By Sid Perkins
Analyses of seismic waves that travel deep within Earth may resolve a decades-old debate about the thickness of the planet’s continents.
Some studies have suggested that the major landmasses in Earth’s rigid outer shell–including the planet’s crust and the upper layers of its mantle–are between 200 and 250 kilometers thick. To arrive at those estimates, researchers considered phenomena such as heat flow from within the planet.
Other investigations, particularly analyses of seismic waves traveling through and just under those landmasses, have indicated that the continents may be up to 400 km thick.
Now, Barbara Romanowicz, a geophysicist at the University of California, Berkeley, and her colleagues contend that some of those seismic-wave studies missed an important factor known as seismic anisotropy, a phenomenon in which some earthquake waves travel more quickly than others. Specifically, earthquake waves with side-to-side ground motions tend to travel faster through Earth’s shell than do those with up-and-down movements, Romanowicz explains.
With their new model, which incorporates seismic anisotropy and is described in the April 17 Nature, Romanowicz and her coworkers estimate that Earth’s continents are 200 to 250 km thick. Previous estimates typically had considered only the faster, side-to-side seismic waves, Romanowicz says.
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