Anisotropy

Anisotropy is responsible for large variations in seismic velocities; changes in the orientation of mantle minerals, or in the direction of seismic waves, cause larger changes in velocity than can be accounted for by changes in temperature, composition or mineralogy. Plate-tectonic processes, and gravity, create a fabric in the mantle. Anisotropy can be microscopic – orientation of crystals – or macroscopic – large-scale laminations or oriented slabs and dikes. Discussions of velocity gradients, both radial and lateral, and chemistry and mineralogy of the mantle must allow for the presence of anisotropy. Anisotropy is not a second-order effect. Seismic data that are interpreted in terms of isotropic theory can lead to models that are not even approximately correct. Slab anisotropy can cause artifacts in tomographic models. A wealth of new information regarding mantle structure, history, mineralogy and flow is becoming available as the anisotropy of the mantle is becoming better understood.

Introduction

In the first edition of Theory of the Earth there were sections that are largely missing in this edition. There was a very large section on anisotropy since it was a relatively new concept to seismologists.