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7.0 earthquake: If the big one hits Utah's Wasatch front

Utah could see thousands dead, survivors without power, water, phones

Published: Sunday, Jan. 17 2010 12:00 a.m. MST

Dave Marshall is dwarfed by the rotunda tier girder system that will help the Capitol withstand an earthquake during retrofit work on the Capitol March 22, 2006.

Tom Smart, Deseret News

SALT LAKE CITY — Scientists say Utah's Wasatch Front faces a 1-in-7 chance of being hit by a magnitude-7.0 earthquake — the size of the tremor that devastated Haiti last week — sometime in the next 50 years.

If it hits at the worst-possible time — in the middle of a winter night — government computer models estimate that it could kill about 3,000 people, injure another 42,000, damage more than a third of all buildings and cause at least $35 billion in economic loss.

More than 60,000 people would need public shelter during freezing temperatures. Most power, water and communications would be out initially, and may not be restored in some areas for months. Fires could be widespread.

Police and firefighters would be so overwhelmed that they could not reach many people for days, so neighbors would need to rely on each other for help.

That may not be as bad as what's now unfolding in Haiti — where the Red Cross fears 50,000 people may have died and most buildings were damaged in some cities. Still, a 7.0 quake in Utah would be horrific. That said, damage and suffering could be reduced if residents plan and take precautions now.

QUAKE ODDS

"It's been about 1,300 years — plus or minus 650 — since the Salt Lake City segment (of the Wasatch fault) had an earthquake of that magnitude," said University of Utah seismologist Jim Pechman.

He adds an ominous statistic: "They happen there, on average, about every 1,300 years," or about the same length of time since the last one occurred.

So does that mean Utah is due for a "big one"?

"They don't go off like clockwork," Pechman said, and it depends how much pressure has built up on the fault. "There's a lot of variability in that."

He said the probability of a large quake on the Salt Lake City segment of the Wasatch fault (one of 10 independent segments of that fault system) is somewhere between 1-in-10 to 1-in-7 in the next 50 years. "If you look at the Wasatch Front as a whole, it would be higher," he said, about 1-in-7 odds overall.

Pechman notes that the Salt Lake City segment of the fault has been expanding an estimated 1.2 millimeters a year recently. Scientists say as pressure builds along a fault, it is like slowly stretching a rubber band until it suddenly breaks.

QUAKE DESCRIPTION

Scientists say a worst-case scenario quake in Utah would be centered in mid-Salt Lake Valley, and would be felt in seconds from Brigham City in the north to Santaquin in the south, and from Tooele in the west to Coalville in the east. It would affect 80 percent of Utah's population.

Shaking in Salt Lake County would be so severe that no one would be able to stand without falling. Unsecured hot-water heaters would shake loose in many homes, and broken gas lines would start fires. Bookcases, furniture and debris would fall and injure many. Some people trying to run out of buildings would be killed by falling debris. Collapsing buildings would kill many.

A 7.0 quake would quickly create a ledge or "scarp" that could stretch for nearly 30 miles along the main fault — roughly following Wasatch Drive in the south valley, and following Highland Drive and 1300 East in the central and northern parts of the valley.

Pechman said that scarp, or ledge, would be about 3 feet high — but would be up to 9 feet tall in places. As the scarp forms, it could snap utility lines and block transportation across it, and topple houses around it.

Bob Carey, earthquake preparedness director for the Utah Division of Homeland Security, says two small valley faults would likely also shake "in sympathy" with the larger Wasatch fault, and also form smaller scarps.

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