TY - JOUR
T1 - Independent estimate of velocity structure of Earth's lowermost outer core beneath the northeast Pacific from PKiKP - PKPbc differential traveltime and dispersion in PKPbc
AU - Ohtaki, Toshiki
AU - Kaneshima, Satoshi
N1 - Funding Information:
We thank Nozomu Takeuchi for providing the program to calculate synthetics, the staff of the Hi-net, F-net, and J-array, and Ryohei Iritani for providing their model. The Hi-net data were obtained from the National Research Institute for Earth Science and Disaster Prevention (NIED), Japan (http://www.hinet.bosai.go.jp/?LANG=en). The F-net data were also obtained from NIED (http://www.fnet.bosai.go.jp/top.php?LANG=en). The J-array data were obtained from the Earthquake Research Institute, University of Tokyo, Japan (http://jarray.eri.u-tokyo.ac.jp/). Constructive comments by two anonymous reviewers and the Associate Editor improved this paper. We used the Generic Mapping Tools (GMT) [Wessel and 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. This research was supported in part by JSPS KAKENHI grant 15H05832.
PY - 2015/11
Y1 - 2015/11
N2 - The presence of a low-velocity layer at the base of Earth's outer core has been proposed. However, the seismic profile of the basal layer indeed has been poorly constrained. In previous seismic studies the model parameters of the layer are substantially nonunique and there are tradeoffs between the seismic velocity of the layer and the properties of the mantle and inner core. A more tightly constrained profile of the layer helps further examine the composition and dynamics of the layer. In this study we obtained the P wave velocity profile of the basal layer beneath the northeast Pacific based on two new seismic observations by analyzing seismograms of the Hi-net in Japan. The new observations are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the Earth: (1) the frequency dispersion in P waves that graze or are diffracted at the inner core boundary (PKPbc and PKPc-diff) and (2) differential traveltimes between the P waves reflected from the inner core boundary (PKiKP) and those that turn above the boundary (PKPbc). The resulting velocity model of the lowermost outer core (called "F layer velocity model for the Western Hemisphere" (FVW)) has P wave velocities that lie between those of AK135 and the preliminary reference Earth model (PREM), and a velocity gradient that is slightly gentler than that of PREM. Models with a uniform P wave velocity value within the layer are not supported by the observations for the region investigated, which appears not to support the presence of a thick basal layer that is Fe rich and dense there.
AB - The presence of a low-velocity layer at the base of Earth's outer core has been proposed. However, the seismic profile of the basal layer indeed has been poorly constrained. In previous seismic studies the model parameters of the layer are substantially nonunique and there are tradeoffs between the seismic velocity of the layer and the properties of the mantle and inner core. A more tightly constrained profile of the layer helps further examine the composition and dynamics of the layer. In this study we obtained the P wave velocity profile of the basal layer beneath the northeast Pacific based on two new seismic observations by analyzing seismograms of the Hi-net in Japan. The new observations are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the Earth: (1) the frequency dispersion in P waves that graze or are diffracted at the inner core boundary (PKPbc and PKPc-diff) and (2) differential traveltimes between the P waves reflected from the inner core boundary (PKiKP) and those that turn above the boundary (PKPbc). The resulting velocity model of the lowermost outer core (called "F layer velocity model for the Western Hemisphere" (FVW)) has P wave velocities that lie between those of AK135 and the preliminary reference Earth model (PREM), and a velocity gradient that is slightly gentler than that of PREM. Models with a uniform P wave velocity value within the layer are not supported by the observations for the region investigated, which appears not to support the presence of a thick basal layer that is Fe rich and dense there.
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U2 - 10.1002/2015JB012140
DO - 10.1002/2015JB012140
M3 - Article
AN - SCOPUS:84955204653
VL - 120
SP - 7572
EP - 7586
JO - Journal of Geophysical Research
JF - Journal of Geophysical Research
SN - 0148-0227
IS - 11
ER -