Small vibrations lend new insights on Earth's crust

With further refinement, the approach is expected to lead to vast improvements in a range of earth-science activities - from generating more-detailed maps of the crust and assessing earthquake risks to monitoring the ban on nuclear-weapons tests.

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Ordinarily, scientists build 3-D images of the planet's rocky skin and molten interior by analyzing vibrations from earthquakes as they pass through the planet. In some cases, they have detonated explosives to generate a seismic signal. Vibrations from other sources, such as ocean waves crashing onto a coast or the subtle changes in air pressure as storms pass, were widely viewed as seismological "garbage."

But a US-French team is now using that garbage to build detailed maps of major geological features in the crust beneath California. The team's work, described in the current edition of the journal Science, relies on seismographs used in a separate project, called USArray. Scientists in the United States are now deploying nearly 3,000 seismographs to probe the continent's crust. A handful of them will be installed permanently as a backbone to a national seismic network; the rest will lead a more nomadic existence over the project's 10-year life span.

The crustal-imaging team gathered a month's worth of seismic "noise" from USArray stations in California. Some of this noise, such as the thump of waves crashing into the coast, has a short period. It creates a seismic "wave" every five to 10 seconds. Other noise, such as the subtle seismic waves generated by changes in atmospheric pressure as storms pass, have longer periods, ranging from tens to hundreds of seconds.

These waves tend to get trapped near Earth's surface - the longer the period, the deeper the layer through which they travel. These short and long-period surface waves also respond differently to subsurface rock and sediment formations, allowing researchers to build their maps of the crust's underlying structure. The beauty of the approach, the researchers say, lies in the adjustable level of detail. If you want to see smaller structures, just pack more seismographs into the area. The approach is expected to plumb the crust to depths of 60 miles or more.

The new imaging technique also promises to allow geophysicists to exploit the USArray 24/7, rather than wait for an earthquake to happen. This would lead to a much richer trove of data than originally envisioned. Once the data are archived, "you'll no longer have to ask: How will I gather the data? The data will already be there," explains David Simpson, president of Incorporated Research Institutions for Seismology, a Washington-based consortium focused on studying Earth's interior.

For areas in the US beyond the West Coast, "we really don't understand the earth's crust - how variable it is, or how strong it is," says Michael Ritzwoller, a geophysicist at the University of Colorado at Boulder and member of the group.

That variability holds clues to how the crust formed and the processes that altered it over billions of years. The information also is expected to improve estimates of earthquake hazards in the East, where quakes are less frequent, but potentially more devastating, and where much less is known about the crust.