Geoscience Reference
In-Depth Information
Chapter 20
Fabric of the mantle
Anisotropy
this edition. There was a very large section on
anisotropy since it was a relatively new concept
to seismologists. There was also a large section
devoted to the then-novel thesis that seismic
velocities were not independent of frequency,
and that anelasticity had to be allowed for in esti-
mates of mantle temperatures. Earth scientists
no longer need to be convinced that anisotropy
and anelasticity are essential elements in Earth
physics, but there may still be artifacts in tomo-
graphic models or in estimates of errors that
are caused by anisotropy. At the time of the
first edition of this topic -- 1989 -- the Earth
was usually assumed to be perfectly elastic and
isotropic to the propagation of seismic waves.
These assumptions were made for mathemati-
cal and operational convenience. The fact that
a large body of seismic data can be satisfacto-
rily modeled with these assumptions does not
prove that the Earth is isotropic or perfectly
elastic. There is often a direct trade-off between
anisotropy and heterogeneity, and between fre-
quency dependence and depth dependence of
seismic velocities. An anisotropic structure can
have characteristics, such as travel times, normal-
mode frequencies and dispersion curves, that are
identical, or similar, to a different isotropic struc-
ture. A layered solid, for example, composed of
isotropic layers that are thin compared to a seis-
mic wavelength will behave as an anisotropic
solid -- the velocity of propagation depends on
direction. The effective long-wavelength elastic
constants depend on the thicknesses and elastic
properties of the individual layers. The reverse is
also true; an anisotropic solid with these same
elastic constants can be modeled exactly as a
And perpendicular now and now
transverse, Pierce the dark soil and as
they pierce and pass Make bare the
secrets of the Earth's deep heart.
Shelley, Prometheus Unbound
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 pro-
cesses, and gravity, create a fabric in the mantle.
Anisotropy can be microscopic -- orientation of
crystals -- or macroscopic -- large-scale lamina-
tions 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 cor-
rect. Slab anisotropy can cause artifacts in tomo-
graphic 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
