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Published April 10, 1982 | Published
Journal Article Open

Global Q estimates from antipodal Rayleigh waves


Global average estimates of the group velocity and attenuation of long-period (120–300 s) Rayleigh waves were made using seismograms from the epicenter's antipode (Δ≃180°). Focusing at the antipode produced amplified arrivals with favorable signal-to-noise ratios. The high-quality data yielded very stable attenuation values, with excellent agreement between the results from successive Rayleigh arrivals for a single event and between the results for two different events. Lateral heterogeneities in earth structure can cause systematic biasing of attenuation measurements based on antipodal records. The initial, uncorrected results therefore provide a lower bound estimate of global Q. An ellipsoidal perturbation in shape was used to simulate the effects of lateral velocity heterogeneities on Rayleigh wave propagation. Using the agreement of repeated attenuation measurements as a constraint, we estimated both the bias in those measurements and the splitting widths of the Rayleigh modes. At a period of 200 s, the estimated splitting width is 0.30% this agrees closely with calculations by Luh (1974) for an earth model with different continental and oceanic velocity profiles. The estimated bias varied from 30% to zero over the 120- to 260-s band. After correcting for bias, the antipodal Q values range from 108 at 120 s to 188 at 260 s. These Q are within the range of previous measurements but are lower than the mean values from typical great circle studies, implying that the globally averaged upper mantle is slightly more attenuative than has been generally recognized.

Additional Information

© 1982 American Geophysical Union. Received June 12, 1981; revised January 22, 1982; accepted January 29, 1982. Paper number 2B0176. We are indebted to Jose Rial, whose work with the antipodal body waves of the PTO record directly inspired this study. He introduced E.P.C. to the project and offered many valuable suggestions. We would also like to thank Tony Dahlen and Adam Dziewonski for pointing out the need to consider the bias in antipodal Q measurements. Ray Buland kindly provided mode tapes for model 1066A. Hiroo Kanamori and Jeff Given reviewed the manuscript and suggested improvements. We thank Adam Dziewonski and Joseph Steim for a timely preprint which lent support to our results. This research was supported by the National Science Foundation under contract EAR77-14675. E. P. C. was supported in part by a National Science Foundation graduate fellowship. Contribution 3644, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

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