The Earth's inner core as a conglomerate of anisotropic domains

The conceptual framework of a uniform cylindrical anisotropy in Earth's inner core (IC) with the fast axis parallel to the Earth's spin axis has become widely accepted in the geophysical community during the last twenty-five years. Such a concept is appealing to researchers oriented to both body-wave and normal-mode studies because the predictions based on a simple model of cylindrical anisotropy can explain both the observed variations in the differential travel times of PKP waves and the anomalous splitting of normal modes. Namely, PKPbc-PKPdf differential travel times (for ray geometry see inset of Figure 1) for the ray paths traversing the IC nearly parallel to the Earths spin axis (polar paths) were observed to be faster than those in the quasi-equatorial plane (equatorial paths) [e.g. Poupinet et al., 1983; Morelli et al., 1986, Creager, 1992; Shearer, 1994]. In particular, travel times associated with South Sandwich Islands (SSI) earthquakes observed in Alaska depart significantly from predictions. Core-sensitive normal modes exhibit more splitting of eigen-frequencies than predicted from Earth's rotation, shape and heterogeneity [e.g. Giardini et al., 1986; Tromp, 1993].
Newly collected travel-time residuals from seismic waves from the SSI region that sample only the Earth's mantle (PcP and P waves) have a comparable range to the PKP differential travel-time residuals, yet they are insensitive to core structure. This observation suggests that mantle structure affects PKP travel time residuals more than previously acknowledged and challenges the existing conceptual framework of a uniform inner core anisotropy. The inner core could be a conglomerate of anisotropic domains, and the PKP travel times are most likely influenced by the geometry of inner core sampling and inhomogeneous mantle structure. This concept reconciles observed complexities in travel times while preserving a net inner core anisotropy that is required by observations of Earth's free oscillations.

Map of locations of the SSI earthquakes used in this and in the previous study of PKP travel times (stars). Reflection points of PcP waves at the core-mantle boundary are projected to the surface (ellipses) in different colors corresponding to the observed PcP-P differential travel-time residuals. Piercing points of PKPdf and PKPbc waves in the IC are projected to the surface (small and large diamonds) with the corresponding PKPdf-PKPbc differential travel-time residuals using the same color scheme. Travel-time residuals are relative to the model ak135 by Kennett et al. [1995]. PKP and PcP ray-paths projected to the surface are shown in white and black lines. GSN stations PLCA and TRQA are highlighted. Yellow lines indicate a corridor in which some of the largest departures from theoretical predictions in PKPdf-PKPbc and PcP-P travel times are observed. A schematic representation of Earth's cross-section and ray-paths of seismic phases PKP, PcP and P waves used in the study is shown in the inset.

PKPbc-PKPdf differential travel time residuals, plotted as a function of the angle between PKPdf leg in the IC and the Earths rotation axis for (a) the data set from [Leykam et al., 2010] along with the 2.2% and 3.5% uniform cylindrical anisotropy predictions, and (b) only the SSI earthquakes observed in Alaska. (c) The data from (b) plotted as a function of the Alaskan stations longitude. (d) PcP-P differential travel times plotted as a function of azimuth towards recording stations.

A schematic representation of three distinct anisotropic domains in the IC where the strength and orientation of fast crystallographic axes are shown as straight lines. Two different PKPdf ray paths are shown sampling different domains. A represents a semi-constant anisotropy domain with a predominant alignment of fast anisotropic axes; B is a transitional domain with a mixed orientation of fast anisotropic axes, and C is an isotropic or a weakly anisotropic domain. The arrow in the middle represents the net direction of the fast axis of anisotropy.

The quasi-eastern hemisphere of the IC can on average be faster for the equatorial PKP waves than the quasi-western hemisphere, likely due to the domain arrangement. The top 100 km of the IC is isotropic, and has a pronounced hemispherical pattern observed for isotropic velocities. This could be because the anisotropic domains are smaller and erratic as a result of a more recent solidification. Spatial and temporal variations of the geomagnetic field and the lowermost mantle heterogeneity via the outer core can contribute to the complex structure of the IC. Columnar convection and convective heat flux in the outer core result in heat transfer variations, which influences IC growth and crystal alignment [e.g. Yoshida et al., 1996], and this has been suggested through the observation of variations in crystal alignment [Creager, 1999], texture [Cormier, 2007] and modeling of randomly oriented anisotropic patches [Calvet and Margerin, 2008]. Thus, only for certain geometries of sampling, the accumulated travel time anomaly will be strong enough to be detected at the surface. Contrary, if elastic anisotropy in the IC is weak or cancels out in the domains sampled by body waves, then some very anomalous travel times with respect to spherically symmetric models of Earth for those ray paths are likely to be a result of inhomogeneous or anisotropic structure outside the IC, such is probably the case for the SSI earthquakes.

This is an electronic version of an article published by Geophysical Research Letters; Copyright (2010) American Geophysical Union:
Tkalčić, H., Large variations in travel times of mantle-sensitive seismic waves from the South Sandwich Islands: Is the Earths inner core a conglomerate of anisotropic domains?, 37, L14312, doi:10.1029/2010GL043841

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