Kilometer-scale imaging on the core-mantle boundary at the source of the Hawaiian mantle plume

Abstract:

The Hawaiian mantle plume rises from the core-mantle boundary, where it is rooted in the largest known 'Ultra-Low Velocity Zone' (ULVZ, Cottaar et al. 2012). This structure is formed of an unknown material exhibiting extremely slow seismic velocities. We focus on the core-diffracted phases, which are very sensitive to core-mantle boundary structure. We collect several events which generate core diffracted rays that sample the Hawaiian ULVZ from two different directions. Our result shows that the Hawaiian ULVZ is located to the southwest of the surface volcanism and is highly axisymmetric in nature.

We also retrieve a rare and weak high-frequency postcursor signal buried in noise. This enables us for the first time, to bring the vertical resolution of the ULVZ to the kilometer scale and to map its internal structure. The higher frequency postcursor is delayed by 60 to 80 seconds relative to the main Sdiff phase, which indicates the velocity reductions just above the core-mantle boundary are slower than previous estimates.

Using recent computational advances in AxiSEM3D full waveform modeling (Leng et al. 2020), we model the synthetic global wavefield down to 3s periods and compare synthetic seismograms to data observations. The observations can be reproduced with a ULVZ that exhibits a drastic decrease in shear wave velocity with depth from 10% 20 km above the core-mantle boundary to 30% at the core-mantle boundary. This new observation supports a chemically layered ULVZ with increasing iron content closer to the core-mantle boundary.