Seismic Anisotropy and Age-Dependent Structure of the Upper Oceanic Crust

  1. John M. Sinton
  1. Gerard J. Fryer,
  2. Daniel J. Miller and
  3. Patricia A. Berge

Published Online: 19 MAR 2013

DOI: 10.1029/GM057p0001

Evolution of Mid Ocean Ridges

Evolution of Mid Ocean Ridges

How to Cite

Fryer, G. J., Miller, D. J. and Berge, P. A. (1989) Seismic Anisotropy and Age-Dependent Structure of the Upper Oceanic Crust, in Evolution of Mid Ocean Ridges (ed J. M. Sinton), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM057p0001

Author Information

  1. Hawaii Institute of Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii 96822

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875904580

Online ISBN: 9781118666449

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Keywords:

  • Sea—floor spreading—Congresses;
  • Mid-ocean ridges—Congresses

Summary

Seismic anisotropy in oceanic layer 2 resulting from a preferred alignment of fractures has been widely recognized, but all experiments to date have sought to measure only the weak azimuthal variation of elastic properties resulting from tectonically controlled systems of vertical fractures. From ocean drilling data, however, especially from DSDP Hole 504B, we know that layer 2 is composed of interleaved massive flows and breccia units, and that the massive units have a very strong concentration of horizontal fractures. Layer 2's pronounced horizontal fabric of low-velocity “layers” (fractures and/or breccia zones) permeating an otherwise high-velocity matrix, will cause P-waves to travel faster horizontally than vertically. This anisotropy has no azimuthal expression, and so cannot easily be recognized in seismic data, but it may lead to overestimation of the thickness of upper crustal layers by as much as 30% in young crust. Further, the anisotropy affects P and S waves differently, so where shear-wave data are available, Poisson's ratio may be substantially underestimated. The widespread observation of a low Poisson's ratio zone in the upper few kilometers of young crust is almost certainly an artifact of ignoring anisotropy. As the crust ages, fractures and voids are filled by chemical alteration and precipitation, the velocity contrast between rock and void-filling material is reduced, and the anisotropy decreases. The errors introduced by assuming isotropy thus show an inverse relationship to crustal age, so that thickness measurements from old crust are probably no more than 10% in error. This explains a long-standing enigma of marine seismology: the apparent thinning of upper crustal layers with age.