A powerful earthquake could erupt at Panguitch tomorrow or centuries from now.
All scientists know for sure is that the Sevier Fault Zone, which is beneath Panguitch, is more active than suspected earlier. It is capable of unleashing a temblor of magnitude 6.5 to 7 and sets off a big quake about every 3,000 or 5,000 years, says William R. Lund, a senior scientist with the Utah Geological Survey.
Lund is the author of the recent UGS report "Paleoseismic Reconnaissance of the Sevier Fault, Kane and Garfield Counties, Utah," which is the 16th in the series "Paleoseismology of Utah." He is also the editor of the series.
The 155-mile Sevier Fault Zone stretches through southern Utah, passes beneath the Grand Canyon in Arizona and ends just south of the Grand Canyon. About 67 miles of it are in Utah; the UGS study focused on this part.
To date earthquakes along the infamous Wasatch Fault that runs through Salt Lake City, geologists look for scarps, "places where the ground has been displaced in recent geological time," Lund said in a telephone interview. They can cut trenches across scarps and search for organic material buried in earlier earthquakes, then date the debris through radiocarbon techniques.
But the Sevier Fault, which isn't as active, doesn't show recent scarps. Another way had to be found to date ground movements. "We found two places where volcanoes had erupted and where lava had crossed the fault," he said. They were about 25 miles apart. "And then those lava flows had been displaced."
The displacement was caused by earthquakes after the lava flowed. They showed how far the ground had moved since the volcanoes erupted. When the lava flows were checked through argon dating techniques, they were found to be to about 5.1 million years ago and about 4.96 million years old.
Volcanic rocks were displaced 600 feet at Red Canyon, Garfield County, the northern site, and 75 feet at Black Mountain, Kane County, to the south.
Using the dates of the lava flows and the distance the fault had moved since that time, the geologists were able to calculate that the fault's movement is an average of 0.0157 inches per year for the northern section and about one-tenth that for the more southerly.
Both are poky beside the Wasatch Fault at Salt Lake City, which moves about 0.04 inch to 0.08 inch per year, with large earthquakes expected around every 1,300 years. The last in this area, said Lund, was about 1,250 or 1,300 years ago.
At the part of the Sevier Fault studied, the interval between big earthquakes of magnitudes 6.5 to 7.5 seems to be about once every 5,000 to once every 33,000 years, according to a news release by state officials.
Is an earthquake overdue?
"We don't have that data" from which that can be calculated, Lund said. Scientists weren't able to determine when the fault moved last. But they know that it does slip, so the zone is considered active.
"It wouldn't surprise me at all to see an earthquake on the Sevier Fault with a magnitude of 6.5 to 7," he added.
Each magnitude is 10 times as powerful as the previous number, so an earthquake of 7.0 releases 10 times the force of a 6.0. The recent Wells, Nev., earthquake was measured at magnitude 6.0.
"Panguitch lies to the west of the Sevier Fault," a few miles from where the fault reaches ground level, Lund added. Because the fault is west-dipping, that means it extends to beneath the Garfield County town. That could put the epicenter of an earthquake on the Sevier Fault below it or a few miles away.
"The hazard to Panguitch is ground-shaking," he added. Farther off are Bryce Canyon National Park and Ruby's Inn, where shaking would be felt but not as dangerous.
Visitors to Zion National Park also could feel a Panguitch earthquake, "very much like happened in Salt Lake City when we had the Wells event." If a 7.0 magnitude quake hit, it would cause damage to unreinforced chimneys near the national park, he said. "St. George would know it, Kanab would know it, Richfield would know it" by the jolts residents would feel.
But Panguitch is the closest town, "which implies that the ground-shaking there would be greatest. ... Panguitch has a lot of risk because they have a lot of URMs, unreinforced masonry buildings."
Still, not knowing when the last earthquake hit, Lund added, "we don't know where we are in the cycle. That's a complete unknown to us."
The lack of recent scarps suggests it's been a long time since a earthquake. Pressure could be building for a good-sized shake.
"Panguitch could be tomorrow's Wells," Lund said, "or Panguitch may not have an earthquake for 500 years." Further studies may help refine scientists' understanding.The study is available at the state's Natural Resources Map and Bookstore, 1594 W. North Temple, for $14.95. The outlet can be reached by e-mail at [email protected].
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