Model Of Upper Mantle Research Articles

Upper mantle heterogeneities in the Indian Ocean from waveform inversion

A waveform inversion method is applied to 156 Love and Rayleigh wave seismograms to build up a 3‐D model of the shear velocity in the upper mantle beneath the Indian Ocean. The first step of the method consists in finding, for each path, a radially stratified upper mantle model compatible with the Love and Rayleigh wave seismograms relative to that path. The fundamental mode and few higher modes are modelled in the waveform inversion. In a second step, the models related to the different paths are used in a tomographic inversion to map the 3‐D upper mantle structure. The 3‐D velocity model has a lateral resolution of 1000 km. No significant velocity anomalies are found below 300 km, although the resolution is still good. Continental roots are confined to the upper 300 km and low velocities below mid‐oceanic ridges to the upper 250 km, depending on their spreading rates. At shallow depths (<80 km), we find a positive velocity anomaly beneath the West Indian ridge near the Rodriguez triple junction, where gravimetric and bathymetric data indicate a reduced volcanic activity. At greater depth (around 200 km), a low velocity signature is found beneath the hot‐spot of Réunion‐Mauritius islands and the Central Indian ridge. It could reflect a real connection between the two structures.

P wave velocity of Proterozoic upper mantle beneath central and southern Asia

P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source‐receiver path of one event crosses central Africa and lies outside the African superswell while the source‐receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL‐Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL‐Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s−1, velocities between 8.3 and 8.5 km s−1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s−1, velocities between 7.9 and 8.4 km s−1 at a depth of 130 km, and velocity gradients between −0.001 and 0.001 s−1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150–200 km, whereas for southern Africa they bottom at ∼100–150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not consistent with explanations for the African superswell invoking extensive amounts of lithospheric thinning. If extensive lithospheric thinning had occurred beneath southern Africa, as suggested previously, then upper mantle P wave velocities beneath southern Africa would likely be lower than those in our models.

Mass transfer of helium, neon, argon, and xenon through a steady-state upper mantle

We have examined the steady-state upper mantle model for helium, neon, argon, and xenon following the mass transfer approach presented by Kellogg and Wasserburg (1990) for helium and Porcelli and Wasserburg (1995a) for xenon. The model explains the available observational data of mantle helium, neon, argon, and xenon isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, subduction of rare gases, and mantle rare gas concentrations. Rare gases in the upper mantle are derived from mixing of rare gases from the lower mantle, subducted rare gases, and radiogenic nuclides produced in situ. Isotopic shifts in the closed system lower mantle are due to decay of uranium and thorium decay series nuclides, 40K, 129I, and 244Pu over 4.5 Ga, while isotopic shifts in the steady-state upper mantle are due to decay of uranium and thorium series nuclides, and 40K over a timescale of ∼1.4 Ga. The model predicts that the shift in 21Ne/ 22Ne in the upper mantle relative to that in the lower mantle is the same as that for 4He/ 3He between the mantle reservoirs. This is compatible with the available data for MORB and ocean islands. Subduction of atmospheric helium and neon is not significant. All of the 40Ar in the lower mantle has been produced by 40K decay in the lower mantle. In the upper mantle, 40K decay further increases the radiogenic 40Ar from the lower mantle by a factor of ∼3. The calculated minimum lower mantle 40 Ar 36 Ar ratio is substantially greater than the atmospheric ratio. The inferred rare gas relative abundances of the lower mantle are different from those of the atmosphere and are consistent with possible early solar system reservoirs. Both the calculated 3 He 22 Ne and 20 Ne 36 Ar ratios of the lower mantle are within the range for meteorites with ‘solar’ neon isotope compositions. The 130 Xe 36 Ar ratio of the lower mantle is greater than that of the atmosphere, and may be possibly as high as the ratio found for meteoritic “planetary” rare gases. The model treats the atmosphere as a separate reservoir with rare gas isotope compositions that are distinct from those in the mantle. If the Earth originally had uniform concentrations of rare gases as represented by those in the lower mantle, then degassing of the upper mantle would have provided only a small proportion of the nonradiogenic rare gases presently in the atmosphere. The remainder may have been derived from late-accreted material with a much higher concentration of rare gases than the lower mantle. However, the amount of radiogenic 129Xe and 136Xe in the atmosphere as well as the lower mantle implies a substantial loss of rare gases. It is most likely that rare gases have been lost during late accretion and/or during; the hypothesized moon-forming impact. The nonradiogenic rare gases in the atmosphere were then supplied by subsequently accreted material with nonradiogenic xenon, possibly in comets. Fractionation of atmospheric xenon isotopes relative to other early solar system components must have occurred either on the late-accreting materials or during subsequent loss from the Earth.

Elasticity of forsterite to 16 GPa and the composition of the upper mantle

NEARLY 60 years ago, Bernal1 proposed that a polymorphic phase transformation in olivine might be responsible for the seismic velocity discontinuity near 410 km depth in the mantle. Phase equilibria experiments2,3 have since shown that the olivine (α) to wadsleyite (β) transition in (Mg,Fe)2SiO4 occurs at the appropriate pressure (13.8 GPa) under mantle conditions. Comparison of laboratory measurements of the acoustic velocity contrast in the α–β system to the magnitude of the seismically observed discontinuity at 410 km provides a way to constrain the olivine content of the mantle at this depth. Here we report measurements of the full set of elastic moduli of single-crystal forsterite (α-Mg2SiO4) at pressures between 3 and 16 GPa, using Brillouin scattering in a diamond anvil cell. At 13.8 GPa, the aggregate compressional-and shear-wave velocities of α-Mg2SiO4 are 2.7 ± 0.7% lower than predicted from earlier low-pressure data4,5. From our data, and assuming a homogeneous mantle composition, the seismic velocity contrast at 410 km depth can be satisfied only by a mantle containing less than ~40% olivine. This is well below the olivine abundance assumed in peridotite-based upper-mantle models.

Crust and upper mantle models along the active Tyrrhenian rim

ABSTRACTThe volcanic complexes from the Eolian islands to the Campania/Roman regions and Tuscany further north, rest on lithospheric sectors which overlie the Adriatic continental lithosphere sinking along the Apennine‐Maghrebian orogenic belt. Evidence for this stems from the melting, at mantle depth, of upper crustal materials as indicated by the widespread interaction of S‐type and K‐alkaline melts. The genesis of atypical magmas of the Roman Province (central‐southern Italy) appears to be the result of an important block faulting and deep lithospheric rifting of the Apennine continental margin lying parallel to and above relic sinking slabs. Intermediate and deep‐focus earthquakes indicate that the lithospheric slab is still seismically active under the Eolian‐Calabrian area and, sporadically, at the southern end of Campania. On the other hand, in the Roman/Tuscan region, it seems to be almost inactive, few earthquakes having been located with hypocentral depths not exceeding 150 km. The analysis of the spectral content of seismic sources supports the existence of two distinct zones of lithospheric shortening in correspondence of Tuscany and South Tyrrhenian sea, which are separated by a tensional region, which extends from Latium to Calabria. The existence of distinct lithospheric slabs along the Tyrrhenian rim is supported by surface wave dispersion and scattering measurements as well as P‐wave residuals, and is confirmed by the trend of long‐wavelength gravity anomalies. Bidimensional gravity models along transects in the Tyrrhenian sea and italian peninsula interpreted within the geometrical constraints imposed by the results of the interpretation of aeromagnetic, seismic and seismological data have been used to delimit the spatial distribution of the density contrasts in the upper mantle which might be due to the existence of the above‐mentioned lithospheric slabs.

Direct Solution Method による地球内部3次元構造の決定

We summarize recent results of our work on inversion for laterally heterogeneous earth structure using the Direct Solution Method (DSM). We obtained a 3-D upper mantle S-wave velocity model, PUM0 (Preliminary Upper Mantle model, version 0) by iterative linearized waveform inversion of surface wave data. The deep roots of ridges and cratons can be identified as the geological features responsible for the lateral heterogeneity of PUM0 at depths of 300-400km. We discuss the necessity of iterative linearized waveform inversion based on our calculations. We also discuss the future prospects for application of the DSM to waveform inversion of body wave data for the 3-D structure of the earth's deep interior.

Characterizing Long-Period Seismic Effects of Long-Wavelength Elastic and Anelastic Models

SUMMARY We present the results of a synthetic investigation designed to characterize the effects of long-wavelength elastic and anelastic models on the amplitudes and phases of long-period normal mode multiplets and Rayleigh wavepackets. Normal mode synthetics are created for recently constructed long-wavelength elastic and anelastic aspherical models of the Earth's upper mantle, using both the multiplet self-coupling approximation and the more accurate ±5 multiplet-multiplet coupling of the Galerkin method. Amplitude and phase measurements of the normal mode spectral peaks between 2 and 9 mHz and of the first eight Rayleigh wavepackets for 331 source-receiver pairs are compiled for each type of synthetic. the effects of anelastic and elastic structures are compared quantitatively with one another and with the predictions of zeroth order (in 1/l) asymptotic normal mode theory and linearized ray theory (LRT), and difficulties and advantages of applying these theoretical simplifications are identified and discussed. Although anelastic structures have only a minor effect on phases, long-wavelength models of anelastic and elastic structure each perturb amplitude measurements, with anelasticity accounting for up to ∼1/3 of the normal mode perturbations and up to ∼1/2 of the surface wave amplitude effect. Zeroth-order asymptotic theory and LRT predict that elastic and anelastic amplitude effects should qualitatively differ from one another, and thus should be separable in the data. While synthetics display qualitative agreement with the predictions of the approximations, for both normal mode spectra and surface wave measurements significant quantitative departures from zeroth-order asymptotic theory and LRT are observed. the part of the synthetic elastic amplitude signal not forecast by the approximate theories obscures the effects of aspherical anelasticity, particularly for normal modes, and can severely bias estimates of anelastic structure based solely on the approximations. In contrast, if an a priori model of aspherical elastic structure is assumed, the transfer functions that map amplitude anomalies from the elastic model to those for a model which includes anelastic asphericity are much more accurately forecast by zeroth-order asymptotic theory and LRT. Asymptotic theory accounts for over 85 per cent of the variance of such transfer functions for normal modes, and LRT predicts 67 per cent of the variance of surface wave transfer functions. Therefore, with the assumption of a priori elastic models, or in joint inversions of amplitude and phase data for elastic and anelastic structure, the approximations considered should prove useful for estimating models of aspherical attenuation.

Constraints on North Atlantic upper mantle anisotropy from S and SS phases

Several analyses of SSH‐SH differential travel‐times disagree on whether these data contain evidence for azimuthal anisotropy in the North Atlantic upper mantle. To explore possible explanations for these discrepant results, we calculated the azimuthal variation in SSH‐SH travel‐times predicted by an anisotropic upper mantle model. Theoretical travel‐times were obtained by cross‐correlating S and SS phases on transverse‐component reflectivity synthetic seismograms. We determined that predicted azimuthal travel‐time patterns are sensitive to the relative amplitudes of radial and transverse particle motions incident on the anisotropic medium. If a large variation in the relative incident amplitudes exists in a dataset, the overlap of different azimuthal travel‐time patterns may obscure anisotropic signatures.We analyzed shear‐wave splitting in a dataset of 12 SS and 10 S phase observations to constrain the presence of anisotropy in the North Atlantic upper mantle. A comparison of SS splitting parameters with S and SKS particle motions suggests that shear wave splitting apparent in the SS phases is at least partially due to anisotropy in the vicinity of their bounce‐points, although contributions from anisotropy in the source and station regions cannot be ruled out. Observed SS splitting is consistent with an olivine model in which horizontal a‐axes are aligned at an average of N24°W in the central North Atlantic (15°–40°N), and at an average of N23°E in the northern North Atlantic (40°–50°N).

Born seismograms using coupled free oscillations: the effects of strong coupling and anisotropy

SUMMARY We have developed an expression for the first-order Born waveform perturbation for long-period seismic records on an aspherical earth, using an anelastic monopole reference model. We have derived both an explicitly anelastic first-order Born term u’(r, t) and an approximate, but much simpler, expression in which quasi-degenerate modal coupling is represented by secular terms in the time domain. We designate this expression the ‘strong’ Born approximation to identify the presence of ‘strong’ quasi-degenerate coupling. Earlier applications of Born theory to surface wave seismograms have typically neglected the coupling between different modal dispersion branches. The strong Born approximation is linear in the aspherical model, but the secular terms restrict its accuracy to ‘short’ times after event onset, typically fewer than 10 hr. Additional accuracy can be gained by treating the self-coupling of isolated multiplets explicitly by formally summing the Born series using a projection of the aspherical operator 2” onto the multiplet in question. We call this the ‘stronger’ Born approximation, which is a non-linear functional of the aspherical model. The stronger Born approximation is similar to, but formally less accurate than, the subspace projection algorithm. We verified the accuracy of synthetic seismograms calculated with the ‘strong’ and ‘stronger’ Born approximations against Galerkin coupled-mode synthetics using a zonally symmetric, weakly anisotropic upper mantle model. The Born seismograms replicate successfully the ‘quasi-Love’ waveforms that diagnose spheroidal-toroidal coupling, suggesting that the strong Born approximation is adequate to model the interaction of free-oscillation coupling partners that are closely spaced in the frequency domain, at least for short records. We performed a waveform inversion test using three-component synthetic seismograms from 297 source-receiver pairs. Once spheroidal-toroidal coupling is incorporated in the inverse formulation, the waveform perturbations associated with the anisotropic parameters are clearly distinct from waveform perturbations associated with the isotropic parameters. Although the model space used in this experiment is highly restricted compared to the real Earth, these results suggest that the trade off between anisotropy and isotropic lateral structure may be less problematic in a fully-coupled waveform inversion than in a tomographic inversion of surface wave phase delays.

Upper mantle anisotropy and coupled-mode long-period surface waves

SUMMARY We investigate surface wave propagation effects caused by P and S-wave velocity anisotropy and its orientation in the upper mantle. We compare coupled-mode synthetic seismograms, constructed from sums of free oscillations for different types of upper mantle models, in order to understand better the observable aspects of anisotropy and its fast direction in the earth. Our calculations suggest that upper mantle anisotropy, which is not transversely isotropic, of few per cent can generate significant waveform anomalies in long-period seismic records of 0–20 mHz. Those anomalies are termed ‘quasi-Love’ waves and are caused by spheroidal—toroidal free-oscillation coupling. Large quasi-Love waveforms are difficult to produce with levels of isotropic heterogeneity consistent with global tomographic models of the mantle. Coupled-mode waveform anomalies in long-period seismic records at a single station can be used to diagnose the presence of anisotropic structure in the upper mantle. Because 0Sl, —0Tl coupling pairs have weak sensitivity to aspherical wavenumbers s 4 mHz, smoother/rougher structure generates longer/shorter-period fundamental-branch waveform anomalies. Anisotropy in our models is assumed to have hexagonal symmetry, that is, a single axis of symmetry. However, the symmetry axis can be arbitrarily oriented. The waveform anomalies associated with spheroidal-toroidal coupling are sensitive to the orientation of the fast-velocity direction of upper mantle anisotropy. In particular, for the same level of velocity perturbation, coupled-mode waveform anomalies increase as the fast axis of anisotropic structure rotates from vertical to horizontal. The orientation of the fast axis can influence both phase shifts and coupled-mode waveforms of long-period surface waves. The coupled-mode waveform anomalies predicted by our theoretical calculations are observed in the data and seem to diagnose the presence of upper mantle anisotropy which is not transversely isotropic.

From tectonic reconstruction to upper mantle model: An application to the Alpine-Mediterranean region

De Jonge, M.R., Wortel, M.J.R. and Spakman, W., 1993. From tectonic reconstruction to upper mantle model: An application to the Alpine-Mediterranean region. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 53–65. The seismic velocity structure of the upper mantle and lithosphere in the Mediterranean region is very complex: many areas with anomalous P-wave velocities can be recognized by means of seismic tomography. In view of the Complicated tectonic evolution of the peri-Mediterranean orogens and basins this is not surprising. In this study we will show results of a modelling approach we have recently developed to predict this structure. In this approach we make a quantitative connection between the tectonic evolution on one hand and the present seismic velocity structure on the other. Our model predicts the evolution of temperature distributions in the lower lithosphere and upper mantle, by simulating the flow of material inferred from a kinematic description of lithosphere-scale processes. From the modelled present thermal state (the endpoint of an evolution) we calculate expected seismic velocity anomalies that can be compared with results obtained by seismic tomography. Thus, the method enables us to compare the implications of geological reconstructions, describing the past tectonic processes, with seismological results for the present structure. The predicted mantle models show a good correlation with the tomographic results obtained for the mantle. This means we can verify many features present in the tectonic scenarios with the seismologically determined deep structure.

Annealing algorithms in computing stress tensors: A set of numerical simulations for local stress tensor recovery

The determination of a local lithospheric stress pattern from low magnitude seismic activity recorded at a loca array is attempted by means of numerical simulations that include waveform inversion and annealing inversion techniques. A set of flat stratified layers are assumed for the propagating medium. The tests show that the annealing algorithm is a useful tool in this kind of study where collected seismic activity at local arrays could help us better understand local stress distributions in the lithosphere. Although real data recorded at local seismic arrays should be processed, assuming more realistic crust and upper mantle models, many characteristics of the annealing process described here could be used when studying real problems.

What patterns of heterogeneity in the Earth's mantle can be revealed by seismic travel time tomography?

We investigate which patterns of seismic heterogeneity in the mantle would be returned reliably by a tomographic inversion in which the model mantle is parametrized by a set of discrete, non-overlapping voxels. We construct synthetic data sets based on real ray sampling of the mantle by introducing spherical harmonic patterns of velocity heterogeneity and perform inversions of the synthetic data. We expand the resulting voxel model in spherical harmonics and compare the power at each degree and in each model layer with the input spherical harmonics in order to determine which patterns produced by inversions of real data may be deemed reliable and to identify patterns which must be viewed with skepticism. We find that while the power input to a particular pattern of heterogeneity in the 0–200 km layer is generally recovered accurately, the pattern itself is poorly determined in this layer. A pattern in the 200–400 km layer is more precisely determined though the power contained in the pattern is consistently underestimated and more leakage occurs to the layers above and below. The transition zone, 400–670 km, shows similarly strong control of lateral heterogeneity patterns but tests return a more accurate estimate of input powre than for the second layer. The l = 2, 4, and 6 components all show accurate recovery in the 400–670 km layer. This result supports previous findings, from inversions with real data, that l = 2 is a significant pattern of heterogeneity in the mantle's transition zone and that l = 4 is not a significant pattern. For the entire upper mantle, l = 6 would be retrieved reliably and its constructive behavior in upper mantle models derived with real data is confirmed. These tests also demonstrate the inability of our inversion procedure to retrieve shorter wavelength features in the lower mantle. The results for our lowermost layer, D″, must be considered suspect due to the inadequate constraints placed on model values by our ray coverage and the sensitivity of these results to noise in the data.

A model of post-seismic recovery induced by a deep-focus earthquake

Post-seismic behavior induced by a deep-focus earthquake in subduction zone is closely investigated numerically on realistic upper-mantle models, taking into account the pressure and temperature conditions. For this purpose, the post-seismic stress changes expected from the deep event are calculated by a two-dimensional finite element method (2D FEM), assuming newtonian and power-law creep behaviors as justified by high P– T experiments. Several cases are investigated in detail for various fault depths and thermal structures. Two cases are considered for the location of the fault: one when an earthquake occurs within oceanic crust overlying a subducting plate, and the other when an earthquake is located within the plate. We also examine the case when the temperature distribution within the plate is lower by about 200°C on average than that in the upper mantle at the same depth. In addition, the post-seismic behavior due to a deep-focus event in a petrological thermal model of a mantle wedge is investigated. The results show that the post-seismic behavior associated with the deep event occurring within the oceanic crust indicates a pattern of relaxation of the coseismic stress changes. Lower temperatures inside the plate would suppress the post-seismic displacements and stress changes effectively. The petrological model also has some influence on the post-seismic flow pattern in the mantle wedge. We also study a possible correlation between deep-focus earthquakes and a shallow large event which has been detected in the northwestern Pacific region. However, effective mechanisms to explain the phenomenon could not be identified from the viewpoint of post-seismic behavior caused by power-law creep.

Are long-period body wave coda caused by lateral heterogeneity?

SUMMARY Data from two broad-band arrays in western Europe (NARS and GRF) are used to study the character of long-period body wave coda. The events studied are at epicentral distances of 40 to 60, in the Hindu Kush region and on the Mid-Atlantic Ridge, sampling the upper mantle to a depth of about lo00 km. The periods studied are 5-50 s. The long-period coda of P at GRF (interstation distance about 10 km) are strongly coherent, whereas the long-period coda of PP-, PPP- and S-waves are incoherent. This indicates that the latter coda consists of scattered waves. To investigate the nature of the scattering process, the data of GRF are analysed for slowness and azimuth variations in the coda intervals. A new beamforming algorithm is presented to deal with the low frequences and relatively short time intervals. The method is based on Backus-Gilbert inverse theory. The results show that the incoherent long-period coda intervals almost entirely consist of surface waves; these waves are scattered from the preceding body waves. Some calculations with linearized theory for body wave to Rayleigh wave conversion at topography at the surface or at the Moho show that realistic scatterers can account for the observed (constant) coda level. The beamforming results show that the phases in the P coda all arrive along the great circle. As scattering calculations point out that body wave to body wave scattering is inefficient, it is concluded that the long-period P coda does not contain a significant amount of scattered energy. Synthetic seismograms obtained with the reflectivity method show that spherically symmetric upper mantle models can explain these coda waves. For events in the Hindu Kush region, an upper mantle with a thick lid overlying a pronounced low-velocity zone (LVZ) is necessary to explain the character of the P coda at GRF. Such an upper mantle agrees with previous studies of similar great circle paths. The strong coherence of the P coda at GRF is lost on the scale of NARS (station separation about 200 km). This suggests lateral variations in the upper mantle at a scale of about 200 km. It appears from previous studies of the upper mantle under Europe that these variations must be sought in the LVZ. It is shown that the long-period P coda is sensitive to variations in the P velocity structure of the LVZ. This suggests the P coda (i.e. PdP phases) as a tool for monitoring lateral variations in the LVZ and possibly to prove the existence or absence of a LVZ in the P velocity.

Crustal and upper mantle structures of the brazilian coast

The Rayleigh wave phase and group velocities in the period range of 24–39 sec, obtained from two earthquakes which occurred in northeastern brazil and which were recorded by the Brazilian seismological station RDJ (Rio de Janeiro), have been used to study crustal and upper mantle structures of the Brazilian coastal region. Three crustal and upper mantle models have been tried out to explain crustal and upper mantle structures of the region. The upper crust has not been resolved, due basically to the narrow period range of the phase and group velocities data. The phase velocity inversions have exhibited good resolutions for both lower crust and upper mantle, with shear wave velocities characteristic of these regions. The group velocity data inversions for these models have showed good results only for the lower crust. The shear wave velocities of the lower crust (3.86 and 3.89 km/sec), obtained with phase velocity inversions, are similar to that (β=3.89 km/sec) found byHwang (1985) to the eastern South American region, while group velocity inversions have presented shear velocity (β=3.75 km/sec) similar to that (β=3.78 km/sec) found byLazcano (1972) to the Brazilian shield. It was not possible to define sharply the crust-mantle transition, but an analysis of the phase and group velocity inversions results has indicated that the total thickness of the crust should be between 30 and 39 km. The crustal and upper mantle model, obtained with phase velocity inversion, can be used as a preliminary model for the Brazilian coast.

Upper mantle models and the thickness of the continental lithosphere

Analysis of the Early Rise traveltimes in the United States shows that on several of the profiles there are features which can be interpreted as indicating a low-velocity zone above 200 km, i.e. that the continental lithosphere thickness is less than 200 km. The explosion data from the Nevada Test Site, GNOME and Early Rise are all consistent with Lehmann's suggestion that there is a discontinuity at a depth of about 200 km. Comparison of the observed data between 15° and 23° with model calculations shows that the major differences between the western and central-eastern United States upper mantles occur above 200 km. It appears that the only reasonable explanation for the 200 km discontinuity is that it represents the termination of a zone of partial melting.

Regional gravity investigation of Honduras, Central America

Abstract A regional gravity study of Honduras was performed as part of a major study of the geothermal resource potential of Honduras. This study was conducted by Los Alamos National Laboratory, in cooperation with the Honduras government. Regional offshore free-air and onshore Bouguer gravity maps, and residual/isostatic gravity maps of Honduras and surrounding regions were produced. From these data several regional crustal and upper mantle models were produced. These models pass through two local geothermal sites, Platanares and San Ignacio. The regional geologic and tectonic implications of the models and their relevance to the geothermal potential of Honduras and to six well known geothermal sites in particular are examined. No obvious regional structures observed in the gravity data can explain the thermal enhancement in general or the specific geothermal sites. More local tectonic or structural conditions must control the distribution of the thermally enhanced areas.

Horizontal motions due to post‐glacial rebound

North American horizontal post‐glacial rebound velocities are computed for three five‐layered, incompressible, spherical Maxwell earth models, using a simple glacial loading cycle centered on Hudson Bay. The reference earth model has a 120‐km thick elastic lithosphere and upper‐ and lower‐mantle viscosities of 1021 Pa·s and 2 × 1022 Pa·s, respectively. The second model has a lower viscosity upper‐mantle (5 × 1020 Pa·s) and a higher viscosity lower‐mantle (1023 Pa·s), and the third has a 250‐km thick lithosphere. The reference model and the low‐viscosity upper‐mantle model calculations are in good agreement with uplift and gravity data, although they may somewhat overestimate the present‐day central uplift rate. The thick lithosphere model fares less well because it fails to exhibit the well‐pronounced peripheral bulge sinking that is required by eastern North America tide gauge data. The reference model generally has the largest horizontal rebound velocities, reaching a maximum of 4 mm/yr towards Hudson Bay at 2400 km from the load center. The effect of a thick lithosphere or of a low‐viscosity upper‐mantle is to reduce present‐day horizontal velocities, especially in the formerly glaciated region. The sensitivity of horizontal motions to lithospheric thickness and upper‐mantle viscosity may assist in discriminating between different models of the earth's rheology once sufficient high‐quality geodetic data exist.

Constrained reference mantle model

A dataset of updated normal mode eigenperiods is used to develop a new average upper mantle model. The starting model takes account of recent body wave and surface wave models and of constraints derived from different petrological models. Different parameterizations with or without anisotropy down to different depths are checked. Anisotropy seems to be required at least down to 200 km and preferably down to 400 km but with a smaller amplitude than in the preliminary reference Earth model (PREM). The new models provide velocity gradients down to 400 km in good agreement with finite strain trajectories. The S-wave velocity jump at 400 km is smaller than previously found from regional studies. If the S-wave and P-wave velocity jumps at the 400-km discontinuity are considered simultaneously, this discontinuity cannot be explained by an upper mantle composed mainly of olivine.