High resolution CMB imaging from migration of short-period core reflected phases

The fine scale structure of the core mantle boundary (CMB) region beneath southern Alaska is studied using core reflected P-waves (PcP) recorded at the Canadian Yellowknife array. Seismic studies of the lowermost mantle have detected a multitude of structures at this contact zone between iron alloy liquid of the core and the silicates of the mantle. These structures offer key constraints for the dynamics of mantle and core. High quality waveform information and migrated array data have been used to infer the existence of thin layering at the CMB in the form of ultra-low velocity zones. The data clearly show the absence of an ultra-low velocity zone thicker than about 7 km in this region while there is evidence for a weak P-wave reflector about 110 km above the CMB beneath the Kenai peninsula. These observations are in good agreement with the hypothesis that ultra-low velocity zones are related to areas with lower than average seismic velocities in tomographic images and absent in regions of fast seismic velocities likely related to subduction processes such as beneath Alaska.