Geoscience Reference
In-Depth Information
Fig. 20.6
Seismic flow map at 280 km depth. This combines
information about shear velocity and polarization anisotropy.
Open symbols are slow, solid symbols are fast. Vertical
diamonds are SV
>
SH. Horizontal diamonds are SH
>
SV.
Slow velocities are at least partially due to high temperatures
and, possibly, partial melting.
the plate motion as a whole. It is in good agree-
ment with models of plate motions. This anoma-
lous band may represent a future plate plate
boundary between the North and South Pacific.
If the mantle is anisotropic, the use of
Rayleigh waves or P-waves alone is of limited use-
fulness in the determination of mantle structure
because of the trade-off between anisotropy and
structure. Love waves and shear waves provide
additional constraints. If it is assumed that avail-
able surface-wave data are an azimuthal average,
we can treat the upper mantle as a transversely
isotropic solid with five elastic constants. The
azimuthal variation of long-period surface waves
is small.
the Samoa--Tonga elbow to the Juan Fernandez
microplate on the East Pacific plate (Montagner,
2002). This band is also reflected in seismic veloc-
ities and anisotropy. This band continues across
the Nazca plate, the Challenger fracture zone and
the Juan Fernandez volcanic chain to the south
endofthevolcanicgapinChile.Tofirstorder,the
azimuthal anisotropy below the Pacific reflects
