Since the first evidence for inner core anisotropy was presented [Morelli et al., 1986; Woodhouse et al., 1986], increasingly complex models have been proposed. It has been documented that anisotropy increases with depth in the inner core, and that it is much weaker in the quasi-eastern than in the quasi-western hemisphere. At the top of the inner core ( ), P-wave velocity may be isotropic and faster in the quasi-eastern hemisphere than in the quasi-western hemisphere.
The above complexity was questioned by several authors [Bréger et al., 2000; Romanowicz et al., 2002; Ishii et al., 2002]. The complex lateral variations in P-wave velocity could be due to mantle, and possibly outer core, heterogeneity [Bréger et al., 2000; Romanowicz et al., 2002]. Ishii et al. (2002) suggested that there need not be an isotropic layer at the top of the inner core and that both body wave and normal mode observations can be explained by a model with constant anisotropy in the inner core.
More recently, the existence of an Innermost Inner Core (IMIC), within which the anisotropic characteristics are distinct, was proposed, respectively based on body wave [Ishii and Dziewonski, 2002] (hereafter referred to as ID02) and normal mode data [Beghein and Trampert, 2003] (hereafter referred to as BT03). However, the structures proposed are inconsistent: not only are the radii of the IMIC different ( [ID02] versus [BT03]), but, more importantly, so are the slowest directions of anisotropy: in one model, the slowest direction is with respect to the earth's spinning axis (ID02); the other is along the spinning axis (BT03). Cormier and Stroujkova (2005) tested the IMIC model of ID02 using PKIKP waveform modeling and suggested a much larger radius (). Because the existence of the suggested IMIC is thought to be closely related to the early stages of inner core formation, it is important to try and clarify this inconsistency through further study.
The ID02 dataset is derived from the International Seismological Center (ISC) bulletins, and their study relies on the statistical analysis of a large noisy dataset. On the other hand, BT03 used normal mode data, the resolution of which decreases towards the center of the inner core. In this paper, we assemble a new dataset of absolute PKIKP (Fig. 2.64) travel time residuals, which are measured on high quality digital broadband seismograms recorded in global and local seismic networks (e.g., GSN, GEOSCOPE, and PASSCAL), to explore the seismic anisotropy in the central part of the inner core.
Berkeley Seismological Laboratory
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