|
|
Fault capable of 7 to 7.5 earthquake runs under nerve
gas storage area in Utah!
Further, the present nerve gas incinerator is located within a mile of this
dangerous fault
Two Articles appear in Salt Lake Newspapers which push Utah's danger from earthquakes even higher!
The Salt Lake Tribune -- Studies Boost Utah's Seismic Hazard
September 29, 1999
BY LEE SIEGEL
THE SALT LAKE TRIBUNE
Scientists have discovered new evidence that prehistoric earthquakes measuring magnitude-7 or more repeatedly ruptured Utah's Wasatch, East Great Salt Lake and southern Oquirrh fault zones.
The findings underscore that multiple faults threaten destruction in the Ogden-Salt Lake City-Provo area. They also suggest quake-generated lake waves could inundate low-lying areas, and indicate infrequent but powerful quakes could be centered beneath Army chemical weapons facilities in Tooele County.
The studies were discussed Tuesday in Salt Lake City during the Association of Engineering Geologists' annual meeting.
University of Utah scientists David Dinter and Jim Pechmann bounced sound waves off lake-bottom sediment layers. They found the layers were broken by at least three magnitude-7 or stronger quakes on the East Great Salt Lake fault in the last 12,000 years, most recently within the last 1,000 years.
"This is a very active fault and poses a hazard to the Salt Lake Valley-Ogden corridor," Dinter said.
Such quakes would trigger waves "that could flood a lot of the low-lying coastal areas in the Farmington area along the I-15 corridor," he added.
The East Great Salt Lake fault runs northwest-southeast along the west side of the Promontory Mountains and Fremont and Antelope Islands. Dinter found that south of Antelope Island, it bends southwest and may link with the Oquirrh fault zone, which runs along the western base of the Oquirrh Mountains.
Six years ago, researchers found the northern Oquirrh fault zone had produced two or three magnitude-7 or stronger quakes in the past 40,000 years. In a new study, the southern Oquirrh fault zone was examined by paleoseismologist Susan Olig of URS Greiner Woodward-Clyde consultants in Oakland, Calif. The zone covers four distinct fault lines, including the Mercur fault near the southwest end of the range.
Olig and her colleagues dug trenches across the Mercur fault last month. They found evidence that four quakes exceeding magnitude-7 ruptured the fault during the past 140,000 years. She said such quakes happened much less often than on the Wasatch fault, but their strength was larger than expected.
The Mercur fault dips west, so epicenters from future quakes could be directly beneath the Deseret Chemical Depot's nerve-gas storage bunkers and the incinerator where chemical weapons now are being destroyed, Olig said.
The incinerator is designed to withstand a 6.5 quake. The bunkers hold chemical weapons on racks that could tip easily during a milder jolt.
Army officials said this week that Olig's discovery provides more reason to incinerate the aging weapons quickly before a strong quake occurs. Olig said her study probably supports that argument.
She also said quakes on the Mercur and other faults in the southern Oquirrhs "would do significant damage to older structures in the Salt Lake and Provo urban areas."
Dinter said the East Great Salt Lake and Oquirrh fault zones together "could be nearly as dangerous as the Wasatch fault."
Jim McCalpin, president of GEO-HAZ Consulting in Estes Park, Colo., this month dug an 80-foot-deep trench across the Wasatch fault near Little Cottonwood Canyon on the east side of the Salt Lake Valley.
Researchers already knew the fault produced four magnitude-7 to 7.5 quakes in the valley in the past 6,000 years, and they happened almost like clockwork on an average of every 1,350 years. The last was about 1,300 years ago, so scientists consider the valley due for a devastating quake.
McCalpin also found evidence of two and possibly four more major quakes on the Wasatch fault between 6,000 and 15,000 years ago. He said they apparently did not happen at regular intervals like the four more recent quakes.
He speculated the irregular quake activity between 6,000 and 15,000 years ago may be due to the fact ancient Lake Bonneville -- which once covered much of western Utah -- started drying up 14,500 years ago, reducing pressure on the Wasatch fault from the weight of the water.
Today, however, "we're back on the regular track of what the fault wants to do without that big lake [Bonneville] out there," so the more recent pattern of major quakes every 1,350 years is likely to continue, he said.
Brigham Young University geophysicist Al Benson found evidence ancient magnitude-7 quakes on the Wasatch fault zone broke the ground along three north-south fault lines that now are buried beneath the flat sediments of Mapleton and may extend north through Provo. The three faults are located west of the known Wasatch fault line, which is at the base of the Wasatch Range.
Benson said his discovery suggests future Wasatch fault quakes in Utah Valley might rip open the ground not at the base of the mountains, but perhaps to the west under downtown Mapleton and Provo.
"These faults pose potential earthquake threats and . . . should be taken into account when planning, zoning and building in Mapleton," Benson concluded.
~~~~~~~~~~~~~~~
Deseret News, Tuesday, September 28, 1999, 12:00 AM MDT
Quake zone runs under incinerator
Seismologists point to huge tremors in the past
By Joe Bauman
Deseret News staff writer
Earthquake fault zones under the Great Salt Lake and to the west of the lake have produced powerful shakes in the past and one of them dips directly under Deseret Chemical Depot, where the Army is destroying millions of pounds of nerve and chemical agent.
That was the message Tuesday from top seismologists who addressed the Association of Engineering Geologists meeting in the the Little America Hotel in Salt Lake City.
Faults discussed Tuesday ù the East Great Salt Lake and West Oquirrh fault systems ù are in addition to the infamous Wasastch Fault that bisects Salt Lake City. Somewhere along the long Wasatch Fault, quakes occur about every 300 or 400 years. One is believed to be overdue.
The latest faults to be described aren't as well understood as the Wasatch Fault. Geologists are still searching for additional data that could help pin down the timing of their quakes. But both seem to produce gigantic earthquakes from time to time.
Susan Olig of URS Greiner Woodward-Clyde Federal Services of Oakland, Calif., described the results of three investigative trenches the company dug in August in the south Oquirrh Mountains fault zone in Tooele County. Fault breaks uncovered in the trenches showed at least four powerful earthquakes shook the region in the past 130,000 to 140,000 years.
These were all quakes strong enough to break the ground, which meant they were at least of magnitude 7, she said. The fault zone is located less than a mile from Deseret Chemical Depot, and the fault dips directly under the incinerator and storage area, with their racks of deadly chemical warfare agent.
"Traces of the fault lie less than 1 kilometer (about 0.6 mile) from the boundary" of the depot, she said. "The incinerator is inside the boundary of the depot, so it's a few kilometers away. The fault dips to the west, and the incinerator lies to the east. The fault dips underneath . . . the bunkers," she said.
"That's where you would expect the epicenter to be."
Besides the danger posed by shaking the ancient weapons, Salt Lake City could be damaged if a big earthquake occurred in Tooele County. However, she said, much of the force would be dissipated by a separate fault segment about 12 miles to the west of Utah's capital.
Older structures could be badly damaged by shaking and ground liquefaction.
A closer and even more active fault zone running through the Great Salt Lake was described by David Dinter, assistant professor of geology and geophysics at the University of Utah. He towed seismic reflector devices across the lake for 10 days last September, making recordings of fault zones deep beneath the lake bed.
Dinter found evidence for at least three magnitude 6.5 or greater earthquakes in the past 20,000 years. The latest may have been within the past 1,000 years.
The Great Salt Lake poses special dangers from earthquakes, besides the severe shaking and liquefaction that could occur in Salt Lake City, he said.
"It could quite easily set up the lake equivalent of a tsunami," a giant tidal wave, he said. "The lake water would rush down into the depressed area to the west, and then it would set up a major . . . wave that could flood a lot of the low-lying coastal areas, Farmington area, along the I-15 corridor."
At the best guess, the lake fault zones may have big earthquakes every 3,000 or 4,000 years, but more studies are needed to pin down the dates. Dinter said core samples will be taken next year to recover organic material that can be dated through radiocarbon studies.
Page Modified September 29, 1999