Testing Inner Core Alignment Processes With Seismic Travel Times
Abstract
Our current understanding of Earth's inner core is based primarily on disciplinary studies: a model is constructed from seismic data, physical properties of core constituents are independently estimated, and processes within the core are invoked to link the mineral physics properties with the seismic models. We undertake an integrated approach in which a general physical process is examined. The critical parameters are analyzed such that models of seismic anisotropy can be determined for any plausible set of parameter values. Finally, the raw seismic observations (differential travel times in this case) are inverted to determine the range of parameter values that are consistent with a particular physical process or class of processes. There is mounting evidence from seismology that the outermost 100 or more km of the inner core is relatively isotropic. Steep gradients exist below that depth to strong anisotropy which may require anisotropic ray tracing to properly interpret. The standard seismological model is to assume the symmetry axis is parallel to the spin axis. This is mathematically simple and provides an adequate explanation of the seismic observations. However, there is no appealing physical model that would produce such a simple pattern to the fabric of the inner core. We examine two different classes of seismological models that represent a host of physical models including dendritic growth and crystal alignment due to magnetic shear stresses along lines of constant latitude. These would align the a or c axes of hexagonal crystals respectively in the directions perpendicular to and going through the rotation axis. In the latter case, if we assume the a axis is randomly oriented with respect to the alignment direction then averaging produces an apparent cylindrical symmetry with respect to the alignment direction. Both of these very different physical mechanisms produce the same symmetry relations, though very different apparent elastic constants for given physical properties of iron. Given these important symmetry considerations, we invert differential travel-time data for three parameters that define the relation between the intrinsic elastic structure of iron and the wave speed, plus a degree of alignment function that varies from 0 to 1 and depends on radius and latitude. Taken at face value, neither of these styles of models provide an adequate fit to existing differential travel-time data. So either our parameterization of the seismological models are too simple to capture all the complexities of the physical processes or we need to consider other physical models that produce different symmetries.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.U41D..01C
- Keywords:
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- 1507 Core processes (1213;
- 8115);
- 3902 Creep and deformation;
- 3909 Elasticity and anelasticity;
- 7203 Body waves;
- 7207 Core (1212;
- 1213;
- 8124)