constraints on the core mantle boundary topography from p4kp pcp differential travel times 1 p4kp pcp differential travel times are examined to infer the core mantle boundary cmb topography a total of 362 p4kp pcp times are measured with a measurement error of 0 5 s the travel times are corrected for the earth s hydrostatic ellipticity and mantle heterogeneity using a p wave tomographic model spherical harmonic expansion up to degree 4 is adopted for model parameterization the p wave velocity heterogeneity is then estimated in the lowermost 150 km of the mantle to overcome the problem of underestimation of the velocity perturbation at the base of the mantle in the global 3 d p wave mantle model subsequently the cmb topography is inferred using the residues of the above processes since the odd degree components of the cmb topography are insensitive to the p4kp pcp times only the components of degrees 2 and 4 are solved for the resultant features indicate that the maximum amplitude of the cmb topography does not exceed 2 km with an uncertainty of less than 0 5 km a numerical test confirms that the pattern of degree 4 is more reliable with less amplitude recovery the obtained degree 4 pattern shows an amplitude of less than 1 km and indicates the presence of depressions under the circum pacific the central pacific and south africa

probing two low velocity regions with pkp b caustic amplitudes and scattering summary we probe low velocity regions of the lower mantle with pkp waves using south pacific earthquakes recorded at stations from the temporary ethiopia afar geoscientific lithospheric experiment array in ethiopia and the german regional seismic network grsn germany the ethiopian stations show strong scattering of pkpab waves which arrive as precursors to pkpdf they also show anomalously large amplitude diffractions from the pkp b caustic in contrast the events recorded at the grsn show scattering of pkpab as precursors to pkpdf but more normal amplitudes in the pkp b caustic diffractions to understand the cause for the observed amplitudes we test a large set of 1 d models with anomalies in p and s wave velocities and density in the lowermost mantle and outer core structures that can cause the observed large pkp b caustic amplitudes are low velocity regions just above the core mantle boundary of the order of 50 km thick for the south pacific to ethiopia path with velocity reductions of around 5 per cent for p and 25 per cent for the s wave velocity alternatively a 30 km thick lamella starting at 50 km above the core mantle boundary with 10 per cent p wave velocity reduction and 25 per cent s wave velocity reduction also produces a good fit to the data we find that the low velocity regions act as waveguides enhancing the amplitude of the pkp b caustic diffracted waves and causing superposition of pkp b diffractions with pkpdf the low velocity regions cannot be smooth layers but must show a certain degree of small scale heterogeneity to produce the high frequency scattering observed in the data the path to germany needs small scatterers in the lowermost mantle but the low velocity region is less strong compared with the south pacific ethiopia path a plausible explanation for these low velocity regions is the presence of melt inclusions perhaps entrained in small scale convective systems that act as high frequency wavefield scatterers in regions where these inclusions or convective systems are abundant they appear as a large scale low velocity zone to lower frequency waves these observations support ideas that ultra low velocity zones at the base of the mantle may be heterogeneous regions of melt rich material

kinematic considerations for mantle mixing recent experimental and computational studies tal and computational mixing studies in highly controlled show that islands u mixedrcgions that donot interactwith flows show that even seemingly inconsequential errors in the surrounding regions are ubiquitous features in chaotically computed velocity field of 3 d or time dependent flows can advecting fluids such is ands uitenaturall account for the geochemically inferred coexistence of apparently distinct long lived geochemical heterogeneity with relatively homo geneous regions of an actively convecting mantle these results also indicate that mixing patterns the set of islands and folds characterizing the large scale material advection are sensitive to small variations in the rheology of the fluid therefore interpretation of numerical simulations of mantle transport and mixing is less straightforward than currently supposed computational studies of analytic flow solutions with systematically introduced and controlled errors indi cate that mantle simulations are unlikely to accurately com pute individual trajectories for even moderate time but that tmjectory ensembles can be accuntely computed for long time significantly computations also indicate that mixing and transport resultsmay not evolve smoothly with increased rheological realism

sound velocity and density of magnesiowustites implications for ultralow velocity zone topography we explore the effect of mg fe substitution on the sound velocities of iron rich mg 1 x fe x o where x 0 84 0 94 and 1 0 sound velocities were determined using nuclear resonance inelastic x ray scattering as a function of pressure approaching those of the lowermost mantle the systematics of cation substitution in the fe rich limit has the potential to play an important role in the interpretation of seismic observations of the core mantle boundary by determining a relationship between sound velocity density and composition of mg fe o this study explores the potential constraints on ultralow velocity zones at the core mantle boundary

mixing of heterogeneities in the mantle effect of viscosity differences the effect of viscosity variations associated with large scale chemical heterogeneities on mixing in the mantle is studied for a model two dimensional problem low viscosity regions are rapidly deformed develop long tendrils and are entrained towards regions of surface divergence very viscous regions are only slowly stretched thus geochemical reservoirs can persist relatively undisturbed for long periods of time if they are o 10 100 times more viscous than the surrounding mantle high viscosity ratio blobs have a tendency to aggregate leading to the formation of large scale heterogeneities from smaller ones

the origin of d reflections a systematic study of seismic array data sets regional seismic studies of the earth s deep mantle indicate the presence of p and s wave reflectors at the top of the d layer such reflectors appear to be laterally intermittent occur at a range of depths and produce waveforms whose amplitude and polarity differ at different locations a number of possible explanations for d reflections have been proposed including a phase change from perovskite to post perovskite thermal or thermochemical discontinuities generated by subducted slabs alignment of anisotropic minerals and global or small scale chemical heterogeneity arising perhaps from core mantle interactions or primitive material however some of the observed heterogeneity in d structure may be related to lack of consistency between the style quality and number of seismic observations in different regions as well as complexity in wave propagation due to 3 d structure in this study we present a compilation of observations of d reflections for regions in which 1 a reflector is seen in both p and s wave arrivals and 2 consistent array seismology techniques have been applied to infer the properties of the reflectors using a recent monte carlo thermodynamic modelling method to convert changes in chemistry and mineralogy into changes in seismic velocity we compare the fit of different isotropic thermochemical structures to the observed seismic properties of d reflections namely the polarity and amplitude of the waveforms our results indicate that it may be possible to discriminate between different possible origins for d reflections on the basis of polarity observations alone amplitude measurements may provide further constraints but must be treated with caution due to a large number of factors which can influence waveform amplitudes in regions such as the caribbean which are characterized by high amplitude negative polarity p waveforms and positive polarity s waveforms a phase change from perovskite to post perovskite provides the most likely explanation for the discontinuity however in other regions notably siberia and the central pacific p and s waveforms both display a positive polarity and multiple explanations for the discontinuity are possible including subduction related chemical heterogeneity and anisotropy

strong seismic scatterers near the core mantle boundary west of mexico pkpphasesobservedinjapan showstrongpre cursors with clear onsets for earthquakes that occurred in south america the onset times of the pkp precursors can allbeexplainedbyexistenceofseismicscatterersinthelow ermost 100 kmof themantlewest of mexico withinan area of 200 km300 km forward modeling of pkp precursor amplitudes indicates that a p wave velocity variation of at least 6 is required to explain the large amplitudes of the observed pkp precursors suggesting that the core mantle boundary region beneath west of mexico is a highly anoma lous region characterized by strong seismic scatterers

seismic evidence for earth s crusty deep mantle abstract seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into earth s lower mantle however the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep earth we present high resolution seismic array analyses of anomalous p waves sampling the deep mantle and deterministically locate heterogeneities in the lowermost 300 km of the mantle spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km the heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle large low velocity provinces llvps consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo chemical piles the size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust these results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust stirred by mantle convection and modulated by long lasting thermo chemical structures the preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo chemical origin of the llvps our observations relate to the mixing behaviour of small length scale heterogeneity in the deep earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle

annual cycle of subsurface thermal structure in the tropical atlantic ocean abstract the subsurface thermal structure in the tropical atlantic ocean 30 n 20 s east of 80 w is studied on the basis of an extensive data bank of subsurface soundings calendar monthly maps are presented showing mixed layer depth base of thermocline thermocline thickness and vertical temperature gradient across the thermocline these maps are complemented by vertical cross sections depicting mixed layer depth base of thermocline and selected isotherms a zonal profile along the equator 50 w 10 e a meridional transect across the eastern atlantic 4 n 18 s and a meridional section across the central atlantic 30 n 18 s the basinwide subsurface thermal structure is dominated by the annual cycle of the surface wind field with extrema around april and august the mixed layer is relatively shallow between 20 n and 10 s with overall grater depth in the western as compared to the eastern portion of the basin two systems of annual cycle variation of mixed layer depth stand out i along the eq

normal mode sensitivity to earth s d layer and topography on the core mantle boundary what we can and cannot see the core mantle boundary cmb is earth s most profound internal boundary separating the liquid iron outer core and the solid silicate mantle the detailed structure near the cmb has a major influence on mantle convection and the evolution of the core seismic observations such as topography on the cmb thin ultra low velocity zones ulvzs seismic anisotropy and the anticorrelation between shear wave and bulk sound velocity heterogeneities have mainly been made using body waves and are still poorly constrained we investigate the sensitivity of earth s free oscillations to these features and specifically show how large individual anomalies must be for them to be observable in addition we discuss the possible trade offs between these different lowermost mantle structures although modes have strong sensitivity to all the structures inserted the results illustrate the limits of what normal modes can resolve our tests show that i even small scale features such as ulvzs with a thickness larger than 19 km can be observed as long as their distribution contains a long wavelength component ii the peak to peak amplitude of cmb topography has a larger influence than its pattern and has to be smaller than 5 km to fit the data iii the effect of scaling between shear wave velocity and density anomalies is less constrained but a laterally varying pattern is implied by a simple test suggesting the presence of chemical variations iv a strong trade off exists between anisotropy in compressional wave velocity and incidence angle whereas shear wave anisotropy is less observable these findings provide valuable information for future normal mode studies on structures in earth s lowermost mantle and their trade offs

anisotropy of heterogeneity scale lengths in the lower mantle from pkikp precursors summary the anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its evect on the high frequency precursors of pkikp scattered by the heterogeneities although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short period precursors the frequency content of broad band pkikp precursors favours a dominantly isotropic distribution of scale lengths precursor coda shapes are consistent with 1 per cent fluctuations in p velocity in the wavenumber band 0 05 0 5 km 1 extending to 1000 km above the core mantle boundary and with a d region open to circulation throughout the lower mantle the level of excitation of pkikp precursors observed in the frequency band 0 02 2 hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber anisotropy of scale lengths may exist in a d layer having larger horizontal than vertical scale lengths and produce little or no detectable evects on pkikp precursors for p velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres and much higher when averaged over scales of hundreds of metres or less

inhomogeneities near the core mantle boundary inferred from short period scattered pkp waves recorded at the global digital seismograph network we analyze short period pkp precursor wave trains recorded at global digital seismograph network stations in the distance range 120 140 to infer the nature of inhomogeneities near the core mantle boundary travel times and particle motions are consistent with predictions of single scattered waves near the cmb the dominant frequencies are around 1 hz the regions best sampled are beneath indonesia north america central africa and south america based on first order scattering theory the characteristics of the wave number spectrum of the structural inhomogeneities are obtained for two hypotheses volume inhomogeneities and topographic irregularities for the range of wavelengths sampled by these data between 10 and 70 km the spectrum of inhomogeneities is best represented by a power law of index 5 3 for volume inhomogeneities and 6 8 for topographic irregularities if the scattering is due only to volumetric inhomogeneities we estimate the strength of the structural variations to be about 0 5 in p wave velocity for a 200 km thick layer if the scattering is due only to topographic irregularities the topography has an rms height of 280 m at present we can not distinguish between these two types of inhomogeneities

bulldozing the core mantle boundary localized seismic scatterers beneath the caribbean sea images of the scattering field near the core mantle boundary cmb beneath the caribbean were obtained from migration processing of pkikp precursors recorded by a broadband seismic array in the southern caribbean from earthquakes in the western pacific strong seismic scatterers were found to be concentrated near the cmb north of colombia and are surrounded by high velocity anomalies associated with the subducted farallon plate the relative location between the imaged scatterers and the surrounding high velocity anomalies leads us to speculate that the observed seismic scatterers are remnants of the subducted slab as the farallon plate penetrated into the lower mantle segregation allowed its basaltic crust to descend more quickly and to pool in front of the major portion of the subducting slab the ponding oceanic crustal material could be further collected or bulldozed by the residual motion of the subducting plate forming a localized region with strong seismic scatterers

strong seismic scatterers near the core mantle boundary north of the pacific anomaly abstract tomographic images have shown that there are clear high velocity heterogeneities to the north of the pacific anomaly near the core mantle boundary cmb but the detailed structure and origin of these heterogeneities are poorly known in this study we analyze pkp precursors from earthquakes in the aleutian islands and kamchatka peninsula recorded by seismic arrays in antarctica and find that these heterogeneities extend 400 km above the cmb and are distributed between 30 and 45 n in latitude the scatterers show the largest p wave velocity perturbation of 1 0 1 2 in the center 160 180 e and 0 5 to the west and east 140 160 e 180 200 e scs s differential travel time residuals reveal similar features we suggest that these seismic scatterers are the remnants of ancient subducted slab material the lateral variations may be caused either by different slabs or by variations in slab composition resulting from their segregation process

core mantle boundary topography from short period pcp pkp and pkkp data abstract we use a total of 839 369 pcp pkpab pkpbc pkpdf pkkpab and pkkpbc residual travel times from bull seism soc am 88 1998 722 grouped in 29 837 summary rays to constrain lateral variation in the depth to the core mantle boundary cmb we assumed a homogeneous outer core and the data were corrected for mantle structure and inner core anisotropy inversions of separate data sets yield amplitude variations of up to 5 km for pcp pkpab pkpbc and pkkp and 13 km for pkpdf this is larger than the cmb undulations inferred in geodetic studies and moreover the pcp results are not readily consistent with the inferences from pkp and pkkp although the source receiver ambiguity for the core refracted phases can explain some of it this discrepancy suggest that the travel time residuals cannot be explained by topography alone the wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and cmb topography in a second experiment we therefore re applied corrections for mantle structure outside a basal 290 km thick layer and inverted all data jointly for both cmb topography and volumetric heterogeneity within this layer the resultant cmb model can explain pcp pkp and pkkp residuals and has approximately 0 2 km excess core ellipticity which is in good agreement with inferences from free core nutation observations joint inversion yields a peak to peak amplitude of cmb topography of about 3 km and the inversion yields velocity variations of 5 in the basal layer the latter suggests a strong trade off between topography and volumetric heterogeneity but uncertainty analyses suggest that the variation in core radius can be resolved the spherical averages of all inverted topographic models suggest that the data are best fit if the actual cmb radius is 1 5 km less than in the earth reference model used i e the average outer core radius would be 3478 km

fine scale ultralow velocity zone structure from high frequency seismic array data 1 variability of core mantle boundary cmb structure is imaged at the kilometer to tens of kilometers scale by intensive waveform modeling of the scp phase recordings of earthquakes in the tonga fiji subduction zone from the small aperture 10 km high frequency 2 4 hz alice springs array in central australia are used to document anomalous scp waveform variations due to the presence of an ultralow velocity zone ulvz at the cmb these data expand coverage from previous results using the warramunga array which is located slightly to the north and indicate the existence of a geographically confined ulvz at the cmb roughly 100 km in lateral extent south of new caledonia and east of australia synthetic modeling indicates seismic velocity reductions of 8 and 24 for p wave and s waves respectively and a well constrained density increase of 10 in a 8 5 km thick layer above the cmb this ulvz patch is well explained by an elevated iron content of partially molten mantle material likely present as divalent oxidized iron in the low spin state the properties of such an iron enriched assemblage are consistent with both an elevated density and bulk modulus of this region as are dictated from our best fit seismic results furthermore a positive velocity gradient within the ulvz may be present which need not be associated with a decrease in melt content within this region but could rather be associated with a shift in the textural distribution of melt within the ulvz

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

dipping low velocity layer in the mid lower mantle evidence for geochemical heterogeneity data from western united states short period seismic networks reveal a conversion from an s to a p wave within a low seismic velocity layer greater than or equal to the 4 percent velocity difference compared to the surrounding mantle in the mid lower mantle 1400 to 1600 kilometers deep east of the mariana and izu bonin subduction zones the low velocity layer about 8 kilometers thick dips 30 to 40 southward and is at least 500 kilometers by 300 kilometers its steep dip large velocity contrast and sharpness imply a chemical rather than a thermal origin ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low velocity layer because of its broad thin extent

record of large late pleistocene outburst floods preserved in saanich inlet sediments vancouver island canada abstract two anomalous gray silty clay beds are present in odp cores collected from saanich inlet vancouver island british columbia canada the beds which date to about 10 500 14c yr bp 11 000 calendar years bp contain tertiary pollen derived from sedimentary rocks found only in the fraser lowland on the mainland of british columbia and washington just east of the strait of georgia abundant illite muscovite in the sediments supports a fraser lowland provenance the clay beds are probably distal deposits of huge floods that swept through the fraser lowland at the end of the pleistocene muddy overflow plumes from these floods crossed the strait of georgia and entered saanich inlet where the sediment settled from suspension and blanketed diatom rich mud on the fiord floor the likely source of the floods is late pleistocene ice dammed lakes in the fraser and thompson valleys which are known to have drained at about the time the floods occurred

rolling hills on the core mantle boundary recent results suggest that an iron rich oxide may have fractionally crystallized from a primordial magma ocean and settled on the core mantle boundary cmb based on experimental results the presence of only a few percent of fe rich oxide could slow seismic waves down by several percent this heavy layer can become highly undulating as predicted from dynamic modeling but can remain as a distinct structure with uniform velocity reductions here we use the large usarray seismic network to search for such structures strong constraints on d 00 are provided by the core phase sks where it bifurcates containing a short segment of p wave diffractions pd when crossing the cmb called sksd synthetics from models with moderate velocity drops less than 10 involving a layer with variable thickness perhaps a composite of sharp small structures with strong variation in thickness can explain both the observed sksd waveforms and large scatter in differential times between skks and sks a smooth 3d image is obtained from inverting sksd waveforms displaying rolling hills with elongated dome like structures sitting on the cmb the most prominent one has an 80 km height 81 length and 41 width thus adding still more structural complexity to the lower mantle we suggest that these results can be explained by a dynamically stabilized material containing small amounts 5 ironrich mg fe o providing a self consistent physical interpretation

pkp precursors implications for global scatterers precursors to the core phase pkp are generated by scattering of seismic energy from heterogeneities in the mantle here we examine a large global data set of pkp precursors in individual seismograms and array data to better understand scattering locations the precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase pkikp and mantle phase pp we find and correct for a hemispherical asymmetry in the precursor pkikp amplitudes resulting from inner core structure using ray tracing we locate scatterers in our array data and use these to infer scattering locations in the individual data the scattering strength displays regional variation however we find no relationship with long scale core mantle boundary velocity structure scattering is observed in all regions of data coverage as are paths with no precursors this indicates that scattering occurs from various small scale heterogeneities including but not limited to ultralow velocity zones or partial melt and slabs

large angle reflection from a dipping structure recorded as a pkikp precursor evidence for a low velocity zone at the core mantle boundary beneath the gulf of mexico abstract we observed a clear phase like arrival prior to the pkikp wave at a broadband seismic array in eastern tibet from an intermediate depth earthquake occurring in guatemala the measured incident angle and back azimuth of this phase indicate that it originates from scattering near the core mantle boundary cmb of the source side this phase however was not observed from another earthquake that is only 60 km away suggesting that scattering depends strongly on the angles of the incident waves ray tracing and diffraction migration indicates that the precursor is a large angle reflection from a dipping structure in the lowermost mantle east of mexico the seismic reflector dips northward by 52o and is centered at 93 31ow and 23 89on with an east west extension of 100 km a decrease of p wave velocity by a few to 10 is required to explain the amplitude and polarity of the phase it is unlikely to explain the large p wave velocity contrast and the large dipping feature with the post perovskite phase transition the reflector is located in a region where the lowermost mantle is marked by a high velocity anomaly related to the subducted farallon slab previous numerical modeling suggested that a substantial amount of hot mantle could be trapped beneath a slab over long periods of time leading to the formation of a mega plume thus the observed sharp dipping boundary here might correspond to the edge of a low velocity zone below the subducted farallon slab mixed results on the presence of an ultra low velocity zone in the region have been reported by previous seismic studies we speculated that the low velocity zone below the cold slab was developed as a consequence of long term heating from core

on the dynamics of equatorial subsurface countercurrents abstract the equatorial subsurface countercurrents sscc are strong steady geostrophically balanced eastward flows situated below the high speed core of the equatorial undercurrent euc at 3 5 n and s the dynamics of these currents are explored using a continuously stratified vertically diffusive linear steady state ocean model forced by zonal winds with effectively no wind stress curl model results agree favorably with observations in that both euc and sscc like structures are generated a diagnosis of the model momentum vorticity and continuity balances at various depths and latitudes reveals that the sscc lie outside a vertically diffusive equatorial momentum boundary layer so that both components of velocity are geostrophically balanced they are however located at the poleward of a broader diffusive equatorial vorticity boundary layer within this boundary layer cyclonic vorticity associated with the euc diffuses to the level of the sscc where it is balanced by poleward advection of pla

a compositional origin to ultralow velocity zones we analyzed vertical component short period scp waveforms for 26 earthquakes occurring in the tonga fiji trench recorded at the alice springs array in central australia these waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow velocity zone ulvz layering beneath the coral sea we used the viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival scsp and the precursor arrival spcp relative to scp we compare our measurements to a database of 340 000 synthetic seismograms finding that these data are best fit by a ulvz model with an s wave velocity reduction of 24 a p wave velocity reduction of 23 a thickness of 8 5 km and a density increase of 6 this 1 1 vs vp velocity decrease is commensurate with a ulvz compositional origin and is most consistent with highly iron enriched ferropericlase

intense seismic scattering near the earth s core mantle boundary beneath the comoros hotspot i report strong seismic pkp precursors recorded in the seismic stations in tanzania for earthquakes occurred in the fiji subduction zone the observed pkp precursors show clear onsets and rapid variations of amplitude for different events and stations i develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies the travel time analysis indicates that these recorded pkp precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the comoros hotspot the amplitudes of the observed pkp precursors suggest a rather complex seismic structure with a p velocity variation of at least 8 likely associated with partial melt

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

a new paradigm for earth s core mantle boundary understanding the boundary between earth9s rock mantle and the molten core is one of the key challenges in geoscience in his perspective garnero discusses recent geophysical discoveries that are forcing the adoption of a new paradigm for studying the core mantle boundary long thought to be a simple division between solid silicate rock and liquid iron alloy the core mantle boundary is now emerging as an active complex and heterogeneous region of earth9s interior

small scatterers in the lower mantle observed at german broadband arrays seismograms of earthquakes from the south pacific recorded at a german broadband array and network show precursors to pkpdf these precursors mainly originate from off path scattering of pkpab or a nearby pkpbc to p for receiver side scattering or from scattering of p to pkpab or pkpbc on the pkpdf path for source side scattering standard array processing techniques based on plane wave approximations such as vespagram or frequency wavenumber analysis are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 300 km for short period waves we therefore develop a migration method to estimate the location of scatterers in the mantle at the core mantle boundary and at the top of the outer core with our method we are able to find isolated scatterers at the source side and the receiver side although the depth of the scatterer is not well constrained however from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place scattering volume we believe that the location of the scatterers is in the lowermost mantle since we have detected scatterers in regions where ultralow velocity zones have been discovered recently we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle comparing results from broadband and band pass filtered data the detection of small scale structure of the ultralow velocity zones becomes possible

locating scatterers in the mantle using array analysis of pkp precursors from an earthquake doublet abstract well separated individual pkp precursors observed at the yellowknife seismic array yk for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation based on the comparison of the waveforms of stacked individual precursors and those of pkikp phases we are able to determine that most of these precursors originate from scattering of the pkpbc rather than the pkpab branch above the b caustic on the receiver side this allows a reliable location of the scatterers in the lower mantle their depths range from 2890 km the cmb to 2270 km scattering angles range from 45 8 to 16 0 and surface projections range from southern ontario to northern saskatchewan in canada these locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north america this suggests that the subducted slab remnants under north america have retained their compositional signature the fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended which should be confirmed using broadband data average differences in measured slowness and back azimuth of the doublet precursors are as small as 0 08 s deg and 1 4 deg respectively our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays

analysis of pkp scattering using mantle mixing simulations and axisymmetric 3d waveforms abstract the scattering of pkp waves in the lower mantle produces isolated signals before the pkikp phase we explore whether these so called pkikp precursors can be related to wave scattering off mid ocean ridge basalt morb fragments that have been advected in the deep mantle throughout geologic time we construct seismic models of small scale 20 km heterogeneity in the lower mantle informed by mantle mixing simulations from brandenburg et al 2008 and generate pkikp precursors using 3d axisymmetric waveform simulations up to 0 75 hz we consider two end member geodynamic models with fundamentally different distributions of morb in the lower mantle our results suggest that the accumulation of morb at the base of the mantle is a viable hypothesis for the origin of pkp scattering we find that the strength of the pkikp precursor amplitudes is consistent with p wave speed heterogeneity of 0 1 0 2 as reported previously the radial distribution of morb has a profound effect on the strength of pkikp precursors simulation of pkikp precursors for models with an increasing morb concentration in the lowermost 500 km of the mantle appears to reproduce most accurately the strength of pkikp precursors in global seismic network waveforms these models assume that morb has an excess density of at least 7 additional simulations of more complex geodynamic models will better constrain the geodynamic conditions to explain the significant variability of pkp scattering strength

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

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

an analysis of large scale variations in small scale mantle heterogeneity using global seismographic network recordings of precursors to pkp high frequency precursors to the core phase pkp are caused by scattering off heterogeneities in the lowermost mantle and d regions and they provide a unique window into the small scale structure of the deep earth we study lower mantle scattering by analyzing 412 high quality pkp precursor records at ranges between 120 and 137 5 as obtained from the global seismic networks during the last 10 years to examine regional variations in scattering strength we compare individual records with the globally averaged pkp precursor stack of hedlin et al 1997 we identify strong differences in apparent scattering strength among specific source receiver paths inversion of these data for scattering source regions is complicated by ambiguity between source and receiver side scattering and the sparse and uneven data coverage synthetic tests however suggest that inversions with applied smoothness constraints can resolve large scale differences in scattering strength over significant parts of the lower mantle we use a conjugate gradient method based on an approximation to rayleigh born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle our results indicate particularly strong scattering beneath central africa parts of north america and just north of india whereas weaker scattering is seen beneath south and central america eastern europe and indonesia some regions of strong scattering correlate roughly with large scale anomalies revealed by seismic tomography including the african plume and the tethys trench these correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source receiver ambiguity in all areas further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations

observations of eastward currents in the tropical south atlantic ocean 1978 1980 data from four cruises to the tropical south atlantic ocean are used to describe the current distribution at 25 w and 27 5 28 w two distinct eastward flowing countercurrents separated by a branch of the westward flowing south equatorial current sec were observed during all the cruises the south equatorial undercurrent located between 3 s and 5 s has a subsurface velocity core with maximum speeds of about 0 4 m s geostrophic volume transports of the seuc from the surface to 1000 m range from 5 to 23 sv surface flow above the core of the seuc can be to the east or west the south equatorial countercurrent secc is located typically between 7 s and 9 s although weaker than the seuc with transports ranging from 3 to 7 sv the secc also has a subsurface velocity core maximum speeds of 0 1 m s are observed at about 275 m surface flow above the core of the secc is typically to the east upper layer waters of the secc are saltier and warmer than waters within similar density layers to the north

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

seismic structure and ultra low velocity zones at the base of the earth s mantle beneath southeast asia abstract we constrain seismic structure and ultra low velocity zones near the earth s core mantle boundary cmb beneath southeast asia we first determine the average shear velocity structure near the cmb in the region based on travel time analysis of s scs p and scp phases we then map seismic scattering in the lowermost mantle using the pkp precursors observed at the usarray the inferred average shear velocity perturbations in the lowermost 200 km of the mantle range from about 6 to 6 and exhibit a complex geographic distribution of alternate low and high velocity patches adjacent to each other surrounded by a high velocity anomaly in the south the inferred strong seismic scatterers exhibit a crescent shape distributed from the south china sea to the maluku islands and coincide with the westernmost low velocity patch suggesting that the strong scatterers represent ultra low velocity zones ulvzs we suggest that the seismic structure in the region likely results from a complex interaction between a downwelling and a low velocity region near the cmb the downwelling the high velocity patches displaces the low velocity region into many low velocity patches and pushes the ulvzs to the edge of the low velocity region

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

melting of subducted basalt at the core mantle boundary the geological materials in earth9s lowermost mantle control the characteristics and interpretation of seismic ultra low velocity zones at the base of the core mantle boundary partial melting of the bulk lower mantle is often advocated as the cause but this does not explain the nonubiquitous character of these regional seismic features we explored the melting properties of mid oceanic ridge basalt morb which can reach the lowermost mantle after subduction of oceanic crust at a pressure representative of the core mantle boundary 135 gigapascals the onset of melting occurs at 3800 kelvin which is 350 kelvin below the mantle solidus the sio2 rich liquid generated either remains trapped in the morb material or solidifies after reacting with the surrounding mgo rich mantle remixing subducted morb with the lowermost mantle