Degrees 2, 4, 6 inferred from seismic tomography

Abstract

Lateral heterogneities of geophysical fields (gravity, magnetic, seismic velocities...) are usually expended into spherical harmonics. Free oscillation and geoid data show that heterogeneity in the deep Earth's mantle is dominated by degree 2. However, its geographical pattern and its location at depth is still questionable. Recent tomographic models of seismic velocity, anisotropy and anelasticity obtained from GEOSCOPE and GDSN data, make it possible to gain more insight on this problem. They show that at depth larger than 400km ± 100km, the degree 2 and t a less extent, the degree 6 arise as the most important features. These new models also display a large degree 4 for radial anisotropy at large depths. A simple flow pattern can explain the predominance of these different degrees 2, 4, 6 and it is shown that degrees 2, 4, 6 in the transition zone are not independent. They are compared to the corresponding degrees of the hotspot distribution and geoid. A possible relation between the degree 2 of velocity and the degree 4 of radial anisotropy is also displayed. However, the different location of degree 2 in the upper mantle and the lower mantle is still puzzling.