SPICE-Research Training Network - UiO > Anisotropy
An introduction to
Continuation of the Introduction
Locations of anisotropy:
Anisotropy can be entcountered everywhere under the earth's surface and even at greater depths. We'll take the example in Fig. 4 of the Indian Ocean to show that maps of anisotropy can be drawn in order to have a better understanding of the Earth's structure:
Fig. 4: The colors show the mean velocity variation at two different depths (50 and 100 km). Red areas are slow-velocity areas, blue ones are high-velocity areas. The dashes show in which direction the velocities are fastest. (after J.J. Lévêque, E. Debayle and V. Maupin, Anistropy in the Indian Ocean upper mantle from Rayleigh- and Love-waveform inversion, Fig. 3, GJI, 133, 529-540)
At depths greater than 400 km both seismological investigations and analysis based on mineralogical data show that there are several regions of the Earth where seismic anisotropy may exist, but it is clear that more investigations are necessary to confirm what presently still relies mainly on speculations.
The possible sources of anisotropy:
Two questions have to be answered: at what depth is anisotropy occuring and which mecanism makes a structure becoming anisotropic?
Effects of cracks: practically all rocks, even dense crystalline varieties, contain small cracks. Oriented systems of cracks may cause a velocity anisotropy, with the most prominent velocity reductions in the direction normal to the plane of oriented cracks. In the crust, cracks are believed to be the probable origin of anisotropy.
Effects of preferred mineral orientation: due to higher confining pressures, the cracks cannot explain anisotropy in the mantle. Elastic anisotropy in the mantle is commonly attributed to the preferred orientation of minerals, especially olivine crystals. These are particularly vulnerable to dislocation slips that have as consequence anisotropy. In the upper mantle, anisotropy varies from 3 to 10 per cent in general.
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