High-frequency Shear Wave Diffraction at Hawaii ULVZ

The main arrival and post-cursor of shear waves diffracted (Sdiff) along the core-mantle boundary have been used to constrain the seismic nature of ultra-low velocity zone(ULVZ) beneath Hawaiian hotspot (Cottaar and Romanowicz 2012). Previous waveform studies exploited Sdiff post-cursors with period above 10s. For the first time, we directly observe the Sdiff post-cursors at a much shorter period around 3s. The result of slowness and backazimuth from beamforming further confirms the observation is caused by ULVZ. The observed post-cursors show a more extreme travertine delay but a similar incoming backazimuth comparing to those of short-period postcursors . This observation has given us an opportunity to probe the the geometric shape and seismic contrast in a finer length scale. The properties at the base of Hawaii ULVZ offer new clues on the origin and formation of ULVZ, e.g. presence of increased Fe composition or increased partial melting at bottom of ULVZs.