The Applied Technology Council, based in California, estimated that a 7.5 earthquake in Salt Lake County could cause direct losses of $9 billion to $15 billion. Indirect losses such as casualties, refugees and shelter needs increase the figure to $11 billion to $18 billion — and nobody can quantify the suffering and bereavement.

Other segments of the fault are vulnerable, too. A magnitude 7 quake hits somewhere on the fault's five segments, from Brigham City to Nephi, about once every 350 years.

"The last one happened about 600 years ago near Provo," the report adds.

Debris flows and landslides

In late August 2001, land on the plain below Thurston Peak slipped in east Layton. Utah Geological Survey researchers concluded that a wet period reactivated an ancient landslide, causing a landslide 1,030 feet wide and 450 feet long. Three houses were destroyed and three were damaged.

Costs to homeowners, insurance companies and Layton city amounted to more than $1 million.

"Anytime you're moving a large piece of land, especially if it associates itself with a neighborhood, then it tears up infrastructure," said Bob Carey, epicenter manager for the Natural Hazards Section, Utah Division of Comprehensive Emergency Management. Besides homes, damage can be severe to "roads, storm drains, sewer lines and ground lines," he said.

Landslides are different from debris flows in that slides are movements of sections of ground. Debris flows are the rip-snorting tumble of rocks, soil, brush and trees, washed into canyons or gullies by storms.

A year ago in Santaquin, a thunderstorm soaked unstable soil that had been exposed by a forest fire. Floodwaters and hundreds of cubic yards of debris flowed off Dry Mountain and into two subdivisions, causing $500,000 damage, the Utah Geological Survey estimated.

In 1991, Carey said, a debris flow in North Ogden destroyed one home and significantly damaged another.

Debris flows roared off mountains in Davis County in 1983, a year of severe flooding. A fast snowmelt pushed through canyons, and some of the worst flows hit Farmington.

Forest fires

As long as trees have grown across mountainsides, wildfires have whipped through the forests. But the danger is greater with each passing year because more people live and work in the Wasatch Mountains.

"You have this growth that's into areas that normally were forests," Carey said. "People have put their houses in the middle of a nice tree stand, and this creates an opportunity for fire to come."

Lightning strikes, careless smokers, people building campfires that get away from them, arsonists — all can cause devastating fires in Wasatch Mountain forests.