the interaction of the s wavefield with upper mantle heterogeneity summary many methods for the analysis of long period and broad band s waveforms depend on a representation of the seismic wavefield in terms of a sum of modes of a reference structure in addition it is frequently assumed that the propagation of the modal contributions from source to receiver may be regarded as independent this assumption may not be warranted if there is significant heterogeneity in seismic properties along the propagation path the interaction between modal components in different styles of heterogeneity model for the upper mantle is examined using a coupled mode propagation technique kennett 1984 which allows direct construction of reflection and transmission matrices with full allowance for intermode interactions for 2 d heterogeneity structures the first type of structure considered has been proposed to explain amplitude and traveltime anomalies in body wave studies of upper mantle phases this heterogeneity has an amplitude of about 1 per cent and is distributed with a horizontal scale of around 300 400km and a vertical scale which increases from 70km in the uppermost mantle to 200 km at 900 km depth horizontally travelling s waves are hardly affected by this class of heterogeneity for frequencies less than 0 07 hz the second heterogeneity model was based on the wepl3 model proposed by nolet 1990 from waveform inversion for the structure under the nars array in western europe the heterogeneity reaches 5 per cent deviation from the reference model premc in organized regions 700 km or more in length for this structure surface wave modes with group velocity below 4 2kms can be regarded as propagating independently up to 0 020hz the body wave group of modes with higher group velocity succumbs to significant interaction above 0 040 hz the frequency limit for largely independent propagation for the body wave group of modes can be extended to about 0 05 hz for a velocity model with up to 1 per cent additional variability superimposed on wepl3 such a composite heterogeneity model would be consistent with both body wave and surface wave behaviour the errors introduced into the analysis methods by ignoring mode interactions above these frequency limits will depend on the distribution of energy across the modes imposed by the source and the criterion used for waveform matching between observed and theoretical seismograms theoretical seismograms for the heterogeneity structures based on wepl3 including full allowance for intermode coupling show a distinct phase shift for the fundamental mode when compared with the corresponding calculations for the reference model premc as is indeed observed at the nars stations the parts of the seismograms which show the largest influence from the presence of lateral heterogeneity are those which depend on interference phenomena such as su the disruption of the phase patterns can have a profound influence on the appearance of the waveforms the presence of small scale heterogeneity has very little influence on the seismograms below 0 035 hz but becomes more important as the frequency increases especially for the body wave phases

seismic wavefield calculation for laterally heterogeneous earth models ii the influence of upper mantle heterogeneity summary there is increasing evidence that the earth s mantle is laterally heterogeneous on a broad range of scales but the character of smaller scale heterogeneity has to be deduced indirectly the aim of the present paper is to examine the influence of a variety of stochastic representations of heterogeneity on seismic wave behaviour to help constrain the nature of the variations in seismic properties in the upper mantle for each of the models the seismic wavefield is simulated using a pseudospectral method in a 2 d cylindrical coordinate system the presence of stochastic heterogeneity is particularly important for those parts of the seismic wavefield where a significant portion of the propagation path in the upper mantle is close to horizontal such as the pp and ss phases and fundamental mode and higher mode surface waves the effects are noticeable traveltime anomalies and waveform changes for the body waves particularly associated with phase triplications and significant phase shifts for rayleigh waves a variety of styles of stochastic heterogeneity models are compared for the same source and station configurations using wavefield snapshots and the character of the calculated seismograms the influence of heterogeneity on body waves and on longer period rayleigh waves increases as the scale length increases compared to the wavelength of the seismic waves the aspect ratio of the heterogeneity has a pronounced effect on the coherence and amplitude of traveltime fluctuations and waveform changes across stations at the surface which depend on the structures encountered along the propagation paths to the specific receivers the effect of nearly isotropic heterogeneity is to induce small short scale variations in traveltime fluctuations and waveform changes as the heterogeneity becomes more plate like the fluctuations are on a broader scale and of larger amplitude because the individual patches of heterogeneity have a stronger influence the effects of broad scale and stochastic heterogeneity are compared for a model built from a slice through a tomographic model derived from delay time inversion for the himalayan region as would be expected the influence of the deterministic heterogeneity derived from the tomography study has the result of introducing systematic traveltime variations for body waves and noticeable phase shifts for surface waves when compared with the results for the background reference model the addition of a moderate level of small scale stochastic heterogeneity which could not be resolved in the tomography study has a limited effect on the seismic wavefield at longer periods but is much more significant for periods of less than 4 s when the heterogeneity scale is of the order of 40 km

elastic wave velocities of forsterite and its β spinel form and chemical boundary hypothesis for the 410 km discontinuity a method for measuring elastic wave velocities at pressures up to 15 gpa has been developed velocities of forsterite and spinel were measured using a piston cylinder apparatus with a pulse echo overlap method for pressures up to 4 gpa and using a split sphere apparatus with a pulse transmission technique for higher pressures to validate the results volume compression incompressibility and rigidity were estimated from measured travel times and comparison was made between the present estimates and data from x ray brillouin and acoustic measurements the v p and k values for spinet are 3 and 8 higher than previous measurements at 10 gpa respectively velocity jumps in v p and v s due to olivine to spinel transition were about 1 34 and 0 80 km s 1 respectively very different from seismological observations of velocity jumps at the 410 km discontinuity which were concentrated around 0 45 km s 1 for the p wave and 0 22 km s 1 for the s wave velocity jumps in mantle peridotite were calculated using the experimental results and comparison was made between these estimates and seismological observations at the 410 km discontinuity the evidence does not support the phase change hypothesis and a chemical boundary was proposed as the origin of the 410 km discontinuity an eclogitic mineral assemblage might be an alternative for the composition of the transition layer

the velocity structure and heterogeneity of the upper mantle abstract in a particular region a smoothed and averaged representation of the velocity structure in the upper mantle can be derived from long period body wave and higher mode surface wave observations the vertical resolving power of such techniques is limited by the relatively long wavelengths used in contrast short period studies show considerable complexities in postulated one dimensional models which are unresolvable in the long period band however these may be associated with the mapping of lateral heterogeneity into a vertical profile to assess the extent to which lateral heterogeneity in the mantle can be resolved we have attempted to model the features seen on recordings of earthquakes in the indonesia new guinea region at large aperture 400 1000 km arrays of portable seismic recorders with horizontal heterogeneity scales of the order of 300 500 km and velocity perturbations up to 2 the amplitude deviations from those predicted for a stratified model can be considerable and fit in well with the pattern of observations for long period waves such laterally heterogeneous structures have only a modest effect with weak inter mode coupling resulting from the heterogeneity as a result a one dimensional model may still be a useful approximation for long period studies but will not be adequate for broad band or short period work

limitations of waveform modelling of long period seismograms summary in recent years the use of synthetic seismograms calculated for radially stratified models has gained increasing popularity as a means of placing further constraints on the velocity structure of the earth s mantle such synthetics do however have a number of limitations at short periods 1 s the amplitudes as well as the wave shapes of travel time branches are affected by seismograph siting the structure immediately beneath the seismograph and any laterally heterogeneous structure in the mantle later arrivals can also be masked by signal generated noise and by extended source times at longer periods 20 s the larger wavelengths reduce the sensitivity of amplitudes and waveforms to contarninating effects as a result the use of long period synthetics can only lead to the resolution of the gross features of the earth s interior

an investigation of the upper mantle beneath nw australia using a hybrid seismograph array summary a large array of short period portable seismographs was operated in the northern territory of australia for 3 months in 1986 in order to record earthquakes in the island arcs to the north and east of australia the array consisted of 18 digital recorders 28 analogue tape recorders and the 20 stations of the permanent warramunga wra seismic array an unusual aspect of this experiment was the variable station spacing and apertures of the different elements of the hybrid array the wra and portable digital arrays had station spacing of 2 2 km and 5 13 km respectively and allowed confident identification of phases returned from the upper mantle whilst the analogue array had station spacing of 40 km and allowed us to track mantle phases across its 500 km aperture seismograms from 17 shallow earthquakes mb 4 3 5 1 in the indonesian arc are used to investigate the p wave velocity structure of the upper mantle beneath nw australia we combine seismograms from these events into a composite record section covering ranges from lo00 to 2600km strong phases following the first p wave arrivals in this composite record section clearly indicate the presence of significant structure in the upper mantle lateral heterogeneity in the upper mantle causes the timing and amplitudes of mantle phases to vary across the array and among earthquakes in order to minimize the effects of lateral heterogeneity we filter and stack the data and concentrate on features in the data that are seen for many individual seismograms and for several earthquakes we calculate wkbj and reflectivity seismograms in order to construct a vertical velocity profile that fits the observed traveltimes and waveforms in an average sense our preferred model nwb 1 includes second order velocity discontinuities of 3 7 per cent near a depth of 200km 6 1 per cent near 400km and 3 7 per cent near 620km in order to reproduce the amplitudes of the later phases the low amplitudes of the first arrivals in the range from 1600 to 2200 km require either a low velocity zone below 230 km depth or a low velocity gradient between 230km and the 400 km discontinuity model nwb 1 is smoother than some models that have been previously proposed for this region which may have mapped lateral heterogeneity into vertical velocity profiles

a note on lateral variation in upper mantle shear wave velocity across the alpine front summary we have detected what appear to be strong lateral variations in upper mantle shear wave velocities under europe and western eurasia the evidence suggests that the alpine belt of southern europe is underlain by a tectonic upper mantle with a thin lid and thick low velocity layer whereas the eastern europe western russia precambrian platform is underlain by shield like upper mantle the transition zone between the two appears to be sharply defined and coincident with the northern boundary of the alpine belt the techniques used in the analysis include the construction of synthetic seismograms for long period s and ss waves in the distance range 30 60 where differences in upper mantle shear wave velocity most strongly affect the differential times ss s and the ratio of ss to s amplitudes as a function of epicentral distance a comparison with upper mantle shear wave structure in western north america as reported by grand helmberger indicated that under the alpine belt the upper mantle is considerably faster than under western north america a situation that may be interpreted as due to the much larger and widespread thermal anomalies under western north america as a consequence of the existence ofa subducted ridge

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

upper mantle velocity structure beneath the siberian platform summary we present a new velocity model for the continental upper mantle beneath central siberia based on observations of the 1982 rift deep seismic sounding dss profile three peaceful nuclear explosions pne were detonated to provide energy for the 2600 km long profile that extends from the yamal peninsula to the mongolian border se of lake baikal in this paper we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle transition zone discontinuities the analysis combines forward traveltime modelling and waveform matching using reflectivity synthetic seismograms our model for the lithosphere has velocities of 8 25 8 20 km s from the moho to 117km depth between 117 and 123km depth a strong velocity gradient 8 30 8 53 km s is required while a moderate gradient 8 53 8 55 km s l exists between 123 and 136km depth a low velocity zone from 136 to 210km depth terminates this phase arrival branch the gradient again rises between 210 and 233 km and depth culminating in a high gradient zone 8 63 8 80 km s between 233 and 235 km depth below the high gradient zone more moderate gradient 8 80 8 85 km s is required from 235 to 253 km depth terminating in a zone of lower velocity 8 62 8 64 km s from 253 to 400 km depth the upper mantle transition zone consists of two high gradient zones separated by a more moderate gradient the upper zone is best modelled as a 35 km thick velocity gradient 8 64 9 45 km s from 400 to 435 km depth the existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance a model with a first order discontinuity predicts significant arrivals at ranges closer than 1580 km our observations contain no compelling evidence for a 520 km discontinuity although a small discontinuity cannot be ruled out the lower transitionzone discontinuity is modelled as a 4 km thick gradient zone 10 25 10 62 km s i from 655 to 659 km depth

observational and theoretical constraints on crustal and upper mantle heterogeneity abstract the effects of crustal and mantle heterogeneity are reflected in the relative complexity of short period seismograms at regional to teleseismic distances observations of events at regional ranges from indonesia and new guinea at the warramunga array in northern australia allow a separation of the propagation through the mantle by concentrating on the coherent signal across the medium aperture array most of the incoherent features on the seismograms arise from signal generated noise in the general vicinity of the array with scale lengths of the order of 1 2 km or smaller partially coherent phases with relatively rapid changes in waveform across the array are to be associated with organised heterogeneity this is most likely to arise from features in the crust or at the crust mantle boundary near the receiver but could in some cases be due to structural effects near the source the coherent signals show considerable complexity for phases travelling large portions of their path above 80 km as the depth of sources increases this coherent portion of the seismic field tends to become simpler which suggests that the scale length of heterogeneity is larger at depth the use of horizontally stratified models has provided a convenient framework for the understanding of many seismic wave phenomena via the calculation of synthetic seismograms however further development is needed for calculating the response of the laterally heterogeneous earth the convenience of physical interpretation provided by working with the reflection and transmission properties of the medium can be retained if the structure can be split into a stratified reference model with superimposed heterogeneity with a plane wave decomposition of the wavefield the effect of lateral variations can be introduced by forcing mixing of wavenumber components a wide variety of phenomena can be simulated by varying the statistical description of the heterogeneity field in the wavenumber domain this varies the cross coupling between wavenumber components and so changes the nature of the resulting theoretical seismograms such an approach allows a quantitative study of the degradation of the major seismic phases compared to the predictions for stratified models and also the characterisation of the codas of these phases

variations in upper mantle structure under northern australia summary temporary array deployments of short period seismometers in northern australia have been used to build up composite record sections for waves interacting with the upper mantle stable measures of the seismic wavefield are provided by stacking the complex envelopes of all the seismic waveforms falling in a 10km distance interval away from the source two groups of sources a along the flores arc indonesia with propagation under northwestern australia and b in new guinea with paths to the nne of the array have been used to construct composite record sections for both p and sv waves over the distance range 1300 2800 km the timing and amplitude distributions for p waves from the two regions show noticeable differences detailed modelling of the record sections yields velocity models with significant variation in velocity for the two sets of propagation paths for which the midpoints are separated by about 1000km the short period sv wave sections indicate efficient propagation of highfrequency s waves in a lithosphere extending down to 210km arrivals from the deeper mantle cannot be correlated with confidence because of a loss in high frequency content revealed by broad band observations this requires a significant attenuation zone for s beneath 210 km

seismic velocity gradients in the upper mantle the radial velocity gradients for p and s waves in the upper mantle are of considerable interest for petrological studies of the upper mantle but are difficult to constrain directly comparison of three different styles of velocity model constructed within a common parametrisation to give a good fit to the travel times of p and s waves to 30 provide a useful framework for constructing bounds on the radial gradients for depths from 250 1000km a somewhat subjective set of bounds are suggested for the velocity gradients and velocity contrasts at the major discontinuities in an average mantle one of the most significant factors affecting the estimation of radial gradients is the presence of small scale heterogeneity in the mantle modest heterogeneity around 1 percent perturbations on scale lengths of about 300km can give rise to horizontal velocity gradients which can overwhelm the radial gradients in the shallower parts of the mantle and still be a quarter of the size of the radial gradients at greater depths however such scales of heterogeneity are unlikely to simulate the presence of additional upper mantle discontinuities for ray paths travelling near vertical

paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic evaporite deposits in the western qaidam basin abstract uranium u provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments its proxy potential under suboxic oxic conditions associated with salinization of surface water in closed hydrologic systems however has not been adequately explored this study examined the abundance and distribution of u in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality 938 5 m long core drilled in the western qaidam basin ne tibetan plateau this study examined u concentrations in calcareous clastic sediments including carbonates acid insoluble fractions as well as the amounts of iron and manganese oxides oxyhydroxides and salt layers in the calcareous clastic sediments variations in u concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations providing a tool for tracing redox conditions in paleolake bottom water correlations between the variations in carbonate content with u and sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the u distribution in carbonates in the salt layers u and al concentrations correlate closely implying that both derived mainly from the detrital fraction in the salt layers however enrichments of u and mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine these results indicate that during the process of carbonate precipitation carbonate might effectively remove u from lake water during the subsequent stage of salt mineral precipitation u behavior is conservative and u resides for a long period in dissolved forms within the saline water giving rise to enhanced u enrichment in the more concentrated brines and the associated precipitated salt minerals our study suggests however that u related redox proxies should be interpreted with caution in suboxic oxic environments during the long term paleolake evolution

velocity structure of upper mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms summary we present velocity constraints for the upper mantle transition zones beneath central siberia based on observations of the 1982 rift deep seismic sounding dss profile the data consist of seismic recordings of a nuclear explosion in north western siberia along a 2600 km long seismic profile extending from the yamal peninsula to lake baikal we invert seismic data from the mantle transition zones using a non linear inversion scheme using a genetic algorithm for optimization and the wkbj method to compute the synthetic seismograms a statistical error analysis using a graphbinning technique was performed to provide uncertainty values in the velocity models our best model for the upper mantle velocity discontinuity near 410 km depth has a two stage velocity gradient structure with velocities increasing from 8 70 9 25 km s over a depth range of 400 415 km a gradient of 0 0433 s and from 9 25 9 60 km s over a depth range of 415 435 km a gradient of 0 0175 ski this derived model is consistent with other seismological observations and mineral physics models the model for the velocity discontinuity near 660 km depth is simple sharp and includes velocities increasing from 10 15 km s l at 655 km depth to 10 70 km s at 660 km depth a gradient of 0 055 s

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

upper mantle shear velocity beneath eastern australia from inversion of waveforms from skippy portable arrays in 1993 the research school of earth sciences of the australian national university commenced a nationwide seismometry project skippy for a comprehensive study of the upper mantle beneath the australian continent we applied a waveform inversion technique to broad band data recorded while the skippy portable arrays were positioned in eastern australia in order to construct a 3 d model of shear velocity in the upper mantle and transition zone beneath eastern australia and the adjacent oceanic regions the skippy data were augmented by data from the permanent seismological observatories in the region the first step of the waveform inversion used involved the matching of the waveforms of fundamental and higher mode rayleigh waves with waveforms synthesized from radially stratified models in the second stage the linear constraints on radial variations in shear velocity were combined in a tomographic inversion for aspherical variations in shear velocity the preferred model reduces the data variance by 90 per cent owing to the dense data coverage structural features with dimensions larger than 250 km laterally and 50 km vertically are resolved the major structural features inferred from the tomographic images are 1 a substantial increase in the thickness of the high velocity lid from the oceanic region to the phanerozoic continental and proterozoic continental regions 2 a pronounced low velocity zone centred at 140 km depth beneath the part of eastern australia often referred to as the tasman fold belt which confirms previous inferences from rayleigh wave dispersion curves 3 a prominent zone of low wave speeds beneath the eastern margin of the continent that coincides with locations of cenozoic volcanism and regions of enhanced heat flow 4 deep low wave speed anomalies beneath both the coral and tasman seas and 5 localized high wave speed perturbations in the mantle transition zone 410 660 km depth beneath eastern australia in particular beneath the new england fold belt and the mount isa block from the images we infer that the eastern boundary of the precambrian shields does not coincide with a sharp seismic contrast unless this boundary is located further to the east than is commonly assumed the structure of the upper mantle and transition zone beneath eastern australia as inferred from the images is more complex than that beneath the precambrian cratons of central and western australia which could be due to subduction beneath the eastern continental margin prior to opening of the tasman sea if correct this interpretation would imply that part of the upper mantle has moved along laterally with the lithosphere during the relatively fast northward motion of the plane

velocity structure of the upper mantle discontinuities beneath north america from waveform inversion of broadband seismic data using a genetic algorithm we present a velocity structure of the mantle discontinuities beneath the united states from a full waveform inversion of broadband seismic data the data consist of 10 moderate earthquakes in the western united states that were recorded on 13 broadband seismometers across the united states operated by the u s national seismic network a total of 36 seismograms with distance ranging from 1150 to 2700 km were used a full waveform inversion based on two dimensional modeling and one dimensional inversion methods using a genetic algorithm was adopted the inversion method is objective provides error bounds on the model and does not require any a priori information about these discontinuities we find that the upper discontinuity is 10 km thick from 395 to 405 km where the velocity increases from 8 80 to 9 15 km s 1 with a velocity gradient of 0 035 s 1 the lower discontinuity is 5 km thick occurring over a depth range of 650 655 km where the velocity increases from 10 20 to 10 70 km s 1 our results are in agreement with previous studies of the lower discontinuity but do not agree for a sharp 5 km upper mantle discontinuity

inhomogeneities of the eurasian mantle structure from the traveltimes of the nuclear explosions recorded by the finnish seismic network during 1961 1985 summary during the period 1961 1985 hundreds of nuclear explosions from the area of the soviet union and china were well recorded at finnish seismological stations the seismic waves recorded on the baltic fennoscandian shield penetrated through the mantle of the siberian platform the ural mountains and the east european platform from the known crustal models the functions describing the increase of the average velocities of sedimentary cover and crystalline complex of the crust with thickness were found the corrections for sedimentary cover and crustal thickness were included the large amount of data permitted the calculation of traveltimes of p waves for three sectors up to distances of about 5000 km in all mantle models the boundaries 400 and 700 km were found comparison of the results shows a difference in the traveltimes of the order of 5 s at a distance of about 4000 km which reflects the mantle structure differentiation for depths greater than 700 km lowest velocities for latitudinal direction and highest velocities for longitudinal direction the average s wave velocity model of the mantle was obtained using the traveltimes of s wave first arrivals high values of the vp vs ratio were found in the depth interval 200 400 km while in other depth intervals they were close to 1 73 our 1 d models are compared with and discussed in connection with other models of the east european and siberian platforms as well as with global tomographic solutions

variability of p660s phases as a consequence of topography of the 660 km discontinuity abstract records from the broad band nars nl array deployed in the netherlands western germany and belgium have been analyzed for p to s converted phases from the 660 km discontinuity p660s no individual seismogram from this array shows a clear p660s phase stacks of up to 113 crosscorrelograms show p660s energy that is weak in comparison with similar stacks elsewhere very high amplitude observations of p660s in single seismograms from the same region have been made in a previous study paulssen 1985 geophys res lett 12 709 712 paulssen 1988 j geophys res 93 10489 10500 such variations may be due to relief on the 660 km discontinuity in this paper we quantify the effects of different topographies on the time delays amplitudes and waveforms of the direct p wave as well as on the p660s converted wave synthetic p and p660s waveforms are calculated by applying a kirchhoff helmholtz integration over the wave fields at the 660 km discontinuity interface for topography on scales of a few hundreds of kilometers the effects on the p wave are very small compared with those on the p660s phases time delays focused and defocused amplitudes and waveform distortion of p660s phases influenced by such topography diminish the coherence of p660s in synthetic stacks our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of 15 km to 25 km is consistent with the pattern of p660s observations in single seismograms and stacks