When seismometers recorded a magnitude 2.2 earthquake in early January, few people felt it. But several geysers in Yellowstone National Park, including Old Faithful, have not been the same since.
"It's the greatest change in Old Faithful in years," said Ann Deutch, a Yellowstone ranger naturalist and geyser watcher. "In a practical sense, it has changed the way the geyser is being predicted."At 1:50 a.m. on Jan. 9, seismometers recorded the earthquake about a mile northwest of Old Faithful, and within a few hours, Cascade Geyser, virtually unknown since the late 1800s, began erupting. Vault Geyser, dormant since 1988, began spouting so much water that it washed channels in the sinter chips that surround it.
About eight days later, Old Faithful shook off its traditional eruption pattern so effectively that park naturalists lost their touch for predicting its crowd-pleasing shows.
In the four months prior to the earthquake, Old Faithful was erupting more frequently, once every 74 minutes on average. Since then, the geyser has averaged more than 80 minutes between eruptions, the longest average since records have been kept.
Old Faithful has never erupted like clockwork. Instead, it erupts for two minutes, then waits about an hour to erupt again, or it erupts for three to five minutes and then waits about 90 minutes before spouting again.
"There is a lot more going on out there," said Mike Keller, a longtime geyser gazer. "Old Faithful is only part of it."
Following the January tremor, park rangers have had a harder time predicting Old Faithful eruptions.
The proportions of long intervals have increased, said Lynn Stephens, a statistician and professor at Eastern Washington University.
Once during the winter, rangers watched the geyser wait 115 minutes between eruptions, an interval almost unheard of in the past.
"The curve has shifted, and the shorter periods are now less common," Stephens said. "It's nearly as predictable as it was before - it's just that we've had to change the prediction model."
Because of the change in eruption patterns, geyser gazers are debating whether the earthquake juggled the geysers or whether the geysers and their subterranean plumbing set off the earthquake.
"I think there may well be something bigger going on at depth," said T. Scott Bryan, a geology professor and author of a guide to Yellowstone geysers. "This earthquake may have been just one part of the picture."
Geologists have long known that earthquakes shake up the tunnels beneath geysers that carry the water supply. Old Faithful has been known to react to earthquakes by speeding up before tremors or slowing down afterward, depending on their size and proximity to Yellowstone.
After a magnitude 7.3 earthquake near Borah Peak, Idaho, in 1983 a geologist reported Old Faithful's average interval rose from about 69 minutes to 78 minutes. The geologist attributed the slowdown to a shift in hot water away from Old Faithful.
Increasing pressure in hot-water reservoirs that feed the park's geysers may have fractured the ground with enough force to cause the earthquake, opening new conduits to the surface at the same time, Bryan said.
But Keller said it may be the other way around with the earthquake causing the water flow to change.