Shaking Things Up in Vegas

Catherine Snelson takes an explosive approach to gauging seismic risks in a fault-ridden city.

sci1203blast_A_438.gif

Illustrations by John MacNeill

Las Vegas has an earthquake problem. Recent geologic and geophysical studies indicate that no less than eight active fault lines cut through the valley in which it sits; together they create the potential for a magnitude 6.5 or greater temblor. While no one can predict when such a quake might strike, Catherine Snelson, an assistant professor of geophysics at the University of Nevada, Las Vegas, might soon be able to predict which areas will be hardest hit when the big one comes — a critical step in directing emergency response. We tagged along as she and a handful of sun-baked graduate students trekked into the Nevada desert to set off 4,000 pounds of explosives in an attempt to home in on Vegas’s seismic “hot spots.”

Popular Science: Should I be nervous about all those explosives?

Catherine Snelson: Not at all. Near the city, we’ve packed about 50 pounds of them into holes that are 70 to 100 feet deep, and filled in the rest of each hole with gravel — all you’ll hear from those is a firecracker-like sound. The holes farther away from Vegas, however, hold 1,000 pounds of dynamite. There you’ll feel the earth move and hear a loud bang. But we keep people 200 to 300 feet from the blasting hole to make sure everyone is safe. Plus, anyone using explosives in Nevada has to be certified. The most dangerous part is when I hook up the blasting cap between the charge cord and the detonation cord. At that point I’m holding enough explosive material to blow my hand off, and as little as one amp of energy from, say, a cellphone or a static charge could set it off.

PS: How do the explosions play into your research?

CS: We’ve placed 800 sensors, called Texans, every 100 to 200 meters along the valley floor. Each one records the velocity of the shock waves from the blast as they travel through the Earth’s crust. The waves speed up or slow down depending on the density of the material they encounter. From this data we’ll create a three-dimensional image of the subsurface — kind of like a CAT scan.

PS: What do you do with the 3-D image?

CS: When we merge our data with geologic and geotechnical data — things like fault structure and soil stiffness — compiled by other groups, we’ll be able to create a “shake map” of the valley. This will help us identify hot spots where the ground shaking will be most intense, such as areas with loose unconsolidated sediments where shock waves are trapped and intensified. We should have a shake map of the Las Vegas Basin in place by next fall.

PS: So how does the shake map get used?

CS: Emergency managers will be able to direct their workers to the areas of greatest damage within minutes of an earthquake. The logic used to be that the epicenter was where the most damage occurs. But that’s wrong — it varies depending on the geology.

PS: When was the last big quake?

CS: We know there have been major earthquakes within the last 7,000 to 10,000 years, which is fairly recent from a geologic perspective, but the last good-size quake was a 3.5 in 2001. It didn’t cause any damage, but it definitely woke people up to the fact we could have them.

PS: Give it to us straight: Is Vegas in trouble?

CS: We may never have an earthquake in our lifetime, or we may get one tomorrow. We just do not know. But if we had a
magnitude 6.9 quake right now, we would have losses and damages exceeding $11 billion and injuries and casualties in the tens
of thousands.