Utah Due for a Big Quake, Experts Warn

Scientists are discovering more evidence that an ancient rhythm of immense earthquakes could strike northern Utah again. Their findings raise the danger of widespread destruction for the heavily populated 120-mile Wasatch corridor around Salt Lake City.

The 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 that a large quake could occur tomorrow or a century from now -- a span that represents a mere blip in geologic time.

The geologists are doing field work with newfound urgency, most recently digging into a fault complex at Mapleton, near Provo, 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 7-magnitude or greater going back at least 12,000 years. The digging gives an idea of the kind of violence that accompanies sudden releases 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, in greater Salt Lake City, which has been virtually undisturbed since the arrival of Mormon settlers in 1847.

Simple timing underscores why geologists are putting a target on Salt Lake City: 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.

All dates 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 is also poised to release a powerful quake.

Over the last 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 has determined.

Keaton measured as much as 62 feet of drop on the zone’s east side over the last 140,000 years -- movement that would have been produced 11 to 13 quakes of magnitude 6.5 to 6.7. At least six of those quakes occurred in the last 12,000 years.

To illustrate the dangers, URS 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.

Utah’s earthquake hazard is being driven by forces deep in the Earth that scientists have yet to fully explain.

But scientists know that the Wasatch Front is at the ripping edge of the Great Basin, an immense mile-high desert that extends nearly 500 miles to Reno, Nev.

The Great Basin has been subject to millions of years of stretching, an almost continuous creep measured at nearly half an inch a year westward that is widening the continent, said University of Utah geophysicist James Pechmann, an expert on the basin’s crustal structure.

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.

Any way geologists look at it, northern Utah is due for a big one. Pechmann puts the odds that Salt Lake will suffer a large quake at 1-in-3 over the next 50 years.

“We know these earthquakes have been going on for millions of years,” he said.

“There’s no reason to think they’re going to stop just because people moved into the valley.”