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Science 24 April 2009:
Vol. 324. no. 5926, pp. 499 - 502
DOI: 10.1126/science.1169499
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Reports

Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates

Hitoshi Kawakatsu,1,* Prakash Kumar,1,{dagger} Yasuko Takei,1 Masanao Shinohara,1 Toshihiko Kanazawa,1 Eiichiro Araki,2 Kiyoshi Suyehiro2

The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

1 Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
2 Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan.

{dagger} On leave from National Geophysical Research Institute, Uppal Road, Hyderabad-500007, India.

* To whom correspondence should be addressed. E-mail: hitosi{at}eri.u-tokyo.ac.jp

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