UC Berkeley researchers reveal deep earth dynamics with "CT scans"

Researchers with the University of California, Berkeley (UC Berkeley), have used the largest array of seismometers ever deployed on the seafloor and hundreds of others in the continental United States to create "CT scans" of the Juan de Fuca plate and part of the earth's mantle directly below it.

The massive tectonic plate is grinding under the North America continent along an 800-mile ( 1,290-kilometer) swath that runs from northern California in the United States to Vancouver Island in Canada, known as the Cascadia subduction zone.

The 3-D imaging process, known as seismic tomography, has revealed with unprecedented clarity a huge, buoyant, sausage-shaped region of the upper mantle, or asthenosphere, pressing up on the oceanic plate from about 100 miles (160 kilometers) under. The imaging casts new light on the competing hypotheses about the drivers of plate tectonics, a dynamic earth process that has been studied for more than 50 years but is still poorly understood.

Different evidence has led to three different plate movement scenarios: either the plates are pushed from mid-ocean ridges; or they are pulled from their subducting slabs; or their movement is driven by the drag of the viscous mantle material that lies directly below, said a news release from UC Berkeley.

The new research suggests that the third scenario does not apply to the Cascadia subduction zone. Rather, it reveals that a distinct, thin -- and difficult to observe -- layer separates the plate from the mantle beneath, at least in the Cascadia subduction zone. The layer acts as a kind of berm that the plate rolls over before descending beneath the continent, says UC Berkeley seismologist Richard Allen, leader of the research and co-author of a paper appearing in the journal Science this week.

"What we observe is an accumulation of low-viscosity material between the plate and the mantle. Its composition acts as a lubricant, and decouples the plate's movement from the mantle below it," Allen, director of the Berkeley Seismological Laboratory and professor and chair of Earth and Planetary Science at UC Berkeley, was quoted as saying by the release, adding that the plates may move independently of the mantle below.

The researchers obtained 3-D images of the earth's interior by measuring how differences in shape, density, rock type and temperature affect the path, speed and amplitude of seismic waves traveling through the planet from an earthquake.

Coupled with data from the largest-scale ocean-floor deployment to complement the onshore data already available, researchers succeeded in generating the best images of the region so far.

Much as in medical CT scans, which use X-rays as the energy source, in seismic tomography, computers process differences in energy from seismic waves measured at the receiving end to infer interior 3-D detail.

"It is the motion of the plates that causes earthquakes," Allen said. "Models like this help us understand that linkage so we can be better informed of the coastal hazards."

"First though, we need to learn if what we find here is typical of subduction zones across the planet, or if it is unique for some reason," he added.

The researchers expect that the new study will help refine models of plate tectonic dynamics, aiding the long-range effort to understand the connection between tectonics and earthquakes. Endi