Despite its location away from major population centers, a swarm of earthquakes in the far northwest corner of Nevada is attracting attention as it continues to rumble residents this week with several magnitude 3.0 earthquakes, a 4.0-magnitude earthquake on Nov. 21 and a magnitude 4.3 on Nov. 17.
"The activity has quieted down somewhat this week, but it has had slow down periods throughout, so we are still closely monitoring the sequence," Ken Smith, seismic network manager in the University of Nevada, Reno's Nevada Seismological Laboratory, said. "The sequence, although slowing down somewhat, is still not over."
The thousands of earthquakes in the last several months is the strongest of the swarm-type sequences recorded in Nevada in recent history with its current count of 12 magnitude 4.0-plus events since it began in July.
"We've located about 1,350 earthquakes, but thousands more that are taking place just can't be located because of the small number of seismic stations in that part of the state," Smith said. "So far there have been 112 earthquakes greater than or equal to magnitude 3.0. We'd have to go back to a similar sequence near Hawthorne, Nev. in 2011 and the 2008 west Reno swarm for anything comparable."
The Nevada Seismological Lab will be collecting more data from the swarm with the installation of additional real-time seismographs completed this week. The stations are closer to the activity and will improve locating the seismic events, Smith said.
"We've been in contact with local residents, and they've been very helpful in finding locations to install this additional instrumentation" Graham Kent, director of the University's Seismological Lab, said. "Residents expressed an eagerness to help, as they are feeling the daily barrage of magnitude 3 and 4 earthquakes. Many Reno residents recall the 2008 swarm, and feeling the large number of earthquakes from that swarm, and especially the larger events."
The added equipment in the remote region will help scientists better understand earthquake swarm behavior and apply that knowledge to other areas of the state that may experience similar sequences of earthquakes in the future.
"One positive from these types of swarms we see in Nevada is that residents get ample time to prepare for possible larger events," Smith said. "That's why we closely monitor all current activity and try to get the word out quickly if these sequences of smaller earthquakes appear to increase in intensity."
This ongoing swarm is located in the Sheldon National Antelope Refuge about 40 miles southeast of Lakeview, Ore., 40 miles northeast of Cedarville, Calif. and about 250 miles north of Reno. Farm, ranch and grazing lands dominate the landscape.
The largest two events (magnitude 4.7) occurred just after midnight Nov. 6 and Nov. 7. A Magnitude 4.1 earthquake struck the area on Nov. 12, at 6:42 a.m.
"People have been feeling the larger quakes just north of Fort Bidwell and on the east side of Massacre Lake in Nevada," Jean Bilodeaux, a reporter with the Modoc Times told the Nevada Seismology Lab. "One man on the east side of Surprise Valley, east of Cedarville, felt and heard his home shake. Some dogs north of Cedarville got panicky just before a 3.0 or larger quake even though the owners didn't feel anything. Checking the times listed for the quakes seem to confirm this."
Some residents have asked if the tremblers are related to an extinct volcano in the Sheldon Refuge.
"We know the Sheldon seismicity swarm is in the shallow crust and seems to have character that is consistent with the regional faults," Bill Hammond, with the University's Nevada Geodetic Laboratory, said.
Using GPS satellites and ground stations, the University's Geodetic Lab measures the shape of the Earth and monitors tectonic and volcanic activity. They track any uplift or subsidence in the range of millimeters all across the globe. The University maintains the world's largest GPS data processing center, able to process information from about 13,000 stations around the world continuously, 24/7.
"We have looked at data from stations that are about 50 kilometers from the swarm and see no anomaly," Hammond said. "We would not expect to, given the size of the swarm's earthquakes and distance to those stations, so we have recently installed more stations near the epicenters and have plans for more."
In recent years, they have measured background plate deformation in the broader northern Basin and Range, including the Sheldon area.
"We see tectonic stretching and shearing of the crust in the vicinity of the swarm at a rate of about one millimeter a year over the 100 kilometers centered on the swarm," Hammond said. "This stretching drives earthquake occurrence in the crust which can be expressed as vigorous swarms we sometimes get in Nevada. However, conclusively ruling out a volcanic source will require the additional seismic and geodetic measurements closer to the events."
Following any sequence of earthquakes similar to what is occurring in northwest Nevada, there is a small increase in the probability of a larger event. Whether a larger event will occur in the northwest Nevada swarm cannot be predicted or forecast. However, large earthquakes can happen anywhere in Nevada, and officials encourage citizens to take steps to prepare for the potential for strong ground shaking.
"Right now, it's not making much impact on the nearest communities, but if this gets into the magnitude 5 range a couple of communities will start to see an impact, and if it reaches magnitude 6.0, which is always a possibility in Nevada, we could see some impacts on people and damage to structures," Kent said. "If this intensifies, the shaking will become more prominent in the communities nearest the epicenter. As with a couple of other swarms we've seen in recent years, this gives the community some warning and time to prepare in case a damaging quake happens. It's a good reminder for all of us that we live in earthquake country."
Nevada is the third most seismically active state in the United States, behind California and Alaska, yet hasn't had a magnitude 7.0 earthquake in 60 years.
"The first 54 years of the 20th century we had seven earthquakes of magnitude 6.5 or larger and haven't had one since," Kent said. "We haven't had the big one since the Dixie Valley/Fairview Peak sequence in 1954 east of Fallon, which was two magnitude 7s four minutes apart. Since then, Nevada's population has grown 13-fold."
The Wells, Nev., earthquake in 2008 was a magnitude 6.0, the largest event in Nevada in 42 years. Historically, Nevada can expect to have three magnitude 7.0 earthquakes per century and one magnitude 6.0 or larger every decade according to Kent.
The Nevada Seismological Laboratory and the Nevada Geodetic Laboratory at the University of Nevada, Reno, the Pacific Northwest Seismic Network at the University of Washington, the Oregon Department of Geology and Mineral Industries and the U.S. Geological Survey are closely monitoring this earthquake activity.