Jessica C. E. Irving

Regional Variation of Inner Core Anisotropy from Seismic Normal Mode Observations

Clearing Up the Inner Core The behavior of Earths core controls the planets heat budget and magnetic field, yet its structure remains enigmatic. For instance, the seismic properties of the solid inner core suggest hemispherical structural asymmetry, but questions remain as to how these variations arose (see the Perspective by Buffett). Monnereau et al. (p. 1014, published online 15 April) modeled grain sizes of crystalline iron—the predicted dominant mineral phase in the core—and found that a slow translational motion eastward may trigger melting in the Eastern Hemisphere and solidification in the Western Hemisphere, creating a lopsided core. Deuss et al. (p. 1018, published online 15 April) examined the normal-mode seismic structure of the inner core, collected from 90 large earthquakes, which reveal not just simple hemispherical variations, but more nuanced regional structures. The overlap of the seismic data with Earths magnetic field suggests that directionally dependent crystal alignment in the inner core formed during the solidification of the core or as a consequence of strong forces exerted by magnetism. Seismic data from the inner core reveal that anisotropic regions overlap with gravitational anomalies. Earth’s solid inner core is surrounded by a convecting liquid outer core, creating the geodynamo driving the planet’s magnetic field. Seismic studies using compressional body waves suggest hemispherical variation in the anisotropic structure of the inner core, but are poorly constrained because of limited earthquake and receiver distribution. Here, using normal mode splitting function measurements from large earthquakes, based on extended cross-coupling theory, we observe both regional variations and eastern versus western hemispherical anisotropy in the inner core. The similarity of this pattern with Earth’s magnetic field suggests freezing-in of crystal alignment during solidification or texturing by Maxwell stress as origins of the anisotropy. These observations limit the amount of inner core super rotation, but would be consistent with oscillation.

Seismically determined elastic parameters for Earth’s outer core

Seismic properties and equation-of-state parameters of the liquid iron alloy in the outer core are inferred from normal mode data. 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.

Earth's soft heart

A modern seismological method raises questions about the properties of Earths inner core Earths inner core has proven to be a challenging region for geophysicists to investigate, more than 80 years after its discovery (1). It grows slowly from the liquid iron alloy that constitutes the outer core, and its solidification is the result of the planets cooling over the course of its history. The inner core provides an important part of the energy budget for the geodynamo—the mechanism that generates Earths magnetic field—as latent heat is released and light elements are preferentially segregated into the fluid outer core. Despite the inner cores importance in the cores energy budget (2), its composition and material properties are difficult to ascertain. On page 329 of this issue, Tkalčić and Phạm (3) employ a new understanding of method that has been successfully used to study the crust and upper mantle to detect energy that has traveled as shear waves through the inner core. The shear-wave velocity (VS) reveals a softer inner core composition than previously thought.