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An introduction to anisotropy.
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?

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Universitetet i Oslo - Institutt for geofag / SPICE 2004