Big earthquakes can trigger other quakes far from their geographical center at least 9 percent of the time, a statistical analysis by a U.S. researcher shows.

With a number of huge earthquakes in recent years -- in Sumatra, Indonesia, in December 2004, Chile in February 2010 and Japan in 2011 --leading many to question whether one large quake can cause another on the other side of the world, Tom Parsons of the U.S. Geological Survey surveyed catalogs of seismic activity on every continent except Antarctica going back to 1979.

Of the 260 earthquakes of magnitude 7 or greater during that period, small earthquakes on separate fault systems followed in the wake of 24 of them, triggered by seismic waves passing through distant lands, he said.

"It's a small hazard, but there is a risk," he said.

Parsons, who presented his results Friday at the Seismological Society of America annual meeting in Salt Lake City, says his next step will be to investigate the 24 quakes that caused far-off events and see if there is anything special about them.

"So far they look fairly ordinary. So we're going to have to really dig into them," he said.

Seismic activity during deadly Utah mine collapse yields insights
Salt Lake City (UPI) Apr 19, 2013 - Analysis of seismic activity recorded in a 2007 deadly Utah mine collapse shows its extent was greater than previous studies indicated, researchers say.

The owner of the Crandall Canyon coal mine initially blamed the collapse, which killed six miners and three rescue workers, on an earthquake but University of Utah researchers say analysis of the recordings of the tremor and hundreds of small aftershocks suggests they were a result of mining activity and the subsequent collapse.

"We can see now that, prior to the collapse, the seismicity was occurring where the mining was taking place and that, after the collapse, the seismicity migrated to both ends of the collapse zone," said Tex Kubacki, a graduate student in mining engineering.

Mapping the locations of the aftershocks "helps us better delineate the extent of the collapse at Crandall Canyon," he said.

A previous University of Utah study, based on far fewer aftershocks, said the epicenter of the collapse was near where the miners were working and the aftershocks showed the collapse area covered 50 acres.

The new study, based on data of hundreds of additional aftershocks, has extended the area of the collapse to the full extent of the western end of the mine, Kubacki said.

"It's gotten bigger," he said.

Most of the seismic activity before the collapse was due to mining, the researchers said, although they are investigating whether any of those small jolts might have been signs of the impending collapse.

So far, however, "there is nothing measured that would have said, 'Here's an event [mine collapse] that's ready to happen,'" said Michael "Kim" McCarter, a mining engineering professor and the study's co-author.