Some problems with S, SKS and ScS observations and implications for the structure of the base of the mantle and the outer core

Main Article Content

V. F. Cormier


Complicated radially symmetric models of the seismic velocity structure at the base of the mantle (Bullen's D" region) and the uppermost outer core have been inferred from analyses of the waveforms and relative amplitudes of S, SKS and ScS phases. Using radially symmetric structure, it has been difficult to construct physically realizable models of the rheology of D" that simultaneously satisfy P and S amplitudes and slownesses in the core shadow. These data are reviewed in the light of an increasing body of evidence that the structure of D" is characterized by heterogeneities having a broad spectrum of scale lengths. Depending on the region and range interval of D" sampled, S waveforms can be found that support either a radially simple or complex model of D". The complex models have one or more first-order discontinuities in velocity. The particle motion measured by three-component recordings of some S + ScS waveforms is consistent with a discontinuous increase in S velocity 250--300 km above the core-mantle boundary. The observed particle motion in these examples cannot readily or alternatively be explained by either general anisotropy or by strong lateral velocity gradients in D". Sufficient variability in S waveforms and travel times exists, however, that any radially symmetric model having a strong degree of complexity should be accepted with caution until all of the competing effects of lateral heterogeneity and possible anisotropy in D" are fully investigated. The distribution and scale lengths of heterogeneities in D" may account for regional differences in the properties of D" inferred from waveform data, including features that mimic intrinsic attenuation and anisotropy.

Google Scholar        ARK:



Article Details

How to Cite
Cormier, V. F. (1985). Some problems with S, SKS and ScS observations and implications for the structure of the base of the mantle and the outer core. Journal of Geophysics, 57(1), 14-22. Retrieved from


Aki, K. (1980) Scattering and attenuation of shear waves in the lithosphere. J. Geophys. Res. 85, 6496-6504

Anderson, D.L., Hanks, T. (1972) Formation of the Earth's core. Nature 237, 387-388

Backus, G.E. (1962) Long wave elastic anisotropy produced by horizontal layers, J. Geophys. Res. 67, 4427-4440

Bolt, B.A., Niazi, M. (1984) S velocities in D" from diffracted SH-waves at the core boundary. Geophys. J. R. Astron. Soc. 79

Bullen, K.E. (1950) An Earth model based on a compressibility-pressure hypothesis. Mon. Not. R. Astron. Soc. Geophy. Suppl. 6, 50-59

Burdick, L.J. (1978) t* for S waves with a continental ray path. Bull. Seismol. Soc. Am. 68, 1013-1030

Choy, G.L. (1977) Theoretical seismograms of core phases calculated by a frequency-dependent full wave theory, and their interpretation. Geophys. J. R. Astron. Soc. 51, 275-312

Cleary, J .R. (1974) The D" region. Phys. Earth Planet. Int. 9, 13-27

Comer, R.P., Clayton, R.W. (1984) Tomographic reconstruction oflateral velocity heterogeneity in the earth's mantle. EOS Trans. Am. Geophys. Un. 65, 236

Cormier, V.F. (1984) The polarization of S waves in a heterogeneous isotropic whole Earth model. J. Geophys. 56, 20-23

Cormier, V.F., Richards, P.G. (1977) Full wave theory applied to a discontinuous velocity increase: the inner core boundary. J. Geophys. 43, 3-31

Crampin, S. (1981) A review of wave motion in anisotropic and cracked elastic-media. Wave Motion 3, 343-391

Cram pin, S. (1984) Seismic anisotropy- the state of the art: II. Geophys. J. R. Astron. Soc. 76, 1-16

Der, Z.A., McElfresh, T.W., O'Donnell, A. (1982) An investigation of the regional variations and frequency dependence of anelastic attenuation in the United States in the 0.5-4 Hz. band. Geophys. J. R. Astron. Soc. 69, 67-100

Doornbos, D.J. (1978) On seismic-wave scattering by a rough core-mantle boundary. Geophys. J. R. Astron. Soc. 53, 643-662

Doornbos, D.J., Mondt, J.D. (1979) P and S waves diffracted around the core and the velocity structure at the base of the mantle. Geophys. J. R. Astron. Soc. 57, 381-395

Dziewonski, A.M. (1984) Mapping the lower mantle: determination of lateral heterogeneity up to degree 6. J. Geophys. Res. 89, 5929--5952

Dziewonski, A.M., Anderson, D.L. (1981) Preliminary reference Earth model. Phys. Earth Planet. Int. 25, 297-356

Dziewonski, A.M., Hales, A.L., Lapwood, E.R. (1975) Parametrically simple Earth models consistent with geophysical data. Phys. Earth Planet. Int. 10, 12--48

Frankel, A., Clayton, R.W. (1984) A finite difference simulation of wave propagation in two-dimensional media. Bull. Seismol. Soc. Am. 74,2167-2186

Fukao, Y. (1984) Evidence from core-reflected shear waves for anisotropy in Earth's mantle. Nature 309, 695-698

Haddon, R.A.W. (1982) Evidence for inhomogeneitites near the coremantle boundary. Phil. Trans. Roy. Soc. Lond. A-306, 61-70

Haddon, R.A.W., Cleary, J.R. (1974) Evidence for scattering of seismic PKP waves near the mantle-core boundary. Phys. Earth Planet. Int. 8, 211-234

Hales, A.L., Roberts, J. (1970) The travel times of S and SKS. Bull. Seismol. Soc. Am. 60, 461-489

Harvey, D., Choy, G.L. (1982) Broadband deconvolution of GDSN data. Geophys. J. R. Astron. Soc. 69, 659-668

Husebye, E.S. King, D.W., Haddon, R.A.W. (1976) Precursors to PKIKP and seismic wave scatterring near the mantle-core boundary. J. Geophys. Res. 81, 1870-1882

Jeanloz, R., Richter, F.M. (1979) Convection, composition, and the thermal state of the lower mantle. J. Geophys. Res. 84, 5497-5504

Jones, G.M. (1977) Thermal interaction of the core and the mantle and long-term behavior of the geomagnetic field. J. Geophys. Res. 82, 1703-1709

Kind, R., Miiller, G. (1977) The structure of the outer core from SKS amplitudes and travel times. Bull. Seismol. Soc. Am. 67, 1541-1554

Lay, T., Heimberger, D.V. (1983a) A lower mantle S-wave triplication and the shear velocity structure of D". Geophys. J. R. Astron. Soc. 75, 799-838

Lay, T., Heimberger, D.V. (1983b) The shear-wave velocity gradient at the base of the mantle. J. Geophys. Res. 88,8160-8170

Mitchell, B.J., Heimberger, D.V. (1973) Shear velocities at the base of the mantle from observations of S and ScS. J. Geophys. Res. 78, 6009-6020

Mula, A.H. (1981) Amplitudes of diffracted long-period P and S waves and the velocities and Q structure at the base of the mantle. J. Geophys. Res. 86, 4999--5011

Mula, A.H., Miiller, G. (1980) Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle. Pure Appl. Geophys. 118, 1270-1290

Murtha, P.E. (1985) Seismic velocities in the upper part of the Earth's core. Ph.D. thesis, University of California, Berkeley

Richards, P.G., Menke, W. (1983) The apparent attenuation of a scattering medium. Bull. Seismol. Soc. Am. 61, 1675-1692

Ruff, L.J., Anderson, D.L. (1980) Core formation, evolution and convection: a geophysical model. Phys. Earth Planet. Int. 21, 181-201

Ruff, L.J., Heimberger, D.V. (1982) The structure of the lowermost mantle determined by short-period P-Wave amplitudes. Geophys. J. R. Astron. Soc. 68, 95-120

Schlittenhardt, J., Schweitzer, J., Miiller, G. (1985 ) Evidence against a discontinuity at the top of D". Geophys. J. R. Astron. Soc. (in press)

Schweitzer, J. (1984) Laufzeiten und Amplituden der Phasen SKS und SKKS und die Struktur des auseren Erdkerns, Diplomarbeit, Institut fur Meteorologie und Geophysik der Johann Wolfgang Goethe Universitat, Frankfurt a.M.

Stacey, F.D., Loper, D.E. (1983) The thermal boundary layer interpretation of D" and its role as a plume source. Phys. Earth Planet. Int. 33, 44-55

Strelitz, R. (1975) The September 5, 1970 Sea of Okhotsk earthquake: a multiple event with evidence of triggering. Geophys. Res. Lett. 2, 124-127

Wright, C., Lyons, J.A. (1984) Further evidence for radial velocity anomalies in the lower mantle. Pageoph. 119, 137-162

Wright, C., Muirhead, K.J., Dixon, A.E. (1985) The P wave velocity structure near the base of the mantle. J. Geophys. Res. 90, 623-634