Velocity and Attenuation in the Earth’s Inner Core - fig.1
Velocity and Attenuation in the Earth’s Inner Core - fig.1
Credit:
Vernon F. Cormier, Anastasia Stroujkova• University of Connecticut; Xu Li • Massachusetts Institute of Technology/IRIS Consortium
Description
Q-1(1 Hz) as a function of radius and ray direction at the turning point of the PKIKP ray path for global data. Note the steep rise in attenuation at the radius of 600 km.
Broadband velocity waveforms of PKIKP in the distance range 150° to 180° are inverted for a model of inner core attenuation due to forward scattering by a three-dimensional heterogeneous fabric. A mean velocity perturbation of 8.4%±1.8% and a scale length of heterogeneity of 9.8±2.4 km are determined from 262 available PKIKP ray paths. The velocity perturbations are larger for polar than equatorial paths, decrease with depth, and show anisotropy in both global and regional data (Figure at right). 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 is roughly similar to that obtained assuming a viscoelastic origin, 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 satisfies the observed depth dependence and anisotropy of attenuation requires solidification of iron crystals having high (>10%) intrinsic anisotropy, which are preferentially aligned in time and depth. Since weak velocity dispersion, elastic anisotropy, attenuation anisotropy, and their depth dependence agree with that predicted by such a fabric, we suggest that scattering attenuation is not a small fraction but rather the predominant mechanism of attenuation in the inner core in the 0.02 to 2 Hz frequency band.
The structure of the uppermost 100 km of the inner core was examined from PKIKP and PKiKP waveforms in the distance range of 118°–140°. We found evidence of a low-velocity layer in the uppermost inner core in the equatorial region predominantly located between longitude 20°W to 140°E (Figure at left). In the latitudinal direction the anomaly is detectable from 35°S beneath the Indian Ocean to 60°N underneath Asia. The maximum thickness of the low-velocity layer inferred from waveform modeling is 40 km with velocity jump of about 3%. We speculate that this layer may represent newly solidified core in the area where vigorous compositional convection in the outer core coincides with new crystal growth in the inner core.
Cormier, V.F., and X. Li. Frequency dependent attenuation in the inner core: Part II. A scattering and fabric interpretation, J. Geophys. Res., 107, 10.1029/2002JB1796, 2002.
Stroujkova, A., and V.F. Cormier. Regional variations in the uppermost 100 km of the Earth’s inner core, J. Geophys. Res., 109, doi: 10.129/2004JB002976, 2004.
Photographer / Contributor: Vernon F. Cormier, Anastasia Stroujkova• University of Connecticut; Xu Li • Massachusetts Institute of Technology
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