Bokelmann, G.H.R., 2002, Convection-driven motion of the
North American craton: Evidence from P-wave anisotropy,
Geophysical Journal International, 248, 2, 278-287
We study the effect of anisotropy on P-wave travel times and generalize
Backus' (1965) well-known expression of azimuthal velocity dependence
to the sphere.
The variation of P-wave travel times on the lower hemisphere due to anisotropy
is described by a set of 15 parameters.
For single-crystal olivine a set of 6 parameters describes about 98%
of the
variation and for practical purposes of mantle seismology this set of
6 parameters should be sufficient.
We find a coherent pattern of anisotropy throughout the North American
craton.
The anisotropy is characterized by fast directions dipping
toward the Southwest, roughly in the direction of absolute plate
motion, and shallow dip angles (<45 degrees).
This is consistent with a simple-shear deformation of the deep
lithospheric roots,
due to the relative motion between plates and deeper mantle.
If this is correct, it implies that the deeper mantle convects faster
than the plate velocity, and that the mantle helps to
drive the motion of the North American plate.
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