The core-mantle boundary under the Gulf of Alaska: NoULVZ 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 fl ow boundary. Previous observations of very slow compressional wavespeeds su ggest that thin ultra-low-velocity zones (ULVZs), possibly composed of a mi xture of molten iron and silicates, exist at the base of the mantle. A molt en or partially molten layer would cause a large shear wavespeed decrease; however this velocity drop has not been observed. Here, we use core reflect ed ScP phases to investigate the shear properties of ULVZs. These phases re veal 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 o f up to 30 times larger than calculated with average Earth models. The larg e amplitudes suggest a combination of focusing by CMB topography, high shea r wavespeeds at the bottom of the mantle, and low attenuation (high Q) alon g 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 wav espeeds (ultra-low V-p) has previously been observed in this region; howeve r, 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 U LVZ under the Gulf of Alaska or? if it does exist, it is restricted to comp ressional wavespeed changes, precluding interpretation as partial melt but rather suggesting a chemical origin. (C) 2000 Published by Elsevier Science B.V. All rights reserved.