evidence for a sharp lateral variation of velocity at the core mantle boundary from multipathed pkpab rapid changes in differential travel times between the various pkp branches have been attributed to strong lateral velocity variations at the base of the mantle differential time pkpab df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to pkpab here we report on some well documented normal and multipathed pkpab phases along with rapidly varying differential time residuals observed for fiji events with paths sampling the northern edge of the mid pacific large slow structure beneath hawaii systematic analyses of these data with two dimensional models suggest the presence of a ridge shaped structure at the core mantle boundary containing abrupt reductions of p wave velocity of up to 10 this narrow ultralow velocity zone ulvz is located at the boundary between a large scale low velocity and high velocity structure corroborating recent dynamic models which propose that ulvzs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at hawaii

large scale structure at the core mantle boundary from diffracted waves at the base of the earth s mantle is a region called d which serves as a thermal and chemical boundary layer between the silicate mantle and the liquid iron outer core1 2 tomographic models of mantle compressional wave velocity vp have their worst resolution in d owing to limited ray sampling3 5 but have hinted at large scale lateral variations there here i use a new technique which has its greatest resolution within d to produce a map of the large scale vp variations within d the technique compares the arrival times of waves that have been refracted across and diffracted around the core mantle boundary cmb the diffracted waves travel a long way in d making them excellent probes of this layer and the differential technique removes many effects arising from ray paths outside of d ref 6 the new map provides an image of the continent sized variations at the cmb that is complementary to existing shear wave data7 8 and similarities between the map and projections of ancient subducted lithosphere9 verify previous suggestions10 of a strong coupling between surface plate tectonics and the base of the mantle

pkp differential travel times implications for 3 d lower mantle structure differential pkp ab df travel time measurements are compared directly with predicted residuals for mantle tomographic models to investigate the heterogeneity of the lowermost mantle much of the ab df time is accumulated in the lowermost portions of the mantle resulting in heavy weighting of 3 d lowermost mantle structure on the differential travel time perturbations comparisons of the observations with tomographic models show strong model dependence the best variance reductions are achieved using the recent p wave model of van der hilst et al 1997 up to 20 but the heterogeneity in the lowermost mantle may be 8 times larger than what shown by the tomographic model the predictions for present s wave tomographic models correlate poorly with the observations if we assume p velocity anomalies are proportional to s velocity anomalies we argue that s velocity anomalies in the lowermost mantle do not always track p velocity anomalies our study shows no obvious evidence of slab effect on the observed ab df anomalies and thus the use of pkp phases is likely to provide a powerful tool in mapping the p velocity structure in the lowermost mantle

scatter and bias in differential pkp travel times and implications for mantle and core phenomena differential travel times commonly used to eliminate unwanted near source and near receiver heterogeneity from travel time studies can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly in particular d and core structure studies can be biased if differential pkp times are used here we present differential pkp travel time measurements having significant azimuthal travel time anomalies consistent both in size and pattern with a near source slab effect and explore other sources of error in differential times

sediments at the top of earth s core unusual physical properties at the core mantle boundary have been inferred from seismic and geodetic observations in recent years we show how both types of observations can be explained by a layer of silicate sediments which accumulate at the top of the core as earth cools compaction of the sediments expels most of the liquid iron but leaves behind a small amount of core material which is entrained in mantle convection and may account for the isotopic signatures of core material in some hot spot plumes extraction of light elements from the liquid core also enhances the vigor of convection in the core and may increase the power available to the geodynamo

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

architecture and stratigraphy of alluvial deposits morrison formation upper jurassic utah this article presents a unique cross section of a 13 5 km 8 3 mi long by 150 m 492 ft thick stratigraphic interval containing braided stream and associated flood plain deposits the cross section is oriented approximately parallel to depositional strike this cross section is a resource for geoscientists and engineers interested in the measurements of stratigraphic architectural elements such as dimensions and continuity of facies tracts and facies associations stratigraphic and geographic changes in sandstone mudstone proportions net to gross and frequency and cause of vertical fluid communication between superposed reservoirs in addition to presenting this rich data resource observed and documented stratigraphic relationships add to our conceptual understanding of certain attributes of the stratigraphic process response system for example we show that like meandering rivers aggradational braided river systems also build levees and alluvial ridges providing the supra elevation above the adjacent floodplain to create extensive crevasse splay and channel complexes we show that superposed channel sandstone reservoirs may be brought into physical contact not only by the erosion of an upper sand into a lower sand but also by the vertical aggradation of the lower sandstone producing a pyramid on which a younger sandstone is deposited four stratigraphic cycles of increasing and decreasing accommodation sediment supply a s regimes are identified these cycles are recognized from systematic vertical changes in stratigraphic and sedimentologic attributes at the largest scale there is a progressive downhill shift in facies tracts recording a basinward stepping of the four stratigraphic cycles at the intermediate scale there is a systematic change in channel types from laterally amalgamated braided channels to vertically building braided channels to steer head distributary channels encased within lacustrine dominated fines at the smallest scale systematic and repeated vertical successions of facies occur within the three types of channel belts these systematic changes are related to progressive changes in the a s regime that occur during superimposed stratigraphic base level cycles of three different scales

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

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

seismic evidence for ultralow velocity zones beneath africa and eastern atlantic sks waveforms recorded at distances of about 110 are extremely useful to constrain seismic velocity structure at the base of the mantle sks waves near this distance develop a complicated interference pattern with the phases sp dks and skp ds we report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath south america from stations in europe and africa we model these data with two dimensional dome like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30 lower than in the preliminary reference earth model prem the spatial extent of these structures their position with respect to the sks core exit points and their seismic characteristics can not be uniquely determined however the presence of a dipping or a concaved upper interface is a key attribute of successful models models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior the most anomalous data correspond to sampling regions at the base of the mantle beneath the east african rift and beneath the iceland where possibly whole mantle upwellings form

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

tomographic inversion for three dimensional anisotropy of earth s inner core abstract seismological studies generally suggest that the earth s inner core is anisotropic and the anisotropic structure changes significantly both laterally and with depth previous body wave studies of the inner core have relied on ray tracing or waveform modeling using one dimensional 1d models here we present non linear tomographic inversions of the inner core anisotropy using three dimensional 3d ray tracing spline parameterization and a large collection of pkp differential travel times we adapt a pseudo bending ray tracing pbr method in spherical coordinates for seismic rays that traverse the inner core pkp df phase the method iteratively perturbs each discontinuity point and continuous segment of the ray through 3d earth structure so that its travel time is minimum the 3d anisotropic structure of the inner core is approximated to the first order as 3d heterogeneous but isotropic structure for a given ray the data are corrected using a scaled mantle tomographic model the inner core anisotropy model obtained has the following major features 1 the model has strong hemispherical and depth variation the isotropic velocity in the topmost inner core is greater in quasi eastern hemisphere qeh 40 160 e than in quasi western hemisphere qwh other longitudes the anisotropy is weak in qeh to the depth of 600 700 km below the inner core boundary icb while in qwh the anisotropy increases at much shallower depth about 100 200 km below the icb to about 3 4 then remains at about 2 4 throughout the rest of the inner core 2 the anisotropy form changes abruptly over a depth range of about 150 km at the radius of about 600 km slightly less than half of the inner core radius forming a distinct inner inner core iic the velocity in the iic has maximums at equatorial and polar directions and minimum at an angle of about 40 from the equatorial plane the velocity in the outer inner core oic however changes little for ray directions 0 40 from the equatorial plane 3 despite large variation of the anisotropy the isotropic velocity voigt average throughout the inner core is nearly uniform the results suggest that the oic is likely composed of the same type of iron crystals with uniform chemistry but the iic may be composed of a different type of crystal alignment a different iron phase or a different chemical composition our tests on model parameterization mantle correction and linear and non linear inversion suggest the main features of our model are very robust however fine scale structures are likely to differ particularly in the major transition zones e g in the topmost qwh isotropy to anisotropy between oic and iic change in the form of anisotropy and between qeh and qwh in oic difference in anisotropy strength searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

horizontal transition from fast to slow structures at the core mantle boundary south atlantic recent tomographic studies of the deep mantle have revealed large scale low velocity zones beneath africa and relatively fast velocity zones beneath south america here we conduct a concentrated core mantle boundary cmb study across this transition zone from fast to slow beneath the south atlantic deep south american earthquakes recorded in africa provide s and scs waveform data at core grazing distances of 85 105 the waveform interference pattern produced at these ranges provides an excellent sample of the cmb structure in the vicinity of the scs bounce point which can be used to map out regions of low velocity zones 10 reductions the strongest anomalies occur at the western edge roughly beneath the tristat plateau with weaker structures to the east beneath the so called great africa plume these results agree with dynamic models where strong local upwellings develop at the edges of d fast structures

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

constructing synthetics from deep earth tomographic models recent studies of deep mantle structure indicate strong heterogeneity to conduct high resolution waveform modelling of these structures we have developed a new method to construct 2 d synthetics directly from block style tomographic models unlike the wkbj approximation which utilizes rays overshooting and undershooting receivers our method wkm approximation uses rays that arrive at the receiver first the ray paths from the 1 d layered reference model are used to localize each ray segment where the anomalous velocities are applied by overlay as in tomography next new p i t i p i ray parameter ti traveltime are computed to satisfy snell s law along with their numerical derivative p t which is used to construct a synthetic seismogram similar to the wkbj method as a demonstration of the usefulness of this method we generated wkm synthetics for the d region of high velocities beneath central america based on grand s tomography model reasonable fits to broad band data are obtained by condensing his distributed anomalies into his lowermost mantle layer such a 2 d model predicts synthetics containing a laterally varying s cd triplication similar to observations

pkp bc df travel time residuals and short scale heterogeneity in the deep earth differential pkp bc pkp df travel time residuals for quasi polar paths in the earth present large excursions from the trends predicted by simple models of inner core anisotropy we analyze a set of such paths classified according to specific source and station regions and show that observations are compatible with a constant anisotropy model of at most 1 5 in the outer 400 km of the inner core to explain local departures of up to 3 sec from the predictions of such a model strong lateral variations on scale lengths shorter than a few hundred km need to be invoked either in the core or in the mantle independent results of d modelling argue in favor of an important contribution from the latter

interpreting d seismic structure using synthetic waveforms computed from dynamic models we have formulated dynamic models of processes hypothesized to give rise to observed seismological structure at the base of the mantle one dimensional seismic models are determined from thermo chemical convection calculations from which synthetic waveforms are computed of the three scenarios studied 1 a thermal slab 2 a thermal slab interacting with simple chemical layer and 3 a thermal slab interacting with a phase transition the strength of seismic phases computed from 3 best correlates with the global geography of the observed d triplication seismic structures predicted by 1 or 2 are inconsistent with the global geography and strength of the triplication our successful model motivates a new class of 1 d seismic models with two mild gradients and a small discontinuity to test this hypothesis we searched for seismic models that both contain these features and fit d seismic observations our preferred model contains a gentle positive gradient initiated 350 km above the cmb and a 1 jump in s velocity near 200 km a strong negative gradient begins about 100 km above the cmb similar to previous studies representing the lower thermal boundary layer with the reduced first order discontinuity the p wave triplication becomes too small to observe and provides a simple explanation for the observed weakness of the p in proportion to the s triplication

a very slow basal layer underlying large scale low velocity anomalies in the lower mantle beneath the pacific evidence from core phases a multi phase analysis using long period world wide standardized seismograph network and canadian network data has been conducted using core phases for deep focus events from the southwest pacific these include sks s2ks svd iff and sp dks the last phase emerges from sks near 106 and is associated with a p wave diffracting along the bottom of the mantle patterns in s2ks sks differential travel times t s2ks sks correlate with those in sp dks sks t spdks sks t s2ks sks values strongly depend on variations in v s structure in the lower third of the mantle whereas t spdks sks values mainly depend on v p structure and variations in a thin zone 100 km or less at the very base of the mantle anomalously large t s2ks sks and t spdks sks values relative to the preliminary reference earth model prem are present for fiji tonga and kermadec events recorded in north and south america along with anomalously large sv diff amplitudes well into the core s shadow more northerly paths beneath the pacific to north america for indonesian and solomon events display both prem like and anomalous times a model compatible with the observations is presented and contains a thin very low velocity layer at the base of the mantle that underlies the large volumetric lower mantle low velocity regions in the southwest pacific a low velocity layer of 20 100 km thickness with reductions of up to 5 10 relative to prem can reproduce t spdks sks as well as sv diff amplitudes large scale more than 1000 km lower mantle v s heterogeneity 2 4 can explain long wavelength trends in t s2ks sks the exact thickness and velocity reduction in the basal layer is uncertain owing to difficulties in resolving whether anomalous structure occurs on the source and or receiver side of wavepaths at the cmb

the effect of d on pkp ab df travel time residuals and possible implications for inner core structure abstract the hypothesis that the deep inner core is anisotropic is based on pkp travel time observations at large distances and relies on a small number of very anomalous measurements for paths quasi parallel to the earth s rotation axis here we analyze a global dataset of pkp ab df travel times residuals and discuss their significant dispersion 2 s and coherent large scale patterns we show that the trends observed for quasi equatorial paths are consistent with predictions from recent tomographic mantle models when the latter are modified to account for strong heterogeneity at the base of the mantle under the pacific ocean and africa as documented in several recent studies likewise for polar paths we show that a large part of the signal could be explained by deep mantle structure the effects of complex structure in the deep mantle on pkp ab df travel times should be carefully considered in order to reliably estimate the anisotropic structure of the central part of the inner core

pkp bc versus pkp df differential travel times and aspherical structure in the earth s inner core pkp bc versus pkp df differential travel times show coherent patterns of residuals indicating aspherical structure within the earth s inner core using a waveform correlation method we measure 318 differential pkp bc versus pkp df travel times from 5 years of global digital seismograph network gdsn short period data at ranges between 145 and 155 in addition we obtain 19 470 probable bc versus df travel times at these ranges by searching through 23 years of the international seismological centre isc catalog plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths this pattern is robust with respect to range and source depth these anomalies are almost certainly due to aspherical inner core structure since pkp bc and pkp df have nearly coincident ray paths in the mantle and outer core the gdsn and isc residual patterns are in general agreement but the isc residuals underpredict the gdsn residuals by about a factor of 2 the isc data have better spatial coverage particularly for n s ray paths within the inner core these n s ray paths show positive residuals of about 0 5 s indicating faster inner core p velocities in this direction a result consistent with previous studies of absolute pkp df travel times but derived from an independent data set due to the sparse ray coverage of our data either heterogeneity or anisotropy within the inner core with about 1 velocity variations could explain these residual patterns however inversions of the isc data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two parameter anisotropy model

a method for semi automated objective quantification of linear bedforms from multi scale digital elevation models the increasing availability of large detailed digital representations of the earth s surface demands the application of objective and quantitative analyses given recent advances in the understanding of the mechanisms of formation of linear bedform features from a range of environments objective measurement of their wavelength orientation crest and trough positions height and asymmetry is highly desirable these parameters are also of use when determining observation based parameters for use in many applications such as numerical modelling surface classification and sediment transport pathway analysis here we 1 adapt and extend extant techniques to provide a suite of semi automatic tools which calculate crest orientation wavelength height asymmetry direction and asymmetry ratios of bedforms and then 2 undertake sensitivity tests on synthetic data increasingly complex seabeds and a very large scale 39 000 km2 aeolian dune system the automated results arecompared with traditional manually derived measurements at each stage this new approach successfully analyses different types of topographic data from aeolian and marine environments from a range of sources with tens of millions of data points being processed in a semi automated and objective manner within minutes rather than hours or days the results from these analyses show there is significant variability in all measurable parameters in what might otherwise be considered uniform bedform fields for example the dunes of the rub al khali on the arabian peninsula are shown to exhibit deviations in dimensions from global trends morphological and dune asymmetry analysis of the rub al khali suggests parts of the sand sea may be adjusting to a changed wind regime from that during their formation 100 to 10 ka bp

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

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios