Seismically determined elastic parameters for Earth's outer core

Abstract

Turbulent convection of the liquid iron alloy outer core generates Earth's magnetic field and supplies heat to the mantle. The exact composition of the iron alloy is fundamentally linked to the processes powering the convection and can be constrained by its seismic properties. Discrepancies between seismic models determined using body waves and normal modes show that these properties are not yet fully agreed upon. In addition, technical challenges in experimentally measuring the equation-of-state (EoS) parameters of liquid iron alloys at high pressures and temperatures further complicate compositional inferences. We directly infer EoS parameters describing Earth's outer core from normal mode center frequency observations and present the resulting Elastic Parameters of the Outer Core (EPOC) seismic model. Unlike alternative seismic models, ours requires only three parameters and guarantees physically realistic behavior with increasing pressure for a well-mixed homogeneous material along an isentrope, consistent with the outer core's condition. We show that EPOC predicts available normal mode frequencies better than the Preliminary Reference Earth Model (PREM) while also being more consistent with body wave-derived models, eliminating a long-standing discrepancy. The velocity at the top of the outer core is lower, and increases with depth more steeply, in EPOC than in PREM, while the density in EPOC is higher than that in PREM across the outer core. The steeper profiles and higher density imply that the outer core comprises a lighter but more compressible alloy than that inferred for PREM. Furthermore, EPOC's steeper velocity gradient explains differential SmKS body wave travel times better than previous one-dimensional global models, without requiring an anomalously slow ~90- to 450-km-thick layer at the top of the outer core.

Fig. 1. Vinet EoS parameter posterior distributions and trade-offs.

ρ₀ is presented using the fixed molar mass of 0.05 kg and inverted molar volume. The black stars show the median values used for the EPOC-Vinet model in Fig. 2. Numbers on the lower left panels indicate correlation coefficients between the parameters in question.

Fig. 2. The EPOC-Vinet velocity and density models.

P wave or bulk sound velocities (left) and densities (right) for EPOC-Vinet (green) compared to PREM (orange lines). The models produced by the median parameters are shown as the dark green lines, the shaded region encompasses two-thirds of the values, and the dashed lines encompass 95% of the values at each depth.

Fig. 3. SmKS differential travel time predictions.

Predictions made for EPOC-Vinet, PREM, ak135, iasp91, and SP6 (60) are compared to body wave array-based observations from three events and for three phase pairs from (18). The green error bars represent predictions using the velocity ranges in table S3.

Fig. 4. Relevant physical properties of the EPOC-Vinet and PREM models.

Bullen parameter (top left) and squared Brunt-Väisälä frequency (bottom left) as a function of depth. Velocity as a function of density (right). Ranges for velocity and density are calculated when a velocity is predicted by at least 80% of the models. Colors are as in Fig. 2.

Fig. 5. Mode central frequency sensitivity kernels.

Kernels showing sensitivity of mode central frequencies to bulk modulus (top) and density (bottom), for modes that have more than 10% of their sensitivity in the outer core (pink region) for PREM. Modes in teal were not used when PREM was constructed.