SALT LAKE CITY -- Emergency responders across Utah were holding the largest earthquake drill in state history on Tuesday with mass evacuations, mock patients at hospitals and the Utah National Guard deploying 400 soldiers.
"We just had a disaster," said Gov. Gary Herbert at the state Capitol, playing the role of state leader after a powerful jolt. "I would say to everybody: Expect aftershocks. Drop, cover and hold on."
Governments, schools and other organizations practiced evacuations when it was deemed safe for people to leave buildings. Emergency officials tested their communications -- and found some coordination problems, Herbert said.
The National Guard staffed four command posts and was ready to move medical supplies or begin search-and-rescue operations.
"You can imagine looking at Interestate-15 with all the bridges dropped," said Brig. Gen. Jeff Burton, commander of the 7,000-strong Utah Guard.
I-15 follows Utah's heavily populated Wasatch Front, a ticking time bomb for infrequent but powerful earthquakes.
The drill scenario called for a 7-magnitude quake, which occurs on the Wasatch Front about every 350 years.
The last one was about 350 years ago.
We're about due," said Katherine Whidden, research scientist at the University of Utah's Seismograph Stations.
Tuesday's drill brought out some new technology. The Salt Lake City suburb of Draper tested an emergency notification system that alerts residents of a natural disaster based on GPS information from their mobile device.
Experts stressed something that people should not do in an earthquake: run out of buildings, where they could be hit by falling debris.
School kids dived under desks to brace for the first minute of an imagined quake.
Yet experts also say Utah's many unreinforced brick homes are likely to collapse in a powerful earthquake, and they advise people to take the first opportunity to escape into the open.
More modern homes and office buildings are expected to stand up to a magnitude-7 quake but could still suffer damage.
"We could be cut off from medical help," Herbert said. "Hospitals could be destroyed."
The Utah Seismic Safety Commission says 90 percent of the state's population lives in earthquake zones.
"Folks need to harden themselves for an event like this," Burton said.