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Published June 1981 | public
Journal Article

Preliminary reference Earth model


A large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earth's interior. The data set was supplemented with a special study of 12 years of ISC phase data which yielded an additional 1.75 × 10^6 travel time observations for P and S waves. In order to obtain satisfactory agreement with the entire data set we were required to take into account anelastic dispersion. The introduction of transverse isotropy into the outer 220 km of the mantle was required in order to satisfy the shorter period fundamental toroidal and spheroidal modes. This anisotropy also improved the fit of the larger data set. The horizontal and vertical velocities in the upper mantle differ by 2–4%, both for P and S waves. The mantle below 220 km is not required to be anisotropic. Mantle Rayleigh waves are surprisingly sensitive to compressional velocity in the upper mantle. High S_n velocities, low P_n velocities and a pronounced low-velocity zone are features of most global inversion models that are suppressed when anisotropy is allowed for in the inversion. The Preliminary Reference Earth Model, PREM, and auxiliary tables showing fits to the data are presented.

Additional Information

© 1981 Published by Elsevier B.V. Received 3 December 1980, Accepted 5 December 1980, Available online 25 October 2002. Anton Hales was an interested observer at all stages of this study and we gratefully acknowledge his advice. We also acknowledge helpful correspondence with Sir Harold Jeffreys. John Woodhouse participated in numerous discussions related to this work and assisted us in solving many problems. In particular, he derived equations for the travel times in a transversely isotropic medium and his note on this subject accompanies this report. Robert North made available to us his Love wave data prior to publication. This research was supported by National Science Foundation Grants No. EAR78-05353 (Harvard) and EAR77-14675 (California Institute of Technology). Contribution No. 3531 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

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