Quakes Keep Shaking Reno; Scientists Stumped

Sandra Chereb in Reno, Nevada
Associated Press

April 30, 2008

Scientists at the University of Nevada, Reno, are scrutinizing seismic readings and studying damage at residents' homes to try to figure out what's happening beneath Earth's surface under a northwest Reno neighborhood rocked by a seemingly endless string of earthquakes.

What they can't say is whether the hundreds of temblors that have rattled the area for two months—the largest a magnitude 4.7 Friday night—are subsiding or a prelude to bigger things to come.

"You're not going to get an earthquake prediction today," John Anderson, director of Seismology Laboratory at the University of Nevada, Reno, said Tuesday during a briefing with Governor Jim Gibbons and emergency managers on the seismic activity.

Scientists are calling the swarm of temblors that began February 28 the "Mogul earthquake sequence," in reference to the neighborhood where hundreds of mostly minor earthquakes have occurred.

But the shaking is unusual, seismologists say, because the intensity of the quakes has increased over the past few weeks. Generally, earthquakes tend to occur and are followed by smaller aftershocks.

In this case, the earth's rumblings have continued unabated, with barely negligible bumps occurring often minutes apart, followed by occasional larger shakers.

It's impossible to know if the temblors are foreshocks of a bigger quake to come or aftershocks of what has been, experts said.

Day After the Day After Tomorrow?

Up until April 15, sizable quakes that could be felt were occurring about once every third day.

Then the rate increased, with about three, 2.0 or larger incidents occurring daily.

On April 24, when the first 4.2 quake was registered, "all of a sudden we were seeing 20 [of the magnitude] 2s and larger per day," said state geologist Jon Price.

Earthquake magnitudes are calculated according to ground motion recorded on seismographs. An increase in one full number—from 5.5 to 6.5, for example—means the quake's magnitude is 10 times as great.

A quake with a magnitude of 6 can cause severe damage, while one with a magnitude of 7 can cause widespread, heavy damage.

"This is an exceptionally vigorous sequence of earthquakes," Price said.

During the past week alone, more than 500 occurrences have been recorded.

Most recently, two measuring 3.1 and 3.2 in magnitude occurred around 11 p.m. Monday. Another 3.1 was recorded at 9:15 a.m. Tuesday.

The largest so far was a 4.7 quake that was registered at 11:40 p.m. Friday. It was preceded 11 seconds earlier by a 3.3 quake, and followed 3 minutes later by one registering 3.4.

The temblors sent goods flying off shelves, cracked walls, broke glass, and collapsed part of a water flume west of Reno. There were no injuries.

They are mostly shallow, occurring just beneath the surface to within a mile or two.

"Shallow makes us believe this is absolutely not volcanic," Price said.

"Needled by Earthquakes"

Mapping of the quakes shows they are clustered around the Mogul and Somersett neighborhoods in northwest Reno, in an area about 2.5 miles (4 kilometers) long and 0.33 mile (0.5 kilometer) wide (see map of Nevada).

Craig dePolo, with the Nevada Bureau of Mines and Geology, said he understands the anxiety of residents who have lived with the persistent shaking.

"What's going on is extraordinary," he agreed. "People are being needled by little earthquakes ... for months."

"And the best we can say is we don't know what's going to happen."

DePolo, who said he's been through many earthquakes, acknowledged that he, too, is "a little nervous."

The governor and emergency managers urged residents to be prepared by strapping down water heaters or any heavy items that could fall and injure people and to have first aid and food provisions on hand.

Frank Siracusa, head of the Nevada Division of Emergency Management, said state, regional, and local agencies train constantly for disasters and have been in daily contact.

"I'd like to say we're prepared, but we can never be too prepared," he said.

The governor said he's "very concerned about the safety of the public" and stressed that residents need to be prepared to minimize risk in the event of a disaster.

Gibbons, himself a geologist, said the earth's movement is what makes the mountains and Nevada landscape special.

"I find it fascinating about our Earth and how it continually evolves over time," he said.

But with Nevada being the second most seismically active state in the continental U.S., he echoed the advice of experts who said large earthquakes are inevitable.

"At some point ... we are going to have a magnitude 6 or 7," Gibbons said.