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Book Chapter

Weak elastic anisotropy in global seismology

By
Leon Thomsen
Leon Thomsen
Delta Geophysics, 12707 Melvern Court, Houston, Texas 77041, USA, and Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA
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Don L. Anderson
Don L. Anderson
Seismological Laboratory, California Institute of Technology, MS 252-21, Pasadena, California 91125, USA
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Published:
October 01, 2015

It has been known for over 50 years that seismic anisotropy must be included in a realistic analysis of most seismic data. The evidence for this consists of the observed dependency in many contexts (reviewed briefly here) of seismic velocity upon angle of propagation and upon angle of S-wave polarization. Despite this well-established understanding, many current investigations continue to employ less realistic isotropic assumptions. One result is the appearance of artifacts which can be interpreted in terms of details of Earth structure rather than of the restrictive assumptions in the analysis.

The reason for this neglect of anisotropy is presumably the greater algebraic complexity and the larger number of free parameters of anisotropic seismics. However, the seismic anisotropy in the Earth is usually weak, and the equations for weak anisotropy are only marginally more complex than for isotropy. Further, the additional parameters are commonly required to describe the data. Moreover, the parameters of weak anisotropy defined below (combinations of the anisotropic elastic moduli) are less subject to compounding of uncertainty and to spatial resolution issues than are the individual anisotropic moduli themselves. Hence inversions should seek to fit data with these parameters, rather than with those individual moduli. We briefly review the theory for weak anisotropy and present new equations for the weakly anisotropic velocities of surface waves. The analysis offers new insights on some well-known results found by previous investigations, for example the “Rayleigh wave–Love wave inconsistency”, including the facts that Raleigh wave velocities depend not only on the horizontal SV velocity but also on the anisotropy, and Love wave velocities depend not only on the horizontal SH velocity but also on the anisotropy.

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Contents

GSA Special Papers

The Interdisciplinary Earth: A Volume in Honor of Don L. Anderson

Gillian R. Foulger
Gillian R. Foulger
Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
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Michele Lustrino
Michele Lustrino
Dipartimento di Scienze della Terra, Universita` degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Roma, Italy
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Scott D. King
Scott D. King
Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA
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Geological Society of America
Volume
514
ISBN print:
9780813725147
Publication date:
October 01, 2015

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