Hundreds of small quakes leave Reno area residents shaken

Neighborhoods west of Reno are being jolted by a swarm of quakes that are fascinating seismologists but frustrating residents.

"People are very anxious. They're not sleeping well. They're ready for it to be over," said Glenn Biasi of University of Nevada, Reno, Seismological Laboratory.

So far experts can't say what's causing the swarm, how long it will last or what's coming next. One consolation, though, is that a quake much bigger than 5.5 or 6 on the Richter scale seems pretty unlikely.

"Structurally, it doesn't look like the start of something big," Biasi said.

The swarm, which began Feb. 28, hasn't produced any quakes with a bigger initial measurement than Friday's 4.7 event. It has produced hundreds of smaller quakes, though, many so shallow that people notice the shaking even at magnitudes of 1.5 or 2.

Later this week seismologists at the Reno lab hope to place more monitoring equipment just west of Reno, near the communities of Mogul and Somersett.

While the quake region lies north of Interstate 80 roughly in line with the eastern shores of Lake Tahoe, it appears to be a small, concentrated area of disturbance with no potential impact for Tahoe communities.

"There's no obvious connection of any of the faults in the Tahoe basin to this," Biasi said.

"Earthquakes of this size can occur just about anywhere," he added. "You only need an unhappy kilometer or so of rock to pop, and there are a lot of stresses down deep we know nothing about."

The earthquakes, which have been cracking pavement and breaking glass, are originating from a buried fault zone that previously showed no surface signs of its existence.

There's no rift, valley or telltale displacement like those commonly found along the San Andreas or Hayward faults in California. In fact, the land shows signs of enduring stability – balancing rock formations dating back thousands of years. A strong, recent quake would have toppled those precariously perched rocks, Biasi said.

Earthquake swarms – multiple quakes clustered in a small area – are fairly common along the West Coast, spanning anywhere from a few minutes to as long as a year, said John Vidale, director of the Pacific Northwest Seismic Network.

No one is certain what causes them, Vidale said. One theory is that as faults deep in the Earth's crust move, they place added stress on higher areas, which is then released through swarms. Another theory suggests that underground movement of water or magma triggers swarms. Or, it could be that one earthquake starts a disruptive chain, driving another quake and then another.

If seismologists knew what was happening, they could zero in on factors that might help them predict the likelihood of quakes, he said.

While earthquake swarms aren't unusual, this one is "more energetic than most … it's peppy," said Vidale, with an abundance of quakes that people notice. On Monday, one quake registered 4.2.

Swarms are also a reminder that earth scientists still know relatively little about many of the processes that occur well below our feet.

We have only a few decades of detailed quake monitoring data, and when seismologists do venture into predictions, they're usually talking about something that might happen in the next few hundred years, said David Oppenheimer, a seismologist with the U.S. Geological Survey in Menlo Park.

"You see these swarms come out of nowhere, and all of a sudden you start to realize we don't have a very complete picture at all," Oppenheimer said.

"Is there a lot of (ground) strain in the Reno area? We don't even have that answer," he added. "Are the faults close to failure, or are they thousands of years away from failure? We have no idea."

As they track the Reno swarm, scientists should learn more about what can cause a relatively forceful string of shallow quakes, said Nevada seismologist Biasi.

For rattled residents at the far western edge of Reno, who just want to know when the shaking will stop, that's little comfort.

"The maybe not terribly consoling thing is that we're watching it, and we'll let them know if we get indications that we're done," Biasi said.

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