The material erupting at volcanoes near the Tonga and Fiji islands has flowed many hundreds of miles, originating deep in the earth near the Samoa islands to the north. The work is supported by the National Science Foundation. Smith examined seismic anisotropy, a phenomenon where the velocity of a seismic wave depends on the vibration direction of the wave. This variation in velocity is usually due to the alignment of rock crystal orientations. The flow patterns in the earth align the rock crystals deep in the mantle.
Scientists can thus determine the direction of mantle flow by mapping the anisotropy directions.
Seismic shear waves have vibrations that are perpendicular to the direction they are traveling. If the wave is propagating vertically, the vibration direction is horizontal. The vibration in the fast direction travels faster and arrives at the seismograph sooner than the vibration in the slow direction. Smith studies the splitting of the shear wave arrival to determine the direction of mantle flow. Most previous studies of the Earth's mantle flow relied upon seismometers placed on land.