HR: 15:15h
AN: S12D-06 INVITED     [PDF]
TI: Seismic Wave Attenuation and Dispersion: Insights From the Earth and the Laboratory
AU: * Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: U. C. Berkeley, BSL, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Jackson, I N
EM: ian.jackson@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT0200 Australia
AB: The study of the 3D anelastic structure and its frequency dependence using seismological tools, combined with experimental data on Q provide potentially powerful constraints on the rheology and dynamics of the earth's interior. Since the pioneering work of D.L. Anderson and his colleagues in the 1970's and 1980's, progress has been hampered on the seismological side, by the difficulty of separating the effects of intrinsic attenuation from those of elastic scattering and focusing, and on the experimental side, by the difficulty of performing measurements in the combined relevant frequency, temperature, and pressure ranges. The last decade has seen the progressive build-up of a large global dataset of high quality seismic broadband data, as well as significant improvements in processing techniques. In parallel, there has been progress in our ability to model 3D elastic structure as well as its effects on the amplitudes of seismic waves. We focus on discussing recently developed global and regional models of lateral variations in Q in the crust and upper mantle. In particular, we illustrate how, for the first time, 3D global models obtained using different types of waves and processing techniques result in compatible Q distributions in at least the upper 250km of the mantle, which correlate well with global tectonics as well as higher resolution regional results. We discuss future challenges and directions. In parallel with seismological studies, there has been dramatic recent progress in laboratory study of seismic wave dispersion and attenuation - particularly in olivine-dominated upper-mantle materials. Torsional forced-oscillation and microcreep tests of fine-grained synthetic Fo90 polycrystals have revealed a broad absorption band within which $Q^{-1}$ varies smoothly and monotonically with period (with a power-law exponent ~0.3), temperature and grain size. Similar studies of melt-bearing olivine polycrystals typically reveal an enhanced level of background dissipation along with a clearly resolved dissipation peak. Superposition of the frequency dependences associated with the background and the long-period side of the dissipation peak results in nearly frequency-independent $Q^{-1}$ for suitable combinations of period, temperature, grain size and melt fraction. The $Q^{-1}$ peak for the melt-bearing materials is apparently not the result of melt squirt; instead it is plausibly attributed to elastically accommodated grain-boundary sliding. However, the behaviour of the melt-free materials is difficult to reconcile with classical models of grain-boundary sliding - suggesting that these need to be re-evaluated. Seismic-frequency laboratory data, extrapolated to the larger grain sizes and higher pressures of the upper mantle, are finally beginning to provide a robust basis for understanding the wave speed variability and attenuation in the mantle. The state-of-the-art will be illustrated with laboratory-based models for the depth dependence of shear wave speed and attenuation for the oceanic upper mantle.
DE: 3909 Elasticity and anelasticity
DE: 7218 Lithosphere and upper mantle
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
SC: Seismology [S]
MN: 2003 Fall Meeting