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Seismic Structures in the Earth’s Inner Core Below Southeastern Asia

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Abstract

Documenting seismic heterogeneities in the Earth’s inner core (IC) is important in terms of getting an insight into its history and dynamics. A valuable means for studying properties and spatial structure of such heterogeneities is provided by measurements of body waves refracted in the vicinity of the inner core boundary (ICB). Here, we investigate eastern hemisphere of the solid core by means of PKPBC–PKPDF differential travel times that sample depths from 140 to 360 km below its boundary. We study 292 polar and 133 equatorial residuals measured over the traces that probe roughly the same volume of the IC in both planes. Equatorial residuals show slight spatial variations in the sampled IC volume mostly below the level of 0.5 %, whereas polar residuals are up to three times as big, direction dependent and can exhibit higher local variations. The measurements reveal fast changes in seismic velocity within a restricted volume of the IC. We interpret the observations in terms of anisotropy and check against several anisotropy models few of which have been found capable of fitting the residuals scatter. We particularly quantify the model where a dipping discontinuity separates fully isotropic roof of the IC from its anisotropic body, whereas the depth of isotropy–anisotropy transition increases in southeast direction from 190 km below Southeastern Asia (off the coast of China) to 350 km beneath Australia. Another acceptable model cast in terms of localized anisotropic heterogeneities is valid if 33 largest polar measurements over the rays sampling a small volume below Southeastern Asia and the rest of polar data are treated separately. This model envisages almost isotropic eastern hemisphere of the IC at least down to the depth of 360 km below the ICB and constrains the anisotropic volume only to the ranges of North latitudes from 18° to 23°, East longitudes from 125° to 135° and depths exceeding 170 km. The anisotropy strength in either model is about 2 %. Further effective pursuit of the models presents challenges in terms of resolution and coverage and basically requires a significant dataset extension.

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Acknowledgments

Authors thank the anonymous reviewer for helpful comments. This study was partially supported by the Russian Foundation for Basic Research Grant No. 14-05-00447 and personal grant to Dr. Krasnoshchekov from Academia of Finland. We thank IRIS DMC for providing a part of other seismic data. Authors also acknowledge Paul Wessel and University of Hawaii for General Mapping Tools that helped to make the location map. The POLENET/LAPNET project was a part of the International Polar Year 2007–2009 and a part of the POLENET IPY consortium. Organizations participated in the POLENET/LAPNET project are: (1) Sodankylä Geophysical Observatory of the University of Oulu (Finland), (2) Institute of Seismology of the University of Helsinki (Finland), (3) University of Grenoble (France), (4) University of Strasbourg (France), (5) Institute of Geodesy and Geophysics, Vienna University of Technology (Austria), (6) Geophysical Institute of the Czech Academy of Sciences, Prague (Czech republic), (7) Institute of Geophysics ETH Zürich (Switzerland), (8) Institute of Geospheres Dynamics of the Russian Academy of Sciences, Moscow (Russia), (9) The Kola Regional Seismological Centre, of the Russian Academy of Sciences (Russia), (10) Geophysical Centre of the Russian Academy of Sciences, Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (Russia), (11) Swedish National Seismological Network, University of Uppsala (Sweden), (12) Institute of Solid Earth Physics, University of Bergen (Norway), (13) NORSAR (Norway), (14) University of Leeds (UK). Equipment for the temporary deployment was provided by RESIF-SISMOB, FOSFORE, EOST-IPG Strasbourg Equipe sismologie (France), seismic pool (MOBNET) of the Geophysical Institute of the Czech Academy of Sciences (Czech republic), instrument pool of Sodankylä Geophysical Observatory (Finland), Institute of Geosphere Dynamics of RAS (Russia), Institute of Geophysics ETH Zürich (Switzerland), Institute of Geodesy and Geophysics, Vienna University of Technology (Austria), University of Leeds (UK). Funding agencies that provided support for organization of the experiment are: Finland: The Academy of Finland (Grant No. 122762) and University of Oulu. France: BEGDY program of the Agence Nationale de la Recherche, Institut Paul Emil Victor and ILP (International Lithosphere Program). Task force VIII. Czech Republic: Grant No. IAA300120709 of the Grant Agency of the Czech Academy of Sciences Russian Federation: Russian Academy of Sciences (Programs No. 5 and No. 9). POLENET/LAPNET Working Group: Elena Kozlovskaya (1), Helle Pedersen (3), Jaroslava Plomerova (6), Ulrich Achauer (4), Eduard Kissling (7), Irina Sanina (8), Teppo Jämsen (1), Hanna Silvennoinen (1), Catherine Pequegnat (3), Riitta Hurskainen (1), Robert Guiguet (3), Helmut Hausmann (5), Petr Jedlicka (6), Igor Aleshin (10), Ekaterina Bourova (3), Reynir Bodvarsson (11), Evald Brückl (5), Tuna Eken (6), Pekka Heikkinen (2) Gregory Houseman (14), Helge Johnsen (12), Elena Kremenetskaya (9), Kari Komminaho (2), Helena Munzarova (6) Roland Roberts (11), Bohuslav Ruzek (6), Hossein Shomali (11), Johannes Schweitzer (13), Artem Shaumyan (8), Ludek Vecsey (6), Sergei Volosov (8).

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Authors and Affiliations

  1. Institute of Dynamics of Geospheres, Russian Academy of Sciences, Leninsky pr. 38 korp. 1, Moscow, 119334, Russia

    Dmitry Krasnoshchekov, Petr Kaazik & Vladimir Ovtchinnikov

  2. Sodankylä Geophysical Observatory and Oulu Mining School, University of Oulu, POB 3000, 90014, Oulu, Finland

    Elena Kozlovskaya

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  1. Dmitry Krasnoshchekov
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Correspondence to Dmitry Krasnoshchekov.

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Krasnoshchekov, D., Kaazik, P., Kozlovskaya, E. et al. Seismic Structures in the Earth’s Inner Core Below Southeastern Asia. Pure Appl. Geophys. 173, 1575–1591 (2016). https://doi.org/10.1007/s00024-015-1207-6

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