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in order to define anomalous regions. In fact, all
shields are not the same, and velocity does not
increase monotonically with age within oceanic
regions. The region around Hawaii, for example,
is faster than equivalent-age ocean elsewhere at
shallow depth and slower at somewhat greater
depth. From ScS data we know that the average
seismic velocity and attenuation in the mantle
under Hawaii are normal. On average, hotspots
are not associated with anomalous upper mantle,
although visual correlations are often claimed
with the deep mantle.
There are now many models of the veloc-
are also slow. The most pronounced differences
are in the upper 200 km, but substantial differ-
ences between regions extend to about 400 km.
The mantle above 200--300 km depth correla-
tes very well with known tectonic features. There
are large differences between continents and
oceans, and between cratons, tectonic regions,
back-arc basins and different age ocean basins.
High velocities appear beneath all Archean cra-
tons. Platforms are variable. At depths between
800 and 1000 km there is good correlation
of seismic velocities with inferred regions of
past subduction.
Hypothetical hot, narrow
mantle upwellings - the elusive mantle
plumes
-- do not, in general, show up consis-
tently in mantle tomography.
Over most of the Earth, long-period Rayleigh-
and Love-wave dispersion curves are 'inconsistent'
in the sense that they cannot be fit simultane-
ously using a simple isotropic model. This has
been called the
Love-wave--Rayleigh-wave
discrepancy
and attributed to anisotropy.
There are now many studies of this effect --
also called polarization or radial anisotropy, or
transverse isotropy. Independent evidence for
anisotropy in the upper mantle is strong (e.g.
from
receiver functions amplitudes and
shear wave splitting
).
Love waves are sensitive to the SH (hori-
zontally polarized shear waves) velocity of the
shallow mantle, above about 300 km for most
studies. The slowest regions are at plate bound-
aries, particularly triple junctions. Slow velocities
extend around the Pacific plate and include the
East Pacific Rise, western North America, Alaska-
Aleutian arcs, Southeast Asia and the Pacific-
Antarctic Rise. Parts of the Mid-Atlantic Rise and
the Indian Ocean Rise are also slow. The Red Sea-
Gulf of Aden--East Africa Rift (Afar triple junction)
is one of the slowest regions. The upper-mantle
velocity anomaly in this slowly spreading region
is as pronounced as under the rapidly spread-
ing East Pacific Rise. Since it also shows up for
S-delays and long-period Rayleigh waves, this is a
substantial and deep-seated anomaly.
Rayleigh waves are sensitive to shallow Pri-
mary or compressional (P) velocities and SV (ver-
tically polarized shear wave, or polarized in
the plane of the ray) velocities from about 100
ity,
attenuation
and
anisotropy
--
radial
and
azimuthal -- of the upper mantle.
[
tomography upper mantle anisotropy
]
[
mantle tomographic maps
][
global
seismic struture maps
]
[
bullard.esc.cam.ac.uk/
maggi/
Physics_Earth_Planet/Lecture_7/
colour_figures.html
]
∼
Global surface wave tomography
The most complete global maps of seismic het-
erogeneity of the upper mantle are obtained
from surface waves [
global tomographic
images
]. By analyzing the velocities of Love and
Rayleigh waves, of different periods, over many
great circles, small arcs and long arcs, it is pos-
sible to reconstruct the radial and lateral veloc-
ity and anisotropy variations. Although global
coverage is possible, the limitations imposed by
the locations of seismic stations and of earth-
quakes limit the global spatial resolution to
about 1000 km. The raw data consist of ampli-
tude variations, phase delays, travel times, or
average group and/or phase velocity over many
arcs. These averages can be converted to images
using techniques similar to medical tomography.
Body waves have better resolution, but coverage,
particularly for the upper mantle, is poor. The
best global maps of mantle structure combine
surface waves, higher modes, body waves and nor-
mal modes. Even the early surface-wave studies
indicated that the upper mantle was extremely
inhomogenous and anisotropic. Shield paths are
fast, oceanic paths are slow, and tectonic regions
