Mysterious pulsations of the earth — unnoticed until the advent of Global Positioning System receivers but then detected at several seismic zones — have puzzled scientists for several years. Now a Utah State University researcher may have found the cause.

The researcher is Anthony R. Lowry. His paper, "Resonant slow fault slip in subduction zones forced by climatic load stress," was published in the Aug. 17 issue of the scientific magazine Nature.

Lowry is new to USU, having arrived about a week ago to begin work as an assistant professor of geology with a speciality in geophysics; he's starting a geophysics program at the Logan school.

He earned his Ph.D. at the University of Utah and did post-doctorate work at the University of Colorado, Boulder, where he wrote the paper.

Geologists have been tracking subtle movements of earthquake fault zones through GPS satellite receivers placed on the ground. To their surprise, they detected what are called "silent earthquakes" or "slow slip events" at more than a half dozen localities around the world.

"All of us at about the same time started to realize there's something going on in these data we didn't expect," Lowry said in a Deseret Morning News interview.

For most fault zones, the ground displays a "slow, steady motion" that corresponds to the build-up of strains that are released in an earthquake, he said. But some places, including the region near Puget Sound, a site in Japan, a zone in Mexico and other places, showed different patterns.

There, a seismic plate may show the expected slow, omnidirectional motion most of the time. But for a period the movement will be in another direction. Then the normal motion resumes. Depending on the site, the backward motion can last from about a week to about a year.

"But just for that short period of time it'll move backwards. And this was a surprise to us," he said.

If the motion were fast, it would be an earthquake. But it's so slow that no damage results and nobody would even know it took place if GPS receivers didn't record minute changes in position.

What is startling is that the unusual motion happens regularly. The Puget Sound slip happens every 14.7 months, give or take 1.2 months, according to the paper. At Guerrero, Mexico, it's 12 months, plus or minus 0.3 month. It's every six months in Japan's Shikoku area. In northern California, the fluctuations occur at about 10.9-month intervals.

A "slow slip event" in Alaska took place from 1999 to 2001. "That was a really big one," he said. "That released energy equivalent to a magnitude 7.2 earthquake.

"But it was completely harmless."

Lowry focused much of his work on the Mexican events. "Down in southern Mexico we find that these things happen, very regularly, around January-February," he said.