Adam M. Dziewonski

Preliminary reference earth model

A large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earths interior. The data set was supplemented with a special study of 12 years of ISC phase data which yielded an additional 1.75 × 10^6 travel time observations for P and S waves. In order to obtain satisfactory agreement with the entire data set we were required to take into account anelastic dispersion. The introduction of transverse isotropy into the outer 220 km of the mantle was required in order to satisfy the shorter period fundamental toroidal and spheroidal modes. This anisotropy also improved the fit of the larger data set. The horizontal and vertical velocities in the upper mantle differ by 2–4%, both for P and S waves. The mantle below 220 km is not required to be anisotropic. Mantle Rayleigh waves are surprisingly sensitive to compressional velocity in the upper mantle. High S_n velocities, low P_n velocities and a pronounced low-velocity zone are features of most global inversion models that are suppressed when anisotropy is allowed for in the inversion. The Preliminary Reference Earth Model, PREM, and auxiliary tables showing fits to the data are presented.

Degree 12 model of shear velocity heterogeneity in the mantle

We obtain a three-dimensional (3D) model of shear wave velocity heterogeneity of the Earths mantle by inverting a large set of seismic data consisting of 27,000 long-period seismograms and 14,000 travel time observations. About 60% of the data has been collected through the efforts of several research groups and used in earlier studies. The new data, which come from stations of different seismic networks including the Chinese Digital Seismographic Network (CDSN) and Geoscope, are extracted to provide sampling of mantle heterogeneity as uniform as possible. Because of the improved data coverage, we expand our model to degree 12 in spherical harmonics to describe horizontal variations, and to order of 13 in Chebyshev polynomials to describe radial variations. The resulting model shows a clear pattern of slower-than-average shear velocities at shallow depths underlying the major segments of the world-wide ridge system. These anomalies extend to depths greater than 300 km and in some cases appear to continue into the lower mantle. There is also a good correlation between the major continental shields and fast-velocity perturbations at depths extending to 300–400 km. Some of the continental “roots” extend to depths greater than in other studies. The pattern of heterogeneity is more complex in the midmantle, where the power spectrum is almost flat and has a relatively low amplitude; therefore the results in this depth range should be interpreted with caution. The pattern of the heterogeneity indicates a rapid change at a depth of about 1700 km. At this depth, the power spectrum of the model shifts from one which is almost flat in the midmantle to that dominated by degrees 2 and 3; this pattern then continues to the core-mantle boundary (CMB). The model is dominated by a few megastructures of velocity heterogeneity below the depth of 2000 km, in agreement with previous studies. Among these megastructures are the “Pangea Trough,” “Great African Plume,” and “Equatorial Pacific Plume Group.” The model predicts well the large-scale pattern of observed S, SS absolute travel times, and SS-S, ScS-S differential travel times. It also predicts well the waveforms of mantle wave and body wave. We compare our model with several other recently published models. There is generally a good agreement in the long-wavelength pattern of the models, especially at shallow depths and near the CMB. However, the amplitude as well as the pattern of shorter-wavelength features are in some cases quite different.

The unique anisotropy of the Pacific upper mantle

The development and interpretation of tomographic models of the Earths mantle have usually proceeded under the assumption that fast and slow seismic velocity anomalies represent a spatially heterogeneous temperature field associated with mantle convection. Implicit in this approach is an assumption that either the effect of anisotropy on seismic velocities is small in comparison with isotropic thermal or compositional effects, or that the tomographic results represent the average isotropic heterogeneity, even if individual seismic observations are affected by anisotropic structure. For example, velocity anomalies in the upper portions of the oceanic mantle are commonly interpreted in terms of the progressive cooling, (and localized reheating) of amechanical and thermal boundary layer consisting of rigid oceanic lithosphere and an underlying, less viscous, asthenosphere. Here, however, we present results from a global three-dimensional tomographic model of shear-wave velocity which shows that the uppermost mantle beneath the central Pacific Ocean is considerably more complicated than this simple model. Over a broad area, with its centre near Hawaii, the seismic data reveal a regional anomaly in elastic anisotropy which produces variations of seismic velocities that are at least as large as those due to thermal effects. Because seismic anisotropy is an indicator of strain in Earth materials, our tomographic results canbe used to put constraints on both buoyancy forces (thermal effects) and flow patterns in the upper mantle.

SIMULTANEOUS INVERSION FOR 3-D VARIATIONS IN SHEAR AND BULK VELOCITY IN THE MANTLE

Until recently, most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in P- or S-wave velocities. The S-wave velocity variations are larger and hence provide stronger signal on long-period waveforms. The direct P travel times, being the first arrivals, on the other hand, are most frequently reported in the International Seismological Centre (ISC) Bulletins. In mineral physics experiments, the variation in bulk velocity is more often measured. To better understand the differences between δvP and δvP patterns and better link the results from mineral physics to those of seismic tomography, we formulate the inverse problem in terms of relative perturbations in the shear velocity vS = (μϱ)12 and bulk sound velocity vΦ = (Kϱ)12. We use a large data set which consists of waveforms, waveform-derived travel times and travel times from the ISC Bulletins. The earthquakes are relocated using corrections for lateral heterogeneity. The events which cannot be reliably determined are discarded. The model is defined as spherical harmonics to Degree 12 horizontally and as Chebyshev polynomials to order 13 radially, for both shear and bulk sound velocity. The inversion is performed under smoothness constraint. The resolution tests and bootstrapping analysis indicate that the model is well recovered, particularly at long wavelength. The results indicate a much larger variability of shear than bulk sound velocity. The model explains observations well. The most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle. The explanation is not very clear. From the mineral physics point of view, it is not unlikely that this could be explained in terms of thermal variation, even though we are unwilling to rule out the possibility of large wavelength compositional variations.

Centroid-moment tensor solutions for January–March, 1984

Abstract Centroid-moment tensor solutions are presented for 148 earthquakes that occurred during the first quarter of 1984. The solutions are obtained using corrections for aspherical Earth structure.

Centroid-moment tensor solutions for April–June, 1983

Abstract Centroid-moment tensor solutions are presented for 99 earthquakes that occurred during the second quarter of 1983.

Global Images of the Earth's Interior

The three-dimensional maps of the earths interior now span regions from the bottom of the crust to the inner core of the earth. Although a wealth of new information on the dynamics of the earth has been discovered, the inner core offers the greatest surprise: it appears to be anisotropic with the axis of symmetry aligned with the axis of rotation.

Global seismicity of 1982: centroid-moment tensor solutions for 308 earthquakes

Abstract The centroid-moment tensor analysis is applied to digital recordings of 308 moderate and large earthquakes that occurred during 1982 and were recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. The tables contain numerical results for all events. A graphical representation of the source geometry is shown for the full moment tensor solution, as well as for the “best double couple”.

European–Mediterranean regional centroid-moment tensors: 1997–2000

The Mediterranean region is seismically very active and undergoing rapid deformation. Earthquakes as strong as M≥6 are present, but relatively rare. On the other hand, moderate-magnitude seismicity (4.5<M<5.5) is widespread and, often, damaging. An improved knowedge of moderate-magnitude earthquakes can contribute significantly to a better understanding of active tectonics in the region. Harvard centroid-moment tensors (CMT) today represent the most widely used reference for reliable source mechanisms, but modelling seismic sources with the CMT technique for earthquakes with magnitude less than M≈5–5.5 is quite difficult. Since 1997, we are using an extension of the standard CMT algorithm to compute regional centroid-moment tensors (RCMT) for moderate-magnitude earthquakes (MW as low as 4.0), analyzing intermediate-period surface waves from regional and teleseismic seismograms. We routinely apply the algorithm to European–Mediterranean seismicity to produce a catalog of seismic moment tensors for events with magnitude between 4.5 and 5.5, which generally are not included in the Harvard CMT catalog. In this paper, we show the results for earthquakes that occurred between 1997 and 2000, consisting of 252 solutions, and we discuss the characteristics of the catalog and the areas where these new data are most relevant.

Models of the mantle shear velocity and discontinuities in the pattern of lateral heterogeneities

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.