Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle
10.1126/science.286.5443.1326
Crossref journal-article
American Association for the Advancement of Science (AAAS)
Science (221)
Abstract

A sharp discontinuity at the base of Earth's mantle has been suggested from seismic waveform studies; the observed travel time and amplitude variations have been interpreted as changes in the depth of a spatially intermittent discontinuity. Most of the observed variations in travel times and the spatial intermittance of the seismic triplication can be reproduced by a ubiquitous first-order discontinuity superimposed on global seismic velocity structure derived from tomography. The observations can be modeled by a solid-solid phase transition that has a 200-kilometer elevation above the core-mantle boundary under adiabatic temperatures and a Clapeyron slope of about 6 megapascal per kelvin.

Bibliography

Sidorin, I., Gurnis, M., & Helmberger, D. V. (1999). Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle. Science, 286(5443), 1326–1331.

Authors 3
  1. Igor Sidorin (first)
  2. Michael Gurnis (additional)
  3. Don V. Helmberger (additional)
References 44 Referenced 160
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  23. The tomography model (13) is smoothed by interpolating the anomalies between the centers of the blocks. The smoothed structure is mapped onto a mesh with a 0.2° by 0.2° by 2 km resolution. A 1.5% velocity jump (31) is added to each vertical column of this fine mesh at a depth predicted by the phase transition characteristics (16). The addition of the discontinuity affects the predicted travel times and to make our composite model consistent with the original tomography model we applied a negative velocity gradient at the base of the mantle. In dynamic models such a gradient is produced by a thermal boundary layer above the CMB (14). We used a constant velocity of 7 km/s (Fig. 2B) at the CMB and computed the gradient to preserve the travel time across the vertical column (32).
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  25. The tomography model (13) has the poorest resolution at the very base of the mantle so T ScS-Scd is less certain than T Scd-S because it includes the travel time of the ScS phase that traverses the D" layer. In addition our modification of the tomography model is restricted to the region below the imposed discontinuity (Fig. 2). This produces the largest disturbance on ScS travel times (32) further increasing the uncertainty in T ScS-Scd .
  26. The quality of agreement between the model predictions and observations can be characterized by the rms difference between predicted and observed T Scd-S differential travel times 〈δTScd­S〉=1N ∑1N (TScd­Ssynth−TScd­Sdata)21/2where N is the number of considered ray paths.
  27. For Eurasia Alaska and India we used the ray paths studied in (7 9). To explore the spatial variation of the triplication beneath the central Pacific Ocean and Central America ray paths from three Fiji-Tonga and three South America events (33 8) to the World Wide Standard Seismograph Network (WWSSN) and the Canadian National Seismograph Network (CNSN) stations in North America in the 68° to 85° distance range were considered.
  28. The D" triplication has also been extensively studied beneath Central America. However the structure beneath this region appears to be highly variable [for example (8 11)] and no single comprehensive data set is currently available.
  29. X. F. Liu J. Tromp and A. Dziewonski [ Earth Planet. Sci. Lett. 160 343 (1998)] have suggested that the D" discontinuity may be produced by the local gradients caused by thermal variations alone without a need for a first-order discontinuity. Although such a possibility has long been acknowledged by the seismological community (10) it is unlikely that thermal gradients of sufficient sharpness (5) can be produced. Dynamically consistent seismic velocity gradients constrained by mineral physics data (11) are not capable of producing a triplication consistent with observations. Similarly no triplication is produced by the seismic velocity gradients obtained from tomography inversions (18).
  30. This is not immediately obvious considering some degree of smoothing that occurs in tomographic inversions [for example
  31. Bréger L., Romanowicz B., Vinnik L., Geophys. Res. Lett. 25, 5 (1998); (10.1029/97GL03359) / Geophys. Res. Lett. by Bréger L. (1998)
  32. ]. To test if a flat discontinuity combined with larger amplitude anomalies can produce travel time variations consistent with observations we globally scaled the anomalies by a factor of 1.5. However this did not significantly improve the fit to the observations.
  33. Dynamic modeling incorporating a chemically distinct dense layer at the base of the mantle [for example (11 14)] demonstrates that this layer is depressed beneath cold downwellings (likely corresponding to fast seismic velocity regions) and elevated beneath hot upwellings (slow velocity regions).
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  36. For example L. Stixrude and M. S. T. Bukowinski [ J. Geophys. Res. 95 19311 (1990)] have suggested that (Mg Fe)SiO 3 may break down into oxides under D" conditions; A. M. Hofmeister (in preparation) argued that the velocity jump at the top of D" may be due to a MgO transition to NaCl or CsCl structure.
  37. 10.1016/0031-9201(81)90046-7
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  39. It has been demonstrated (11) that a jump of as little as 1% in velocity may explain the observations of the shear wave D" triplication provided it is accompanied by sufficiently large gradients. Here we use a slightly higher value on the premise that tomographic inversion smears the structure so that the gradients provided by the tomography models are somewhat lower than in the real structure.
  40. Adding the discontinuity and the low-velocity compensation at the base of the mantle only conserves travel times of phases that do not travel through D" or cross D" at steep angles. The imposed low-velocity zone would disturb the travel times of phases such as ScS especially at large (>80°) distances when the phase almost grazes the CMB. At shorter distances however only a small portion of the path is affected by the basal low-velocity zone and so the travel time perturbation is less significant.
  41. Garnero E. J., Helmberger D. V., Grand S., Phys. Earth Planet. Inter. 79, 335 (1993). (10.1016/0031-9201(93)90113-N) / Phys. Earth Planet. Inter. by Garnero E. J. (1993)
  42. The seismic 1D reference models for different regions were obtained in (7–9) to approximate the observed differential travel times for each particular region.
  43. The normalization is required for a meaningful comparison of the relative strength of Scd phase for paths with different epicentral distances since Scd / S increases with distance even for a constant seismic velocity structure (7–9). We obtain the distance trend by averaging Scd / S amplitude ratios for all ray paths with a given epicentral distance.
  44. We thank S. Grand for access to his tomography model and T. Lay and E. Garnero for providing the D" triplication travel time data. Two anonymous reviewers provided a number of valuable comments and suggestions. Supported by NSF grant EAR-9809771. This is contribution no. 8664 Division of Geological and Planetary Sciences California Institute of Technology.
Dates
Type When
Created 23 years, 1 month ago (July 27, 2002, 5:37 a.m.)
Deposited 1 year, 7 months ago (Jan. 13, 2024, 5:29 a.m.)
Indexed 2 months, 2 weeks ago (June 13, 2025, 3:26 a.m.)
Issued 25 years, 9 months ago (Nov. 12, 1999)
Published 25 years, 9 months ago (Nov. 12, 1999)
Published Print 25 years, 9 months ago (Nov. 12, 1999)
Funders 0

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@article{Sidorin_1999, title={Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle}, volume={286}, ISSN={1095-9203}, url={http://dx.doi.org/10.1126/science.286.5443.1326}, DOI={10.1126/science.286.5443.1326}, number={5443}, journal={Science}, publisher={American Association for the Advancement of Science (AAAS)}, author={Sidorin, Igor and Gurnis, Michael and Helmberger, Don V.}, year={1999}, month=nov, pages={1326–1331} }