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Consider the Wells earthquake a wake-up call.

The 6.0 magnitude quake that hit the Nevada city early Thursday morning left the city strewn with the wreckage of older buildings - and that's the kind of damage to expect from a Wasatch Fault earthquake, a University of Utah seismologist said Thursday.

Geophysics professor Robert B. Smith, former director of the university's seismograph station, said the Wasatch Fault that runs between Brigham City and Levan stretches the Earth's crust four inches every year.

That constant elasticity is "pulling the West apart," he said. "It's a rubber band being loaded."

And it will snap.

Mathematical calculations, based on regional earthquake history, show a one-in-four chance of a big quake of magnitude 6.5 to 7.5 will happen here in the next 50 years.

Or maybe tomorrow.

The Wells quake, which the U.S. Geological Survey says likely occurred on what is called the Independence Valley fault system, is considered moderate. But even a moderate earthquake of 5.5 to 6.5 magnitude, if its epicenter were in a Wasatch Front city, would be devastating, Smith said.

The Wasatch Fault is is 350 kilometers long - about 217 miles - and divided in segments, Smith said. Those segments in a large earthquake would act as if the loaded rubber band shot a rock into a windshield: break lines speed outward from the ding until the whole thing shatters.

"The crack propagates and has a velocity of roughly the speed of sound," Smith said.

That means a fracture would travel a 20-mile fault segment in six seconds, collapsing unreinforced brick buildings and homes in an instant and possibly shearing underground water, electric, sewer and natural gas lines along the way and causing billions of dollars of damage.

That doesn't mean post-earthquake cities would be smoldering ruins, as newer buildings are engineered to withstand seismic activity, Smith said.

Still, the Wells quake, moderate as it was, "is going to be remembered as something that caused a lot of damage," Smith said.

Unfortunately, he added, memories are short.

"We all say we're going to make preparations, but we don't. People have the tendency with earthquakes [to remember] for about a week," he said.

There's not much people can do to defend themselves in a 7.5 magnitude quake, but they can allay damage by retrofitting masonry homes to make them safer.

Smith said people also should pull together the standard, basic emergency 72-hour kits of food, water and batteries and make plans for how to contact families and friends - the sorts of advice available from multitudes of public-safety and church Web sites.

But the best preparedness comes from land-use planning and government. Officials understand they have a huge responsibility, but tend to treat earthquakes as a problem far in the future they won't have to deal with, Smith said.

"They want to know what's going to happen in the next year or so, not the next 100 years," he said.

* Earthquakes of magnitude 6.5 to 7.5 are considered large - and likely to occur in Utah. While it's not possible to predict with certainty when such a quake might occur in Utah, they could shake it up on any of several active sections of the Wasatch Fault between Brigham City and Levan.

* Large quakes have occurred on the Wasatch Fault once every 400 years during the past 6,000 years. Seismologists figure there's a 25 percent chance a big one could hit in the next 50 years.

* Future large earthquakes will break segments of the fault about 20 to 40 miles long and cause extensive damage up to 50 miles from the epicenter, including soil liquefaction, landslides, rock falls and even permanent tilts in valley floors that could cause the Great Salt Lake and Utah Lake to flood Salt Lake City and Provo.

* Seismologists have predicted that a big one would wreak havoc with utilities, water lines, sewer systems, highways, bridges, airports and railways. Should such a quake occur in the central part of the Wasatch Fault, damage costs would be in the billions of dollars.

* At particular risk are Utah's brick buildings and homes built before 1960, because the unreinforced masonry crumbles easily, even during earthquakes of smaller magnitude. The loss of those structures likely would account for three-quarters of the damage.

* Moderate earthquakes of 5.5 to 6.5 magnitude occur somewhere in Utah once every seven years on average. Should one of those quakes' epicenters be in a major metropolitan area, the effects would be devastating.

- Compiled by Patty Henetz

Source: University of Utah Seismograph Stations,