If a 6.0 earthquake similar to what struck rural Nevada Thursday ever directly hits the populous Wasatch Front, state officials predict it will kill some people, collapse older brick buildings, break water and sewer lines, knock out power and create widespread damage from falling walls, chimneys and items from shelves.
Still, that damage is only considered moderate. A bigger 7.0 or so quake which experts have said has about a 1 in 5 chance of hitting the Wasatch Front in the next 50 years could kill up to 6,200 Utahns, injure 90,000 more, at least moderately damage 42 percent of all local buildings and cause $40 billion in economic losses.
"It all depends on when an earthquake hits and where, and the size," said Bob Carey, the earthquake preparedness director for the Utah Division of Homeland Security, who has performed computer modeling of likely damage from different sizes of quakes here.
Carey said a 6.0 in Salt Lake County would likely cause "some loss of life, but it would be minimal" depending on the time of day it hit. "If it happens at night when people are home and asleep, it would be less of a problem" than if people are at work or school.
He said a 6.0 would cause major damage to "unreinforced masonry" buildings, built of bricks and mortar and no reinforcing steel. "Those buildings would take a pretty good hit," and many might collapse, he said. Of note, some schools are made of that type of construction.
"It's a recognized problem, but it's also a funding issue. You can only go as fast (to upgrade schools) as funding allows you to," Carey said.
Of note, just an hour or so after the Nevada quake hit on Thursday, the Utah Legislature's House Government Operations Committee endorsed a bill to require schools to conduct seismic evaluations of their buildings that could lead to fixing the most needy buildings first.
Carey said most structural collapses and serious damage would occur relatively near the epicenter of a 6.0 quake. But "non-structural" damage could occur far from it.
For example, Carey said, "With some of the older buildings downtown, I would expect to see their decorative cornices have problems. You may see some failure there. You will definitely see parapet damage from this. With that parapet damage, you always see partial wall failure."
A parapet is a low wall along the edge of a roof or balcony.
He adds that damage may be seen on higher floors of buildings because swaying is more severe there.
Carey said he also would expect some water and sewer lines to be knocked out because the lines tend to be brittle and break with shaking. He adds he would expect some power blackouts. "Power is pretty delicate," he said. "If it does go down, they'll leave it down for a time to make sure they don't damage it by bringing it back up."
He said some liquefaction or soils acting like liquids, or quicksand, where structures may tip over is possible with a 6.0.
"A lot of that depends on the duration of the event," he said. "If you see it, you'll likely see it along someplace like the Jordan River so you could have some problems with bridges crossing the Jordan (River)."
Carey also notes that a 6.0 would have been big enough to cause major damage to some buildings before recent seismic retrofitting. "If a 6.0 hit the state Capitol before its retrofit, it would have caused major damage to the dome. Now, I expect it would cause no problem."
Still, all of the damage expected from a 6.0 is only considered moderate, Carey said, and local governments could probably handle all needed response, with assistance from emergency officials on the edge of the quake-affected area.
That would not hold true with a large quake, such as 7.0.
Carey said computer modeling done two years ago and reported by a 2006 Deseret Morning News series predicted a worst-case 7.0 in Salt Lake County could kill 6,200 people, injure 90,000 more, at least moderately damage 42 percent of all local buildings and cause $40 billion in economic losses.
Carey said the only change in those numbers in two years is that because the population has increased, "we might expect even more casualties."
It would swamp emergency responders, and many would need to depend on neighbors, community emergency training and their emergency supplies for survival for five days or longer.A 7.0 releases 32 times more energy than a 6.0, and creates 10 times more ground shaking, Carey said.
Contributing: Arthur Raymond