Core-mantle boundary science spans the deep Earth disciplines of seismology, dynamics, geochemistry, geomagnetism, and mineral physics. An outstanding question in deep Earth science is whether or not the iron core and silicate mantle chemically react with each other. These chemical reactions at the core-mantle boundary (CMB) would result in fine-scale outermost core and lowermost mantle layering that could be detected using seismic waves. Recent geochemical, dynamical, and geomagnetism considerations have supported core-side layering, but such a layer has not been unambiguously imaged. Better resolving outermost core structure has the potential of (a) spawning research activities in a number of fields that depend on accurate knowledge of the core, and (b) helping geoscientists better resolve issues relating to the current chemical state of the planet.
Seismic studies of the outermost core rely on SmKS waves: mantle S-waves that convert to P-waves upon entering the core, reflect m-1 times from the underside of the CMB, then convert back to an S-wave for the final mantle leg. SmKS phases are the only seismic waves that have turning depths in outermost core. Past work indicates reduced P velocities at the top of the outer core, however, SmKS phases can be significantly contaminated by lower mantle heterogeneities. Also, a stably stratified outer core layer must be less dense, and hence predicts that the velocity of such a layer will be elevated, not decreased. Thus, the investigators seek to better image the outermost core, while accounting for anomalous mantle structure. Our goal is to seismically study the fine scale structure of Earth?s outermost core in unprecedented detail, to detect and characterize (if present) any outer core layering (or establish its absence). We first seek to better characterize the heterogeneous lowermost mantle that affects SmKS. Building on an existing whole mantle tomographic model, we seek to refine our understanding of the lowermost mantle by including a vastly increased data set of SmKS arrivals from (a) new SmKS data from recent regional and global seismic deployments obtained using a clustering algorithm optimized for processing large data volumes, and (b) existing data sets of two other seismologists. We will simultaneously invert for 3D mantle structure and 1D outer core velocity to assess the need for an updated 1D reference model while accounting for deep mantle heterogeneity. We will also forward model aspects of the broadband SmKS wavefield that depend strongly on the P-wave velocity and density in the outermost 10?s of km of the core. These experiments will include modeling S4KS-S3KS and S3KS-S2KS times, and the ?birth? of the S2KS and S3KS waves. These waveform-modeling experiments will include 1- and 2-D synthetic seismogram predictions, array analyses, and deconvolution algorithms to sharpen arrivals ? each of which will permit us to map any fine scale structure just beneath the CMB. This project will reveal properties of core-mantle reactions which are fundamental to understanding the evolution of the Earth. A unique feature of this proposal is combining the tools of tomography with those of detailed waveform modeling which offers cross-training for most of the proposal participants.
