The core–mantle boundary under the Gulf of Alaska: No ULVZ for shear waves

The Earth’s core–mantle boundary (CMB) marks the boundary between the hot, molten iron core and the silicate mantle and is a thermal, chemical, and flow boundary. Previous observations of very slow compressional wavespeeds suggest that thin ultra-low-velocity zones (ULVZs), possibly composed of a mixture of molten iron and silicates, exist at the base of the mantle. A molten or partially molten layer would cause a large shear wavespeed decrease; however this velocity drop has not been observed. Here, we use core reflected ScP phases to investigate the shear properties of ULVZs. These phases reveal at least two distinct regions: one region under Central America which is distinctly average (PREM and iasp91-like) and a second region under the entire Gulf of Alaska that produces large ScP reflections with amplitudes of up to 30 times larger than calculated with average Earth models. The large amplitudes suggest a combination of focusing by CMB topography, high shear wavespeeds at the bottom of the mantle, and low attenuation (high Q) along the ScP path; high shear wavespeeds and low attenuation are opposite from what would be expected from a shear wave ULVZ. A ULVZ in compressional wavespeeds (ultra-low Vp) has previously been observed in this region; however, in addition to the large amplitudes, the short-period ScP waveforms show no complexity that can be related to a ULVZ. Thus, either there is not a ULVZ under the Gulf of Alaska or, if it does exist, it is restricted to compressional wavespeed changes, precluding interpretation as partial melt but rather suggesting a chemical origin.