Geoscience Reference
In-Depth Information
Depth 2500 km
FAST:
SLOW:
ds = 0.0001
ds =
−
0.0001
region data (Baig and Dahlen, 2004; Shearer and
Earle, 2004). Small scale structures (
Fig. 11.13
Compressional velocity at 2500 km depth. Note
the large low-velocity zone under Africa. This was called a
superplume by later workers but it has high density and high
bulk modulus; it is a compositional feature.
8km)in
the upper 200 km appear to be an order of mag-
nitude greater than elsewhere. The strongly scat-
tering upper mantle is also strongly anisotropic,
a possible manifestation of some organized het-
erogeneity, and high attenuation, also consistent
with scattering, although other explanations for
these are available.
Controversial evidence for strong
seismic scattering in the lower man-
tle has been used to support the whole
mantle convection model but newer and
more powerful techniques
allow a different
conclusion. It appears that the upper mantle is
the stronger scatterer of seismic energy and the
lower mantle -- below 1000 km depth -- is rather
bland except in D
. The same features that scat-
ter energy may also be responsible for the pro-
nounced anisotropy and anelasticity of the upper
mantle.
∼
'superplumes,' which implies a thermal, active
upwelling with low-density and low-bulk mod-
ulus. The existence of large lateral changes
in chemistry and lithology makes suspect all
attempts to infer composition and temperature
from 1D mantle models such as PREM.
Seismic scattering
Because of resolution limitations global seismic
models are smooth and heterogeneities of order
1 to 10 km are not imaged. High-frequency tech-
niques, however, are available to map small-scale
heterogeneities throughout the mantle. Early
measurements of precursors to multi-reflected
core phases showed that the upper 300 km of
the mantle was complex, giving both coherent
and incoherent reflections (Whitcomb and Ander-
son, 1970). Recent results confirm the multiple
reflections in the mantle (Figure 9.3). Scattering
is minor in the lower mantle compared with the
upper mantle and the upper 200 km contain
more or stronger scatterers than the transition
Color images
Experienced geodynamicists are fond of saying
that one mantle convection simulation is worse
than none. This is true in all branches of
non-linear mechanics and thermal convection.
Complex
systems
are
sensitive
to
changes
in
