A seismologist at Washington University in St. Louis has made the first 3-D model of seismic wave damping - diminishing - deep in the Earth's mantle, and in doing so, has revealed the existence of an underground water reservoir at least the volume of the Arctic Ocean.
The research, to be published by the American Geophysical Union, provides the first evidence of water existing in the Earth's deep mantle.
Michael Wysession, Ph.D., Washington University professor of earth and planetary sciences in Arts and Sciences, working with former graduate student Jesse Lawrence, analyzed 80,000 shear waves from more than 600,000 seismograms and found a large area in Earth's lower mantle beneath eastern Asia where water is damping out, or attenuating, seismic waves from earthquakes.
The traditional method seismologists use to image the Earth is to measure the speed of seismic waves. It provides a sort of CAT scan of the Earth's core and mantle. Using wave speeds alone is a problem, however, because they cannot distinguish between temperature and composition variations.
Analyzing Damped-out Waves
An increasingly popular method, which Wysession used, is to analyze the way waves damp out from their source. Attenuation data tell seismologists how stiff a region is, which is a function of how hot it is and how much water it contains. Looking at the seismic wave speeds and attenuation at the same time can tell whether an anomaly is due to temperature or water.In analyzing the data, Wysession first saw large patterns associated with known areas where the ocean floor is sinking down into the earth. Beneath Asia, the fallen Pacific sea floor piles up at the base of the mantle. Right above that, he observed an "incredibly highly attenuating region, that is both very damping and slightly slow. Water slows the speed of waves a little. Lots of damping and a little slowing match the predictions for water very well," he reveals.
Previous predictions calculated that a cold ocean slab sinking into the earth at 745 miles to 870 miles beneath the surface would release water in the rock that would escape the rock and rise up to a region above it, but this phenomenon never previously was observed.
Beijing Anomaly
"That is exactly what we show here, the exact depth and high attenuation amounts right above it," Wysession says. "I call it the Beijing anomaly. Water inside the rock goes down with the sinking slab and it's quite cold, but it heats up the deeper it goes, and the rock eventually becomes unstable and loses its water. The water then rises up into the overlying region, which becomes saturated with water."If you combine the volume of this anomaly with the fact that the rock can hold up to about 0.1 percent of water, that works out to be about an Arctic Ocean's worth of water."
