Wei-jia Su

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.

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.

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.

Planet within a planet : Rotation of the inner core of Earth

The time dependence of the orientation of Earths inner core relative to the mantle was determined using a recently discovered 10-degree tilt in the axis of symmetry of the inner cores seismic-velocity anisotropy. Two methods of analyzing travel-time variations for rays traversing the inner core, on the basis of 29 years of data from the International Seismological Centre (1964–1992), reveal that the inner core appears to rotate about 3 degrees per year faster than the mantle. An anomalous variation in inner-core orientation from 1969 to 1973 coincides in time with a sudden change (“jerk”) in the geomagnetic field.

Inner core anisotropy in three dimensions

The purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the PKIKP phase. We use the arrival times reported in the International Seismological Centre Bulletins for years 1964–1990. We select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations. The earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three-dimensional mantle model. We use a total of 313,422 observations of the DF branch of PKP travel time anomalies within the epicentral distance ranges of 120°–140° and 150°–180° reported by 2335 stations for 26,377 earthquakes. We process the data using an averaging procedure, which we call cylindrical anisotropy stacking, that enhances the effects of anisotropy, but is expected to suppress those due to lateral heterogeneity and random errors. The processed residuals show a remarkably consistent pattern. This confirms the dominance of the cylindrical elastic anisotropy in the inner core with, approximately, a constant axis of symmetry. This axis of symmetry is found to be tilted 10.5°±1° from the Earths rotation axis in the direction 160°E±5° in the northern hemisphere. In this new coordinate system we determine a four-layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick. The model shows that the anisotropy is strongest (>3%) within the innermost part of the core. The travel time anomalies show significant (±1.5 s) longitudinal variations, even when the tilt of the axis of symmetry is considered. There is a substantial increase in the amplitude of the longitudinal variations, which are dominated by the second and fourth harmonics, for rays with bottoming depths exceeding 400 km below the inner core boundary. The measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy. The increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200–300 km of the Earths inner core.

On the scale of mantle heterogeneity

Abstract A data set comprising 3313 residuals measured from digital records of 429 earthquakes is analyzed for the heterogeneity of the Earths mantle. The smoothed SS residuals reveal a large pattern of heterogeneity and agree well with the major tectonic features on the Earths surface and the residual patterns predicted by upper mantle and whole mantle models. The spectrum of SS residuals indicates that the heterogeneity in the Earths mantle, which probably reflects the patterns of mantle convection, is dominated by long-wavelength features. Our analysis of the power of SS residuals as a function of harmonic degree shows a distinct difference in the rate of decrease of the power below and above degree 8. The rate is first slower than l −1 , and then it exceeds l −2 . This is also true for SS residuals predicted for model SH 425.2 and Love wave phase velocities at 100 s measured by other researchers. This indicates that there is a preference for features with dimensions larger than 2500–3500 km. We find that the power spectrum of the velocity anomalies associated with the subducted slabs is two orders of magnitude lower for 1 ≤ l ≤ 8 than that of velocity anomalies obtained by inversion of waveform data. For the ocean basins, correlation of the SS residuals with the square root of age of the oceanic lithosphere is highly significant. However, the age correlation predicts only 10% of the variance. This means that the thermal signature of the spreading center represents only a small fraction of the thermal anomalies introduced into the upper mantle by convection.