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Utah - A Place Waiting For An Earthquake to Happen!
Two Deseret News Stories of January 18, 1999 tell the deadly story that someday will happen
Fault causes suspension of Salt Palace expansion
Experts don't know if it extends under existing building
By Jason Swensen
Deseret News staff writer
An unwelcome reminder of the Wasatch Front's seismic past has temporarily suspended
construction of the expansion of the Salt Palace Convention Center.
Geotechnical engineers recently discovered a small tectonic fault in the 25-foot-deep
construction excavation site south of the Salt Palace (100 S. West Temple). The fault line is
visible about halfway between West Temple and 200 W.
County officials opted Wednesday to shut down excavation and construction of the expansion for 30 days to allow further studies of the fault by the project's geologic consultants and project designers.
A second opinion on the fault has also been commissioned, Salt Lake County Commissioner
Brent Overson said. "This is a public safety matter; we want to look at all our options," Overson
said. Depending on the findings of the ongoing study, those options could range from
abandoning the current Salt Palace expansion construction site to simply building around the
fault.
The Salt Palace expansion was expected to be completed in July of 2000. What is not certain is
if the fault extends north into the existing Salt Palace Convention Center building. The fault was
discovered under about 25 feet of fill dirt and appears to be at the surface of native soil.
Geological study was not conducted that deep during the excavation of the first phase of Salt
Palace construction.
Lee Allison, director of the Utah Geological Survey, said the geologic feature
discovered at the expansion site may be the southern tip of the Warm Springs Fault, a branch of
the Wasatch Fault. "I think we feel pretty confident that (the fault) is under the existing Salt
Palace and continues north," Allison said.
Geologist David Simon, the consultant who discovered the fault, said the geological studies of
the first phase were "diligent and prudent," adding the project managers did all they could to
identify any geologic problems.
The current Salt Palace building was designed to seismic zone 3 standards, which reportedly
means it would remain standing in a Richter scale 7 earthquake. "We upgraded the steel in the
(Salt Palace) exhibit hall to accommodate what was seen in the Northridge earthquake," said
Salt Palace project manager Ken Ament.
Although the existing Salt Palace is designed to withstand shaking, it is uncertain how the
building would react if there was a rupture along the fault, said Allison, acknowledging that he is
a geologist, not a structural engineer.
Excavation teams plan to dig trenches closer to the existing Salt Palace building to see if the
fault extends that far north.
The fault is considered small because it appears displacement is only about one to three feet,
Simon said.
Geologists are concerned about the fault's liquefaction features. When liquefaction occurs, soil
and sand behave like dense fluid rather than a wet solid mass during an earthquake, assuming
quicksand-like properties. Liquefaction occurs when soils that are granular or sandy, with a loose
density, are waterlogged and are subjected to seismic activity.
A fault is considered active if it is less then 10,000 years old. Although carbon dating is needed
to confirm a fault's age, Allison said preliminary evidence suggests the Salt Palace fault is active.
Allison, who applauded Salt Lake County officials for being upfront about the fault, said he
would still rather be in the Salt Palace during an earthquake then many older area structures built
away from a fault.
Temporarily suspending construction of the expansion project will not be costly in the short
run, Overson said. The halt will cost about $300,000, less than 1 percent of the project.
Long-range costs can only be determined after gathering the findings of the ongoing study.
Ament said dealing with faults is an unavoidable aspect of construction work along the Wasatch
Front, saying several Salt Lake-area buildings are built on fault lines.
The $47.5 million expansion project is expected to add about 200,000 square feet of exhibit
hall, ballroom, circulation and support areas to the Salt Palace. Six hundred underground
parking stalls are also planned. The Salt Palace Convention Center, including the expansion, is
expected to house the main media center during the 2002 Winter Games. If the expansion could
not be built, some redesign would be needed to accommodate the thousands of journalists
expected to cover the Games, said Salt Lake Organizing Committee spokeswoman Shelley
Thomas.
"It would make things a little tighter, but we can make it work," Thomas said.
Wasatch quake is due anytime [Image] Geologists say temblors come in historic cycles
By Ray Boren Deseret News staff writer
Salt Lake area appears to have major quakes every 1,200 to 1,300 years. And Brigham City
area may be the most overdue for a quake.
Until some 18 million years ago, says M. Lee Allison, director of the Utah Geological
Survey, the western portion of the North American continent was being shoved east in a
collision with the tectonic plate under the Pacific Ocean.
He compares the result especially the Great Basin's compressed pattern of mountain ranges
and valleys seemingly ad infinitum across Nevada and western Utah to an accordion.
Or, better still, a throw rug.
"It's like when you push a rug with your foot it folds," he said.
But then the titanic foot eased up. Like a gradually settling rug, "a thousand miles of
continent has started to relax," Allison said.
To those of us in Utah and to our neighbors in the Great Basin, that means an occasional
earthquake.
"It's said that 18 million years ago, Reno and Salt Lake were 75 miles closer than they are
today," he said. "The state geologist of Nevada says four acres a year are being created" in the
Great Basin province between the Wasatch Front and Sierras. "He's arbitrarily assigned it to the
Utah/Nevada border and wants to arm wrestle or something to assign it."
At the same time, some force deep within the Earth perhaps an arc of magma deep beneath
the mantle is pushing upward beneath us, adding to the complex geology of the interior West,
he said.
The Wasatch Fault, stretching about 240 miles from Malad, Idaho, to Fayette in Sanpete
County and home to about 1.6 million people, has several segments. The five central portions
the Brigham City, Weber-Davis, Salt Lake City, Provo and Nephi segments are the best
researched.
In the 1980s, the U.S. Geological Survey and Utah Geological Survey employed a scientific
"SWAT team approach" to dive into the seismic past, bolstering crews, mapping the faults,
digging a series of trenches to better document and understand them and calling upon a variety
of high-tech seismic measuring techniques.
Much of what is known about the Wasatch Fault today, and particularly the fracture's past
6,000 years, comes from that intensive effort.
Just what do we know or suspect?
A few of the central Wasatch segments revealed cycles of surface-rupturing earthquakes,
usually a temblor of 6.5 on the Richter scale or higher. The Salt Lake segment, for instance,
seems to have been the most regular, experiencing major quakes every 1,200 to 1,300 years
four times over the past 6,000 years.
Ominously, the last such event in the Salt Lake Valley was about 1,200 to 1,300 years ago.
The Brigham City segment, too, showed a pattern, one that ended more than 2,000 years
ago.
"Some people think that's the most overdue," Allison said. "There's some concern that stress
is really building up there."
Many people tend to think damage from a big quake will be concentrated along the known
faults. That's not necessarily so, he said.
"Everyone along the Wasatch Fault is subject to some danger," Allison said, including fault
slippage but also shaking, landslides, slope failures and liquefaction.
For one thing, faults aren't even and angular, like the edge of a block or a table. Most
apparently curve possibly two, three or even 10 miles beneath the ground. So an earthquake's
epicenter ground zero could actually be under the center of the Salt Lake Valley.
For another, the valleys along the Wasatch Front are former lake beds they were, until
12,000 years ago, the bottom of Lake Bonneville, a creation of the vanished Ice Age. So most
communities strung along I-15 and its motorway tributaries are sitting not on top of bedrock but
upon unconsolidated "fill" rocks, soil and sand.
When water tables are high or soil is saturated, such material can liquefy, or temporarily act
like a liquid, during the vibrations of a really big quake.
A detailed map of geologic hazards in Salt Lake County, including dashes and dots
representing known and inferred faults, brands patches of the valley according to their potential
for liquefaction, from VL for "very low" to H for "high," with "L-low" and "M-moderate" in
between. Territory along the high benches, for example, tends to be rated "very low"; spots near
the Great Salt Lake and along the Jordan River seem to get the "H."
Utah records about 700 earthquakes every year, but only a half-dozen are of a Richter
magnitude of 3.0 or greater. Destructive, surface-breaking tremors of 7.0 to 7.5 have not
occurred within historic times but are believed to rattle the Wasatch Fault about once every 150
to 350 years.
The relative quietude along Utah's quake corridor is nothing to be reassured by, Allison said.
"I think it's misleading. People say, 'Hey, I've never experienced a big earthquake. My
grandfather never had an earthquake. So we're pretty safe here.'
"We've never had a major earthquake in the time Utah has been settled," Allison said. "But
it's going to happen. The stresses are building up and at some point the rocks are going to fall."
Page Modified January 20, 1999