By Dr David Whitehouse
BBC News Online science editor
A major earthquake in Alaska in 2002 set off lots of smaller quakes in the Yellowstone National Park about
3,000 km away, say scientists.
Castle Geyser erupted less often after the quake
Within hours geysers in the park changed their eruption patterns, according to the journal Geology.
Researchers believe that earthquakes keep geysers alive by periodically shaking loose clogged channels.
The Alaskan earthquake was one of the strongest in North America in the past 150 years.
Thousand smaller quakes
Robert Smith, of the University of Utah, says his team's study shows that large earthquakes at great distances can have profound effects on the Yellowstone geysers.
"We did not expect to see these prolonged changes in the hydrothermal system," he said.
The geysers showed changes just a few hours after the shock waves from the 3 November Alaskan earthquake passed through.
More than a thousand minor local earthquakes were triggered by the shock waves, many of them near hot springs and geysers.
They altered water and steam pressure in the geysers, opened new channels and unclogged others.
In the study, the researchers looked at the eruption patterns of 22 geysers during the winter of 2002-3, noticing that eight geysers showed major changes.
One of them - Daisy Geyser - erupted more often but returned to its normal pattern after a few weeks.
The geysers Castle, Plate and Plume also displayed short-term irregularities that lasted for a few days.
"Several small hot springs, not known to have geysered before, suddenly surged into a heavy boil with eruptions as high as one metre," the researchers say.
"The temperature at one of these springs increased rapidly from about 42C to 93C and became much more acidic than normal.
"In the same area, another hot spring that was usually clear showed muddy, turbid water."
Scientists once believed that an earthquake in one location could not trigger earthquakes at distant sites.
That view was altered after the 1992 Landers Earthquake (magnitude 7.3) in California's Mojave Desert triggered a spate of quakes more than 1,200 km away at Yellowstone.
Professor Smith believes that the Alaskan quake focused its energy southeast towards Yellowstone meaning that the stresses rippling through the ground at Yellowstone were 200-300 times greater than if the quake's waves were aimed elsewhere.