In his pioneering 1961 paper on seismic anisotropy in a layered earth, Don L. Anderson (hereafter referred to as DLA) introduced a parameter often referred to in global seismology as η without providing any reasoning. This note hopes to clarify the significance of η in the context of the dependence of body wave velocities in a transversely isotropic system on the angle of incidence, and also its relation with the other well-known anisotropic parameters introduced by Leon Thomsen in 1986.

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy to derive the best rotation pole of the Pacific plate in the uppermost 200 km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3, and NNR). However, when dividing the Pacific plate into two subplates separated by what we refer to as the megagash, an east-west low-velocity and low-anisotropy band extending across the Pacific plate from Samoa-Tonga to the Easter–Juan Fernández Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the “classical” rotation pole of the Pacific plate. This result suggests a differential motion between the North and South Pacific and an ongoing reorganization of plates in the Pacific Ocean. The megagash might be a future plate boundary between the North and South Pacific plates, associated with the intense volcanism along this band.