Wasatch Front residents: Need a simple reminder that you're living in an earthquake alley? Look east. See those towering Wasatch Mountains? If you lived in a tremor-free zone, you could probably see Heber on a clear day.
The fact is, we're perched over a sequence of faults.Much of the Beehive State sits atop a junction where stable North America (from the Rocky Mountains east into the Atlantic) and tectonically active North America (the Great Basin and the West Coast) grind together, says M. Lee Allison, director of the Utah Geological Survey.
And we haven't been particularly picky about where we've put our homes, schools, hospitals, churches and other buildings.
Salt Lake County officials were jolted by the Wasatch Front's seismic past when a fault was discovered where crews have been excavating the Salt Palace Convention Center expansion site south of the existing building.
Construction has been halted for several weeks until geologists can determine the age and extent of the fault zone, visible in several places in trenchs and cuts between West Temple and 200 West.
At Faultline Park, near 1050 E. 400 South, Allison paused recently to recount and contemplate the Salt Lake Valley's seismic history.
"You have 6,000 to 7,000 years of offset visibility at this spot," he said. In other words, the park sits high on a steep slope -- a fault-born scarp -- with benchlands to the east and downtown Salt Lake City due west.
In geologic terms, this scarp is recent. Multiply the zone's effect over 6 million to 7 million years or, even better, 15 million to 18 million years, "which is how long we think this fault has been active," and instead of a small scarp you get the Rocky Mountains, he said.
Looking west and southwest, Allison pointed out where the Warm Springs, North Oquirrh, Stansbury, West Valley and Taylorsville faults would be.
Faultline Park, with its small playground and great view of the city, "is a good example of appropriate land-use planning," considering its placement smack atop a known fault, Allison said. A single-story apartment complex downhill is nicely situated, too, with buildings "set back far enough that they would probably survive a major earthquake." But the geologist looked askance at multistory apartment buildings to the south.
"And what's really scary," he said, "is there. Look north and what do you see? A hospital."
A further tour of the valley's seismic hot spots with Allison underscored the tenuous nature ofour placement upon the land.
Washington Elementary School at 420 N. 200 West, he noted, sits beside another dramatic scarp, one created by the Warm Springs Fault. Interestingly, slippery slides for the children make good use of the scarp's slope.
The school is relatively new, essentially a two-building complex rebuilt in the 1970s to the earthquake standards of that time. Under the Salt Lake City School District's current earthquake-safety rebuild/retrofit plan, Washington's main building is to be upgraded by 2002, said Gregg Smith, the district's buildings and grounds manager.
Allison said the Warm Springs Fault beside Washington Elementary starts in North Salt Lake and extends south through the Beck Street gravel quarry into the downtown area. "Our best guess is the Salt Palace is part of it."
The East Bench Fault, of which Faultline Park is a part, stretches in segments roughly from the University of Utah to Cottonwood Mall, mostly following the course of several arterial highways. Or vice versa.
"There's a reason you have those steep hillsides along 1300 East," Allison said. Still, "I'll bet less than 1 percent of the people who drive up and down these hills have any idea that this is an earthquake zone.
"I love the houses in this area," he said, admiring the variety of architectural styles employed in early decades of the 20th century. But were they built to withstand earthquakes? "Not too well," he said. "They're mostly unreinforced brick. Some are wood-frame, which can bend and twist. But brick construction and mortar -- it just comes down."
Between the Warm Springs and East Bench faults, of course, is most of downtown Salt Lake City. This is a "transfer zone," Allison said, with little evidence of surface faulting, the sign of a truly big quake. But that might be deceiving in a way.
The city's vigorous growth over the past 150 years has displaced tons of soil. Decades of construction, excavating and building mean the downtown terrain has been altered. Remember, geologists found the Salt Palace fault 25 feet underground, but the fault was likely near the top of the native soil. Fill dirt from past excavations long ago buried the original surface.
The groundwater level downtown has dropped over time, from wells and pumping (even inside today's modern Salt Palace), which may have helped stabilize the area. Nevertheless, many of the older buildings are built of unresilient brick and stone, the geologist noted.
The main Wasatch Fault hugs the base of the mountain range along the southeastern side of the valley from Mount Olympus to Traverse Ridge. In some places, as at the mouth of Little Cottonwood Canyon, "the fault zone is 100 feet wide, with all kinds of traces and splays," Allison said, every one of them probably marking a different earthquake.
The glacial moraine below Lone Peak's Bells Canyon offers particularly dramatic evidence of the fractures. One ridge is broken by multiple faults and by a classic graben, a spot where the earth was pulled apart and debris fell into the gap.
The area is sprinkled with pricey houses and condominiums, all sitting on unconsolidated moraines, cobbles and dirt "carved out by the glaciers and dumped here on the fault zone" more than 20,000 years ago.
"This is the problem: Everyone wants to live on a high place with a view -- that's what I did," admitted Allison, whose home sits on a more stable segment of the East Bench. "I didn't look into it, and my house isn't wood-framed."
Because the Salt Palace site was being closely scrutinized, the ancient evidence of what might be the southern tip of the Warm Springs Fault was discovered. However, most Wasatch Front homeowners and businesses have no idea where the faults are, and though maps are available, disclosure of geologic hazards is not required in most instances when structures are built or sold, Allison noted.
Just how recent the Warm Springs quake proves to have been is key to the Salt Palace expansion project. If the rupture is more than 10,000 years old, fire up the backhoes and get back to work. It is inactive.
But geologists suspect this evidence is more recent, marking a major quake sometime within the past 6,000 years -- making it both "active," in historic terms, and a major headache for anyone associated with the Salt Palace expansion. Depending on the findings of the ongoing study, building managers will have to decide whether to abandon expansion plans or, at best, how to safely build around the fractures.
Even then, the position of the now-exposed fault "certainly suggests that it continues under the middle of the existing Salt Palace," Allison said.
Excavation projects like this one occasionally reveal evidence of past quakes, as do research-oriented trenches. But in many instances, his office still uses U.S. Geological Survey aerial photos from the 1930s to recall the lay of the land, to pinpoint surface anomalies that may be faults in the Salt Lake Valley, Allison said.
That works fine if you're studying West Jordan and other areas that were predominantly agricultural 60 years ago, but much of Salt Lake City was already built at that time. It is impossible to spot displacement on soil that has been bulldozed, flattened and paved.
"It's just difficult to look for faults in the city," Allison said, though mapping of the fractures and the valley's bedrock underpinning continues.
Remnants of the Bonneville shoreline add to the challenge of precisely mapping faults along the Wasatch Front.
"You wonder," Allison said, "is that a beach or an old faultline scarp?"