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Normal mode study of the D'' region and core-mantle boundary topography.

By P.J. Koelemeijer


The core-mantle-boundary (CMB) is the largest thermal and compositional interface in the Earth and a key feature in understanding the evolution of the core and its relation to the mantle and the Earth’s magnetic field. Anomalous seismic velocity structures found in the lowermost mantle (D” region), such as thin (50 km) ultra-low-velocity-zones (ULVZ's) and an anticorrelation between shear wave velocity and density heterogeneities, are comparable in complexity with the structures found in the lithosphere and are still not well understood. These features may be due to the existence of partial melting or compositional heterogeneity, but convincing seismological evidence is still lacking. The majority of these structures has been found using either body wave observations, which are limited by the uneven station and earthquake distribution or by normal mode studies which have concentrated on larger scale lower mantle tomography and the CMB topography. A more detailed understanding of the velocity structures in the lowermost mantle (the D” region) and of the CMB topography is a prerequisite for the integration of results from seismology with mineral physics and convection modelling. In this study I investigate the sensitivity of the normal modes to these small scale features and CMB topography by computing synthetic spectra and misfit values for the Bolivia earthquake of 1994 using self and full coupling theory. In addition, to study specific structural characteristics of the lower mantle structures, I compute self coupled splitting functions. Results are presented in this report for a range of models in which the distribution and parameters governing the D” anticorrelation, the distribution, thickness and properties of ULVZ’s, and the pattern and amplitude of CMB topography models has been varied. The results show sensitivity to all the inserted structures including ULVZ’s and particularly splitting functions prove to be useful, allowing determination of an optimum model in some cases. The D” anticorrelation seems to be constrained by a limited number of normal modes and is laterally varying in possibly a ring around the Pacific pattern consistent with lower mantle tomography models. Sensitivity to ULVZ’s is observed mainly in higher order overtones and coefficients and can even be observed for local models with thicknesses smaller than 30 km. The results imply furthermore that the peak-to-peak amplitude of the CMB topography is smaller than 5 km and that the pattern of the topography is more difficult to constrain. However, non-linear components and trade-offs between the topography and the D” heterogeneities might be present. It is therefore important to combine normal mode results with body wave data to obtain a complete picture of the lowermost mantle

Topics: Geowetenschappen, Seismology, Core-mantle boundary, D'' region, Normal modes, Ultra-low-velocity-zones, Core-mantle boundary topography.
Year: 2010
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