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A 120-foot gash dug near the northwest tip of Salt Lake City reveals new evidence of four magnitude-6.5 earthquakes along the West Valley Fault.

Now geologists are scrambling to determine how active the fault is and whether it is connected to the better-known Wasatch Fault rimming Salt Lake Valley's east bench. If it is, scientists warn a future earthquake would be even more severe than historical projections.

The low level of the Great Salt Lake and lower than usual groundwater allowed researchers to analyze in unprecedented detail a 10-foot-deep trench on the fault west of Salt Lake City International Airport at roughly 5000 West and 1000 North.

Touring the trench with reporters Wednesday, geologists said lab work will be available by spring that should pinpoint when the quakes occurred and how big a threat the fault still poses.

So, could the seismic activity dash the hopes of planners and land owners who have ambitious plans to develop the neighboring Northwest Quadrant?

"We suspect the two faults are structurally related," said Mike Hylland, senior geologist at the Utah Geological Survey. "When the Salt Lake segment moves and generates a big earthquake, there's a good likelihood that the West Valley Fault moves at the same time."

Standing alongside the trench — in the wind-swept but peaceful grassland with panoramic mountain views — it's hard to imagine the setting as the source of such violence. But Hylland says "squirrelliness" in the sediment layers reveals ruptures of at least 6.5 and possibly magnitude-7 quakes. The four now known nearly match the "five or six" major quakes scientists reference from the Wasatch Fault.

The West Valley Fault runs roughly 10 miles from 1700 North to 4800 South between Redwood Road and 5600 West. Development obscures much of the fault line except at its north end near the Great Salt Lake.

Hylland says little is known about the West Valley Fault. Until now, it had only been studied twice, compared with dozens of studies of the Wasatch Fault throughout the decades.

While modern building codes ensure that most structures will survive a magnitude-6.5 quake, project geologist Chris DuRoss says a 6.5 to 7.0 event would cause widespread shaking and "extensive damage."

"The key question is whether these two faults are separate sources of earthquakes," DuRoss said. He said significant quakes typically occur every 1,300 years.

"We're right on schedule," DuRoss said.

During the past several decades, Kennecott Utah Copper has spent millions of dollars addressing the threat that old tailings ponds could pose to Magna should an earthquake strike along the West Valley Fault.

Now planners for the Northwest Quadrant may have to re-evaluate any large-scale development penciled in two miles to the west of the fault.

"Oh, absolutely," said Barry Welliver, a structural engineer with BHW Engineers and past chairman of the Utah Seismic Safety Commission. "The games do change, based on new science."

Welliver says building codes are generally conservative enough to ensure new development falls in the acceptable range for quake risk. But he calls the risk of serious land failure and liquefaction "game changers."

"Certainly, any approving body would be foolhardy" he said, "to ignore or minimize the fact that there has not been a lot of research done along the fault line there."

Even so, Salt Lake City Planning Director Wilf Sommerkorn said susceptible lake-bed sediment stretches across the valley, toward downtown, and as far as 1300 East.

"If we're concerned about development there, we would also have to be concerned about development anywhere," he said. "You get to the question of how much acceptable risk is there? We've done that by just building a city here."

Sommerkorn says the liquefaction map for the quadrant clearly identifies the fault. Before any construction could begin, he says special geo-technical reports would be required to identify hazards.

Hylland says the new data soon will be included on National Seismic Hazard Maps, a basic tool engineers rely on before building.

If the maps reflect changes, Sommerkorn says city ordinances would automatically require more specific geological surveys. —

Northwest Quadrant geology

Evidence of four significant, magnitude-6.5 earthquakes west of the Salt Lake City International Airport has raised new questions about whether a massive development envisioned for up to 100,000 people in the barren Northwest Quadrant makes sense. Salt Lake City is working on a master plan for the quadrant that concerns urban planners and environmentalists. A trench revealing earthquake activity during the past 15,000 years is located at 5000 West and 1000 North, roughly two miles east of the potential housing and 25 schools in the quadrant. Geologists are still studying whether the West Valley Fault and Wasatch Fault are connected. If they are, a future earthquake would be more severe for the entire Salt Lake Valley. A prominent structural engineer says the new evidence "absolutely" should give west-end developers pause, calling the threat "foolhardy" to ignore. —

Map pinpoints West Valley Fault

The West Valley Fault runs roughly 10 miles from 1700 North to 4800 South between Redwood Road and 5600 West. Development obscures much of the fault line, except at its north end near the Great Salt Lake. Geologists believe it has been the site of four major quakes, ranging in magnitude from 6.5 to as great as 7.0. See a map. › B1