New insights into the P- and S-wave velocity structure of the D″ discontinuity beneath the Cocos plate

P- and S-waves from earthquakes in South America recorded at Californian network stations are analysed to image lateral variations of the D �� -discontinuity beneath the Cocos plate. We apply two array processing methods to the data set: a simplified migration method to the P-wave data set and a double-array method to both the P- and S-wave data sets, allowing us to compare results from the two methods. The double-array method images a dipping reflector at a depth range from 2650 to 2700 km in the southern part of the study area. We observe a step-like topography of 100 km to a shallower reflector at about 2600 km depth to the north, as well as evidence for a second (deeper) reflector at a depth range from 2700 to 2750 km in the north. Results from the simplified migration agree well with those from the double-array method, similarly locating a large step in reflector depth in a similar location (about 2650 km depth in the south and about 2550 km in the north) as well as the additional deeper reflector at the depth of about 2750 km in the north. Waveform modelling of the reflected waves from both methods suggests a positive velocity contrast for S waves, but a negative velocity contrast for P waves for the upper reflector in agreement with predictions from mineral physical calculations for a post-perovskite phase transition. The data also show some evidence for the existence of another deeper reflector that could indicate a double intersection of the geotherm with the post-perovskite stability field, that is, the back-transformation of post-perovskite to perovskite close to the core‐mantle boundary.

Related papers:

title: Lateral variations in D″ thickness from long‐period shear wave data
abstract: 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.

title: Lateral variations in D″ below the Caribbean
abstract: Broad-band data from the Canadian National Seismograph Network (CNSN) shows evidence for a laterally-varying S-wave discontinuity in the lowermost mantle beneath the Caribbean. The magnitude of the velocity contrast across this D″ discontinuity varies inversely with its distance from the core-mantle boundary (CMB). Clear evidence for a 2.75% increase in shear velocity 250 km above the CMB is observed in a region 10°N and 60–85°W; a thicker D″ layer (290 km) with a less pronounced topside discontinuity (2.45%) is interpreted in the region 80°–90°W and 20°N; and there is no evidence for a D″ discontinuity in the region 65°–80°W and 25°N. Considerable 3D structure within the D″ region is suggested by travel-time scatter in ScS arrivals and S arrivals which turn within D″ . Transverse-component SKS signals due to upper-mantle anisotropy can be confused with D″ signals; however, we show that false interpretations of D″ structure can be mitigated through the careful analysis of travel-time moveout and low-pass filtering.

title: Modelling D″ structure beneath Central America with broadband seismic data
abstract: 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/dΔ 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.

title: Lateral variation of the D″ discontinuity beneath the Cocos Plate
abstract: Broadband shear wave signals from 15 deep South American earthquakes reveal small-scale variations of the D″ shear velocity discontinuity beneath the Cocos Plate. Deconvolution by average event source wavelets allows multiple event and California station combinations to be double-array stacked in 4 geographically separate reflection point bins with lateral dimensions of about 200 km. Reflections from the top of D″ are apparent in all 4 bins, with significant variations in timing and strength relative to ScS. The data are compatible with a discontinuous shear velocity increase 264 km above the core-mantle boundary that varies laterally in strength from 0.9% to 2.6%, with corresponding lateral velocity variations within D″.

title: High‐resolution investigation of shear wave anisotropy in D″ beneath the Cocos Plate
abstract: Broadband shear wave signals from 16 deep South American earthquakes are used in a high-resolution analysis of lowermost mantle structure beneath the Cocos Plate. Shear wave splitting of 162 ScS phases is generally compatible with vertical transverse isotropy (VTI) in the D″ region, with little path length or spatial dependence across the 200 × 800 km2 area sampled. Assuming a 250 km thick D″ layer, the VTI has an average value of 0.63%. ScS-S differential travel time anomalies for the same data indicate a lateral gradient in positive D″ transverse component velocity perturbations, increasing northward. S-wave triplication arrivals from a shear velocity discontinuity at the top of D″ show splitting similar to the ScS phases, but with smaller magnitude. Vertical resolution remains limited, but the data are consistent with VTI material being concentrated in just the upper portion or at the top and bottom of D″.

title: Small scale lateral variations in azimuthally anisotropic D'' structure
abstract: Shear waves from intermediate and deep focus South American events recorded by California broadband seismometers reveal complex anisotropy in the lowermost mantle. ScS waves densely sample the D″ region beneath the Cocos Plate, just west of Central America (1–16°N; 267–277°E). Our data show evidence for significant near-source anisotropy, which we constrain for each event by performing polarization analyses on an expanded dataset of S waveforms. ScS waveforms are corrected for upper mantle and near-source anisotropy, and estimates of D″ anisotropy are made using the covariance method. We find splits between fast and slow components of ScS ranging from 0.4 to 2.5 s, with an average of 1.6 s. Fast polarization directions in the southern portion of our study area are approximately orthogonal to the raypaths. North of 7° latitude there is a rapid change in fast polarization direction to nearly raypath parallel. Beyond 10° latitude fast directions are more highly scattered than in southern regions. Past work in the region has inferred strong lateral variations in shear wave velocity, but fairly uniform transverse isotropy. In contrast, our results indicate that the lowermost mantle beneath the Cocos plate has azimuthal anisotropy that varies laterally over scales of 100–200 km. In addition, our data suggest connections between previously imaged topography on the D″ discontinuity and the character of anisotropy.

title: Seismic detection of folded, subducted lithosphere at the core–mantle boundary
abstract: Seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the Earth's lower mantle. 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 regions. The D″ discontinuity itself is probably the result of a phase change in relatively low-temperature magnesium silicate perovskite. 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.

title: D shear velocity heterogeneity, anisotropy and discontinuity structure beneath the Caribbean and Central America
abstract: The D �� region in the lowermost mantle beneath the Caribbean and Central America is investigated using shear waves from South American earthquakes recorded by seismic stations in North America. We present a large-scale, composite study of volumetric shear velocity heterogeneity, anisotropy, and the possible presence of a D �� discontinuity in the region. Our data set includes: 328 S(Sdiff )-SKS differential travel times, 300 ScS-S differential travel times, 125 S(Sdiff ) and 120 ScS shear wave splitting measurements, and 297 seismograms inspected for Scd, the seismic phase refracted from a high-velocity D �� layer. Broadband digital data are augmented by high-quality digitized analog WWSSN data, providing extensive path coverage in our study area. In all, data from 61 events are utilized. In some cases, a given seismogram can be used for velocity heterogeneity, anisotropy, and discontinuity analyses. Significant mid-mantle structure, possibly associated with the ancient subducted Farallon slab, affects shear wave travel times and must be corrected for to prevent erroneous mapping of

title: Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D″ layer
abstract: The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3 (ref. 1). But it has not been possible to explain many unusual properties of the lowermost ∼150 km of the mantle (the D″ layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D″ layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D″ layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.

title: High‐ and low‐resolution images of the Earth's mantle: Implications of different approaches to tomographic modeling
abstract: Recently published images of the Earth's mantle are characterized by a nominal resolution much higher than that used in previous studies, where substantially different techniques were employed. The agreement between such “high-resolution” and “low-resolution” images often seems very poor. In an attempt to determine the reason for this discrepancy, we analyze how the choice of inversion algorithm (exact or iterative), regularization (norm or roughness minimization), and parameterization (spherical harmonics up to a variable degree, blocks) affects a tomographic model. We also investigate the effects of the varying density of the data coverage on the final solution. In our experiments we employ two seismic data sets: Rayleigh wave phase velocity at 75 s period and P wave travel times. We construct a new model of P velocity in the mantle (BDP98) based on the International Seismological Center bulletins 1964–1992. We use our findings in an evaluation of recent mantle models, including our own, focusing on similarities and discrepancies between models of different nominal resolution. In all the models the long-wavelength component is the most stable. However, consistent high-resolution details, probably corresponding to features of the real Earth, are also seen. In general, we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem, rather than from actual errors or approximations.

title: Seismic imaging of structural heterogeneity in Earth's mantle: evidence for large-scale mantle flow.
abstract: Systematic analyses of earthquake-generated seismic waves have resulted in models of three-dimensional elastic wavespeed structure in Earth's mantle. This paper describes the development and the dominant characteristics of one of the most recently developed models. This model is based on seismic wave travel times and wave shapes from over 100,000 ground motion recordings of earthquakes that occurred between 1980 and 1998. It shows signatures of plate tectonic processes to a depth of about 1,200 km in the mantle, and it demonstrates the presence of large-scale structure throughout the lower 2,000 km of the mantle. Seismological analyses make it increasingly more convincing that geologic processes shaping Earth's surface are intimately linked to physical processes in the deep mantle.

title: Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities
abstract: Resolution of the pattern of large-scale shear velocity variations above and below the known and postulated mantle discontinuities could provide constraints on the nature of mineral phase transitions, changes in composition, and the scale of mantle convection. To achieve good resolution across a full range of depths, we use a diversified data set consisting of body and mantle wave waveforms, travel times, and surface wave phase velocities. Our main focus is on the 670-km discontinuity, long presumed to be an important barrier, or impediment, to whole mantle convection. Our data set has a relatively high radial resolution throughout the mantle; in the transition zone and some 200 km below it, the long period waveforms, dominated by multiple surface reflections, make a particularly important contribution. We use a local spline support to parameterize the model; this allows us to obtain a smooth model (twice differentiable) and simplifies calculation of the model and its derivatives in applications such as three-dimensional ray tracing. In one inversion we use a continuous radial representation throughout the mantle; in the other, a discontinuity is allowed across the 670-km boundary. Both models suggest that the long-wavelength anomalies of the transition zone and the mantle below 750 km are significantly different. Near 670 km these two models display notable differences in the peak amplitudes and lateral scales of major anomalies. The degree 2 spherical harmonic, which dominates the large-scale shear velocities in the transition zone, is strongly attenuated at the top of the lower mantle where the power spectrum is essentially white. Resolution tests show that these results are robust, which suggests a possible reorganization of the flow between the upper and lower mantle. At other depths the power spectra of our models as a function of depth indicate a modest change near 400 km, where the dominating effect of degree 5 (shields) is replaced by degree 2 (slabs). The power of the heterogeneity in at mid-mantle depths is low and nearly flat as a function of spherical harmonic degree up to 𝓁 = 12, with no detectable change near 1000 km. The increase of the power in the lowermost mantle is rather gradual, not characteristic of a discontinuous change. Cross sections of our models at major subduction zones indicate that major downwelling of cold slab material may occur at some locations. On the other hand, there are numerous examples of an abrupt change of the sign of the velocity anomalies across the 670-km discontinuity.

title: Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)
abstract: 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.

title: A comparison of tomographic and geodynamic mantle models
abstract: We conduct a comprehensive and quantitative analysis of similarities and differences between recent seismic tomography models of the Earth's mantle in an attempt to determine a benchmark for geodynamic interpretation. After a spherical harmonic expansion, we find the spectral power and radial correlation of each tomographic model as a function of depth and harmonic degree. We then calculate the correlation, at the same depths and degrees, between all possible pairs of models, to identify stable and model-dependent features (the former being usually of longer spatial wavelength than the latter). We can therefore evaluate the degree of robust structure that seismologists have mapped so far and proceed to calculate ad hoc mean reference models. Tomographic models are furthermore compared with two geodynamic subduction models that are based on plate motion reconstructions. We find systematically low intermediate-wavelength correlation between tomography and convective reconstruction models and suggest that the inadequate treatment of the details of slab advection is responsible. However, we confirm the presence of stable, slab-like fast anomalies in the mid-mantle whose geographic pattern naturally associates them with subduction. This finding, in addition to our analysis of heterogeneity spectra and the absence of strong minima in the radial correlation functions besides the one at ~700 km, supports the idea of whole mantle convection with slab penetration through the 660 km phase transition, possibly accompanied by a reorganization of flow.

title: Mantle shear–wave tomography and the fate of subducted slabs
abstract: A new seismic model of the three–dimensional variation in shear velocity throughout the Earth's mantle is presented. The model is derived entirely from shear bodywave travel times. Multibounce shear waves, core–reflected waves and SKS and SKKS waves that travel through the core are used in the analysis. A unique aspect of the dataset used in this study is the use of bodywaves that turn at shallow depths in the mantle, some of which are triplicated. The new model is compared with other global shear models. Although competing models show significant variations, several large–scale structures are common to most of the models. The high–velocity anomalies are mostly associated with subduction zones. In some regions the anomalies only extend into the shallow lower mantle, whereas in other regions tabular high–velocity structures seem to extend to the deepest mantle. The base of the mantle shows long–wavelength high–velocity zones also associated with subduction zones. The heterogeneity seen in global tomography models is difficult to interpret in terms of mantle flow due to variations in structure from one subduction zone to another. The simplest interpretation of the seismic images is that slabs in general penetrate to the deepest mantle, although the flow is likely to be sporadic. The interruption in slab sinking is likely to be associated with the 660 km discontinuity.

title: Global seismic tomography: A snapshot of convection in the Earth: GSA Today
abstract: Two new global high-resolution models of the P-wave and S-wave seismic structure of the mantle were derived independently using different inversion techniques and different data sets, but they show excellent correlation for many large-scale as well as smaller scale structures throughout the lower mantle. The two models show that high-velocity anomalies in the lower mantle are dominated by long linear features that can be associated with the sites of ancient subduction. The images suggest that most subduction-related mantle flow continues well into the lower mantle and that slabs may ultimately reach the core-mantle boundary. The models are available from anonymous ftp at maestro.geo.utexas.edu in directory pub/grand and at brolga.mit. edu in directory pub/GSAtoday.

title: THE DYNAMICS OF CENOZOIC AND MESOZOIC PLATE MOTIONS
abstract: Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present-day plate motions. A fuller understanding, how- ever, can be derived from consideration of the history of plate motions. Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate-driving forces. We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions. The present-day field is not equipartitioned in poloidal and toroidal compo- nents; toroidal motions account for only one third of the total. The toroidal/poloidal ratio has changed substan- tially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone. We develop a self- consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) cal- culating the induced plate motions for each stage of the Cenozoic. The "slab" heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core-mantle boundary. The slab model pre- dicts the observed geoid extremely well, although com- parison between predicted and observed dynamic topog- raphy is ambiguous. The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present. The plate motion model itself successfully predicts Cenozoic plate motions (glob- al correlations of 0.7-0.9) for mantle viscosity structures that are consistent with a variety of geophysical studies. We conclude that the main plate-driving forces come from subducted slabs (! 90%), with forces due to litho- spheric effects (e.g., oceanic plate thickening) providing a very minor component (" 10%). For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs. Unfortunately, we cannot re- produce the toroidal/poloidal partitioning ratios ob- served for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries. The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawai- ian-Emperor seamount chain. Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian-Emperor bend. However, we find that this collision has no effect on the motion of the Pacific plate, implying that impor- tant plate boundary effects are missing in our models. Future progress in understanding global plate motions requires (1) more complete plate reconstruction infor- mation, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc man- tle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity con- straints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change.

title: Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties
abstract: Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D″) is important to understand the formation and dynamics of the Earth's mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core–mantle interaction, the presence of remnants of subducted material and that D″ is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core–mantle boundary temperature and pressure conditions. Here we explore the efficacy of this ‘post-perovskite’ phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D″ discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.

title: Azimuthal anisotropy in the D″ layer beneath the Caribbean
abstract: The lowermost mantle beneath Central America has anisotropic seismic velocity structure manifested in shear wave splitting of signals from South American earthquakes recorded at North American broadband recording stations. Prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy (VTI), which is sufficient to explain a general trend of early tangential (SH) component arrivals. However, VTI models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region. After accounting for splitting effects of upper mantle anisotropy beneath the recording stations, we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic D″ layer. This is the first attempt to quantitatively model a large data set including azimuthal anisotropy in D″. The models include transverse isotropy with either a vertical or tilted symmetry axis, the latter resulting in azimuthal anisotropy. For some initial shear wave polarizations, tilted transverse isotropy (TTI) produces small, reversed polarity arrivals on the SV components at the arrival time of SH, consistent with the data. Geographical variations in the azimuth of the TTI symmetry axis are indicated by the data. The lack of azimuthal coverage prevents unique resolution of the TTI orientation and also precludes distinguishing between TTI and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals. Nonetheless, our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than VTI for this region.

title: Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific
abstract: 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.

title: Evidence for normal and inhomogeneous lowermost mantle and core-mantle boundary structure under the Arctic and northern Canada
abstract: Seismic P waves from a total of about 200 nuclear explosions in the USA, the former USSR and China, observed at 10 arrays and four networks in Europe, Canada and the USA, are used to analyse the structure at the base of the mantle and the core-mantle boundary (CMB). 
 
 
 
The simple waveforms and well-controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration. A new array technique (double beamforming; Kruger et al. 1993) integrating both concepts is applied, which increases the slowness resolution considerably. 
 
 
 
A total of 56 source-receiver combinations (i.e. reflection points in the lower mantle or on the CMB) could be analysed. In five regions, anomalous arrivals (PdP) with slowness and arrival times between those of P and PcP are observed. One of these five areas (Svalbard region) shows short-period PcP/P amplitude ratios, which are about three times higher than those predicted by standard earth models. In the Severnaya Zemlya region, where PdP and PcP precursors were observed previously (Kruger et al. 1993), PcP shows azimuth deviations of up to 10°. For some other regions, deviations of the PcP waveform from the direct P waveform are also observed. 
 
 
 
These anomalous phases and the PcP waveform distortions cannot be explained with standard 1-D earth models. They are probably produced by inhomogeneities in the lowermost mantle. The observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges. The first is the CMB and its immediate vicinity of a few tens of kilometres; the second region is the depth range between about 200 and 300 km above the CMB. Maps of the North Pole region, giving the distributions of inhomogeneities in the lower mantle and on the CMB, are presented. These maps show evidence of strong heterogeneity of the D″ boundary layer and possibly also of the CMB in the same area.

title: Vespa Process for Analysis of Seismic Signals
abstract: This article describes applications of the Vespa process to study P waves from seismic events and also to extract seismic signals arriving at the same time but from different approach angles.

title: 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.

title: Seismic Evidence of Partial Melt Within a Possibly Ubiquitous Low-Velocity Layer at the Base of the Mantle
abstract: Three source regions show evidence of a low-velocity layer that is less than 15 kilometers thick on top of the core-mantle boundary and require about a 3:1 ratio of shear-to-compressional velocity reduction, which is consistent with partial melt. Layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity. These observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core-mantle boundary. Low viscosity accompanying partial melt can localize the upwelling of warmed mantle, making the low-velocity layer a plausible source of mantle plumes.

title: Noise reduction and detection of weak, coherent signals through phase-weighted stacks
abstract: We present a new tool for efficient incoherent noise reduction for array data employing complex trace analysis. An amplitude-unbiased coherency measure is designed based on the instantaneous phase, which is used to weight the samples of an ordinary, linear stack. The result is called the phase-weighted stack (PWS) and is cleaned from incoherent noise. PWS thus permits detection of weak but coherent arrivals. The method presented can easily be extended to phase-weighted cross-correlations or be applied in the z-p domain. We illustrate and discuss the advantages and disadvantages of PWS in comparison with other coherency measures and present examples. We further show that our non-linear stacking technique enables us to detect a weak lowermantle P-to4 conversion from a depth of approximately 840 km on array data. Hints of an 840 km discontinuity have been reported; however, such a discontinuity is not yet established due to the lack of further evidence.

title: Small scatterers in the lower mantle observed at German broadband arrays
abstract: Seismograms of earthquakes from the South Pacific recorded at a German broadband array and network show precursors to PKPdf. These precursors mainly originate from off-path scattering of PKPab or a nearby PKPbc to P (for receiver-side scattering) or from scattering of P to PKPab or PKPbc on the PKPdf path (for source-side scattering). Standard array processing techniques based on plane wave approximations (such as vespagram or frequency-wavenumber analysis) are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 × 300 km for short-period waves. We therefore develop a migration method to estimate the location of scatterers in the mantle, at the core-mantle boundary and at the top of the outer core. With our method we are able to find isolated scatterers at the source side and the receiver side, although the depth of the scatterer is not well constrained. However, from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place (scattering volume), we believe that the location of the scatterers is in the lowermost mantle. Since we have detected scatterers in regions where ultralow-velocity zones have been discovered recently, we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle. Comparing results from broadband and band-pass-filtered data the detection of small-scale structure of the ultralow-velocity zones becomes possible.

title: Small-scale changes of core-mantle boundary reflectivity studied using core reflected PcP
abstract: Short-period PcP phases from earthquakes in the western Aleutians recorded at the Canadian Yellowknife array (YKA) sample the core-mantle boundary (CMB) beneath the Alaskan Kenai peninsula and the Cook inlet. The PcP -to- P amplitude ratios for these events show large variability with some amplitude ratios more than an order of magnitude larger than predicted by radial Earth models such as IASP91 or PREM. The amplitude ratios vary laterally with PcP core-mantle boundary reflection points showing a ∼1° region with exceptionally large amplitude ratios. It is likely that the variability in relative PcP amplitudes is due in large part to CMB topography. Other mechanisms, such as radiation pattern; mantle attenuation; focusing and defocusing; and core-mantle boundary velocity and density perturbations could cause variations on scale-lengths of less than a degree as found in this study, but are unable to produce variations as large as those we observe.

title: Variable Azimuthal Anisotropy in Earth's Lowermost Mantle
abstract: A persistent reversal in the expected polarity of the initiation of vertically polarized shear waves that graze the D″ layer (the layer at the boundary between the outer core and the lower mantle of Earth) in some regions starts at the arrival time of horizontally polarized shear waves. Full waveform modeling of the split shear waves for paths beneath the Caribbean requires azimuthal anisotropy at the base of the mantle. Models with laterally coherent patterns of transverse isotropy with the hexagonal symmetry axis of the mineral phases tilted from the vertical by as much as 20° are consistent with the data. Small-scale convection cells within the mantle above the D″ layer may cause the observed variations by inducing laterally variable crystallographic or shape-preferred orientation in minerals in the D″ layer.

title: Phase transition in MgSiO3 perovskite in the earth’s lower mantle
abstract: A new polymorph of MgSiO3 more stable than the Pbnm-perovskite phase has been identified by first-principles computations. It has the CaIrO3 structure with Cmcm symmetry and consists of SiO3 layers intercalated by eightfoldcoordinated Mg ions. High-temperature calculations within the quasiharmonic approximation give a volume change of f 1.5% and a Clapeyron slope of f 7.5F 0.3 MPa/K at f 2750 K and f 125 GPa. These pressure–temperature (P–T) conditions are close to those in which a phase transition in MgSiO3-perovskite has been observed by in situ angle dispersive X-ray diffraction measurements. This transition appears to be associated with the DU discontinuity. D 2004 Elsevier B.V. All rights reserved.

title: Segregation of subducted oceanic crust in the convecting mantle
abstract: Subducted oceanic crust, transformed into dense mineral assemblages at high pressure, may gravitationally segregate at the bottom of the convecting mantle, for example, the D″ layer. Here it could be stored for a long enough time to develop an “enriched” isotopic signature, before it is recycled in mantle plumes and hence control the geochemical character of hot-spot basalts. We study both the geodynamical and geochemical aspects of this hypothesis in two-dimensional numerical convection models, in which plate motion is imposed by a velocity boundary condition. About 250,000 tracer particles are used to identify the basalt fraction in the oceanic crustmantle system. High-, average-, and low-tracer densities indicate basalt or eclogite, peridotite, and harzburgite, respectively. The tracers are negatively buoyant to account for the density differences between the rock types. At surface divergence zones, crust formation is simulated by extracting tracers and transferring them into a thin layer at the surface. The tracers carry a certain amount of the relevant nuclides of the U-Pb and Sm-Nd systems, which are fractionated between the basalt tracers and a second species of residue tracers during crust formation. Using reasonable parameter values, we find that of the order of 1/6 of the subducted crust accumulates in pools at the bottom, which reside underneath thermal plumes. After 3.6 Ga, the [207Pb]/[204Pb], [206Pb]/[204Pb], and [143Nd]/[ 144Nd] ratios in various parts of the model cover the observed HIMU-MORB range. A systematic study of the influence of control parameters on the results indicates that the amount of segregation and the diversity of isotope ratios (1) increases strongly with Rρ, the ratio of chemical to thermal buoyancy; (2) decreases moderately with the Rayleigh number Ra, where Ra = 106 is the highest value that we employed; (3) increases strongly with the degree of temperature dependence of the viscosity; and (4) is not very sensitive to the partitioning between internal and bottom heating. The largest uncertainty in applying our model results to the Earth lies in the lack of accurate density data for conditions at the core-mantle boundary. We conclude that, if our density estimates are correct, segregation and reentrainment of subducted crust is of fundamental importance for the dynamics and chemistry of mantle plumes.

title: SeismicHandler: programmable multichannel data handler for interactive and automatic processing of seismological analyses
abstract: SeismicHandler is an analysis program for digital seismograms. It operates on various types of data records such as single traces, 3-component records, array seismograms, sets of globally distributed stations of arbitrary sample rates, and record sections of refraction seismograms. The user interface is based on a command language which is designed particularly for the handling of seismic traces. The predefined instruction set can be extended by more complex commands using command procedures. The main part of the program is machine independent, thus SH is portable to any computer type without much effort. Versions for VAX/VMS, SUN/UNIX, and ATARI TT already are in use.