Imaging Structure at and Near the Core Mantle Boundary With a Generalized Radon Transform of Broad Band ScS and SKKS Coda Waves

Seismic constraints on the core-mantle boundary (CMB) and on the character of any nearby structures and interfaces are of great importance for understanding the mineralogy, phase chemistry, and dynamics in Earth's lowermost mantle. The recently discovered post-perovskite transition is an obvious target of our research, but our technique could also reveal hitherto unknown structures. The exponentially growing data sets available through, e.g., IRIS call for powerful inversion methods. In order to produce accurate images of scatterers or interfaces near the CMB from seismic body waves such as ScS and SKKS - and their codas - we have developed a generalized Radon transform (GRT) from 'exact' asymptotic analysis and the theory of Fourier integral operators. We enhance the images, estimate uncertainty, and infer scaling properties of the interfaces using mixed-model statistics (i.e., properties of the GRT imaging operator are used for the characterization and reduction of noise). Our inverse scattering approach is set up to extract information from tens of thousands of broad-band waveforms. We present preliminary results of GRT imaging of the CMB beneath Central America (from -115 to --65W and -10S to 40N), using some 65,000 ScS phases generated by earthquakes with an Mb magnitude larger than 5.2. Specifically, we will present a ~2000 km great circle transect from (-105W, 0) to (-75W, 30N), showing a sharp CMB and a more gradual interface approx. 280 km above it, with hints of structure in between. From this we can begin to retrieve the lateral variation in depth (related to the Clapeyron slope in case of a phase transition) and the regularity of interfaces in the bottom 300 km or so of the mantle. These lateral variations will be discussed in the context of wavespeed variations inferred from global tomography.