Experts: No seismic link to rockslide
Experts are discounting the possibility of any connection between recent seismic activity in western Colorado and Thanksgiving Day’s rockfall in Glenwood Canyon.A magnitude 2.5 earthquake struck Friday afternoon 15 miles southeast of Glenwood Springs, a day after the rockfall that knocked holes in Interstate 70 and closed it for some 30 hours. While earthquakes sometimes are preceded by slight tremors, the U.S. Geological Service National Earthquake Information Center reports no Garfield County quakes in the days prior to the rockfall. And a scientist there says it would have taken a larger quake than Friday’s to shake loose rocks of the size that came down Thursday.”There is no way there is any connection,” said Waverly Person, a geophysicist at the center.Another minor quake occurred in the Paonia/Crawford area on Nov. 23, two days before the rockfall, but that was way too small and too far away to have been a cause, Person said.Even Friday’s quake would not have been close enough to affect Glenwood Canyon, Person said.Anything that would have caused the rockfall would have done other damage as well, he said.Residents in Missouri Heights and up Cattle Creek reported feeling Friday’s quake, but there were no reports of damage.Ty Ortiz, who manages the Colorado Department of Transportation’s rockfall program, shares Person’s view that it would take a bigger earthquake than Friday’s to cause the kind of rockfall that occurred last week.”That magnitude of earthquake is pretty small as far as earthquakes go,” he said. “Had there been a large-magnitude earthquake in the canyon, yeah, certainly, that could stir things up a bit.”Colorado has a history of earthquakes, but almost all have been minor ones. In Ortiz’s opinion, the freeze-thaw cycle creates a far bigger threat of rockfall. That cycle, suspected in Thursday’s incident, causes moisture between rocks to expand like a jack, throwing the rocks off-balance.”Eventually they just get to the point that they can’t support themselves,” he said. “It’s almost unpreventable, something like that.”Typical techniques such as bolting rocks to cliff-sides wouldn’t have stopped a massive rockfall such as Thursday’s. Even dozens of rockfall fences probably wouldn’t have made a difference, Ortiz said.”It was several orders of magnitude above what’s typically protected from rockfall fences these days,” he said.No one was injured in Thursday’s incident, but falling rocks have killed people before in the canyon.Over the years, CDOT has spent probably about $6 million in Glenwood Canyon on fencing, netting and other rockfall mitigation work, Ortiz estimated. The highway’s alignment was partly dictated by an attempt to minimize the risk posed by overhanging rock.CDOT rates some 750 sites statewide for rockfall danger, but views Glenwood Canyon as a special condition with its own rockfall rating. The agency evaluates the danger there on a continuous basis. But some rockfalls can’t be predicted or prevented, Ortiz said.”Geologically speaking these things occur from time to time,” he said. “If the mountain’s going to hiccup, the mountain’s going to hiccup. … There’s really not much you can do about it.”Contact Dennis Webb: 945-8515, ext. firstname.lastname@example.org
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