Investigating the base of the mantle using differential travel times

Several techniques using differential seismic travel times to map lateral structure in the lowermost mantle are discussed. Results are shown for recent studies involving the established techniques of core-reflected phases (ScS-S and PcP-P) and diffracted phase profiles (Sdiff), and new techniques involving the differential times of both core-transmitted and core-diffracted phases (PKP-Pdiff and Sdiff-SKS-SKKS) are described. The recent databases of digital seismograms have allowed for a study of D″ velocities in the Eastern Hemisphere using ScS-S and sScS-sS differential times from the many Western Pacific earthquakes. The result is an image at a resolution of a few hundred kilometers of a slow velocity anomaly of 2500 km width beneath Micronesia (-2% relative to the Preliminary Reference Earth Model (PREM)) that is surrounded on three sides by fast D″ rock that is 3% faster than PREM. A study using the differential arrivals of core-diffracted S waves (Sdiff) from digital records is providing information about long-wavelength variations in D″ shear velocities, though the rigorous earthquake-station geometry requirements limit the study to particular regions of the globe. Another study is using over 40 000 PcP-P differential travel times as reported to the International Seismological Centre to map global P velocities at the base of the mantle, and it shows that global coverage of the core-mantle boundary (CMB) is very poor. Though there are some regions (Northern Asia, Northern Pacific, Central America) with enough data sampling to allow a quantification of average D″ P velocities (with a total robust range of 4% lateral variation), they cover only a small portion of the total CMB. As a means of increasing our understanding of the long-wavelength variations of seismic velocities, a description is given of two techniques that will take advantage of totally different sets of earthquake-station geometries from the core-reflected phase studies. In the distance range from 120° to beyond 165° the differential times of PKP and Pdiff can be used to map long-wavelength D″ P velocities. These two phases are very different in shape and frequency content, so the differential times are found by a waveform cross correlation with reflectivity synthetic counterparts. In the distance range from 105° to beyond 135° the differential times of Sdiff, SKS, and SKKS can be examined simultaneously to test models of velocity structure above and below the CMB, also through comparison with synthetic counterparts.