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

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

global anisotropic phase velocity maps for higher mode love and rayleigh waves summary it is well established that the earth s uppermost mantle is anisotropic but there are no clear observations of anisotropy in the deeper parts of the mantle surface waves are well suited to observe anisotropy since they carry information about both radial and azimuthal anisotropy fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometres and therefore do not provide information on anisotropy below higher mode surface waves have sensitivities that extend to and beyond the transition zone and should thus give insight about azimuthal anisotropy at greater depths we have measured higher mode love and rayleigh phase velocities using a model space search approach which provides us with consistent relative uncertainties from measurement to measurement and from mode to mode from these phase velocity measurements we constructed global anisotropic phase velocity maps prior to inversion we determine the optimum relative weighting for anisotropy we present global azimuthal phase velocity maps for higher mode rayleigh waves up to the sixth higher mode and love waves up to the fifth higher mode with corresponding average model uncertainties the anisotropy we derive is robust within the uncertainties for all modes given the ray theoretical sensitivity kernels of rayleigh and love wave modes the source of anisotropy is complex but mainly located in the asthenosphere and deeper our models show a good correspondence with other studies for the fundamental mode but we have been able to achieve higher resolution

improved methods for observing earth s time variable mass distribution with grace using spherical cap mascons we discuss several classes of improvements to gravity solutions from the gravity recovery and climate experiment grace mission these include both improvements in background geophysical models and orbital parameterization leading to the unconstrained spherical harmonic solution jpl rl05 and an alternate jpl rl05m mass concentration mascon solution benefitting from those same improvements but derived in surface spherical cap mascons the mascon basis functions allow for convenient application of a priori information derived from near global geophysical models to prevent striping in the solutions the resulting mass flux solutions are shown to suffer less from leakage errors than harmonic solutions and do not necessitate empirical filters to remove north south stripes lowering the dependence on using scale factors the global mean scale factor decreases by 0 17 to gain accurate mass estimates ocean bottom pressure obp time series derived from the mascon solutions are shown to have greater correlation with in situ data than do spherical harmonic solutions increase in correlation coefficient of 0 08 globally particularly in low latitude regions with small signal power increase in correlation coefficient of 0 35 regionally in addition to reducing the error rms with respect to the in situ data reduction of 0 37 cm globally and as much as 1 cm regionally greenland and antarctica mass balance estimates derived from the mascon solutions agree within formal uncertainties with previously published results computing basin averages for hydrology applications shows general agreement between harmonic and mascon solutions for large basins however mascon solutions typically have greater resolution for smaller spatial regions in particular when studying secular signals

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

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

on the scale of mantle heterogeneity abstract a data set comprising 3313 residuals measured from digital records of 429 earthquakes is analyzed for the heterogeneity of the earth s mantle the smoothed ss residuals reveal a large pattern of heterogeneity and agree well with the major tectonic features on the earth s surface and the residual patterns predicted by upper mantle and whole mantle models the spectrum of ss residuals indicates that the heterogeneity in the earth s mantle which probably reflects the patterns of mantle convection is dominated by long wavelength features our analysis of the power of ss residuals as a function of harmonic degree shows a distinct difference in the rate of decrease of the power below and above degree 8 the rate is first slower than l 1 and then it exceeds l 2 this is also true for ss residuals predicted for model sh 425 2 and love wave phase velocities at 100 s measured by other researchers this indicates that there is a preference for features with dimensions larger than 2500 3500 km we find that the power spectrum of the velocity anomalies associated with the subducted slabs is two orders of magnitude lower for 1 l 8 than that of velocity anomalies obtained by inversion of waveform data for the ocean basins correlation of the ss residuals with the square root of age of the oceanic lithosphere is highly significant however the age correlation predicts only 10 of the variance this means that the thermal signature of the spreading center represents only a small fraction of the thermal anomalies introduced into the upper mantle by convection

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

effect of sharp lateral heterogeneity on the earth s normal modes when inverting normal mode data for global large scale heterogeneity possible biases due to small scale structure are commonly ignored the authors conducted two experiments in the first one they calculated with a recently developed first order scattering formalism the effect of a simplified subduction zone model on the eigenfrequencies of the fundamental spheroidal mode branch the frequency shifts induced by this single subduction zone appear to be smaller by only a factor of 3 to 4 than shifts observed on long period seismograms or than shifts induced by existing models of global upper mantle inhomogeneity the second experiment involved a model of a hot spot plume on the core mantle boundary solving the variational problem with many coupling terms included the authors calculated the effect of such a plume on the amplitude of some low angular order modes the results suggest an effect at least as large as that due to available large scale mantle modes it thus appears that relatively small scale sharp lateral structure in different depth ranges of the earth may have an important effect on normal mode observations and that these effects may not be ignored in inversions of normal mode data

on ach or how reliable is regional teleseismic delay time tomography abstract ach named after aki et al 1976 bull seismol soc am 66 501 524 aki et al 1977 j geophys res 82 277 296 is a standard widely used method for three dimensional seismic imaging of the earth the fundamental hypothesis which underlies the method is that the time residuals generated outside the given target volume from the seismic source to the bottom of the modelled zone are approximately constant across the seismic array the main purpose of this study is to check this assumption we computed travel times for a given station and event distribution using a three dimensional global earth model taken from seismic tomography we found that the relative residuals generated outside the target volume are not negligible and that the fundamental hypothesis underlying ach is thus not verified these deviations are generated in the lower and or upper mantle and the corresponding proportions are entirely dependent on the raypaths the bias in the inverted model is statistically similar to the input model outside the target volume we thus recommend caution in any interpretations involving ach generated models a secondary somewhat independent outcome of this study is that fermat s principle used to linearise the inverse problem in ray theory based tomography seems to be valid without any restrictions given our input model is representative for the true earth for rays with turning points in the lower mantle for rays with turning points in the upper mantle the constant raypath approximation is probably not true this applies to global tomography as well

what patterns of heterogeneity in the earth s mantle can be revealed by seismic travel time tomography abstract we investigate which patterns of seismic heterogeneity in the mantle would be returned reliably by a tomographic inversion in which the model mantle is parametrized by a set of discrete non overlapping voxels we construct synthetic data sets based on real ray sampling of the mantle by introducing spherical harmonic patterns of velocity heterogeneity and perform inversions of the synthetic data we expand the resulting voxel model in spherical harmonics and compare the power at each degree and in each model layer with the input spherical harmonics in order to determine which patterns produced by inversions of real data may be deemed reliable and to identify patterns which must be viewed with skepticism we find that while the power input to a particular pattern of heterogeneity in the 0 200 km layer is generally recovered accurately the pattern itself is poorly determined in this layer a pattern in the 200 400 km layer is more precisely determined though the power contained in the pattern is consistently underestimated and more leakage occurs to the layers above and below the transition zone 400 670 km shows similarly strong control of lateral heterogeneity patterns but tests return a more accurate estimate of input powre than for the second layer the l 2 4 and 6 components all show accurate recovery in the 400 670 km layer this result supports previous findings from inversions with real data that l 2 is a significant pattern of heterogeneity in the mantle s transition zone and that l 4 is not a significant pattern for the entire upper mantle l 6 would be retrieved reliably and its constructive behavior in upper mantle models derived with real data is confirmed these tests also demonstrate the inability of our inversion procedure to retrieve shorter wavelength features in the lower mantle the results for our lowermost layer d must be considered suspect due to the inadequate constraints placed on model values by our ray coverage and the sensitivity of these results to noise in the data

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

automatic seismic event detection using migration and stacking a performance and parameter study in hengill southwest iceland we investigate the performance of a seismic event detection algorithm using migration and stacking of seismic traces the focus lies on determining optimal data dependent detection parameters for a

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

low wavenumber signatures of time dependent mantle convection abstract spectral analysis is applied to idealized two dimensional numerical models of mantle convection examining the spectral signature of known model temperature fields represents the forward problem corresponding to the inverse problem of inferring the unknown temperature field in the mantle from the spectral components of lateral heterogeneity obtained by seismic tomography previous two dimensional fourier analyses of steady convection cells indicate that with current levels of resolution tomographic techniques have a much better chance of detecting horizontal boundary layers than vertical plumes from an examination of the shape of the spectral envelopes of the power spectrum of lateral heterogeneity at various depths in steady convection cells it has been suggested that evidence for thermal boundary layers in the mantle may be contained in the published spectral amplitudes of lateral heterogeneity this possibility is expanded on in the present study here we examine the effects on the spectral signature of lateral heterogeneity of averaging the model temperature field over vertical intervals of a few hundred kilometers of large amounts of internal heating of time dependent flow and of aspect ratios greater than unity the vertical averaging is an attempt to mimic the finite vertical resolution of tomographic inversions the various complications considered here are found to render the distinction between the spectra of lateral heterogeneity inside and outside of thermal boundary layers less clear than in the simpler models considered previously nevertheless the boundary layer spectra continue to contain significant information on the dominant scale of the model flow solutions

history mass loss structure and dynamic behavior of the antarctic ice sheet antarctica contains most of earth s fresh water stored in two large ice sheets the more stable east antarctic ice sheet is larger and older rests on higher topography and hides entire mountain ranges and ancient lakes the less stable west antarctic ice sheet is smaller and younger and was formed on what was once a shallow sea recent observations made with several independent satellite measurements demonstrate that several regions of antarctica are losing mass flowing faster and retreating where ice is exposed to warm ocean waters the antarctic contribution to sea level rise has reached 8 millimeters since 1992 in the future if warming ocean waters and increased surface meltwater trigger faster ice flow sea level rise will accelerate

predominance of long wavelength heterogeneity in the mantle analysis of travel time anomalies of long period shear waves recorded by the global digital seismographic network indicates that mantle heterogeneities which probably reflect patterns of mantle convection occur most prominently on length scales greater than about 6 000 km when projected onto a two dimensional map at the earth s surface models of mantle convection will need to be able to produce a similar power spectrum

waveform inversion of mantle love waves the born seismogram approach summary normal mode theory extended to the slightly laterally heterogeneous earth by the first order born approximation is applied to the waveform inversion of mantle love wave 200 500 s for the earth s lateral heterogeneity at l 2 and a spherically symmetric anelasticity q structure the data are from the global digital seismograph network gdsn the l 2 pattern is very similar to the results of other studies that used either different methods such as phase velocity measurements and multiplet location measurements or a different data set such as mantle rayleigh waves from different instruments the results are carefully analysed for variance reduction and are most naturally explained by heterogeneity in the upper 420 km because of the poor resolution of the data set for the deep interior however a fairly large heterogeneity in the transition zones of the order of up to 3 5 per cent in shear wave velocity is allowed it is noteworthy that love waves of this period range cannot constrain the structure below 420 km and thus any model presented by similar studies below this depth are likely to be constrained by rayleigh waves spheroidal modes only the calculated modal q values for the obtained q model fall within the error bars of the observations the result demonstrates the discrepancy of rayleigh wave q and love wave q and indicates that care must be taken when both rayleigh and love wave data including amplitude information are inverted simultaneously anomalous amplitude inversions of g2 and g3 for example are observed for some source receiver pairs this is due to multipathing effects one example near the epicentral region which is modelled by the obtained l 2 heterogeneity is shown

3 d upper mantle shear velocity and attenuation from fundamental mode free oscillation data summary we present the results of global measurements of spheroidal and toroidal fundamental mode eigenfrequencies as well as fundamental spheroidal mode attenuation made from a data set consisting primarily of records from the geoscope network data processing includes variable filtering and account is taken of higher order effects along the dispersion branch local frequency and attenuation maps are first obtained as a function of angular order by a regionalization scheme based on the introduction of a correlation length in a second step after correction for shallow layer effects shear velocity maps at different depths in the mantle are obtained by simultaneous inversion of spheroidal and toroidal local frequency maps the results confirm the correlation of shear velocity with large scale tectonic features in the first 200 km of the mantle below that depth the maps are dominated by a degree 2 pattern whose power is peaked around a depth of 300 km shallower than inferred in the first 3 d global studies of the upper mantle and correlated with a degree 2 in attenuation thus confirming the results of our earlier study of degree 2 in the upper mantle we also report estimates of spherically averaged eigenfrequencies for fundamental spheroidal and toroidal mode branches

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

the effect of small scale structure on normal mode frequencies and global inversions presently very little is known about the characteristic length scales of the structures in the upper mantle in generating synthetic seismograms and in large scale inversions one usually assumes that the upper mantle is smooth so that ray theory can be used however it is known that structures in the mantle exist on a scale of a few hundred kilometers notably near subduction zones and other tectonic features in this paper a model of all the subduction zones and spreading ridges is used to investigate the effect of small scale structures on normal mode frequency shifts and on global inversions the normal mode frequency shifts for this model have the same characteristics as observed normal mode frequency shifts furthermore the global models obtained from inversions of synthetic data for the model of subduction zones and spreading ridges are similar in character to the models obtained from global inversions of real data for a realistic set of wave paths the model obtained from the synthetic inversions differs appreciably from a smoothed version of the true model this is due to an uneven path coverage and to a lesser extent due to neglecting scattering effects this means that the models obtained from global inversions of seismological data may contain appreciable artifacts

snowfall driven mass change on the east antarctic ice sheet 1 an improved understanding of processes dominating the sensitive balance between mass loss primarily due to glacial discharge and mass gain through precipitation is essential for determining the future behavior of the antarctic ice sheet and its contribution to sea level rise while satellite observations of antarctica indicate that west antarctica experiences dramatic mass loss along the antarctic peninsula and pine island glacier east antarctica has remained comparably stable in this study we describe the causes and magnitude of recent extreme precipitation events along the east antarctic coast that led to significant regional mass accumulations that partially compensate for some of the recent global ice mass losses that contribute to global sea level rise the gain of almost 350 gt from 2009 to 2011 is equivalent to a decrease in global mean sea level at a rate of 0 32 mm yr over this three year period

the upper mantle degree 2 constraints and inferences from global mantle wave attenuation measurements we present the results of an analysis of global lateral variations in anelasticity of the upper mantle as measured from very long period rayleigh waves observed on the geoscope network four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account in a linear manner focussing effects due to lateral variations in elastic parameters local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length as an alternative to spherical harmonics expansion comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high q and high velocity regions respectively low q and low velocity both for great circle and minor arc data at periods around 200 s the detailed analysis of the degree 2 pattern well resolved in the attenuation maps around this period shows that it originates in the depth range 250 500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin possibly related to the topmost part of the large scale convective system corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid this study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods before performing any comparison with short period data or other geophysical parameters

inversion for laterally heterogeneous upper mantle s wave velocity structure using iterative waveform inversion summary in contrast to previous work which treats the earth s lateral heterogeneity as an infinitesimal perturbation to a spherically symmetrical starting model we conduct iterative linearized waveform inversion for the earth s laterally heterogeneous structure we use the direct solution method dsm geller et al 1990a to calculate synthetic seismograms and their partial derivatives for a laterally heterogeneous earth model we invert surface wave data from the ida and geoscope networks we expand the lateral heterogeneity of rigidity in spherical harmonics up to angular order number 8 and use three parameters to specify the depth dependence of each harmonic giving us a total of 240 unknowns the shortwavelength lateral heterogeneity s 4 6 and 8 in the deeper part of the upper mantle obtained by this study differs significantly from m84a the relative improvement in the variance reduction as compared with model m84a is about 20 per cent for the ida data and more than 100 per cent for the geoscope data

global earth structure inference and assessment summary a suite of compressional and shear phase arrival times p s pp pcp ss scs pkpab pkpbc pkpdf sksac and two sets of differential times ss s410s ss s660s are used to infer lateral variations in p and s velocity structure in the entire earth crust mantle and core volumetric heterogeneity is parametrized by 22 depth layers each of which is subdivided into a grid of cells in addition to volumetric hetero geneity we determine topography on the major internal velocity discontinuities station corrections event relocation parameters and inner core anisotropy a total of 96 300 unknowns the model parameters are estimated using an iterative lanczos algorithm to obtain a partial singular value decomposition a total of 5000 lanczos values and vectors are used to construct model parameter estimates as well as measures of model parameter resolution and covariance the lanczos resolution estimates provide lower bounds on conventional svd based measures throughout most of the mantle we find that regions beneath the continents particularly in the northern hemisphere are well resolved with the exception of africa velocity variations are moderately well resolved in that portion of the outer core lying beneath the continents velocity heterogeneity is poorly resolved in the inner core where cell dimensions are small and deviations in velocity heterogeneity trade off with variations in anisotropy variations in the uppermost mantle 2 3 per cent are correlated with surface tectonics while at greater depths high velocity variations coincide with regions of past and present subduction the high velocity anomalies extend into the lower mantle although with a reduced amplitude 0 5 1 0 per cent until the lowest mantle 2 5 per cent at the base of the outer core a large scale pattern of heterogeneity 0 8 per cent is observed slower at the poles in the inner core we infer a peak anisotropy of 2 3 per cent which varies laterally and with depth however the magnitude and distribution of the anisotropy trades off strongly with velocity heterogeneity

on the presence of intermediate scale heterogeneity in the upper mantle summary power spectra of tomographic models of the entire earth and of models of europe and the mediterranean area are compared this comparison reveals a discrepancy a factor 15 30 between the power of both kinds of models at intermediate spherical harmonic degrees around l 30 two possible explanations for this discrepancy are given first the fact that europe is more complex than the global average and second the lack of resolution in the inversions another approach to the problem of the power spectrum of heterogeneities of intermediate length scale is the study of multipathing of long period surface waves the multipathing observed in the recordings of these waves which manifests itself as an increasing complexity of their amplitude spectra due to increasing interference of spectral components with increasing epicentral distance cannot be explained by present day global earth models the gradients in the models are simply not strong enough however random models with more power at intermediate scales and consequently stronger gradients are able to explain the observed multipathing it is concluded that the global models underestimate the power in the heterogeneities of intermediate scale structure in the earth finally an explanation is given why current global models although they do not contain sufficient power at shorter scales do explain the traveltime and dispersion data used for the construction of the models

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs

comparison between tomographic structures and models of convection in the upper mantle summary gruneisen s and third order finite strain theories are used to compute the density and seismic wave velocities of minerals assuming the minealogical model of ito takahashi 1987 seismic velocities of the upper mantle are calculated using the hashin shtrikman averaging procedure 1 d profiles are first obtained along adiabats and compared to the iasp91 model different adiabats are considered in order to take into account the thermal effect of phase transitions the best results are found with adiabats initiated at 1473 1573 and 1613 k for olivine and spinel respectively the incorporation of thermal effects resulting from phase transitions gives velocity jumps at discontinuities close to those of iasp91 next a model of convection constructed by dupeyrat sotin parmentier 1995 incorporating plate tectonics is used to compute 1 d profiles and 2 d fields of seismic anomalies in the upper mantle averaged profiles show seismic velocity gradients very close to those of iasp91 but individual values are much too high suggesting that the mean temperature profile of the convection model is too cold by 400 k when low pass filtered to the resolution scale of presently available tomogrphic models both the amplitude and shape of the computed seismic anomalies are consistent with the results of tomographic studies the amplitude of the anomalies ranges between 2 7 and 3 8 for p wave slownesses and from 3 3 to 4 5 for s wave slownesses these anomalies correspond to lateral temperature variations of 465 to 520 k these calculations are used 1 as an aid to the interpretation of global tomographic models for instance by computing spectra of lateral heterogeneities and 2 to test the adequacy of the basic assumptions used in the computation of numerical models of mantle convection and to build a theoretical temperature profile that would give the best fit to iasp91 in the uppermost mantle this theoretical model has a shape close to both the convection model and the 1473 k adiabat but in the transition zone the profile is highly subadiabatic the spectra obtained for the synthetic seismic anomalies resemble that of tomographic studies with most of the energy contained within gravest angular orders l and a fast decrease of energy with increasing l the spatial filtering has clearly different effects on heterogeneities depending on their respective wavelengths it is suggested that the change of decreasing rate observed in tomographic models at l 7 is closely related to the filter wavelength and may correspond at a lesser extent to a characteristic wavelength to mantle heterogeneities

the three dimensional shear wave structure in the mantle by overtone waveform inversion i radial seismogram inversion summary the three dimensional 3 d shear wave structure of the mantle down to the depth of about 900 km is obtained by inverting waveforms of radial component seismograms radial component seismograms contain large amplitude overtone signals which circle the earth as wave packets and are sometimes called x1 x2 x3 we use data which contain r1 x1 and x2 and filtered between 2 and 10mhz it is shown that unless each seismogram is weighted all seismograms are not fitted uniformly only data from large earthquakes are fitted and the final velocity anomalies are biased by the small number of large earthquake data resolution is good at shallow depths becomes worse in the intermediate depth range between about 400 and 500 km and then becomes better at greater depth ranges 600 900km even though we use only spheroidal mode data velocity anomalies in the shallow structure show excellent correlation with the age of the surface rocks of the earth in the deeper regions between about 600 and 900km south america shows a fast velocity anomaly which may indicate the slab penetration beyond 700 km there another region which shows a fast velocity anomaly is the mariana trench but other subduction regions do not show such features

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained