Investigating the base of the mantle using differential travel times

• Published in: Physics of the earth and planetary interiors 1995-11, Vol.92 (1), p.67-84
• Main Authors: Wysession, Michael E., Valenzuela, Raul W., Zhu, An-Ning, Bartkó, Lilla
• Format: Article
• Language: English
• ISSN: 0031-9201; 1872-7395
• DOI: 10.1016/0031-9201(95)03062-2

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 compariso