HR: 11:35h
AN: U41C-06    [PDF]
TI: Lateral variations in the structure of the thermal boundary layer at the base of the mantle from the dispersion of core-diffracted waves
AU: * Wysession, M E
EM: michael@wucore.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: Aleqabi, G I
EM: ghassan@ocean@wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: Shore, P J
EM: patrick@wups.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: Lawrence, J F
EM: jfisher@levee.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AU: Fischer, K M
EM: Karen_Fischer@Brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AB: The thermal boundary layer at the bottom of the mantle (D'') plays an extremely important role in controlling the dynamics of Earth's interior. However, very little a actually known of its vertical structure and lateral variations. The lateral and vertical variations in the thermal boundary layer at the top of the mantle (the lithosphere) have been largely mapped by the dispersion of surface waves. We perform a similar study for D'', using the dispersion of core-diffracted Pdiff waves, which can propagate for great distances along the core-mantle boundary. Pdiff dispersion is not nearly as great as for surface Rayleigh waves, but in the right circumstances, it can be used to forward-model velocity structures in D''. Using the linear arrays MOMA (Missouri-to-Massachusetts) and FLED (Florida-to-Edmonton), we examine the dispersion of Pdiff waves across the arrays in order to model regional variations in the vertical structure of D''. Good results were obtained with 12 earthquakes recorded at MOMA for Pdiff, and 5 earthquakes for the surface-reflected pPdiff. The dispersion curves are generated by measuring the varying ray parameter values across the array at a discrete number of bandpass intervals ranging from 0.005 and 0.2 Hz. Many regions are well-modeled by a PREM-type structure (essentially zero velocity gradient) or models with a mild negative velocity gradient. Unfortunately, this method can not reliably distinguish between a thermal boundary layer structure with a negative velocity gradient, and a structure with a zero gradient but lower than average value. There are certain regions, however, that are best modeled by a discontinuous increase in velocity at the top of D'', underlain by a negative velocity gradient. This kind of structure has been inferred from precursors to core-reflected body waves, but the support of such models by such a radically different type of data lends significant validation to their existence.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
SC: U
MN: 2003 Fall Meeting