Detection of an ultralow velocity zone at the core-mantle boundary using diffracted PKKPab waves

摘要

Seismic phases diffracted around Earth's core contain information about lowermost mantle wave speeds. By measuring the slowness of incident diffracted energy from array recordings, seismic velocity along the diffracted path can be estimated. Here we apply this principle to diffraction of the major arc seismic phase PKKP ab recorded at the Canadian Yellowknife array to estimate P wave velocity variations along the core-mantle boundary. We observe PKKP ab diff about 7.5° past the ray theoretical cutoff distance for PKKP ab . We utilize 330 western Pacific rim earthquakes that allow us to probe the core-mantle boundary beneath the North Atlantic and the South Pacific oceans using PKKP ab diff . Slowness and back azimuth are measured by frequency–wave number analysis. Mapping PKKP ab diff slowness variations suggest 4–19% P wave velocity reductions relative to PREM, in good agreement with the magnitude of velocity reductions previously mapped in ultralow velocity zones. The PKKP ab diff slowness and back-azimuth variations combined with results from previous ULVZ studies using SP diff KS imply that the lowered velocities occur at the base of the mantle beneath the North Atlantic Ocean, along the receiver side of raypaths. PKKP ab diff array measurements thus hold important potential for mapping ultralow velocity zone structure in so far unprobed regions of the lower mantle as well as for providing additional and independent information about lower mantle structure.