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

interaction of mantle dregs with convection lateral heterogeneity at the core mantle boundary preliminary numerical models indicate that chemically denser material dregs at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle and might well form discontinuous aggregations below mantle upwellings a model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude in reasonable accord with recent seismological observations of vertical and lateral structure heat flux out of the core is probably strongly modulated laterally by mantle convection while mantle dregs will complicate and possibly amplify this effect such modulation may be relevant to long term 10 7 10 8 year variations in the magnetic field

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

melting curve of mg fe sio3 perovskite to 96 gpa evidence for a structural transition in lower mantle melts the melting temperature of mg fe sio3 perovskite the dominant mineral phase of the earth s lower mantle has been measured to a pressure of 96 10 gpa below 60 gpa we find good agreement with the previous measurement of heinz and jeanloz 1987a and above 60 gpa the melting curve of silicate perovskite has a small positive slope of 19 5 5 5 k gpa the melting point of mg0 9fe0 1 sio3 perovskite is 3800 300 k at 96 gpa by extrapolation to 136 gpa it is 4500 500 k at the core mantle boundary these values provide an upper limit to the geotherm through the solid mantle and they are compatible with recent estimates of the temperature in the core being high 4500 k at the core mantle boundary our melting curve implies that the volume change on melting increases at 60 gpa from vm 0 0 0 2 cm mole to vm 0 16 0 03 cm mole we derive an equation of state for the melt from our data and find that a highly coordinated structure is required to explain the molar volume of liquid mg fe sio3 at pressures above 20 gpa

systematic analysis of deep seismicity 200 centroid moment tensor solutions for earthquakes between 1977 and 1980 summary a systematic study of deep seismicity is performed using waveform data from the gdsn between 1977 and 1980 and the centroid moment tensor method of dziewonski et al two hundred new cmt solutions are found for events deeper than 100km their moments obtained using the same algorithm range between 7 x and 1 4 x lo2 dyne cm a factor of 20 000 a statistical analysis of the cmt parameters provides important conclusions among these the shift in origin time for deep events is found to be almost independent of the seismic moment providing an indication of the complexity of deep earthquakes the deviation from the double couple model appears to be correlated to the state of stress of the subducting slabs mechanisms showing down dip compression are generally dominated by the eigenvalue corresponding to the compression axis and down dip tension correlates with the predominance of the tensional eigenvalue comparison with moment tensors obtained by other authors using different waveform data shows good agreement between different methods comparison with published focal plane mechanisms confirms the precision and reliability of the cmt method examples of cmt solutions used to identify patterns of seismicity associated with deep fault planes are shown

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

dynamical effects on the core mantle boundary from depth dependent thermodynamical properties of the lower mantle a common assumption in modelling dynamical processes in the lower mantle is that both the thermal expansivity and thermal conductivity are reasonably constant recent work from seismic equation of state leads to substantially higher values for the thermal conductivity and much lower thermal expansivity values in the deep mantle the dynamical consequences of incorporating depth dependent thermodynamic properties on the thermal mechanical state of the lower mantle are examined with the spherical shell mean field equations we find that the thermal structure of the seismically resolved anomalous zone at the base of the mantle d is strongly influenced by these variable properties and in particular that the convective distortion of the core mantle boundary cmb is reduced with the decreasing thermal expansivity such a reduction of the dynamically induced topography from pure thermal convection would suggest that some other dynamical mechanism must be operating at the cmb

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

modelling d structure beneath central america with broadband seismic data deep south american events recorded at distances between 70 and 96 are used to constrain the lower mantle structure beneath central america employing data from the terrascope bdsn iris and cnsn canadian national seismic network broadband arrays we obtain a 1 d shear model with a 200 km thick d layer with a 3 velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of sh polarized s scs and scd the shear structure above d is similar to the preliminary reference earth model prem dziewonski and anderson 1981 preliminary reference earth model physics of earth planetary interiors 25 297 356 in terms of dt d and s sks differential times sv synthetics for this model assuming a prem core produce a satisfactory fit to the data except at the larger ranges to the north where the waveforms display rapid variation this feature is indicative of lateral variation of the type suggested by grand 1994 where the discontinuity becomes less well defined synthetics computed for a simplified 2 d cross section through this structure produce the scd phase and simple s at larger distances while scd is easily observed in the shear wave data there is little evidence of its counterpart in the p data by testing several models with different p velocity jumps at the same depth as the d jump in shear structure we conclude that the velocity jump must be less than 1 in fact prem provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower mantle structure beneath central america shows clear evidence for a discontinuity in s velocity but not in p

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

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

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

amplitudes of diffracted long period p and s waves and the velocities and q structure at the base of the mantle frequency and time domain analysis of amplitudes of long period 20 s p and s waves diffracted around the earth s core and registered at the wwnss and csn stations show that the velocities at the base of the mantle have slightly positive gradients this result substantiates the previous ray parameter study of mula and muller 1980 such velocity gradients at the base of the mantle imply that there is no significant departure from homogeneity and or adiabaticity of the base of the mantle from the rest of the lower mantle as suggested by for example bolt 1972 jones 1977 jeanloz and richter 1979 and doornbos and mondt 1979b in addition to this the q factor for the base of the mantle is found not to be less than 250

simultaneous stochastic inversion for geomagnetic main field and secular variation 2 1820 1980 we apply the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation to data from the period 1820 1980 producing two time dependent field models one for the period 1900 1980 the other for 1820 1900 we introduce several adaptations of our original formalism of the inverse problem we choose to represent the time dependency of the field by an expansion in chebychev polynomials we apply an alternative regularization condition in the time domain and we adopt a different method from that used in our earlier study for solving the resulting system of equations whereas our earlier study used only twentieth century observatory data we consider almost all the available data from the time interval 1820 1980 we examine the effects of the crustal field on our observations and find that existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances the first is sequences of measurements made at one particular site for example at permanent magnetic observatories in which case the crustal field produces temporal correlation of the errors in the observations we devise a method for accounting for these correlations the other is measurements made at satellite altitude the crustal field producing spatial correlation of the errors we show how using a very simple statistical model of the crustal field these correlations can be calculated rather than attempting to account directly for these correlations which would require without further assumptions massive computation we instead limit our data selection to points sufficiently well separated in space that the effect of the crustal field is minimal we are able to fit the data to within our estimates of the noise in the data yet the resulting models have lower power in the secular variation than previously postulated by some authors many of the features in the field at the core mantle boundary described in recent work by bloxham and gubbins are seen most clearly indeed this time varying representation of the field reinforces many of their earlier conclusions

effect of grain scale alignment on seismic anisotropy and reflectivity of shales the elastic properties and anisotropy of shales are strongly influenced by the degree of alignment of the grain scale texture in general an orientation distribution function odf can be used to describe this alignment which in practice can be characterized by two legendre coefficients we discuss various statistical odfs that define the alignment by spreading from a mean value in particular the gaussian fisher and bingham distributions we compare the statistical models with an odf resulting from pure vertical compaction no shear strain of a sediment the compaction odf may be used to estimate how the elastic properties and anisotropy evolve due to burial of clayey sediments our study shows that the three statistical odfs produce almost identical correspondence between the two legendre coefficients as a function of the spreading parameter so that the spreading parameter of one odf can be converted to the spreading parameter of another odf in most cases it is then sufficient to apply the spreading parameter for the odf instead of the two legendre coefficients the effect of compaction on the odf gives a slightly different correspondence between the two legendre coefficients from that for the other models in principle this opens up the possibility of distinguishing anisotropy effects due to compaction from those due to other processes we also study reflection amplitudes versus angle of incidence ava for all wave modes where shales having various odfs overlie an isotropic medium the ava responses are modelled using both exact and approximation formulae and their intercepts and gradients are compared the modelling shows that the s wave velocity is sensitive to any perturbation in the spreading parameter while the p wave velocity becomes increasingly sensitive to a perturbation of a less ordered system similar observations are found for the ava of the p p and p sv waves modelling indicates that a combined use of the amplitude versus offset of p p and p sv reflected waves may reveal certain grain scale alignment properties of shale like rocks

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

investigation of laterally heterogeneous shear velocity structure in d beneath eurasia seismic waves are used to explore large scale radial structure and lateral heterogeneity of shear velocity in the d region the chemical and thermal boundary layer at the base of the mantle we analyze long period horizontally polarized sh shear waveforms from earthquakes in the northwest pacific recorded in europe and the middle east for evidence of proposed laterally varying shear velocity stratification in d beneath eurasia specifically our data sample the lower mantle extending from beneath the arctic ocean near novaya zemlya ussr to under northern india using a reflectivity technique to model the sh waveforms we determine a locally stratified shear velocity model sgle that provides a good average travel time and waveform fit to over 80 of the data sensitive to d structure sgle is characterized by a rapid 2 75 increase in shear velocity approximately 290 km above the core mantle boundary similar to previously proposed models for this region unlike for other regions analyzed using similar waveform modeling techniques the percentage of data inconsistent with this average radial model is fairly substantial indicating that laterally heterogeneous d velocity structure is important in this region some waveforms lack the reflected arrival expected for a stratified model while others show evidence of two arrivals both possibly originating within d while our data are insufficient for direct inversion of the three dimensional structure we utilize several simple parameterizations of the waveforms to gain some understanding of the region both the relative amplitude and differential travel time variations of the waveforms indicate that d beneath eurasia is characterized by 1 broad regions in which the structure has large scale stratification giving coherent long period reflections with superimposed velocity variations that scatter shorter wavelength waves and 2 smaller regions which are sufficiently heterogeneous to scatter even the long period energy the scale length of the lateral heterogeneity in d that produces these variations is inferred to be somewhat less than 500 km which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle where heterogeneity scale lengths of approximately 500 1000 km appear to dominate these observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in d generating a locally stratified layer in some regions the effects of a diffuse or topographically variable d discontinuity must be further explored using two and three dimensional synthetic calculations before inversions for the detailed structure can be undertaken

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

dynamical influences from thermal chemical instabilities at the core mantle boundary two dimensional time dependent calculations show that the thermal and flow structures can be influenced by the lower mantle thermal chemical instabilities the style of the core mantle boundary cmb deformation in thermal chemical convection is different from that obtained in thermal convection the amplitude of cmb hills is reduced greatly from the depth dependence of thermal expansivity found in recent high pressure experiments

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

global shear velocity heterogeneities in the d layer inversion from sd sks differential travel times a global map of shear velocity in the d layer results from the inversion of 340 differential travel times of diffracted s sh minus sks sv sd sks from long period records of global seismic networks the two phase design reduces contamination from upper mantle heterogeneities and errors in location and origin time of the events additional corrections are made for 1 azimuthal anisotropy at stations where shear wave splitting parameters are available and for 2 travel time perturbations due to lower mantle asphericity although both effects are minor compared with the observed residuals with respect to the preliminary reference earth model prem dziewonski and anderson 1981 the corrected residuals ranging from 16 to 18 s are attributed to anomalies in d sampled by both phases taking these residuals as data and assuming a constant 250 km thick d layer we invert for a lateral velocity variation model of d using spherical harmonics in parameterizing d velocities a high degree expansion l 14 avoids aliasing but only the reliably determined low degree components li l produce optimal models the optimal model li 6 achieves a variance reduction of 56 and mean error 0 019 km s and the velocity is insensitive to the thickness of the d layer assumed a baseline shift of 0 022 km s from prem is resolved the magnitude of variation 0 2 km s or 3 is nearly twice as large as that of tanimoto 1990 and su et al 1994 while comparable to that of liu and dziewonski 1994 which includes s sks data and li and romanowicz 1996 degree 2 variation dominates the d with positive anomalies beneath the asian australian and american continents and southernmost pacific and negative anomalies beneath the pacific western africa atlantic and the southern indian ocean this pattern is similar to that found in previous tomograpic models significant discrepancies in nondegree 2 features are discussed in terms of the constraints from sd the fast and slow regions of d correlate with the inferred paleoslab positions and the major hotspot distribution respectively both at the 99 confidence level implications of these correlations for mantle dynamics are suggested the global model agrees reasonably well with the velocities determined regionally from the sd slowness measurement and the scs s residuals in other studies the latest radial models of d constrained from s triplications at distances of 75 90 provide a completely independent benchmark models for eurasia alaska and the caribbean predict sd travel times corresponding to 0 03 0 08 km s anomalies consistent with our lateral variation model in respective regions these vertical structures are also consistent with this study in scs times indicating that they agree in a vertically averaged sense major inconsistency exists for the india model syll young and lay 1987b which predicts sd and scs arrivals much slower than those expected from the 0 1 km s fast anomaly we found suggesting either the presence of strong lateral heterogeneities or the need for further mapping in this area

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

influence of heating mode on three dimensional mantle convection numerical models of three dimensional thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets as the proportion of internal heating increases the number of upwelling plumes increases and downwelling sheets become more vigorous and time dependent with any amount of basal heating the entire convective pattern during its evolution is anchored to the upwelling plumes as the proportion of internal heating increases the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow downwelling sheets carry only a minor fraction approximately 30 of the basal heat flow even when the shell is entirely heated from below but they advect almost all of the internally generated heat the relatively large number of plumes in the earth s mantle inferred from hotspots the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from within copyright american geophysical union 1989

s velocities in d from diffracted sh waves at the core boundary summary the slowness of the diffracted sh wave around the core boundary is re examined the criticism that measurements from the iranian earthquake of 1968 august 31 are affected by the focal mechanism is shown to be unjustified theoretically and by comparison with analogous recordings from the imperial valley earthquake of 1979 october 15 evidence is presented that an sh ray parameter estimated from first impetus sh diffracted onsets does not represent a shear velocity averaged over a hundred kilometres depth but rather the velocity within a few tens of kilometres of the core mantle boundary in addition to a re examination of the seismograms previously used for the 1968 august 3 1 iran earthquake long period seismograms for 80 150 have been studied for two tonga earthquakes 1967 october 9 and 1977 august 11 exclusion of readings with 105 from the iranian earthquake increases the gradient from 8 68 0 13 to 9 48 0 26s deg 1 the exceptionally large ray parameter is not fully explained but a plausible hypothesis is anisotropy in shear velocity near the core mantle boundary when observations are restricted to clear arrivals at distances exceeding a few degrees from the shadow boundary reliable new estimates are obtained for the sh slowness namely 8 41 0 07s deg 1 for tonga 1967 and 8 39 0 06 for tonga 1977 the statistical uncertainty of the estimates indicates no significant average change in sh velocity at the base of the mantle along two easterly profiles 3000 km apart under the east pacific and north america for a core radius of 3485 km and a slowness of 8 4s dec 1 the average velocity near the core mantle boundary is 7 24km s 1 this value is in agreement with estimates in d by mula muller the observed attenuation of the diffracted sh ground motion amplitudes at periods of 15 20s is similar for the three profiles considered with a reduction of an order of magnitude within a distance of 25 in the shadow zone this corresponds to an equivalent decay parameter 0 10 at the core boundary

observational evidence for diffracted sv in the shadow of the earth s core on the basis of observational evidence it has been assumed for a long time in the seismological literature that the diffracted s wave in the deep shadow of the earth s core is polarized as sh in good agreement with models that do not contain a well pronounced low velocity zone at the base of the mantle while this seems often to be the case broad band horizontal records from the geoscope network show that for some wavepaths the diffracted sv is strong enough to be clearly observed at distances between 107 and 117 moreover in some cases the diffracted sv is shifted with respect to sh by approximately a quarter period such observations should put additional constraints on the structure near the core mantle boundary among other possibilities lateral variations of the gradient at the base of the mantle allowing for regions with negative gradient and possibly azimuthal anisotropy in d should be considered in order to explain these data

gradual entrainment of a chemical layer at the base of the mantle by overlying convection summary a dense chemical layer may exist at the base of the mantle the overlying convection tends to entrain some of this layer reducing its thickness through time the lower more dense fluid is dragged into cusps beneath upwelling limits of the overlying convection and some material is entrained it is obvious that a high density of intrinsically more dense material retards this entrainment an upper limit and probably a reasonable estimate for the amount of entrainment is obtained by considering the carrying capacity of the vertical flow above cusps and ignoring the complications within the cusps themselves flow through conduits formed by the reduced viscosity of hot material is less effective at entrainment than broad scale flow controlled by the background viscosity of the normal mantle cylindrical plumes are less effective at entrainment than 2 d flow intrinsic density contrasts of around 6 per cent are needed for the layer to persist through geological time assuming the thermal expansion coefficients is 2 10 5 in addition the chemical layer may convect internally this causes the layer to entrain less dense material from above and thus aids entrainment from above by reducing the excess density of the layer convection in the chemical layer and stripping of the layer into cusps by the overlying flow also cause much of the temperature contrast to be in the overlying mantle rather than the chemical layer the difficulty of having more than a few 100 k temperature contrast across the layer unless it is internally stratified negates a reason for supposing the layer exists

melting temperature of the earth s mantle and core earth s thermal structure abstract although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs the temperature distribution in the lower mantle is poorly known models depend strongly on the assumptions of the amount of internal heating and the viscosity profile new melting data on iron to pressures of the outer core 2 mbar and the observed strong decrease of eutectic melting depression in the fe feo fes system with increasing pressure however tightly constrain the temperature at the inner core boundary to slightly less than 5000 k this estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements the resulting temperature at the core mantle boundary of about 4000 k then requires a large temperature gradient at the bottom of the lower mantle of about 1500 k recent findings of the very high melting temperatures o

slab diffraction of s waves seismograms are synthesized by superposition of gaussian beams for teleseismic s waves radiated by deep focus earthquakes located within the zone of high seismic velocities that define a descending lithospheric slab the s wave synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the s wave low frequency energy leaks out of the slab and travels to the receivers along paths in the lower velocity region surrounding the slab seismograms synthesized in fully three dimensional models of slab structure show that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab slab diffraction lengthens the pulse width of s waves in the entire azimuthal range on the dipping side of the slab as the azimuth approaches the strike of the slab the diffracted phase may evolve into a higher frequency impulsive phase which arrives after the minimum time arrival some slab models predict the slab phase to become a superposition of multipathed ray theoretical arrivals as the azimuth approaches the strike of the slab as the azimuth varies through the strike of the slab toward the side opposite the direction of dip the slab phase is rapidly extinguished rapid variations in the complexity of s waveforms recorded along azimuths parallel to the strike of the slab are consistent with the predictions of synthetic seismograms evidence of slab diffraction in the s waveforms of the deepest focus earthquakes supports the hypothesis of slab penetration beyond 650 km depth broadband s waveforms from deep focus earthquakes in the kuril kamchatka slab suggest that this slab penetrates below 650 km and that it may suffer some distortion in shape and advective thickening below this depth

lateral variations in p velocity and attenuation in the d layer from diffracted p waves abstract diffracted p waves recorded at two stations along the same great circle are analysed to specify the lateral variations in velocity and velocity gradient in the d layer at the base of the mantle data come from long period and broadband geoscope and gdsn stations the variations in both apparent ray parameter and amplitude decay are studied as a function of the dominant period of the p wave regional variations in the ray parameter indicate a 2 5 variation for the p velocity in d with high velocities beneath the north pacific central america and northern asia and low velocities beneath the south pacific south atlantic and northern indonesia the absence of correlation between these velocity variations and the lateral variations in p attenuation favour a chemical origin rather than a thermal origin for these heterogeneities on the other hand lateral variations in the velocity gradient are suggested from the frequency dependence of both ray parameter and p amplitude decay

constraints on lower mantle composition and temperature from density and bulk sound velocity profiles recent studies comparing density from the seismological model prem to that predicted for various perovskite and magnesiowustite phase assemblages under lower mantle conditions suggest that the lower mantle is a few percent denser than reasonable candidates for upper mantle composition such as pyrolite this has been interpreted as evidence for an iron enriched lower mantle in addition to density bulk sound velocity provides an important constraint upon proposed lower mantle compositions being independent of the shear modulus has several desirable characteristics experimentally it can be determined in the laboratory by density measurements under static compression seismologically it is not affected by attenuative dispersion and is expected to be much less laterally heterogeneous than p or s wave velocities we have calculated and along lower mantle adiabats as functions of iron magnesium content xmg and silica content xpv and compared them to values obtained from prem for a reasonable temperature estimate at the top of the lower mantle tlm of 2000 k we find that the data are most compatible with a lower mantle enriched in fe in agreement with previous studies in addition the data require that silica enrichment accompany this fe enrichment furthermore increasing tlm increases both the required fe and si content with a high assumed tlm 2700 k the data can be satisfied by low xmg values but even a pure perovskite lower mantle is too slow compared to thus assuming no free sio2 in the lower mantle this provides an upper bound on tlm of approximately 2700 k finally conclusions regarding the composition and temperature of the lower mantle are strongly dependent upon uncertainties in estimated thermoelastic parameters and upon choice of a seismological model most importantly neither fe nor si enrichment is required if the zero pressure volume coefficient of thermal expansion of perovskite is about 2 5 10 5k 1 rather than 4 0 10 5k 1 at high temperatures alternatively but less probably fe and si enrichment are not required if the average of the lower mantle is 1 0 less than given by prem the si enrichment is not required if the average of the lower mantle is 0 5 slower than pkem closer to the value given by the body wave model derived from the jeffreys bullen tables or if the lower mantle is anomalously cold with tlm 1700 k

simple models of fluid flow at the core surface derived from geomagnetic field models summary we examine the problem of determining the fluid flow at the core surface given a model of the temporal variation of the magnetic field seeking to fit the field over a finite time interval the flow is assumed steady over the time interval and we adopt the frozen flux approximation we produce a range of solutions of varying complexity and subject to different constraints estimate the errors on the solutions and assess their consistency with the underlying assumptions we find both direct evidence for ageostrophic flow in the form of flow across the geographical equator and indirect evidence in the form of poorer fits to the field when the flow is contained to be geostrophic on account of this evidence we do not adopt the geostrophic approximation however we find that unconstrained flows are poorly determined and consider the various alternatives for producing better determined solutions we choose to restrict our attention to determining the large scale toroidal part of the flow and find a simple pattern of flow which is nearly symmetric about the equator from a sequence of maps of the flow over 5 year time intervals back to 1915 we are still able to identify many of the basic aspects of this simple pattern but changes in the flow pattern are discernible although our models of the flow explain a large proportion of the signal typically 95 per cent of the variance the fits which we obtain are not as good as might be expected failure of the underlying assumptions together with our decision to map only the large scale toroidal part of the flow explains the poorer than expected fit

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

steady flows at the top of earth s core derived from geomagnetic field models select models of the main geomagnetic field and its secular variation are extrapolated to the base of an insulating mantle and used to estimate the adjacent fluid motion of a perfectly conducting outer core the assumption of steady motion provides formally unique solutions and is tested along with that of no upwelling the hypothesis of no upwelling is found to be substantially worse than that of steady motion although the actual motion is not thought to be steady the large scale secular variation at the top of the core can be adequately described by a large scale combined toroidal poloidal circulation which is steady for intervals of at least a decade or two the derived flows include a bulk westward drift but are complicated by superimposed jets gyres and surface divergence indicative of vigorous vertical motion at depth the circulation pattern and key global properties including rms speed upwelling and westward drift are found to be fairly insensitive to variations in modeling parameters

thermodynamically consistent decompression implications for lower mantle composition estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression in which zero pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities we show that this procedure may lead to biased estimates of the zero pressure density and compressibility of the mantle the bias can be eliminated or minimized by subjecting the equation of state fit to a thermodynamic constraint at zero pressure such inversions provide equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models we used this method to evaluate assemblages containing mg fe sio3 perovskite and mg fe o with fe fe mg ratios as high as 0 2 thermodynamic systematics were used to place bounds on the anderson gruneisen parameter s for perovskite we find a fairly large range of acceptable models for the composition of the lower mantle they range from uppermantle like compositions to assemblages that are enriched in both si and fe relative to the upper mantle however a pure perovskite lower mantle whether enriched in fe or not seems improbable small fe enrichments of the order of 1 given density contrasts across the 670 km discontinuity that are probably insufficient to prevent convective mixing of the upper and lower mantles we therefore suggest that unless the mantle is homogeneous in its major oxide components the lower mantle is likely to be enriched in fe by at least several percent

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

lateral variations at the base of the mantle from profiles of digital s diff data average long wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core diffracted sh waves shdiff there is significant lateral variation in the sdiff apparent ray parameters suggesting considerable lateral shear velocity anomalies in d the region just above the core mantle boundary cmb apparent ray parameters are determined by least squares fits through the pulse maxima of the instrument deconvolved ground displacements and heterogeneities are quantified through comparisons with reflectivity synthetic ray parameters corrections are applied to the data to account for the effects of the earth s ellipticity and of mantle heterogeneities along the shdiff upswing paths a total of 161 shdiff profiles were obtained greatly expanding the number of ray parameter measurements reported in previous studies most of the observed slownesses fall in the range between 8 2 and 8 8 s deg these correspond to slowness anomalies of 3 5 the most robust and extensive feature resolved is under the northern and northeastern pacific ocean the slowest values p 4 occur toward the southeast and there is a trend toward fast velocities moving to the northern and eastern rims of the pacific a model of d shear velocities is obtained by converting the slownesses to velocity anomalies superposing the d path profiles onto the cmb and applying a weighted moving cap spatial average this model agrees well with many current tomographic models both at large 5000 km and intermediate 1000 km scales the fact that this occurs with a different type of data and technique of analysis suggests that we are now able to recognize some of the smaller scale lateral variations at the base of the mantle locations of fast and slow velocity anomalies at the cmb are consistent with the model of cold paleoslabs ponding at the cmb and forcing d rock laterally to form hot aggregates that give rise to plumes in the mantle and hot spots at the surface

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

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

visualization of whole mantle propagation of seismic shear energy using normal mode summation correct representation of seismic waveforms propagating through the mantle from a 600 km deep earthquake is presented using graphic interpolation between synthetic seismograms computed across a grid of mantle depths and distances all torsional normal modes with periods above 12 s are summed to create 72 846 seismograms at depths between the surface and the core mantle boundary the resulting time snapshots show the manner by which seismic shear energy propagates around the core away from the source

regional analysis of d velocities from the ray parameters of diffracted p profiles in examining the slownesses of linear profiles of diffracted p arrivals we have discerned regional variations in the compressional velocity structure of d the base of the mantle three methods of determining the slownesses were tested concluding that the waveform cross correlation xc technique is more reliable over a wide range of epicentral distances than methods involving a linear regression through either the first motion or first peak arrival times xc slownesses suggest a statistically conservative range in d average velocities of 3 1 and reveal regions of very fast velocities under asia and the eastern pacific basin and of very slow velocities under the new guinea solomon islands region slownesses were found to correlate well with the tomographic model v3 of morelli and dziewonski 1986 derived from a different technique and data set synthetic seismograms were generated using a summation of spheroidal normal modes and at periods greater than 45 s the diffracted waves show much less variation and are very well modeled by prem

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

lower mantle thermal structure deduced from seismic tomography mineral physics and numerical modelling abstract the long wavelength thermal anomalies in the lower mantle have been mapped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high pressure mineral physics experiments these parameters are the depth variations of thermal expansivity and of the proportionality factor between changes in density and seismic velocity the giant plume like structures in the lower mantle under the pacific ocean and africa have outer fringes with thermal anomalies around 300 400 k but very high temperatures are found in the center of the plumes near the base of the core mantle boundary these extreme values can exceed 1500 k and may reflect large hot thermal anomalies in the lower mantle which are supported by recent measurements of high melting temperatures of perovskite and iron extremely cold anomalies around 1500 k are found for anomalies in the deep mantle around the pacific rim and under south america numerical simulations show that large negative thermal anomalies of this magnitude can be produced in the lower mantle following a catastrophic flushing event cold anomalies in the mid lower mantle have modest magnitudes of around 500 k a correlation pattern exists between the present day locations of cold masses in the lower mantle and the sites of past subduction since the cretaceous results from correlation analysis show that the slab mass flux in the lower mantle did not conform to a steady state nature but exhibited time dependent behavior

the nature of the earth s core the properties of the earth s core are overviewed with emphasis on seismologically determined regions and pressures and seismologically measured density elastic wave velocities and gravitational acceleration attention is given to solid state convection of the inner core and it is noted that though seismological results do not conclusively prove that the inner core is convective the occurrence and magnitude of seismic anisotropy are explained by the effects of solid state convection igneous petrology and geochemistry of the inner core a layer at the base of the mantle and contact metasomatism at the core mantle boundary and evolution of the core mantle system are discussed it is pointed out that high pressure melting experiments indicate that the temperature of the core is ranging from 4500 to 6500 k and a major implication of such high temperature is that the tectonics and convection of the mantle as well as the resulting geological processes observed at the surface are powered by heat from the core as a result of the high temperatures along with the compositional contrast between silicates and iron alloy the core mantle boundary is considered to be most chemically active region of the earth

mantle layering from scs reverberations 4 the lower mantle and core mantle boundary this is the final installment in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms in this paper long period sh polarized multiple scs phases reflected once from a mantle discontinuity first order reverberations are used to search for abrupt shear wave impedance contrasts in the lower mantle beneath the geographic regions sampled the depth interval of 1000 2600 km bullen s region d appears free of any distinct radial layering in agreement with the majority of recent seismic models and the notion of near adiabatic compression of a compositionally homogeneous lower mantle to pass undetected discontinuities in d must be either small sh polarized reflection coefficient r0 1 0 highly intermittent or subject to lateral depth variations well in excess of 50 km at greater depths corresponding to the d region of the lowermost mantle we find evidence for a reflector of long period seismic energy situated an average of 325 km above the core mantle boundary cmb similar to the discontinuity proposed by t lay and coworkers our results indicate a 4 4 increase in shear wave impedance and a 1 7 increase in density if the 2 75 increase in shear velocity proposed by lay and helmberger 1983 holds true for our study area the discontinuity is observed beneath less than one third of the 18 seismic corridors examined and does not appear to be a ubiquitous feature of the lower mantle this plus an observed correlation of discontinuity depth and lower mantle velocity heterogeneity and the lack of experimental evidence for deep mantle phase transformations strongly favor a compositional origin mantle side layering nearer the cmb perhaps associated with a thin chemical boundary layer would mimic the effects of the surface crust producing a complex wave train of first and higher order reverberations superposed upon all other reverberative interval arrivals if present and unaccounted for cmb structure would drive estimates of crustal structure and qscs obtained by multiple scs waveform inversion away from their true values compared to independent estimates our results admit little bias monte carlo tests are used to assess the sensitivity of this technique to cmb layering which decreases with decreasing boundary layer thickness nonetheless the results limit cmb transition zones in excess of 20 km to impedance contrasts less than 12 8 for a 40 km transition beneath much of the western pacific australia melanesia and indonesia