Sidao Ni

Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa

We present data which indicate that the broad, low shear velocity anomaly beneath southern Africa is stronger and more extensive than previously thought. Recordings of earthquakes in the southwestern Atlantic Ocean at an array of broadband seismic stations in eastern Africa show anomalously large propagation time delays of the shear phases S, ScS, and SKS which vary rapidly with epicentral distance. By forward modeling, we estimate that the low velocity anomaly extends from the core-mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3% lower than in standard models such as PREM. Strong velocity contrasts exist at its margins (2% over about 300 km). These seismic characteristic are consistent with recent numerical simulations of lower mantle mega-plume formation.

Seismological constraints on the South African superplume;could be the oldest distinct structure on earth

A recent study of the lower mantle structure beneath Africa revealed strong lateral changes in S-velocity extending upward from the core–mantle boundary to about 1500 km. SKS travel times observed on the South African Array display jumps of about 6 s when ray paths cross these nearly vertical boundaries. Back projecting these delays onto the core–mantle boundary allows a clear image of the horizontal extent of this structure starting at mid-Africa (15°S, 5°E) where it strikes roughly northwest to beyond the tip of South Africa (45°S, 55°E) where it bends toward the Indian Ocean. Waveform sections of S, ScS, SKS, and SKKS are modeled along two corridors, one along strike and one at right angles to establish its uniformity. The structure is about 1200 km wide and has about a 3% drop in S-velocity although some small-scale features are apparent in the roof structure and midsection. If this structure is stabilized by a localized viscosity condition or a dense core as suggested by some joint inversions, gravity, and free oscillations, it may be isolated from mantle stirring and therefore very old with unique chemistry.

Ridge-like lower mantle structure beneath South Africa

Recent (ScS-S) results from probing the deep structure beneath southern Africa display strong delays of up to 10 s at distances beyond 90°. Such delays could be explained by long-period tomographic models containing smooth (weak) features with the addition of rough (strong) D″ structure (3–9% drops in shear velocities). However, these structures cannot explain the (SKS-S) differentials sampling the same region. To explain the (SKS-S) and (ScS-S) data sets simultaneously requires instead a large-scale ridge-like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 1000 km wide and extends at least 1200 km above D″. It is orientated roughly NW-SE and leans toward the east at latitudes from 15° to about 30°. It proves difficult to explain such sharp features with thermal effects alone and, thus, the importance of high-resolution waveform modeling to establish their existence. To derive the above results, we developed a special algorithm by matching simulated synthetics to observed broadband waveforms. This is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data. Tomographic models can then be constructed, or existing tomographic models can be altered, to match these data, and new 2-D synthetics can be constructed as well to better fit the waveform data. These updated synthetics can again be decomposed and reassembled, and the process can be repeated. This algorithm is applied to a combination of analog and digital data along a corridor from South America, producing the high-resolution 2-D model described above.

Seismic evidence for ultralow‐velocity zones beneath Africa and eastern Atlantic

KS waveforms recorded at distances of about 110 o are extremely useful to constrain seismic velocity structure at the base of the mantle.

Evidence for a sharp lateral variation of velocity at the core–mantle boundary from multipathed PKPab

KS waves near this distance develop a com- plicated interference pattern with the phases SPaKS and

Horizontal transition from fast to slow structures at the core–mantle boundary; South Atlantic

KP,& We report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath South America from sta- tions in Europe and Africa. We model these data with two-dimensional dome-like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30% lower than in the Preliminary Reference Earth Model (PREM). The spatial extent of these structures, their position with respect to the SKS core exit points, and their seismic characteristics can not be uniquely determined. However, the presence of a dipping or a concaved upper interface is a key attribute of successful models. Models that invoke flat layers are insuffi- ciently complex to explain the most erratic waveform behavior. The most anomalous data corre- spond to sampling regions at the base of the mantle beneath the East African Rift and beneath the Iceland, where possibly, whole mantle upwellings form.

Low-velocity structure beneath Africa from forward modeling

Rapid changes in differential travel times between the various PKP branches have been attributed to strong lateral velocity variations at the base of the mantle. Differential time PKPab–df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported. Such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to PKPab. Here we report on some well-documented normal and multipathed PKPab phases along with rapidly varying differential time residuals observed for Fiji events with paths sampling the northern edge of the mid-Pacific large slow structure beneath Hawaii. Systematic analyses of these data with two-dimensional models suggest the presence of a ridge-shaped structure at the core–mantle boundary containing abrupt reductions of P-wave velocity of up to 10%. This narrow ultralow velocity zone (ULVZ) is located at the boundary between a large scale low velocity and high velocity structure, corroborating recent dynamic models which propose that ULVZs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at Hawaii.

Direct measures of lateral velocity variation in the deep Earth

Recent tomographic studies of the deep mantle have revealed large-scale low-velocity zones beneath Africa and relatively fast velocity zones beneath South America. Here we conduct a concentrated core–mantle boundary (CMB) study across this transition zone from fast to slow beneath the South Atlantic. Deep South American earthquakes recorded in Africa provide S and ScS waveform data at core-grazing distances of 85–105°. The waveform interference pattern produced at these ranges provides an excellent sample of the CMB structure in the vicinity of the ScS bounce point which can be used to map out regions of low-velocity zones ( 10% reductions). The strongest anomalies occur at the western edge roughly beneath the Tristat Plateau with weaker structures to the east beneath the so-called ‘Great Africa Plume’. These results agree with dynamic models where strong local upwellings develop at the edges of D″ (fast) structures.

Probing an ultra-low velocity zone at the core mantle boundary with P and S waves

Seismic waveforms observed in South Africa containing the first arrival crossover of S to SKS (70° to 110°) are analyzed. The data consist of analog records from the World Wide Seismographic Station Network (WWSSN) of deep events beneath South America. The S-waves arrive 2 to 3 s early relative to PREM at ranges from 70° to 95° and then become increasingly delayed, becoming 5 to 6 s late at 110°. The SKS phase is late by 3 to 5 s over the entire range. This pushes crossover between S and SKS, normally observed at about 81°, out about 2° to 3°, which is the most anomalous shift ever reported. To model such features, we modified Grands tomography model [Grand et al., GSA Today 7 (1997) 1–7], and generated 2D synthetics to match the data. The overall shape and position of the lower mantle low-velocity anomaly proposed by Grand predicts good results if lower mantle anomalies are enhanced to a level of about 4%. This results in a complex tabular structure extending upward from the core–mantle boundary about 1500 km into the mantle. These features appear to be consistent with a large young plume which is erupting off the CMB.

Approximate 3D Body-Wave Synthetics for Tomographic Models

Current tomographic models of the Earth display perturbations to a radial stratified reference model. However, structures in the deep mantle that are chemically dense with low Rayleigh numbers can develop enormous relief, perhaps with boundaries closer to vertical than to radial. Such features are hard to detect with present tomographic modeling techniques because the timing anomalies are based on long-period filtered waveforms with complexity removed. Here we develop a new tool for processing array data on the basis of a decomposition referred to as a multipath detector, which can be used to distinguish between horizontal structure (in-plane multipathing) and vertical (out-of-plane multipathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in southern Africa, which are compared with synthetic predictions from a metastable dynamic model containing sharp edges. Both data and simulations produce timing gradients larger than 2 s/deg in azimuthal changes for S waves, where only minor effects are obtained for P waves. These results further validate the case for distinct chemistry inside the African Low Shear Velocity Province.