TY - JOUR
T1 - Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth
AU - Ohtaki, Toshiki
AU - Kaneshima, Satoshi
AU - Ichikawa, Hiroki
AU - Tsuchiya, Taku
N1 - Funding Information:
We thank Shin-ichi Takehiro and Shigeo Yoshida for helpful discussion and Nozomu Takeuchi for providing the direct solution method program (DSM; Takeuchi et al., 1996) to calculate synthetic waveforms. We also thank the staff of the Hi-net for providing access to the seismic waveforms, which can be obtained from the National Research Institute for Earth Science and Disaster Prevention (NIED), Japan (http://www. hinet.bosai.go.jp/). We used the Generic Mapping Tools (GMT; Wessel & Smith, 1998), the TauP Toolkit (Crotwell et al., 1999), and the Seismic Analysis Code (SAC; Goldstein et al., 2003) for processing data and making figures. Constructive comments by V. Cormier and an anonymous reviewer improved this paper. We thank Rebecca Stewart from Edanz Group (www.edanzediting. com/ac) for editing a draft of this manuscript. This research was sup ported by JSPS KAKENHI grants JP15H05832 and JP15H05834, and by the Joint Usage/Research Center PRIUS, Ehime University, Japan.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
PY - 2018/12
Y1 - 2018/12
N2 - 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.
AB - 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.
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U2 - 10.1029/2018JB015857
DO - 10.1029/2018JB015857
M3 - Article
AN - SCOPUS:85060225673
VL - 123
SP - 10,903-10,917
JO - Journal of Geophysical Research
JF - Journal of Geophysical Research
SN - 0148-0227
IS - 12
ER -