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| Sunday, April 11, 1999
Copyright © Las Vegas Review-Journal Valley faults capable of healthy jolt A quake causing rampant damage could happen in Las Vegas along one of a number of faults around the valley. By Keith Rogers
Review-Journal Earthquake faults in the Las Vegas Valley have the potential to deliver as much ground shaking as the deadly 1994 Northridge quake in California, geologists say.
"Those fault systems could be a Northridge-type hazard that could have strong shaking right here in the basin," according to Craig dePolo, research geologist with the Nevada Bureau of Mines and Geology.
He said the most prominent threat in the valley is Frenchman Mountain Fault, which could deliver a magnitude-6 to magnitude-7 earthquake based on its 20-mile length and the fact that the fault has shifted before, leaving rock layers offset by as much as 6 feet. Based on preliminary examination of the fault, geologists believe an earthquake of that caliber happens every 10,000 to 50,000 years.
The magnitude-6.7 Northridge earthquake struck at 4:31 a.m. on Jan. 17, 1994, in the San Fernando Valley region of Los Angeles. Fifty-seven people were killed by the quake, which caused widespread structural damage to hundreds of homes and offices, including apartment complexes and buildings at California State University, Northridge some 20 miles northwest of Los Angeles. Ground motion was felt as far away as Las Vegas, 275 miles northeast of the epicenter.
Earthquakes are caused by the gradual movement of huge, tectonic plates, or rock layers, that make up the Earth's crust. Magma, or molten rock from the mantle, drives the movement of the plates as it pours through vents in ocean floors, forming new rock that, like dominoes falling against each other, pushes against the continental plates and builds up strain. Large mountain ranges and plateaus collapsing under their own weight also can stress the crust and cause earthquakes.
Like a pencil that someone grips at each end and pulls steadily downward until it bows upward in the middle and then snaps, an earthquake occurs when strain is relieved in a similar way on the plates deep within the crust.
The energy released when that happens can be enormous. For example, the magnitude-4.7 earthquake that rocked the Nevada Test Site on Jan. 27 released enough energy to knock pictures off walls and leave file drawers ajar. It packed about the same energetic punch that would be delivered from detonating about 5,100 tons of TNT.
The magnitude-6.7 Northridge earthquake released about 900 times the energy as the magnitude-4.7 earthquake, or the same amount that would be generated from exploding more than 5 million tons of TNT.
In the Northridge quake, one side of the fault pushed upward, against gravity, extending over the other side of the fault and causing much damage. Geologists call that type of movement a reverse-slip.
Fortunately, geologists say, movement on Nevada faults is likely to be normal-slip, where one side of the fault falls downward with gravity. In other cases, Nevada faults move in a strike-slip fashion, where the two sides slide past each other. Or the fault movement can be a combination of the two. In either case, damaging ground motions would tend to be less than a Northridge-type quake.
Geologist Burt Slemmons, a member of the Nevada Earthquake Safety Council and professor emeritus at the University of Nevada, Reno, estimates the Las Vegas Valley faults move about one-tenth of a millimeter per year. In contrast, California's notorious San Andreas Fault slips northwest at a much faster pace, 38 millimeters per year near Taft.
DePolo said movement of the Frenchman Mountain Fault would cause the Las Vegas Valley to stretch slightly westward, as it has for millions of years, pulling apart from its eastern rim.
According to Slemmons, there are seven fault zones in the Las Vegas Valley. They are:
--Frenchman Mountain Fault
--Whitney Mesa Fault
--Cashman Fault
--Valley View Fault
--Decatur Fault
--Eglington Fault
--West Charleston Fault
With the exception of the West Charleston Fault, which has an offset of only a few feet, the other six have displaced the surface from between 40 feet and 125 feet during the past few hundred thousand years, according to Slemmons.
Besides the Frenchman Mountain Fault zone, the other fault lines also pose an earthquake hazard, as do so-called background earthquakes -- those that aren't linked to any known fault and don't rupture the surface.
The magnitude-2.7 earthquake that rattled residents in the northwest Las Vegas Valley on Dec. 14 was a background quake. Background earthquakes, dePolo said, are generally magnitude 6 or less.
Another example is the magnitude-5.6 Little Skull Mountain earthquake, which occurred June 29, 1992, at the Nevada Test Site, 24 hours after the Landers, Calif., earthquake sequence. The Landers sequence included a magnitude-7.4 quake near Yucca Valley, a magnitude-6.5 quake near Big Bear and hundreds of aftershocks.
The Little Skull Mountain earthquake occurred on a subsurface fault, or what's sometimes called a blind fault. The epicenter was about three miles from the closest trace of the Rock Valley Fault. At the surface, seismic waves knocked out windows and cracked walls at the Yucca Mountain Project field operations center, some 12 miles away.
"It would not surprise us at all if an earthquake like the one at Little Skull Mountain occurred in Las Vegas," dePolo said.
On March 4 scientists using seismic data gathered for drilling commercial oil wells in the Los Angeles Basin discovered such a buried fracture, the Puente Hills Fault, lurking beneath the heavily populated downtown Los Angeles area.
A fourth seismic hazard to the Las Vegas Valley, dePolo said, are earthquakes that could occur outside the valley with strong enough ground shaking to cause damage within it. Such could be the case, he said, if a magnitude-7.4 earthquake erupted along the Furnace Creek Fault in Death Valley, some 90 miles west and north of Las Vegas.
Another system there, the Central Death Valley Fault, is capable of a magnitude-7.2 temblor. He said geologists estimate such strong earthquakes occur in Death Valley every 500 to 1,000 years.
The 1932 Cedar Mountain earthquake, a magnitude 7.2, shook the Las Vegas basin, but there weren't many structures built at the time, and little damage was reported.
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