Scientists are scrutinizing seismic readings and studying damage to residents' homes trying to figure out what's happening beneath the earth's surface to cause a troubling swarm of hundreds of earthquakes in northern Nevada.
"We're looking at just about everything," said Diane dePolo, a network seismologist at the University of Nevada, Reno's Seismological Laboratory. "We haven't been able to associate it with any known fault."
During the past week alone, more than 500 mostly minor quakes have been recorded in the area. The two most recent widely felt quakes measured 3.1 and occurred about 11 p.m. Monday.
Scientists are calling the swarm of temblors that began Feb. 28 the "Mogul earthquake sequence," in reference to the northwest Reno neighborhood that's at the epicenter of the earthquakes.
Gov. Jim Gibbons was to be briefed later Tuesday by emergency management officials and UNR seismologists, who have said it's impossible to predict whether the temblors are foreshocks of a bigger quake to come, or aftershocks of what has been.
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.
So far the largest was a 4.7 quake that hit at 11:40 p.m. PT Friday. It was preceded 11 seconds earlier by a 3.3 quake, and followed three 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 quakes are mostly shallow, occurring just beneath the surface to within a mile or two.
DePolo said mapping appears to indicate the quake activity is "lining up in a northwest trend."
She said researchers are surveying evidence of ground movement as well as damage at homes to try to determine how the earth is moving.
Much can be learned, experts said, by how homes are positioned and noting which walls pictures fell from, as opposed to others where hangings remained intact.
"What happens at the surface may be different than what happens at a little bit of depth," dePolo said. "We're trying to put the pieces of this puzzle together."
The area where the quakes are occurring is small, she said, measuring about 2.5 miles long and one-third of a mile wide.
"A very small volume of earth is breaking," dePolo said. "The majority of the events have been very localized."
While damage has been minimal, residents' nerves are frazzled.
"You're just so on edge, you just don't know what to do," Cindy Thomas told the Reno Gazette-Journal.
"I can't get it out of my head."
She and her husband, like others in the Mogul and Somersett neighborhoods, packed up a few belongings and went to stay with friends or relatives.
Nevada is the third most seismically active state in the nation behind California and Alaska. Reno's last major quake measured 6.1 on April 24, 1914.
Earthquake magnitudes are calculated according to ground motion recorded on seismographs. An increase in one full number — from 6.5 to 7.5, 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.