Finding Fault

Geophysics professor Catherine Snelson is leading research on earthquake activity by detonating controlled explosions across the Las Vegas Valley. She also monitors the area’s reactions to worldwide seismic activity via computer programs.

Seismologist sets off explosives to evaluate earthquake hazards

By Gian Galassi

When UNLV seismologist Catherine Snelson wanted to measure the impact of ground motion on the Las Vegas basin for a research project, she knew she couldn’t just wait around for an earthquake to occur. She had to make her own.

Using more than 7,500 pounds of ammonium nitrate-based explosives, Snelson, along with more than 40 students, colleagues, and community volunteers, detonated 11 carefully controlled explosions, or “shot points,” to produce seismic waves similar to what a small earthquake might produce. Professors Wanda Taylor of geology and Barbara Luke of civil and environmental engineering also lended their expertise to the project.

The August experiment marked the final refraction survey of the Las Vegas Seismic Response Project, a two-year collaborative study designed to evaluate the seismic hazards in the Las Vegas Basin.

“Explosives are the best way for us to record seismic waves without actually recording an earthquake because they create the same effect without producing any of the damage,” says Snelson, an assistant professor of geophysics. “Although explosives are usually equated with their destructive qualities, we use them in a very benign manner. Most people don’t even know that we’ve done anything at all.”

n Visit the Department of Geosciences site at  http://www.unlv.edu/Colleges/Sciences/Geoscience/

The team drilled holes approximately 150 feet deep that crisscrossed the valley at 10 km intervals from Henderson to Summerlin and from Nellis Air Force Base to Red Rock Canyon. A shot point was detonated in each hole and the ground motion created by these explosions was recorded using 840 seismic sensors. That data will help scientists develop a more detailed profile of the basin’s geophysical composition.

The project began last year when researchers from UNLV, University of Texas at El Paso, and the Lawrence Livermore National Laboratory measured the effects of dynamite blasts from a nearby rock quarry and a chemical blast at the Nevada Test Site. Preliminary analysis showed the Las Vegas basin to be much deeper than previously thought, indicating that ground motion created by an earthquake would be both slow and sustained – factors that contribute to extensive earthquake damage. Other studies uncovered at least eight tectonic faults that are capable of producing an earthquake of magnitude 6.5 to 7.0 on the Richter scale.

Should such an earthquake occur, says Snelson, the valley could suffer more than $11 billion in damages and a significant number of deaths and injuries.

Ultimately, the researchers hope to better educate the public and government officials about earthquake preparedness. “The data we’re compiling will provide a geologic and geophysical model of the Las Vegas basin that will not only be used by academics but also by city, county, and state officials who need to evaluate locations on which to build,” Snelson said. “This research will provide information necessary to change existing building codes as well as plan for mitigation in the event that a large earthquake were to occur.”

The researchers will present their preliminary analysis at the American Geophysical Union meeting in December. The data will be incorporated into a computer model being developed by Lawrence Livermore and UNR scientists that estimates the effects of earthquakes on structures.

The data will also be critical in the development of “shake maps,” which identify areas that would be the most adversely affected by such an event. Officials can then plan how to dispatch emergency resources to the areas likely to need them most.
The research is not limited to how the basin will react to an earthquake in the immediate Las Vegas area; it also studies how ground motion from distant seismic events will affect the valley.

“If a large earthquake occurs within 150 kilometers of Las Vegas, it’s going to shake us pretty good and possibly cause some drastic results,” says Snelson. “It’s much more likely that we will experience a quake like that in our lifetime than we will one in our valley.”
Snelson is quick to point out, however, that although her research does not suggest a greater likelihood of a large earthquake occurring in the Las Vegas Valley, there really is no way of knowing when one might hit. “Every place has a local hazard, and earthquakes just happen to be one of ours,” she said. “We may have been lucky so far, but we can’t expect that our luck will always hold out.”