Mapping the lowermost mantle using core‐reflected shear waves

Abstract

A map of laterally varying D″ velocities is obtained for the region from 50°S to 50°N in latitude and 70°E to 190°E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS‐S and 266 sScS‐sS phase pairs. The long‐period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D″ paths, and the remaining residuals are interpreted as the result of D″ heterogeneity. Ray‐tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies.The resulting map reveals significant long‐wavelength D″ structure including a 3% low‐velocity region beneath northeastern Indonesia, surrounded by three identified high‐velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3–5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low‐velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D″ rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high‐velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core‐mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron‐poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher‐pressure polymorph.