Las Vegas Sun

March 28, 2024

Earthquake a real possibility

To Nevadans who have grown complacent about the potential for a devastating temblor rocking the region, a state earthquake-activity report presents an unsettling conclusion.

Nevada ranks third among the states, behind Alaska and California, in terms of seismic activity - defined by the magnitude of earthquakes that occur on average once per year, according to a report issued last week by UNR's Nevada Seismological Laboratory.

Nevada's ranking may be a surprise, given that the state has been shaken by only 19 significant earthquakes between 1868 and 1994. But a repeat of those earthquakes, which generated a total of $17 million in damage at the time, would cause $1.57 billion in damage today because of how the state has grown, based on estimates by the Nevada Earthquake Safety Council.

Catherine Snelson, UNLV assistant professor of seismology, says a quake causing significant damage in Las Vegas "is very possible."

Some of the eight faults running through the Las Vegas Valley are capable of triggering an earthquake up to magnitude 7 , Snelson says.

A magnitude 7 quake can cause serious damage over a large area. By comparison, a magnitude 3.5 is about the smallest that can be felt by most people and a magnitude 6 can damage buildings.

UNLV researchers have reported that the region could experience an earthquake that could kill hundreds of people and do more than $10 billion in damage.

Snelson says the faults under Las Vegas typically would produce a magnitude 7 earthquake every 1,000 to 10,000 years. The ability to precisely predict earthquakes eludes scientists.

The potential for an earthquake isn't the only seismological threat to the area. A fault system that is believed by geologists to be more active than those in Nevada lies beneath Death Valley in California, 150 miles from Las Vegas but with the potential of causing serious structural damage here.

"That system is close enough to us that it could be rather devastating," Snelson says.

John Anderson, director of the Nevada Seismological Laboratory, says there are about 60 monitoring stations around the state to study earthquakes here and around the world.

About half of those are funded by the Energy Department to monitor seismic activity in conjunction with its efforts to put a high-level radioactive waste dump at Yucca Mountain, 90 miles northwest of Las Vegas.

Anderson says more data are needed to better understand how earthquakes work throughout the state, but one consequence of thwarting the waste dump would be that many of the data-collection points would be lost.

Nevada's earthquakes are triggered largely by the annual, half-inch northward creep of the Sierra Nevada Mountains while Nevada remains mostly in place. When push comes to shove, earthquakes happen.

Southern Nevada's geology makes the region even more susceptible to earthquake damage, Snelson and others say, because development has occurred atop sediment that has washed down into the valley from the surrounding mountains. In parts of the valley, it is hundreds of feet deep.

Ground waves from hundreds of miles away can reverberate through the bowl of the Las Vegas Valley in a process called "amplification."

Also, the water table under much of the valley is just 50 feet below the surface.

That combination of sediment-causing amplification and the underlying water can lead to "liquefaction," resulting in serious damage.

"We live in the nice big valley, and we kind of look at it like a bowl of Jell-O," Snelson says. The parts of San Francisco that saw the greatest damage after the 1989 Loma Prieta earthquake were those areas that had the greatest amplification and liquefaction.

The same geological pairing played out in a devastating 1985 earthquake in Mexico that killed thousands of residents 200 miles away in Mexico City.

Aly Said, a UNLV structural engineer, says the kind of damage seen in the Mexico City earthquake is not likely in Las Vegas because of better-constructed buildings. Although intuitively the last place most people would want to be is the top of a high-rise building, those buildings in Las Vegas are among the most stable because they were designed to handle strong lateral pressure from wind and earth motion, Said says - even if falling glass and other materials jeopardize people below.

Snelson says risk assessments show the most dangerous buildings would be the four- to nine-story buildings, because they don't absorb the ground waves triggered by earthquakes as effectively.

Said says the same design elements that allow tall buildings to survive high winds also provide lateral strength to overcome earthquakes.

Single-story structures are also generally safe because there is less material to collapse.

A 1999 federal study put Nevada fifth in the country for potential economic loss from earthquakes, echoing the conclusions of a host of studies that put the state close to the top in terms of potential risk.

"We're constantly getting new information that further defines our earthquake risk," says Ron Lynn, head of Clark County's building division and chairman of the Nevada Earthquake Safety Council. "No matter how you look at it, we do have a risk. And we have been addressing that risk."

This year, Clark County and its cities adopted new building codes from the International Code Council that specify building materials, design standards and reinforcing elements within new buildings. The new rules go into effect in May, but rules designed to limit the damage are already in place, Lynn says. High rises, he adds, are "eminently survivable."

Steel and cement in commercial building and wood frames in homes are relatively flexible, providing a measure of survivability to most Clark County buildings, he says. Most vulnerable are older, unreinforced masonry buildings - of which there are none in Clark County, Lynn says, and about five dozen statewide. Most such buildings, such as the state government buildings in Carson City, have been retrofitted for earthquakes.

Lynn, Snelson and others active with the Nevada Earthquake Safety Council say people can at least take safety precautions in their own homes. The biggest killer during and after earthquakes is falling objects, including bookshelves and other tall furniture that can tip in a quake.

Snelson says the admonition to "drop, cover and hold on" is still the best advice: Get close to the ground; get under a heavy structure such as a table, desk or doorway; and stay there until the quake is over.

Earthquake preparedness should include filling an emergency kit with food and water for five days, can openers, portable radio, flashlight, batteries, prescription medications, a first aid kit and cash.

The kits should be refreshed annually.

Snelson says that as Hurricane Katrina dramatically illustrated, disasters happen.

"All we can really do is be prepared," she says. "We can't ignore it."

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