wave scattering theory in analysis of p wave anomalies at norsar and lasa summary observed travel time and amplitude anomalies at the norsar and lasa arrays are significantly larger than the measurement errors and cannot be explained in terms of deterministic crust and upper mantle structural models for the respective siting areas in this paper the chernov theory is used in modelling the crust and upper mantle as a random medium and also to estimate to what extent such models can account for the observed anomalies we find that the random medium modelling can explain 50 60 per cent of the variance of the time and amplitude anomalies at norsar the corresponding results for lasa are 50 60 per cent and 25 per cent respectively in contrast reasonable deterministic models cannot account for even 25 per cent of this variance while the random medium model can provide a satisfactory explanation of the observational data extensive tests have revealed that the parameters characterizing the random medium cannot be uniquely determined from the data the reasons underlying this limitation are discussed

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

regional travel time and slowness measurements for p waves in the distance range 40 57 a method for analysing travel times measured at a large array or a network of seismographs from many earthquakes within a specific region has been developed approximate relative station corrections are calculated from the residuals on a least squares line or least squares quadratic form fitted through the times for each earthquake and may be improved by iteration after a preliminary travel time curve has been derived accurate relative baseline corrections for each earthquake are also calculated iteratively and an optimum slowness distance curve is determined from the combined corrected travel times from all earthquakes using a trade off procedure calculations using synthetic travel time data suggest that abrupt changes in slowness of 0 4 s deg 1 due to the presence of triplications are generally resolvable provided that the effects of lateral variations are small even with random epicentre mislocations in the range 0 5 slowness measurements at a network of temporary stations deployed across australia do not show any discontinuities in slowness greater than 0 2 s deg 1 in the distance range 45 54 similar measurements at the warramunga array from the same source regions however suggest the presence of complexity in the slowness curve at distances close to 50 relative arrival times at the temporary network generally have standard deviations less than 0 25 s thus suggesting that details of structure finer than those derived from conventional travel time studies can be resolved

further evidence for radial velocity anomalies in the lower mantle wright andlyons 1981 used a least squares matching technique lmm and an adaptive processing method adp to study the behaviour of slowness and azimuth measurements made on two synthetic interfering wavelets having different arrival vectors and onset times we have applied these results to the analysis of real array seismograms some of the effects generated synthetically are frequently observed on real seismograms of earthquakes recorded at yellowknife at distances close to 50 and 90 we have also processed sufficient data to illustrate how the interference phenomenon can be used to confirm the presence of radial velocity anomalies in the lower mantle numerousp arrivals from south american earthquakes at distances between 78 and 98 suggest the presence of two radial velocity anomalies at depths close to 2400 and 2730 km below the caribbean region these anomalies also appear to vary laterally

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

the measurement of apparent velocity and azimuth using adaptive processing techniques on data from the warramunga seismic array summary adaptive processing of multichannel data can be achieved by cross correlating the signal on each channel with a velocity and azimuth filtered composite signal in an iterative manner in this way it is possible to accurately determine the arrival times of the wavefront at each sensor this technique which is currently used for the autematic analysis of seismic events recorded at the lasa and norsar arrays has been used as the basis of a software system developed for processing data from the medium aperture warramunga wra array in northern australia this system has been developed for a small and relatively inexpensive computer installation and permits the rapid direct measurement of apparent velocity and azimuth of signals coherent across most or all of the array the position of wra relative to the earth sactive seismic zones and in particular the belt north of australia results in multiple p wave signals from single events being frequently recorded emphasis was therefore placed on assessing the capabilities of adaptive processing for making measurements not only on the first arrival but also on its immediate coda the effects of signal multiplicity on parameter measurement were studied for both real and synthetic data using a number of different processing variables such as convergence criteria parts of waveform used and window positioning the results show that several measurements must be made across a waveform in order to identify systematic drifts shown to result from interference effects experiments using inverse filtering were performed in an attempt to improve the resolution

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

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

localized double array stacking analysis of pcp d and ulvz structure beneath the cocos plate mexico central pacific and north pacific abstract a large high quality p wave data set comprising short period and broadband signals sampling four separate regions in the lowermost mantle beneath the cocos plate mexico the central pacific and the north pacific is analyzed using regional one dimensional double array stacking and modelling with reflectivity synthetics a data screening criterion retains only events with stable pcp energy in the final data stacks used for modelling and interpretation this significantly improves the signal stacks relative to including unscreened observations allows confident alignment on the pcp arrival and allows tight bounds to be placed on p wave velocity structure above the core mantle boundary cmb the pcp reflections under the cocos plate are well modelled without any ultra low velocity zone from 5 to 20 n at latitudes from 15 to 20 n we find evidence for two p wave velocity discontinuities in the d region the first is 182 km above the cmb with a ln vp of 1 5 near the same depth as a weaker discontinuity

the p wavespeed structure in the lowermost 700 km of the mantle below the central part of the indian ocean abstract earthquakes from the indonesian region recorded at distances between 78 and 104 from broad band stations of the kaapvaal network were used to investigate the p wavespeed structure of the lowermost 700 km of the mantle below the central part of the indian ocean analysis of arrival times indicates a maximum in slowness gradients at a distance of 88 and wavespeeds in the lowermost 700 km of the mantle that are lower than in the reference model iasp91 these results suggest that the lowermost mantle is relatively hot measurements of p wave amplitudes also show a pronounced maximum at a distance close to 88 phase weighted stacking of waveforms from three deep focus earthquakes do not show any later arrivals that might identify a triplication in travel times associated with the presence of a discontinuity at the top of d however synthetic seismograms produced by both the wkbj and reflectivity methods for models having a small discontinuity at the top of d show the presence of an amplitude maximum due to constructive interference between the closely spaced signals near the cross over point of a triplication whose branches are not clearly resolvable visually the observed amplitude maximum however is too large to be explained by the smooth model midind01 fitted to the travel times stacking of synthetic seismograms constructed from models with small discontinuities at the top of d gives similar results to those for the real seismograms the observed amplitudes travel times results of phase weighted stacking and synthetic seismograms together suggest the presence of a small p wave discontinuity of 0 4 0 6 that defines the top of d 210 20 km above the core mantle boundary

abyssal currents and advection of resuspended sediment along the northeastern bermuda rise abstract an integrated study using current meters time series nephelometers and water samplers for measurements of particulate matter concentration revealed an intense nepheloid layer centered on the slopes of the eastward scarp the northeastern boundary of the bermuda rise these slopes also contained extensive furrows truncated seismic reflectors and wisconsin age and older surface sediments the abyssal circulation in this region appears to be accelerated when it encounters the steep topography of the eastward scarp eroding sediments creating the erosional furrows and moving eroded sediment upslope instruments on top of the bermuda rise recorded episodic events benthic storms where particle concentrations at 25 m above the bottom increased from a background level of 60 g 1 1 to maximum of 251 g 1 1 and current velocities were nearly 20 cm s 1 net transport of both water and entrained sediment was to the west northwest during the largest benthic storm water temperature decreased by 0 1 c consistent with a 300 600 m upslope transport of water and resuspended sediments from the upper slopes of the eastward scarp when currents carry resuspended sediment over the rim of the eastward scarp to the plateau region the current slows depositing its sediment load thus the plateau is growing upwards slowly at the expense of the surrounding scarps

penetration of anthropogenic carbon into organic particles of the deep ocean 1 in the late 1980s bomb 14c was present in suspended particulate organic carbon poc from the north central pacific ncp and sargasso sea ss throughout most of the water column demonstrating that deep poc had exchanged with atmospheric co2 in the past 30 years upon reoccupation of these sites in 1999 and 2000 respectively we observed that the 13c values of suspended poc were lower than those measured a decade earlier this demonstrates that anthropogenic co2 from fossil fuel and biomass burning has penetrated a major organic matter pool in the deep ocean 14c measured in the suspended poc showed similar or higher values in the deep sargasso sea and decreased values in the deep ncp compared to those measured previously we use a box model to show that the differences in the radiocarbon results are likely due to the presence of resuspended sediment that is laterally advected from the continental margin to the deep sargasso sea

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

decade time scale variability of ventilation in the north atlantic high precision measurements of bomb radiocarbon in banded corals the first high precision radiocarbon measurements for the upper ocean are presented for banded corals from two sites in the north atlantic ocean the striking dissimilarities between the post 1950 records at bermuda in the sargasso sea and the florida straits in the gulf stream illustrate the different mixing processes in the upper ocean at each site convective overturn associated with 18 degree water formation during late winter in the northern sargasso sea facilitates storage of considerable quantities of bomb radiocarbon at depth which accounts for the damping of the 14c signal at bermuda during the 1960s a multibox isopycnal mixing model is used to estimate the ventilation rate of the upper 700 m of the water column in the sargasso sea from 1950 to 1983 an inverse model is used that is the water mass renewal rate was calculated for the post bomb period in order to satisfy the bomb radiocarbon time history in the corals sea water radiocarbon measurements made during the geosecs 1972 1973 and transient tracers in the ocean 1980 1981 surveys are used to constrain the subsurface radiocarbon values calculated by the model results show that the rate of water mass renewal in the sargasso sea was high during 1963 1964 decreased during the late 1960s and remained low during most of the 1970s the 14c derived record of water mass renewal precedes by about 4 years that derived from isopycnal salinity in the sargasso sea jenkins 1982 illustrating that the coral 14c record is controlled to a large extent by changes in ocean circulation rather than by atmospheric exchange of co2

chronology of miocene terrestrial deposits and fossil vertebrates from quebrada honda bolivia abstract at 3500 m above sea level in the eastern andean cordillera quebrada honda basin exposes more than 300 m of middle miocene sedimentary strata developed on distal alluvial fan sediments interstratified with volcanoclastic deposits overlying paleozoic basement these deposits are rich in exotic vertebrate fauna adapted to a wetter and warmer climate at lower paleoelevation as contrasted with the present high elevation cold and dry andean conditions this study provides new magnetostratigraphic and 40ar 39ar age control of the quebrada honda rio rosario and huayllajara fossiliferous areas demonstrating that they are 13 1 12 4 ma part of the succession evidences cyclical deposition of calcretes and reddish mudstones which may be a response to precessional orbital forcing climatic processes paleomagnetic measurements show a significant clockwise tectonic rotation related to postdepositional tectonism associated with evidence of an important uplift after the deposition of quebrada honda sediments this uplift of the andean eastern cordillera was associated with the formation of the subandean fold thrust belt and the development of the amazon and la plata drainage basins proposed uplift rates of 200 m ma are compatible with well established rates of uplift recorded in contemporaneous alpine sections

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

stable nitrogen and carbon isotope δ15n and δ13c variability in shallow tropical pacific soft coral and black coral taxa and implications for paleoceanographic reconstructions abstract soft corals and black corals are useful proxy tools for paleoceanographic reconstructions however most work has focused on deep water taxa and few studies have used these corals as proxy organisms in shallow water

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

the core shadow zone boundary and lateral variations of the p velocity structure of the lowermost mantle abstract the recent determination of high quality short period p wave amplitude profiles near the core shadow zone for three source receiver combinations allows an exploration of lateral variations in p velocity structure at the base of the mantle various radially symmetric models are tested by comparison of the data with amplitudes measured from generalized ray theory synthetics the assumption necessary to justify one dimensional modeling that each of the profiles is most sensitive to scales of heterogeneity larger than the discrete regions sampled is well supported by the coherence of the individual profiles the observed amplitude versus distance profiles exhibit significant regional variations of the apparent shadow zone boundary with as much as a 5 shift in onset distance but it is possible to model the overall behavior using simple regionally varying positive p velocity gradients in the lowermost mantle velocity models with pervasive negative velocity gradients in the d layer are not consistent with the data the modeling indicates that d velocities beneath the north pole are 2 slower than those beneath the central pacific while velocities beneath the north pacific are 1 faster although the simplest class of successful models begins to deviate from the prem reference earth model as much as 690 km above the core mantle boundary these models do not violate global mantle velocity constraints and the theoretical slowness values calculated for the three models are consistent with slowness measurements for the same general regions more complex multiple gradient lower mantle velocity structures with a thinner zone of lateral heterogeneity may be compatible with the p wave data but such detailed structures cannot be resolved by our modeling though bounds can be placed on viable structures the strong lateral variations required by the data support the presence of compositional heterogeneity in d and the data require that at least in several locations the predominant p velocity gradients in d are positive

spatial and temporal offsets between proxy records in a sediment drift chronologies for late quaternary marine sediment records are usually based on radiocarbon ages of planktonic foraminifera signals carried by other sedimentary components measured in parallel can provide complementary paleoclimate information a key premise is that microfossils and other indicators within a given sediment horizon are of equal age we show here that haptophyte derived alkenones isolated from bermuda rise drift sediments are up to 7000 years older than coexisting planktonic foraminifera this temporal offset which is apparently due to lateral transport of alkenones on fine grained particles from the nova scotian margin markedly influences molecular estimates of sea surface temperatures more broadly the observation raises questions about both the temporal and the geographic fidelity of paleoenvironmental records encoded by readily transported components of sediments

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

variability of the δ13c of dissolved inorganic carbon at a site in the north pacific ocean abstract we present a depth profile of 13 c dic dissolved inorganic carbon and a time series of surface water 13 c dic from seawater samples collected at a single site during the eve cruise in the north pacific in june of 1987 our deep water results confirm those reported by kroopnick 1985 for geosecs station 213 10 west of eve site we observed a small but significant decrease in the 13 c of dic in the upper 500 m of the water column over 13 years since the geosecs survey this is likely due at least in part to the input of additional fossil fuel derived co 2 to the upper ocean we observed a significant change in surface water 13 c over a short time period 2 days that was the result of a change in the shallow surface water mass this change emphasizes the need for caution when conducting seasonal studies

seismic heterogeneous structure in the lowermost mantle beneath the southwestern pacific 1 the p and s wave velocity structure of the d layer beneath the southwestern pacific was investigated by using short period data from 12 deep events in the tonga fiji region recorded by the j array and the hi net two large aperture seismic arrays in japan reflected wave beam forming rwb and a migration method were used to extract weak signals originating from heterogeneities in the lowermost mantle in order to acquire high resolution a double array method was applied to the data the results of the rwb method indicate that seismic energy is reflected at discontinuities near the depths of 2520 and 2650 km which have a negative p wave velocity contrast of 1 at the most in addition there is a positive seismic discontinuity at a depth of 2800 km in the case of the s wave reflected energy is produced almost at the same depth 2550 km depth an apparent depth shift 50 km of the discontinuity at the depth of 2850 km may indicate that the s wave velocity reduction in the lowermost mantle is 2 3 times stronger than that of p a two dimensional cross section constructed with the rwb method suggests that the observed discontinuities can be characterized as intermittent lateral heterogeneities whose lateral extent is a few hundred kilometers the migration shows weak evidence of scattering objects which belong to the seismic discontinuities detected by the rwb method these anomalous structures may represent a part of hot plume generated beneath the southwestern pacific in the lowermost mantle