Scientists have agreed that strong earthquakes produce aftershocks within the region, but many did not believe that temblors of magnitude 7 and above could trigger smaller quakes on the other side of the world - until now.

A recent study of major earthquakes since 1992 - the kind that can generate widespread damage - revealed that such events routinely trigger smaller jolts around the planet, including areas that are not prone to quakes.

One example was the 1992 Landers earthquake that was a magnitude 7.3 and shook up the Southern California desert. The surprise was that it caused small jolts as far away as Yellowstone National Park - some 800 miles away.

"Earthquakes have been thought of to be very isolated events," Aaron Velasco, lead author of the study and University of Texas at El Paso associate professor of geological sciences, said. "Now we're seeing a relationship between big earthquakes and smaller events around the world."

Velasco and Stephen Hernandez, a UTEP undergraduate student, analyzed 15 major earthquakes stronger than magnitude 7, and found that at least 12 of them triggered small quakes hundreds and even thousands of miles away.

The pair was joined on the study by seismologists Kris Pankow of the University of Utah and Tom Parsons of the U.S. Geological Survey in Menlo Park, Calif. The team's analysis was published online Sunday, May 25, 2008, in the journal, Nature Geoscience.

The team analyzed data from more than 500 seismic recording stations five hours before and five hours after earthquakes that registered more than 7 on the "moment magnitude" scale, which scientists say is the most accurate scale for large earthquakes.

The data was obtained from the Incorporated Research Institutions for Seismology, a consortium of universities, from 1992 through 2006. It included the Landers quake, the magnitude 7.9 Denali fault quake in Alaska in 2002, and the magnitude 9 Sumatra-Andaman Islands quake near Indonesia in 2004.

That last quake generated a catastrophic tsunami that washed out villages and was responsible for many of the quake's 227,898 deaths in 11 countries that share a coastline with the Indian Ocean.

Scientists previously noted that those three major quakes triggered not only nearby aftershocks, but small quakes at great distances. In the case of the 2004 quake, there is evidence that it produced temblors in Ecuador half a world away.

"This phenomenon has been documented for several isolated events in certain regions, but now we're able to show that this is occurring everywhere," Velasco said. "As a result, our ideas about how earthquakes occur and how we react have to change."