Marie Calvet

Lopsided Growth of Earth's Inner Core

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. The asymmetry of the inner core is explained by iron crystallization on one side and melting on the other. Hemispherical asymmetry is a prominent feature of Earth’s inner core, but how this asymmetry relates to core growth is unknown. Based on multiple-scattering modeling of seismic velocity and attenuation measurements sampling the whole uppermost inner core, we propose that the growth of the solid core implies an eastward drift of the material, driven by crystallization in the Western Hemisphere and melting in the Eastern Hemisphere. This self-sustained translational motion generates an asymmetric distribution of sizes of iron crystals, which grow during their translation. The invoked dynamical process is still active today, which supports the idea of a young inner core.

Constraints on grain size and stable iron phases in the uppermost inner core from multiple scattering modeling of seismic velocity and attenuation

We propose to model the uppermost inner core as an aggregate of randomly oriented anisotropic ``patches. A patch is defined as an assemblage of a possibly large number of crystals with identically oriented crystallographic axes. This simple model accounts for the observed velocity isotropy of short period body waves, and offers a reasonable physical interpretation for the scatterers detected at the top of the inner core. From rigorous multiple scattering modeling of seismic wave propagation through the aggregate, we obtain strong constraints on both the size and the elastic constants of iron patches. We perform a systematic search for iron models compatible with measured seismic velocities and attenuations. An iron model is characterized by its symmetry (cubic or hexagonal), elastic constants, and patch size. Independent of the crystal symmetry, we infer a most likely size of patch of the order of 400~m. Recent {it bcc} iron models from the literature are in very good agreement with the most probable elastic constants of cubic crystals found in our inversion. Our study (1) suggests that the presence of melt may not be required to explain the low shear wavespeeds in the inner core and (2) supports the recent experimental results on the stability of cubic iron in the inner core, at least in its upper part.

Lapse‐Time Dependence of Coda Q: Anisotropic Multiple‐Scattering Models and Application to the Pyrenees

Abstract The coda quality factor of short‐period S waves ( Q c ) excited by local earthquakes in the Pyrenees has been measured as a function of the length of the coda window ( L W ) for different choices of the onset time of the coda ( t W ). In the 2–16xa0Hz frequency band, we observe a transient regime characterized by an increase of Q c with L W , followed by a stabilization around a plateau the value of which depends on the central frequency of the signal. Using Monte Carlo simulations of wave transport in a variety of random media (≈1200 models), we demonstrate that the lapse‐time dependence of Q c in the Pyrenees may be modeled by multiple anisotropic scattering of seismic waves, without invoking any depth dependence of the attenuation properties in the crust. In our model, anisotropic scattering is quantified by the ratio between the transport mean free path and the mean path ( l */ l ). At 6xa0Hz, the data require an anisotropy factor l */ l ≥5, a transport mean free path l *≈400u2009u2009km, and an intrinsic quality factor Q i ≈800. From the frequency‐dependent plateau of Q c at large lapse time, we infer an intrinsic quality factor of the form Q i ≈400 f 0.4 in the Pyrenees. We also show how the rapid increase of the lapse‐time dependence of Q c with frequency may be exploited to put constraints on the power spectrum of heterogeneities in the crust.

Velocity and attenuation of scalar and elastic waves in random media: A spectral function approach

This paper investigates the scattering of scalar and elastic waves in two-phase materials and single-mineral-cubic, hexagonal, orthorhombic-polycrystalline aggregates with randomly oriented grains. Based on the Dyson equation for the mean field, explicit expressions for the imaginary part of Greens function in the frequency-wavenumber domain (ω, p), also known as the spectral function, are derived. This approach allows the identification of propagating modes with their relative contribution, and the computation of both attenuation and phase velocity for each mode. The results should be valid from the Rayleigh (low-frequency) to the geometrical optics (high-frequency) regime. Comparisons with other approaches are presented for both scalar and elastic waves.