Arwen Deuss

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.

Distinct layering in the hemispherical seismic velocity structure of Earth's upper inner core

The existence of hemispherical variation in the Earths inner core is well-documented, but consensus has not yet been reached on its detailed structure. The uppermost layers are a region of particular importance, as they are directly linked to the growth processes and post-solidification mechanisms of the inner core. Here, we use a large PKIKP-PKiKP differential travel time residual data set to derive a model for the upper inner core, providing new constraints on its isotropic and anisotropic velocity, and the amount of scattering. We find that the eastern and western hemisphere are separated by sharp boundaries. This is incompatible with the recently proposed inner core translation model, but might be explained by differences in outer core convection and inner core solidification rates. The eastern hemisphere displays weak anisotropy of 0.5%–1.0%. The western hemisphere, on the other hand, is characterized by the presence of an isotropic upper layer with a thickness of 57.5 km, with anisotropy of 2.8% appearing at deeper depths. The boundary between the isotropic layer and the deeper anisotropy appears sharp. We also detect, for the first time, a high velocity layer at the top of the eastern hemisphere with a thickness of 30 km, which we interpret as being due to an increased amount of light elements. There appears to be no relationship between the layered structure in the two hemispheres, with abrupt changes in velocity with depth in one hemisphere without any significant change at the same depth in the other hemisphere. Our results indicate that there is a difference in composition and mineral structure between the hemispheres, resulting in differing responses to external processes.

Reflectivity of the 410-km discontinuity from PP and SS precursors

A number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long-period seismograms on a surface reflection phase before stacking. In particular, SS and PP precursors from the 410-km discontinuity (termed P410P and S410S) have revealed long-wavelength topography of this discontinuity. Here we extend these techniques to examine the reflection coefficient of the 410-km discontinuity. Using our measurements of P410P and S410S amplitudes, we constrain the impedance contrasts across the 410-km discontinuity. We also show lateral variations in the P wave impedance contrast at 410 km, which is typically low under North America and China and higher beneath the North Pacific. The S wave impedance contrast shows less variability on the regional scale. However, analysis of P410P and S410S amplitudes over smaller areas (by binning traces into spherical caps) shows that the S wave reflection coefficient varies over much shorter scale lengths than that for P waves. The different patterns of variation for P410P and S410S reflection amplitudes could be due to the presence of melt, water, or other chemical heterogeneities in the transition zone. Other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes, but they would be expected to lead to correlated variations, and so they cannot explain all the variation that we observe.

Splitting function measurements for Earth's longest period normal modes using recent large earthquakes

Recent megathrust earthquakes, such as the 23 June 2001 Peru event, the Sumatra events of 2004 and 2005 and the 27 February 2010 Chile event, have given us the opportunity to measure splitting of the longest period normal modes. We use wave spectra to make robust measurements for modes 0S2, 0S3, 0S4, 2S1 and 1S2. Singlet frequencies of these modes have been measured previously using gravimeters, but here we use seismic records to observe splitting functions for 0S2 and 2S1 for the first time. Cross-coupling with nearby modes is included to account for ellipticity and rotation of the Earth and results in significantly improved splitting function measurements for 0S3, 0S4 and 1S2 compared with previous studies. The new splitting function measurements can easily be implemented in future tomographic modelling of aspherical velocity and, particularly, density structure.

Density structure of Earth’s lowermost mantle from Stoneley mode splitting observations

Advances in our understanding of Earths thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density. The large-low-shear-wave velocity provinces (LLSVPs) atop the core–mantle boundary beneath Africa and the Pacific are the largest structures in the lower mantle, and hence severely affect the convective flow. Here, we show that anomalous splitting of Stoneley modes, a unique class of free oscillations that are perturbed primarily by velocity and density variations at the core–mantle boundary, is explained best when the overall density of the LLSVPs is lower than the surrounding mantle. The resolved density variations can be explained by the presence of post-perovskite, chemical heterogeneity or a combination of the two. Although we cannot rule out the presence of a ∼100-km-thick denser-than-average basal structure, our results support the hypothesis that LLSVPs signify large-scale mantle upwelling in two antipodal regions of the mantle.

Large‐scale mantle discontinuity topography beneath Europe: Signature of akimotoite in subducting slabs

The mantle transition zone is delineated by seismic discontinuities around 410 and 660 km, which are generally related to mineral phase transitions. Study of the topography of the discontinuities further constrains which phase transitions play a role and, combined with their Clapeyron slopes, what temperature variations occur. Here we use P to S converted seismic waves or receiver functions to study the topography of the mantle seismic discontinuities beneath Europe and the effect of subducting and ponding slabs beneath southern Europe on these features. We combine roughly 28,000 of the highest quality receiver functions into a common conversion point stack. In the topography of the discontinuity around 660 km, we find broadscale depressions of 30 km beneath central Europe and around the Mediterranean. These depressions do not correlate with any topography on the discontinuity around 410 km. Explaining these strong depressions by purely thermal effects on the dissociation of ringwoodite to bridgmanite and periclase requires unrealistically large temperature reductions. Presence of several wt % water in ringwoodite leads to a deeper phase transition, but complementary observations, such as elevated Vp/Vs ratio, attenuation, and electrical conductivity, are not observed beneath central Europe. Our preferred hypothesis is the dissociation of ringwoodite into akimotoite and periclase in cold downwelling slabs at the bottom of the transition zone. The strongly negative Clapeyron slope predicted for the subsequent transition of akimotoite to bridgmanite explains the depression with a temperature reduction of 200-300 K and provides a mechanism to pond slabs in the first place.

PKP precursors : Implications for global scatterers

Precursors to the core phase PKP are generated by scattering of seismic energy from heterogeneities in the mantle. Here we examine a large global data set of PKP precursors in individual seismograms and array data, to better understand scattering locations. The precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase PKIKP and mantle phase PP. We find and correct for a hemispherical asymmetry in the precursor/PKIKP amplitudes, resulting from inner core structure. Using ray tracing, we locate scatterers in our array data and use these to infer scattering locations in the individual data. The scattering strength displays regional variation; however, we find no relationship with long-scale core-mantle boundary velocity structure. Scattering is observed in all regions of data coverage, as are paths with no precursors. This indicates that scattering occurs from various small-scale heterogeneities, including but not limited to ultralow velocity zones or partial melt, and slabs.

The existence of radial anisotropy in Earth's upper inner core revealed from seismic normal mode observations

As we strive to understand the most remote region of our planet, one critical area of investigation is the uppermost inner core since its structure is related to solidification of outer core material at the inner core boundary (ICB). Previous seismic studies have used body waves to show that the top ∼100 km of the inner core is isotropic. However, radial anisotropy cannot be uniquely determined by body wave observations. Alternatively, normal mode center frequencies are sensitive to spherically symmetric Earth structure, therefore may provide a constraint on the existence of radial anisotropy in the inner core. Here we show that normal mode center frequency measurements are compatible with 2-5% radial anisotropy in the top ∼100 km of the inner core with a fast direction radially outward and a slow direction along the ICB. Given the uncertainties in the mineral physics and processes that produce anisotropy, the observed radial anisotropy may be reconciled with predictions based on either solidification processes or from texturing due to anisotropic growth.

Crustal Formation on a Spreading Ridge Above a Mantle Plume : Receiver Function Imaging of the Icelandic Crust

Iceland sits astride a mid‐ocean ridge underlain by a mantle hot spot. The interplay of these two geological processes has the potential to generate a complex and laterally variable crustal structure. The thickness of the Icelandic crust is a long running and controversial debate, with estimates ranging from a thin 20‐km crust to a thick 40‐km crust. We present new images of the first‐order seismic discontinuity structure of the Icelandic crust based on a joint inversion of receiver function and ambient noise‐derived surface wave dispersion data. Inversion results are validated through comparison to receiver functions multiphase common conversion point stacks across the densely instrumented Northern Volcanic Zone. We find a multilayered crustal structure consisting of a 6‐ to 10‐km‐thick upper crust underlain by either one or two discontinuities. The shallower discontinuity is found at depths of ≈20 km throughout Iceland. The deeper discontinuity is only present in some regions, defining the base of a lens‐like lower layer with maximum depths of 44 km above the center of the mantle plume. Either of these two discontinuities could be interpreted as the seismic Moho, providing an explanation why previous estimates of crustal thickness have diverged. Such structure may form via underplating of a preexisting oceanic crust as has been hypothesized in other ocean island plume settings. However, we demonstrate with a simple petrological model that variability in seismic discontinuity structure can also be understood as a consequence of compositional variation in melts generated with distance from the plume center. Plain Language Summary When tectonic plates pull apart, magma wells up between them forming new oceanic crust. Iceland sits astride one of these mid‐ocean ridges, but unlike most others which are found on the ocean floor, it is raised above sea level. This is caused by a hot area of the Earths mantle raising the area up, thought to be caused by a mantle plume (a convective upwelling rising from the Earths core). In this study we try and understand what crust formed in this special setting, where mid‐ocean ridge and mantle plume interact, looks like. We make observations of the Icelandic crust using distant earthquakes that are recorded in Iceland, extracting information that earthquake signals carry about the material they travel through on their journey through the Icelandic crust. This gives us a new picture of Icelands crust: it is much thicker than normal mid‐ocean ridge crust, thickest in the center above the plume and thinning outward, and is made up of several layers. By analyzing crystal content of lavas erupted in Iceland at different distances from the plume, we construct a model that explains the structure we observe by variation in the types of magma available for crustal formation in different locations.

Correction to “Distinct layering in the hemispherical seismic velocity structure of Earth's upper inner core”

[ 1 ] In the paper “ Distinct layering in the hemispherical seismic velocity structure of Earth ’ s upper inner core ” by L. Waszek and A. Deuss ( Journal of Geophysical Research , 116 , B12313, doi:10.1029/2011JB008650, 2011), an incorrect version of Table 3 was published. The correct Table 3 and its caption are shown here.