investigation of upper mantle discontinuity structure beneath the central pacific using ss precursors 1 using a high quality broadband seismic data set of precursors to the phase ss we investigate the structure of upper mantle discontinuities beneath the central pacific including the hawaiian hot spot we image structure by stacking over 4000 records into geographic bins retaining periods down to 5 s we consider the effects of correcting for four separate tomographic models of mantle heterogeneity excluding data at distances containing phases that potentially interfere with precursors we find evidence for peak to peak topography of 15 20 km on the 670 km discontinuity and 7 28 km of topography on the 400 km discontinuity weak reflections are detected from discontinuities near 220 and 520 km depth the average transition zone thickness beneath our region is approximately 242 3 km very similar to previous estimates of the global average lateral transition zone thinning and thickening weakly correlate with reduced and increased transition zone shear velocity respectively consistent with a thermal origin to topographical variations on the discontinuities within our study region the transition zone beneath hawaii and to the east of the line island chain is thinned by up to 20 km in a province spanning nearly 1000 km suggesting an excess mantle temperature of 200 k in the oldest crustal regions of our study area 100 ma the 400 km discontinuity is relatively shallow and the transition zone is relatively thick 250 255 km a possible explanation for this pattern includes small scale convection in the upper mantle bringing colder material into the transition zone

seismic detection of folded subducted lithosphere at the core mantle boundary seismic tomography of the d region beneath the caribbean reveals a 100 km vertical step in an otherwise flat d shear velocity discontinuity and localized low velocities at the edge of the slab material which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the earth s lower mantle1 2 the fate of these slabs has remained unresolved but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle known as the d region relatively cold slab material may account for high seismic velocities imaged in d beneath areas of long lived plate subduction and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions3 4 the d discontinuity itself is probably the result of a phase change in relatively low temperature magnesium silicate perovskite5 6 here we present images of the d region beneath the cocos plate using kirchhoff migration of horizontally polarized shear waves and find a 100 km vertical step occurring over less than 100 km laterally in an otherwise flat d shear velocity discontinuity folding and piling of a cold slab that has reached the core mantle boundary as observed in numerical and experimental models can account for the step by a 100 km elevation of the post perovskite phase boundary due to a 700 c lateral temperature reduction in the folded slab we detect localized low velocities at the edge of the slab material which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer

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

seismic reflections from the upper mantle discontinuities beneath the mid atlantic ridge observed by a seismic array in hokkaido region japan following a recent big deep event mb 6 8 in the fiji islands region a seismic array in hokkaido region japan recorded clear impulsive precursors to the p p phases p is another symbol for pkp and p p is the surface reflection of p in the epicentral distance range of 70 72 4 reflection points of the phases are located 1 5 3 5 west of the mid atlantic ridge 13 s 15 w the precursors were also recorded at two wwssn stations in japan travel times and slowness measured by the array are best explained as the precursors being underside reflections from a depth of 660 km the amplitude ratios of p 660p p p are about 0 1 a value similar to previous observations the observation suggests that there exists a sharp discontinuity at a depth of 660 km beneath the area close to the mid atlantic ridge

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

buckling instabilities of subducted lithosphere beneath the transition zone a sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds recent tomographic images beneath subduction zones together with quantitative fluid mechanical scaling laws suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth beneath subduction zones such as central america and java the fast anomalies in elastic wavespeed are too broad 400 km to be explained by simple compressional thickening of subducted lithosphere that is initially 50 100 km thick by contrast a periodic buckling mechanism can account for both the characteristic wedge shapes of the anomalies and for their widths which agree closely with those predicted by the scaling laws in addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions interpretation of geoid anomalies and modeling of secular changes in earth s moment of inertia

reflections from a distant subduction zone accurate models of the distribution of heterogeneities in the earth s mantle especially of subduction zones are crucial for our understanding of the composition and dynamics of the earth here we present broad band bb seismograms with p waves reflected at the subduction zone under southern japan at about 84 distance from german regional seismic network the earthquakes occurred in the mariana islands region at 100 to 105 distance array techniques allow the separation of this wave with an asymmetric path p1p2 from reflections and conversions under the source the receivers and the midpoint between the source and the receivers our observation demonstrates that subduction zones can act as strong seismic reflectors and can be mapped with a depth resolution of 20km with bb arrays at teleseismic distances the analysis of such data will help to derive a more detailed picture of the structure of the earth s mantle

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 to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

are long period body wave coda caused by lateral heterogeneity summary data from two broad band arrays in western europe nars and grf are used to study the character of long period body wave coda the events studied are at epicentral distances of 40 to 60 in the hindu kush region and on the mid atlantic ridge sampling the upper mantle to a depth of about lo00 km the periods studied are 5 50 s the long period coda of p at grf interstation distance about 10 km are strongly coherent whereas the long period coda of pp ppp and s waves are incoherent this indicates that the latter coda consists of scattered waves to investigate the nature of the scattering process the data of grf are analysed for slowness and azimuth variations in the coda intervals a new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals the method is based on backus gilbert inverse theory the results show that the incoherent long period coda intervals almost entirely consist of surface waves these waves are scattered from the preceding body waves some calculations with linearized theory for body wave to rayleigh wave conversion at topography at the surface or at the moho show that realistic scatterers can account for the observed constant coda level the beamforming results show that the phases in the p coda all arrive along the great circle as scattering calculations point out that body wave to body wave scattering is inefficient it is concluded that the long period p coda does not contain a significant amount of scattered energy synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves for events in the hindu kush region an upper mantle with a thick lid overlying a pronounced low velocity zone lvz is necessary to explain the character of the p coda at grf such an upper mantle agrees with previous studies of similar great circle paths the strong coherence of the p coda at grf is lost on the scale of nars station separation about 200 km this suggests lateral variations in the upper mantle at a scale of about 200 km it appears from previous studies of the upper mantle under europe that these variations must be sought in the lvz it is shown that the long period p coda is sensitive to variations in the p velocity structure of the lvz this suggests the p coda i e pdp phases as a tool for monitoring lateral variations in the lvz and possibly to prove the existence or absence of a lvz in the p velocity

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

boundary layer model of mantle plumes with thermal and chemical diffusion and buoyancy summary we present a boundary layer model for mantle plumes driven by thermal and chemical diffusion and buoyancy the problem is solved for a boussinesq newtonian fluid with infinite prandtl number and constant physical properties we focus on axisymmetric mantle plumes but also solve 2 d plumes due to line sources for comparison the results show that chemical plumes are much thinner than thermal plumes because of small chemical diffusivity in the mantle when pressure release partial melting occurs in a thermal chemical plume at least two mantle components may be involved one from the chemical plume and one from the ambient mantle a buoyant chemical boundary layer in the plume source region tends to cause narrow and strong plumes a dense chemical source would have the opposite effect the effects of chemical buoyancy diminish as the lewis number the ratio of thermal to chemical diffusivity increases for fully developed mantle plumes the effects of chemical buoyancy may be insignificant the physical parameters of mantle plumes may be estimated using surface information deduced from swell models the total heat input from the hawaiian plume source is about 1 3 x lo1 w nearly 5 10 per cent of the total heat loss from the core the depth of the hawaiian plume source is constrained to be near the core mantle boundary our results show that 2 d plumes are generally stronger than axisymmetric plumes

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

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

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

frequency dependence of the visibility and depths of mantle seismic discontinuities the visibility and apparent position of a seismic gradient zone depends on the seismic wave frequency at which it is observed which we illustrate by showing how the visibility and depth of the olivine phase transformation 410 km discontinuity change as temperature varies temperature variations change bottomside reflection coefficients by factors of 2 4 in the 0 5 1 0 hz frequency band which can explain spatial variability in short period p 410p observations and the small apparent variability in observed 410 km discontinuity depths discontinuity depths inferred from the time lags of these reflections are also frequency dependent leading to a frequency dependence of estimates of mantle lateral thermal variability based on discontinuity displacements the frequency dependent seismic clapeyron slopes for the olivine phase change we compute are 2 04 mpa k 1 at 0 2 hz and 2 12 mpa k 1 at 0 05 hz and differ significantly from thermodynamic slope for the mg2sio4 end member

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

steeply reflected scsh precursors from the d region we present evidence for precursors to the scs and sscs sds and ssds phases observed at epicentral distances smaller than 30 these precursors are intermittently observed in broadband recordings from the six incorporated research institutions for seismology stations used western pacific region and south america they appear approximately 35 50 s ahead of the scs arrival on the transverse component and are independent of hypocentral depth the observed precursors are characterized by relatively large amplitudes in the frequency band from 0 05 to 0 2 hz a more detailed analysis is restricted to a subsidiary data set from station snzo new zealand the coherency frequency dependence slowness polarity and polarization of the precursors are discussed many explanations for sds can be rejected and we conclude that a reflector 180 km above the core mantle boundary causes these occasionally strong precursors the large sds scs amplitudes at low frequencies require an unrealistically large impedance contrast for a one dimensional model we test the possibility of focusing the sds phase by a discontinuity with topography and show that structure with scale lengths of l9 25 1200 1600 km i e larger than the fresnel zone can account for large intermittently observed sds amplitudes with their geometrical reflection points within the same fresnel zone this is surprising since it is often assumed that scale lengths smaller than the fresnel zone must be responsible for variations within a fresnel zone the limited data set permits no conclusions about the global properties of this phase or its implied reflector

sensitivity kernels of pp precursor traveltimes and their limitations for imaging topography of discontinuities we analyze the sensitivity of pp precursor traveltimes that are often used to infer lateral variation in the depths of the 410 and 660 km discontinuities in the mantle previous results were inconclusive due to complex wave phenomena such as multiple energy conversions and focusing defocusing that hamper their interpretation using spectral element synthetics and frechet derivatives calculated with adjoint methods we compute sensitivity kernels for volumetric and boundary parameters in a 1 d model for representative epicentral distances of past studies and a dominant period of 11 25 s our results indicate that the boundary sensitivity of pp precursors is low and that these phases are not coherently seen in exact synthetics our most important finding is the strong sensitivity to both shear and compressional wave speeds indicating that wave interference and wave conversions are dominant the pp precursor traveltimes appear more sensitive to structural parameters that is compressional and shear wave speed than to the boundaries therefore they are unlikely sources for valuable insight into discontinuity topography

reflectivity of the 410 km discontinuity from pp and ss precursors a number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long period seismograms on a surface reflection phase before stacking in particular ss and pp precursors from the 410 km discontinuity termed p410p and s410s have revealed long wavelength topography of this discontinuity here we extend these techniques to examine the reflection coefficient of the 410 km discontinuity using our measurements of p410p and s410s amplitudes we constrain the impedance contrasts across the 410 km discontinuity we also show lateral variations in the p wave impedance contrast at 410 km which is typically low under north america and china and higher beneath the north pacific the s wave impedance contrast shows less variability on the regional scale however analysis of p410p and s410s amplitudes over smaller areas by binning traces into spherical caps shows that the s wave reflection coefficient varies over much shorter scale lengths than that for p waves the different patterns of variation for p410p and s410s reflection amplitudes could be due to the presence of melt water or other chemical heterogeneities in the transition zone other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes but they would be expected to lead to correlated variations and so they cannot explain all the variation that we observe

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

subducted lithosphere beneath the kuriles from migration of pp precursors abstract we seismically image both thermal and chemical heterogeneity of the mantle beneath the kurile subduction zone using p wave energy reflected from the underside of discontinuities arriving as precursors to the seismic phase pp we take advantage of new broadband seismic data provided by the high lava plains seismic experiment and earthscope s usarray collecting a dataset of 31 high quality sumatran earthquakes sampling beneath the kuriles we employ high resolution array analysis techniques including migration and vespagrams to identify precursory arrivals and study lateral variations in discontinuity depth sharpness and impedance of the mantle transition zone we find the 410 km boundary is at 395 km near the subducting kurile slab though the boundary is 410 425 km deep elsewhere in regions away from subduction we do not detect a laterally continuous underside reflection of p waves from the 660 km discontinuity however in the vicinity of the subducting kurile slab we detect robust p660p reflections from interfaces near 620 670 km depth signifying an increase in the impedance contrast at 660 km depth we also detect deeper reflectors down to 720 km depth beneath the kurile slab in a localized area cold aluminum depleted harzburgitic lithosphere residing at the base of the transition zone best explains the local enhancement of the 660 km discontinuity p wave impedance contrast our new discontinuity measurements support the hypothesis of cold depleted lithosphere stagnating at the 660 km discontinuity beneath the kuriles subduction zone and imply the 660 km boundary can locally impede mantle flow and produce chemical heterogeneity within the transition zone

moho offset across the northern margin of the tibetan plateau anomalous double pulse teleseismic p wave arrivals were observed at one station near the northern margin of the tibetan plateau the azimuthal dependence of the waveform distortion and its absence at nearby stations indicated that the distortion was produced by receiver side crustal heterogeneity modeling of the three component data revealed a 15 to 20 kilometer moho offset that occurs over a narrow lateral range of less than 5 kilometers this east west striking offset separates the thick tibetan plateau crust from the qaidam basin crust such a sharp crustal thickness change implies a weak tibetan plateau crust that thickens vertically in response to penetration by india from the south and to blockage caused by a strong qaidam basin crust to the north

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

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

sharp 400 km discontinuity from short period p reflections p wave data from the short period around 1 hz southern california seismic network reveal pre critical reflections from the 410 km discontinuity below the gulf of california appearing as a clear and continuous feature in the data to epicentral distances as small as 10 deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving p waves the reflections are used to estimate the width of the gradient zone near 410 km depth constraining the velocity jump across the discontinuity using critically refracted waves the velocity jump across the 410 km discontinuity is 6 2 5 which is in agreement with existing estimates of the velocity jump for this region the clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km with most of the velocity increase occurring over about 4 km or less for the reflections to be visible in short period data applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least 410 km depth

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

imaging upper mantle discontinuity topography using underside reflection data this paper presents a method to invert underside reflection p d p or s d s arrivals data for lateral depth variations of upper mantle discontinuities combining traveltime and amplitude data the method greatly improves the resolution of small scale undulations obtained by existing imaging methods and does not suffer from the long wavelength biases that are likely to be present in currently available models existing inversion methods account for the large size of the fresnel zone of underside reflections but not for its complexity arising from the mini max traveltime nature of pp and ss related waves this neglect results in long wavelength artefacts from small scale undulations of the discontinuities obscurring true long wavelength depth variations the inversion method presented in this paper uses a complex valued sensitivity kernel derived from the representation of underside reflections through a kirchhoff integral formulation the sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the fresnel zone the method is applied to a large synthetic data set the data set consists of p d p amplitudes and traveltimes the results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the fresnel zone of individual underside reflections but larger than the dominant wavelength retaining the resolution of large scale variations the results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad band p670p data sets

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

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

an explanation for earth s long term rotational stability paleomagnetic data show less than approximately 1000 kilometers of motion between the paleomagnetic and hotspot reference frames that is true polar wander during the past 100 million years which implies that earth s rotation axis has been very stable this long term rotational stability can be explained by the slow rate of change in the large scale pattern of plate tectonic motions during cenozoic and late mesozoic time provided that subducted lithosphere is a major component of the mantle density heterogeneity generated by convection therefore it is unnecessary to invoke other mechanisms such as sluggish readjustment of the rotational bulge to explain the observed low rate of true polar wander

transport slopes sediment cover and bedrock channel incision in the henry mountains utah 1 field data from channels in the henry mountains of utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds the cover effect we compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit navajo sandstone but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources bedrock channels that contain abundant clasts diorite rich have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment diorite poor the diorite poor tributaries have incised more deeply to lower average slopes and have more reach scale slope variability which may reflect bedrock properties longitudinal sediment sorting and incision at lower sediment supply diorite rich channels have less bedrock exposed and smoother longitudinal profiles than diorite poor channels we find that 1 coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision validating the hypothesized cover effect in fluvial incision models 2 the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock suggesting that the slope of incising bedrock channels can become adjusted to the sediment load 3 when abundant sediment is available transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope and 4 cover effects can be important even when moderate amounts of bedrock are exposed in channel beds

reflections of p p from upper mantle discontinuities beneath the mid atlantic ridge summary precursors to p p observed in japan from a large deep earthquake mb 6 4 in the fiji islands are analysed to investigate reflection properties of upper mantle discontinuities beneath an area close to the mid atlantic ridge mar in the equatorial atlantic ocean traveltime analysis indicates that the precursors are interpreted as underside reflections of p p from discontinuities at depths of 655 and 415 km in the mantle the latter reflection is much weaker and more intermittent than the former slant stacking and beam forming are attempted to measure the slowness and incidence azimuth of p p and its precursors the obtained slownesses are 3 4 and 2 8 s deg 1 for p p and p p respectively little deviations of the ray incidence off the diametral azimuth are found for both phases amplitude ratio p dp p p here d 655 or 415 is studied as a function of the wave period and the distance of reflection point of p p to the axis of mar the overall trend of the spectral amplitude ratio for p 655p is consistent with the theoretical value for the 670 km discontinuity of the prem model dziewonski anderson 1981 p 415p signals were very weak and it was difficult to obtain an unbiased estimate of p 415p p p the amplitude ratios p 655p p p of filtered seismograms with a passband of 0 5 4 0 s have a mean value of 0 13 and a standard deviation of 0 03 this amplitude ratio is in good agreement with those of previous studies on the reflections from the 650 km discontinuity beneath antarctica and surrounding seas when the reflection points of p p are calculated under the assumption of a spherically symmetric earth the observed p 655p p p seems to decrease as the points become close to the axis of mar thus it could be inferred that some changes in the reflectivity of the discontinuity might occur at or near the spreading center another interpretation may also be possible the low velocity beneath the mar region would cause focussing of p p rays

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

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

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

upper mantle converted waves beneath the nars array several registrations of teleseismic events on the nars array show a phase which can be identified as a p to s conversion at the 670 km discontinuity beneath western europe variations in the amplitude and arrival time of the so called ps phase are observed for different azimuths a single station multiple event stacking technique was applied to the data to determine the consistency of the phase the results are indicative of amplitude arrival time and slowness variations of the converted phase on a local scale these observations are most logically explained by a model with depth variations of the 670 km discontinuity of the order of 20 km over a distance of 200 km

lateral variations in mantle velocity structure and discontinuities determined from p pp s ss and ss sds travel time residuals on the basis of p pp s ss arrival times and ss s410s ss s660s differential times we construct models of mantle p and s velocity structure and boundary topography of the 410 km and 660 km discontinuities events from the catalog of the international seismological centre isc are relocated relative to the international association of seismology and physics of the earth s interior 1991 iasp91 velocity model using both p and s arrival times the arrival times are corrected for ellipticity and the pp and ss residuals are corrected for the topography at the bounce point the cap averaged pp p and ss s differential time residuals plotted at the pp and ss surface reflection points form broad coherent patterns the geographic distribution of the cap averaged residuals agrees quite well with pp p and ss s differential time residuals derived from long period global digital seismograph network gdsn data a robust lp inversion scheme is used to infer global mantle structure synthetic tests indicate that for regions well sampled by ss s410s and ss s660s differential times the velocity estimates are not seriously contaminated by the topography of the 410 and 660 km discontinuities however estimates of boundary deflections may be influenced by extensive p and s velocity variations of 3 or greater we find the 410 km discontinuity to be depressed by as much as 24 km beneath north america conversely the discontinuity is deflected upward underneath eurasia in some regions the topography of the 660 km discontinuity is quite distinct from that of the 410 km discontinuity but the two appear to be positively correlated a series of depressions are found at several intersections of the 660 km discontinuity with known subduction zones the elevated topography in the 410 km discontinuity beneath europe is underlain by a trough in the 660 km discontinuity a number of subduction zones are characterized by a thinning of the transition zone negative p and s velocity anomalies underlying back arc basins and tectonically active continental regions encircle the pacific where they are resolved the stable continental cratons are systematically positive velocity features that extend below 200 km with the inclusion of pp and ss travel time residuals we are better able to constrain midmantle structure most notably in the depth range 35 660 km beneath the northwest pacific we observe high p velocity where they are resolved mid ocean ridges are most clearly imaged as low velocity features in the s model the northern portion of the mid atlantic ridge is underlain by negative s velocity anomalies in the pacific the east pacific rise is an extensive low s velocity anomaly

precursors to p p re examined using broad band data summary observations of underside p wave reflections from discontinuities deep within the upper mantle which precede the p p phase have provided important constraints on the existence and physical properties of these dynamically critical features in this study we extend earlier work by examining for the first time broad band recordings of those earthquakes for which one would expect to observe p p and its associated precursors p dp an exhaustive search of 20 yr of carnegie data and 11 yr of grafenberg data uncovered a number of fine p p observations but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth these null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of p velocity near 670 km or alternatively the scale length of discontinuity topography which acts to defocus the precursor this has been done by using the reflectivity and gaussian beam methods to compute synthetic seismograms for a suite of 1 d and 2 d earth models which vary slightly from prem we find that the null data may be satisfied either by i reducing the prem impedance jump at 670 km from 8 5 to 4 5 per cent ii smoothing this 8 5 per cent jump linearly over a zone 15 30km thick or iii imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength for the one case of an observed precursor the relative size of the underside reflection and main p p phase suggests that in this instance the precursor s amplitude is correctly predicted by prem

broadband observations of ps conversions from the upper mantle in eurasia summary the delay and sum method by vinnik 1977 to detect weak ps conversions in the p coda is applied to german czechoslovakian and chinese broadband data clear phases a factor 4 5 above the noise level are observed in the 3 11 s period band from the 670 and 400 km discontinuities after summing several hundred traces the decisive step to obtain such a good signal to noise ratio was the summation of records from stations in different regions the summation of a large number many hundreds of traces of a single station does not result in similar improvements because of coherent signal generated noise due to scattering below the station there are weak indications of a 520 km discontinuity in our data the delay times relative to p of the converted phases are stable within 1 s between central europe and china

detection of underside p reflections at mantle discontinuities by stacking broadband data underside p reflections at mantle discontinuities are difficult to observe directly on seismograms mainly because of their low amplitude it is shown that stacking seismograms obtained at several epicentral distances but having their pp bounce points in the same region enables the identification of such phases this method is applied to data from japanese and taiwan events recorded by the nars array and other broadband european stations discontinuities at about 670 and 300 km are revealed beneath northern eurasia

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

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

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

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

development of finite strain in the convecting lower mantle and its implications for seismic anisotropy 1 seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle a combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics specifically it is investigated whether lattice preferred orientation lpo can explain the anisotropy observed in regions of paleosubduction since lpo is caused by dislocation creep and is destroyed by diffusion creep we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric strain is then calculated in these regions and is related to mineral physics experiments combined with high pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range the effect of variations in transition stress activations parameters and strength of slabs on fabric development is investigated it is shown that lpo is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs