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NEW TECHNIQUE PAINTS PICTURE OF TECTONIC PLATES PLUNGING BENEATH MOUNTAINS IN SOUTHERN CALIFORNIA

* This press release is embargoed until 6 p.m. EDT Thursday, June 29, 1995. The study will appear in the June 30 issue of the journal Science.

SANTA CRUZ, CA--Using the waves from earthquakes as sonar-like "pings" to probe below the earth's crust, a seismologist has learned surprising new details about giant slabs of rock that dive into the planet at the roots of southern California's dramatic Transverse Ranges.

The study seems to verify what researchers suspected: A kink in the San Andreas fault near Los Angeles forces parts of two tectonic plates to collide and plummet nearly straight down. Evidence suggests that one of these slabs, the Pacific plate, is sheared and rended apart as it dives, thanks to severe stresses created by the bend in the fault. But the other slab, the North American plate, appears thicker and more mature. The stresses disrupt the plate much less severely during its vertical journey.

These findings do not significantly change estimates of earthquake hazard in the region. However, the imaging technique does offer seismologists a new way to explore the inner workings of slabs, which cause some of the world's largest quakes as they sink into the earth. Other methods can locate these slabs, but only as undefined blobs of cold rock; the new method begins to see structures within the blobs that previously escaped notice.

"The best analogy is the device that hangs off a boat to scan for fish in the water," says author Justin Revenaugh, assistant professor of earth sciences at the University of California, Santa Cruz. "Earthquakes send out a complex ping, and that ping scatters off changes in material properties at various depths in the earth. Instruments at the surface pick up the echoes. This technique reveals small-scale features that other methods just can't see."

Revenaugh, a member of the Crustal Imaging Laboratory at the UC Santa Cruz Institute of Tectonics, published his work in the June 30 issue of the journal Science.

Looming over the study site are the Transverse Ranges, which Revenaugh calls "some of the most spectacular topography in North America." Marching east from the sea near Santa Barbara, the mountain ranges stretch past L.A., San Bernardino, and into the Joshua Tree National Monument. They include the Santa Ynez Mountains, San Bernardino Mountains, and the San Gabriel Mountains, which soar so abruptly north of Los Angeles.

Unlike most other coastal mountain ranges in California, which mirror the northwest-trending San Andreas fault, these mountains run east to west--transverse to the rest. The culprit is a sweeping 20-degree bend in the San Andreas. The bend makes it impossible for the Pacific and North American plates to simply lurch past each other, as they do elsewhere. Rather, the uppermost part of the planet, the crust, gets twisted and piled up as it careens (in geologic time) around the curve.

"There are bends in other strike-slip faults, but this is far and away the largest," Revenaugh says. "The result is that these are among the fastest growing mountains in the world." The mountains also crumble quickly, he notes, in a classic competition between tectonics and erosion.

Earth's crust is brittle, so it fractures as it gets compressed in the bend. The fractures trigger events such as the 1994 Northridge earthquake. But below the crust in the upper mantle, rock tends to flow, not break. Because of the bend, the bottom parts of the Pacific and North American plates can't flow past each other. Instead, they sink in the process known as subduction.

Seismologists see the subduction by charting the speeds of seismic waves, which travel faster through cold rock. As a slab subducts, it stays colder than the mantle around it for millions of years. Beneath the Transverse Ranges, some seismic waves move 3.5 percent faster, implying rocks colder by about 1,000 degrees F. However, this method--called seismic tomography--cannot discern features inside the slab, which is about 40 miles wide.

To get finer resolution, Revenaugh adapted a technique that researchers have used to monitor nuclear tests. It models the upper part of the earth as a swarm of points that scatter energy, like fish in the water scatter sonar. Quakes from across the globe send waves through the swarm. Seismographs receive the echoes; their intensity and timing tells Revenaugh which points scatter most strongly. Pronounced scattering means that some abrupt change in the earth exists at that point. The technique unmasks those blips at a scale of a mile or two.

Revenaugh's data encompassed 120 Pacific Rim quakes from 1980 to 1993, measured by 232 stations in the Southern California Seismographic Network. He saw a lot of seismic energy scattered from the southern part of the slab under the Transverse Ranges, corresponding to the Pacific plate. This was most clear at depths between 50 and 125 miles.

The scattering, Revenaugh believes, arises when cold parts of the slab are exposed to the warmer mantle relatively quickly. That would happen if the slab gets disrupted on its way down by the tortured stresses imposed by the sinuous fault at the surface. Like a sliced-and-diced sausage with a hot middle, cold and warm bits of slab would mingle, reflecting seismic waves.

Strikingly, the northern part of the slab, the remnants of the North American plate, scattered virtually no energy. Revenaugh offers a theory: "The Pacific plate moves through the Salton Sea area, where there is hot material ascending from the mantle. That warms up the plate and rejuvenates it, effectively making it thinner. When it goes down, it's easier to shear that plate and mess it up." In contrast, the North American plate seems colder and thicker, so tectonic shearing does not expose its inner layers to the warm mantle. As a result, the temperature contrasts are much less severe.

Revenaugh hopes to extend his technique to other fault areas and to refine it by using different types of seismic waves for his analysis.

Editor's note: You may reach Revenaugh at (408) 459-3055 or .

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