Evidence for Lateral Heterogeneity at the Core-Mantle Boundary from the Slowness of Diffracted S Profiles

  1. D. E. Smylie and
  2. Raymond Hide
  1. Michael E. Wysession and
  2. Emile A. Okal

Published Online: 29 MAR 2013

DOI: 10.1029/GM046p0055

Structure and Dynamics of Earth's Deep Interior

Structure and Dynamics of Earth's Deep Interior

How to Cite

Wysession, M. E. and Okal, E. A. (1988) Evidence for Lateral Heterogeneity at the Core-Mantle Boundary from the Slowness of Diffracted S Profiles, in Structure and Dynamics of Earth's Deep Interior (eds D. E. Smylie and R. Hide), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM046p0055

Author Information

  1. Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208

Publication History

  1. Published Online: 29 MAR 2013
  2. Published Print: 1 JAN 1988

ISBN Information

Print ISBN: 9780875904504

Online ISBN: 9781118666562

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Keywords:

  • Earth—Core—Congresses;
  • Geodynamics—Congresses

Summary

A regional study of SH velocities in D″ has been conducted through the comparison of the apparent ray parameters of diffracted SH waves [Sd] from large earthquakes and synthetic pulses generated by normal mode summation for three different velocity structures. This technique has the advantage of automatically including all non-geometric optics effects in the calculation of the synthetics and of avoiding the complications due to varying waveform frequency content that arise when trying to directly convert apparent slownesses into mantle velocities.

We computed the excitation functions of torsional normal modes up to 0.05 Hz for the smooth model PREM, a model PREM(HVZ) incorporating Lay and Helmberger's [1983a] proposed high-velocity zone, and a PREM(LVZ) low-velocity structure. The apparent slownesses for the path profiles were obtained by cross-correlations of both the synthetic Sd arrivals, corrected for ellipticity using the modal approximations of Dahlen [1975], and the data, which were filtered and corrected for upper mantle heterogeneity (using the tomographic SH velocity models of Woodhouse and Dziewonski [1983], and Tanimoto [1987] so as to be compatible with the synthetics). Comparisons of the cross-correlated slownesses revealed one region on the CMB with D″ velocities slower than those of PREM, one region compatible with PREM, two zones distinctly faster than PREM and one zone that strongly represents a region of high velocity. This high-velocity zone was also identified as such by the Lay and Helmberger [1983a] study.