SEISMIC TOMOGRAPHY YIELDS INCREASINGLY DETAILED 3-D IMAGES OF EARTH'S UPPER MANTLE

Note: This release is embargoed until Tuesday, April 5, 1994, when Yu-Shen Zhang will present his research at a meeting of the Seismological Society of America at the Pasadena Convention Center.

PASADENA, CA--The symptoms: Skin that shifts, cracks, and erupts. The diagnosis: Stubborn lumps, hot and cold spots, and churnings under the surface. The complexion of a pitiful adolescent? No, it is the complexity of the upper layers of our planet earth, which seethe with enough unrest to raise mountains, spout volcanoes, and unleash devastating earthquakes.

Many geophysicists probe these layers, using seismic waves from earthquakes as their x-rays. One powerful technique, "seismic tomography," has flourished in the last decade as a means of exploring three-dimensional structures within the earth.

Yu-Shen Zhang of the Institute of Tectonics at the University of California, Santa Cruz, has emerged as a leading tomographer in recent years. Zhang, a postgraduate research geophysicist, uses a tomographic model called "RG5.5" that reveals subtle temperature variations in the earth's interior at a level of detail surpassing other models. For instance, RG5.5 has exposed:

Concentrated regions of hot and cold temperatures in the upper mantle, the rocky part of the planet directly beneath its thin crust. The hot regions underlie active areas such as midocean ridges, where earth's oceanic crust forms; subduction zones, where the crust is destroyed; and many of the isolated boils of volcanic activity known as "hot spots."

Marked differences in the 3-D structures beneath midocean ridges and hot spots.

Apparent connections between several hot spots and nearby midocean ridges.

"The more we can understand these structures, the better we can hope to predict future activity at the earth's surface," says Zhang. He will discuss RG5.5 and present several applications during a special session on global tomography on April 5 at a meeting of the Seismological Society of America in Pasadena.

Zhang developed RG5.5 for his Ph.D. thesis at the California Institute of Technology in 1991 with Toshiro Tanimoto, then at Caltech and now at UC Santa Barbara. They have used RG5.5 to study the earth with several colleagues, including Don Anderson of Caltech and Thorne Lay, director of UCSC's Institute of Tectonics.

Here's how RG5.5 works: Earthquakes of magnitude 5.5 or larger send seismic waves around the globe. Certain long-period vibrations travel along the planet's surface but also penetrate hundreds of kilometers into the upper mantle. The speed of each wave depends on several factors, including the temperature of the rock through which it travels. In general, the wave speeds up in cold patches, where rock is brittle, and slows down in warm patches, where rock flows more easily.

Computer analysis of many thousands of such seismic waves can tease out these velocity changes for slices of the earth's mantle. Although the relationship between temperature and velocity is not exact, a difference in speed of just 1 percent in one patch might translate to a temperature change of about 100 degrees celcius. By putting the slices together, the researchers end up with 3-D maps of hot and cold blobs.

Other tomographic techniques can detect such blobs only if they are at least 3,000 kilometers across. That is sufficient to show that the upper mantle has large swaths of warm and cold material. However, RG5.5 can image regions just 1,000 kilometers across and 80 kilometers thick. This effectively lets Zhang and his colleagues peel back the planet's crustal plates and look underneath at smaller features in the upper mantle that contribute to tectonic activity at the surface.

For instance, most models illuminate a close relationship between extremely cold blobs in the upper mantle--where seismic waves travel up to 5 percent faster than the global average--and the most stable parts of continents at the planet's surface, such as central Canada. It is harder to find a correspondence between hot regions (up to 5 percent slower) and both hot spots and midocean ridges. With RG5.5, Zhang and his colleagues find that correspondence quite clearly. The effect is especially dramatic in the mantle's uppermost 100 kilometers for the "ring of fire" around the Pacific Ocean and for midocean ridges in the Atlantic and Indian Oceans.

But there are differences in the structures under hot spots and midocean ridges. RG5.5 shows that the low-velocity areas under midocean ridges are very shallow: The hottest regions are less than 50 kilometers deep. According to Zhang, this may indicate that new crust forms at ridges as a result of passive spreading of the oceanic plates, not because of active upwelling of material from deep within the mantle. Not all geophysicists agree with that conclusion, he notes.

Under hot spots, such as Hawaii and Iceland, RG5.5 reveals much deeper warm patches--as deep as 200 kilometers. This is consistent with a theory that hot spots are the tops of narrow "plumes" of volatile material from the deep mantle that break through to the surface. However, RG5.5 does not see the plume columns themselves--if they exist, they are too small to resolve.

Intriguingly, the different structures under hot spots and midocean ridges appear linked. For instance, RG5.5 finds regions of slow velocity between Hawaii and the East Pacific Rise, thousands of kilometers away, and between several hot spots in the Atlantic Ocean and the Mid-Atlantic Ridge. Channels in the mantle of an unknown nature, says Zhang, may transport material from hot spots to ridges.

Another interesting aspect of RG5.5 is its ability, in Zhang's words, to "look back in time." Conditions at the surface of the earth change far more quickly than in the mantle, where heat takes tens of millions of years to dissipate. For instance, the Mid-Atlantic Ridge has gradually migrated westward since the Pangaea supercontinent broke up about 100 million years ago. Near the surface, RG5.5 shows that the hottest regions of the mantle are under today's ridge. Deeper in the mantle, however, heat from the ancient position of the Mid-Atlantic Ridge still is visible as a "ghost ridge" hundreds of kilometers to the east.

Zhang is now improving his model by including more earthquake records and longer-period seismic waves that penetrate deeper into the mantle. But even with finer resolution, he emphasizes, seismic tomography will remain just one of several techniques with which geophysicists can explore the roots of earth's unrest. "The inside of the earth is so complicated," he says, "that we can't expect one model to explain everything."

Editor's note: You may reach Zhang at (408) 459-3653 or yushen@earthsci.ucsc.edu.

This release is also available on UC NewsWire, the University of California's electronic news service. To access by modem, dial (209) 244-6971.

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