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Geologists pondering mystery in park By MICHAEL MILSTEIN Gazette Wyoming Bureau YELLOWSTONE NATIONAL PARK – Geologists are trying to tell whether a swarm of hundreds of small earthquakes that rattled Yellowstone Park’s Norris Geyser Basin in late July and early August may be connected to an earlier upsurge in temperatures and energy of some of the basin’s hot springs and geysers.
The minor earthquakes – most registering between magnitude 1 and 2 and too small to be noticeable to park visitors – began jiggling Norris about July 25, according to the University of Utah Seismograph Station, which maintains a network of earthquake monitoring units in Yellowstone. The number of tremors peaked July 28-29 and has gradually tailed off since then, but continued into this week. Such earthquake swarms hit commonly atop Yellowstone’s ancient volcanic plateau, but this summer’s swarm at Norris interests geologists because it follows increasing thermal activity that began last fall on the 100 Springs Plain, a dynamic section of Norris that lies beyond the boardwalks frequented by visitors. Norris regulars late last year began noticing a dramatic burst of activity and increasing temperatures on the 100 Springs Plain, with new geysers appearing, hot springs heating up and trees dying from the rising heat. Norris is the hottest and most dynamic of Yellowstone’s famous geyser basins, leading many scientists to suspect it has the most direct conduit to the reservoir of hot water and rock beneath the national park. Geologists have long puzzled over the relationship between earthquakes and changes in Yellowstone’s steaming geyser basins. It’s a geological chicken-and-the-egg kind of mystery: Do surges of thermal water moving toward the surface lubricate or fracture underlying rock, triggering earthquakes? Or do earthquakes fracture the rock, creating new channels for hot water to push to the surface? “The question is a cause-and-effect question and we don’t know which is which,” said Robert Smith, a professor of geophysics at the University of Utah and authority on Yellowstone geology. “But there is clearly a correlation here and you can’t ignore that.” Smith, his students and Yellowstone Park geologists are trying to compare the timing of the earthquake swarm with the initial and later changes in thermal activity at Norris to shed more light on the relationship between the two. The largest earthquakes were only slightly larger than magnitude 2.5, just barely strong enough for people to notice. Earthquakes in the swarm were tightly clustered around the southeast flank of Mount Holmes, to the northwest of Norris Geyser Basin, and struck at depths of anywhere from just below the surface to about five kilometers, or about three miles, deep. There was no shift in the epicenters of the earthquakes over time, Smith said. For more details about the earthquake swarm, including maps showing earthquake epicenters, visit http://www.mines.utah.edu/UUSATRG/ Norrisswarm.html. Michael Milstein can be reached at (307) 527-7250 or at gazette@wavecom.net
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