Seismic vibes gauge Earth's crust.

New seismic observations are filling in scientists' knowledge about the thickness of Earth's crust, especially in the Southern Hemisphere.

Because vibrations traveling through any material are deflected by its irregularities, scientists can use seismic waves traveling through our planet to examine its structure. Specifically, researchers can exploit ground motions produced by large earthquakes or deliberately detonated explosions, for example, to discern the boundary between Earth's rigid crust and its softer, more viscous mantle below.

Using data collected over the past 65 years, geophysicists have estimated the crust's thickness at almost 7,500 locations, says Shane T. Detweiler of the U.S. Geological Survey in Menlo Park, Calif. More than one-third of those estimates have been produced in the past 2 years, many from data gathered by scientific expeditions in the Southern Hemisphere.

About 50 percent of Earth's crust is less than 10 kilometers thick, Detweiler and his colleagues estimate. Most thin areas are created at midocean ridges and therefore lie beneath oceans, the researchers say.

The crust on continents and their submerged fringes, continental shelves, is generally from 20 km to about 50 km thick. Only in two locales, the Himalayas of southern Asia and the central Andes of South America, is the crust thicker than 70 km, says Detweiler.--S.P.