Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle
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Edited by Russell J. Hemley, Carnegie Institute of Washington, Washington, DC, and approved March 7, 2007 (received for review September 15, 2006)
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Fig. 1.A map of events and stations (triangles) used in the construction of a 2D model along a corridor through the African Superplume. We use data from two arrays, South African and the new Ethiopia/Kenya array (19) of events arriving along a great circle in this study. SKS and SKKS exist points at the CMB are given in colored triangles, with blue indicating no delay and red >5 s. To produce the sharp jumps requires a monolithic structure denoted in heavy green lines as the ALVS, where the S velocity inside the box is reduced by 3% relative to PREM (18). We have included some example ray paths S, SKS, and SCS. The background tomographic model is the most recently updated Grand model (27).
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Fig. 2.Display of a 2D section through a metastable thermo-chemical structure along with predicted V P and V S velocities (14). Only the bottom 1,200 km of the model is shown. Within the anomaly, the material has a larger bulk modulus (6% larger than ambient) and higher zero pressure density (2.25%). The layer forms a single dynamic structure with the average density near neutral. Note the plumes along the edges and the down-welling near the middle. (a) Nondimensional temperature. (b) Density anomaly, δρ (%). (c) V P anomaly, δVp (%). (d) V S anomaly, δVs (%).
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Fig. 3.Presentation of seismic predictions by inserting the HBMS model into the earth beneath Africa, essentially replacing Fig. 1 b by Fig. 2 d and comparing results against seismic observations. We have included the ALVS results for comparison. (a) The geometry and ray paths along a 2D cross-section (East Pacific Rise to the South Africa Array) sampling the anomalous structure, SKS (red) and Sd (light blue). (b) The differential timing derived by cross-correlating the observed waveforms (5), with synthetics relative to predictions from the 1D reference earth PREM. Because the structure is roughly symmetric, we included predictions from both sides with those from the left (heavy line) and from the right (light line). (c) A comparison of travel time predictions generated from HBMS synthetics against those observed at the South Africa Array (Kaapvaal array) (11) are displayed for diffracted S (Sd, ▴) and P (Pd, ▵). Because the diffracted waves sample the top edge of the structure first, the anomalous travel times have a gradual onset as predicted by the solid curves, heavy from the left, light from the right. Both the data and synthetic predictions display considerable scatter indicative of possible embedded fine structure, which is likely to be time-dependent. However, the magnitude of the anomalous S delays relative to P are well matched.
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Fig. 4.Comparison of SCS-S and PCP-P predictions with observations (5). (a) The ray paths of SCS (mangenta), S (cyan), and P (yellow) are shown sampling the HBMS model. (b) Comparison of differential SCS-S (solid line) and PCP-P (dot line) predicted by the HBMS and ALVS models. The modeled PCP-P differential time is shifted up by 1 s, considering the possible base-line shift for origin time correction.
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Fig. 5.Modeling a D” triplication from the Ethiopia/Kenya array from a deep Sandwich Island event. (a) The recording geometry with solid circles indicating the SCS sampling points at the CMB. (b) We assume that the ALVS inside the green box in Fig. 1 b is 1D and replace the velocities near the bottom with our preferred structure (b). We have included a model with a sharp 4% jump (CM), which is discussed in SI Text. (c) Geometric ray paths with arrival times given as lines on the data-synthetic record sections. (d) The comparison with observed waveforms against predictions from the HBMS model. (e) The comparison with observed waveforms against predictions from the hybrid model containing structure b.
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Fig. 6.Map of the northern edge of the Pacific Superplume displaying where anomalous seismic data has been studied relative to Grand's tomographic model (27). The rectangle indicates the position of a sharp change in shear velocity structure, which appears to be quite compatible with the edge of the HBMS (22). The piercing points of PKP(AB) three Fiji-Tonga events (red, green, and blue) are indicated relative to a dotted line where the dotted PKP arrivals are 2 s earlier than those to the north. The symbols indicted by ∗ or triangles are locations where PKP(AB) display complexities (multipathed). The white trapezoid region shows rapid lateral variation of D″ (23).
Footnotes
- †To whom correspondence should be addressed. E-mail: helm{at}gps.caltech.edu
- © 2007 by The National Academy of Sciences of the USA
