Findings about this ancient rhythm of immense quakes raise the threat of widespread destruction for the heavily populated 120-mile Wasatch corridor around Salt Lake City.
Geologists cannot say with certainty when the next devastating earthquake will hit the Wasatch Front, but they say the threat is real and constant and by some measures long overdue.
University of Utah geophysicist James Pechmann, an expert on the Salt Lake basin's crustal structure, put the odds that Salt Lake will suffer a large quake over the next 50 years at 1-in-3.
"We know these earthquakes have been going on for millions of years," Pechmann said. "There's no reason to think they're going to stop just because people moved into the valley."
Geologists are doing field work with newfound urgency, most recently digging into a fault complex at Mapleton, Utah, where they are studying Utah's most recent devastating quake, which snapped about 600 years ago.
The Mapleton trench also revealed at least four previous temblors of magnitude 7 or greater going back 12,000 years. The digging gives an idea of the kind of violence that accompanies the sudden release of pent-up energy, said paleoseismologist Susan Olig of San Francisco consultants URS Corp.
Gary Christensen, hazards program manager for the Utah Geological Survey, said the risk is much greater one mountain basin to the north of Mapleton, in greater Salt Lake City, which has been virtually undisturbed since the arrival of Mormon settlers in 1847.
Simple timing explains why geologists believe Salt Lake City is likely to be struck next. The Wasatch fault here last slipped with a violent shudder about 1,283 years ago - and the intervals between each of the four most recent prehistoric quakes ranged from 1,269 to 1,441 years.
The estimates, derived from radiocarbon dating of organic matter in fault zones, are qualified by different margins of error, but the implication is clear.
"The earthquake strain has been accumulating all this time and there is sufficient energy to release the big one," said Walter Arabasz, director of the University of Utah's Seismograph Stations.
"What's overdue is strong shaking. Since 1847 we've just been lucky," he said.
A magnitude 7.5 quake could kill 7,600 people in the Salt Lake basin, injure 44,000 others and cause $12 billion in building damage alone, a pair of Stanford University engineers calculated in 1994.
The main Wasatch fault, which snakes along the base of the Wasatch Range, isn't the only threat to Utah's largest city. Out on the basin floor, the West Valley fault group also is poised to release a powerful quake.
Over the past 12,000 years, the West Valley complex produced a powerful jolt every 1,700 to 2,000 years - most recently about 2,000 years ago, geologist Jeffrey Keaton determined.
Keaton measured as much as 62 feet of drop on the zone's east side over the past 140,000 years - movement that would have been produced by 11 to 13 quakes of magnitude 6.5 to 6.7. At least six of those quakes happened during the past 12,000 years.
To illustrate the dangers, URS Corp. produced a map of the Salt Lake basin that gives a block-by-block look at the shake hazards from a typical magnitude-7 quake originating 10 miles underground.
It isn't a pretty picture.
Most of the Salt Lake metropolis sits on unconsolidated sediments washed down from the mountains. Those sediments will amplify instead of dampen the shock waves of a quake. The map shows few safe zones, with most areas rated for strong or violent shaking.
"People say, 'Yeah, I heard earthquakes can happen here,' but that's about as much as they know," says structural engineer Charles Richardson, who advises homeowners converting attics or replacing roofs on ways to strengthen, or at least not weaken, houses. It often comes down to "the value of your life versus what you want to spend on this house."
Richardson spent thousands of dollars bolting his own roof to the walls, the single most important way to make houses safer from earthquakes.
Utah's earthquake hazard is being driven by forces deep in the earth that scientists have yet to fully explain. But they know the Wasatch Front is at the ripping edge of the Great Basin desert, which extends nearly 500 miles to Reno, Nev.
The Great Basin has been stretching westward for millions of years, an almost continuous creep measured at nearly a half-inch a year that is widening the continent, said Pechmann.
This upsets the basin's angular crustal blocks, making for slips that can raise mountains and lower basins. Southern California has a different earthquake problem at the San Andreas Fault, which is ripping the coastline along a horizontal plane, but in Utah the action is nearly vertical.
The return of an ancient, lumbering earthquake that violently shakes the ground for hundreds of miles could originate under Brigham City in northern Utah first.
Brigham City is sitting on ground that hasn't seen such a quake for 2,125 years - but the average interval for prehistoric quakes on that fault line is only 1,750 years.
By Paul Foy