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Shear wave velocities in the lower mantle

DEPARTMENT OF GEOLOGY AND GEOPHYSICS MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS

Abstract

The Large Aperture Seismic Array in eastern Montana was used to measure the travel times and dT/d of earthquake-generated shear waves for the purpose of determining lower-mantle shear velocities. The data were limited to epicentral distances between 27 and 95 degrees. To convert uncertainties in the dT/d measurements to uncertainties in the computed velocities, the data were inverted by a Monte Carlo procedure. The computer randomly generated velocity models, and those which satisfied prescribed travel time and dT/d-versus- limits were considered as possible models for the real earth. The dT/d curve possessed anomalously steep slopes between 27-30 and 65-75 degrees, and the corresponding velocity models had increased gradients between the approximate depths of 700-800 and 1550-2100 kilometers. The slope of the dT/d curve also decreased at about 88 degrees which may indicate a decrease in velocity gradient several hundred kilometers above the core-mantle boundary. For epicentral distances greater than 27 degrees the observed travel times to LASA were greater than the expected J-B times. Although vague, a systematic increase in the observed residuals appeared to exist between 40 and 60 degrees which agreed with the J-B residuals of the computed models.

Footnotes

^* Now at Chevron Oil Field Research Company, La Habra, California.

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