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Hemispherical variations in seismic velocity at the top of the Earth's inner core

Nature volume 410, pages 1081–1084 (2001)Cite this article

Abstract

Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for inner-core growth1,2,3. Previous global seismic studies4,5,6,7,8,9 have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions10,11,12. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals—observed on both global seismograph stations and several regional seismic networks—are systematically larger, by about 0.8 s, for waves that sample the ‘eastern hemisphere’ of the inner core (40° E to 180° E) compared to those that sample the ‘western hemisphere’ (180° W to 40° E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.

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Figure 1: Map view of PKiKP minus PKIKP travel-time residuals displayed as lines along ray segments through the inner core and symbols at the turning points.
Figure 2: PKiKP - PKIKP travel-time residuals as a function of turning depth and angle.
Figure 3: Examples of seismograms.
Figure 4: Distribution of hit points of PKIKP and PKiKP rays at the CMB.

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Acknowledgements

We are grateful to the IRIS, J-array Data Center, W. Shannon and L. Saumure for supplying the data. We thank S. Tanaka for providing us their PKP-PKIKP data. We also thank H. Kawakatsu, S. Sacks and P. Silver for comments on the manuscript. Discussions with J. Aurnou, M. Fouch, W. Holt, S. Solomon, I. Sumita, S. Tanaka and D. Weidner were helpful in preparing the manuscript. This work is supported by the fellowship of the Carnegie Institution of Washington and an NSF grant.

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  1. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, 20015, DC, USA

    Fenglin Niu

  2. Department of Geosciences, State University of New York at Stony Brook, Stony Brook, 11794, New York, USA

    Lianxing Wen

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  1. Fenglin Niu
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  2. Lianxing Wen
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Correspondence to Fenglin Niu.

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Niu, F., Wen, L. Hemispherical variations in seismic velocity at the top of the Earth's inner core. Nature 410, 1081–1084 (2001). https://doi.org/10.1038/35074073

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