Seismic detection of rigid zones at the top of the core

Data from earthquakes in the Tonga-Fiji region recorded at a seismic array in northern Australia show evidence for rigid zones at the top of the outer core. The ScP waveforms can be modeled by thin (0.12 to 0.18 kilometer) zo nes of molten iron mixed with solid material with a small, but positive, S- wave velocity (0.6 to 0.8 kilometer per second) that enables the propagatio n of S-waves in the outermost core. The zones may be topographic highs of t he core-mantle boundary filled by light core sediments and might be importa nt for variation of Earth's nutation and for convection of the outer core.