Seismological Modeling of Inner Core Boundary Topography

Earth's solid inner core is created by the slow freezing of a well-mixed, vigorously convecting, iron-rich outer core. The structure near inner core boundary (ICB) has a significant effect on core dynamics including the mechanisms behind the growth of inner core and the compositional convection driving the geodynamo. A dynamically unstable process of compaction of solids and expulsion of fluids at a solidifying boundary can produce small-scale corrugations of the inner core boundary with heights on the order of 1km, consistent with a number of previous observations of body waves interacting with the ICB. We determine topographic models of a rough ICB that match the observed PKiKP and PKP-Cdiff waveforms. In order to constrain parameters of the modeled topography, the observations are compared with synthetic seismograms generated using a boundary element method that exploits a dense discretization along the surface of the ICB. This method of modeling is more computationally efficient and flexible than finite difference methods previously used in these studies, which in turn allows us to make our calculations more accurate. The implementation of the modeling procedure starts by setting up boundary element method for a two-layered homogeneous interior and exterior of ICB system and later adds the real Earth's radial inhomogeneity to the exterior where propagation of rays are calculated using ray theory. An initial test is carried out to identify the parametric limits of models where ICB topography begins to impose observable effects to the PKiKP coda at approximately 50° great circle distance.