an automated time window selection algorithm for seismic tomography we present flexwin an open source algorithm for the automated selection of time windows on pairs of observed and synthetic seismograms the algorithm was designed specifically to accommodate synthetic seismograms produced from 3 d wavefield simulations which capture complex phases that do not necessarily exist in 1 d simulations or traditional traveltime curves relying on signal processing tools and several user tuned parameters the algorithm is able to include these new phases and to maximize the number of measurements made on each seismic record while avoiding seismic noise our motivation is to use the algorithm for iterative tomographic inversions in which the synthetic seismograms change from one iteration to the next hence automation is needed to handle the volume of measurements and to allow for an increasing number of windows at each model iteration the algorithm is sufficiently flexible to be adapted to many tomographic applications and seismological scenarios including those based on synthetics generated from 1 d models we illustrate the algorithm using data sets from three distinct regions the entire globe the japan subduction zone and southern california

investigation of upper mantle discontinuity structure beneath the central pacific using ss precursors 1 using a high quality broadband seismic data set of precursors to the phase ss we investigate the structure of upper mantle discontinuities beneath the central pacific including the hawaiian hot spot we image structure by stacking over 4000 records into geographic bins retaining periods down to 5 s we consider the effects of correcting for four separate tomographic models of mantle heterogeneity excluding data at distances containing phases that potentially interfere with precursors we find evidence for peak to peak topography of 15 20 km on the 670 km discontinuity and 7 28 km of topography on the 400 km discontinuity weak reflections are detected from discontinuities near 220 and 520 km depth the average transition zone thickness beneath our region is approximately 242 3 km very similar to previous estimates of the global average lateral transition zone thinning and thickening weakly correlate with reduced and increased transition zone shear velocity respectively consistent with a thermal origin to topographical variations on the discontinuities within our study region the transition zone beneath hawaii and to the east of the line island chain is thinned by up to 20 km in a province spanning nearly 1000 km suggesting an excess mantle temperature of 200 k in the oldest crustal regions of our study area 100 ma the 400 km discontinuity is relatively shallow and the transition zone is relatively thick 250 255 km a possible explanation for this pattern includes small scale convection in the upper mantle bringing colder material into the transition zone

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

constraints on seismic anisotropy in the mantle transition zone from long period ss precursors the mantle transition zone mtz of earth is demarcated by solid to solid phase changes of the mineral olivine that produce seismic discontinuities at 410 and 660 km depths mineral physics experiments predict that wadsleyite can have strong single crystal anisotropy at the pressure and temperature conditions of the mtz thus significant seismic anisotropy is possible in the upper mtz where lattice preferred orientation of wadsleyite is produced by mantle flow here we use a body wave method ss precursors to study the topography change and seismic anisotropy near the mtz discontinuities we stack the data to explore the azimuthal dependence of travel times and amplitudes of ss precursors and constrain the azimuthal anisotropy in the mtz beneath the central pacific we find evidence for 4 anisotropy with a se fast direction in the upper mantle and no significant anisotropy in the mtz in subduction zones we observe 4 anisotropy with a trench parallel fast direction in the upper mantle and 3 anisotropy with a trench perpendicular fast direction in the mtz the transition of fast directions indicates that the lattice preferred orientation of wadsleyite induced by mtz flow is organized separately from the flow in the upper mantle global azimuthal stacking reveals 1 azimuthal anisotropy in the upper mantle but negligible anisotropy 1 in the mtz finally we correct for the upper mantle and mtz anisotropy structures to obtain a new mtz topography model the anisotropy correction produces 3 km difference and therefore has minor overall effects on global mtz topography

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

do double ss precursors mean double discontinuities summary stacks of ss precursors have been widely used in the past two decades to investigate the existence and characteristics of upper mantle discontinuities on a global scale as well as in several regional cases here we present observations of ss precursors from an mb 6 7 earthquake recorded at the us transportable array in 2010 in this particular case the s660s precursors on the transverse component are strong enough to be identified on individual seismograms across the array without any stacking procedures two s660s precursors are observed seeming to suggest double discontinuities around 660 km depth in the bounce point region through careful analysis of 1 d and 3 d synthetic seismograms we however discover that although they have arrival times and slownesses that are very close to the theoretical values of ss precursors the apparent double precursors are artefacts because of mantle heterogeneity in the upper mantle near the receivers away from the bounce point region this suggests that caution must be taken about appropriate azimuthal coverage at the ss bounce point before interpreting double ss or pp precursors in terms of complex mineralogical transitions

seismic observations of splitting of the mid transition zone discontinuity in earth s mantle the transition zone of earth9s mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers here we investigate the detailed structure between 410 and 660 kilometers depth by making use of regional stacks of precursors to the ss phase the previously observed discontinuity at about 520 kilometers depth is confirmed in many regions but is found to be absent in others there are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth an effect which can be interpreted as a splitting of the 520 kilometer discontinuity these observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone

effects of latent heat release at phase boundaries on flow in the earth s mantle phase boundary topography and dynamic topography at the earth s surface abstract mantle flow models that do not consider the effects of latent heat on phase boundaries typically predict dynamic surface topography too large to be compatible with observations here these effects were implemented in a mantle flow model and resulting changes in dynamic topography and topography of phase boundaries were computed inclusion of these effects was found to reduce the rms amplitude of dynamic topography by about 50 60 m still leaving a substantial misfit from observations in most cases considered generally it also leads to a slight improvement of correlation with dynamic surface and phase boundary topography constrained by observations results thus indicate that the model implemented here is applicable to the earth but that other effects need to be considered as well in order to fully explain phase boundary and dynamic surface topographies

the pacific lithosphere asthenosphere boundary seismic imaging and anisotropic constraints from ss waveforms 1 the lithosphere asthenosphere boundary lab separating the rigid lid from the underlying weaker convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics however the exact depth and defining mechanism of the lab interface remain poorly understood the ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70 of earth s surface area and have a relatively simple geological history seismically imaging the oceanic lab at high resolution has proved challenging yet several studies have recently increased resolution with provocative results we summarize recent imaging of discontinuity structure beneath much of the pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean continent margins ss precursors and waveform modeling of multiple phases including multiple bounce s waves scs reverberations and surface waves overall there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the lab some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths yet when the results are considered together no single age depth relationship is illuminated there are also puzzling discrepancies in where the discontinuity is detected which again suggests greater complexity here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure we stack ss waveforms with bounce points in the central pacific into azimuthal bins we use two methods one that inverts for discontinuity structure based on subtle variations in the character of the ss waveform and another that considers ss at higher frequency we find azimuthal variation in the amplitude of the waveform including a polarity reversal we suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate and must be carefully considered to constrain the age depth dependence and defining mechanism of the oceanic lithosphere

the geometry of aleutian subduction three dimensional kinematic flow model we construct a steady state kinematic model of slab flow along the aleutian arc assuming the slab s geometry is known the slab geometry is constrained by local network data where the slab is seismically active and by p wave residual sphere analysis of 11 earthquakes and one nuclear explosion where it is aseismic the slab is approximated by a thin newtonian sheet subducting in a less viscous and incompressible mantle particles enter the trench at known rates of relative plate motion and are required to subduct along the predefined slab geometry for discrete but small 0 1 m y time steps we determine the speed and direction of each slab particle that minimizes dissipation power as the spherical shell oceanic lithosphere deforms to obtain the observed slab shape because the dip of the slab in the central aleutians is steep relative to the radius of curvature of the trench the slab must stretch along arc to obtain its present configuration our kinematic model represents one internally consistent flow that results in the observed geometry furthermore it provides a lower bound on the total amount of internal in plane deformation required by the shape and an estimate of stream lines and of the orientation and spatial distribution of the deformation rate the predicted in plane strain rate field is dominated by along arc extension reaching values as large as 10 15s 1 in the central aleutians and decaying by an order of magnitude to the east and west the slab accumulates an in plane strain of 10 by the time it reaches the depth of the seismicity cutoff along arc variations in both the magnitude and orientation of calculated strain rates are consistent with the spatial distribution of seismic moment release and source mechanism orientations of intermediate depth earthquakes suggesting that in order to understand intermediate focus earthquakes one must consider along arc deformation three dimensional analyses of subduction in a spherical geometry such as the present study are required to model along arc effects relative plate motions along the trench vary from nearly normal in the eastern aleutians to transform farther west coincident with this change the maximum depth of seismicity within the descending slab decreases from 250 to 50 km and active andesitic volcanism ceases the kinematic flow model predicts that the slab subducted obliquely along the central portion of the aleutian arc is transported laterally westward producing cold slab material to a depth of at least 300 km along the entire arc after only 15 m y of subduction the lack of volcanic activity in the western aleutians is apparently related to the lateral transport of the slab perhaps indicating that the slab is dehydrated and incapable of producing the volatiles necessary for arc magmatism the along arc variation in the maximum depth of seismicity can be explained thermally we construct temperature estimates within the slab using a quasi three dimensional finite difference scheme including entrained mantle flow stream lines from the kinematic model and the time history of the age of the subducting lithosphere as a function of position along the trench the theoretical temperature at the seismicity cutoff depth is remarkably close to a constant fraction of the depth dependent mantle solidus along the entire arc this observation is consistent with the hypothesis that seismicity terminates because of the initiation of high temperature steady state creep and provides an explanation for the lack of intermediate focus earthquakes in the western aleutians even though slab material exists there

mantle transition zone beneath eurasia mantle pds fig 1 converted phases are detected in the records of 11 seismograph stations in easternmost russia and china these data reveal neither a strong depression on the 660 km discontinuity nor a layer of partial melting atop the 410 km discontinuity that were found previously beneath this region in studies using long period underside sh reflections and multiple scs reverberations apparently a significant deepening of the 660 km discontinuity occurs only if the subducted plate penetrates the discontinuity which is not the case in the study region most estimates of thickness of the mantle transition zone mtz in eurasia based on the pds data are a few kilometres larger than the standard value 250 km and suggest that the average thickness of the mtz beneath eurasia is about 10 kilometres more than beneath the surrounding oceans

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

on the relevance of born theory in global seismic tomography 1 does the application of seismic born theory as opposed to simpler ray theory lead to an improvement in tomographic images of the earth in recent publications montelli et al 2004a 2004b and van der hilst and de hoop 2005 among others have expressed opposite opinions we propose a quantitative approach to the comparison of tomographic images which we apply to the case of surface wave phase velocity maps derived with born vs ray theory

sharp mantle transition from cratons to cordillera in southwestern canada the western canada sedimentary basin marks the transition from the old north american continental lithosphere to young accreted terranes earlier studies in this region have suggested a large number of intricate basement domains as well as major seismic velocity gradients in the mantle to investigate the effect of the accretion and subduction on the mantle structure beneath the western margin of the north american craton we analyze p to s converted waves from upper mantle discontinuities from the canadian rockies and alberta network a regional broadband seismic array based in alberta canada the depths of the 410 and 660 km seismic discontinuities are correlated and on average are 9 km and 7 km greater than their respective global estimates the largest depression is observed beneath the rocky mountain foreland belt in southern alberta which highlights a steep southward westward structural gradient from the cratons to cordillera at lithospheric mantle depths the severity of the depressions especially in the southernmost alberta may be triggered by diffuse partial melt or increased water content above the 410 km discontinuity this result is corroborated by locally increased impedance contrast across the 410 km discontinuity and a strongly depressed 660 km discontinuity a relic mesozoic slab fragment may be partially responsible for the deep olivine phase boundary at the base of the upper mantle

a comparison of synthetic seismograms for 2d structures semianalytical versus numerical teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great circle degrees when observed at broadband arrays these rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid and deep mantle although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths it is important to validate their accuracy against the predictions of numerical methods here we compare synthetics from an approximate ray based method wkbj modified wkm against the pseudospectral method for a 2d model of the s velocity anomaly associated with the south african plume this model consists of a uniform 3 decrease in s velocity over a broad 10 region of the mid and deep mantle beneath south africa contiguous at its bottom with a thin 100 to 200 km thick zone of low velocity extending 30 westward toward south america along the core mantle boundary transitions between anomalous and radially symmetric structures of the test model are sharp occurring over l0 km or less sv and sh wave fields synthesized by the wkm and pseudospectral methods in this model generally agree with each other well slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray based wkm method

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

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

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

imaging the upper mantle transition zone with a generalized radon transform of ss precursors we demonstrate the feasibility of using inverse scattering for high resolution imaging of discontinuities in the upper mantle beneath oceanic regions far from sources and receivers using broadband wavefield observations consisting of ss and its precursors the generalized radon transform grt that we developed for this purpose detects in the broadband data signals due to scattering from elasticity contrasts in earth s mantle synthetic tests with realistic source receiver distributions demonstrate that the grt is able to detect and image deep mantle interfaces even in the presence of noise depth phases ghosts phase conversions and multiples generated by reverberation within the transition zone as a proof of concept we apply the grt to 50 000 broadband seismograms to delineate interfaces in the depth range from 300 to 1000 km beneath the northwest pacific we account for smooth 3d mantle heterogeneity using first order perturbation theory and independently obtained global tomography models the preliminary results reveal laterally continuous but undulating scatter zones near 410 and 660 km depth and a weaker broader and more complex structure near 520 km depth the images also suggest the presence of multiple laterally intermittent interfaces near 350 km and between 800 and 1000 km depth that is above and below the transition zone sensu stricto filtering of the data we consider four pass bands 20 50 s 10 50 s 5 50 s and 2 50 s reveals a prominent frequency dependence of the magnitude width and complexity of the interfaces in particular of the scatter zone near 520 km depth such dependencies may put important constraints on the mineralogy and phase chemistry of the transition zone

remote sensing of hydrogen in earth s mantle hydrogen1 in earth s interior is known to play a key role in a number of processes consequently inferring the distribution of hydrogen is a critical step in our study of dynamics and evolution of earth usually hydrogen distribution is inferred from two types of samples at the surface first a magma may contain hydrogen water and under some conditions the water content of the magma can be quenched upon cooling in these cases measurements of the water content of the magma provide us with some constraints on the hydrogen water content of the source region if we know the partitioning of hydrogen between magmas and the source rocks and the degree of melting second hydrogen content of some xenoliths transported by magma can be measured they provide a direct clue as to the hydrogen content in a region from which a xenolith has been carried however these direct petrologic approaches have two major problems firstly the sampling is limited by the distribution of volcanoes and even if there are volcanoes that carry rocks from earth s interior the depth extent that volcanoes sample rocks is limited usually 200 km secondly there is no guarantee that the hydrogen content that one measures on these samples actually represents the hydrogen content in a region where these samples came from for example hydrogen is known to diffuse very easily so hydrogen dissolved in minerals could diffuse out during the transport of a rock or conversely a piece of rock may acquire extra hydrogen during its ascent also hydrogen atoms dissolved in minerals may precipitate in the mineral to form fluid inclusions or micro scale hydrous minerals in summary the direct method to infer the distribution of hydrogen from rock samples has major limitations and an alternative approach i e remote sensing hydrogen content from geophysical

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

simultaneous inversion for mantle shear velocity and topography of transition zone discontinuities summary until now modelling of three dimensional 3 d velocity variations in the mantle and topography of the transition zone discontinuities have been considered separately velocity models were obtained assuming that the radii of the discontinuities are constant then the travel time data sensitive to the topography such as the ss precursors were corrected for the effect of 3 d seismic structure and inverted for depth variations of a discontinuity such a procedure is unsatisfactory as it may introduce artefacts that could significantly affect the topographic results the opposite trade off is less likely to introduce conceptually important changes in the velocity distribution but should also be considered in this study we bring together the same set of s velocity sensitive data as used by gu et al and combine it with a large set of differential travel times of ss s400s s s670s and direct measurements of s400s s670s we formulate the inverse problem in terms of the volumetric 3 d and topographic 2 d perturbations for both the 400 and 670 km discontinuities the best fitting model of the joint inversion significantly improves the variance reduction of ss s400s and ss s670s residuals the velocity distribution in the resulting model topos362d1 is very similar to that in model s362d1 with correlation coefficients 0 9 throughout the mantle which indicates that lateral variations of discontinuity depths have only minor influence on global modelling of velocity important changes however have been made to the topography of the 400 and 670 km discontinuities with respect to those obtained earlier assuming an existing velocity model the overall undulation of the 400 km discontinuity is considerably less than that reported by earlier global studies in topos362d1 its maximum variation does not exceed 12 km the strong degree 1 component before the joint inversion has decreased such that the correlation between the velocities above the discontinuity and the shape of the discontinuity itself has substantially diminished spatially this result means a significant diminution in the strength of the earlier reported depression of the 400 km discontinuity under the pacific the power spectrum of the topography of the 670 km discontinuity has been enriched in long wavelength component especially in degree 2 the range of depth variations is 18 km and its shape correlates well with the radially averaged velocity perturbations in the transition zone at wavelengths greater than 1000 km there is little correlation between the depth perturbations of the 400 and 670 km discontinuities the topography of the 400 km discontinuity does not appear to be strongly influenced by thermal structures potentially associated with subduction processes and plumes this implies that thermal influence on the olivine to phase transformation may not fully account for the observed depth variations dynamical effects and potential variations in composition may be important near the top of the transition zone

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

ps conversions from the upper mantle transition zone beneath the canadian landmass broadband teleseismic p waveforms recorded on the canadian national seismograph network have been processed to isolate near receiver ps conversions originating from upper mantle discontinuities at approximately 410 and 660 km depth a simultaneous deconvolution procedure which removes source effects and exploits the move out characteristics of the weak ps phases is used to raise signal above the noise levels in the p coda large amplitude arrivals exhibiting times and slownesses consistent with an interpretation as ps converted phases from the 410 and 660 km discontinuities are observed at 18 and 20 of the 24 stations examined respectively the arrival times corrected for crustal heterogeneity show strong correlation with upper mantle structure above 410 km depth as determined in previous studies stations in the cordillera above low velocity mantle are characterized by residuals up to 3 3 s slow relative to the iasp91 model whereas sites overlying the high velocity root below the canadian shield are advanced by as much as 1 9 s the difference between the p660s and p410s times provides information on the structure of the transition zone beneath canadian stations the trend of the data indicates that topography on the discontinuities is at or below resolvable levels 5 km over most of the region in general accord with a previous study of the transition zone beneath continental stations but in contrast with results for the pacific basin

sh velocity and compositional models near the 660 km discontinuity beneath south america and northeast asia 1 we constrain sh wave velocity structures near the 660 km discontinuity beneath south america and northeast asia using triplicated phases near the discontinuity recorded in the epicentral distance range of 10 35 for three deep events we then explore mineralogical and compositional models appropriate for explaining the inferred seismic structures between the two regions sh velocity structures near the 660 km discontinuity are found to be different in the two regions beneath south america the velocity gradient above the 660 km discontinuity is larger than that of preliminary earth reference model prem while the velocity jump across the discontinuity is the same as prem beneath northeast asia the velocity gradient above the 660 km discontinuity is the same as that of prem while the velocity jump across the discontinuity is larger than prem both regions are characterized by a large velocity gradient extending about 80 km deep below the 660 km discontinuity the different velocity structures require different mineralogical models in the transition zone in the two regions the larger velocity gradient above the 660 km discontinuity beneath south america requires existence of the ilmenite phase in the bottom of transition zone while that beneath northeast asia can be explained by the temperature and pressure dependence of elastic properties of the major mantle mineral assemblages the observed large velocity gradients in the top of the lower mantle can be explained by gradual transformation of garnet to perovskite persisting to greater depths the velocity jump across the 660 km discontinuity beneath south america in the presence of the ilmenite phase in the bottom of the transition zone requires a bulk composition of more garnet than the pyrolite model while a larger velocity jump across the discontinuity beneath northeast asia requires a larger fraction of garnet transforming to perovskite across the discontinuity than what is required for explaining prem these different mineralogical models can be caused by different mantle temperature or composition especially the aluminum content in mantle composition the presence of garnet 80 km below the 660 km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3 4 the existence of ilmenite in the bottom of the transition zone beneath south america and the absence of ilmenite beneath northeast asia can be explained by either a difference in mantle temperature of about 100 c with that beneath south america being lower between the two regions assuming a uniform mantle composition or alternatively a difference in aluminum content of about 1 with that beneath south america being lower between the two regions without invoking a temperature difference between the two regions we also discuss conditions of mantle composition and temperature that double discontinuities may occur near the 660 km depth as well as the depth separation and velocity jumps of the double discontinuities for the inferred mantle temperature and composition beneath south america and northeast asia the maximum separation of the double discontinuities is 20 km and cannot be resolved by the sh wave data

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

transition zone velocity gradients and the 520 km discontinuity stacks of long period global digital seismograph network gdsn seismograms at 110 to 180 epicentral distance reveal precursors to ss that result from underside reflections off upper mantle seismic discontinuities the 410 and 660 km discontinuities are obvious in these stacks but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410 and 660 km reflections these sidelobes result from the limited bandwidth of the gdsn instrument responses and the effect of crustal reverberations on the ss reference phase the crustal effects can be minimized by restricting the records to oceanic bounce points where the 6 km thick crust has little effect on the long period waveforms over 2000 long period transverse component seismograms with oceanic ss bounce points recorded by the gdsn from 1976 to 1991 are manually edited aligned on ss and then stacked using a new procedure that weights the records by data quality the resulting image shows a clear reflection from a 520 km discontinuity that cannot be explained as a sidelobe artifact confirming earlier results of shearer 1990 1991 and revenaugh and jordan 1991 by stacking along the expected travel time curves for discontinuity phases the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time the features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces using geometrical ray theory and assuming a constant velocity versus density scaling relationship i fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion the inverse problem is addressed by performing a deconvolution of the profile with the ss reference phase after a correction for attenuation followed by a direct mapping of reflectivity versus time into velocity versus depth velocity depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models except that the ss precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660 km discontinuity estimated discontinuity shear impedance changes are 6 7 1 1 at 420 km 2 9 0 7 at 519 km and 9 9 1 5 at 663 km the impedance change near 520 km is consistent with current mineral physics results for the olivine to phase change and places constraints on the fraction of olivine in the transition zone

seismic imaging of transition zone discontinuities suggests hot mantle west of hawaii the hawaiian hotspot is often attributed to hot material rising from depth in the mantle but efforts to detect a thermal plume seismically have been inconclusive to investigate pertinent thermal anomalies we imaged with inverse scattering of ss waves the depths to seismic discontinuities below the central pacific which we explain with olivine and garnet transitions in a pyrolitic mantle the presence of an 800 to 2000 kilometer wide thermal anomaly tmax 300 to 400 kelvin deep in the transition zone west of hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward hawaii and perhaps other islands this implies that geochemical trends in hawaiian lavas cannot constrain lower mantle domains directly

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

global seismic data reveal little water in the mantle transition zone abstract knowledge of the earth s present water content is necessary to constrain the amount of water and other volatiles the earth acquired during its formation and the amount that is cycled back into the interior from the surface this study compares 410 and 660 km discontinuity depth with shear wave tomography within the mantle transition zone to identify regions with seismic signals consistent with water the depth of the 410 and 660 km discontinuities is determined from a large updated dataset of ss s410s and ss s660s differential travel times known as ss precursors the discontinuity depths measured from binning and stacking the ss precursor data are then compared to the shear velocity model hmsl s06 in the transition zone mapping all the possible combinations very few locations match the predictions from mineral physics for the effects of water on discontinuity depth and shear velocity the predictions although not yet measured at actual transition zone temperatures and pressures are a shallow 410 km discontinuity a deep 660 km discontinuity and a slow shear velocity only 8 of the bins with high quality data are consistent with these predictions and the calculated average water content within these bins is around 0 6 wt a few isolated locations have patterns of velocity topography that are consistent with water while there are large regional scale patterns consistent with cold hot temperature anomalies combining this global analysis of long period seismic data and the current mineral physics predictions for water in transition zone minerals i find that the mantle transition zone is generally dry containing less than one earth ocean of water although subduction zones could be locally hydrated the combined discontinuity and velocity data show no evidence that wadsleyite or ringwoodite have been globally hydrated by subduction or initial earth conditions

wet subduction versus cold subduction 1 in situ x ray diffraction study of post spinel transformation in hydrous peridotite 2 wt h2o indicates that the phase boundary is shifted to higher pressures by 0 6 gpa relative to anhydrous peridotite at 1473 k whereas it shows no obvious shift at high temperature around 1873 k a linear equation for the boundary is p gpa 0 002 t k 26 3 which is applicable for temperatures below 1800 k the present data shows that a significant part of the depressions seen in the 660 km seismic discontinuity may be affected by existence of water it is also well resolved that the olivine wadsleyite phase transformation corresponding to the 410 km seismic discontinuity is shifted to lower pressures by 1 2 gpa by the addition of water thus the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction

global scale p wave tomography optimized for prediction of teleseismic and regional travel times for middle east events 2 tomographic inversion 1 we construct a model of three dimensional p wave velocity structure in the crust and mantle that is global in extent but with detailed upper mantle heterogeneities throughout the greater middle east region fully three dimensional ray tracing is employed to achieve accurate travel time predictions of p and pn arrivals requiring the characterization of irregular and discontinuous boundaries therefore we explicitly represent undulating seismic discontinuities in the crust and upper mantle within a spherical tessellation modeling framework the tessellation based model architecture is hierarchical in that fine node sampling is achieved by recursively subdividing a base level tessellation determining the required node spacing to effectively model a given set of data is problematic given the uneven sampling of seismic data and the differing wavelengths of actual seismic heterogeneity to address this problem we have developed an inversion process called progressive multilevel tessellation inversion pmti that exploits the hierarchical nature of the tessellation based design and allows the data to determine the level of model complexity pmti serves as an alternative to existing multiresolution approaches and robustly images regional trends while allowing localized details to emerge where resolution is sufficient to demonstrate our complete modeling concept we construct a velocity model based on teleseismic p travel time data for global events and regional pn travel time data for events occurring throughout the middle east input data are a product of the statistical procedure called bayesloc that simultaneously models all components of a multievent system including event locations origin times and arrival times described in the myers et al 2011 companion paper the initial tomographic image provides a new glimpse of the complex upper mantle velocity anomalies associated with the convergence of the arabian and indian plates with eurasia more important for event monitoring the model accurately predicts both teleseismic and regional travel times for events occurring within the middle east region

shear and compressional velocity models of the mantle from cluster analysis of long period waveforms summary we present a new technique for the efficient measurement of the traveltimes of long period body wave phases the technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event this allows the application of cross correlation techniques that are usually used in a regional context to measure precise global differential times the analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity this allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique this technique can be applied to any seismic phase that is observed over a reasonably large distance range here we present the results of applying the algorithm to the long period channels of all data archived at the iris dmc from 1976 to 2005 for the seismic phases s and p from 23 to 100 and ss and pp from 50 to 170 the resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of s and p velocity the resolution of the new model is enhanced particularly in the mid mantle where ss and pp turn we find that slow anomalies in the central pacific and africa extend from the core mantle boundary to the upper mantle but their direct connection to surface hotspots is beyond our resolution furthermore we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid mantle

an analysis of ss precursors using spectral element method seismograms summary the traveltimes of underside shear wave reflections i e ss precursors are widely used data for mapping the topography of mantle discontinuities in the upper mantle here we examine the accuracy of ray theory in estimating the contribution of shear velocity heterogeneity in the mantle to ss s400s ss s670s and s400s s670s traveltimes we analyse stacks of spectral element method waveforms computed for 34 shallow earthquakes and 16 020 globally distributed virtual seismometers the waveforms are computed for three versions of shear velocity model s20rts with variable strengths of the volumetric shear velocity perturbations within the layered structure of the preliminary reference earth model we find that ray theoretical corrections account for only 50 per cent of the traveltime variation due to large scale velocity heterogeneity for current tomographic models as represented by s20rts this translates into unpredictable long and short wavelength errors in maps of mantle discontinuity depths of about 5 km this amounts to roughly 10 per cent of published variations in mantle discontinuity depths however relative errors will be significantly larger if ray theoretical traveltime corrections are based on forthcoming models of shear velocity with much stronger heterogeneity at the smallest scales

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

the effect of topography of upper mantle discontinuities on ss precursors using the spectral element method we explored the effect of topography of upper mantle discontinuities on the traveltimes of ss precursors recorded on transverse component seismograms the latter are routinely used to infer the topography of mantle transition zone discontinuities the step from precursory traveltimes to topographic changes is mainly done using linearised ray theory or sometimes using finite frequency kernels we simulated exact seismograms in 1 d and 3 d elastic models of the mantle in a second simulation we added topography to the discontinuities we compared the waveforms obtained with and without topography by cross correlation of the ss precursors since we did not add noise the precursors are visible in individual seismograms without the need of stacking the resulting time anomalies were then converted into topographic variations and compared to the original topographic models based on the correlation between initial and inferred models and provided that ray coverage is good we found that linearised ray theory gives a relatively good idea on the location of the uplifts and depressions of the discontinuities it seriously underestimates the amplitude of the topographic variations by a factor ranging between 2 and 7 real data depend on the 3 d elastic structure and the topography all studies to date correct for the 3 d elastic effects assuming that the traveltimes can be linearly decomposed into a structure and a discontinuity part we found a strong non linearity in this decomposition which cannot be modelled without a fully non linear inversion for elastic structure and discontinuities simultaneously

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

thickness estimates of the upper mantle transition zone from bootstrapped velocity spectrum stacks of receiver functions summary we modify the receiver functions stacking technique known as velocity spectrum stacking vss so as to estimate combinations of velocity model v p and v s and depth that stack the ps conversion from upper mantle discontinuities most coherently we find that by estimating the diverences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of p410s and p660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities we present two examples indicating that the transition zone beneath obninsk russia is 252 6 km thick and that beneath pasadena california is only 220 6 km thick

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

seismically deduced thermodynamics phase diagrams for the mantle transition zone abstract seismic discontinuities at 410 and 660 km depth are usually attributed to solid phase changes within the olivine component of the mantle the clapeyron slopes 410 and 660 i e the thermal dependence of the depths of reactions have been shown experimentally to be of opposite signs yet their values are not well constrained by laboratory measurements seismological observations have not been more precise due to the difficulty to separate the competing effects of background wave velocities and of temperature on the topography of discontinuities in this study we use conversion imaging of interfaces under western us we propose a new approach to derive a seismological estimate of the clapeyron slopes with respect to 410 for the major and minor phase changes of the transition zone we obtain 660 3 mpa k 1 for 410 3 mpa k 1 we construct seismic phase diagrams of the transition zone that can be directly compared with experimental phase diagrams we also apply a z transform to better constrain the clapeyron slopes of the minor phase changes although tenuous signals in seismic phase diagrams suggest that minor phase transitions both in the olivine and the non olivine component of the mantle have visible seismic expressions they can tentatively be described as follows the 410 is overlaid at low temperature by an interface corresponding to a decrease of velocity with depth and 3 mpa k 1 the 660 widens at high temperature and is preceded at low temperature by an interface the 620 with 7 mpa k 1 a 520 is suggested with 2 3 mpa k 1 these last two interfaces correspond to velocity increases with depth at last near 590 km depth an interface may be associated with a velocity reduction showing a weak dependence on temperature 0 mpa k 1

sharpness of the mantle discontinuities seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep earth but these data are difficult to obtain we explore a possibility to determine sharpness of the major mantle discontinuities those at depths around 400 and 650 km from the broad band records of phases converted from p to s underneath the receiver our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the p waves in the teleseismic records of brv kazakhstan grf germany nre0 norway and ykw canada the data indicate that the shape of the pulse converted at the 400 km discontinuity is close to that of the p wave whereas the pulse of the 650 km conversion is substantially broader we infer from this comparison that the 400 km discontinuity is sharp width of the transition zone is less than 5 7 km the 650 km transition is modelled by a linear gradient zone 20 30 km thick

imaging upper mantle discontinuity topography using underside reflection data this paper presents a method to invert underside reflection p d p or s d s arrivals data for lateral depth variations of upper mantle discontinuities combining traveltime and amplitude data the method greatly improves the resolution of small scale undulations obtained by existing imaging methods and does not suffer from the long wavelength biases that are likely to be present in currently available models existing inversion methods account for the large size of the fresnel zone of underside reflections but not for its complexity arising from the mini max traveltime nature of pp and ss related waves this neglect results in long wavelength artefacts from small scale undulations of the discontinuities obscurring true long wavelength depth variations the inversion method presented in this paper uses a complex valued sensitivity kernel derived from the representation of underside reflections through a kirchhoff integral formulation the sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the fresnel zone the method is applied to a large synthetic data set the data set consists of p d p amplitudes and traveltimes the results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the fresnel zone of individual underside reflections but larger than the dominant wavelength retaining the resolution of large scale variations the results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad band p670p data sets

reconciling pacific 410 and 660 km discontinuity topography transition zone shear velocity patterns and mantle phase transitions a r t i c l e i n f o article history we observe that the depths of the 410 and 660 km seismic discontinuities are on average slightly positively correlated globally this is due in large part to a modestly depressed 660 km discontinuity and a large depression of the 410 km discontinuity across the paci c the clapeyron slope dp dt the change in pressure depth of a phase transition with a change in temperature can be used to predict the depth of a phase transformation assuming lateral temperature variations the phase change of olivine to spinel is well understood experimentally almost certainly produces the 410 km discontinuity and has a positive clapeyron slope at the base of the transition zone both the olivine component of the mantle spinel and the pyroxene component garnet transform to perovskite and periclase observations of 660 km discontinuity depths are often consistent with the negative clapeyron slope of the spinel to perovskite and periclase transition with an apparent anti correlation with the depth of the 410 km discontinuity however under the paci c the depression of the 410 km discontinuity and slow seismic velocities indicate that the mantle is warmer than average using a negative clapeyron slope for the perovskite forming reaction the 660 km discontinuity is predicted to be shallow in this region however we observe that it is either depressed or has little de ection from its average depth we nd that if the clapeyron slope associated with the 660 km discontinuity changes sign from negative to positive between 1920 and 2020 k we can explain the correlation of discontinuity structure with seismic velocities in the transition zone this shift in sign is in accord with the dominant 660 km transition forming reaction shifting from spinel to garnet near this temperature

interpreting late precambrian microfossils chia wei li et al describe microfossils found in doushantuo phosphate rocks from guizhou province south china that are about 580 million years old 1 1 well preserved macroscopic multicellular fossils associated with prokaryotic and eukaryotic microfossils from the same source rocks have been

sensitivity analysis of seismic waveforms to upper mantle discontinuities using the adjoint method using spectral element simulations of wave propagation we investigated the sensitivity of seismic waveforms recorded on transverse components to upper mantle discontinuities in 1 d and 3 d background models these sensitivity kernels or frechet derivatives illustrate the spatial sensitivity to model parameters of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper we focus on the boundaries at 400 and 670 km depth of the mantle transition zone ss precursors have frequently been used to infer the topography of upper mantle discontinuities these seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110 160 this distance range is chosen to minimize the interference from other waves we show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110 160 range the sensitivity kernels are computed with the adjoint method using synthetic data from our simulations we can draw three main conclusions i the exact frechet derivatives show that in all time windows and also in those centred on the ss precursors there is interference from other waves this explains the difficulty reported in the literature to correct for 3 d shear wave speed perturbations even if the 3 d structure is perfectly known ii all studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of ss precursors can be linearly decomposed into a 3 d elastic structure and a topography part we recently showed that such a linear decomposition is not possible for ss precursors and the sensitivity kernels presented in this paper explain why iii in agreement with previous work we show that other parts of the seismograms have greater sensitivity to upper mantle discontinuities than ss precursors especially multiply bouncing s waves exploiting the s wave triplications due to the mantle transition zone these phases can potentially improve the inference of global topographic variations of the upper mantle discontinuities in the context of full waveform inversion in a joint inversion for an elastic parameters and topography

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

mantle discontinuity structure beneath the southern east pacific rise from p to s converted phases receiver functions derived from teleseismic body waves recorded by ocean bottom seismometers on the southern east pacific rise reveal shear waves converted from compressional waves at the mantle discontinuities near 410 and 660 kilometer depth the thickness of the mantle transition zone between the two discontinuities is normal relative to the global average and indicates that upwelling beneath the southern east pacific rise is not associated with an excess temperature in the mantle transition zone

imaging mantle transition zone thickness with sds ss finite frequency sensitivity kernels summary we invert differential sds ss traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410 and 660 km discontinuities this approach yields higher resolution images of transition zone thickness than previous stacking methods which simply average smooth over topographic features apparent structure measured using simple stacking is highly dependent upon the bin size of each stack by inverting for discontinuity topography with a variety of bin sizes we can more accurately calculate the true structure the inverted transition zone model is similar to simple stack models with an average thickness of 242 km but the lateral variations in thickness are larger in amplitude and smaller in scale fast seismic velocities in 3 d mantle models such as sb4l18 correlate with areas of thicker transition zone the elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone the anomalously thin transition zone regions are laterally narrow and not broadly continuous if these variations in transition zone thickness are interpreted as thermal in nature then this model suggests significant temperature variations on small lateral scales

one dimensional physical reference models for the upper mantle and transition zone combining seismic and mineral physics constraints one dimensional seismic reference models are known to be a nonunique solution to global seismic data hampering an interpretation in terms of physical structure here we test the compatibility of the simplest hypothesis of a mantle convecting as a whole with a constant pyrolitic composition with phase transitions directly against the kind of seismic data that went into global seismic reference models focusing on upper mantle structure down to 800 km depth by randomly varying the elastic and anelastic parameters of the main mantle minerals within their uncertainty bounds we generate a set of 100 000 adiabatic pyrolitic models a small number of these models 0 1 give a fit to far regional p and s travel times reprocessed isc catalog u 18 5d 26d and fundamental spheroidal and toroidal modes reference earth model web page 1 60 that is as satisfactory as the fit of preliminary reference earth model prem or ak135 f although the accepted models have widely different combinations of the mineral parameters there is a preference for a relatively high olivine shear modulus and its pressure and temperature derivatives relatively low wadsleyite bulk and shear parameters and relatively high ringwoodite bulk modulus derivatives the resulting seismic profiles are very similar and compared to prem or ak135 have lower velocities above 400 km larger jumps near 410 lower transition zone gradients lower jumps around 660 and stronger gradients directly below such physical models that fit seismic data well enough to be useful as a seismic reference model can significantly facilitate physical interpretation of seismic structures

receiver functions in the western united states with implications for upper mantle structure and dynamics 1 investigations into mechanisms driving surface tectonics commonly search for mantle sources but few observations constrain flow in the upper mantle and transition zone here variations in the upper mantle discontinuities at 410 km and 660 km below the western united states are revealed through mapping depths of compressional to shear wave conversions recorded by broadband seismometers the resulting image exhibits 20 and 30 km of topography on the 410 and 660 km discontinuities similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone the pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the rocky mountains colorado plateau or basin and range province providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region at least at scale lengths of a few hundred kilometers furthermore we find no clear correlation between the depths of the 410 and 660 km discontinuities undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km if the topography were due only to lateral temperature differences such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone alternatively the topography may reflect lateral variations in composition variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth

seismic waveform inversion for core mantle boundary topography the topography of the core mantle boundary cmb is directly linked to the dynamics of both the mantle and the outer core although it is poorly constrained and understood recent studies have produced topography models with mutual agreement up to degree 2 a broad band waveform inversion strategy is introduced and applied here with relatively low computational cost and based on a first order born approximation its performance is validated using synthetic waveforms calculated in theoretical earth models that include different topography patterns with varying lateral wavelengths from 600 to 2500 km and magnitudes 10 km peak to peak the source receiver geometry focuses mainly on the pdiff pkp pcp and scs phases the results show that pkp branches pcp and scs generally perform well and in a similar fashion while pdiff yields unsatisfactory results we investigate also how 3 d mantle correction influences the output models and find that despite the disturbance introduced the models recovered do not appear to be biased provided that the 3 d model is correct using cross correlated traveltimes we derive new topography models from both p and s waves the static corrections used to remove the mantle effect are likely to affect the inversion compromising the agreement between models derived from p and s data by modelling traveltime residuals starting from sensitivity kernels we show how the simultaneous use of volumetric and boundary kernels can reduce the bias coming from mantle structures the joint inversion approach should be the only reliable method to invert for cmb topography using absolute cross correlation traveltimes

resolving crustal thickness using ss waveform stacks we image lithospheric interfaces using variations in the character of ss waveform stacks a method we term ss lithospheric interface profiling sslip the variations are caused by reflected phases that is underside reflections ss precursors and topside multiples ss reverberations created at velocity discontinuities near the midpoint of the ss ray path stacks from continental versus oceanic bounce point regions produce distinctly different ss waveforms consistent with the large continent ocean difference in crustal thickness to investigate the potential of ss waveform stacks to constrain moho depths under continents we develop a method to fit continental bounce point stacks with a reference ss waveform convolved with a crustal operator the sslip inferred moho depths agree with the crust 2 0 model in asia for those regions where the ss bounce point density is the highest the sslip depths are correlated correlation coefficient 0 82 with the crust 2 0 values averaged over sample bins of 10 radius the sslip method has broad lateral resolution in comparison to most other methods for resolving crustal thickness but has the potential to sample regions where station coverage may be sparse

upper mantle anisotropy and transition zone thickness beneath southeastern north america and implications for mantle dynamics a variety of models for mantle flow beneath southeastern north america have been proposed including those that invoke westward driven return flow from the sinking farallon slab small scale convective downwelling at the edge of the continental root or the upward advective transport of volatiles from the deep slab through the upper mantle we use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern united states to test several of these proposed mantle flow geometries near the coast stations exhibit well resolved null no splitting behavior for sks phases over a range of back azimuths consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry farther inland we identify splitting with mainly ne sw fast directions consistent with asthenospheric shear due to absolute plate motion apm lithospheric anisotropy aligned with appalachian tectonic structure or a combination of these phase weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations at most stations we find transition zone thicknesses that are consistent with the global average 240 km with two stations showing evidence for a slightly thickened transition zone 250 km our results are relevant for testing different models for mantle dynamics beneath the southeastern united states but due to the sparse station coverage we are unable to uniquely constrain the pattern of mantle flow beneath the region our sks splitting observations support a model in which mantle flow is primarily vertical either upwelling or downwelling beneath the southeastern edge of the north american continent in contrast to the likely horizontal apm driven flow beneath the continental interior however our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material we expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks including the transportable array and flexible array components of usarray

mapping mantle transition zone discontinuities beneath the central pacific with array processing of ss precursors we image mantle transition zone mtz discontinuities beneath the central pacific using 120 000 broadband ss waveforms with a wave packet based array processing technique curvelet transform we improve the signal to noise ratio of ss precursors and remove interfering phases so that precursors can be identified and measured over a larger distance range removal of interfering phases reveals possible phase shifts in the underside reflection at the 660 that is s 660 s which if ignored could lead to biased discontinuity depth estimates the combination of data quantity and improved quality allows improved imaging and uncertainty estimation time to depth conversions after corrections for bathymetry crustal thickness and tomographically inferred mantle heterogeneity show that the mean depths of 410 and 660 beneath the central pacific are 420 3 km and 659 4 km respectively the mean mtz thickness 239 2 km is close to global estimates and suggests an adiabatic mantle temperature of 1 400 c for the central pacific depth variations of the 410 and 660 appear to be relatively small with peak to peak amplitudes of the order of 10 15 km the 410 and 660 are weakly anticorrelated and mtz is thinner beneath hawaii and to the north and east of the hotspot and thicker southwest of it the relatively small discontinuity topography argues against the presence of large scale more than 5 wide thermal anomalies with excess temperatures over 200 k across the transition zone the data used cannot exclude stronger thermal anomalies that are of more limited lateral extent or that are not continuous across the mtz

topography of the 410 km and 660 km discontinuities beneath the japan sea and adjacent regions by analysis of multiple scs waves the northwest pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities due to the sparse distribution of seismic stations in the sea previous studies mostly focus on mantle transition zone mtz structures beneath continents or island arcs leaving the vast area of the japan sea and okhotsk sea untouched in this study we analyzed multiple scs reverberation waves and a common reflection point stacking technique was applied to enhance consistent signals beneath reflection points a topographic image of the 410 km and 660 km discontinuities is obtained beneath the japan sea and adjacent regions one dimensional and 3 d velocity models are adapted to obtain the apparent and true depth we observe a systematic pattern of depression 10 20 km and elevation 5 10 km of the 660 with the topography being roughly consistent with the shift of the olivine phase transition boundary caused by the subducting pacific plate the behavior of the 410 is more complex it is generally 5 15 km shallower at the location where the slab penetrates and deepened by 5 10 km oceanward of the slab where a low velocity anomaly is observed in tomography images moreover we observe a wide distribution of depressed 410 beneath the southern okhotsk sea and western japan sea the hydrous wadsleyite boundary caused by the high water content at the top of the mtz could explain the depression the long history trench rollback motion of pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

improved regional and teleseismic p wave travel time prediction and event location using a global 3d velocity model a global validation dataset of 116 seismic events and 20 977 associated pn and p arrivals is used to assess travel time prediction and event location accuracy for the global scale 3d p wave velocity model called llnl g3dv3 simmons et al 2012 strong regional trends that are observed for ak135 travel time residuals are largely removed when llnl g3dv3 is used for prediction the 25th 75th quantile spread of travel time residuals is reduced by 30 40 at teleseismic distances and the spread is reduced by 60 at regional distances 16 epicenter error decreases when more data are used to constrain event locations until more than 40 arrivals times are used at which point epicenter error reduction tends to plateau median epicenter errors for the ak135 and llnl g3dv3 models plateau at 8 0 and 5 5 km respectively for teleseismic p datasets median epicenter errors for the ak135 and llnl g3dv3 models plateau at 12 0 and 4 0 km respectively for regional pn datasets we demonstrate that spatially correlated travel time residual errors for the ak135 model lead to increasing epicenter error when 40 to 100 pn arrivals are used to constrain the location the effect of correlated error is mitigated by llnl g3dv3 for which epicenter error steadily decreases to 4 km when 100 pn arrivals are used the median area of 0 95 epicenter probability bounds for ak135 and llnl g3dv3 are 1811 and 758 km2 respectively the ak135 ellipses are inflated to achieve the desired rate of true events occurring inside the probability region whereas llnl g3dv3 error ellipses based on empirical residual distributions cover the true location at the expected rate because location bias is minimal

the upper mantle discontinuities correlated or anticorrelated the discontinuities in the earth s upper mantle at 410km and 660km are frequently considered as phase changes which are exothermic and endothermic respectively from this follows that at laterally changing temperatures both discontinuities move in opposing vertical directions that means they are negatively correlated we attempt to test this hypothesis using travel times of seismic waves converted at both discontinuities underneath seismic stations the broadband stations used are located in areas of high e g western us and low heat flow e g central asia sclater et al 1980 we observe a clear positive correlation of the conversion times of both discontinuities which can however be explained with known lateral velocity changes above 410km from the scatter of our data we conclude that the limit of a possibly existing and negatively correlated topography of the two discontinuities is about 28km this is in contrast to recently claimed observations of about 35 km anticorrelated topography in the western pacific revenaugh jordan 1991 stammler kind 1992

depth of the upper mantle discontinuities beneath the east pacific rise we observe discontinuities at depths of 415 and 655 km beneath the northern east pacific rise epr the data consist of precursors to ss waves from 71 tangential component seismograms ranging in epicentral distance from 107 to 167 the mid points of the paths are beneath the northern epr region with about half the data bouncing beneath ocean younger than 9 ma the ss phases were aligned and then time shifted to account for the ocean age at the bounce point the data were then stacked to remove theoretical moveout for underside reflections from the transition zone the stacked results show clear underside reflections these values are slightly more shallow than recently derived average values for oceans but the depth difference between the discontinuities is almost identical to the average for all oceans shearer 1996 although the absolute depths are dependent on the velocity model used in the estimate the depth difference in the discontinuities is not sensitive to velocity structure our results suggest there are not unusually hot temperatures within the transition zone beneath the epr relative to average ocean this implies that deep upwelling beneath the ridge if it exists is passive

seismic observation of narrow plumes in the oceanic upper mantle 1 we have found seismic evidence for the existence of narrow about 200 km diameter and hot plumes in the upper mantle above the 410 km discontinuity they are located mainly under ocean islands not however under the central parts of major continents these results are obtained from comparing globally the observed travel time differences between ps conversions and ss precursors from the discontinuity at 410 km depth using published data and new observations from ocean island stations comparison of ss precursors and ps conversions however leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth than in the upper mantle above 410 km depth