a study of mantle layering beneath the western pacific the reverberative interval of the seismogram defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc provides an excellent window in which to observe subcritical reflections from mantle discontinuities on an sh polarized seismogram the intervals between scsn and sscsn wave groups zeroth order reverberations are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities first and higher order reverberations we have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth and first order reverberations and have applied them to a data set of long period digital seismograms drawn from a number of tectonic regions in and around the western pacific the results place important constraints on the nature of the transition zone discontinuities measurements of the two way vertical shear wave travel time from the free surface to the 400 km and 650 km discontinuities 400 and 650 respectively yield estimates of transition zone travel time tz largely uncontaminated by outside heterogeneity variations in the path averaged estimates of tz are of the order of 5 4 s indicating significant lateral heterogeneity in the transition zone correlations of tz with 400 and lm travel time through the lower mantle are consistent with but do not require the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km with 12 km of long wavelength 500 5000 km topography on each the amount of topography estimated for the 650 km discontinuity is significantly less than predicted for a rigorously stratified mantle 100 km in a search for other mantle discontinuities we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle average depth of 86 km due to ambiguity inherent to the method this feature can also be interpreted as an impedance increase in the d region of the lower mantle 70 170 km above the core mantle boundary no evidence is found for a 220 km discontinuity or the d discontinuity proposed by lay and helmberger 1983 suggesting that if such features are present in the western pacific they are either small normal incidence sh reflection coefficient r 1 5 or intermittent or have lateral depth variations well in excess of 50 km

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

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

evidence for a sharp 670 km discontinuity as inferred from p to s converted waves the fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short period and broadband seismograms using a stacking technique to analyze the p wave coda of teleseismic records evidence is found for coherent near receiver p to s converted phases generated by the 400 and 670 km discontinuity beneath a number of the stations used in this study variations in travel time slowness and amplitude of these phases as observed among the stacks for stations of the regional seismic test network in the united states and of the network of autonomously recording seismographs in western europe are very likely the expression of upper mantle heterogeneity observations of coherent converted phases from the 400 km discontinuity are fewer in number than of phases converted at the 670 km discontinuity suggesting that the latter is more pronounced some of the seismograms especially of station rscp show extremely high amplitude p to s converted phases from the 670 km discontinuity these seismograms allow a detailed waveform comparison of the converted phase with the direct p phase and present evidence for a sharp 670 km discontinuity

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

imaging global body wave phases by stacking long period seismograms stacked record sections combining over 5 years of long period global digital seismograph network gdsn seismograms from shallow events show dozens of seismic body wave phases separate images are obtained for the vertical radial and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking these images provide an indication of the relative signal to noise characteristics of different parts of the gdsn data set core mantle boundary diffracted phases are particularly prominent in these stacks which show pdiff to 150 pkpdiff to 200 ppdiff to 230 sdiff to 160 and ssdiff to 220 high order s surface multiples can be seen extending through the surface wave train up to ranges of 540 two examples of possible upper mantle discontinuity reflected phases are visible one following ppp between about 95 and 125 the other following scs3 between about 210 and 240 an apparent offset between sh and sv arrivals is seen which becomes larger for the higher order s multiples sss ssss etc and appears consistent with models containing transverse isotropy in the upper mantle

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

upper mantle seismic anisotropy and lithospheric decoupling the lithosphere underlying stable continental shield regions is thought to extend to a depth of 200 km compressional wave velocity models derived from a recent approximately ns refraction profile in northern australia include a pronounced high velocity zone at a depth of around 200 250 km conventional petrological interpretations in terms of mineralogical or compositional changes cannot readily explain this feature calculated velocities for the garnet pyrolite model using a derived geotherm fit the velocities above and below this high velocity zone but cannot account for the high velocity 8 6 km s 1 zone itself this feature is explained in terms of velocity anisotropy which might reflect mechanical decoupling of the continental lithosphere from the underlying mantle

occurrence and significance of magnesite in upper permian guadalupian tansill and yates formations delaware basin new mexico magnesite mgco 3 occurs pervasively in a 270 ft 82 m cored interval of upper permian guadalupian shelf deposits from the northern rim of the delaware basin portion of the permian basin new mexico in our core example magnesite is found in tidal flat lagoon and pisolite shoal dolomites and siltstones of the tansill and uppermost yates formations the interval is overlain by magnesite bearing anhydrite and a thick halite section of the ochoan salado formation the basinwide extent of magnesite is unknown the texture of magnesite in core slabs thin sections and sem scanning electron microscope mounts is nearly identical to that of dolomite the magnesite consists of anhedral to euhedral crystals ranging in size from 1 to 4 mm within the dolomite intervals of the core magnesite occurs in three forms 1 patches of crystals and as replacement of allochems both in an otherwise dolomite matrix 2 pervasive replacement of host dolomudstone and peloidal and pisolitic dolowackestone packstone and 3 discrete layers interbedded with anhydrite magnesite rarely fills pore spaces in dolomite but within siliciclastic beds magnesite occurs both as a first generation cement and as a replacement of carbonate grains magnesite and dolomite are mutually exclusive in siliciclastic beds wi hin the core and magnesite occurs only where anhydrite is abundant magnesite may have formed either 1 during ochoan deposition or thereafter after burial of the tansill and yates formations from dense brines originating from the overlying salado evaporites or less likely 2 syndepositionally with the tansill and yates sediments preliminary measurements of stable carbon and oxygen isotopes for magnesite yield normal permian values for sigma 13 c averaging 6 84 parts per mil pdb and slightly evaporitic values for sigma 18 o averaging 1 04 parts per mil pdb corrected 87 sr 86 sr isotope composition averages 0 70687 because a high content of associated uranium in the magnesite rich part of the core causes large gamma ray deflections similar to those for shale and because the density of magnesite is close to that of anhydrite the presence of magnesite could lead to improper evaluation of lithology and porosity from logs and could ultimately result in failure to recognize potential reservoir zones

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

can the differential sensitivity of body wave mantle wave and normal mode data resolve the trade off between transition zone structure and boundary topography abstract large scale seismic models of the three dimensional 3d variations in elastic properties will be biased by topography on mantle boundaries to the extent that volumetric and topographic structures produce similar effects in the data to date seismic inversions for global scale 3d elastic models of the mantle have largely ignored the effect that topography on the major mantle discontinuities would have on estimating these models in this paper we address three questions 1 to what extent does unmodeled structure on the 410 and 660 km boundaries bias volumetric structure in inversions based on normal mode mantle wave structure coefficients absolute s wave travel times and differential ss s travel times 2 can the differences in the sensitivity of s waves ss s phase pairs and normal mode mantle wave data be exploited to estimate transition zone volumetric models that are relatively unbiased by topographic structure 3 have current volumetric models resolved the trade off that exists between volumetric and topographic structures to address question 1 synthetic experiments were performed which show that volumetric models inferred from normal mode mantle wave data can be biased by an average value of 20 25 in r m s amplitude in the transition zone relative to current aspherical earth models depending on the model parametrization employed the average transition zone volumetric bias of models inverted from absolute travel times from both transition zone and lower mantle bottoming s rays that are imprinted with the same topographic signatures is reduced by at least by a factor of five relative to models inverted from normal mode mantle wave data alone a reduction in bias by a factor of about four is obtained using ss s phase pairs in which the ss legs bottom in both the transition zone and lower mantle the use of lower mantle bottoming s rays or ss s phase pairs with the ss legs bottoming in the lower mantle reduces the bias only by an average factor of about two to three relative to the normal mode mantle wave inversion these estimates of bias reduction can vary with the type of damping or smoothness constraints that are applied in the inversion with respect to question 2 these results suggest that in principle absolute s wave travel time data can be used to desensitize volumetric inversions to the bias caused by topography on the transition zone boundaries in practice however near source structure that contaminates s wave travel times would diminish this capability the use of ss s differential times for ss waves which bottom in the transition zone and mantle waves sensitive to transition zone structure but insensitive to the 660 km boundary should be the most effective means of overcoming the trade off to address question 3 we adopt the criterion that the combination of an unbiased volumetric model with an accurate topographic model should provide a better fit to normal mode structure coefficients than the volumetric model alone the boundary model topo660a when added to the volumetric model s12 wm13 fits the normal mode structure coefficients significantly better than the volumetric model alone however more recent models of 410 and 660 km boundary topography degrade the fit of the volumetric models s12 wm13 and sh 10c 17 to the normal mode structure coefficients suggesting that there is not yet a conclusive answer to this question

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

stable isotope study of carbonate sediments from the coorong area south australia the mineralogy and isotope geochemistry of carbonate minerals in the coorong area are determined by the water chemistry of different depositional environments ranging from seawater to evaporitically modified continental water the different isotopic compositions of coexisting calcite and dolomite suggest that each of the above two minerals was formed from water of composition and origin unique to that specific mineral in addition the dolomite was not formed by simple solid state cation exchange the occurrence of two types of dolomite was shown by isotope analysis and sem observations the dolomite which is isotopically light 13c 1 to 2 0 18o 3 to 5 0 and of fine grain size 0 5 m probably precipitated under the influence of evaporitically modified continental water coarser grained dolomite up to 4 m is isotopically heavier 13c 3 to 4 0 18o 5 to 6 0 contains mg in excess of ca and was formed in or close to equilibrium with atmospheric co2 probably by the dolomitization of aragonite

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

topography of the 400 km discontinuity from observations of long period p400p phases summary new evidence is presented for lateral variation of the depth of upper mantle discontinuities this evidence is derived from an analysis of amplitude of p400p phases underside reflections from the 400 km discontinuity for great circle paths with bounce points in the upper mantle near the hawaii emperor hotspot chain the data are long period from wwssn sro and rstn stations the epicentral distance ranges from 95 to 115 which ensures that no other major phases arrive in the time window of interest observed p400p amplitudes are compared with synthetic amplitudes for both a laterally homogeneous model and models including 3 d topography on the discontinuities the observations permit models of topography that can be characterized by undulations of the 400 km discontinuity with wavelengths in the range 600 2000 km and peak to peak amplitude of the order of 30 40 km such long wavelength structure causes quite strong de focusing of pdp phases synthetic pdp waveforms reveal that topography not only affects its amplitude but also smears out the pdp wave causing overlap between different pdp phases this mechanism is proposed as the reason for the lack of well developed pdp phases reported in the literature the fact that the waveforms can be seriously distorted prohibits the use of pdp phases in a waveform inversion the curvature of the discontinuity can be explained as the signature of convection currents crossing the discontinuity although the data cover the hawaii hotspot no signature of a plume structure can be detected in the data probably because the resolving power of the data is too low

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

seismic wave theory in the us 1991 1994 the early 1990s saw many interesting contributions to the field of seismic wave propagation from a variety of us researchers even as many wave propagation specialists shifted their research focus away from new methods for seismogram synthesis toward data acquisition and inversion in part this shift was a natural consequence of the explosive growth in the amount and availability of high quality digital seismic data ready to be processed with interpretation tools developed in previous decades as a result observational seismology has recently made dramatic contributions toward imaging global geodynamics revealing the great variety of crustal structures and laying the foundation for a collaborative global network of broadband seismometers to monitor earthquakes and other seismic events however recent observational studies have brought to light new problems in wave propagation theory and have made some older problems more difficult to ignore

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

seismic tomography of the earth s mantle the analysis of information contained in seismograms is the most promis ing tool for retrieving the three dimensional structure of the earth s mantle and thereby improving our understanding of its dynamics the procedure through which three dimensional images of different parts of the earth s interior arc obtained by inversion of a set of observables measured on the surface of the earth is commonly termed tomography by analogy with the medical procedure with which it shares many aspects of methodology but with much less favorable experimental conditions ideally one would like to be able to model entire seismograms over a wide frequency band for a large number of uniformly distributed sources and stations in order to obtain an optimal illumination of the inhomogeneities appropriate for tomographic inversion in practice a number of difficulties and limitations arise onc is the nonuniform distribution of sources and receivers seis micity is concentrated at plate boundaries and the distribution of con tinental and island areas leaves large portions of the oceans unin strumented another difficulty is our limited ability to accurately forward model the effects of wave propagation in a realistic earth which presents departures from a spherically symmetric referencc model of both elastic anc1astic and isotropic anisotropic nature this is why until now the simplest kind of tomography travel time tomography has prevailed relying on simplifying assumptions concerning the interaction of asphericity with propagating seismic waves ray theory and the laws of geometrical optics in their

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

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

long period surface waves and mantle boundary undulations summary we address two questions with synthetic experiments 1 is there sufficient signal in long period surface wave data to provide useful constraints on topography on the internal boundaries in the upper mantle and around the transition zone 2 what is the magnitude and nature of the bias caused by undulations on internal discontinuities on inversions for volumetric structures that do not also include boundary structures the synthetic experiments rest on models of boundary topography on three upper mantle and transition zone discontinuities at 220 400 and 660 km depth the 660 km topographic model is that of shearer masters 1992 we consider the effect of two different 400 km models one is identical to shearer and master s model model a and the other is their model sign flipped model b the 220 km model is ad hoc but has been motivated by the work of revenaugh jordan 1989 1991b who argue that the 220 km discontinuity is not a global feature at all but has a continental affinity for simplicity our 220 km model is global but the jump in material properties across the discontinuity has been reduced relative prem by a factor of two and is based on a continent ocean model which is depressed up to 25 km below continents and ascends 10 km under oceans these resulting boundary models have a significant effect on long period fundamental spheroidal mode structure coefficients with a magnitude on average of 15 30per cent relative to the volumetric models m84a lo2 56 between harmonic degrees 20 l 75 inversion for volumetric structures of the structure coefficients computed with the boundary models alone devoid of volumetric structure shows that simple mantle volumetric structures can be found to fit the boundary structure coefficients nearly perfectly this demonstrates the essential trade off between boundary and volumetric mantle structures using surface wave data alone destructive interference between the effects of the 660km boundary and model b of the 400 km boundary requires a somewhat oscillatory volumetric mantle model to fit the structure coefficients but the resulting volumetric model is no more oscillatory than current models of the upper mantle both boundary models a and b produce on average a 30 per cent volumetric bias relative to m84a lo2 56 from the base of the crust to 1000 km depth the relative nature of the 400 km and 660 km undulations strongly affects the radial distribution of the volumetric bias but has less of an effect on the average size of the bias

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

the upper mantle degree 2 constraints and inferences from global mantle wave attenuation measurements we present the results of an analysis of global lateral variations in anelasticity of the upper mantle as measured from very long period rayleigh waves observed on the geoscope network four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account in a linear manner focussing effects due to lateral variations in elastic parameters local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length as an alternative to spherical harmonics expansion comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high q and high velocity regions respectively low q and low velocity both for great circle and minor arc data at periods around 200 s the detailed analysis of the degree 2 pattern well resolved in the attenuation maps around this period shows that it originates in the depth range 250 500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin possibly related to the topmost part of the large scale convective system corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid this study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods before performing any comparison with short period data or other geophysical parameters

evidence for a seismic discontinuity near 200km depth under a continental margin summary we present evidence for a seismic discontinuity near 200 km depth the lehmann discontinuity under the passive continental margin of north west australia where continental lithosphere merges into oceanic lithosphere the velocity contrast across the discontinuity is 0 2 0 3 km s and is similar to the contrast across discontinuities at similar depths in seismic models for purely continental paths to the east under central australia the discontinuity has been shown to be present under continents oceans and now at continental margins and is probably a worldwide feature

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

a mineral physics test of a pyrolite mantle an homogeneous upper mantle chemical composition defined by the pyrolite model is compatible with the observed seismic velocities such a test is now possible for depths to 670 km as a result of new mineral elasticity and phase equilibria data three specific criteria for comparing the mineralogical velocity model with that of the mantle include i the velocity jump at the 400 km discontinuity ii the velocity increase between 400 and 670 km and iii the absolute value of velocity in the 300 to 400 km region shallower than the discontinuity the first criterion is sensitive to the olivine content of the mantle and the pressure derivatives of the elastic properties of the high pressure phase the application of this criterion suggests a mantle with 40 65 olivine by volume the second criterion is sensitive to the details of the pyroxene to garnet transformation and the relative abundances of the pyroxenes and garnet the third criterion depends on details of this transformation as well as iron content and the temperature of the mantle