New normal mode constraints on bulk inner core velocities and density

Abstract

Inner core elastic parameters Vp, Vs and density (ρ) inform both experimental and theoretical studies of inner core composition and potential light element candidates. Meanwhile, constraints on the density jump at the inner core boundary offer much-needed observational insight into the processes powering the geodynamo. Thus, obtaining accurate observational constraints on the elastic structure of the inner core is key to gaining better understanding of its composition and dynamics.While body wave phases provide information on inner core P velocity and anisotropy structure, constraining shear velocity and density currently relies predominantly on core-sensitive normal mode observations. Since the construction of the Preliminary Reference Earth Model (Dziewonski and Anderson, 1981), which was largely based on such data, the database of observed normal modes has become both larger and more accurate, and new model exploration methodologies have become computationally feasible.Here we use recent normal mode center-frequency catalogues to simultaneously explore average shear (Vs) and compressional (Vp) velocities, as well as density (ρ), in the inner core via a Monte Carlo parameter-space search.We find best-fitting Vp values (11,160–11,180 m/s) in good agreement with PREM, while Vs values (3560–3590 m/s) favor a minor reduction (<1% relative to PREM). We show that a recently proposed reduction of 2.5% in Vs in the inner core, while keeping the other two parameters constant, is incompatible with normal mode data, and demonstrate the importance of simultaneously exploring the parameters to address trade-off effects. Meanwhile, we show that ρ is the most poorly-constrained parameter, though mode data without any additional constraints favor a density reduction of ~0.7–1.9% with respect to PREM. We also show that the need for this reduction is largely independent of various assumed parameters, including: the choice of reference mantle and outer core model, observational catalog, and the depth of the ICB.The presented results may be of interest for laboratory and ab-initio studies aiming at constraining IC composition and for studies that consider the energy available to power the geodynamo via compositional convection.