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

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

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

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

internal loading of an inhomogeneous compressible earth with phase boundaries summary we use an approach described by dehant wahr 1991 to estimate the geoid and boundary topography caused by mass loads inside the earth we show how the estimates are affected by compressibility and a radially varying density distribution and by the presence of phase boundaries with density discontinuities we compare results for earth models that have a chemical boundary at 670 km depth with those that have a phase boundary at that depth we find that the 670 km topographies predicted for models with a chemical boundary are several times larger than for models with a phase boundary depending principally on the value of the clapeyron slope in the phase case the geoid predicted in the chemical boundary case is about 30 40 per cent smaller than that predicted in the phase case the effects of compressibility and radially varying density are likely to be small we estimate the laterally varying structure inside the fluid core by computing the topography on constant potential surfaces within the core we conclude that this structure is not likely to be a complicating factor when using nutation and earth tide observations to estimate the flattening of the core mantle boundary finally we compute the inner core outer core icb topography for loading inside the mantle and for loading inside the inner core we show that in the first case the icb topography is small and dominated by long wavelengths in the second case we conclude that if the density heterogeneities inside the inner core are two orders of magnitude smaller than in the mantle the icb topography can be about 100 m peak to peak

compressional and em wave velocity anisotropy in a temperate glacier due to basal crevasses and implications for water content estimation we have conducted a series of experiments designed to investigate elastic and electromagnetic em velocity anisotropy associated with a preferentially aligned fracture system on a temperate valley glacier in south central alaska usa measurements include a three dimensional compressional wave p wave seismic reflection survey conducted over a 300 m 300 m survey patch with uniform source grid and static checkerboard receiver pattern additionally we acquired a multi azimuth multi offset polarimetric ground penetrating radar gpr reflection experiment in a wagon wheel geometry with 948 of azimuthal coverage results show azimuthal variation in the p wave normal moveout velocity of 3 3765 and 3630 m s 1 in the fast and slow directions respectively and difference of nearly 5 between the fast 0 164 m ns 1 and slow 0 156 m ns 1 em velocities fracture orientations estimated from the gpr and seismic velocity data are consistent and indicate a preferred fracture orientation that is 30 458 oblique to glacier flow these measurements agree with borehole observations anisotropic analysis of the polarimetric data gives a single volumetric water content estimate of 0 73 0 11 we conclude that meaningful estimates of physical properties in glaciers based on em or seismic velocity measurements require collecting data such that the presence of anisotropy can be evaluated and an anisotropic analysis employed when necessary

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

viscoelastic mantle density heterogeneity and core mantle topography summary starting with the model of mantle density heterogeneity derived by ricard et al 1993 which uses plate motion reconstructions under the assumption that subducted slabs sink vertically into the mantle we model the temporal evolution of these internal loads we investigate the viscoelastic deformation of the earth due to this time dependent excitation source using a love number formalism for a linear viscoelastic mantle with a maxwell model of rheology we are especially interested in the deep earth viscoelastic deformation such as the radial displacement that appears at the core mantle boundary and we compare the calculated topography with that proposed by seismologists

deformation of the core mantle boundary induced by spherical shell compressible convection we derive the single mode mean field equations for spherical shell convection with variable viscosity and compressibility included core mantle boundary cmb topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at cmb results for long wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters in order to obtain excess ellipticity of about 1 km the activation energy and volume in the lower mantle should be low and the temperatures at cmb around 4 500 k in accord with recent experimental findings

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

arsenic in a fractured slate aquifer system new england usa influence of bedrock geochemistry groundwater flow paths redox and ion exchange abstract elevated as levels have been reported by the vermont geological survey in groundwater from public and domestic bedrock wells in northwestern new england usa the study area in southwestern vermont is underlain by pyrite rich organic rich slates that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the ordovician taconian orogeny and the distribution of wells with elevated as shows that they were completed in slates hydrochemical and bedrock geochemical analysis indicates that elevated as in the aquifer system is controlled by the following 1 the presence of black slates that are rich in arsenian pyrite 200 2000 ppm as 2 release of as via the dissolution of as rich pyrite 3 geochemically reducing and slightly alkaline conditions where high as values occur at eh 7 and 4 physical hydrogeological parameters that foster low eh and high ph particularly long groundwater flow paths and low well yields i e high residence time which provides high rock to water ratios where all four factors affect as contents in groundwater 72 of wells in a zone of distal groundwater flow low relief topography exceed 10 g l ppb and 60 of wells in this zone exceed 25 ppb as where flow paths are shorter in slates and groundwater has higher eh and lower ph i e in regions of higher relief topography closer to recharge zones only 3 of wells contain 10 ppb as and none contain 25 ppb overall 28 50 176 of low elevation wells

comment on reproducing earth s kernel uncertainty of the shape of the core mantle boundary from pkp and pcp travel times by p b stark and n w hengartner

p and pcp travel time tomography for the core mantle boundary pcp arrival times reported to the international seismological centre isc are inverted for vertical travel time anomaly at the core mantle boundary cmb we first invert about 2 106 isc first p arrival data for the three dimensional 3 d structure of the mantle and hypocenter locations iteratively the pcp data are corrected for the relocated hypocenters and the 3 d structure this correction has reduced the observed pcp residuals the corrected pcp data are then inverted for the vertical travel time anomalies at the cmb which are due either to velocity anomaly or topographic anomaly the spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the cmb by 3 if the layer is 20 km thick or a lowering of the cmb by 3 km the heterogeneity pattern of the cmb is drastically different from that of the overlying d layer negative vertical travel time anomalies appear in the northern high latitude region and positive anomalies appear under southeast asia and middle to south america the lateral dimensions of these anomalies are of the order of 40 the anomalies can be interpreted as either a velocity perturbation in the boundary layer 7 if the layer is 20 km thick or an undulation of the cmb 7 km various tests including the error and resolution estimations indicate that these three anomalies are the significant resolvable features of the cmb these anomalies are also corroborated by the result of the joint inversion of p and pcp for both the 3 d mantle and the 2 d cmb assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core the pcp data are also inverted for the isostatic figure of the boundary layer the velocity perturbation in this case is 5 6 if the layer is 20 km thick and the cmb undulation is only 1 2 km regardless whether the cmb heterogeneity is due to velocity and or topographic perturbations it is dominant in the zonal component of degree 2 while the degree 2 pattern in the lower mantle including the d is dominant in sectorial component

seismic velocities at the core mantle boundary inferred from p waves diffracted around the core the very base of the mantle is investigated with core diffracted p wave pdiff travel times published by the international seismological centre isc for the period 1964 1987 apparent slownesses are computed for two station profiles using a difference method as the short period pdiff mostly sample a very thin layer above the core mantle boundary cmb a good approximation of the true velocity structure at the cmb can be derived from the apparent slownesses more than 27000 profiles are built and this provides an unprecedented pdiff sampling of the cmb the overall slowness distribution has an average value of 4 62 s deg which corresponds to a velocity more than 4 lower than that of most mean radial models an analysis of the residuals of absolute isc p and pdiff travel times is independently carried out and confirms this result it also shows that the degree of heterogeneities is significantly higher at the cmb than in the lower mantle a search for lateral velocity variations is then undertaken a first large scale investigation reveals the presence of coherent slowness anomalies of very large dimensions of the order of 3000 km at the cmb a tomographic inversion is then performed which confirms the existence of pronounced 8 10 lateral velocity variations and provides a reliable map of the heterogeneities in the northern hemisphere the influence of heterogeneity in the overlying mantle of noise in the data and of cmb topography is evaluated it seemingly proves minor compared with the contribution of heterogeneities at the cmb our results support the rising idea of a thin low velocity laterally varying boundary layer at the base of the d layer the two principal candidate interpretations are the occurrence of partial melting or the presence of a chemically distinct layer featuring infiltrated core material

observations of pkkp precursors used to estimate small scale topography on the core mantle boundary stacks of global seismic data reveal the time and distance dependence of high frequency 1 hertz precursors to the seismic phasepkkp synthetic seismogram modeling shows that scattering from random small scale topography at the pkkpcore mantle boundary reflection point generates precursory arrivals similar to those seen in the data models with a root mean square core mantle boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data however a systematic range dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors

few 2 50 km corrugations on the core mantle boundary short period pcp amplitudes have been observed to vary by a factor of about 20 in the epicentral range 40 85 this behavior is due to the reflection coefficient at the core mantle boundary which varies from a maximum of about 0 5 at an angle of incidence of about 65 to zero at an angle of incidence of about 85 corresponding to an epicentral ranges of about 60 90 we present physical model experiments in which we study the effect of corrugations of the core mantle boundary on pcp amplitudes we find that corrugations with 2 50 km scale lengths would radically alter the amplitude range behavior of short period pcp reducing its variation with range the observed pattern of short period pcp amplitudes indicates that the core mantle boundary is smooth on these scales with amplitudes less than a few hundred meters

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

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

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

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