50 100 km the results show that the obtained vp and vs are 10 15 lower than those expected for the fully hydrated mid ocean ridge basalt suggesting the existence of aqueous fluids by 1 vol in the crust at this depth range our observations suggest that overpressurized fluids channeled in the subducting crust plays as a dominant factor for facilitating the genesis of crustal earthquakes at intermediate depths

crustal evolution of the southwestern kuril arc hokkaido japan deduced from seismic velocity and geochemical structure abstract the southwestern kuril arc which is not affected by a series of accretion and collision processes is one of the most important experimental sites among the world arc trench systems for investigating the evolution of arc crust published petrological and seismic studies have shown that in south central hokkaido the upper part of the kuril arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction among several hypotheses for formation of continental crust we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models seismic velocity structure is obtained from an onshore offshore seismic survey across the southwestern kuril arc trench system the structure in the back arc area shows a thick accretionary complex formed by paleogene accretion and collision based on published seismic results this accretionary complex is considered to continue on land to the cretaceous accretionary complex observed in the hidaka belt the structure of the kuril arc appears characteristic of upper crustal andesitic rocks 6 0 6 2 km s and thick middle to lower crust 6 5 7 3 km s based on published geochemical study we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the kuril arc each volume of andesitic basaltic and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model crustal composition estimated from the geochemical model and major element chemistry of the volcanic rocks in the kuril arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust

existence of a seismic belt in the upper plane of the double seismic zone extending in the along arc direction at depths of 70 100 km beneath ne japan 1 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

dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust intermediate depth earthquakes in cold subduction zones are observed within the subducting oceanic crust as well as the mantle in contrast intermediate depth earthquakes in hot subduction zones predominantly occur just below the mohorovicic discontinuity these observations have stimulated interest in relationships between blueschist facies metamorphism and seismicity particularly through dehydration reactions involving the mineral lawsonite here we conducted deformation experiments on lawsonite while monitoring acoustic emissions in a griggs type deformation apparatus the temperature was increased above the thermal stability of lawsonite while the sample was deforming to test whether the lawsonite dehydration reaction induces unstable fault slip in contrast to similar tests on antigorite unstable fault slip that is stick slip occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed microstructural observations indicate that strain is highly localized along the fault r1 and b shears and that the fault surface develops slickensides very smooth fault surfaces polished by frictional sliding the unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate a thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones indicating the potential for unstable frictional sliding within natural lawsonite layers

p wave attenuation in the pacific slab beneath northeastern japan revealed by the spectral ratio of intraslab earthquakes abstract we investigate p wave attenuation q p 1 in the pacific slab beneath northeastern ne japan adopting for the first time the spectral ratio technique for intraslab earthquakes when seismograms of two earthquakes are recorded at a station and their ray paths to the station are largely overlapped station dependent amplification and structural effects on the overlapped rays can be canceled out from the ratio of the spectral amplitudes of the seismograms therefore adopting the spectral ratio technique for intraslab earthquakes has a great advantage for the precise evaluation of q p 1 in the slab because the structural effects above the slab including the high attenuation mantle wedge are removed for estimating the intraslab q p 1 we determined corner frequency of the intraslab earthquakes using the s coda wave spectral ratio as the first step then we evaluated the inter event path attenuation t from the ratio of the spectral amplitudes of p waves the obtained result shows that p wave attenuation in the pacific slab marks q p 1 of 0 0015 q p of 670 at depths of 50 250 km this indicates that the p wave attenuation in the pacific slab is weaker than that in the mantle wedge the relatively high q p 1 is correlated with the distributions of intraslab earthquakes suggesting that the p wave amplitude is more attenuated around active seismicity zones in the slab therefore our observations likely indicate the presence of fractures hydrous minerals and dehydrated fluid around seismogenic zones in the slab at intermediate depths

shoreline change mapping using crowd sourced smartphone images abstract shoreline change information is critical for effective management of the coastal zone this study presents a low cost method for mapping shoreline change that harnesses smartphone images collected by the community and uploaded to social media platforms a smartphone camera cradle installed overlooking a coastal region is used to constrain the crowd sourced camera extrinsic parameters and accompanying signage instructs participants on how to share their image to social media using a site specific hashtag identifier surveyed ground control points solve for the focal length of the smartphone lens and more accurately resolve the camera extrinsic parameters shoreline position is subsequently mapped on georectified images using an edge detection technique based on the red and blue colour channels a validation of the method was conducted at two sandy beaches in se australia and resulted in strong community participation 400 images submitted over 7 months by 198 individual contributors concurrent shoreline surveys using rtk gnss indicated that shoreline accuracy using this crowd sourced approach is comparable to that of established coastal imaging systems with cross shore shoreline accuracy best for these two elevated validation sites camera elevation 17 3 m 27 1 m above msl in the camera nearfield rmsd 1 4 m and rmsd ranging between 2 6 and 3 9 m over coastal stretches spanning up to 1 km minimal differences in shoreline accuracy were observed between low resolution images characteristic of those uploaded to social media and higher resolution images sourced from the smartphone the successful application of this low cost approach combined with the proliferation of smartphones and social media usage open up new possibilities for crowd sourced shoreline change mapping at suitable coastal locations worldwide

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

evidence from high frequency seismic waves for the basalt eclogite transition in the pacific slab under northeastern japan abstract seismic multi pathing effects attributed to a contrast in seismic attenuation between the back arc mantle wedge and subducted crust are detected in central honshu northeastern japan we observe an initial broadened p wave which is followed by a delayed higher frequency p wave signal their discrepant frequencies are best explained by attenuation effects delayed p wave signals travel in the low attenuation oceanic crust and therefore contain more high frequency components the time separation between the initial broadened p waves and the delayed p wave signals are affected by the seismic velocity in the subducted oceanic crust we observe systematic variation in the delay times of the later waves indicating an increase in seismic velocity in the oceanic crust relative to the mantle wedge at 130 150 km depth high frequency seismic simulations incorporating mineral physics derived models show that a 4 vp increase due to the blueschist decomposition and a 9 vp increase associated with the lawsonite talc eclogite transition replicate the observed delay time variation the blueschist breakdown may occur at a depth of 100 km and the lawsonite talc eclogite transition might be linked with the reduced seismicity level at depths greater than 150 km distinct from traditional guided waves the multi pathing effects in this study are mainly controlled by attenuation contrast and therefore may not require the oceanic crust to have low velocity and any special decoupling mechanism the multi pathing effects offer us another important tool to image subducted oceanic crust below back arc mantle wedges especially where guided waves are not observable in this study we demonstrate the value of observing and simulating high frequency seismic waves 20 hz in advancing our understanding of subduction zones

shear wave splitting beneath the southwestern kurile arc and northeastern japan arc a new insight into mantle return flow 1 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

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs