Using Unique Seismometer Array, Seismologists Map Mantle Flow

Using a unique array of sea floor seismometers deployed in 1994, Gideon Smith, Ph.D., senior research scientist, and Douglas Wiens, Ph.D., professor of earth and planetary sciences in Arts & Sciences at Washington University have mapped out how the mantle flow pattern varies near the Tonga subduction zone, where a plate on the earth's surface descends into the mantle. The results of this study help scientists to understand the path followed by material that is erupted at volcanoes. 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 research was published in the April 27, 2001 issue of Science. 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. As the wave travels vertically, the horizontal vibration becomes split -- one part vibrates in the fast direction, the other in the slow direction. 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. Such studies are limited because they cannot map mantle flow beneath the ocean, which covers two-thirds of the earth's surface. Smith's work, on the other hand, makes use of the most extensive array of ocean bottom seismometers deployed in such a location.

Smith's results match geochemical studies of the region that compared ratios of helium isotopes to understand mantle flow. The helium isotopes suggest that the mantle material originated beneath the Samoa islands, about 400 miles to the north. The flow direction inferred from the shear wave splitting suggests that the material flowed southward from Samoa to Tonga through a gap in the subducting Pacific plate. This is the first time that such geochemical studies have been confirmed by extensive seismic mapping of the flow direction.

"The most exciting things to come from this study is that the anisotropic fast direction rotates from an east-west direction far from the Tonga arc to north-south flow close to the trench," Smith said. "Unlike many seismic anisotropy studies, in this work we can clearly see that the shear wave splitting results from mantle flow in the Tonga region. This observation is particularly interesting as it correlates well with the mantle flow directions determined from geochemical studies of helium isotopes."