Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth

Toshiki Ohtaki, Satoshi Kaneshima, Hiroki Ichikawa, Taku Tsuchiya

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)


The Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light-element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower-velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04 km/s, which corresponds to 0.8 wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light-element concentration beneath Australia.

Original languageEnglish
Pages (from-to)10,903-10,917
JournalJournal of Geophysical Research: Solid Earth
Issue number12
Publication statusPublished - Dec 2018

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology
  • Geophysics
  • Space and Planetary Science
  • Earth and Planetary Sciences (miscellaneous)


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