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Published May 10, 1982 | Published
Journal Article Open

Absorption Band Q Model for the Earth


Body wave, surface wave, and normal mode data are used to place constraints on the frequency dependence of Q in the mantle. With a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range. The quality factor Q_s (ω) is proportional to ω^α in the band and to ω and ω^(−1) at higher and lower frequencies, respectively, as appropriate for a relaxation mechanism with a spectrum of relaxation times. The parameters of the band are Q(min) = 80, α = 0.15, and width, 5 decades. The center of the band varies from 10^1 seconds in the upper mantle, to 1.6×10^3 seconds in the lower mantle. The shift of the band with depth is consistent with the expected effects of temperature, pressure and stress. High Q_s regions of the mantle are attributed to a shift of the absorption band to longer periods. To satisfy the gravest fundamental spheroidal modes and the ScS data, the absorption band must shift back into the short-period seismic band at the base of the mantle. This may be due to a high temperature gradient or high shear stresses. A preliminary attempt is also made to specify bulk dissipation in the mantle and core. Specific features of the absorption band model are low Q in the body wave band at both the top and the base of the mantle, low Q for long-period body waves in the outer core, an inner core Q_s that increases with period, and low Q_P/Q_S at short periods in the middle mantle. The short-period Q_s increases rapidly at 400 km and is relatively constant from this depth to 2400 km. The deformational Q of the earth at a period of 14 months is predicted to be 463.

Additional Information

© 1982 American Geophysical Union. Received September 29, 1981; revised February 10, 1982; accepted February 26, 1982. Paper number 2B0308. We thank Robert Hart for use of a program and for help during the early stages of this project. Thorne Lay, Bernard Minster, Anton Hales, and Ichiro Nakanishi reviewed the manuscript and provided helpful comments. Research was supported by the Division of Earth Sciences, National Science Foundation grant EAR77-14675 and National Aeronautics and Space Administration grant NSG-7610, Contribution 3695, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.

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