A-blast lights up core-mantle boundary.

It was a shot heard halfway round the world. In May 1992, an underground nuclear test in China punched a 1-second seismic pulse thousands of miles into the Earth. Meanwhile, scientists on the other side of the Pacific Ocean had their "ears" to the ground, hoping to record telltale changes in the pulse as it grazed the boundary between Earth's rocky mantle and its molten, metallic core.

The scientists' ears consisted of a continent-spanning network of 1,062 seismometers scattered among 14 earthquake-listening sites in Canada and the United States, forming the most geographically widespread network ever focused on a single source of seismic waves. Using hundreds of measurements; from these sites, John E. Vidale. and Harley M. Benz of the U.S.. Geological Survey in Menlo Park, Calif., have identified a distinct island of material that lies some 2,700 kilometers be- neath northern Alaska, they report in the Feb. 11 NATURE. The newly identified structure measures approximately 300km across and 130 km thick, the smallest feature yet pinpointed near the important coremantle boundary, Vidale says. The scientists could detect this relatively small structure because seismic waves passing through it speeded up slightly, indicating a difference in temperature or chemical composition between it and the surrounding mantle material.

Vidale attributes the improved measurements to the nature of the Chinese test explosion and to the number of widely dispersed seismometers listening to it. The blast, .a short, powerful pulse of known geographical origin, enables scientists to identify more precisely details of the core-mantle boundary "We have a high-resolution picture of one [small] place now, using this very sharp, powerful explosion:' Vidale says.

By combining measurements of earthquakes, researchers had previously shown that the 250- to 300-km-thick layer over the core-mantle boundary varies significantly in its physical properties (SN: 6/11/88, p. 378). Geoscientists have turned their attention to these variations, or heterogeneities, seeking to understand more clearly the interactions between Earth's mantle and its core.

Geoscientists aren't sure whether the heterogeneities stem from chemical reactions between the core and mantle or from the downward flow of cooler, denser material from the upper mantle. But resolving this question may help them explain some important geologic processes assumed to originate at the core-mantle boundary, Vidale says.

"That we can actually identify one structure will hopefully help us to figure out what this heterogeneity is:' he notes. "It's really the highest resolution picture [of a structure near the core-mantle boundary] so far."