a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

joint inversions of seismic and geodynamic data for models of three dimensional mantle heterogeneity the seismic models of three dimensional 3 d mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow related observables such as the nonhydrostatic geoid the current models of global scale shear velocity heterogeneity provide reasonably good fits to the long wavelength nonhydrostatic geoid data 60 70 variance reductions but rather poor fits to the corresponding free air gravity anomalies 30 40 variance reductions this major difference is due to two factors 1 the very different amplitude spectrum of the gravity anomalies which is nearly flat for degrees l 3 the largest mismatch between the pattern of predicted and observed gravity anomalies is in the southern hemisphere this observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle in contrast the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the mantle we therefore perform a series of experiments in which we simultaneously invert a large set of seismic data which includes long period waveforms ss s and scs s differential travel times and normal mode structure coefficients and the long wavelength gravity anomaly data we thus determine whether it is possible to derive new 3 d heterogeneity models which satisfy both data sets the gravity anomaly data are interpreted in the context of spherically symmetric viscous flow models of the mantle in these inversion experiments we test several radial viscosity and ln ln profiles and thereby assess their plausibility the joint seismic geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free air gravity anomalies with variance reductions of 80 90 readily accessible while preserving the fit to the seismic data this improvement is achieved with some adjustment to the heterogeneity in the depth range 1500 2500 km where the seismic data constraints appear to be weakest the joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone

the proportionality between relative plate velocity and seismicity in subduction zones the magnitude and rate of seismicity differ between subduction zones calculations of background seismicity rates based on a global model of subduction zone seismicity reveal a positive correlation between relative plate velocity and background seismicity yet only the seismically quieter zones seem capable of generating magnitude 9 earthquakes

spatiotemporal distribution of interplate coupling in southwest japan from inversion of geodetic data 1 we determine the spatiotemporal distribution of slip or slip deficit on the subduction interface of the nankai trough over an entire earthquake cycle using geodetic data including leveling triangulation and trilateration sea level and gps surveys obtained during the past 100 years in southwest japan we develop a new inversion method that accounts for long term crustal deformation coseismic earthquake displacements and stress relaxation of the viscoelastic asthenosphere from this analysis we obtain a model that shows postseismic afterslip on the deeper part of the plate interface following the 1946 nankaido earthquake significant afterslip is found beneath central shikoku that totals about 0 8 m the slip deficit rate during the interseismic period is 5 6 cm year in the n50 w n60 w direction which is consistent with the relative plate motion between the philippine and amurian plates a fully locked region is found in the shallower portion 30 km and the slip deficit rate has a maximum at a depth of about 20 km the plate interface deeper than 30 km is slowly slipping the amount of slip deficit reaches about 3 3 m off shikoku and about 2 m off the kii peninsula 50 years after the 1946 earthquake this suggests that fault healing and full interplate coupling has occurred by this time along the nankai trough

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

a high resolution p wave model for the top 1200 km of the mantle this work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the international seismological centre p wave data the processing includes summary ray sorting and hypocentral redetermination to cope with the extreme variation of ray coverage a multicell inversion is used to simultaneously constrain anomalies in overlapping cells the multicell solutions are summed after inversion to form the final model named p1200 at cell size of 1 1 and about 50 km in thickness the lateral resolution of the model is about 1 2 in major subduction zones and about 5 15 in most places major features in the model withstood tests on signal coherency and different reference models the long wavelength parts of the p1200 model correlate well with previous global models especially the s12 model of shear wave among the large features in the transition zone and below there are two fast bands one from canada to south america and another from siberia through east asia and the philippine sea to western australia there are also slow patches under the central pacific and from india to eastern africa though most subduction zones have slab like high velocity anomalies they are sandwiched by broad slow velocities at shallow depths some of the slabs appear to penetrate the 660 km discontinuity and stagnant slabs are also seen in the transition zone plume like slow anomalies exist below some prominent hot spots like hawaii iceland yellowstone and french polynesia

geodetic imaging of plate motions slip rates and partitioning of deformation in japan 1 interseismic deformation in japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces we use spherical linear block theory constrained by geodetic observations from densely spaced global positioning system gps stations to estimate plate motions fault slip rates and spatially variable interplate coupling on the japan kuril sagami and nankai subduction zones the reference model developed in this paper consists of 20 blocks produces a mean residual velocity magnitude of 1 84 mm yr at 950 stations and accounts for 96 of the observed interseismic deformation signal we estimate fault slip rates in excess of 15 mm yr along the niigata kobe tectonic zone and itoigawa shizuoka tectonic line through central japan confirming their hypothesized roles as major tectonic boundaries oblique convergence across the nankai trough is partitioned with 3 4 of the 30 mm yr of trench parallel motion accommodated by strike slip motion on the subduction interface and the remaining 1 4 accommodated by right lateral slip on the median tectonic line in contrast our models suggest negligible slip partitioning in eastern hokkaido where oblique slip on the japan kuril subduction interface accommodates all of the trench parallel component of relative plate motion inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes

geology and tectonics of japanese islands a review the key to understanding the geology of asia abstract the age of the major geological units in japan ranges from cambrian to quaternary precambrian basement is however expected as the provenance of by detrital clasts of conglomerate detrital zircons of metamorphic and sedimentary rocks and as metamorphic rocks intruded by 500 ma granites although rocks of paleozoic age are not widely distributed rocks and formations of late mesozoic to cenozoic can be found easily throughout japan rocks of jurassic age occur mainly in the jurassic accretionary complexes which comprise the backbone of the japanese archipelago the western part of japan is composed mainly of cretaceous to paleogene felsic volcanic and plutonic rocks and accretionary complexes the eastern part of the country is covered extensively by neogene sedimentary and volcanic rocks during the quaternary volcanoes erupted in various parts of japan and alluvial plains were formed along the coastlines of the japanese islands these geological units are divided by age and origin i e paleozoic continental margin paleozoic island arc paleozoic accretionary complexes mesozoic to paleogene accretionary complexes and cenozoic island arcs these are further subdivided into the following tectonic units e g hida oki unazuki hida gaien higo hitachi kurosegawa south kitakami nagato renge nedamo akiyoshi ultra tamba suo maizuru mino tamba chichibu chizu ryoke sanbagawa and shimanto belts the geological history of japan commenced with the breakup of the rodinia super continent at about 750 ma at about 500 ma the paleo pacific oceanic plate began to be subducted beneath the continental margin of the south china block since then proto japan has been located on the convergent margin of east asia for about 500 ma in this tectonic setting the most significant tectonic events recorded in the geology of japan are subduction accretion paired metamorphism arc volcanism back arc spreading and arc arc collision the major accretionary complexes in the japanese islands are of permian jurassic and cretaceous paleogene age these accretionary complexes became altered locally to low temperature and high pressure metamorphic or high temperature and low pressure metamorphic rocks medium pressure metamorphic rocks are limited to the unazuki and higo belts major plutonism occurred in paleozoic mesozoic and cenozoic time early paleozoic cambrian igneous activity is recorded as granites in the south kitakami belt late paleozoic igneous activity is recognized in the hida belt during cretaceous to paleogene time extensive igneous activity occurred in japan the youngest granite in japan is the takidani granite intruded at about 1 2 ma during cenozoic time the most important geologic events are back arc opening and arc arc collision the major back arc basins are the sea of japan and the shikoku and chishima basins arc arc collision occurred between the honshu and izu bonin arcs and the honshu and chishima arcs

thermal and chemical heterogeneity in the mantle a seismic and geodynamic study of continental roots abstract chemical differences in the mantle below continents and oceans are investigated in the context of a recent seismic model of three dimensional mantle structure this analysis of the existence and magnitude of deep continent ocean chemical differences is based on a model of a viscous flowing mantle and it suggests that this continent ocean heterogeneity is internally supported and thus produces little flow this stabilization reduces the vertical flow of mass and heat across the upper mantle and results in a large continent ocean signal in the surface gravity and topography fields the stabilization of continent ocean heterogeneity also provides an explanation for the large amplitude of the nonhydrostatic ellipticity in the earth s geoid

crustal deformation and interplate coupling in the shikoku district japan as seen from continuous gps observation abstract data from the nationwide gps continuous tracking network which has been operated by the geographical survey institute gsi of japan since april 1996 were used to study crustal deformation in the shikoku district japan the crustal deformation data are inverted to estimate interplate coupling between the subducting philippine sea phs plate and the overriding continental plate for this purpose we first extracted site coordinates from daily sinex files for the period from 1 april 1996 to 31 march 1998 and estimated average site velocities by linear regression estimated velocities under itrf96 are converted into a kinematic reference frame k heki j geophys res 101 1996 3187 3198 to discuss the crustal deformation relative to the stable interior of the eurasian eur plate to segregate the signal and noise in the thus obtained velocity vectors we employed the least squares prediction technique estimated signals displacement vectors were used to reconstruct the strains dilatations maximum shear strains principal axes of strains and to estimate the interplate coupling in the area results of crustal strains clearly portray tectonic straining of the shikoku island 1 dilatational strains show that the shikoku island is under a compressive strain regime induced by the subducting phs plate 2 maximum shear is high in the island which may be due to the oblique subduction 3 the principal axes of the strains indicate that the shikoku island is under the influence of the converging oceanic plate beneath the island in contrast the results of the inversion analysis show 1 there is a high coupled region extending from 10 to 30 km deep 2 the average back slip rate in this region reaches 43 5 mm yr and 3 the direction of the plate convergence at the nankai trough is n42 6 w

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

inferring the viscosity and the 3 d density structure of the mantle from geoid topography and plate velocities summary in a dynamic earth mantle mass heterogeneities induce gravity anomalies surface velocities and surface topography these lateral density heterogeneities can be estimated on the basis of seismic tomographic models recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates the relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle here we use this model combined with a generalized least squares method in order to infer the viscosity profile of the earth from the surface observations and to get some new insight into the 3 d density structure of the mantle the computed radial viscosity profile presents a continuous increase of more than two orders of magnitude the asthenosphere has a viscosity close to 2 x lo2 pa s no sharp discontinuity is requested at the upper lower mantle interface the largest viscosity 7 x 10 pas is reached in the middle of the lower mantle at greater depth approaching the core mantle boundary the viscosity decreases by one order of magnitude the model suggests that the well known degree 2 and order 2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs it also slightly increases the degree 2 and order 0 in the lower mantle and decreases it in the upper mantle in other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes

on the geometry of mantle flow beneath drifting lithospheric plates summary the convective flow pattern beneath the oceanic lithosphere is strongly influenced by the movement of the lithospheric plate on top assuming upper mantle convection the wavelengths geometric structure and time of onset for ridge perpendicular rolls as functions of the rayleigh number and the plate velocity have been investigated by numerical modelling the related gravity anomalies have been estimated and compared to observations from the se pacific which show ridge perpendicular anomalies with wavelengths of 150 220 km and amplitudes of about 10 mgal for constant viscosity the type of secondary circulation perpendicular to the diverging ridge depends on the relation between imposed plate velocity vs and maximum velocity in the free convection case vhfree if vs is less than vhfree the flow is dominated by off ridge up and downwellings if vs is about 10 times larger than vhfree the flow remains 2 d if vs is between these extrema a chain of ridge plumes develops from the lower thermal boundary layer with adjacent ridge perpendicular walls that lead to roll like circulation richter rolls throughout the model box the wavelength of these rolls is a function of the rayleigh number and the plate velocity for parameters realistic for the earth rolls with considerable amplitude first develop after more than 50 myr with wavelengths of at least 250 km for temperature and pressure depth dependent viscosity the wavelengths of the evolving rolls are determined by the rayleigh number related to the maximum viscosity in the interior of the convecting system and not to the average viscosity for models with a strong decrease in viscosity to values of about 1019 pa s in a 100 km wide asthenospheric channel below the lithosphere ridge perpendicular rolls which only fill the low viscosity layer appear after a few million years after a transitional stage of some tens of millions of years rolls in the whole upper mantle also evolve the asthenospheric rolls develop from instabilities of the upper thermal boundary layer and have wavelengths of about 220 280 km the time needed for alignment of these rolls to the direction of plate movement is a few million years and the distance from the ridge to the point of onset of the rolls is about 200 500 km both are functions of the plate velocity while the formation time for rolls in the whole upper mantle is quite large and their existence is related largely to two layered convection this type of flow might not be very relevant for the earth although it has been used to account for 3 d structures at oceanic ridges to explain the observed gravity anomalies in the se pacific roll like circulation in a low viscosity asthenospheric layer is the preferred candidate where the modelled amplitudes and wavelengths of gravity anomalies fit the observations

geoid low and highs of the indian ocean and western pacific implications to mantle convection abstract as geoid anomalies have capacity to unravel deep seated density structures it has acquired special significance in recent years for this purpose geoid anomalies of india and se asia are characterized by the indian ocean geoid low and geoid highs of the western pacific the spectral analysis of the indian ocean geoid low iogl suggests depths of 1300 700 and 340 km for relatively large wavelength components they are related to middle mantle and transition zones in the upper mantle respectively where olivine changes to spinel with sharp increase in the velocity and density this geoid low is caused by the subducted indian lithosphere that rolled back after subduction and got accumulated in this section in this process the subducting slab would acquire same temperature as surroundings and would be characterized by low velocity and density as it primarily consists of the low density rocks of continental lithosphere the dehydration of subducting slab of different temperatures at different levels would set in convection cells in this case mid to upper mantle circulation convection is envisaged that has caused central indian ocean deformation zone with large scale faults folds high heat flow and seismic activity presence of such convection at the tail end of the indian plate drives it relatively faster 5 cm year the iogl may mark the site of first collision between the indian and the asian plates the geoid highs of the western pacific coincide with the subduction zones in this sector its spectral analysis provided average depths to the large wavelength sources as 2335 845 and 523 km they are attributed to sources above d layer and the transition zones in the upper middle mantle within the error limit of 10 15 depending on the wavelengths deeper level of sources for the geoid highs of pacific ocean may be attributed to high angle of subduction in this sector where cold subducting slab of high velocity and density can get easily detached and sink into the lower mantle in this case due to high angle of subduction of oceanic lithosphere the subducting slab would be at lower temperature compared to surroundings maintaining it brittle even in the transition zone that has given rise to deep focus earthquakes almost up to 700 km the high angle subduction along the subduction zones in the western pacific causes faster subduction that drives the pacific plate faster 4 10 cm year in this case the dehydration of the subducting slab would set in whole mantle convection between the lower and the upper mantle in the back arc regions of the mantle wedge it has plausibly given rise to extensional regime forming back arc basins on large scale such as japan sea philippine sea lau basin and others

numerical simulations of deformation and dynamics of horizontally lying slabs summary recent results of seismic tomography have revealed that configurations of subducted slabs are varied some lie horizontally on the boundary between the upper and the lower mantle and others penetrate into the lower mantle in this study to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle a 2 d numerical model was constructed for this purpose the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle incorporating density and viscosity jumps and a clapeyron slope for a pv mw phase transformation of olivine obtained from high pressure and hightemperature experiments at a depth of 660 km into the model we investigate temperature and velocity distributions state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km when the slabs are treated to move freely just after their bending at the 660 km phase boundary the slabs temporarily appear to lie horizontally on the phase boundary however the slabs penetrate the phase boundary and eventually sink into the lower mantle the length of the horizontally lying portion of the slab becomes longer as descending velocity increases the lengths are about 1500 km 1200 km and 800 km for subducting velocities of 8 cm yr 1 4 cm yr 1 a nd 2c m yr 1 respectively when a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle for all the models in which the guide was removed at a depth of 400 km the slabs penetrate into the lower mantle without lying horizontally on the phase boundary these results indicate that it is difficult for the slabs to bend at the pv mw phase boundary and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle the calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs which is inconsistent with seismologically observed vertical directions of the p axes there in order to explain the observed configurations and state of stress within the horizontally lying slabs it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail

low frequency earthquakes and tomography in western japan insight into fluid and magmatic activity abstract low frequency lf microearthquakes are detected in the lower crust and uppermost mantle beneath western japan which may reflect fluid and magmatic activity in the subduction zone in this work we combine seismic tomography and the lf earthquakes to study the crustal and upper mantle structure and fluid and magmatic activity in western japan high resolution tomographic images under this region are determined using high quality arrival time data in kyushu the subducting philippine sea slab is detected clearly as a high velocity zone and the arc magma related low velocity low v anomalies under the active arc and back arc volcanoes are imaged clearly prominent low v zones are visible in the crust and uppermost mantle beneath the quaternary volcanoes along the japan sea coast and lf events occur actively under some of those volcanoes lf events are also detected in the source areas of the 1995 kobe earthquake m 7 2 and the 2000 tottori earthquake m 7 3 supporting the hypothesis that the large crustal earthquakes were triggered by the crustal fluids these results indicate that fluid and magmatic activities exist widely in western japan which are caused by dehydration of the subducting philippine sea slab and corner flow in the mantle wedge

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

thermal expansion measurements at very high pressure systematics and a case for a chemically homogeneous mantle new data on thermal expansivity of iron to over 400 kbar in laser and electrically heated diamond anvil cells and on mgo and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats boehler 1982 imply that at high pressure may be described by a constant value of 1n 1nv t 5 5 0 5 this behavior is supported by previous measurements at lower pressures on a large variety of materials densities of magnesio wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and at high pressures using the above systematics match those in prem through the entire lower mantle making a strong case for a uniform mantle

correlation analysis between subduction in the last 180 myr and lateral seismic structure of the lower mantle geodynamical implications we have calculated the correlation between four recent tomographic models and reconstructions of past subduction sites to estimate the present positions of the lithospher subducted during the cenozoic and the mesozoic a significant correlation is found at a depth of about 1000 km for subduction younger than 120 myr while remnants of subduction older than 120 myr are detected close to the core mantle boundary cmb somewhere in the depth range of 1300 2600 km a gap exists where no significant correlation is found in any time the results suggest that the slabs do not pass through the lower mantle continuously it is possible that both regimes the catastrophic and the steady state may be important for different time periods fast seismic anomalies close to the cmb may be associated with a past catastrophic event while the good correlation in the upper part of the lower mantle may be explained by a steady state model other explanations for the very good correlation found at a depth of about 1000 km are either a predicted perovskite phase transition or a chemical boundary suggested by seismic array analysis

formation of a future supercontinent through plate motion driven flow coupled with mantle downwelling flow series of high resolution numerical simulations of three dimensional mantle convection were performed to examine the interaction between the drifting continental lithospheres and the underlying mantle structure for 250 m y from the present and to predict the configuration of the future supercontinent the density anomaly of the mantle interior was determined by the seismic velocity anomaly from global seismic tomography data sets which contain well resolved subducting slabs the present day plate motion was imposed for the first stage of the simulation as a velocity boundary condition at the top surface boundary instead of a shear stress free condition the switching time from the plate motion boundary to shear stress free conditions was taken as a free parameter the results revealed that australia eurasia north america and africa will merge together in the northern hemisphere to form a new supercontinent within 250 m y from the present the continental drift was assumed to be realized by plate scale mantle flow rather than large scale upwelling plumes that is continuously moving plates at the surface for the first stage of the simulation are mechanically coupled with the subducting slabs in the mantle this enhances the underlying mantle downwelling flow as a result persistent continental drift can be reproduced for long future time periods even though top surface boundary conditions may switch in response to shear stress free conditions the configuration of the numerically reproduced future supercontinent in this study is broadly consistent with the hypothetical model of amasia as indicated by previous findings from geological correlations and a paleogeographic reconstruction

crustal deformation and strain analysis in nepal from gps time series measurement and modeling by arma method high precision global positioning system gps measurements from 2002 to 2017 in nepal has been analyzed and modeled with autoregressive moving average arma method the measured velocities of the gps sites in itrf 2014 indicate 6 12 mm year n s convergence inside the nepal between india and tibet this convergence occurs in the southern and western border of nepal but it is negligible to the eastern border of nepal arma modeled result of india fixed velocities is able to successfully model within 0 5 mm year the current measured velocities in nepal synthetic or modeled velocities using arma method and measured velocity have difference 2 mm year at some sites this indicates the possibility of very small level of a ductile deformation in the present best fit arma model the modeled velocities 8 10 mm year convergence taken up from northern to southern boundaries of nepal indicates slip along active main himalayan thrust mht which is the cause of change in tectonic strain and may be able to trigger earthquakes inside the country dislocation modeling of active mht is also carried out in the study using gps vertical displacement the geometric model of the mht is characterized by a rectangular plane with dip angle of 9 5 and the reverse kinematics with a slip of 19 mm year the thrust is located at a depth of 20 km in addition the regional deformation of the nepal has been checked by baseline measurement and coordinate stability of gps sites for the different zones the baseline divergence and convergence of nearest gps sites have been tested using triangular sites the summary of regional deformation implies n s convergence only and rules out any type of e w convergence finally the investigation of observed and modeled strain confirms that there is no significant strain accumulation due to localized regional deformation or earthquakes specific to the active dislocations and faults this suggests that the convergence from northern to southern boundaries of nepal is associated with the slip along the mht only

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

seismic imaging of southwest japan using p and pmp data implications for arc magmatism and seismotectonics abstract to better understand the relationship between crustal heterogeneity and seismotectonics and arc magmatism we determined a detailed p wave velocity structure of the crust under a north south belt area in southwest japan by using the first arriving p waves and the reflected p waves from the moho discontinuity pmp from 60 crustal earthquakes recorded by 29 seismic stations our results revealed significant low velocity anomalies beneath the fault zone of the median tectonic line and the daisen volcano which suggest that daisen is potentially an active volcano in the lower crust we detected low velocity anomalies overlying the subducting philippine sea plate which may indicate the existence of fluids released by the slab dehydration the 1955 southern tokusima earthquake m 6 4 occurred between low and high high velocity zones the 2000 western tottori earthquake m 7 3 and other nearby strong earthquakes occurred around the daisen volcano which may be related to the upwelling magma and fluids

the three dimensional seismological model a priori constrained confrontation with seismic data we compare the predictions of an a priori model of the upper mantle with seismic observations of surface waves and eigenmodes the 3 dimensional seismological model a priori constrained 3smac has been developed by natal and ricard 1996 it is based on the interpretation by geodynamicists of the near surface layers of the earth on distributions of temperature pressure and composition as a function of depth and then on estimates of seismic parameters density velocities attenuations from solid state laboratory measurements as a function of temperature and pressure the 3smac predictions are confronted with observations consisting of phase velocities for love and rayleigh waves in the period range of 70 250 s montagner and taniraoto 1990 we first show that tomographic inversions applied to 3smac synthetics induce a strong smoothing of the heterogeneities this ca sts doubt on the meaning of the spectra of mantle heterogeneities revealed by tomography we then show that most of the love and rayleigh fundamental mode observations for periods less than 200 s are satisfactorily predicted by 3smac the major differences come from the seismic velocities under the red sea and southeast china which are much slower than what is estimated from 3smac as well as those under greenland which are not as fast as the other cratonic areas because the lithosphere is thinner than 100 km under oceans and thinner than 300 km under continents in 3smac we suggest that the existence of deeper lithospheric anomalies as proposed in many tomographic models is mostly due to a spurious effect of the inversion rather than implied by surface wave data half of the variance of the degree 2 anomaly mapped by low degree eigenmode observations can be explained by lithospheric velocity structures the other half is highly correlated with the distribution of deep slabs but its amplitude is a factor of 3 or 4 larger than that predicted by 3smac the lithospheric anomalies present a degree 6 pattern well correlated with the distribution of hotspots even when the thermal anomalies that could be associated with plumes are not included in 3smac our results emphasize the importance of giving very close attention to surface corrections in tomographic models

the role played by a low viscosity zone under a 660 km discontinuity in regional mantle layering abstract our previous work from regional geoid inversion over oceans has shown that there exists a low viscosity zone lvz under the 660 km boundary nonlinear inversion has been applied for fitting to the intermediate wavelength oceanic geoid without invoking a priori any impermeable boundary in this study we have conducted a systematic search of the dependence of the depth of this regional boundary from 660 to 1000 km we found that the use of this boundary at the depth of the lvz exerts little influence on the inversion the lvz can act regionally as an impermeable boundary and enhances very fast lateral flow at the uppermost lower mantle on the other hand impermeable boundary alone is not caused by a lvz as shown also by analysis of the 2d flow patterns driven by internal density anomalies inferred from tomography from the intermediate wavelength geoid we can not resolve the existence of layering when the lvz exists beneath the 660 km boundary however a lvz is needed to fit the geoid this lvz can have profound influence on the geodynamical behavior of mantle circulation and the dispersal of geochemical anomalies and the generation of seismic anisotropy

effect of the mid mantle viscosity and phase transition structure on 3d mantle convection abstract recent geophysical evidence shows some sort of layering in the lower part of the mantle transition zone down to a depth of 1000 km seismic observations have revealed a sharp reflector surface at around 900 or 1000 km depth for which a possible explanation can be given in terms of a new phase transition of the lower mantle constituent minerals furthermore new results from the inversion of the oceanic geoid show the existence of a second low viscosity zone lvz somewhere between 660 and 1000 km depth the existence of the second lvz may be linked to the mid mantle phase transitions the phase and viscosity stratification of the transition zone have been included in a series of 3d convection simulations in a 4 4 1 rectangular box with a surface rayleigh number of 2 10 7 beneath the well known 400 and 660 km phase changes we assumed a hypothetical weak endothermic transition at 1000 km in some of our models the principal controlling factor of the style of mantle convection is still the 660 km endothermic transition which sets up a partial or full barrier to flow causing stratified circulation we used various viscosity profiles with emphasis on the model containing the second lvz the main consequence of this zone is to enhance flow layering many plumes can emanate from the transition zone and small scale instabilities develop in the second lvz when the 1000 km endothermic phase transition is included these instabilities can grow only at a few places but then they form strong downwellings two distinct types of penetrative deep downwellings can be present at the same time one which crosses the whole transition zone and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily this can explain seismological observations which suggest that subducted slabs can be retarded not only by the 660 km boundary but also by some deeper obstacle near 1000 km depth

new perspectives on mantle dynamics from high resolution seismic tomographic model p1200 recently a high resolution tomographic model the p1200 based on p wave travel times was developed which allowed for detailed imaging of the top 1200 km of the mantle this model was used in diverse ways to study mantle viscosity structure and geodynamical processes in the spatial domain there are lateral variations in the transition zone suggesting interaction between the lower mantle plumes and the region from 600 km to 1000 km some examples shown here include the continental region underneath manchuria ukraine and south africa where horizontal structures lie above or below the 660 km discontinuity the blockage of upwelling is observed under central africa and the interaction between the upwelling and the transition zone under the slow icelandic region appears to be complex an expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60 for degrees exceeding around 10 the spectra at various depths decay with a power law like dependence on the degree with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2 6 these spectral results may suggest the time dependent nature of mantle convection details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high resolution tomographic model we have considered only the intermediate degrees l 12 25 in the nonlinear inversion with a genetic algorithm approach several families of acceptable viscosity profiles are found for both oceanic and global data the families of solutions for the two data sets have different characteristics most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere in other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump other families evidence a viscosity decrease across the 660 km discontinuity solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere the other right under 660 km depth all of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle the presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection numerical modelling of mantle convection with two phase transitions and a realistic temperature and pressure dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self consistently in time dependent situations involving a partially layered configuration in an axisymmetric spherical shell model

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

effects of the postperovskite phase change on the observed geoid in the lowermost mantle seismic velocity variations beneath pacific margins have been related to the perovskite to postperovskite ppv phase transition we investigate the influence of this phase transformation on the geoid using 3 d spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle we demonstrate that the geoid anomalies are strongly affected by the presence of ppv because of phase dependent viscosity changes relative to the surrounding mantle whereas geoid heights above subduction zones are increased for high viscosity ppv the presence of weak ppv reduces them thereby improving the fit to the observed geoid an investigation using two different tomography models different ppv density contrasts and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrates the various effects of weak ppv on the geoid

the 2011 magnitude 9 0 tohoku oki earthquake mosaicking the megathrust from seconds to centuries geophysical observations from the 2011 moment magnitude m w 9 0 tohoku oki japan earthquake allow exploration of a rare large event along a subduction megathrust models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places sources of high frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip relative to the m w 8 8 2010 maule chile earthquake the tohoku oki earthquake was deficient in high frequency seismic radiation a difference that we attribute to its relatively shallow depth estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

inferences of viscosity from the oceanic geoid indication of a low viscosity zone below the 660 km discontinuity abstract we have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans in order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents we have considered only the intermediate degrees of the oceanic geoid l 12 25 a genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660 km boundary somewhere in the depth range of 400 1000 km a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle unfortunately the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel

subducting slabs jellyfishes in the earth s mantle the constantly improving resolution of geophysical data seismic tomography and seismicity in particular shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle this observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle the distribution and character of seismicity reveal however that slabs undergo vertical extension in the upper mantle and compression near the transition zone in this paper we demonstrate that during the subduction process the shape of low viscosity slabs 1 to 100 times more viscous than the surrounding mantle evolves toward an inverted plume shape that we coin jellyfish results of a 3d numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head the head is linked to the body of the subducting slab by a thin tail a complete parametric study reveals that subducting slabs may achieve a variety of shapes in good agreement with the diversity of natural slab shapes evidenced by seismic tomography our work also suggests that the slab to mantle viscosity ratio in the earth is most likely to be lower than 100 however the sensitivity of slab shapes to upper and lower mantle viscosities and densities which remain poorly constrained by independent evidence precludes any systematic deciphering of the observations

isostatic deformations and polar wander induced by redistribution of mass within the earth the effects at the surface of the earth of time dependent mantle mass anomalies are analyzed within the framework of a viscoelastic mantle with maxwell rheology the implications for the earth s rotation are developed using the linearized liouville equations valid for small polar displacements our approach is appropriate for a simplified modeling of subduction the displacement of the earth s axis of rotation called true polar wander is very sensitive to the viscosity profile of the mantle and to the nature of the 670 km seismic discontinuity phase change models generally yield a huge amount of polar wander except for large viscosity increases for chemically stratified models true polar wander is drastically reduced as a consequence of dynamic compensation of the mass anomalies at the upper lower mantle interface when the source is embedded in the upper mantle in the proximity of the chemical density jump the polar wander stops after a finite time controlled by the slowest isostatic mode

long and short wavelengths of indian ocean geoid and gravity lows mid to upper mantle sources rapid drift and seismicity of kachchh and shillong plateau india abstract spectral analysis of the indian ocean geoid low provides depth to the large wavelength sources as 1300 700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels the three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine spinel transformation of successively increasing fe mg ratio as depth increases known as transition zones the first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field the low density rocks in this section appear to be related to the subducted indian tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments the relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere asthenosphere boundary lab under the indian continent and surrounding oceans respectively a low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows the subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid to upper mantle which is likely to set in baby plumes presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section they would make this region buoyant that appears to be responsible for the central indian ocean deformation zone with large scale folding faulting seismicity and high heat flow the same also appear to be responsible for the rapid drift of the indian plate short wavelength component of the indian ocean geoid low reflects most of the surface shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose kachchh and the shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries tectonics and residual geoid anomalies of the indian continent and adjoining regions suggest that the geoid highs of kachchh and the shillong plateau along the nw and the ne corners of india are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries geoid highs indicating high density mafic rocks suggest their connections extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries besides both these regions are affected by i prominent lithospheric flexure of the indian plate ii large scale mafic intrusions and iii interaction of proterozoic mobile belts with effects of present day himalayan orogeny that make them more vulnerable for seismic activity

seismic anisotropy and mantle dynamics beneath china abstract we analyzed the shear wave splitting at 138 permanent seismograph stations to study seismic anisotropy and mantle dynamics under mainland china to obtain reliable results we used three different methods to measure the shear wave splitting parameters using core phases sks skks skiks and pks as well as the direct s waves from regional and distant earthquakes our results show that the fast orientations of the anisotropy wnw ese in eastern china are generally consistent with the absolute plate motion apm direction of the eurasian plate suggesting that the anisotropy is mainly located in the asthenosphere resulting from the lattice preferred orientation of olivine due to the shear deformation there the fast axes in western china generally agree with the strikes of the orogens and active faults while they are perpendicular to the direction of the maximum horizontal stress suggesting that the anisotropy in the lithosphere contributes significantly to the observed shear wave splitting the fast axes in western china are also consistent with the apm direction suggesting that the apm driven anisotropy in the asthenosphere is another source of the shear wave splitting there these results suggest that apm driven anisotropy commonly exists under continents similar to that under oceanic regions even though the continental lithosphere has suffered extensive deformation

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

the fate of slabs inferred from seismic tomography and 130 million years of subduction the volume and location of subducted plate since 130 ma are reconstructed from magnetic anomalies and updated finite rotation parameters describing relative motions in the ocean basins and between plates and hotspots the area of subducted plate is calculated from the relative motions between overriding and overridden plates along the length of convergence in the hotspot reference frame in 5 ma intervals ridge locations are reconstructed by rotating magnetic anomalies from their present to their former positions the distances between trenches and ridges at a certain point of time in the spreading direction are thus estimated and are used to determine the ages of trenches according to the spreading history recorded in present day oceans the thickness of the oceanic plate is obtained from the age based on the half space cooling model about 3 45 10 10 km 3 of oceanic plate has been subducted during the past 130 ma with maximum accumulation beneath southeast eurasia at spherical harmonic degree l 2 where subduction flux peaks excellent correlation is found between four seismic tomographic models at the top of the lower mantle about 800 1100 km deep and predicted slab locations for some seismic models excellent correlation is also found at other degrees in that depth range some models also have good correlations with predicted slab locations in the deep mantle at l 2 direct comparison between subduction and seismic tomographic patterns at 800 1100 km shows that subduction history correlates with tomography very well in terms of location and amplitude of anomalies cold downwellings which may be related to slabs appear to be trapped in the mesosphere or middle mantle correlations between seismic tomography and subduction over different time periods support this conclusion there are also some correlations in the upper mantle and in the deep lower mantle although they are generally not as significant as those in the 800 1100 km depth range there may be a significant boundary in the mantle near the 800 1100 km depth from a geodynamic and chemical point of view the lower mantle may start at a depth closer to 1000 km than to 670 km

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

mantle viscosity and ice age ice sheet topography ice age paleotopography and mantle viscosity can both be inferred from observations of earth s response to the most recent deglaciation event of the current ice age this procedure requires iterative application of a theoretical model of the global process of glacial isostatic adjustment results demonstrate that the iterative inversion procedure converges to a paleotopography that is extremely close to that from the ice 4g model the accompanying mantle viscosity profile is furthermore shown to reconcile the requirements of aspherical geoid anomalies related to the mantle convection process thus resolving a fundamental issue concerning mantle rheology the combined model also explains postglacial sea level histories for the east coast of the united states

the statistics of pink noise on a sphere applications to mantle density anomalies summary this paper shows that the power spectrum of the earth s gravity field is very well modelled by white noise signals originating at just four depths within the earth being able to estimate both the lateral and the radial position of anomalous density gives a new and independent way of imaging mantle structure the success of a four source depth model undermines the evidence for the long held picture that a surface observation of gravity reflects density anomalies distributed indistinguishably at all depths within the earth a picture largely based on an inappropriate definition of the power spectrum all spherical harmonics of gravity observed at the earth s surface with degree greater than about 60 originate within the lithosphere degrees less than 4 come from a poorly constrained source near the core mantle boundary all other wavelengths come from sources centred near white noise depths of 315 km and between 1100 and 1500 km these depths are maxima the same statistical model describes sources at a shallower depth but now representing a random process with a finite correlation length the form of the spectrum requires mantle heterogeneity to increase by about two orders of magnitude between the asthenosphere and the core mantle boundary it is suggested that the variance of density inhomogeneities is related to mantle viscosity

secular crustal deformation and interplate coupling of the japanese islands as deduced from continuous gps array 1996 2001 abstract data from the nation wide gps continuous tracking network that has been operated by the geographical survey institute of japan since april 1996 were used to study crustal deformation in the japanese islands we first extracted site coordinate from daily sinex files for the period from april 1 1996 to february 24 2001 since raw time series of station coordinates include coseismic and postseismic displacements as well as seasonal variation we model each time series as a combination of linear and trigonometric functions and jumps for episodic events estimated velocities were converted into a kinematic reference frame heki k 1996 horizontal and vertical crustal movements from three dimensional very long baseline interferometry kinematic reference frame implication for reversal timescale revision j geophys res 101 3187 3198 to discuss the crustal deformation relative to the stable interior of the eurasian plate a least squares prediction technique has been used to segregate the signal and noise in horizontal as well as vertical velocities estimated horizontal signals horizontal displacement rates were then differentiated in space to calculate principal components of strain dilatations maximum shear strains and principal axes of strain clearly portray tectonic environments of the japanese islands on the other hand the interseismic vertical deformation field of the japanese islands is derived for the same gps data interval the gps vertical velocities are combined with 31 year tide gage records to estimate absolute vertical velocity the results of vertical deformation show that 1 the existence of clear uplift of about 6 mm yr in shikoku and kii peninsula whereas pattern of subsidence is observed in the coast of kyushu district this might reflect strong coupling between the philippine sea plate and overriding plate at the nankai trough and weak coupling off kyushu 2 no clear vertical deformation pattern exists along the pacific coast of northeastern japan this might be due to the long distance between the plate boundary japan trench and overriding plate where gps sites are located 3 significant uplift is observed in the southwestern part of hokkaido and in northeastern tohoku along the japan sea coast this is possibly due to the viscoelastic rebound of the 1983 japan sea mw 7 7 and the 1993 hokkaido nansei oki mw 7 8 earthquakes and or associated with distributed compression of incipient subduction there we then estimate the elastic deformation of the japanese islands caused by interseismic loading of the pacific and philippine sea subduction plates the elastic models account for most of the observed horizontal velocity field if the subduction movement of the philippine sea plate is 100 locked and if that of the pacific plate is 70 locked however the best fit for vertical velocity ranges from 80 to 100 coupling factor in southwestern japan and only 50 in northeastern japan since horizontal data does not permit the separation of rigid plate motion and interplate coupling because horizontal velocities include both contributions we used the vertical velocities to discriminate between them so we can say there is strong interplate coupling 80 100 over the nankaido subduction zone whereas it is about 50 only over the kurile japan trench

mantle viscosity derived by genetic algorithm using oceanic geoid and seismic tomography for whole mantle versus blocked flow situations abstract we have applied the genetic algorithm ga technique a nonlinear global optimization method to determine the radial viscosity structure of the mantle from regional geoidal patterns from numerical simulations of 2 d mantle convection we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition the differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths this result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole mantle and blocked flow situations in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth in order to confirm the robustness of this study two different density models have been used which were constructed from three tomographic models and appropriate velocity to density scaling relations based on recent results from mineral physics we have analyzed only oceanic geoid spanning between spherical harmonic degree l 12 25 the correlation of the predicted geoid with the observations over the atlantic indian and pacific oceans have been employed as the fitting function in our ga approach which has been modified from the common algorithm in constructing the families of suitable viscosity profiles we have used 100 parents which have been iterated for 100 generations and have been started with 10 different sets of initial parents convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole mantle and layered models in some cases multiple viscosity solutions are found acceptable by using the correlation criteria the outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones one lying under the lithosphere the other right under the bottom of the spinel to perovskite phase change the solutions for the whole mantle model can fit better and are preferred over the solutions with the layered boundary condition which generally result in unrealistic viscosity profiles our results would suggest a more complex mantle viscosity structure which has not been detected previously from geoid signals with longer wavelengths and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

confidence regions for mantle heterogeneity least squares tomographic models of mantle p and s velocity structure from travel times have shown large scale variations correlated with surface tectonic features as well as coherent structures in the lowermost mantle the reliability of these global features of velocity models depends on whether velocity throughout the features can be estimated well simultaneously one needs to be able to say with confidence that a feature involving many voxels is likely to be real we find a lower bound on how wide a 95 simultaneous confidence region for least squares estimates of mantle p or s velocity perturbations must be as a function of position in the mantle suppose perhaps optimistically that summary ray travel time errors are independent with mean zero and standard deviation 0 25 s for direct p phases and standard deviation 0 5 s for all other phases regardless of distance if summary travel time residuals are fitted by least squares to a mantle model parametrized by 4872 10 by 10 voxels the half width of a 95 confidence region for p velocity using 889 909 summary p rays 84 046 summary pp rays 67 228 summary pp rays 23 024 summary pcp rays 11 239 summary pkpab rays 47 227 summary pkpbc rays and 141 843 summary pkpdf rays ranges from 0 025 to 99 8 km s 1 it is wider than 0 193 km s 1 in half of the mantle and wider than 0 593 km s 1 in a quarter of the mantle by volume under the same assumptions and using the same parametrization a 95 confidence region for s velocity based on 163 354 summary s rays 13 781 summary ss rays 7441 summary scs rays and 12 494 summary ss rays ranges from 0 031 to km s 1 is wider than 0 254 km s 1 in half of the mantle and is wider than 0 554 km s 1 in a quarter of the mantle with this ray set parametrization and error model the 95 confidence region around the least squares estimate of p velocity includes the radially symmetric iasp91 model for 86 6 of the mantle s volume a 95 confidence region around the least squares s estimate includes iasp91 for 88 9 of the mantle s volume on a global scale the mantle s velocity structure is nearly consistent with the iasp91 radially symmetric model smaller voxels more realistic assumptions about the errors more accurate modeling of the physics and three dimensional structure outside the mantle would make the confidence regions still wider

interplate seismogenic zones along the kuril japan trench inferred from gps data inversion stress accumulation between earthquakes results from slip that is insufficient to fully accommodate plate movement an inverse analysis of gps data from the kuril japan trench reveals a trench parallel belt of stress accumulation with six peaks in the depth range of 10 40 km suggesting potential source regions for future earthquakes in the subduction zones around japan where four plates interact with one another large earthquakes have occurred repeatedly1 these interplate earthquakes are part of the process of tectonic stress accumulation and release that is driven by relative plate motion2 3 4 stress accumulation between earthquakes results from slip deficit slip that is insufficient to fully accommodate plate movement for the prediction of large earthquakes it is therefore important to monitor the distribution of slip deficit on plate interfaces here we apply an inversion method based on bayesian modelling using direct and indirect prior information on the magnitude and distribution of fault slip5 to horizontal and vertical velocities from global positioning system data for the seismically calm period between 1996 and 2000 we obtain a precise distribution of slip deficit rates on the interface between the north american and pacific plates around japan which reveals a trench parallel belt of slip deficit with six peaks in the depth range of 10 40 km these peaks agree with the source regions of past large interplate earthquakes along the kuril japan trench we conclude that the slip deficit zones identified with our method are potential source regions of large earthquakes

high strain rate zone in central honshu resulting from the viscosity heterogeneities in the crust and mantle abstract a linear zone with high strain rates along the backarc side of central honshu the so called niigata kobe tectonic zone has been revealed by the dense gps array geonet operated by the geographical survey institute of japan in order to explore the origin of this zone we examine rheological heterogeneities in the crust or mantle above a subducting slab which possibly contribute to the high strain rates we calculate velocity profiles at the surface of the upper plate due to loading and unloading of the subducting philippine sea plate using a 2 dimensional finite element method from the numerical experiments we find that the high strain rates can be reproduced by viscosity heterogeneities either in the crust or the mantle if viscosity heterogeneity in the crust is an origin the conditions to be satisfied are i an effective elastic thickness of 5 km for short term loading 10 2 10 3 yr ii a low viscosity 19 pa s in the lower crust beneath the zone with a normal viscosity 10 21 pa s in the ambient lower crust and iii a uniform viscosity of 10 19 pa s in the upper mantle on the other hand if viscosity heterogeneity in the mantle is an origin the conditions to be satisfied are i an effective elastic thickness of 30 km for short term loading ii a low viscosity 10 18 pa s at least down to a depth of 60 km in the zone and to a depth of 10 km further seaward below the moho depth and iii a viscosity of 10 20 pa s in the ambient mantle the crust heterogeneity model contradicts the 15 km cut off depth of intraplate seismicity and the 20 km effective elastic thickness previously inferred from gravity and topography in central honshu the fact that quaternary active faults and intraplate earthquakes are not particularly concentrated in the zone also favours the mantle viscosity heterogeneity model thus we prefer the mantle heterogeneity model we suggest that upward movement of water dehydrated from the subducting philippine sea and pacific plates partial melting of the mantle above the pacific plate and serpentinization in the wedge mantle above the philippine sea plate are possible origins of the low viscous upper mantle possible low viscosities in the upper mantle wedge presented in this study seem to provide important constraints for subduction processes of oceanic plates and strain accumulation processes in the upper plate

contemporary deformation and seismicity analysis in southwest japan during 2010 2018 based on gnss measurements a significant amount of work has been conducted in the japanese islands on high precision positioning surveying natural disaster mitigation reference frame and datum definitions however there is substantial scope for exploring the current seismotectonic activity of japan which would provide further awareness of disaster management the short recurrence time of great inter plate earthquakes and well recorded geodetic observations provide one of the best examples to analyze the combination of contemporary deformation and seismic intensity in southwest sw japan in the current study we analyzed three districts of sw japan which include mainly kyushu district and parts of chugoku and shikoku districts we predicted global navigation satellite system gnss measurements based on amurian fixed reference frame aref velocities with the help of singular spectrum analysis ssa method and examined the regional deformation from april 01 2010 to december 31 2018 in addition we analyzed the earthquakes clustered calculated by the nearest neighborhood distance method which showed prominently bimodal seismicity of sw japan and revealed the existence of two statistically distinct earthquake populations the events inside a cluster were normally close to their nearest neighbors whereas events from distinct clusters were comparatively aloof from each other finally the paper provides the spatial strain and rotational rate variation for selected seismotectonic districts that are in relatively good agreement with the seismic deformation inferred from earthquake focal mechanisms we believe that our integrated approaches would provide a better insight into the active deformation of sw japan

continuous gps array and present day crustal deformation of japan a gps array with about 1 000 permanent stations is under operation in japan the gps array revealed coseismic deformations associated with large earthquakes and ongoing secular deformation in the japanese islands based on daily coordinate data of the gps stations strain rate distribution is estimated most regions with a large strain rate are related to plate boundaries and active volcanoes in addition the niigata kobe tectonic zone nktz is recognized as a region of large strain rate along the japan sea coast and in the northern chubu and kinki districts this newly found tectonic zone may be related to a hypothetical boundary between the eurasian or amurian and the okhotsk or north america plates precise observation of crustal deformation provides important boundary conditions on numerical modeling of earthquakes and other crustal activities appropriate computation methods of continuous deformation field are directly applicable to data assimilation for such numerical simulations

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

ultrafast mantle plumes and implications for flood basalt volcanism in the northern atlantic region abstract recent geochemical and geochronological data on paleocene flood basalts from west greenland se greenland and the british isles show that volcanic activity at these widely separated locations commenced nearly simultaneously around 61 62 ma ago and that the duration of this initial phase of flood basalt magmatism was of the order of a few million years or less a small fast moving upper mantle plume which rapidly spreads out horizontally on encountering the base of the lithosphere appears to be a viable mechanism to explain these observations however in order to reconcile the idea of an ultrafast plume with the observed plate velocities a physical mechanism is needed for inducing a separation of timescales between the plume speed and the surrounding mantle circulation thermal convection with a non newtonian temperature and depth dependent rheology provides such a mechanism wherein extremely fast plumes ascending at velocities between one to tens of meters per year can be produced in an otherwise slowly convecting mantle moving at cm yr this transport mechanism is capable of bringing up very hot matter from the transition zone to the lithosphere the fast upward velocities lead to the production of high viscous heating rates surrounding the plume upon impinging the lithosphere it is thus possible to explain the near simultaneous onset of magmatism in west greenland se greenland and the british isles in the early tertiary by a fast moving mantle plume spreading out horizontally with a velocity of around 0 5 m yr for resolving numerically the thermal mechanical state of these strongly time dependent mantle flows extremely high spatial resolution on the kilometer scale is required finally we suggest the possibility that the source of the fast upper mantle plume under iceland may be rooted in the lower mantle this is consistent with the recent findings by seismic tomography of a deep mantle plume under iceland

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

skewness of s wave velocity in the mantle we analyze tomographic models of the s wave velocity in the mantle the depth variation of laterally heterogeneous structure is parametrized in terms of the skewness which yields information on mantle convection recent models show negative skewness in the lower half of the mantle assuming that the velocity anomaly is related to the temperature anomaly this suggests that there is significant heat flow from the core

crustal deformation in kyushu derived from geonet data 1 an investigation was conducted on the horizontal crustal deformation in kyushu using the coordinate data from gps earth observation network geonet for the 4 years from 1998 to 2002 for which the annual variations are corrected we examined the spatial differences in the displacement rate relative to the amurian plate by subtracting the velocity of the amurian plate at each of the global positioning system gps stations models for interplate coupling in hyuganada and the dilatational source beneath sakurajima are obtained the crustal deformation in eastern kyushu is well explained by the model of interplate coupling in hyuganada which is strong in the north and weak in the south when the effects of the interplate coupling and the dilatational source are removed a southward extensional field is recognized in and to the south of the beppu shimabara graben in western kyushu in southern kyushu to the south of around latitude 32 n a southeastward movement is conspicuous it must be noted that the displacement field is extensional indicating that southern kyushu is being dragged at the base by a flow in the mantle not moving like a raft on the flow it is probable that the flow in the mantle is driven by the rising of hot materials in the okinawa trough and a retreat of the trench where the philippine sea plate is subducting a gap in the magnitude of the eastward displacement rate around latitude 32 n implies that the velocity of the flow in the mantle decreases substantially to the north of the line the beppu shimabara graben may be a continuation of the okinawa trough but the significant southwestward displacement rate observed in the eastern part suggests that the deformation in the graben at least to the east of aso volcano is affected by the westward movement of the forearc sliver produced by the oblique subduction of the philippine sea plate on the nankai trough off shikoku island we think that the strong coupling in the bungo channel in our model is probably not real but might be a result of our assumption that all the westward displacements in northeastern kyushu a part of which are actually due to the movement of the forearc sliver are produced by interplate coupling

dynamic topography compared with residual depth anomalies in oceans and implications for age depth curves ocean depth is affected by dynamic topography caused by mantle flow regional bathymetric deviations from the cooling plate age depth curve called residual depth anomalies can therefore be used as indicators of dynamic topography in oceans in this work we first evaluated the oceanic residual depth anomalies we then compute the dynamic topography for the flow induced by density perturbations converted from seismic tomography models by assuming we found that the predicted dynamic topography correlates with the depth anomalies when the density perturbations in the shallow part of the upper mantle were inferred from slab densities not from tomographic models we estimated a new age depth curve based on depths corrected for dynamic topography this corrected age depth curve shows that the corrected depths for old seafloor 70 110 ma are a few hundred meters deeper than those uncorrected

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

dynamical consequences of mid mantle viscosity stratification on mantle flows with an endothermic phase transition recent geophysical evidences from seismology and geoid inversions have pointed out the existence of some layering between 900 and 1000 km depth and the possibilities of a low viscosity zone between 660 and 1000 km depth using a finite difference and spectral code we have conducted 3 d calculations of thermal convection with the endothermic phase transition at 670 km depth and a family of viscosity profiles with emphasis on the presence of two low viscosity zones one in the upper mantle between 100 and 250 km depth and the second one between 670 and 1000 km depth below 1000 km the viscosity increases with depth the surface rayleigh number used was in the range of 10 7 the dominating effect is that due to the second low viscosity zone below the phase change layered convection is induced by the presence of this second asthenosphere which causes horizontal flow to develop in there alternatively a viscosity increase at 670 km depth would shift convection to the single layer regime the potential sharp variations of the viscosity structure between 670 and 1000 km depth can greatly influence the global flow dynamics of the mantle

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

global love wave phase velocity variation and its significance to plate tectonics abstract global love wave phase velocity variation was constructed for periods between 80 and 200 s by using approximately 9000 paths from 971 earthquakes with m 5 5 the data set was from gdsn and geoscope networks between 1980 and 1987 we examined both the spherical harmonic expansion method and the block parameterization method with a simple constant damping parameter approach synthetic tests showed that more accurate results were obtained by the block parameterization method than by the spherical harmonic expansion method we adopted the block inversion method with a nearly equal area block 5 5 near the equator discretization the general pattern of the resulting maps were consistent with previous global and local studies with a discretization of 5 by 5 the maps were detailed enough to test some plate tectonic models for the pacific atlantic and indian oceans surface wave velocities increased smoothly to plate ages older than 100 ma simple forward modeling showed that seismic phase velocity variation with a continuous thickening of lithosphere up to about 150 ma fits the present observation disagreeing with the model deduced from the heat flow and ocean depth data which change variations at about 60 80 ma the seismic phase velocity variations in different oceans showed systematic differences at younger ages and convergence beyond 100 ma the difference at younger ages implies a failure of scaling derived from a simple thermal boundary layer model for oceanic plates age seismic phase velocity relationships on each side of ridges were also examined and asymmetric velocity variations were found which suggests differences in thermal states from one side of the ridge to the other in order to further examine the thermal state of the lithosphere average age phase velocity relations were established for each ocean and subtracted from phase velocity variation maps the results indicated broad low velocity regions in the south pacific super swell region the south and west indian ocean and high velocity regions east of the east pacific rise and in the north to northeast indian ocean the results reflect the asymmetry of phase velocity variation about ridges there is some correlation between hot spot locations and low velocity anomalies but additional large scale thermal anomalies exist under old oceanic plate

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

sound velocities in olivine at earth mantle pressures the independent elastic constants of an upper mantle mineral san carlos olivine mg1 8fe0 2 sio4 were measured from 0 to 12 5 gigapascals evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400 kilometer discontinuity and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers in the absence of high temperature data at high pressures estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers in contrast with low pressure laboratory data the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle

structural insights from geodetic global positioning system measurements in the darjiling sikkim himalaya abstract deformation in active convergent wedges like the himalaya occurs over different time and space scales requiring integrated datasets for holistic insights into wedge scale deformation a new long time series 1998 2009 of geodetic static global positioning system gps dataset suggests that active deformation in the darjiling sikkim himalaya dsh is not confined to the himalayan front as previously believed instead the 9 mm yr n s active convergence here is divided almost equally within the lesser himalayan duplex lhd and the higher himalaya due to aseismic reverse slip of 18 mm yr on the himalayan decollement main himalayan thrust mht dipping 6 006 and locked at 16 km depth slip transfer along the mht causes minor to moderate earthquakes at the brittle ductile transition in the dsh measured east velocities show no systematic variation indicating that strike slip seismicity though dominant in the region does not alleviate the decollement mht seismic hazard continued slip along the mht therefore will cause a great decollement earthquake in the region also gps velocities vary in the eastern and western dsh suggesting segmentation of the himalayan arc along a transverse fault over geological to decadal time scales our integrated approach can provide better insight into active deformation in convergent wedges worldwide

a large intraplate earthquake triggered by latent magmatism 1 a magnitude mj 7 3 intraplate earthquake occurred in the western tottori area southwest japan on 6 october 2000 where there was no apparent prefaulting subsurface indication of the source fault of the 2000 earthquake magnetotelluric soundings were taken in and around where the aftershocks occurred in order to image three dimensional electrical resistivity structure at depths of up to 40 km which could indicate an anomalously conductive body in the middle crust to the upper mantle on the southwestern side of the source fault free gas and dissolved gases collected from groundwater wells around the seismic source region are characterized by 3he 4he ratios several times higher than the atmospheric value the highest value of 5 1ra is similar to those of typical arc related volcanic gases although alkali basalts of early pleistocene age are sparsely distributed in the western tottori area the observed 3he 4he ratios are higher than the calculated 3he 4he ratio derived from the ancient magmatism considering postextrusive radiogenic ingrowth of 4he by decay of u and th included in the magma therefore it is concluded that the geophysical anomaly imaged to the southwest of the source fault is attributed to latent magmatism in the present day subduction system aqueous fluids separated from the cooling crustal magma could cause deep low frequency earthquakes around the moho discontinuity and migrate into the brittle upper crust in addition the presence of aqueous fluid is expected to weaken the crustal materials locally anelastic deformation implying notable compressive deformation in the e w direction was observed in the region where aftershocks were distributed under overpressure conditions the existing fault could serve as a pathway for aqueous fluids expelled from magma with high 3he 4he ratios so the upwelling of overpressurized fluids toward the earth s surface results in the emanation of groundwaters with high 3he 4he ratios along the trace of the source fault segments

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow