UC San Diego

Seismic anisotropy, the dependence of velocity on direction, is often induced by mantle flow. Here, I studied the influence of a proposed mantle plume beneath Hawaii on the azimuth dependence of Rayleigh wave phase velocity. I used a two-layer forward modeling code to explore how the orientation of a transversely isotropic Pyrolite mantle model controls the fast direction and strength of azimuthal anisotropy. Two layers are assumed because plate motion of the Pacific plate rearranged about 45 Million years ago. It is thought that the fossil spreading direction was ‘frozen’ into parts of the lithosphere while the asthenosphere below carries the signature of current mantle flow. Depending on how different the horizontal orientation of Pyrolite is in both layers, the strength of anisotropy can vanish for some frequencies but not others. This can ultimately be used to estimate the thickness of the anisotropic layers and the orientation of Pyrolite.

In the second part, I forward-modeled data collected for the Hawaiian PLUME project. At high frequencies, the overall pattern of azimuthal anisotropy follows the fossil spreading direction while this coherency breaks down at low frequencies. I find that anisotropy in the upper lithosphere is largely intact, but the pattern is incoherent in the lower lithosphere and asthenosphere. These results provide strong evidence for the presence of a mantle plume beneath Hawaii.