Seismic Imaging of the Inner Core Boundary Region

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The aim of the proposed work is to achieve a better understanding of the boundary between the Earth's solid inner core and liquid outer core by analyzing earthquake-generated seismic waves. Seismic analysis of the inner core boundary (ICB) region provides unique constraints that complement laboratory and computational experiments on the properties of core components at high pressure and temperature. The fine structure of the ICB region is particularly relevant to understanding the dynamics and evolution of the core because it represents a phase boundary within the Fe, Ni, light-element chemistry of the core. Seismic constraints on the ICB are also relevant to geodynamicists who study Earth's magnetic field because they guide the selection of boundary conditions and driving forces for models of convection in the outer core. The proposed study involves diffracted (PKP_Cdiff}) and scattered (precritical PKiKP coda) seismic energy. The PKP_Cdiff} data set is being constructed from the many temporary and permanent broadband seismic arrays that exist around the world. Apparent PKP_{Cdiff} ray parameters are being estimated at various frequencies, and the corresponding dispersion curves are being inverted for radial velocity models of the lower outer core, using full wave theory to do the forward problem. Short period ray parameter estimates are being analyzed in a differential sense (using PKP_{DF} as a reference) to search for lateral structural variations in the ICB region. Such differential ray parameter estimates are much less sensitive to lower mantle heterogeneities than corresponding differential travel time observations. The PI's data base of precritical PKiKP waveforms is being updated using the various small aperture array stations of the International Monitoring System, and a non-linear curve-fitting technique is being applied to quantify variations in coda decay rates. The PI is synethesizing PKiKP coda envelopes with single-scattering techniques to model the data and to compare the relative contributions of four potential sources of precritical PKiKP coda waves: heterogeneities in the lower mantle, CMB topography, heterogeneities in the upper inner core, and ICB topography.Most of the proposed funds are being used to support a PhD student and several undergraduate research assistants. The students will have the opportunity to travel to scientific meetings and become authors or co-authors of peer-reviewed publications. The results from the proposed work, including relevant data sets, will be made available on the web. Techniques related to the proposed work will be discussed and applied in graduate geophysics classes taught by the PI. The subject of the proposed work will be discussed in lay terms to various K-12 groups as the PI performs his role as the outreach representative for the Geosciences section of the Department of Earth and Atmospheric Sciences.