Proposed thirteen years ago [Morelli et al., 1986; Woodhouse et al., 1986], the anisotropy of the earth's inner core seems today firmly established, the fast axis of iron crystals being roughly aligned with the earth's axis of rotation. Yet, there is still no consensus as to what the exact distribution and strength of this anisotropy are. Most studies of the structure of the inner core are based on PKP travel times, and in particular differential travel times such as PKP(AB-DF) and PKP(BC-DF), which are often used in order to reduce mantle effects. We have showed that AB-DF differential travel times could actually be largely explained by deep mantle structure, including the trend with angle of the path with respect to the earth's rotation axis, usually attributed to inner core anisotropy [Bréger et al., 1999]. Although BC-DF travel times are in principle much less sensitive to mantle structure, it is nevertheless important to quantify the effect due to heterogeneity in the earth's mantle to reliably estimate the signal due to the inner core.
By fixing a station and comparing residuals in a given source region, it is possible to gain insight into small scale spatial variations of BC-DF residuals. Station COL, in Alaska, is unfortunately the only one for which a relatively large dataset of hand-picked BC-DF travel times on quasi-polar paths is available. International Seismological Center (ISC) bulletins provide a large dataset spanning several decades. One drawback of this dataset is that it is very noisy. However, it has recently been improved by systematic elimination of misidentified phases and earthquake relocation [Engdahl et al., 1998]. From this improved catalog, we have extracted BC and DF absolute travel time residuals with respect to the ak135 model [Kennett et al., 1995] for a number of key stations corresponding to polar paths and classified them by source region (Figure 21.1). A second drawback of the ISC dataset is that measurements of both BC and DF travel times for the same event/station pair are infrequent. However, when absolute DF and BC residuals show systematic trends as a function of epicentral distance and angle , it is possible to estimate the corresponding variations of BC-DF and their standard deviations in a "composite" fashion.
In Figure 21.2 are summarized the trends which we observed for 10 quasi-polar paths. We have plotted the composite BC-DF travel time anomalies as a function of angle (Figure 21.2), compared to predictions from different simple models of inner core anisotropy. Figure 21.2 shows large excursions, both locally and over the range of considered, from a trend that is compatible with the predictions of a model of anisotropy with strength on the order of 1.5%, at least for the top 400km of the inner core.
In order to explain our observations, a very complex model of inner core anisotropy, possibly associated with heterogeneity, needs to be invoked. The inner core structure would change on a hemispherical scale [Tanaka and Hamaguchi, 1997], but also over distances of a few hundreds of kilometers, from one station to another, and on even shorter scales, when the station is fixed. Some very large residuals are sometimes observed: hand-picked residuals at station COL vary from 2s to exceptional values of some 5s. An anisotropy of more than 5% would be required in order to speed up DF by 5s at 154o. This is extremely high, but could be compatible with the results of recent high-pressure experiments [Mao et al., 1998] ,and the existence of a complex convective pattern [Jeanloz and Wenk, 1988].
A complex lowermost mantle structure could be an alternative explanation to this complex behaviour. BC-DF travel times are not anomalous for polar paths for roughly one third of the globe, over Asia and Russia [Tanaka and Hamaguchi, 1997]. Interestingly, the deep mantle beneath these regions seems weakly anomalous. On the other hand, some of the largest residuals are obtained at station NRIL, for which there could be a strong interaction of the PKP paths with the base of the African plume (Figure 21.1).
The rapid variations of BC-DF residuals on scales of a few hundreds kilometers can be understood as an effect of very complex anisotropic structure at the top of the inner core, or mantle heterogeneity, or a combination of both. Lowermost mantle structure happens to geographically correlate with the complex behaviour of BC-DF residuals, in locations where it is well documented. Whatever their origin, the existence of such strong variations shed a new light on the complexity of the deep earth.
We are particularly grateful to A. Souriau and G. Poupinet, M. Wysession, P. Richards, and S. Tanaka and H. Hamaguchi for providing us with their data, and E. R. Engdahl, R. van der Hilst, and R. Buland for making their ISC dataset available. We benefited from discussions with H.R. Wenk and R. Jeanloz. We also thank the IRIS, CNSN, Geoscope, GRSN, Mednet, BDSN, SCSN, and GRSN teams.
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