On Nov. 3, 2002,
Alaska’s Denali fault ruptured, causing a magnitude-7.9 earthquake. Within
less than an hour, surface waves rolled through Yellowstone National Park, about
1,900 miles to the southeast, triggering more than 250 smaller-magnitude quakes
over the next 24 hours — and altering the eruption behavior of many of
the park’s famed geysers.
Castle Geyser at Yellowstone National
Park, Wyo., erupted less frequently after seismic waves from the Denali earthquake
almost 2,000 miles away in Alaska moved through Yellowstone in November 2002.
Scientists say that the hydrothermal activity is connected to the far-reaching
seismic effects. Image courtesy Stephan Husen, University of Utah.
The fact that small earthquakes often occur in areas with hydrothermal activity
shortly after larger quakes has led scientists to theorize that high-pressure
fluids in the crust are one mechanism behind the phenomenon, called remotely
triggered seismicity. Until now, however, no solid evidence had been found to
connect triggered seismicity and hydrothermal activity. “Nobody has been
able to prove that fluids are the culprit,” says Stephanie Prejean, a seismologist
with the U.S. Geological Survey at the Alaska Volcano Observatory in Anchorage.
Seismologist Stephan Husen, currently at the Swiss Seismological Survey of the
Swiss Federal Institute of Technology in Zurich, and colleagues from the University
of Utah and the National Park Service make that connection in the June Geology.
The altered behavior of multiple geysers across Yellowstone, accompanied by
the unusual occurrence of simultaneous earthquake swarms at different geyser
basins, strongly suggests triggered seismicity as a result of changes in fluid
pressure, the authors write.
“The great thing about the Yellowstone case after Denali,” Prejean
says, “is that they do see changes in the hydrothermal system at the same
time that the triggered earthquakes are happening; it’s very good evidence
that they are in fact linked.”
Yellowstone National Park, spread across the states of Montana, Idaho and Wyoming,
is home to some 10,000 geysers, hot springs and fumaroles. As the surface waves
from Denali passed by Yellowstone’s 100 Spring Plain (a hot spring system
in Norris Geyser Basin), several small springs begin to boil and erupt, spurting
water up to a meter out of the ground. Other normally clear springs became muddy
and turbid, Husen’s team reported.
Within hours, many geysers experienced either an increase or decrease in their
eruption intervals. Of the 22 that researchers were monitoring, eight displayed
perceptible changes: Five began erupting more frequently, and three began erupting
less frequently. Why some erupted more and others less may be explained by the
jarring loose of minerals that had been clogging channels, the researchers say.
In some cases, this would have opened more steam conduits, thus increasing local
permeability and, consequently, the frequency of eruptions. In other cases,
it would have widened the channels, thus decreasing the pressure and slowing
the eruptions.
The changes observed during and after the quake lasted from just hours or days
up to several weeks. Lone Pine Geyser in the West Thumb Geyser Basin, for example,
had a gradually increasing eruption interval — it erupted less and less
frequently, until it peaked three weeks after the earthquake. Hundreds of tiny
earthquakes continued to occur in the week after the Denali quake, but then
seismic activity within the park returned to normal levels after 30 days.
The Denali event is not the first time in history such events have occurred
at Yellowstone. “The 1959 earthquake at Hebgen Lake caused many changes
to the hydrothermal features at Yellowstone, but it was a fairly local earthquake,”
says geophysicist Jake Lowenstern, scientist-in-charge at the Yellowstone Volcano
Observatory.
Although scientists had observed that earthquakes like Hebgen Lake triggered
smaller quakes and altered hydrothermal activity near the epicenter, they did
not believe that earthquakes could trigger distant seismicity until 1992. That
year, a magnitude-7.3 earthquake occurred in the Mojave Desert near Landers,
Calif., and triggered smaller quakes in geothermal areas all over the western
United States, including 800 miles away in Yellowstone.
The 2002 Denali quake, however, surpassed all others in activating quakes at
great distances. “It’s just a spectacular example of triggered seismicity,”
Lowenstern says.
Sara Pratt
Geotimes contributing writer
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