Researchers led by Caroline Beghein, assistant professor of Earth, planetary and space sciences in UCLA's College of Letters and Science, used a technique called seismic tomography to study the structure of the Pacific Plate — one of eight to 12 major plates at the surface of the Earth. The technique enabled them to determine the plate's thickness, and to image the interior of the plate and the underlying mantle (the layer between the Earth's crust and outer core), which they were able to relate to the direction of flow of rocks in the mantle.

http://newsroom.ucla.edu/portal/ucla/new-insights-on-plate-tectonics-250174.aspx