HR: 08:00h
AN: T21A-01    [PDF]
TI: Tangent Cylinder within the Outer Core and the Constraints from Normal-Mode Data
AU: * Ishii, M
EM: ishii@seismology.harvard.edu
AF: Scripps Institution of Oceanography, 8800 Biological Grade, La Jolla, CA 92037 United States
AU: Dziewo\'{n}ski, A M
EM: dziewons@eps.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: Recently, the tangent cylinder within the outer core has received a renewed interest as a structure that may explain observations of seismic waves travelling through the core. This is an important issue, not only because seismically detectible heterogeneity within the outer core has implications to its dynamics, but also because inferences of inner core anisotropy depends upon outer core structure. Instead of using travel time measurements, we rely mainly upon normal-mode splitting data to investigate the velocity and density difference between regions within and without the tangent cylinder. The lateral heterogeneity within the Earth, such as the tangent cylinder, is observed in seismic spectra as the splitting of normal-mode resonance peaks. Because isolated modes are standing waves, splitting measurements of these modes are only sensitive to even-degree structure. This limitation does not affect the investigation of the tangent cylinder, since this structure consists only of spherical harmonics at even degrees. In addition, biasing of data due to uneven coverage is less prominent in splitting data than in travel-time measurements, hence the normal-mode data are better suited to study large-scale structure. The normal-mode data are divided into subsets based upon their sensitivity to different parts of the Earth. Inversions of these data sets with various model parametrisations are performed to obtain and test the robustness of the strength of the tangent cylinder. The resulting ranges of velocity and density heterogeneity within the cylinder are then used to analyse the effects on travel-time measurements and on inner-core anisotropy.
DE: 7207 Core and mantle
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Tectonophysics [T]
MN: 2003 Fall Meeting