Morning earthquake; Southern Utah is earthquake country

ST. GEORGE – An earthquake of local magnitude 2.7 occurred at 5:32 a.m., Tuesday, about 90 miles southeast of Cedar City in northern Arizona. The U.S. Geological Survey also lists the quake, giving it 3.0 magnitude reflecting its use of a different rating system. A micro-earthquake of local magnitude 2.0 occurred last Wednesday 5 miles south of Hurricane; and another 2.0 occurred Sunday 13 miles north-northwest of Ivins.

“The fact that we’ve had a few small events is just an average for the year,” Bill Lund of the Southern Utah Geological Survey said. “Utah has an average of 700 smaller earthquakes per year.” And he said today’s earthquake is unrelated to the swarm of earthquakes in the Cedar City area earlier this year.

UUSS uses “local magnitude” or mwhich is akin to California’s Richter scale measurements, duty seismologist Mark Hale with the University of Utah Seismograph Stations, said. UUSS has authority over the seismic region and uses different magnitude scales for different purposes when it comes to describing earthquakes.

The last earthquake to occur near to today’s epicenter in the central Kaibab fault system was a local magnitude 4 in 1991 some 60 miles away.

“This was a pretty straight forward earthquake,” Hale said. “It looks like an isolated incident. In Utah, we just don’t have the frequency, we don’t really know if there’s a precursor event.”

There’s absolutely no way to tell if the smaller quakes are a precursor of something larger that’s coming down the road, Lund said. Basically 99.9 percent of the time they’re not a precursor, it’s just that the earth is always adjusting.

Hale said that seismic activity has only been recorded in Utah for about 100 years, and of that century the first 50-60 years included data from only a few seismographs. It’s only in the past 30-40 years that Utah’s been recording with station density.

Today’s earthquake occurred on the western part of the Colorado Plateau – a large block of the North American tectonic plate that extends across much of eastern Utah and western Colorado and a small portion of northern Arizona. Larger earthquakes, magnitude 5-6, have occurred in northern Arizona – and in Southern Utah.

Earthquake country

Although the impact of these small earthquakes on Southern Utahns is negligible, if even felt, they provide a good reminder that Utah and northern Arizona are earthquake country. In Utah, earthquakes come less frequently than in California, but there are a number of faults entirely capable of having magnitude 7 quakes and we shouldn’t ignore those, Lund said.

Seismologists study earthquakes by trenching fault scarps, an active fault with surface rupture. When the geomorphic setting of the fault is such that sedimentation is preserved, data on geological shifts that have occurred over time can be gathered by dating – through carbon and other dating methods – past seismic events and the intervals between them.

“So, for instance,” Lund said, “on the Salt Lake City portion of the Wasatch fault, we know they happen about every 1,300 years and it’s been 1,400 years.” This may suggest the area is overdue for an earthquake.

In another instance, Lund said that the last major earthquake on the Nephi segment of the Wasatch fault was 370 years ago; since those historically occur about every 1,300 years it is not expected there will be another major earthquake in Nephi for quite some time.

However, both Lund and Hale said that despite the data available, they would not be surprised if Utah had a major earthquake tomorrow and they would not be surprised if it didn’t have one for another 300 years. In other words, earthquakes are unpredictable.

Southern Utah faults

The second largest and arguably second most active fault in Utah, Lund said, belongs to Southern Utah – the Hurricane fault. It’s not as long or as active as the Wasatch fault, he said, but the Hurricane fault and “its little brother, the Washington fault,” are both capable of generating large earthquakes.

Lund said that more data is available on the Washington fault than on the Hurricane fault because of the geology of the Hurricane fault. But there is ample evidence along the Hurricane fault that it is a big active fault.

The last major event in Southern Utah was the St. George earthquake of 1992 at local magnitude 5.8.

“It did a lot of damage,” Lund said, “it damaged a lot of older brick buildings, triggered the Springdale landslide which closed SR-9 for awhile, homes built on that landslide were destroyed, and that was on a fault that didn’t break the ground’s surface.”

At risk

Lund said that if Southern Utah has another 5.5 or 5.6 in Southern Utah it won’t surprise him and it shouldn’t surprise local residents either.

Preparedness and awareness are important.

Old unreinforced brick buildings, or unreinforced masonry – also called URMs – are a huge problem when sizable earthquakes hit.  Lund said that Southern Utah has tens of thousands of URMS among 160,000 statewide. And they present a dilemma to the URM owner. There is such a thing as a seismic retrofit but the cost relative to the value of the home or building is usually prohibitive.

Risk of one sort or another accompanies geography. The awareness to be heightened among Southern Utahns is that they have put down roots in earthquake country. Knowing the risks and knowing ways to respond in the event of an earthquake make for good preparedness.

University of Utah Seismograph Stations has prepared an informative booklet, provided here: Putting Down Roots in Earthquake Country.

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Email: jkuzmanic@stgnews.com

Twitter: @JoyceKuzmanic

Copyright St. George News, SaintGeorgeUtah.com LLC, 2013, all rights reserved.

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