Magazine article Oceanus

Listening Closely to 'See' into the Earth

Magazine article Oceanus

Listening Closely to 'See' into the Earth

Article excerpt

A new national facility of cutting-edge seafloor seismographs probes Earth's interior

Chemists can monitor reactions in test tubes in their labs. Ecologists can go into the field to make observations. But Earth scientists interested in the structure of Earth's deep interior don't have the luxury of seeing their subject close at hand. Without a way to travel through the Earth, we have had to rely on ways to "see" Earth's structure from a great distance.

To accomplish that, our method employs sound, rather than sight. Whenever an earthquake occurs, scientists can tune in and "listen" to it. We use seismometers and seismographs that measure and record earthquake-generated seismic waves that travel along Earth's surface and through its interior. By analyzing these waves, we can infer a great deal about the characteristics of the materials the waves are traveling through. (See "Earthquakes and Seismic Waves," page 18.)

Within the last decade, two factors have helped make seismology the preeminent tool for determining Earth's hidden interior structure. First, more permanent and temporary seismographs to record seismic waves have been distributed around the globe. Second, international policies have made archives of seismic data freely and widely available on the Internet to any investigator.

Today, excitement and anticipation are growing because of new generations of seismographs designed for use in the oceans. These new instruments are being built at Woods Hole and two other oceanographic institutions, and they will comprise a new national pool of instruments for use by the scientific community. This new national facility will let us monitor more of the planet with the precision we've long wished for, and thus enhance our ability to answer fundamental questions about our planet.

Mapping Hawaii's plume

Until now, almost all seismic observatories have been located on land, which accounts for only 30 percent of Earth's surface. The lack of evenly distributed seismograph coverage in the oceans has limited our understanding of Earth's structure at both regional and global scales.

On a regional scale, consider the task of understanding the structure of the upper mantle beneath the small geographic region of Hawaii. Hawaii is thought to be the surface expression of a buoyant plume of hot rock in the upper mantle, possibly originating at the core-mantle boundary at a depth of about 2,900 kilometers (1,800 miles). We don't know the width, temperature, or depth of the plume.

To find out, we will need to record seismic waves that travel from a known source, through the plume, to receiving seismographs. But the waves must travel through as little as possible of the rest of the Earth, so that we can attribute any anomalies or alterations in the waves entirely to their passing through the plume.

Hawaii's landmass is so small, however, that land seismic stations cannot be positioned in places to intercept waves that travel through the plume at depth. So we haven't been able to measure how deep the plume is. Nor are the islands close enough together to let us accurately measure the plume's diameter. The only way to determine the plume's structure is by placing seismographs in appropriate locations to intercept waves that have traveled only through the plume-and those locations are on the seafloor.

Seismic waves through the Earth

To probe Earth's interior on a more global scale, the most useful earthquakes to study are large and deep. But such earthquakes do not occur uniformly over the planet. They happen only in specific geographic areas called subduction zones, where tectonic plates plunge back into the mantle.

Seismic waves from these earthquakes radiate through Earth's deep interior to the other side of the globe. But because existing land-based seismic stations aren't distributed uniformly on Earth's surface, we don't receive any information from many areas. …

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