HR: 08:00h
AN: S21A-01 INVITED     [PDF]
TI: Q in Earth's Core
AU: * Cormier, V F
EM: vernon.cormier@uconn.edu
AF: University of Connecticut, Geology and Geophysics, 354 Mansfield Road, Storrs, CT 06269-2045 United States
AB: Observations of high frequency PKnKP waves, multiply reflected by the underside of the core-mantle boundary, place a lower bound of 10,000 on the Q$_{K}$ of the outer core. Hence, for most applications Q$_{K}$ of the outer core can be assumed to be infinite, consistent with the expected behavior of a low viscosity fluid. Observations of high ($>$1 Hz) frequency PKPPKP coupled with the high Q$_{K}$ of the outer core, suggest a strong ($\omega^{-1}$) frequency dependence of attenuation in the mid-mantle begins between 0.1 to 1 Hz. In contrast to the outer core, significant seismic attenuation is observed in the inner core over a broad band of frequencies, with a mean Q$_{\alpha}$ at 1 Hz of 307$\pm$90 determined from waveform modeling of PKIKP in the distance range $130\deg$ to $180\deg$. A strong depth dependence of Q$_{\alpha}$ is observed, with attenuation much stronger (Q$_{\alpha}$ much lower) in the upper 300 km. The attenuation of PKIKP waveforms is frequency dependent with very weak velocity dispersion. Either the effects of viscoelasticity or scattering can model the pulse broadening and dispersion of PKIKP. Observations of the backscattered coda of PKiKP suggest that a significant fraction of the attenuation in the short-period (1 Hz) band may be due to scattering. A pure scattering model requires velocity perturbations on the order of 6 to 10% and scale lengths on the order of 1 to 10 km. The velocity perturbations are larger for polar than equatorial paths, decrease with depth, and show anisotropy in both global and regional data. For paths beneath North America, the smallest scale lengths (1-5 km) tend to lie in either the upper 200 km of the inner core or along paths close to the rotational axis. The depth dependence of attenuation inferred from a scattering mechanism is roughly similar to that inferred from a viscoelastic mechanism, except a more abrupt transition is seen between higher attenuation in the upper inner core and lower attenuation in the lower inner core. This transition may be sharp enough to produce either a first or second order discontinuity with depth in the long-wavelength (composite) elastic moduli. A fabric that may satisfy the observed depth dependence and anisotropy of attenuation is one composed of iron crystals having high ($>$10%) intrinsic anisotropy, which are progressively ordered with increasing depth in the inner core. Reported hemispherical differences in the attenuation and seismic velocities of the upper inner core, together with a high regional variance in measured Q$_{\alpha}$ emphasize the importance of globally dense ray-path coverage of the inner core for imaging the viscoelastic and elastic fabric of the inner core, which has been left as the signature of its solidification from the liquid outer core and the operation of a compositionally driven dynamo.
UR: http://www.sp.uconn.edu/~cormier
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
DE: 8115 Core processes (1507)
SC: Seismology [S]
MN: 2003 Fall Meeting