INVESTIGATING THE BASE OF THE MANTLE USING DIFFERENTIAL TRAVEL-TIMES

Several techniques using differential seismic travel times to map late ral structure in the lowermost mantle are discussed. Results are shown for recent studies involving the established techniques of core-refle cted phases (ScS-S and PcP-P) and diffracted phase profiles (Sdiff), a nd new techniques involving the differential times of both core-transm itted and core-diffracted phases (PKP-Pdiff and Sdiff-SKS-SKKS) are de scribed. The recent databases of digital seismograms have allowed for a study of D'' velocities in the Eastern Hemisphere using ScS-S and sS cS-sS differential times from the many Western Pacific earthquakes. Th e result is an image at a resolution of a few hundred kilometers of a slow velocity anomaly of 2500 km width beneath Micronesia (-2% relativ e 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 u sing the differential arrivals of core-diffracted S waves (Sdiff) from digital records is providing information about long-wavelength variat ions in D'' shear velocities, though the rigorous earthquake-station g eometry requirements limit the study to particular regions of the glob e. 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 covera ge of the core-mantle boundary (CMB) is very poor. Though there are so me regions (Northern Asia, Northern Pacific, Central America) with eno ugh data sampling to allow a quantification of average D'' P velocitie s (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 unders tanding of the long-wavelength variations of seismic velocities, a des cription is given of two techniques that will take advantage of totall y different sets of earthquake-station geometries from the core-reflec ted phase studies. In the distance range from 120 degrees to beyond 16 5 degrees the differential times of PKP and Pdiff can be used to map l ong-wavelength D'' P velocities. These two phases are very different i n 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 tim es of Sdiff, SKS, and SKKS can be examined simultaneously to test mode ls of velocity structure above and below the CMB, also through compari son with synthetic counterparts.