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Experts ‘cautious’ about YNP quakes
By Carole Cloudwalker
This document was published online on Friday, January 02, 2009
Portions of Yellowstone Park were shaken by a swarm of small earthquakes for the fourth day Wednesday, but experts say they are not concerned, just cautious.
“They’re well within the range of normal earthquake activity,” Yellowstone geologist Hank Heasler said Wednesday.
He said the most recent quakes occurred Wednesday, but no damage was reported.
The strongest of the quakes up to that time was ranked as magnitude 3.9; it occurred at 10:15 p.m. Saturday, Dec. 27.
It was the only one of the current swarm “ or group of earthquakes occurring in the same general spot and time “ that was felt at Old Faithful.
Most of the earthquakes in the swarm have only been felt at Lake and Grant Village.
The epicenter of the recent earthquake activity has been at the northern end of Yellowstone Lake near Stevenson Island, south of Fishing Bridge, Heasler said.
He said the swarm, while of interest to scientists and residents, has shown “no indication” the park’s famous caldera is likely to erupt.
“We’re not packing,” another park employee said Wednesday, while Heasler quoted Interpretative Ranger Matt Johnson, who is stationed at Lake, as saying winter residents there “are becoming familiar with the Richter Scale.”
Heasler said the quakes are “interesting, and they tell us about how the volcanic hot water system works “ we’re intensely interested, but not concerned.”
Heasler added that nevertheless, “due diligence is warranted.”
Scientists watched closely to see whether the more than 250 tremors, which reached a maximum magnitude of 3.9, were a sign of something bigger to come.
Swarms of small earthquakes happen frequently in Yellowstone, but it’s unusual for so many earthquakes to take place during several days, said Robert Smith, a professor of geophysics at the University of Utah.
“They’re certainly not normal,” Smith said. “We haven’t had earthquakes in this energy or extent in many years.”
Smith directs the Yellowstone Seismic Network, which operates seismic stations around the park. He said the quakes have ranged in strength from barely detectable to one of magnitude 3.9 that happened Saturday. A magnitude 4 quake is capable of producing moderate damage.
“This is an active volcanic and tectonic area, and these are the kinds of things we have to pay attention to,” Smith said. “We might be seeing something precursory.
“Could it develop into a bigger fault or something related to hydrothermal activity? We don’t know. That’s what we’re there to do “ to monitor it for public safety.”
All the quakes were centered beneath the northwest end of Yellowstone Lake.
A park ranger based on the north side of the lake reported feeling nine quakes in a 24-hour period during the weekend, according to park spokesman Stacy Vallie. No damage was reported.
Smith said it’s difficult to say what might be causing the tremors. He pointed out Yellowstone is the caldera of a volcano that last erupted 70,000 years ago.
He said Yellowstone remains geologically active “ and the park’s famous geysers and hot springs are a reminder that a pool of magma still exists 5-10 miles underground.
“That’s just the surface manifestation of the enormous amount of heat that’s being released through the system,” Smith said.
Heasler said in 1985 a swarm of earthquakes occurred during a three-month period, near West Yellowstone. The strongest of those was magnitude 3.7.
Yellowstone has had significant earthquakes as well as minor ones in recent decades.
In 1959, a magnitude 7.5 quake near Hebgen Lake just west of the park triggered a landslide that killed 28 people.
Though earthquakes can trigger avalanches in snow country, Heasler said that does not appear to have occurred in this case.
He estimated an earthquake likely would have to be magnitude 4 or greater before geologists would caution the park’s avalanche experts.
The majority of the current swarm of quakes were rated as magnitude 2, he said.
(Carole Cloudwalker can be reached at carole@codyenterprise.com.)
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Logan wrote on Jan 4, 2009 6:52 AM:
Is it true that the next caldera-forming eruption of Yellowstone is overdue?
No. First of all, one cannot present recurrence intervals based on only two values. It would be statistically meaningless. But for those who insist... let's do the arithmetic. The three eruptions occurred 2.1 million, 1.3 million and 0.64 million years ago. The two intervals are thus 0.8 and 0.66 million years, averaging to a 0.73 million-year interval. Again, the last eruption was 0.64 million years ago, implying that we are still about 90,000 years away from the time when we might consider calling Yellowstone overdue for another caldera-forming eruption. Nevertheless, we cannot discount the possibility of another such eruption occurring some time in the future, given Yellowstone's volcanic history and the continued presence of magma beneath the Yellowstone caldera. "