Edition: U.S. / Global

SCIENCE WATCH; Deep Quakes Show Speed Under Pressure

Published: August 13, 1991

CALIFORNIA scientists have found that deep earthquakes, those below 185 miles, develop as much as twice as fast as shallower ones. It is the first physical evidence that deep earthquakes are different from shallower ones.

Shallower earthquakes, like those along the San Andreas fault in California, occur when strain across a section of rock causes it to snap and slide in a shearing motion. Scientists have long known that deep earthquakes could not occur in the same way because of high pressures deep underground. But they were unable to find any evidence of a difference.

To investigate the mystery, geophysicists at the University of California at Davis analyzed data from a global network that monitors movements beneath the earth's surface. The geophysicists, Dr. Heidi Houston and Dr. Quentin Williams, found that deep earthquakes reach their peak motion in as little as half a second as against more than a second for shallower quakes. They published their findings in the current issue of Nature.

"It's the first real difference in the physical characteristics of deep earthquakes," said Dr. Williams. "We could have a very different beast than what we see closer to the surface."

The scientists say that deeper earthquakes may be caused by a mechanism called phase transformation. The quake would be precipitated by olivine, the most common material in the earth's crust, being squeezed to transform into the denser mineral known as spinel. When the transformation occurs, the heat that builds up or the change in materials could cause sections of crust to slip.