Shock waves from a powerful earthquake in Alaska in 2002 changed the behavior of some of Yellowstone's geysers, demonstrating the tremendous reach seismic events have on hydrothermal features, scientists wrote in the June issue of the journal Geology.

Within a week of the 7.9 magnitude Denali temblor, more than 1,000 quakes were reported in Yellowstone National Park, University of Utah seismologist Robert Smith and others wrote in the journal Geology.

Those tiny quakes, they reported, seem linked to a change in the timing and intensity of some of the park's 10,000 geysers, hot spring and steam vents.

"Several small hot springs, not known to have geysered before, suddenly surged into a heavy boil with eruptions as high as 1 meter," Smith and co-authors Henry Heasler and Ralph Taylor of Yellowstone and Stephan Husen, of the University of Utah, wrote.

The temperature of one spring soared from 108 degrees to 199 and its acidity dropped. Another spring in the same area that was usually clear became muddy.

While other large earthquakes have been known to alter the activity of nearby geysers and hot springs, the Denali fault earthquake of Nov. 3, 2002, centered 800 miles north of Yellowstone, is the first known to have changed the behavior of hydrothermal features at such great distances, Smith cited in a press release.

The activity of 22 geysers in the park were monitored that winter. Eight of the 22 "displayed notable changes in their eruption intervals" after the big earthquake. Of those:

* Geysers that erupted more frequently that winter included Daisy, Depression, Plume and Riverside geysers in Upper Geyser Basin and Pink Geyser in Lower Geyser Basin.

* Geysers that erupted less frequently included Castle and Plate in Upper Geyser Basin and Lone Pine Geyser in West Thumb Geyser Basin.

Most geysers returned to their normal timing days to months after the Denali quake.

Some geysers notably affected by earlier nearby earthquakes, such as Grand Geyser and Old Faithful in Upper Geyser Basin, were not affected by the Denali earthquake.

The study team suspects that it may not have been the earthquakes that triggered the change in the geysers, but the other way around.

Smith wrote that the Denali shock waves, directed as they were south toward Yellowstone, affected the geysers by changing water pressure in underground conduits that feed the geysers.

The pressure change - which may have made hot water "flash" explosively into steam - could have altered pressure on adjacent faults, triggering small earthquakes nearby.

That would explain why the quakes were clustered around geyser basins, he wrote.

Smith said that an obvious question related to the group's study is whether a large earthquake closer to Yellowstone could trigger a huge hydrothermal explosion such as those that occur in the Yellowstone caldera about every 1,000 years.

There is no evidence prehistoric quakes triggered those blasts, Smith wrote. And such explosions were not triggered by the magnitude-7.5 Hebgen Lake, Mont., quake in 1959 or the magnitude-7.3 Borah Peak, Idaho, quake in 1983.

Nevertheless, a big quake near Yellowstone with its surface waves aimed the right way conceivably might "cause large hydrothermal eruptions," says Smith. "I would hypothesize that is certainly possible."