What is seismic anisotropy?
Most earth minerals are not perfectly symmetrical. When a seismic wave passes through a cluster of minerals, the crossing of particle motion across grain boundaries impedes the velocity of the wave. So when a seismic wave travels through a cluster of non-symmetric minerals, it can have fewer grain boundaries per some distance in one direction than in another. This is what we call seismic anisotropy, seismic energy traveling faster in one direction than in another direction.
We can then use this information to help determine the orientation of minerals deep in Earth. In some of the figures you may see on this or other pages, we represent the long-axis of the minerals (the fast-axis of the seismic energy) as a parallel vector, or line segment. So the line segment shows the average direction of the minerals.
This information can be useful in determining modern and/or ancient tectonic deformation and helps us build a picture of tectonics through time.