Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

Abstract

Please cite this article as: Irving, J.C.E., Deuss, A., Stratified anisotropic structure at the top of Earth's inner core:a normal mode study, Physics of the Earth and Planetary Interiors (2010), doi:10.1016/j.pepi.2011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Page 1 of 40A c c e p t e d M a n u s c r i p tWe probe the isotropic layer at the top of Earth's inner core using normal modes. Normal mode data are compatible with an isotropic layer of up to 250km thick. Normal modes can therefore be reconciled with previous body wave studies. We further show seismic data support the presence an S-wave isotropic layer. *HighlightsPage 2 of 40 A c c e p t e d M a n u s c r i p t AbstractBody wave studies support the presence of an isotropic layer at the top of the inner core. Recent normal mode models of inner core anisotropy do not contain such a layer, but instead have anisotropic structure extending to the inner core boundary. Here, we use full-coupling of normal mode oscillations sensitive to inner core structure in order to investigate the discrepancy between models of the inner core developed using these two different types of data. We impose an isotropic layer onto existing normal mode models of inner core anisotropy, and calculate frequencies, quality factors and synthetic seismograms for radial modes, PKIKP modes and PKJKP modes. Using full-coupling allows us to make the first simulations of the effect of an isotropic layer on radial modes.The presence of an uppermost isotropic layer has an effect on the frequencies and attenuation of normal modes. By calculating the misfit between data from four large earthquakes and synthetic seismograms created for models of the inner core with isotropic layers of varying thickness, we find that normal mode data are compatible with the presence of an isotropic layer of up to 250km thickness at the top of the inner core. Thus, normal modes can be reconciled with previous body wave studies. The influence of such a layer on PKJKP modes is the only way that S-wave isotropy at the top of the inner core can currently be studied;we show, for the first time, that seismological data support the presence an S-wave isotropic layer.

Deep Earth Structure: The Earth’s Cores

“…1.23.7.2.2.1 The isotropic layer There is a general agreement about a low level of P-wave anisotropy (<1%) in the uppermost 50-150 km of the inner core, with even perhaps no anisotropy at all (Creager, 1999(Creager, , 2000Garcia and Souriau, 2000b;McSweeney et al, 1997;Ouzounis and Creager, 2001;Shearer, 1994;Helmberger, 1995b, 1998;. Irving and Deuss (2011a) show that it is isotropic for both P-and S-waves. Irving and Deuss (2011a) show that it is isotropic for both P-and S-waves.…”

“…The splitting of the modes in multiplets gives information on the anisotropy, once the contributions of the Earth's rotation and ellipticity are corrected (e.g., Durek and Romanowicz, 1999;Dziewonski and Gilbert, 1971;Irving and Deuss, 2011a;Laske and Masters, 2003;Tromp, 1993). They avoid the problem of sparse sampling due to body waves.…”

Heterogeneity and Anisotropy of Earth's Inner Core

“…Normal modes preferentially place the strongest anisotropy near the top of the inner core, which is where they have the largest sensitivity; this is in contradiction with the body waves, which require an isotropic layer of at least 60-80 km at the top. However, it is possible to fit the normal modes with the imposition of an isotropic layer at the top (Durek & Romanowicz 1999, Irving & Deuss 2011b, thus reconciling the normal modes and body waves.…”

Theory and Observations - Seismic Anisotropy

“…This anisotropy is observed in the splitting of the core-sensitive normal modes (Irving and Deuss, 2011;Deuss et al, 2010;Morelli et al, 1986;Woodhouse et al, 1986) and the traveltimes of PKIKP waves for polar paths, close to the Earth's rotation axis, contrasted with paths in the equatorial plane (Leykam et al, 2010;Morelli et al, 1986;Souriau et al, 2003). P waves following polar paths arrive on the average about 3 s earlier than those following the equatorial paths.…”