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Hazard maps: Utah's worst-case quake could be a deadly 7.4 magnitude

Published April 22, 2008 1:25 am

This is an archived article that was published on sltrib.com in 2008, and information in the article may be outdated. It is provided only for personal research purposes and may not be reprinted.

Government geologists have a clearer picture of what The Big One might look like in Utah.

The U.S. Geological Survey published new, nationwide hazard maps Monday which show that the worst-case earthquake has been elevated to a magnitude 7.4 on the 240-mile Wasatch Fault that runs from Malad, Idaho, through the state's population centers to Gunnison, and that the devastation could span several Wasatch Fault segments all at once. Previous maps called for less ground shaking.

They also reveal that tall buildings probably would shake less than previously expected, but one- and two-story buildings might shake harder than thought before.

And new faults have been discovered, including one south of Tooele, one under Utah Lake and one in the Cache Valley.

"This is the science community's best estimate of shaking hazards in Utah," said USGS's Mark D. Petersen.

The latest version shows much of the United States is on shaky ground, with 46 states facing serious seismic threats. The new maps - you can see them at earthquake.usgs.gov/research/hazmaps/ - come the week after a magnitude 5.2 earthquake in southern Illinois.

USGS last published nationwide hazard maps in 2002 and 1996. But research done in the years since has allowed the federal agency to work with experts at universities, professional groups and state agencies, including the Utah Geological Survey, to bring the most up-to-date information to its earthquake maps.

"These maps are the basis of building codes in Utah," said Peterson, noting that engineers will be poring over them to determine how to construct the safest-possible structures.

"These maps are vitally important to us as a community," added Bill Lund, a senior scientist with Utah Geological Survey's geologic hazards program.

He noted that, based on their own findings about the 170 segments of quaternary faults in the state, Utah geologists found many of their suggestions incorporated into the new, national maps. Among the Utah faults making their debut on the latest USGS maps, said Lund, are the Clarkston Fault in the Cache Valley of northeastern Utah, the Southern Oquirrh Mountain Fault at the base of the Oquirrh Range and south of Tooele, and the Utah Lake Faults and folds in Utah County.

"Every year we know more about Utah's active faults," Lund said.

USGS timed the release of the new maps with a national revision of the model building codes used by state and local government to make sure buildings, bridges, highways and utilities can resist earthquake damage.

The National Seismic Hazard Maps are really a series of maps and data sets that project the ground shaking that might happen at points throughout the country.

Insurance companies use the data to set rates in some places, USGS noted. Engineers use it to forecast landslides and the stability of hillsides.

In the federal government, environmental regulators use it to ensure waste-treatment facilities will hold up. Emergency planners use the information in deciding how to allocate funding for education and preparedness.

fahys@sltrib.com

Shaking things up

The U.S. Geological Survey hazard maps for Utah show:

* The worst-case earthquake is a magnitude 7.4 - not the smaller shaking projected before - on the 240-mile Wasatch Fault that runs from Malad, Idaho, through the state's population centers to Gunnison.

* The devastation could span several Wasatch Fault segments all at once.

* Tall buildings probably would shake less than previously expected, but . . .

* One- and two-story buildings might shake harder than thought before.

*New faults, including one south of Tooele, one under Utah Lake and one in the Cache Valley.