Friday, July 23, 2004 | 11:14 a.m.
The ground in the Las Vegas Valley could increase the power of an earthquake should the "big one" every hit, according to a new study of seismic activity in Southern Nevada.
A team of researchers spent two years studying eight known faults in the valley and found that a major earthquake would leave significant damage here, although scientists said such a quake was unlikely in the near future.
The study, conducted by a group of researchers from UNLV, University of Nevada, Reno, and Lawrence Livermore National Laboratory in California, is is the first major study of the faults in the valley in more than a decade and showed how the ground would react in an earthquake, a key factor in what kind of damage an earthquake would do.
The research found that the ground is less stable than previously thought and more prone to shaking in the event of an earthquake, particularly in the northeast part of the valley.
The ground could also amplify the magnitude of an earthquake here, even one that registers at a lower number on the Richter scale outside the valley.
"Basins (like the Las Vegas Valley) shake by factors of 10 when an earthquake passes through them," said Jim O'Donnell, a geophysicist and a member of the Nevada Earthquake Safety Council who has monitored the data collected from the study. "A magnitude 5 earthquake elsewhere would be like a 6 here."
Cathy Snelson, a University of Nevada Las Vegas seismologist who led a portion of the two-year study, said that while Nevada is among the most seismically active states in the nation, the valley doesn't see many earthquakes.
Snelson said a "major event" is not likely and said while there's no anecdotal evidence of such a quake in the last 100 years. But she said there's "a lot of potential here for damage."
Scientists are not very good at predicting earthquakes, O'Donnell said.
"We may not have one tomorrow, but we could have one at anytime," O'Donnell said.
Snelson noted the last noticeable earthquake in the valley was a 3.5 magnitude earthquake in 2001, which was centered on the west side. She called it small, but because of the ground it was felt across the valley.
Of the eight known faults in the valley, one runs directly under the Strip at its southern edge, which could cause a great deal of structural damage to buildings and casualties if the earthquake hit at least 5.9 on the Richter scale, O'Donnell said.
"There will be a lot of shaking going on on the Strip if the focal point of the earthquake is near those faults running under it," he said. "We're not in a high seismic area, but were are at high risk for damage; we have the potential for magnitude 6 or 7 earthquake that could look like a 7 or 8."
For the first time since 1992, when the Energy Department removed several instruments from the valley that were used to monitor explosions at the Nevada Test Site, seismographs were used to monitor movement in the earth's crust in the valley.
That's an important achievement in a fast-growing city where new buildings are being built every day, said O'Donnell.
A 2001 estimate using software from the Federal Emergency Management Agency revealed that a 6.9 magnitude earthquake here could cause $11 billion in economic losses and thousands of deaths.
That, in part, is because the Las Vegas Valley's geological structure acts like a bowl of jelly, Snelson said.
Even a "short and sweet" earthquake here could send strong ripples through the sediment that sits inside the bowl, shaking the Las Vegas Valley at varying degrees of severity depending on the earth's firmness at the different locations, Snelson said.
In August 2003, Snelson and several of her university colleagues set off a series of underground explosions designed to mimic seismic activity. The experiment was designed to help geologists map the subsurface of the valley.
After several months analyzing the data her team collected, Snelson's group determined that the types of rocks in some parts of the valley are less stable and would be more prone to shaking in the event of an earthquake.
As a result, the scientists determined that the northeastern portion of the valley would feel the effects of an earthquake more intensely than the southwestern area would. But the likelihood of a major earthquake in the valley is low, she added.
In explaining why an earthquake isn't likely, Wanda Taylor, a UNLV geologist with the project, said the interval between earthquakes is typically thousands of years.
Though it would be possible to determine when earthquakes last shook the various known fault lines in the Valley, geologists here have not had the funding to do that yet, Taylor said.
But using something called fault scrapes on some of the fault lines Taylor estimated that the most recent earthquakes happened recently enough that they likely would not re-occur in the near future.
Single-family homes with wood frames would be less vulnerable to earthquakes than large steel structures like hotels along the Strip, Snelson said.
Nonetheless, residents should prepare their families for an earthquake emergency, she said.
Helping Las Vegas and its public officials better prepare for a potential earthquake is one goal of the scientists' work.
Currently, only 17 seismographs monitor seismic activity across the valley. For geologists to better predict when an earthquake may occur, they need more equipment Snelson said.
Nevada is the third most seismically active state, after Alaska and California, Snelson said. Most Silver State earthquakes have happened in the northern part of Nevada, but the potential for large-scale damage exists in the south, where more people live, she said.
Ron Lynn, Clark County's Building Services director, said new buildings in Las Vegas follow the International Building Code for the year 2000, and he said the building standards included earthquake regulations that were fairly strict.
Gathering new information on earthquakes is key to setting better building code standards, he said.
"This valley has been under-analyzed," said Lynn, who is also the chairman of the Nevada Earthquake Safety Council, referring to the area's seismic history. "We are only now starting to get information."
Rather than worry about earthquakes, however, people should prepare, Snelson said.
"People in California live with this all their lives," she said. "They don't stop living, nor should we."
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