Publications

Export 76 results:
Sort by: Author Title Type [ Year  (Desc)]
2017
Walpole, J, Wookey J, Kendall JM, Masters TG.  2017.  Seismic anisotropy and mantle flow below subducting slabs. Earth and Planetary Science Letters. 465:155-167.   10.1016/j.epsl.2017.02.023   AbstractWebsite

Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements - spanning similar to 40,000 km of subduction zone trenches - over the complete range of available source depths (4 to 687 km) - and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth <50km) - suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite. Crown Copyright (C) 2017 Published by Elsevier B.V. All rights reserved.

2016
Ma, ZT, Masters G, Mancinelli N.  2016.  Two-dimensional global Rayleigh wave attenuation model by accounting for finite-frequency focusing and defocusing effect. Geophysical Journal International. 204:631-649.   10.1093/gji/ggv480   AbstractWebsite

In this study, we obtain a set of 2-D global phase velocity and attenuation maps for Rayleigh waves between 5 and 25 mHz. Correcting the effect of focusing-defocusing is crucial in order to obtain reliable attenuation structure. Great circle linearized ray theory, which has been used to date, can give useful predictions of this effect if careful attention is paid to how the phase velocity model is smoothed. In contrast, predictions based on the 2-D finite-frequency kernels are quite robust in this frequency range and suggest that they are better suited as a basis for inversion. We use a large data set of Rayleigh wave phase and amplitude measurements to invert for the phase velocity, attenuation, source and receiver terms simultaneously. Our models provide 60-70 per cent variance reduction to the raw data though the source terms are the biggest contribution to the fit of the data. The attenuation maps show structures that correlate well with surface tectonics and the age progression trend of the attenuation is clearly seen in the ocean basins. We have also identified problematic stations and earthquake sources as a by-product of our data selection process.

2015
Ma, ZT, Masters G.  2015.  Effect of earthquake locations on Rayleigh wave azimuthal anisotropy models. Geophysical Journal International. 203:1319-1333.   10.1093/gji/ggv369   AbstractWebsite

Various factors need to be considered when inverting for surface wave azimuthal anisotropic structure. This paper focuses on the 2 phi terms for Rayleigh wave azimuthal anisotropy and shows that the uncertainties of earthquake locations also have significant impacts on the resulting anisotropic structure. We use the global Rayleigh wave phase velocity data set collected in a previous study to demonstrate this effect. The differences between azimuthal anisotropic patterns with and without source relocations are greatest near plate boundaries. Large differences around the South American plate are also identified. Although most of the earthquakes are shifted by less than 15 km from the CMT locations, earthquakes near the Andes can be systematically shifted by more than 30 km. Our final epicentres for earthquakes on ridge-transform fault systems better match the plate boundaries.

2014
Ma, ZT, Masters G, Laske G, Pasyanos M.  2014.  A comprehensive dispersion model of surface wave phase and group velocity for the globe. Geophysical Journal International. 199:113-135.   10.1093/gji/ggu246   AbstractWebsite

A new method is developed to measure Rayleigh- and Love-wave phase velocities globally using a cluster analysis technique. This method clusters similar waveforms recorded at different stations from a single event and allows users to make measurements on hundreds of waveforms, which are filtered at a series of frequency ranges, at the same time. It also requires minimal amount of user interaction and allows easy assessment of the data quality. This method produces a large amount of phase delay measurements in a manageable time frame. Because there is a strong trade-off between the isotropic part of the Rayleigh-wave phase velocity and azimuthal anisotropy, we include the effect of azimuthal anisotropy in our inversions in order to obtain reliable isotropic phase velocity. We use b-splines to combine these isotropic phase velocity maps with our previous group velocity maps to produce an internally consistent global surface wave dispersion model.

Ma, ZT, Masters G.  2014.  A new global Rayleigh- and Love-wave group velocity dataset for constraining lithosphere properties. Bulletin of the Seismological Society of America. 104:2007-2026.   10.1785/0120130320   AbstractWebsite

We present a new and efficient method to measure Rayleigh- and Love-wave group velocity over a broad frequency range. This technique starts in a similar fashion to the traditional frequency-time analysis, but instead of making measurements for all frequencies for a single source-station pair, we apply cluster analysis to make measurements for all recordings from a single event at a single target frequency. We also develop an inversion method with laterally varying smoothnesses to generate 2D group velocity maps with uniform errors. These maps match large scale geologic features and fit our data very well. This dataset will be used to constrain lithospheric structure globally.

Walpole, J, Wookey J, Masters G, Kendall JM.  2014.  A uniformly processed data set of SKS shear wave splitting measurements: A global investigation of upper mantle anisotropy beneath seismic stations. Geochemistry Geophysics Geosystems. 15:1991-2010.   10.1002/2014gc005278   AbstractWebsite

Anisotropy in the Earth's upper mantle is a signature of past and present deformation. Here we present a new data set of similar to 50,000 uniformly processed SKS shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0.02-0.1 Hz. The data set consists of measurements obtained at similar to 2000 seismic stations from similar to 2000 events. We identify several stations characterized by an apparent absence of shear wave splitting (so-called "null stations"). Station-averaged measurements are obtained by stacking shear wave splitting error surfaces. The stacked data set shows excellent agreement with a compilation of previous SKS measurements. The average amount of splitting beneath seismic stations (after error surface stacking) is 0.8 s, slightly lower than that found previously by vectorial averaging of non-null measurement splitting parameters. The data set disagrees, however, with an azimuthally anisotropic surface wave tomography model (DKP2005), suggesting that caution should be exercised when using such models for geodynamic interpretation, especially in continental regions. Studying our data set in detail, we find evidence that flow in the asthenosphere exerts partial control over SKS splitting in orogenic regions globally. In the active orogenic environment of the western USA, where we have the densest coverage, our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east.

Pasyanos, ME, Masters TG, Laske G, Ma ZT.  2014.  LITHO1.0: An updated crust and lithospheric model of the Earth. Journal of Geophysical Research-Solid Earth. 119:2153-2173.   10.1002/2013jb010626   AbstractWebsite

We present the LITHO1.0 model, which is a 1 degrees tessellated model of the crust and uppermost mantle of the Earth, extending into the upper mantle to include the lithospheric lid and underlying asthenosphere. The model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers, such as water, ice, sediments, crystalline crust, lithospheric lid, and asthenosphere. LITHO1.0 is created by constructing an appropriate starting model and perturbing it to fit high-resolution surface wave dispersion maps (Love and Rayleigh, group and phase) over a wide frequency band (5-40mHz). We examine and discuss the model with respect to key lithospheric parameters, such as average crustal velocity, crustal thickness, upper mantle velocity, and lithospheric thickness. We then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency. It appears that LITHO1.0 represents a reasonable starting model of the Earth's shallow structure (crust and uppermost mantle) for the purposes in which these models are used, such as traveltime tomography or in efforts to create a 3-D reference Earth model. The model matches surface wave dispersion over a frequency band wider than the band used in the inversion. There are several avenues for improving the model in the future by including attenuation and anisotropy, as well as making use of surface waves at higher frequency.

2008
Gubbins, D, Masters G, Nimmo F.  2008.  A thermochemical boundary layer at the base of Earth's outer core and independent estimate of core heat flux. Geophysical Journal International. 174:1007-1018.   10.1111/j.1365-246X.2008.03879.x   AbstractWebsite

Recent seismological observations suggest the existence of a approximate to 150-km-thick density-stratified layer with a P-wave velocity gradient that differs slightly from PREM. Such a structure can only be caused by a compositional gradient, effects of a slurry or temperature being too small and probably the wrong sign. We propose a stably stratified, variable concentration layer on the liquidus. Heat is transported by conduction down the liquidus while the light and heavy components migrate through the layer by a process akin to zone refining, similar to the one originally proposed by Braginsky. The layer remains static in a frame of reference moving upwards with the expanding inner core boundary. We determine the gradient using estimates of c(o), the concentration in the main body of the outer core, and c(b), the concentration of the liquid at the inner core boundary. We determine the depression of the melting point and concentrations using ideal solution theory and seismologically determined density jumps at the inner core boundary. We suppose that co determines Delta rho(mod), the jump from normal mode eigenfrequencies that have long resolution lengths straddling the entire layer, and that cb determines Delta rho(bod), the jump determined from body waves, which have fine resolution. A simple calculation then yields the seismic, temperature, and concentration profiles within the layer. Comparison with the distance to the C-cusp of PKP and normal mode eigenfrequencies constrain the model. We explore a wide range of possible input parameters; many fail to predict sensible seismic properties and heat fluxes. A model with Delta rho(mod) = 0.8 gm cc(-1), Delta rho(bod) = 0.6 gm cc(-1), and layer thickness 200 km is consistent with the seismic observations and can power the geodynamo with a reasonable inner core heat flux of approximate to 2 TW and nominal inner core age of approximate to 1 Ga. It is quite remarkable and encouraging that a model based on direct seismic observations and simple chemistry can predict heat fluxes that are comparable with those derived from recent core thermal history calculations. The model also provides plausible explanations of the observed seismic layer and accounts for the discrepancy between estimates of the inner core density jumps derived from body waves and normal modes.

Houser, C, Masters G, Flanagan M, Shearer P.  2008.  Determination and analysis of long-wavelength transition zone structure using SS precursors. Geophysical Journal International. 174:178-194.   10.1111/j.1365-246X.2008.03719.x   AbstractWebsite

Global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 degrees rather than 10 degrees radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.

Houser, C, Masters G, Shearer P, Laske G.  2008.  Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms. Geophysical Journal International. 174:195-212.   10.1111/j.1365-246X.2008.03763.x   AbstractWebsite

We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique. This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23 degrees to 100 degrees) and SS and PP (from 50 degrees to 170 degrees). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core-mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.

2007
Masters, G.  2007.  Core density. Encyclopedia of geomagnetism and paleomagnetism. ( Gubbins D, Herrero-Bervera E, Eds.).:82-84., Dordrecht: Springer Abstract
n/a
2006
Montelli, R, Nolet G, Dahlen FA, Masters G.  2006.  A catalogue of deep mantle plumes: New results from finite-frequency tomography. Geochemistry Geophysics Geosystems. 7   10.1029/2006gc001248   AbstractWebsite

New finite-frequency tomographic images of S-wave velocity confirm the existence of deep mantle plumes below a large number of known hot spots. We compare S-anomaly images with an updated P-anomaly model. Deep mantle plumes are present beneath Ascension, Azores, Canary, Cape Verde, Cook Island, Crozet, Easter, Kerguelen, Hawaii, Samoa, and Tahiti. Afar, Atlantic Ridge, Bouvet(Shona), Cocos/Keeling, Louisville, and Reunion are shown to originate at least below the upper mantle if not much deeper. Plumes that reach only to midmantle are present beneath Bowie, Hainan, Eastern Australia, and Juan Fernandez; these plumes may have tails too thin to observe in the lowermost mantle, but the images are also consistent with an interpretation as "dying plumes'' that have exhausted their source region. In the tomographic images, only the Eifel and Seychelles plumes are unambiguously confined to the upper mantle. Starting plumes are visible in the lowermost mantle beneath South of Java, East of Solomon, and in the Coral Sea. All imaged plumes are wide and fail to show plumeheads, suggesting a very weakly temperature-dependent viscosity for lower mantle minerals, and/or compositional variations. The S-wave velocity images show several minor differences with respect to the earlier P-wave results, including plume conduits that extend down to the core-mantle boundary beneath Cape Verde, Cook Island, and Kerguelen. A more substantial disagreement between P-wave and S-wave images reopens the question on the depth extent of the Iceland plume. We suggest that a pulsating behavior of the plume may explain the shape of the conduit beneath Iceland.

Lawrence, JF, Shearer PM, Masters G.  2006.  Mapping attenuation beneath North America using waveform cross-correlation and cluster analysis. Geophysical Research Letters. 33   10.1029/2006gl025813   AbstractWebsite

We measure seismic attenuation beneath North America using waveform cross-correlation and cluster analysis, and obtain images of the laterally varying anelastic structure of the upper mantle. Cluster analysis improves attenuation measurements by systematically comparing only highly similar waveforms, which reduces bias from scattering, directional differences in source functions, and source-side structure. While lacking station coverage in many areas, the P- and S-wave results are correlated (R-2 >= 0.5) in both travel time and attenuation. Much weaker correlations are observed between travel-time and attenuation measurements. Similarities and differences between attenuation and travel times may be used to infer the source of the observed anomalies. The observed anelastic structure has a long-wavelength pattern crudely similar to that of seismic velocity, which likely indicates higher temperatures beneath western North America than in the east. Shorter-wavelength structure suggests complex variations requiring alternate explanations such as variable water content.

2005
Lebedev, S, Nolet G, Meier T, van der Hilst RD, Masters G.  2005.  Automated multimode inversion of surface and S waveforms. Geophysical Journal International. 162:951-964.   10.1111/j.1365-246X.2005.02708.x   AbstractWebsite

Inversion of the surface, S, and multiple-S waveforms is an effective means of constraining the structure of the upper mantle, including the transition zone. Exploiting the resolving power of the enormous volume of presently available data requires efficiency of data processing and waveform modelling. An established method for rapid generation of synthetic seismograms is the summation of surface-wave modes under an assumption that the effect of seismic-wave scattering is negligible. This assumption is valid at best for portions of a broad-band signal, the portions being generally different for different seismograms. Interactive selection of such parts of the signal is impractical for large data sets of tens or hundreds of thousand seismograms. Here we present a fully automated waveform inversion technique with selection of signal uncontaminated by scattered waves implemented as its integral element. Waveforms are inverted using non-linear optimization and the results are put in the form of orthogonalized linear constraints on average elastic structure along the source-station paths. Structural information from waves of different amplitudes and different types is balanced by means of time- and frequency-dependent weighting, also used to suppress the propagation of errors in the data. The equations obtained from processing thousands of seismograms can be inverted together for high-resolution upper-mantle models. The technique has been applied to a large Western Pacific data set. Analysis of the results suggests that it has been effective and, in particular, confirms that 'chance' fits of scattered waves or noise do not pass the automated selection procedure. Results of the processing also provide an empirical mapping of the field of the JWKB-approximation validity for modelling the propagation of surface waves. While there is no sharp boundary of the JWKB regime, the probability of the approximation validity decreases with increasing distance and frequency.

Davis, P, Ishii M, Masters G.  2005.  An assessment of the accuracy of GSN sensor response information. Seismological Research Letters. 76:678-683. AbstractWebsite
n/a
2004
Gubbins, D, Alfe D, Masters G, Price GD, Gillan M.  2004.  Gross thermodynamics of two-component core convection. Geophysical Journal International. 157:1407-1414.   10.1111/j.1365-246X.2004.02219.x   AbstractWebsite

We model the inner core by an alloy of iron and 8 per cent sulphur or silicon and the outer core by the same mix with an additional 8 per cent oxygen. This composition matches the densities of seismic model, Preliminary Reference Earth Model (PR-EM). When the liquid core freezes S and Si remain with the Fe to form the solid and excess 0 is ejected into the liquid. Properties of Fe, diffusion constants for S, Si, 0 and chemical potentials are calculated by first-principles methods under the assumption that S, 0, and Si react with the Fe and themselves, however, not with each other. This gives the parameters required to calculate the power supply to the geodynamo as the Earth's core cools. Compositional convection, driven by light O released at the inner-core boundary on freezing, accounts for half the entropy balance and 15 per cent of the heat balance. This means the same magnetic field can be generated with approximately half the heat throughput needed if the geodynamo were driven by heat alone. Chemical effects are significant: heat absorbed by disassociation of Fe and 0 almost nullify the effect of latent heat of freezing in driving the dynamo. Cooling rates below 69 K Gyr(-1) are too low to maintain thermal convection everywhere; when the cooling rate lies between 35 and 69 K Gyr(-1) convection at the top of the core is maintained compositionally against a stabilizing temperature gradient; below 35 K Gyr(-1) the dynamo fails completely. All cooling rates freeze the inner core in less than 1.2 Gyr, in agreement with other recent calculations. The presence of radioactive heating will extend the life of the inner core, however, it requires a high heat flux across the core-mantle boundary. Heating is dominated by radioactivity when the inner core age is 3.5 Gyr. We, also, give calculations for larger concentrations of O in the outer core suggested by a recent estimation of the density jump at the inner-core boundary, which is larger than that of PREM. Compositional convection is enhanced for the higher density jumps and overall heat flux is reduced for the same dynamo dissipation, however, not by enough to alter the qualitative conclusions based on PREM. Our preferred model has the core convecting near the limit of thermal stability, an inner-core age of 3.5 Gyr and a core heat flux of 9 TW or 20 per cent of the Earth's surface heat flux, 80 per cent of which originates from radioactive heating.

Montelli, R, Nolet G, Dahlen FA, Masters G, Engdahl ER, Hung SH.  2004.  Finite-frequency tomography reveals a variety of plumes in the mantle. Science. 303:338-343.   10.1126/science.1092485   AbstractWebsite

We present tomographic evidence for the existence of deep-mantle thermal convection plumes. P-wave velocity images show at least six well-resolved plumes that extend into the lowermost mantle: Ascension, Azores, Canary, Easter, Samoa, and Tahiti. Other less well-resolved plumes, including Hawaii, may also reach the lowermost mantle. We also see several plumes that are mostly confined to the upper mantle, suggesting that convection may be partially separated into two depth regimes. All of the observed plumes have diameters of several hundred kilometers, indicating that plumes convey a substantial fraction of the internal heat escaping from Earth.

Montelli, R, Nolet G, Masters G, Dahlen FA, Hung SH.  2004.  Global P and PP traveltime tomography: rays versus waves. Geophysical Journal International. 158:637-654.   10.1111/j.1365-246X.2004.02346.x   AbstractWebsite

This paper presents a comparison of ray-theoretical and finite-frequency traveltime tomography for compressional waves. Our data set consists of 86 405 long-period P and PP-P traveltimes measured by cross-correlation. The traveltime of a finite-frequency wave is sensitive to anomalies in a hollow banana- shaped region surrounding the unperturbed ray path, with the sensitivity being zero on the ray. Because of the minimax nature of the surface-reflected PP wave, its sensitivity is more complicated. We compute the 3-D traveltime sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation. We compare tomographic models with the same chi(2) fit for both ray theory and finite-frequency analysis. Depending on the depth and size of the anomaly, the amplitudes of the velocity perturbations in the finite-frequency tomographic images are 30-50 per cent larger than in the corresponding ray-theoretical images, demonstrating that wave front healing cannot be neglected when interpreting long-period seismic waves. The images obtained provide clear evidence that a limited number of hotspots are fed by plumes originating in the lower mantle.

Schubert, G, Masters G, Olson P, Tackley P.  2004.  Superplumes or plume clusters? Physics of the Earth and Planetary Interiors. 146:147-162.   10.1016/j.pepi.2003.09.025   AbstractWebsite

It is proposed that the broad, seismically slow, mantle structures under Africa and the Pacific, often identified as superplumes, are instead spatial clusters of smaller plumes. Seismic data, including ScS-S differential travel time residuals and tomographic inversions using ScS-S and deep turning S data, show the breakup of so-called superplume regions into smaller structures. For example, the superplume under Africa is clearly formed by at least two and possibly three distinct plumes while the superplume under the Pacific consists of at least six smaller plumes. Enhanced seismic resolution may reveal even smaller-scale structures in the superplume regions. Dynamical considerations argue for the plausibility of superplume regions being clusters of smaller plumes whose heads might have merged into a large region of hot and buoyant material. Alternatively, the superplumes may simply be large, passively upwelling regions that are seismically distinct. (C) 2004 Elsevier B.V. All rights reserved.

2003
Gubbins, D, Alfe D, Masters G, Price GD, Gillan MJ.  2003.  Can the Earth's dynamo run on heat alone? Geophysical Journal International. 155:609-622.   10.1046/j.1365-246X.2003.02064.x   AbstractWebsite

The power required to drive the geodynamo places significant constraints on the heat passing across the core-mantle boundary and the Earth's thermal history. Calculations to date have been limited by inaccuracies in the properties of liquid iron mixtures at core pressures and temperatures. Here we re-examine the problem of core energetics in the light of new first-principles calculations for the properties of liquid iron. There is disagreement on the fate of gravitational energy released by contraction on cooling. We show that only a small fraction of this energy, that associated with heating resulting from changes in pressure, is available to drive convection and the dynamo. This leaves two very simple equations in the cooling rate and radioactive heating, one yielding the heat flux out of the core and the other the entropy gain of electrical and thermal dissipation, the two main dissipative processes. This paper is restricted to thermal convection in a pure iron core; compositional convection in a liquid iron mixture is considered in a companion paper. We show that heat sources alone are unlikely to be adequate to power the geodynamo because they require a rapid secular cooling rate, which implies a very young inner core, or a combination of cooling and substantial radioactive heating, which requires a very large heat flux across the core-mantle boundary. A simple calculation with no inner core shows even higher heat fluxes are required in the absence of latent heat before the inner core formed.

Masters, G, Gubbins D.  2003.  On the resolution of density within the Earth. Physics of the Earth and Planetary Interiors. 140:159-167.   10.1016/j.pepi.2003.07.008   AbstractWebsite

Roughly 30 years have passed since the last publication of a linear resolution calculation of density inside the Earth. Since that time, the data set of free oscillation degnerate frequencies has been completely re-estimated taking into account the biassing effects of splitting and coupling due to 3D structure. This paper presents a new resolution analysis based on the new data and focuses on two particular issues: (1) the density jump at the inner-core boundary which is important in discussions of the maintenance of the geodynamo; and (2) a possible density excess in the lowermost mantle which might be indicative of a "hot abyssal layer". We find that the density jump at the inner-core boundary is 0.82 +/- 0.18 Mg m(-3) which is significantly larger than previously thought. We also find little support for an excess density in the lowermost mantle though an increase of 0.4% is possible. (C) 2003 Elsevier B.V. All rights reserved.

Laske, G, Masters G.  2003.  The Earth's free oscillations and the differential rotation of the inner core. Earth's core : dynamics, structure, rotation ; [The origin of this work derives from a union session that we organized at the Fall 2000 AGU meeting in San Francisco: "Core dynamics, structure, and rotation"] Geodynamics Series. ( Dehant V, Ed.).:5-21., Washington, D.C.: American Geophysical Union Abstract
n/a
2001
Schulte-Pelkum, V, Masters G, Shearer PM.  2001.  Upper mantle anisotropy from long-period P polarization. Journal of Geophysical Research-Solid Earth. 106:21917-21934.   10.1029/2001jb000346   AbstractWebsite

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long-period P onsets. The horizontal polarization of the initial P particle motion can deviate by > 10 degrees from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near-source structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from P-n travel times and SKS splitting. The results of the comparison are consistent, with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from P,, and Pp., in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Bolton, H, Masters G.  2001.  Travel times of P and S from the global digital seismic networks: Implications for the relative variation of P and S velocity in the mantle. Journal of Geophysical Research-Solid Earth. 106:13527-13540.   10.1029/2000jb900378   AbstractWebsite

We present new data sets of P and S arrival times which have been handpicked from long-period vertical and transverse component recordings of the various global seismic networks. Using events which occurred from 1976 to 1994 results in similar to 38,000 globally well-distributed measurements of teleseismic P and similar to 41,000 measurements of S. These data are particularly useful for looking at the relative variation of S and P velocities in the lower mantle. We describe both the measurement techniques and the gross characteristics of the data sets. The size of our data sets allows us to exploit the internal consistency of the data to identify outliers using a summary ray analysis. Since the polarity of each arrival is also known, we can construct fault plane solutions and/or compare with polarities predicted by the Harvard centroid moment tensor solutions to further diagnose phase misidentification. This analysis results in similar to5% of the data being identified as outliers. An analysis of variance indicates that the S residual travel times are dominated by the effects of three-dimensional structure but the P data have comparable contributions from noise and source mislocation effects. The summary ray analysis reveals the basic character of lower mantle structure, and there are large-scale patterns in both the S and P data sets that correlate quite well with each other. This analysis suggests that on average, d ln v(s)/d ln v(p) is an increasing function of depth in the mantle going from a value of similar to1.7 at the top of the lower mantle to an apparent value of 4 near the base of the mantle. This latter extreme value of R seems to result mainly from data which sample one region in the lowermost mantle under the central Pacific, where large positive S residuals are associated with very small P residuals. Such an anomaly cannot be thermal in origin.

2000
Masters, G, Laske G, Gilbert F.  2000.  Matrix autoregressive analysis of free-oscillation coupling and splitting. Geophysical Journal International. 143:478-489.   10.1046/j.1365-246X.2000.01261.x   AbstractWebsite

The study of the splitting and coupling of free oscillations can potentially provide some unique information about the Earth-in particular about how density varies laterally relative to sheer velocity and bulk sound speed. Mode splitting has been studied for many years but it is clear that the time is ripe to revisit this field. Tn particular, the rapid expansion of the global seismic network and the occurrence of many large (and deep) earthquakes in the past few years means that mode-splitting analyses are capable of much higher precision than in the past. Some studies have already argued that 3-D density can be recovered (and that density variations are negatively correlated with shear velocity in the lower mantle). This result is controversial and we present some experiments that indicate that claims of density recovery with the current mode data set are premature. However, we believe that, with expanded data sets and new analysis techniques, the precision needed to recover the 3-D density structure of the Earth and its 3-D anelastic structure is now within our reach.