title: Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates.
abstract: 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.
authors.affiliation: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. hitosi@eri.u-tokyo.ac.jp
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Kawakatsu
authors.forename: Prakash
authors.initials: P
authors.lastname: Kumar
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Takei
authors.forename: Masanao
authors.initials: M
authors.lastname: Shinohara
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Kanazawa
authors.forename: Eiichiro
authors.initials: E
authors.lastname: Araki
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Suyehiro