SHEAR ATTENUATION AND DISPERSION IN MGO

An understanding of anelastic effects on seismic wave propagation is c rucial for developing accurate seismic models of the mantle. The lower mantle is thought to contain a significant volume fraction, approxima tely 20%, of magnesiowustite, (Mg,Fe)O, in addition to the dominant si licate perovskite. Weaker magnesiowustite may dominate the rheological response of the lower mantle. It may also account for the shear atten uation observed in the lower mantle. We have measured the complex tors ional modulus of a single crystal of the magnesiowustite end member pe riclase, MgO, at small strains, seismic frequencies, and high temperat ure. The attenuation is approximately 0.030 (Q approximately 33) at 0. 003 Hz and 1500K with a dislocation density of approximately 5x10(9) m (-2). The corresponding dispersion reduces the shear velocity of MgO a t 0.001 Hz and 1500K by approximately 4% relative to ultrasonic values . To the extent that magnesiowustite behaves similarly to MgO, it appe ars to be a good candidate to account for a major portion of lower-man tle attenuation.