LAUSR

Energy loss varies with frequency in the lithosphere but not in the asthenosphere Ocean basins record the life history of a tectonic plate—its creation at a mid-ocean ridge, its thickening over time, and its consumption at a subduction zone. The movement of tectonic plates is possible because the lithosphere, Earth's stiff outermost shell, slides on top of a weak asthenosphere. Despite its fundamental role in facilitating plate tectonics, the nature of the lithosphere-asthenosphere boundary is poorly understood. The asthenosphere is on average warmer than the lithosphere, but the temperature contrast alone may not provide the necessary viscosity reduction. Previous work has also proposed a dehydrated lithosphere and damp asthenosphere (1), and a solid lithosphere and partially molten asthenosphere (2). On page 1593 of this issue, Takeuchi et al. (3) present an analysis of aftershocks of the 2011 Tohoku earthquake and show how the attenuation of seismic waves has a different frequency response in the lithosphere versus the asthenosphere.