investigation of a lower mantle shear wave triplication using a broadband array sh signals recorded by an array of four broadband benioff 1 90 instruments run by caltech are inspected for evidence of a lower mantle shear wave velocity discontinuity previously detected using wwssn data deep focus earthquakes beneath argentina span the distance range 78 to 82 from the array where the triplication in the travel time curve produced by the discontinuity is expected to result in an arrival between the direct shear wave s and the reflection off of the core mantle boundary scs the sh data for these events do show evidence of the arrival however the broadband signals also show significant receiver structure complexity empirical receiver functions are determined for each station by stacking observations for events in south america closer than 75 from the array incorporating these receiver functions into the synthetic modeling demonstrates that lower mantle shear velocity models with a discontinuity fit the observations better than models without one

the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

p velocity heterogeneities in the lower mantle determined with the german regional seismic network improvement of previous models and results of 2d modelling abstract p waves recorded at the german regional seismic network grsn the grafenberg array the german experiment seismic system geress mini array and two czech stations are used to analyse the inhomogeneous lowermost mantle d under northern siberia and the nansen basin events with a good signal to noise ratio are used to determine the extension of the anomaly relative travel time residuals compared with a 1d model with a relfector in d give information on the shape of the anomaly improved travel time residuals are derived by applying a source or and station correction this correction which was not previously applied reduces the size of the inferred peak to peak topography at the top of d by 20 km if the sources are near lateral inhomogeneities e g near the bend of a subduction zone such corrections are necessary otherwise models of d structure can be severely biased modelling of waveforms for such laterally inhomogeneous structures in d show strong effects on the wave field at dominant periods of 1 to 5 s as multipathing of pdp the p wave reflected at the top of d travel time differences and strong variations in amplitude in agreement with the observations typical dimensions of such hills and valleys are 400 to 1200 km laterally and 10 to 100 km in topograph

analysis of asymmetric multipathing with a generalization of the double beam method abstract an array method integrating source and receiver array beamforming is presented the method is a generalization of previous work kruger et al 1993 to independent variation of slowness and azimuth in both the source and the receiver array compared to the analysis capabilities of a single array the resolution is enhanced and asymmetric scattering configurations and multipathing can be studied the generalized double beam method dbm is applied to a data set consisting of nuclear explosions detonated in eastern kazakhstan and recorded at the yellowknife array in northern canada the multipathing found for pcp and pdp is interpreted in terms of lateral inhomogeneity in the lowermost mantle below severnaya zemlya indications for rayleigh to p wave scattering observed for the shagan river and degelen subsites of the former main test site of the ussr in eastern kazakhstan can be related to the structure in the vicinity of the two subsites

double beam analysis of anomalies in the core mantle boundary region seismic p waves from nuclear explosions at the kazakhstan test site in eastern kazakhstan kts observed at the yellowknife array yka canada are used to analyze the structure at the base of the mantle under severnaya zemlya arctic sea 18 events at the degelen subsite and 9 events at the shagan river subsite of kts are studied the simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration a new method double beamforming integrating both concepts is presented this method increases the resolution considerably for the degelen yka path one anomalous arrival 1 9 s before pcp slowness between p and pcp slowness is observed smaller anomalous arrivals are found 6 5 s degelen yka and 8 1 s before pcp shagan river yka these anomalous phases cannot be explained with standard earth models they are produced by reflections from an inhomogeneity in the lowermost mantle at 82 3n 107 5e under the area north of severnaya zemlya the strong variations between the wavefields for the two source receiver combinations indicate a heterogeneous structure in the lower mantle the anomaly found here is located close to an anomaly in the lowermost mantle beneath the nansen basin

the influence of crust and upper mantle heterogeneity on short period waveform distortion abstract short period teleseismic p waves may suffer considerable distortion owing to the effects of upper mantle crustal heterogeneity this sensitivity suggests the use of p waveforms as an aid to constraining upper mantle structure however this requires a means of separating the relative contributions from these two regimes we investigate the nature of short period waveform distortion as recorded on the washington regional seismograph network wrsn a technique for analyzing short period telescismic p waveform distortion is presented to extract the impulse response function of the underlying crust and upper mantle for a given event station pair the technique is shown to yield highly reproducible results and permits direct comparison of response functions for different source locations short period waveform distortion can be strong and is essentially incoherent between stations minimum separation 20 km we subsequently confine our attention to a single station where waveform distortion is evidenced by strong defocusing and examine the behavior of this distortion as a function of source location in addition to crustal defocusing which persists over a broad range of azimuths and epicentral distances we also find evidence for more subtle variations in the initial waveform which may be attributable to upper mantle structure these observations are modeled in two ways we employ a parabolic approximation method to simulate distortion by deep seated large scale structure as exemplified by a best estimate model of the upper mantle below wrsn derived from travel time inversion the resulting synthetics fail to exhibit strong waveform distortion the principal effects are focusing defocusing which accompany geometrical spreading crustal heterogeneity is probably stronger and exists at smaller scales comparable to the wavelengths of the short period waves hence contributions to waveform distortion through scattering processes become important these contributions are investigated qualitatively using an acoustic finite difference code and models of crustal heterogeneity comprising volume heterogeneity and interface topography waveforms vary markedly over station separations as small as 5 10 km but at a given station are relatively insensitive to changes in source location crustal contamination will seriously complicate the retrieval of information on upper mantle structure from short period waveform distortion however these results suggest that the identification of an upper mantle signature can be achieved by comprehensive analysis of the impulse response at a given station as function of epicentral distance and back azimuth

deconvolution of teleseismic recordings for mantle structure summary delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances this deconvolution problem can be posed in a log spectral domain where the operation of time domain convolution is reduced to an additive form the introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers the system is inherently singular and stabilization is accomplished through the supply of an initial estimate of the source time function although alternative choices exist an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular value decomposition of the suite of aligned seismograms corresponding to a given event the relation of the deconvolution scheme to simultaneous least squares deconvolution is examined application of the methodology to broadband teleseismic p waveforms recorded on the canadian national seismograph network demonstrates the retrieval of effective green s functions including secondary phases associated with upper mantle structure

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

seismology of the lower mantle and core mantle boundary the thermal and chemical processes operating near the core mantle boundary cmb play a major role in the planet s evolution influencing the magnetic field behavior chemical cycling in the mantle irregularities in the rotation and gravitation of the planet and the configuration of mantle convection the past decade has witnessed extensive investigation of the core mantle transition zone using seismological geodynamical geomagnetic and mineral physics experimental techniques the many multidisciplinary contributions on this region have been surveyed in several recent review articles e g loper and lay 1994 jeanloz and lay 1993 bloxham and jackson 1991 which provide a comprehensive survey of the recent literature i confine my attention here to recent seismological evidence for laterally heterogeneous structure in the d region the lowermost 200 km of the mantle while seismic models for d are still low resolution we have attained general constraints on the extent of heterogeneity which raise provocative questions as to the origins of the complex structures the idea that chemical and thermal heterogeneity in d is a manifestation of boundary layer processes is a long standing one bullen 1949 but we are far short of quantifying this notion in particular it is not known whether heterogeneities in d are relics of subducted slabs in situ accumulations of core mantle chemical reaction products primordial residuum from the core formation process or thermal heterogeneities within a hot unstable thermal boundary layer

a shear velocity discontinuity in the lower mantle a lower mantle s wave triplication is detected using short and long period sh seismograms in the distance range 70 sup 0 to 95 sup 0 modeling of the observations with synthetic seismograms indicates that the triplication is produced by a 2 75 0 25 shear velocity increase about 280 km above the core mantle boundary the sh data from intermediate and deep focus events for three distinct source region receiver array combinations show generally consistent travel time and relative amplitude behavior of the triplication there are small systematic travel time shifts between the source regions which suggest regional variations in the depth of the discontinuity of up to 40 km

diagnosis of a jet streak in the vicinity of a severe weather outbreak in the texas panhandle abstract this is a case study of the synoptic and mesoscale aspects of a severe weather outbreak in the texas panhandle we offer circumstantial evidence that the rising branch of a thermally indirect circulation in the exit region of an unusually intense upper level jet streak played a role in storm formation and sustenance the jet streak s vertical circulation could not be accounted for by straight dynamics alone curvature was important especially along the right side of the exit region the geostrophic momentum approximation leads to a reasonable qualitative explanation of the ageostrophic circulation while quasi geostrophic theory does not

seismic discontinuity at the top of d a world wide feature considering the growing number of observations of a seismic discontinuity at the top of d some 300 km above the core mantle boundary we investigate the scenario of whether this feature could be global we show that the mean first arrivals of p waves as used to build global models can be fit by the two branches of a discontinuous model we argue that the intermittence of the detection of the reflector is due to lateral variations of a global discontinuity these observations put strong constraints on the type of chemical layering that could explain the discontinuity we speculate that the properties of the d discontinuity are better explained in terms of a new phase transition

lateral variations in d thickness from long period shear wave data we explore global variations in d shear wave structure by examining 12 years of long period global digital seismograph network gdsn data to identify reflected phases from the d layer in the lowermost mantle we restrict our search to epicentral distances between 63 and 74 where a precritical d reflection will lie between the s and scs arrivals with gdsn long period records a d signal at these ranges is generally obscured by the stronger neighboring phases so to isolate the d reflections we develop a technique for stripping away the interfering s and scs waveforms we interpret travel time variations in the observed sds phases in terms of variations in d thickness and generate maps of inferred d thickness for areas of good seismic coverage the regions of observed d reflections are beneath australasia north central asia the arctic alaska and central america we find considerable variation in d structure in these regions a result consistent with previous studies which have found evidence for lateral heterogeneity in d on average the estimated d thickness is 260 km but there is a fairly uniform distribution of thicknesses between 150 km and 350 km a possible correlation is observed between d thickness and regions of predicted strong horizontal mantle flow

laterally varying reflector at the top of d beneath northern siberia summary we analyse p wave data from the french laboratoire de dctection et gcophysique l d g network of digital short period seismic stations whereas our previous paper houard nataf 1992 showed a few promising kurd events and depicted the methods we proposed to apply an extensive analysis of 32 earthquakes from the kurivkamchatka region has now been carried out by using the deconvolution technique previously described houard nataf 1992 clear and quantitative evidence of intermediate arrivals between the p and pcp waves is shown in the 75 82 distance range our approach is complementary to previous studies in the same region which used broad band data recorded at a dense narrow aperture array weber davis 1990 weber 1993 or long period data recorded at the wwssn stations gaherty lay 1992 for each station record sections of deconvolved signals from different earthquakes are built and lead us to eliminate crustal reverberations beneath l d g stations as a major explanation for these secondary arrivals a compilation of the most conclusive signals has been made it is perhaps one of the most complete pdp wave record sections since the move out with distance can be followed clearly with a good sampling over more than 5 the move out with distance of this extra pdp arrival its residual slowness value eliminates source complexities diffraction or multipathing through the kuril subduction slab as possible explanations the extra arrivals indicate the existence of a lower mantle reflector the structure of the lay discontinuity is investigated using tpdp tp residual traveltimes two different residual time versus distance tendencies are found which correlate to different geographical bounce point regions indicating the existence of lateral variations of the structure the d region is also sampled further south at distances well beyond the theoretical triplication cross over by analysing events from the honshu ryukyu islands region but only one p wave branch is observed the hodochrones and p a curves of the first p arrival are used as a complementary analysis the global prem model dziewonski anderson 1981 predicts correct p wave slownesses though its smooth lower mantle structure does not account for pdp observations this may be an indication that the lay discontinuity is a global feature however the hodochrones of the first p arrival do not allow us to test the existence of the discontinuity in the 82 85 cross over distance range

lateral variations near the core mantle boundary differential travel times between s and sks ts sks and skks and sks tskks sks along with amplitude behavior have been used to study lateral variations at the core mantle boundary the data were recorded by the world wide seismographic station network wwssn and the canadian seismograph network csn from source regions in south america and the fiji tonga area a core mantle boundary region traversed by sks and s beneath the mid pacific from the fiji tonga area to north american stations shows the most anomalous behavior in ts sks sks and s are anomalously separated by up to 8 seconds in addition to ts sks anomalies sks and skks are anomalously separated by 2 to 3 seconds a model fitting ts sks for this anomalous region requires one or more of the following 1 up to 5 increase in the top 300 km of outer core velocity 2 a large scale high vs region 2 increase in the lower mantle ne of tonga 3 lateral variations in d shear wave velocities of up to 3 however 1 above predicts a decrease in tskks sks contradicting the observations this suggests that 2 and 3 are more likely explanations in modeling this data set

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

dynamical influences from thermal chemical instabilities at the core mantle boundary two dimensional time dependent calculations show that the thermal and flow structures can be influenced by the lower mantle thermal chemical instabilities the style of the core mantle boundary cmb deformation in thermal chemical convection is different from that obtained in thermal convection the amplitude of cmb hills is reduced greatly from the depth dependence of thermal expansivity found in recent high pressure experiments

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

short wavelength geoid bathymetry and the convective pattern beneath the pacific ocean summary the geoid and bathymetry data in the pacific ocean are used to shed new light on the structure of upper mantle convection at short wavelengths 200 km the geoid pattern is characterized by parallel lineations lineations however appear less clearly at larger wavelengths 400 or 600 km where the signal due to convective thermal anomalies might be expected the most prominent features among the short wavelength lineations are associated with the well known chains of seamounts and islands other lineations observed in the geoid coincide with computed tracks of other volcanic centres although these tracks are not continuous this suggests that long lived plumes are at the origin of the apparently short chains of seamounts apart from the lineations corresponding to the tracks of classical hotspots there are numerous similar lineations visible on the filtered geoid maps which have the same directions as hotspot tracks these preferential directions and the gravity to bathymetry ratios seem to indicate that the lineations are due to magmas implaced just above or within the lithosphere convective plumes fixed with respect to each other and much more numerous than the classical hotspots could be the source of these magmas a geometrical test of this hypothesis is performed by comparing the lineation pattern with the computed tracks for a set of tentative new hotspots in this way numerous plumes long lived and fixed with respect to each other are found to provide an explanation of the observed small wavelength geoid features

a gradient ratio method for the semi automatic interpretation of gravity map data sets the application of semi automatic interpretation techniques to potential field data can be of significant assistance to a geophysicist this paper generalizes the magnetic vertical contact model tilt depth method to gravity data using a vertical cylinder and buried sphere models the method computes the ratio of the vertical to the total horizontal derivative of data and then identifies circular contours within it given the radius of the contour and the contour value itself the depth to the source can be determined the method is applied both to synthetic and gravity data from south africa the matlab source code can be obtained from the author upon request

evidence for a p wave velocity anomaly in d a number of strong p coda phases have been observed on european wwssn station short period recordings of intermediate magnitude kurile arc earthquakes which occurred at several different depths of focus these phases have been observed for several different events and cannot be explained by known phases such as pcp or source multiplicity a lower mantle model containing a first order discontinuity with a 2 75 p wave velocity contrast and located about 344 km above the core mantle boundary produces synthetic seismograms which approximately reproduce the observed amplitudes and arrival times of the coda phases however the fact that these phases are not always observed and that their amplitudes vary significantly for different events and stations suggests that the discontinuity at the top of d may be a transition zone and may be laterally variable in thickness and contrast

mapping s velocity heterogeneities in the d region from smks differential travel times abstract seismic velocity heterogeneities in the d region are studied using differential travel time residuals between smks waves previous studies indicate that regional variations in the propagation times of these waves are not due to the propagation through the liquid core furthermore they are only weakly influenced by the topography of the core mantle boundary cmb thus the differential travel time anomalies are interpreted as the result of lower mantle heterogeneity we compiled all the available published data and added new observations of travel times from geoscope stations we thus obtained 807 s2ks sks and 209 s3ks s2ks differential times differential travel time residuals with respect to the mean earth model prem preliminary reference earth model are corrected for ellipticity and for mantle heterogeneity down to 2590 km depth 300 km above the cmb using the 3 dimensional s wave velocity model sh12 wm13 this model gives a 10 variance reduction of the residuals the remaining variance of residuals is higher for s2ks sks than for s3ks s2ks which is consistent with a d origin for the heterogeneities the data are inverted for regional variations using a least squares analysis assuming a single layer d structure 300 km thick the earth is divided in 150 equal area sectors about 115 of which are sampled by four rays or more we performed inversions using 18 different geographic grids and averaged the results so as to minimize the artefacts induced by imposing artificial block boundaries the velocity heterogeneity map though affected by the poor sampling of the oceans reveals significant long wavelength structure with velocity perturbations exceeding 3 with respect to a reference velocity which is slightly lower than the prem one this heterogeneity map is consistent with the results from most of previous studies global as well as regional the location of some of the high velocity anomalies coincides with places where subducted slab material may have accumulated at the cmb these high velocity heterogeneities are also generally correlated with places where a discontinuous velocity increase of several per cent has been detected at the top of d

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

traveltime sensitivity kernels for pkp phases in the mantle abstract we investigate the finite frequency effects of perturbations of compressional wave velocity on the traveltimes of pkp phases owing to their long paths in the mantle pkp phases have a large fresnel zone at the cmb and thus sample a large volume of the d layer we compute traveltime frechet derivatives of the three pkp phases in the mantle with respect to the ak135 reference model taking into account the coupling between the different branches pkp kernels do not have a simple banana doughnut shape because the different pkp branches interfere with each other and because of the presence of shadow zones and caustics in addition we compute the differential kernels ab bc bc df and ab df the geometry of pkp differential kernels in d show significant qualitative differences from those predicted by simple ray tracing this suggests that the tomographic techniques commonly used to image lower mantle structures could be improved by using 3d frechet kernels

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

evidence against a discontinuity at the top of d summary we investigate a recent suggestion of lay helmberger that there is a world wide discontinuity in 5 wave velocity about 250 300 km above the core mantle boundary with an increase of 2 5 3 per cent from above to below their model predicts pronounced pulse form changes of long period sh body waves in the epicentral distance range 95 120 a range which has not been studied by them similar pulse form changes are expected in p wave seismograms under the reasonable assumption of a similar increase in p velocity nine long period seismogram sections of sh and p waves from deepfocus earthquakes below fiji west brazil the sea of japan the sea of okhotsk and the banda sea are presented with practically no indication of the complications expected also the decay constants of pdiff as predicted by the new model differ strongly from observed values thus there is clear evidence against this model the pulse form peculiarities between mantle s and scs in transverse component seismograms at distances from 70 to 80 which lay helmberger interpreted as reflections from the discontinuity of their model must have a different cause

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

seismic imaging the d region beneath the central atlantic abstract the lowermost mantle beneath the central atlantic is bounded by the western edge of african large low shear wave province llsvp and a high velocity region beneath the eastern americas which is attributed to the accumulated subducted slab material at the core mantle boundary cmb data recorded at broadband seismic arrays in western europe and morocco allow us to investigate the detailed structure beneath the central atlantic which provides an important constraint on how remnant slab material and llsvp interact and reshape the d layer here we exploit lower mantle triplicated s waveform data from deep earthquakes beneath south america our modelling results show that the d discontinuity is constantly observed from north to south beneath the central atlantic but at varied depths and with varied shear velocities in the lowermost mantle beneath east of caribbean our results indicate a sharp 2 1 shear velocity increase at 340 km above the cmb representing a possible bridgmanite dominant normal mantle d layer whereas the models of northern and southern portions of central atlantic indicate a gradual velocity increase of 4 2 across 200 km starting from 360 km above the cmb downwards we also find a distinct low velocity zone on top of the velocity jump which can be interpreted as an upwelling produced by the dynamic response of ancient slab material reaching the cmb

a search for anomalies in the lowermost mantle using seismic bulletins abstract for deep events and epicentral distances around 75 the bulletins of the international seismological centre bisc contain many secondary arrivals between p and pcp the reflection from the core mantle boundary cmb which cannot be explained by current spherical symmetric earth models assuming that these arrivals in the p coda are reflections from p velocity anomalies in the lower mantle we calculate the associated reflection points geographical coordinates and distance from the cmb and map both strength relative density of reflection points in bins in the lower mantle and distance of the reflectors from the cmb on a global scale the regions with these anomalous bins cover 3 of the 50 of the lowermost mantle that can be analysed with this data set the most significant regions with anomalous bins values in parentheses give a preliminary estimate of the distance of the reflector from the cmb are under the north atlantic ridge 250 km northern central siberia 230 km japan 180 km the central mid atlantic ridge 310 and 260 km and off the coast of ecuador 340 km the presence of reflectors in one of these regions has already been confirmed by detailed seismic studies using waveform data from arrays and networks a slight 2 3 p velocity contrast is sufficient to produce measurable reflection amplitudes in the distance range of 70 80 ten percent of the lower mantle sampled by this study is covered by bins with no or very few secondary arrivals in the p coda the most significant regions with such a normal lower mantle are under central america the caroline and marshall islands in the eastern pacific the area of hawaii and the easter islands and the northern atlantic identification of the regions with anomalous bins serves to locate the areas of the lowermost mantle which should be analysed further by using the full information content of seismograms the study of the travel time the amplitude and the form of the wavefield recorded preferably at seismic arrays dense networks will allow the determination of the anomalous velocity structure of the core mantle transition zone in more detail the most promising areas for such a study are given here but further anomalies are likely to exist

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

fundamental thermodynamic relations and silicate melting with implications for the constitution of d we describe fundamental thermodynamic relations helmholtz free energy as a function of volume and temperature for solids and liquids simple physically based expressions which contain all thermodynamic information about a system the solid fundamental relation consists of debye and birch murnaghan finite strain theory combined in the mie gruneisen framework the liquid fundamental relation is derived by taking the high temperature limit of the solid expression we derive the liquid equation of state which contains only four parameters from the liquid fundamental relation and show that it successfully describes measurements of liquid alkali metals water and liquid diopside over a wide range of pressure and temperature we find optimal fundamental relation parameters for diopside enstatite ilmenite and perovskite and find the solid relation to be in excellent agreement with data including heat capacities thermal expansion and mgsio3 phase equilibria we then combine the liquid and solid fundamental relations to calculate the melting curves of diopside enstatite and perovskite which are found to be in excellent agreement with experiment all predicted melting curves have dt dp slopes which decrease steadily with pressure eventually becoming negative because of liquid crystal density inversion our predicted melting temperature of perovskite in the d region 3750 k at the base of the mantle is thousands of degrees lower than previous estimates yet it is consistent with experimental data the predicted melting curve although consistent with the lack of widespread melting in the lower mantle is much lower than recently proposed geotherms in the d layer at the base of the mantle by combining our results with seismic observations of the deep mantle we propose that the d layer consists of magnesiowustite and silica in the form of stishovite or its recently discovered high pressure modification

modelling d structure beneath central america with broadband seismic data deep south american events recorded at distances between 70 and 96 are used to constrain the lower mantle structure beneath central america employing data from the terrascope bdsn iris and cnsn canadian national seismic network broadband arrays we obtain a 1 d shear model with a 200 km thick d layer with a 3 velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of sh polarized s scs and scd the shear structure above d is similar to the preliminary reference earth model prem dziewonski and anderson 1981 preliminary reference earth model physics of earth planetary interiors 25 297 356 in terms of dt d and s sks differential times sv synthetics for this model assuming a prem core produce a satisfactory fit to the data except at the larger ranges to the north where the waveforms display rapid variation this feature is indicative of lateral variation of the type suggested by grand 1994 where the discontinuity becomes less well defined synthetics computed for a simplified 2 d cross section through this structure produce the scd phase and simple s at larger distances while scd is easily observed in the shear wave data there is little evidence of its counterpart in the p data by testing several models with different p velocity jumps at the same depth as the d jump in shear structure we conclude that the velocity jump must be less than 1 in fact prem provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower mantle structure beneath central america shows clear evidence for a discontinuity in s velocity but not in p

d discontinuity structure beneath the north atlantic from scd observations we analyzed transverse and radial component recordings from the 2010 m6 3 southern spain earthquake depth 620 km recorded on 370 broadband stations in north america we grouped these seismograms into subarrays and applied fourth root vespa processing vespagram analysis in order to enhance low amplitude arrivals these vespagrams show clear scd arrivals which indicate the existence of the d discontinuity beneath the north atlantic ocean 45 60 n 45 55 w these observations are best fit with a 2 4 velocity increase at the top of the d discontinuity at a height above the core mantle boundary of 304 14 km we do not observe scd arrivals at the eastern end of our study region which is consistent with the presence of the easternmost edge of the ancient farallon plate