Seismological Characterization of the Boundary Layers at the Core-Mantle Boundary

Details Seismological Characterization of the Boundary Layers at the Core-Mantle Boundary Seismological Characterization of the Boundary Layers at the Core-Mantle Boundary

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.u41c..04l&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #U41C-04

Mathematics

Logic

7203 Body Wave Propagation, 7207 Core And Mantle, 8120 Dynamics Of Lithosphere And Mantle: General, 8124 Earth'S Interior: Composition And State (Old 8105), 8147 Planetary Interiors (5430, 5724)

Scientific paper

The spatial resolution of seismological structure near the core-mantle boundary (CMB) is improving steadily as the numbers of broadband and high frequency array data increase. The improved spatial resolution is revealing new complexity of the boundary layer at the base of the mantle, and possibly even in the boundary layer in the outermost core. While large-scale seismic tomography has established the predominance of long wavelength heterogeneities in the D" region of the lowermost mantle, the nature of these large provinces can only be assessed by high resolution imaging, which typically involves waveform modeling and array processing of data. Large-scale low velocity provinces in D" have been revealed to have very strong lateral and radial gradients into the low velocity material, challenging the interpretation of these as simple thermal anomalies and bolstering the case for chemical or partial melting contributions to the heterogeneity. A vast region beneath southern Africa, the south Atlantic, and the south Indian Ocean has S velocity reductions of 3 to 5 percent in a 200-300 km thick region with some evidence for an increase in thickness to 800 km below Africa. Margins of the large Pacific anomaly have strong ULVZs and a bumpy S wave discontinuity 230 km above the CMB. Large-scale high velocity provinces in D" have been revealed to have laterally coherent S velocity discontinuities 200-300 km above the CMB, with array processing revealing remarkable topography (or lumpiness) of the discontinuity involving 100 km depth variations over 250 km horizontal scales. Shear wave splitting occurs in these regions, again over large lateral scales, with most data consistent with vertical transverse isotropy or symmetry axes slightly tilted from the vertical. A few localized patches of the outermost core may have finite rigidity or fuzzy transitions with scales of hundreds of meters at the CMB and there still remains a possibility of anomalous global core structure in the outermost 50 km of the core.

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