Similarly, landslides are on the move throughout the state, said geologist Francis Ashland, but "99 percent of them we know nothing about and never will" because it's too difficult to monitor this omnipresent Utah geological hazard. The Geological Survey concentrates on populated areas and regions with known historical landslides, turning to the past to help chart the future.

"We haven't seen non-landslide terrain fail too often," Ashland said. "Almost all the landslides that have happened have been reactivations of pre-existing landslides."

Landslides are usually based on clay soils that slicken when super-saturated with water, as they are this year. Slowly, the ground moves, spurred by the combination of water trickling through soil above and groundwater levels rising from below.

Landslides usually move less dramatically than debris flows and tend to break off from faces of slopes rather than moving in channels as debris flows move. A large landslide creates a scarp, or a scar in the hillside where the dirt dropped and slid. Most landslides are small shifts in the surface that are nearly imperceptible — as little as one inch each year, Ashland said.

"If a landslide moves 100 feet, then the ground cracks and deforms and it's obvious," he said. "If it moves an inch a year, there's not a lot of features that would mark that as an active landslide."

Nonetheless, Ashland and the UGS watch the landslides for signs of dramatic movement and stresses on the environment — heavy rainfall, rapid snow melt, earthquakes or development that shapes or changes a hillside. Signs of a slow landslide include cracks in sidewalks, driveways and foundations, and springs or wet soil where it had been dry.

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E-mail: kswinyard@desnews.com