Geologists are digging trenches along an earthquake fault line that runs beneath Washington City to determine how often the so-called Washington fault has slipped.

Their goal: gauge the potential for future activity.

If a quake the magnitude of 6.5 or larger is possible, such an event could devastate the city and surrounding communities, including St. George, according to William Lund, a geologist with the Utah Geological Survey.

"There would be a lot of damage from shaking," said Lund. "Unreinforced buildings would collapse, interiors would be disrupted and things fall off shelves."

He said there is also danger of broken natural-gas lines causing fires and the liquefaction, or jelly-like shaking of the subsoil, that can trigger landslides.

The biggest "hazards from an earthquake comes from shaking," said Lund.

The Washington fault runs between a point just south of the Utah state line in Arizona north through Washington in southwestern Utah and to a point just north of that city, according to some maps. Geologists on Friday said it could go all the way to Cedar City.

Scientists have dug two trenches along the fault line in Arizona, about three miles south of the state line, where geologic conditions are best suited for a study.

Lund said they have found evidence of two events where the fault has slipped and displaced the surface.

Samples around the fault will be dated to get an idea of how frequently earthquakes occur.

Once the information from what is found in the trenches is logged and documented, it will be incorporated into the U.S. Geographic Seismic Hazard Map compiled by the U.S. Geologic Survey.

"It [the map] shows how much shaking would occur in any place [in the country]," said Lund. "It helps with building safer buildings."

Geologist Tyler Knudsen, who also is studying the fault, said it has not been studied as extensively as the larger Hurricane fault farther to the east that roughly parallels the Washington fault. That fault poses a threat to Hurricane, La Verkin and other areas around Zion National Park.

It is believed the Hurricane fault caused the 5.2 magnitude quake near Springdale in 1992. That event damaged houses and rattled the park.

Knudsen said much of western Utah is prone to quakes because the surface crust is ripping itself apart across the Great Basin, fracturing rock and causing the faults to slip.

Lund said Reno and Salt Lake City grow about a centimeter farther apart a year because of the tearing action.

Studies at the trenches, dug on Bureau of Land Management property, will be conducted for two more weeks and be filled in by the end of June.