Investigation of the velocity- and Q-structure of the lowermost mantle using PcP/P amplitude ratios from arrays at distances of 70°-84°

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J. Schlittenhardt


Investigations of the core reflection PcP at distances in the neighbourhood of the core shadow zone are especially appropriate for the study of isolated regions of D" because PcP for these distances has a large angle of incidence at the core-mantle boundary and is strongly influenced by the structure immediately above the core. A striking result of this investigation of PcP in the distance range 70°–84° is that PcP could be detected only in four cases (from a total of 16 earthquakes and 29 nuclear explosions) in the NORSAR- and GRF-array beams for PcP. This result is found although the P-wave-coda noise levels in the PcP beams are very low, mostly below the amplitude level of PcP predicted by standard Earth models. In the cases where PcP could not be identified, the PcP/P amplitude ratio, which is estimated from the beams with the aid of a cross-correlation procedure, must be regarded as an upper limit for the actual PcP/P ratio. The interpretation of the PcP/P amplitude ratios with one radially symmetric, elastic model proved to be impossible. The generally small PcP/P ratios can be explained by lateral variations in seismic wave absorption within D"Qα values for short-period P waves in D", between 800 (below Usbekistan) and 100 (below the northern Hudson Bay and Central Siberia), were found. Several recent seismological investigations of the P- and S-wave velocity of the lowermost mantle have suggested velocity models with first-order discontinuities (with velocity increases of the order of 1.5%–3.0%) 150–300 km above the core-mantle boundary. Through the calculation of synthetic short-period seismograms and comparison with the array data compiled for the main part of this study, it is shown that such models are very unlikely to be a global feature if the P velocity jump is of the order of 2.5%–3.0%. Even models with reduced P velocity jumps of 1.3% produce clear onsets in short-period seismograms. They are not observed for P waves with ray paths bottoming beneath Central Siberia.

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Schlittenhardt, J. (1986). Investigation of the velocity- and Q-structure of the lowermost mantle using PcP/P amplitude ratios from arrays at distances of 70°-84°. Journal of Geophysics, 60(1), 1-18. Retrieved from


Aki, K., Richards, P.G. (1980) Quantitative seismology, theory and methods. Vol. I and II, Freeman and Co.

Alexander, S.S., Phinney, R.A. (1966) A study of the core-mantle boundary using P waves diffracted by the Earth's core. J. Geophys. Res. 71:5943-5958

Berzon, I.S., Kogan, S.P., Pasechnik, J.P. (1972) The character of the mantle-core boundary from observations of PcP waves. Earth Planet. Sci. Lett. 16:166-170

Bolt, B.A. (1970) PdP and PKiKP waves and diffracted PcP waves. Geophys. J. R. Astron. Soc. 20:367-382

Buchbinder, G.G.R. (1968) Properties of the core-mantle boundary and observations of PcP. J. Geophys. Res. 73:5901-5923

Buchbinder, G.G.R., Poupinet, G. (1973) Problems related to PcP and the core-mantle boundary illustrated by the two nuclear events. Bull. Seismol. Soc. Am. 63:2047-2070

Bullen, K.E. (1949) Compressibility-pressure hypothesis and the Earth's interior. Month. Not. R. Astron. Soc. Geophys. Suppl. 5:355-368

Bullen, K.E. (1950) An Earth model based on a compressibility-pressure hypothesis. Month. Not. R. Astr. Soc. Geophys. Suppl. 6:50-59

Chowdhury, D.K., Frasier, C.W. (1973) Observations of PcP and P at Lasa at distances from 26° to 40°. J. Geophys. Res. 78:6021-6027

Cleary, J.R. (1969) The S velocity at the core-mantle boundary from observations of diffracted S. Bull. Seismol. Soc. Am. 59:1399-1405

Cormier, V.F. (1985) Some problems with S, SKS and ScS observations and implications for the structure at the base of the mantle and the outer core. J. Geophys. 57:14-22

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. (1983) Present seismic evidence for a boundary layer at the base of the mantle. J. Geophys. Res. 88:3498-3505

Doornbos, D.J., Husebye, E.S. (1972) Array analysis of PKP phases and their precursors. Phys. Earth Planet. Inter. 5:387-399

Doornbos, D.J., Mondt, J.C. (1979a) Attenuation of P and S waves diffracted around the core. Geophys. J. R. Astron. Soc. 57:353-379

Doornbos, D.J., Mondt, J.C. (1979b) 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 in P velocity up to degree and order 6. J. Geophys. Res. 89:5929-5952

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

Elsasser, W.M., Olson, P., Marsh, B.D. (1979) The depth of mantle convection. J. Geophys. Res. 84:147-155

Engdahl, E.R., Johnson, L.E. (1974) Differential PcP travel times and the radius of the core. Geophys. J. R. Astron. Soc. 39:435-456

Frasier, C.W., Chowdhury, D. (1974) Effect of scattering on PcP/P amplitude ratios at Lasa from 40° to 84° distance. J. Geophys. Res. 79:5469-5477

Gutenberg, B., Richter, C.F. (1936) On seismic waves. Gerlands Beitr. Geophys. 43:56-133

Haddon, R.A.W. (1982) Evidence for inhomogeneities near the coremantle boundary. Phil. Trans. Roy. Soc. Lond. A306:61-70

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

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

Harjes, H.-P., Henger, M. (1973) Array seimology. J. Geophys. 39:865-905

Ibrahim, A.K. (1971) The amplitude ratio PcP /P and the core-mantle boundary. Pageoph. 91:114-133

Ibrahim, A.K. (1973) Evidence for a low velocity core-mantle transition. Phys. Earth Planet. Inter. 7:187-198

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 behaviour of the geomagnetic field. J. Geophys. Res. 82:1703-1709

Kanamori, H. (1967) Spectrum of P and PcP in relation to the mantlecore boundary and attenuation in the mantle. J. Geophys. Res. 72:559-571

Kanasewich, E.R. (1975) Time sequence analysis in geophysics. University of Alberta Press

Kelly, E.J. (1967) Response of seismic arrays to wide-band signals. Linc. Lab. Techn. Note 30

King, D.W., Haddon, R.A.W., Cleary, J.R. (1974) Array analysis of precursors to PKIKP in the distance range 128° to 142°. Geophys. J. R. Astron. Soc. 37:157-173

Lay, T., Helmberger, D.V. (1983a) A lower mantle S-wave triplication and the shear velocity structure of D". Geophys. J. R. Astron. Soc. 75:799-837

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

Lay, T., Young, C.J. (1986) The effect of SKS scattering on models of the shear velocity structure of the D" region. J. Geophys. 59:11-15

Luke, H.D. (1979) Signalubertragung, Heidelberg: Springer-Verlag

Mitchell, B.J., Helmberger, D.V. (1973) Shear velocities at the base of the mantle from observations ofS and ScS. J. Geophys. Res. 78:6009-6027

Mondt, J.L. (1977) SH-waves: theory and observations for epicentral distances greater than 90 degrees. Phys. Earth Planet. Inter. 15:46-59

Muller, G., Mula, A.H., Gregersen, S. (1977) Amplitudes of long-period PcP and the core-mantle boundary. Phys. Earth Planet. Inter. 14:30-40

Muller, G. (1985) The reflectivity method: a tutorial. J. Geophys. 58:153-174

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., Muller, G. (1980) Ray parameters of diffracted long-period P and S waves and the velocities at the base of the mantle. Pageoph. 118:1270-1290

Okal, E.A., Geller, R.J. (1979) Shear wave velocity at the base of the mantle from profiles of diffracted SH-waves. Bull. Seismol. Soc. Am. 69:1039-1053

Peltier, W.R., Jarvis, G.T. (1982) Whole mantle convection and the thermal evolution of the Earth. Phys. Earth Planet Inter. 29:281-304

Phinney, R.A., Alexander, S.S. (1966) P wave diffraction theory and the structure of the core-mantle boundary. J. Geophys. Res. 71:5959-5975

Sacks, I.S. (1967) Diffracted P wave studies of the Earth's core, 2. lower mantle velocity, core size, lower mantle structure. J. Geophys. Res. 72:2589-2594

Schlittenhardt, J. (1984) Array-Untersuchungen von reflektierten und diffraktierten Kernphasen. PhD thesis. University of Frankfurt

Schlittenhardt, J., Schweitzer, J., Muller, G. (1985) Evidence against a discontinuity at the top of D". Geophys. J. R. Astron. Soc. 81:295-306

Seidl, D. (1980) The simulation problem for broad-band seismograms. J. Geophys. 48:84-93

Spies, T. (1985) Untersuchung der seismischen Geschwindigkeiten an der Basis des Mantels mit Amplituden und Laufzeiten der Kernreflexion PcP. Diploma Thesis. University of Frankfurt

Wright, C., Lyons, J.A. (1981) 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