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Song & Helmberger 1993
Song, X. and Helmberger, D.V. (1993). anisotropy of Earth's inner core. Geophysical Research Letters 20: doi: 10.1029/93GL02812. issn: 0094-8276.

In an effort to confirm inner core anisotropy, we conducted a systematic search for PKP ray paths with various angles from the Earth's spin axis. In particular, we studied paths nearly parallel to the spin axis (polar paths) and those nearly parallel to the equatorial plane (equatorial paths). Data for earthquakes and explosions were collected from Worldwide Standardized Seismograph Network (WWSSN), Long Range Seismic Measurements (LRSM), and Global Digital Seismograph Network (GDSN). Absolute times (DF, BC) and differential times (BC-DF, AB-DF) as well as waveform data were examined. For all polar paths, differential times of BC-DF consistently yield residuals of 1.5 to 3.5 s larger than equatorial paths. Absolute DF time residuals exhibit anomalies of the same magnitude (1 to 4 s) with DF being early along polar paths while BC residuals have no obvious correlation with the differential time anomalies. DF phases appear multi-pathed for polar paths and are relatively simple for equatorial paths. These results coupled with previous studies suggest an axisymmetric anisotropy at the top of the inner core. ¿ American Geophysical Union 1993

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Abstract

Keywords
Seismology, Structure of mantle and core, Seismology, Body waves, Tectonophysics, Composition and state of the Earth's interior
Journal
Geophysical Research Letters
http://www.agu.org/journals/gl/
Publisher
American Geophysical Union
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