global p wave tomography on the effect of various mantle and core phases abstract in this work many global tomographic inversions and resolution tests are carried out to investigate the influence of various mantle and core phase data from the international seismological center isc data set on the determination of 3d velocity structure of the earth s interior our results show that when only the direct p data are used the resolution is good for most of the mantle except for the oceanic regions down to about 1000 km depth and for most of the d layer and pp rays can provide a better constraint on the structure down to the middle mantle in particular for the upper mantle under the oceans pcp can enhance the ray sampling of the middle and lower mantle around the pacific rim and europe while pdiff can help improve the spatial resolution in the lowermost mantle the outer core phases pkp pkikp and pkkp can improve the resolution in the lowermost mantle of the southern hemisphere and under oceanic regions when finer blocks or grid nodes are adopted to determine a high resolution model pp data are very useful for improving the upper mantle structure the resulting model inferred from all phases not only displays the general features contained in the previous global tomographic models but also reveals some new features for example the image of the hawaiian mantle plume is improved notably over the previous studies it is imaged as a continuous low velocity anomaly beneath the hawaiian hotspot from the core mantle boundary cmb to the surface implying that the hawaiian mantle plume indeed originates from the cmb low velocity anomalies along some mid oceanic ridges extend down to about 600 km depth our results suggested that later seismic phases are of great importance in better understanding the structure and dynamics of the earth s interior

lower mantle inhomogeneities inferred from pcp precursors a search of phases arriving between p and pcp the reflection from the core mantle boundary cmb was performed in the bulletins of the international seismological center bisc the distribution of the turning reflection points of anomalous waves in the lowermost mantle indicates the areas which are possibly anomalous regions the regions exhibiting these anomalies cover only a small fraction of the lowermost mantle sampled the areas which can be associated with anomalous arrivals are the lowermost mantle under the north pole region northern siberia central siberia western mongolia afghanistan northeastern china korea and the north atlantic ridge these are the regions which should be studied in more detail by analyzing the seismograms i e the amplitude and waveform information at the corresponding stations in addition to the travel time information reported to the isc

topography at the core mantle boundary analysis of the earth s gravity field using ratio of gravity to geoid g n and cumulative degree contribution curve techniques improves resolution of the mass depth and dimensions of anomalous mass sources within the planet the source for four of the ten major geoid anomalies of the earth has large contributions from harmonic degrees 2 and 3 lie at the core mantle boundary and the greatest has a mass anomaly of 4 2 1022 grams that boundary has a density contrast 4 4 gm cm which is greater than at any other location including the contrast at the earth s surface with space thus relatively small vertical undulations of that boundary produce very large mass anomalies topographic relief of less than 3 km at the core mantle boundary occurs and accounts for the existence of the largest geoid anomalies occurring on the earth

the resolution of whole earth seismic tomographic models summary we evaluate the resolution of whole earth structure achieved by compressional wave traveltime data from the international seismological centre isc the measure of resolution we employ provided by the direct calculation of the model resolution matrix is more rigorous than the traditional and computationally cheaper one obtained through synthetic checkerboard tests our work completes the introductive study of boschi 2003 where only mantle models derived with a very simple regularization scheme were considered here we expand boschi s database with measurements of compressional waves reflected by or refracted through the earth s core mantle boundary cmb and core in analogy with the work of soldati et al 2003 we treat cmb topography and heterogeneous outer core structure as free parameters of our inversions analysing model resolution matrices we attempt to explain the known discrepancy between deep earth structure mapped by seismic waves reflected and refracted by the core

core mantle boundary shape constraints inferred from the pressure torque acting between the core and the mantle summary in the last few years models of the flow at the top of the earth s core and of the related pressure field have been calculated from the secular variation of the geomagnetic field and core mantle topographies have been computed by seismologists a pressure torque results from the action of the pressure field on the core topography which can theoretically be computed from models of both the pressure field and the core mantle interface small scale features of the flow and of the topography are shown to be capable of contributing strongly to the pressure torque it is thus impossible to calculate the exact value of the torque from the knowledge of only the long wavelength components of the models but the interaction between the large scale components generates by itself torques two orders of magnitude larger than the torques inferred from the irregularities of the length of the day it is nevertheless possible to reconcile the topographic coupling mechanism with the length of the day observations keeping the amplitude of the core topography proposed by seismologists if an orthogonality relation between the geometry of the fluid upwellings and downwellings at the top of the core and the topography is satisfied it is shown how to compute such a topography for a given flow and close to an original topography provided by seismic tomography some consequences of the so inferred link between the fluid flow at the top of the core and the core mantle boundary topography are discussed

lower mantle velocity inhomogeneity observed at grf array grf broadband array recordings provide strong support for the existence of anomalous p coda phases along the kuriles european path first observed using wwssn data the data suggest that the phase termed here pdp arrives 3 5s after the direct p wave and has a slowness 0 7 0 8 s deg smaller the latter measurement precludes pdp s misidentification as a source stopping phase and given the magnitude of the slowness difference slab multipathing and diffraction are equally unlikely because the phase is clearly not observed for all kurile events and from timing and geometric considerations crustal reverberations are also untenable as an explanation for pdp s appearance the hypothesis which best explains the pdp arrival time arrival angle and waveform is the presence of a 3 velocity jump in the lower mantle approximately 290 km above the core mantle boundary because of the failure of other studies to observe pdp under favorable recording conditions the reflector must be of limited regional extent beneath northern siberia

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

a possible station bias in travel time measurements reported to isc a station bias in the travel times of p waves may exist in the international seismological centre isc data base it appears that low gain seismographs report systematically late arrival times average station delays confirm this with low gain stations generally slower than high gain stations

iron rich fragments in the yamansu iron deposit xinjiang nw china constraints on metallogenesis abstract volcanic rock hosted iron deposits are among the important iron ores in china however the nature of primary magma and petrogenesis associated with these iron ores remains controversial here we report iron rich fragments irf from the yamansu iron deposit in eastern tianshan mountains nw china which occurs in association with volcanic breccia submarine volcanic breccia and ignimbrite the irf is composed of five types including oligoclase iron oxide type oio oligoclase albite iron oxide type oaio albite iron oxide type aio albite k feldspar iron oxide type akio and k feldspar iron oxide type kio these fragments display typical volcanic fabric features such as porphyritic texture hyalopilitic texture of the groundmass and vesicles filled by minerals to form amygdales the feldspar phenocrysts of irf are dominantly albite the groundmass of irf consists of magnetite and feldspar the magnetite is distributed in between the feldspar laths and together display hyalopilitic texture which could be observed only in volcanic rocks the vesicles are filled with magnetite feldspar chlorite and calcite from the margin to the interior the irf has high si al fe ca ti na and k contents and low mg content the average total fe is 26 wt the magnetite is mostly titanium vanadium magnetite with the tio2 content ranging up to 4 86 wt and v2o3 content up to 3 20 wt the irf probably came from iron rich melts and represent the products of the fenner magma evolution the basaltic magma evolved into the fe na rich residual melts by crystallization under low oxygen fugacity condition in a closed magma chamber after intruding into the shallow crust the fe na rich residual melts were emplaced in hypabyssal environments or erupted generating the orebodies or providing the material source for the generation of the high grade iron ores which were subsequently enriched by the late stage hydrothermal fluids

constraints on core mantle boundary topography from normal mode splitting 1 the frequencies of earth s normal modes are split by rotation ellipticity and internal structure of the earth thus models of mantle heterogeneity and discontinuity topography generate splitting that may be tested against observations we insert maps of core mantle boundary cmb topography which are derived via either a purely seismic or a joint tomographic geodynamic inversion of body waves data on top of tomographic model s20rts we then calculate synthetic splitting functions for normal modes that have been shown to be sensitive to cmb topography and compare these to observed normal mode splitting data the cmb topography maps obtained via geodynamically constrained tomography fit normal mode data better than purely seismic maps in particular when the geodynamic constraint also accounts for the presence of post perovskite in the d 00 region we test the significance of the reduction in misfit using the concept of observability which suggests that normal modes are able to observe the difference between the different cmb topography maps in addition we find that the statistical significance assessed by checking what fraction of 1000 randomly generated cmb models achieve a comparatively good fit as the desired model is higher than 92 for degree 2 and 98 for all degrees in summary we have identified a model of cmb topography that fits body wave data and improves at least to some extent the fit to normal mode data and is coherent with the large scale pattern of deep mantle heterogeneity expected on the basis of convection modeling

stacking gravity tide observations in central europe for the retrieval of the complex eigenfrequency of the nearly diurnal free wobble summary we have used tidal gravity measurements from six stations in central europe to investigate the resonance in the diurnal tidal band caused by inertial coupling between the mantle and outer core of the earth by the use of stacking it was possible to determine the eigenfrequency and quality factor of this eigenmode commonly called the nearly diurnal free wobble we assessed the effect of systematic errors from the ocean correction to the tidal measurements employing a monte carlo method the observed eigenfrequency is 1 1 434 f 7 cycles per sidereal day and is significantly higher than predicted by theories the observed quality factor is 2 8 0 5 x 10

bias in reported seismic arrival times deduced from the isc bulletin summary the accurate timing of seismological data is crucial for most quantitative examinations in seismology we present evidence that traveltime data from many stations contain systematic variations in timing which can be identified by checking the median of station delay times as a function of time this function is expected to be constant but many deviations are found several hundred stations that report arrival times to the isc have been examined the median station delay times of almost 8 per cent of these stations show changes of more than 1 s and thus exceed the structural signal in the data temporal variations of 0 5 1 s are common changes in the distribution of observed earthquakes and other possible explanations of such variations have been tested and fail to explain most of the observations therefore the bulk of the observed changes must be caused by flaws in the timing of the data or by biased picking of arrival times for instance at one station with a strong annual variation of noise level the arrival times are on average picked several tenths of a second later during months with a high noise level because of their systematic nature these errors will not necessarily cancel out by using the large number of traveltimes in the isc bulletin and may therefore introduce a bias in many investigations if the observed timing variations are due to the recording equipment at the stations the errors will be present in the digital waveform data as well tomographic studies could potentially be affected but in particular studies of temporal variations of earth structure based on traveltime data e g inner core rotation need to be looked at with caution as results might be influenced by station effects the exact nature of the bias is study dependent and needs thorough investigation in each individual case

inference of core mantle boundary topography from isc pcp and pkp traveltimes we invert isc pcp and pkp absolute and differential traveltimes in an attempt to infer the long wavelength topography of the core mantle boundary cmb the data selection and processing methods are described and evaluated these travel time data are very noisy and the geographic distribution of the data is highly non uniform inhibiting reliable inference of cmb topography spatial averaging enhances the coherent component of the residual variance related to heterogeneity however the random component of the variance is much larger than the coherent component we show that for pcp data the coherent signal due to mantle heterogeneity overshadows that arising from the cmb and that the effects of mantle heterogeneity are mapped into our inferred cmb solutions the pcp data are not correlated across the spatial averaging bins and seem to have a strong bias due to small scale structure and or noise the non uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the cmb spatial patterns of cmb models inferred from different phases do not agree amplitudes of seismically inferred cmb undulations vary greatly the sensitivity of inferred cmb models to the processing spatial averaging procedure and inversion techniques are investigated topographic amplitudes increase strongly with increasing input residual variance the power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher based on this work we conclude that reliable inference of long wavelength cmb topography is not likely with the current isc data set or with a spherical harmonic expansion truncated to degree and order 6

step wise relocation of isc earthquake hypocenters for linearized tomographic imaging of slab structure a vast volume of seismic phase data computed from routinely determined hypocenter locations is presently available for seismic imaging techniques such as tomographic inversion and residual sphere analysis routinely reported earthquake hypocenters however can be in error by several tens of kilometers these biases in hypocenter location can result in the misidentification of seismic phases and the loss of structural signal in the seismic phase data thus the value of these data in constraining seismic images is significantly reduced furthermore hypocenter mislocations degrade the linearization of the tomographic problem and map into the images produced by tomographic inversion to obtain adequate reference hypocenters for linearized inversion we used iasp91 software and reported arrival times to reprocess isc hypocenters and phase data for northwest pacific earthquakes subsequently we inverted p and pp wave residuals for earth structure and source mis location we describe this step wise relocation of isc hypocenters which underlies the linearization of the tomographic inversion and compare the relocation vectors before and after inversion the hypocenter relocations determined prior to inversion are of the order of 10 km which is significant with regard to both the estimated standard errors and the effect on travel time residuals and systematic with regard to earthquake position in the subducted slab in contrast the spatial components of the relocation vectors determined upon inversion of p and pp residuals are up to an order of magnitude smaller do not reveal a correlation with location in the seismic zone and do not generally exceed the noise level the incorporation of pp data in the relocation was essential to remove the depth bias prior to inversion to retrieve slab signal that might have been absorbed in mislocation and to constrain the focal depth upon seismic inversion non linear inversion schemes will not be efficient in removing the depth bias or in retrieving lost slab signal unless seismic phases that contain additional information about the depth bias like the depth phase pp are used

the trade off between volumetric and topographic structure for seismic traveltimes 660 km topography and mantle structure summary synthetic body wave traveltime inversion experiments were performed to evaluate the effect of unmodelled 660km discontinuity topography on the inference of aspherical volumetric mantle structure synthetic residuals were computed for s waves refracted through a 660 km discontinuity with topography we employed a spherical harmonic parametrization for lateral variations of the topography of the 660 km discontinuity and the mantle velocity structure the radial dependence of the velocity structure in the mantle was parametrized in two ways smooth chebyshev polynomial functions and uniform shells the results of both para metrizations show that significant smearing of the input topographic signal appears in the models of volumetric mantle structure inferred from the synthetic data solving for higher order radial structure with the smooth chebyshev functions reduces the smearing but resolution is limited to the half wavelength of the radial basis functions more of the input synthetic residual variance is absorbed into the solution for volumetric structure by solving for higher order radial structure with smooth polynomials or with thinner shells directly below 660km depth however fundamental differences between the kernels for volumetric and topographic structure restrict recovery of the input signal to approximately 80 per cent this work points to the possible value of relating volumetric and topographic structure when inverting seismic observations this may be addressed by incorporating geodynamic modelling and mineral physics results into the modelling of seismic observations

tomographic inversions for mantle p wave velocity structure based on the minimization of l 2 and l 1 norms of international seismological centre travel time residuals we use international seismological centre isc p arrival data 1964 1987 and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three dimensional p velocity variations in earth s mantle source mislocations and station corrections we find that the maximum velocity perturbations produced by the l1 minimization approximately 4 in our final model are relatively insensitive to smoothing and damping constraints therefore using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum among the well resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the tonga new hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the japanese island arc and eastern asia continuity between these anomalies and shallower fast anomalies is not clear a fast anomaly extending from 670 km to 2070 km depth appears beneath eastern north america the caribbean and north central south america a broad fast anomaly appears beneath eastern asia just above the core mantle boundary as well as several slow anomalies under the pacific basin of comparable size both models contain a circum pacific ring of 2 lower velocities in the depth range 0 200 km associated with back arc basins high velocities over 2 associated with the continental shields tend to disappear below 400 km though a significant region of high velocity remains beneath the siberian platform in the 400 to 670 km depth interval

the possibility of lateral structure inside the core and its implications for nutation and earth tide observations summary we show that there can be lateral structure inside the fluid core caused by gravitational forcing from the mantle from the inner core or from topography on the core mantle or inner core outer core boundaries we describe a method for calculating the internal structure given knowledge of the forcing we discuss the possible effects of this structure on results for the core mantle boundary topography inferred from observations of the earth s forced nutations and diurnal earth tides we consider the possible implications of a thin low density fluid layer at the top of the core

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

analysis procedures at the international seismological centre abstract analysis at the international seismological centre isc falls into three main categories association location and quantification difficulties of association on a global scale are not always appreciated the isc currently analyses 40 50 events a day and as the first arrivals from any particular earthquake may span a time interval of up to 20 min readings from events occurring in different parts of the world may overlap considerably in time the present association algorithm depends on time only and results in many chance mis associations or even the synthesis of fictitious events at present these mis associations are rectified by seismologists intervention but full use of amplitude and period information could help to detect these errors automatically revision of locations follows standard least squares procedures based on jeffrey s method of uniform reduction and jeffreys bullen travel times locations are made from p phases only including crustal phases but other first arrivals and secondary phases are identified and residuals calculated if depth cannot be determined by geometric means a search is made for depth phases or failing this the depth is restrained to that given by another agency or to a conventional value no provision is made for local variations in travel time body wave magnitudes are allocated within the distance range 21 100 from reported readings of a and t or their ratio using gutenberg richter calibration curves surface wave magnitudes are calculated from the prague formula using reports of long period a and t in the distance range 5 160 but only values from stations at distances of 20 or more are used to determine an average ms for a particular event there is no provision for the determination of local magnitude other than to reproduce values assigned by local agencies improvements in these procedures could be made through the automatic association of station readings the introduction of local travel times and better determination of earthquake size particularly for local events

core mantle boundary topography and whole mantle convection the seismically observed topography on the core mantle boundary cmb and the velocities of the surface tectonic plates together constitute important observational constraints on viscous flow models of the earth s mantle we show here that the cmb deflections inferred by morelli and dziewonski 1987 may be explained in terms of a simple model of whole mantle convection and in the process we provide a new method for inferring the penetration depths of subducted slabs

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

detailed investigation of a spot on the core mantle boundary using digital pcp data abstract a number of deep earthquakes from the fiji tonga region were used to study the properties of the core mantle boundary cmb the investigation was restricted to an area with a diameter of only a few hundred kilometers at the cmb the use of a cluster of neighboring events yielding redundant sampling and rather good resolution the components of the investigation are i a classical stacking procedure to search for additional reflectors near the cmb ii a deconvolution of seismograms and a stack of the inferred reflector series for the same purpose iii a matched filter method to determine pcp travel times and pcp p amplitude ratios precisely and iv a comparison with synthetic seismograms to check the consistency of derived travel time residuals and pcp amplitude variations in the presence of cmb topography while we do not see any additional reflector for epicentral distances around 35 we detect by stacking seismograms recorded at 70 an additional phase which might be caused by a reflection from the top of d we show that pcp travel time residuals must not be interpreted as being caused exclusively by cmb topography an additional investigation of synthetics provides some constraints on dominant wavelengths and corresponding amplitudes of cmb undulations for wavelengths of 50 400 km the elevation cannot exceed 2 3 km if pcp is observed in this particular area

lateral variability in shallow marine ichnofabrics implications for the ichnofabric analysis method ichnological analysis commonly undertaken both in the field and in core studies is generally carried out in vertical logged successions this work documents the results of a detailed ichnofabric study of two highly bioturbated sandstone beds separated by a mudstone within the thick lower jurassic succession of the yorkshire coast uk the two sandstone beds contain relicts of hummocky cross stratification and rare bioclasts of bivalves and belemnites bioturbation of the beds is intense and the diversity of trace fossils is considerable trace fossils present include teichichnus rectus teichichnus isp a thalassinoides isp palaeophycus heberti diplocraterion parallelum rhizocorallium rosselia socialis schaubcylindrichnus isp chondrites isp planolites isp and phycosiphon incertum ichnofabrics were studied at 5 m intervals parallel to the modern cliff line which is parallel to the inferred jurassic palaeo coastline the results demonstrate that there is considerable patchiness to ichnofabric even in a comparatively simple depositional setting such as a storm influenced shelf this observation is in line with studies of the modern sea floor the implications of patchiness in ichnofabric are broad ranging but most importantly suggest that caution is needed in the use of ichnofabric analysis for intra regional correlations

p velocity structure of the core mantle boundary region inferred from pkp ab pkp bc differential travel times pkp ab pkp bc differential travel time residuals from the bulletins of the international seismological centre are inverted to retrieve the p velocity anomalies in the lowermost 300 km of the mantle we show that a realistic core mantle boundary cmb topography may have only a marginal contribution to the residuals the block parametrization used allows us to discard poorly sampled areas prior to a least squares inversion the main features are negative anomalies beneath eurasia and north america and a positive anomaly beneath the southwest pacific the inferred map is not consistent with other maps resulting from previous p and s velocity studies which are correlated at long wavelengths with each other the preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the cmb

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

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

amplitude of the core mantle boundary topography estimated by stochastic analysis of core phases abstract the core mantle boundary cmb topography is an important parameter for constraining the models of mantle dynamics and core mantle coupling however the various large wavelength seismological models of the cmb topography which have been obtained up to now are poorly correlated moreover their maximum amplitudes vary considerably from one model to another with values ranging from 4 to 12 km these large discrepancies may be due to the difficulty to separate in the travel time anomalies the contribution of the cmb topography from that of the highly heterogeneous d region at the base of the mantle in order to better constrain the amplitude of the cmb topography we perform a stochastic analysis of the core phases which sample the cmb as transmitted waves and or as reflected waves in particular we analyse underside reflected pkkp phases which help to discriminate between cmb topography and d structure and have in addition a great sensitivity to cmb topography the other phases used are the upperside reflected waves pcp and the transmitted waves pkp the analysis is performed on the travel time residual file obtained by engdahl et al engdahl e r van der hilst r buland r p 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 after earthquake relocation and phase re identification after careful travel time data selection the stochastic analysis allows us to separate coherent signal from random signal in the data at different length scales ranging from 300 to 1500 km then the cmb topography variance is obtained at the different length scales from a joint analysis of the coherent signal in the different core phases in taking into account their different sensitivities to cmb topography the estimated cmb topography variance has a significant signal in the wavelength range that we have investigated showing that 95 of cmb topography amplitude is in the range 4 0 km for wavelengths larger than 300 km this value decreases to 1 5 km for wavelengths larger than 1200 km indicating that the long wavelength cmb topography is significantly lower than previously proposed however it has not been possible to invert the data for deriving a map of the cmb topography a checkerboard pattern analysis reveals that despite the introduction of the pkkp phase the relative contributions of d heterogenieties and cmb topography cannot be separated with the presently available data