Normal mode constraints on inner core structure: a trans­-dimensional Bayesian approach

Obtaining accurate constraints on the elastic structure of the inner core is key to gaining better understanding of it's composition and dynamics. While observations of body wave phases provide information on inner core P velocity and anisotropy structure, constraining shear velocity and density currently relies entirely on core-sensitive normal mode observations. The PREM model (Dziewonski and Anderson, 1981) relied on constraints from core-sensitive modes and is still widely used as a reference for Vs and density in the inner core. However, since then, the normal mode database has grown significantly and new applicable inversion methodologies have become feasible due to increases in computation power. Our goal is to build a probabilistic 1D model of core structure in Vp, Vs and density, taking advantage of an augmented mode center frequency catalog and advances in computational resources. We use the REM catalog of observed frequencies of core sensitive normal modes (http://igppweb.ucsd.edu/ gabi/rem.html), together with the dataset of Deuss (2008), augmented with data from more recent large and deep earthquakes. We also include constraints on the mass and moment of inertia of the Earth. We have implemented a Trans-dimensional Markov chain Monte Carlo method, exploring the model space more fully than ever before while minimizing a-priori inputs and allowing the data to exercise maximum control on the result. In this approach, the number of layers and the properties of each layer are allowed to vary freely to match observations. In a first step, we keep mantle and outer core structure fixed to that of PREM, and explore the space of acceptable models in Vs, Vp and rho in the inner core alone. In a second step, we will allow adjustments in lowermost outer core structure. Ultimately, we will add constraints from mantle modes to investigate trade­offs between mantle and core structure. Here we present preliminary results on structure in the inner core and its vicinity.