imaging the transition between the region of mantle melt generation and the crustal magma chamber beneath the southern east pacific rise with short period love waves 1 beneath the southern east pacific rise resides a crustal magma chamber confined to the immediate vicinity of the rise axis yet the mantle melt production region is broad and asymmetric short period 4 17 s love waves propagating along the rise provide a means of probing the transition from the asymmetric melt production region to the rise centered crustal magma chamber low velocities beneath the rise act as a waveguide for love wave energy we model this complex wave propagation with a gaussian beam representation as part of our nonlinear inversion for velocity structure our results show that the asymmetry exists at the crust mantle interface or moho and increases downward the lowest velocities are located beneath the rise at the moho beneath the rise there is an asymmetric gap in the high velocity lid at the top of the mantle and there are lower asthenospheric velocities beneath the pacific plate than the nazca plate the transition from the broad upwelling region to the narrow crustal magmatic system occurs over a 15 km depth interval at the top of the mantle our observations indicate higher temperatures and greater melt production beneath the pacific plate east of the rise melting appears to abruptly shut off at shallow levels perhaps due to a component of downward mantle flow we suggest that melt percolates upward increasing in concentration until reaching a permeability barrier beneath the lithosphere it then moves up along the lithosphere to accumulate at the moho beneath the rise west of the rise higher temperatures and upward percolation of melt delay growth of the lithosphere

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

electromagnetic induction fields in the deep ocean north east of hawaii implications for mantle conductivity and source fields summary we have analysed a thirty six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north east of hawaii the electromagnetic fields are dominated by tides which have an appreciable oceanic component especially in the east electric and north magnetic components the techniques of data analysis included singular value decomposition svd to remove uncorrelated noise there are three degrees of freedom in the data set for periods longer than five hours indicating a correlation of the vertical magnetic field and the horizontal components suggesting source field inhomogeneity tensor response functions were calculated using spectral band averaging with both svd and least squares techniques and rotated to the principal direction one diagonal component determined mainly by the north electric and east magnetic fields is not interpretable as a one dimensional induction phenomenon the other diagonal term of the response function indicates a rapid rise in conductivity to 0 05 mho m 1 near 160 km no decrease in conductivity below this depth is resolvable polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system

on crustal corrections in surface wave tomography summary mantle models from surface waves rely on good crustal corrections we investigated how far ray theoretical and finite frequency approximations can predict crustal corrections for fundamental mode surface waves using a spectral element method we calculated synthetic seismograms in transversely isotropic prem and in the 3 d crustal model crust2 0 on top of prem and measured the corresponding time shifts as a function of period we then applied phase corrections to the prem seismograms using ray theory and finite frequency theory with exact local phase velocity perturbations from crust2 0 and looked at the residual time shifts after crustal corrections residuals fall within the uncertainty of measured phase velocities for periods longer than 60 and 80 s for rayleigh and love waves respectively rayleigh and love waves are affected in a highly non linear way by the crustal type oceanic crust affects love waves stronger while rayleigh waves change most in continental crust as a consequence we find that the imperfect crustal corrections could have a large impact on our inferences of radial anisotropy if we want to map anisotropy correctly we should invert simultaneously for mantle and crust the latter can only be achieved by using perturbation theory from a good 3 d starting model or implementing full non linearity from a 1 d starting model

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

upper mantle seismic shear discontinuities of the pacific 1 shear wave impedance discontinuities are inventoried for eight paths connecting circum pacific earthquakes with seismic stations on hawaii and oahu in addition to the transition zone discontinuities we observe a consistent impedance decrease at a depth of 80 km that marks the transition from the fast seismic lid to the low velocity zone the interval over which this transition occurs is less than 30 km the requisite impedance decrease ascribed almost entirely to diminished velocities in the low velocity zone exceeds predictions for peridotite chemistries of appropriate lithospheric age in all cases the transition is better matched by lithosphere of roughly half of the true age four paths show clear evidence of the x discontinuity an impedance increase of 3 8 near 300 km depth the only viable explanations would require extensive eclogite or basalt depleted mantle in the upper mantle of the western pacific the impedance contrast of the 410 km discontinuity which depends on the modal fraction of mantle olivine does not signal the presence of either although a mixture of the two remains a possibility we find no evidence of a low velocity layer atop of the 410 km discontinuity if present it is either consistently thin 15 km highly variable in thickness topography in excess of 60 km or has little impedance contrast 2 3 with overlying mantle the apparent absence of melt atop the 410 km discontinuity an intermittent and weak 520 km discontinuity and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central pacific

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

imaging pacific lithosphere seismic discontinuities insights from ss precursor modeling oceanic lithosphere provides an ideal location to decipher the nature of the lithosphere asthenosphere system which is vital to our understanding of plate tectonics it is well established that oceanic lithosphere cools thickens and subsides as it ages according to the conductive cooling models yet this simple realization fails to explain various observations for example old oceanic lithosphere does not subside as predicted further precise imaging of the lower boundary of the oceanic lithosphere has proven challenging here we use ss precursors to image the discontinuity structure across the pacific ocean using 24 years of teleseismic data we image a sharp pervasive velocity discontinuity 3 15 drop over 36 my there is no age depth dependence and we image the discontinuity at an average depth of 60 1 5 km the amplitude and sharpness of the boundary suggests a compositional variation and or layered carbonatitic melt may be required to explain our observations rather than temperature alone the strength and pervasiveness of the boundary suggests that it is likely related to the lithosphere asthenosphere boundary an additional deeper discontinuity at 80 120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth

stratification of anisotropy in the pacific upper mantle 1 on the basis of the use of broadband 25 150 s rayleigh wave group speeds to estimate the 2 component of azimuthal anisotropy we present evidence for a stratification of anisotropy in the uppermost mantle at large scales across the pacific basin we confirm previous surface wave studies that established that the fast axis directions of azimuthal anisotropy for intermediate and long period rayleigh waves approximately align with present day plate motions at shorter periods 25 50 s however fast axes align nearer to the paleospreading or fossil spreading direction which differs from present day plate motion in the old pacific these observations as well as observations of the age dependence of the amplitude of azimuthal anisotropy imply that azimuthal anisotropy in the pacific upper lithosphere 100 km depth is fixed or fossilized on average reflecting the strain conditions extant during the early evolution of the lithosphere rather than the current ambient flow direction in the deeper lithosphere and asthenosphere anisotropic fast axis directions align nearer to present day plate motions apparently having reoriented to conform to the current conditions of mantle flow the mechanism of anisotropy stratification remains unclear but observations are consistent with the anisotropy of the shallow lithosphere being fixed because the shear flows that can produce dislocation creep and a change in anisotropy will occur at increasing depths as the plate ages

a bayesian approach to assess the importance of crustal corrections in global anisotropic surface wave tomography several recent studies have demonstrated the importance of crustal corrections when inverting surface wave data to model lateral variations in mantle radial anisotropy it has also been shown that the choice of the prior crustal model to correct the data can strongly influence the anisotropy model and potentially lead to different geodynamic interpretations in comparing tomographic models of radial anisotropy obtained from different crustal corrections these studies did not however determine quantitative model uncertainties nevertheless mantle models resulting from different prior crustal corrections are statistically different only if the posterior model errors stemming from the non uniqueness of the inverse problem are smaller than the effect of the crustal correction itself here we applied a model space search approach to global fundamental and higher mode rayleigh and love wave phase velocity maps to determine reliable quantitative model uncertainties on seismic velocities and radial anisotropy the technique employed enabled us to describe the model space with a posterior probability density function and therefore to test whether models obtained from different crustal corrections are statistically different we thus assessed the significance of the choice of the crustal model by comparing the posterior model errors to the differences in mantle structure resulting from different crustal corrections we tested prior crustal models crust2 0 crust1 0 and 3smac our study shows that the use of prior crustal corrections from different crustal models yields significant discrepancies in mantle velocities around 50 km depth and in radial anisotropy down to 100 km the impact of the crustal correction on radial anisotropy can extend down to 250 km in some locations we found that choosing 3smac instead of the other crustal models has a stronger influence on the mantle model but that crust1 0 and crust2 0 yield statistically identical anisotropy models at all depths except at a few grid cells importantly the effect of the crustal model is most significant in continental regions and not so much beneath oceans which has important consequences for determining the depth of continental roots our results therefore suggest that improving constraints on crustal structure in continents is essential for our understanding of continent formation our work also demonstrates that the prior crustal model does not significantly affect radial anisotropy and velocities at depths greater than 100 km this implies that if geodynamic interpretations of radial anisotropy below 100 km depth were to account for tomographic model uncertainties they would not depend on the choice of the prior crustal model it is therefore important for geodynamicists and seismologists to work in concert and to put effort into determining quantitative tomographic model uncertainties before interpreting the results our results also caution against the use of 3smac to correct surface wave data for studies of the continental lithosphere and suggest that the solid earth community would benefit from putting some efforts toward building a revised 3smac the discrepancies between mantle models built based on 3smac crustal corrections and those based on crust1 0 or crust2 0 should also help shed light on the validity of the geodynamical assumptions made in the construction of models like 3smac

on the robustness of global radially anisotropic surface wave tomography a number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy as yet there is limited agreement among such studies suggesting significant uncertainties in the models which could lead to divergent geodynamical interpretations in this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10 000 000 fundamental and higher mode surface wave dispersion measurements we carry out inversions for isotropic and radially anisotropic shear wave velocity systematically changing regularization and using three different crustal models to remove the effects of the crust on the data using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling moreover the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations this work suggests that the three dimensional determination of global radial anisotropy in the earth s mantle using surface wave dispersion data is still an ongoing experiment

petrological constraints on seismic anisotropy the purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle the variation with depth and with degree of random orientation is also investigated it is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium azumithally averaged velocities with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated comparison with available seismic data does not yet allow discrimination based on anisotropy between different petrological models it is shown that even if a proportion of random orientation is introduced it should be possible to achieve petrological discrimination from seismic data for some orientation configurations if the inverse problem is well posed i e takes account of the correlations between anisotropic parameters these correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the prem by alternative anisotropic models for the uppermost 220 km

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

scale free distribution of dead sea sinkholes observations and modeling there are currently more than 5500 sinkholes along the dead sea in israel these were formed due to the dissolution of subsurface salt layers as a result of the replacement of hypersaline groundwater by fresh brackish groundwater this process has been associated with a sharp decline in the dead sea water level currently more than one meter per year resulting in a lower water table that has allowed the intrusion of fresher brackish water we studied the distribution of the sinkhole sizes and found that it is scale free with a power law exponent close to 2 we constructed a stochastic cellular automata model to understand the observed scale free behavior and the growth of the sinkhole area in time the model consists of a lower salt layer and an upper soil layer in which cavities that develop in the lower layer lead to collapses in the upper layer the model reproduces the observed power law distribution without involving the threshold behavior commonly associated with criticality

oceanic lithosphere asthenosphere boundary from surface wave dispersion data abstract according to different types of observations the nature of lithosphere asthenosphere boundary lab is controversial using a massive data set of surface wave dispersions in a broad period range 15 300 s we have developed a three dimensional upper mantle tomographic model first order perturbation theory at the global scale this is used to derive maps of the lab from the resolved elastic parameters the key effects of shallow layers and anisotropy are taken into account in the inversion process we investigate lab distribution primarily below the oceans according to different kinds of proxies that correspond to the base of the lithosphere from the shear velocity variation at depth the amplitude radial anisotropy and the changes in azimuthal anisotropy g orientation the estimations of the lab depth based on the shear velocity increase from a thin lithosphere 20 km in the ridges to a thick old ocean lithosphere 120 130 km the radial anisotropy proxy shows a very fast increase in the lab depth from the ridges from 50 km to the older ocean where it reaches a remarkable monotonic subhorizontal profile 70 80 km the lab depths inferred from the azimuthal anisotropy proxy show deeper values for the increasing oceanic lithosphere 130 135 km the difference between the evolution of the lab depth with the age of the oceanic lithosphere computed from the shear velocity and azimuthal anisotropy proxies and from the radial anisotropy proxy raises questions about the nature of the lab in the oceanic regions and of the formation of the oceanic plates

evolution of the lithosphere in the indian ocean from combined earthquake and ambient noise tomography rayleigh wave dispersion extracted from ambient seismic noise has been widely used to image crustal and uppermost mantle structure applications of this approach in continental settings are abundant but there have been relatively few studies within ocean basins by cross correlating ambient noise recorded at broadband seismic stations around the indian ocean we demonstrate the feasibility of extracting high quality long period 10 30 mhz rayleigh waves that traverse the entire ocean basin high quality rayleigh wave cross correlation functions can be obtained from stacking waveforms over less than two years at land stations and less than four years at island stations we show that adding the dispersion information extracted from ambient noise to a global earthquake data set can improve the resolution of phase velocity maps by about 20 in the northern indian ocean where the station distribution is the best we find that a plate cooling model with a potential temperature of 1450 c and plate thickness of 125 km can fit both the seismic observations and seafloor topography the seychelles mascarene plateau is characterized by anomalously slow velocity at 30 mhz the inclusion of ambient noise data in the tomographic inversion shifts the slow velocity anomaly into better agreement with the topographic relief allowing us to estimate its crustal thickness and confirm that the plateau s elevation is supported by thick crust the 10 and 20 mhz phase velocity maps show a strong asymmetry across the central indian ridge that is best explained by eastward asthenospheric flow emanating from nearby hotspots

seismic observations of large scale deformation at the bottom of fast moving plates abstract we present a new tomographic model of azimuthal anisotropy in the upper mantle dr2012 and discuss in details the geodynamical causes of this anisotropy our model improves upon dkp2005 seismic model debayle et al 2005 through a larger dataset expanded by a factor 3 7 and a new approach which allows us to better extract fundamental and higher mode information our results confirm that on average azimuthal anisotropy is only significant in the uppermost 200 250 km of the upper mantle where it decreases regularly with depth we do not see a significant difference in the amplitude of anisotropy beneath fast oceanic plates slow oceanic plates or continents the anisotropy projected onto the direction of present plate motion shows a very specific relation with the plate velocity it peaks in the asthenosphere around 150 km depth it is very weak for plate velocities smaller than 3 cm yr 1 increases significantly between 3 and 5 cm yr 1 and saturates for plate velocities larger than 5 cm yr 1 plate scale present day deformation is remarkably well and uniformly recorded beneath the fastest moving plates india coco nazca australia philippine sea and pacific plates beneath slower plates plate motion parallel anisotropy is only observed locally which suggests that the mantle flow below these plates is not controlled by the lithospheric motion a minimum plate velocity of around 4 cm yr 1 is necessary for a plate to organize the flow in its underlying asthenosphere the correlation of oceanic anisotropy with the actual plate motion in the shallow lithosphere is very weak a better correlation is obtained with the fossil accretion velocity recorded by the gradient of local seafloor age the transition between frozen in and active anisotropy occurs across the typical age isotherm that defines the bottom of the thermal lithosphere around 1100 c under fast continents mostly under australia and india the present day velocity orients also the anisotropy in a depth range around 150 200 km depth which is not deeper than what is observed under oceans

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

inferring upper mantle structure by full waveform tomography with the spectral element method summary mapping the elastic and anelastic structure of the earth s mantle is crucial for understanding the temperature composition and dynamics of our planet in the past quarter century global tomography based on ray theory and first order perturbation methods has imaged long wavelength elastic velocity heterogeneities of the earth s mantle however the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with crustal structure as well as short wavelength or large amplitude mantle heterogeneity the spectral element method on the other hand permits accurate calculation of wave propagation through highly heterogeneous structures and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the earth such as the upper mantle sem importantly sem allows a dramatic improvement in accounting for the effects of crustal structure here we develop and apply a new hybrid method of tomography which allows us to leverage the accuracy of sem to model fundamental and higher mode long period 60 s waveforms we then present the first global model of upper mantle velocity and radial anisotropy developed using sem our model semum confirms that the long wavelength mantle structure imaged using approximate semi analytic techniques is robust and representative of the earth s true structure furthermore it reveals structures in the upper mantle that were not clearly seen in previous global tomographic models we show that semum favourably compares to and rivals the resolving power of continental scale studies this new hybrid approach to tomography can be applied to a larger and higher frequency data set in order to gain new insights into the structure of the lower mantle and more robustly map seismic structure at the regional and smaller scales

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

resolving the lithosphere asthenosphere boundary with seismic rayleigh waves summary seismic surface wave measurements offer tight constraints on shear wave speed values within the lithosphere and asthenosphere it is still a matter of debate however how accurately and under what conditions surface waves can resolve the depth and thickness of the lithosphere asthenosphere boundary lab we investigate the sensitivity of rayleigh waves to lab properties and find that if the lab is associated with a 2 3 per cent shear speed reduction at a depth less than 100 km then 10 20 km variations in its depth translate into phase velocity changes of up to 1 per cent variations in the thickness of the lab from zero to a few tens of kilometres cause much smaller phase velocity changes 0 1 per cent although the signal of the lab thickness variations is enhanced by a distinct frequency dependent pattern of phase velocity increases and decreases for lab depths increasing beyond 100 km larger absolute variations in the lab depth and thickness are needed to generate response of the same amplitude in phase velocities we introduce a grid search inversion of phase velocity curves for the lab depth and thickness defining them as the middle and the width respectively of a depth interval with a linear shear speed decrease in it the inversion comprises dense sampling of the lab depthlab thickness parameter plane and non linear gradient search inversions at every point it accountsforanytrade offsofthelabparameterswithshearspeedvariationsaboveandbelow the lab inversions of phase velocity curves for the lab depth and thickness show that the two parameters have uncorrelated uncertainties with the lab depth constrained better than the lab thickness random errors in phase velocity measurements have a limited effect on lab depth measurements causing errors of only a few kilometres measurements of the lab thickness are less robust and are not possible in the presence of large 1 per cent random errors substantial systematic biases in the measurements errors that persist over broad period ranges of tens of seconds can affect both lab depth and lab thickness estimates phasevelocity curves with such biases are not suitable for inversions for lab properties resolving the lab however in particular the lab depth should already be possible with some of the accurate broad band phase velocity measurements available today applying our method to phase velocity data measured in phanerozoic west central germany we find best fitting lab depths in the 60 80 km range the lab depth always understood to be the middle of the shear speed reduction depth range with minimum misfits achieved near the 70 km lab depth beneath the archean guyana shield we determine a 140 175 km lab depth range with a 160 km best fitting mid lab depth

three dimensional variations in love and rayleigh wave azimuthal anisotropy for the upper 800 km of the mantle we present a new mantle model yb14shani of azimuthal anisotropy for horizontally polarized shear waves sh in parallel with our previously published vertically polarized shear wave sv anisotropy model yb13svani yb14shani was obtained from higher mode love wave phase velocity maps with sensitivity to anisotropy down to 1200 km depth sh anisotropy is present down to the mantle transition zone mtz with an average amplitude of 2 in the upper 250 km and 1 in the mtz consistent with yb13svani changes in sv and sh anisotropy were found at the top of the mtz where olivine transforms into wadsleyite which might indicate that mtz anisotropy is due to the lattice preferred orientation of anisotropic material beneath oceanic plates sv fast axes become subparallel to the absolute plate motion apm at a depth that marks the location of a thermally controlled lithosphere asthenosphere boundary lab in contrast sh anisotropy does not systematically depend on ocean age moreover while upper mantle sv anisotropy is anomalously high in the middle of the pacific as seen in radial anisotropy models sh anisotropy amplitude remains close to the average for other oceans based on the depth at which sv fast axes and the apm direction begin to align we also found that the average thickness of cratonic roots is 250 km consistent with yuan and romanowicz 2010 for north america here we add new constraints on the nature of the cratonic lab and show that it is characterized by changes in both sv and sh anisotropy 2014 american geophysical union all rights reserved

probability of radial anisotropy in the deep mantle article i nfo it is well established that the earth s uppermost mantle is anisotropic but observations of anisotropy in the deeper mantle have been more ambiguous radial anisotropy the discrepancy between love and rayleigh waves was included in the top 220 km of prem but there is no consensus whether anisotropy is present below that depth fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometers and therefore do not provide information on anisotropy below higher mode surface waves however have sensitivities that extend to and below the transition zone and should thus give insight about anisotropy at greater depths but they are very difficult to measure we previously developed a new technique to measure higher mode surface wave phase velocities with consistent uncertainties these data are used here to construct probability density functions of a radially anisotropic earth model down to approximately 1500 km in the uppermost mantle we obtain a high probability of faster horizontally polarized shear wave speed likely to be related to plate motion in the asthenosphere and transition zone however we find a high probability of faster vertically polarized shear wave speed to a depth of 1500 km in the lower mantle we see no significant shear wave anisotropy this is consistent with results from laboratory measurements which show that lower mantle minerals are anisotropic but lpo is unlikely to develop in the pressure temperature conditions present in the mid mantle 2008 elsevier b v all rights reserved

non linear crustal corrections in high resolution regional waveform seismic tomography summary we compare 3 d upper mantle anisotropic structures beneath the north american continent obtained using standard and improved crustal corrections in the framework of non linear asymptotic coupling theory nact applied to long period three component fundamental and higher mode surface waveform data our improved approach to correct for crustal structure in high resolution regional waveform tomographic models goes beyond the linear perturbation approximation and is therefore more accurate in accounting for large variations in moho topography within short distances as observed for instance at ocean continent margins this improved methodology decomposes the shallow layer correction into a linear and non linear part and makes use of 1 d sensitivity kernels defined according to local tectonic structure both for the forward computation and for the computation of sensitivity kernels for inversion the comparison of the 3 d upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected however significant variations are observed in the retrieved 3 d perturbations the largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle as would be expected we suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity

geophysical constraints on partial melt in the upper mantle this paper addresses the conditions under which partial melt can exist in the mantle in order to be observed as a geophysical anomaly typical observed anomalies are high electrical conductivity of the order of 0 1 s m or greater velocity decreases of 7 10 seismic q values less than 100 and a frequency band for seismic effects in the region near 1 hz existing theories of electrical conduction in partial melts and of frequency dependent seismic properties together with recent measurements of melt electrical conductivity viscosity and partial melt texture can be used to establish requirements for melt to be observed by geophysical methods from electrical anomalies mainly sensitive to melt volume and its interconnection one can require a minimum melt fraction of several percent at temperatures close to the solidus 1150 1300 c however seismic models demand only a small volume in very flattened shapes aspect ratio 0 001 melt fraction 0 1 further if melt configuration permits seismic dissipation in bulk that is there exist flattened voids intersecting more or less equant voids then it is possible to infer melt fractions for elastic anomalies that are consistent with the several percent required for electrical anomalies observed equilibrium textures of partly melted peridotite together with inferred melt solid surface energies suggest that melt on a grain size scale in a gravitational field segregates into a strongly anisotropic pattern thus if partial melt causes mantle geophysical anomalies it should exist in a variety of void shapes and probably of sizes while the association of electrical and elastic anomalies with indications of reduced density volcanism and high heat flow makes the hypothesis of partial melting an attractive explanation the minimum physical requirement is for existence of relatively high temperature

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

radial anisotropy and prior petrological constraints a comparative study click here journal of geophysical research vol 115 b03303 doi 10 1029 2008jb005842 2010 for full article radial anisotropy and prior petrological constraints a comparative study c beghein 1 received 31 may 2008 revised 8 september 2009 accepted 6 october 2009 published 6 march 2010 1 radial seismic anisotropy models are traditionally obtained using empirical constraints based on laboratory experiments and petrological considerations we tested the hypothesis that such petrological constraints affect the uppermost mantle models of s wave anisotropy using a statistical approach in addition we were able to determine which model features are constrained by the data and which are dominated by the prior we focused on large scale models and found that the most likely models obtained in both cases are highly correlated this demonstrates that for the best data fitting solution the geometry of uppermost mantle radial anisotropy is not strongly affected by prior petrological constraints the amplitude of the anomalies however can change significantly the best data fitting model obtained without petrological constraints displays stronger amplitudes than the one obtained with prior this could become an issue when quantitatively interpreting seismic anisotropy models and thus emphasizes the importance of accurately accounting for parameter uncertainties and trade offs and of understanding whether the seismic data or the prior constraints the model we showed that model uncertainties are strongly affected by the prior as the relative rms uncertainties were reduced by a factor of 2 in addition we showed that while the model distributions are not necessarily gaussian a priori imposing petrological constraints can force the distributions to be narrower and more gaussian like as expected from inverse theory finally we demonstrated that the age dependence of seismic wave velocities is robust and independent of prior constraints a similar age signal exists for anisotropy but with larger uncertainties without prior constraints citation beghein c 2010 radial anisotropy and prior petrological constraints a comparative study j geophys res 115 b03303 doi 10 1029 2008jb005842 1 introduction 2 accurately modeling mantle seismic anisotropy that is the dependence of seismic wave velocity with the direction of propagation or polarization can help us under stand mantle deformation karato and toriumi 1989 kendall 2000 becker et al 2003 the coupling between lithosphere and asthenosphere silver and holt 2002 becker et al 2006b mantle composition montagner and anderson 1989 rheology becker et al 2008 and the net rotation of the lithosphere becker 2008 however despite numerous efforts to model mantle seismic anisotropy over the past 20 years uncertainties remain on its exact depth extent and lateral variations in the uppermost mantle on its presence in the transition zone and on its global nature in the d 00 layer fouch and fischer 1996 montagner and kennett 1996 ekstro m and dziewonski 1998 lay et al 1998 trampert and van heijst 2002 wookey et al 2002 gung et al 2003 panning and romanowicz 2006 earth and space sciences department ucla los angeles california usa copyright 2010 by the american geophysical union 0148 0227 10 2008jb005842 09 00 beghein and trampert 2004a 2004b beghein et al 2006 panning and romanowicz 2006 zhou et al 2006 marone et al 2007 nettles and dziewonski 2008 beghein et al 2008 3 discrepancies between models can arise for a variety of reasons to fully describe earth s elastic properties one would ideally want to determine the 21 independent ele ments of the fourth order elastic stiffness tensor at a given time and location inside earth in practice this is challeng ing because seismic data are only sensitive to subsets of those 21 elements tanimoto 1986 chen and tromp 2007 beghein et al 2008 and different types of data depend on different subsets of elastic coefficients see summary tables in chen and tromp 2007 and beghein et al 2008 in addition while some data such as shear wave splitting measurements can provide precise constraints on lateral changes in seismic anisotropy their depth resolution is very poor surface wave and free oscillation data are better suited to constrain depth changes in structure but their lateral resolution is lower than that of body waves this can yield apparent discrepancies and make model comparisons diffi cult moreover three dimensional models of seismic anisot ropy are typically obtained by data inversion which is often an ill posed and ill conditioned problem this means that b03303 1 of 17

savani a variable resolution whole mantle model of anisotropic shear velocity variations based on multiple data sets we present a tomographic model of radially anisotropic shear velocity variations in the earth s mantle based on a new compilation of previously published data sets and a variable block parameterization adapted to local raypath density we employ ray theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations our database includes surface wave phase delays from fundamental modes up to the sixth overtone measured at periods between 25 and 350 s as well as cross correlation traveltimes of major body wave phases before inversion we apply crustal corrections using the crustal model crust2 0 and we account for azimuthal anisotropy in the upper mantle using ray theoretical corrections based on a global model of azimuthal anisotropy while being well correlated with earlier models at long spatial wavelength our preferred solution savani additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage this is because the density of the inverse grid ranges between 1 25 in well sampled and 5 in poorly sampled regions allowing us to resolve regional structure better than it is typically the case in global s wave tomography our model highlights i a distinct ocean continent anisotropic signature in the uppermost mantle ii an oceanic peak in above average 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness and iii a long wavelength pattern of 1 associated with the large low shear velocity provinces in the lowermost mantle

lithospheric cooling trends and deviations in oceanic pp p and ss s differential traveltimes 1 the thermal and compositional structure of oceanic lithosphere which exerts an important control on plate behavior is still debated our set of 60 000 pp p and ss s traveltime differences with oceanic pp and ss bounce points provides a good constraint on both compressional and shear wave velocity by calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach we test whether lithospheric cooling can explain pp p and ss s traveltime variations with plate age the pp p and ss s traveltimes have substantial scatter but on average decrease by 0 2 and 0 7 s myr respectively when the pp and ss waves reflect off progressively older oceanic crust both a half space and a plate cooling model with a mid ocean ridge basalt source mantle potential temperature 1315 50 c explain the average values of the pp p and ss s anomalies and their decrease with plate age residual pp p and ss s anomalies relative to a cooling model reveal large scale patterns along a few paths e g tonga fiji to western north america seismic heterogeneity in the deep mantle is responsible for a significant fraction of the pp p and ss s traveltime variation most anomalies probably correspond to broad temperature variations in the upper mantle such as a very slow central northern pacific which may require a 100 c excess temperature and high and low velocity anomalies along the ridges that correlate with deep and shallow bathymetry respectively

minimum speed limit for ocean ridge magmatism from 210pb 226ra 230th disequilibria although 70 per cent of global crustal magmatism occurs at mid ocean ridges where the heat budget controls crustal structure hydrothermal activity and a vibrant biosphere the tempo of magmatic inputs in these regions remains poorly understood such timescales can be assessed however with natural radioactive decay chain nuclides because chemical disruption to secular equilibrium systems initiates parent daughter disequilibria which re equilibrate by the shorter half life in a pair here we use 210pb 226ra 230th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the earth s mantle can be transported accumulated and erupted in a few decades this implies that magmatic conditions can fluctuate rapidly at ridge volcanoes 210pb deficits of up to 15 per cent relative to 226ra occur in normal mid ocean ridge basalts with the largest deficits in the most magnesium rich lavas the 22 year half life of 210pb requires very recent fractionation of these two uranium series nuclides relationships between 210pb deficits 226ra 230th activity ratios and compatible trace element ratios preclude crustal magma differentiation or daughter isotope degassing as the main causes for the signal a mantle melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly the likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied

thickening of young pacific lithosphere from high resolution rayleigh wave tomography a test of the conductive cooling model direct seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges on the east pacific rise at 17 s there were two long term broadband ocean bottom seismometer deployments the melt and glimpse experiments these arrays spanned young pacific plate seafloor ranging in age from 0 to 8 ma combining these two data sets we describe the increase in rayleigh wave phase velocities for 16 100 s period as function of distance from the ridge with two parameterizations an arbitrary function of seafloor age and a simple polynomial function of age on the pacific and nazca plates although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the pacific plate can be resolved at periods of 50 s the three parameter polynomial model fits the data almost as well indicating a simple pattern of the evolution of structure with age to compare our observations to the predictions for the conductive cooling of the lithosphere we convert a thermal half space cooling model to shear velocity using anharmonic and anelastic contributions the patterns in the velocities are not consistent with simple conductive cooling the conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range we observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend the asthenospheric low velocity zone is asymmetric dipping to the west with the lowest velocities beneath the pacific plate west of the spreading center the conductive cooling model also over predicts the shear velocities in the lithosphere by 0 2 km s these anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required

ponded melt at the boundary between the lithosphere and asthenosphere the boundary between earth s rigid lithosphere and ductile asthenosphere is marked by a seismic discontinuity laboratory experiments on basaltic magmas show that melts should pond at pressures that correspond to the lithosphere asthenosphere boundary thus magma ponding could explain the observed seismic discontinuity

age independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere the depth of the oceanic lithosphere asthenosphere boundary lab as inferred from shear wave velocities increases with lithospheric age in agreement with models of cooling oceanic lithosphere on the other hand the distribution of radial anisotropy under oceanic plates is almost age independent in particular radial anisotropy shows a maximum positive gradient at a depth of 70 km which if used as a proxy indicates an age independent lab depth these contrasting observations have fueled a controversy on the seismological signature of the lab to better understand the discrepancy between these observations we model the development of lattice preferred orientation lpo in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate driven mid ocean ridge flows the model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms we find that an age independent distribution of radial anisotropy is the natural consequence of these simple flows the depth and strength of anisotropy is further controlled by the deformation regime dislocation or diffusion creep experienced by crystals during their ascent towards and subsequent motion away from the ridge axis comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume although not excluded additional mechanisms proposed to explain some geophysical signatures of the lab such as the presence of partial melt or changes in water content are not required to explain the radial anisotropy proxy our prediction that the age independent radial anisotropy proxy marks the transition to flow induced asthenospheric anisotropy provides a way to reconcile thermal mechanical and seismological views of the lab

multimode surface waveform tomography of the pacific ocean a closer look at the lithospheric cooling signature summary we present a regional surface waveform tomography of the pacific upper mantle obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode rayleigh waves to constrain the vsv structure down to 400 km depth we have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion we have furthermore improved path coverage in the south pacific region by including rayleigh wave observations from the french polynesian pacific lithosphere and upper mantle experiment deployment this improvement has led to imaging of vertical low velocity structures associated with hotspots within the south pacific super swell region we have produced an age dependent average cross section for the pacific ocean lithosphere and found that the increase in vsv with age is broadly compatible with a half space cooling model of oceanic lithosphere formation we cannot confirm evidence for a pacific wide reheating event our synthetic tests show that detailed interpretation of average vsv trends across the pacific ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region a condition that is difficult to achieve in such a large oceanic basin with current seismic stations

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

reply to comment by a douglas on systematic determination of earthquake rupture directivity and fault planes from analysis of long period p wave spectra summary we respond to the comments by douglas regarding our earlier paper by emphasizing that our automated method was intended to distinguish between the primary and auxiliary fault planes in earthquake focal mechanisms and does not always produce reliable results for rupture velocity and rupture length

global radially anisotropic mantle structure from multiple datasets a review current challenges and outlook abstract since the 1960s seismologists have mapped anisotropy in the uppermost mantle the mantle transition zone and the d region when combined with constraints from mineral physics and geodynamics anisotropy provides critical information on the geometry of mantle flow here we review the theory early work recent tomographic models and experimental constraints on radial anisotropy we discuss current challenges in resolving radial anisotropy seismically in particular we show that it is highly beneficial to use multiple datasets in inversions for anisotropy notably short period group velocity data with strong sensitivity to the crust we present a new whole mantle model of radial anisotropy based on surface wave and body wave travel time data along with incorporated moho perturbations our whole mantle model shares common features with previous global models and is consistent with results from several high resolution regional studies

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data

nonlinear crustal corrections for normal mode seismograms we present an efficient and accurate method for calculating local nonlinear crustal effects on the eigenfrequency of free oscillations of the earth we implement the method in the calculation of synthetic normal mode seismograms and demonstrate that neglecting the nonlinear crustal effects in the tomographic waveform inversion leads to velocity models that appreciably overestimate fast velocity anomalies beneath orogenic belts and cratons at 250 400 km depth online material color versions of figures

olivine anisotropy suggests gutenberg discontinuity is not the base of the lithosphere tectonic plates are a key feature of earth s structure and their behavior and dynamics are fundamental drivers in a wide range of large scale processes the operation of plate tectonics in general depends intimately on the manner in which lithospheric plates couple to the convecting interior current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth thus the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear here we present results from laboratory based torsion experiments on olivine aggregates with and without melt yielding an improved database describing the crystallographic alignment of olivine grains we combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled this model additionally supports the idea that discontinuities in velocity and anisotropy often assumed to be the base of the lithosphere are instead intralithospheric features reflecting a compositional boundary established at midocean ridges not a rheological boundary

new images of the earth s upper mantle from measurements of surface wave phase velocity anomalies 1 the sensitivity of a surface wave to the lateral structure of the earth s mantle depends on the local structure and thickness of the crust particularly at short periods the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave sensitivity functions thus far this effect has not been quantified and tomographic studies of the upper mantle based on surface wave observations have not taken it into account on the basis of a jeffreys wentzel kramers brillouin jwkb description of surface wave propagation we find and discuss the lateral variations of the sensitivity of love and rayleigh waves to mantle seismic anomalies associated with a realistic model of the crust mooney et al 1998 we then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from love and rayleigh wave phase velocity measurements periods from 35 to 300 s in terms of large scale patterns of high and low velocity our model is similar to previously published ones confirming among other features the anomalous anisotropy underlying the central pacific ekstrom and dziewonski 1998 we find however that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies which should not be neglected as we attempt to obtain higher resolution images of the upper mantle