Staff at the center watch USGS’ seismic monitoring network 24 hours a day, 7 days a week. They constantly scan the globe for earthquakes, picking up tens of thousands each year in an effort to alert emergency response systems to damaging quakes as quickly as possible.
Along with the earthquakes, though, the center’s sensors sometimes pick up other events — mining explosions or collapses, gas explosions, and, very rarely, nuclear tests.
Last Sunday, the center detected a 4.7 magnitude event in North Korea.
“While the USGS cannot positively identify the seismic event as a nuclear test, it was shallow and located in the vicinity of the 9 October 2006 North Korean nuclear test,” USGS said in a statement on its Web site.
Less than one percent of the events the USGS seismic monitors pick up are man-made explosions, says William Leith, program manager for the Earthquake Hazard Program.
Those events, he said, look different than earthquakes in several ways.
First, as noted in the statement, they’re “shallow,” or relatively close to the Earth’s surface.
“‘Deep’ for seismologists is many miles underground,” he says — much further down than any man-made explosion would occur.
Also, he explains, earthquakes and explosions produce different wave patterns. The shearing motion that happens in earthquakes when rocks slide against one another causes a kind of wave called an S-wave, while explosions produce compression waves called P-waves.
“They look different to the analyst,” Leith says.
But seismic data alone isn’t enough to confirm a nuclear explosion, Leith explains, and USGS does not officially confirm nuclear tests.
That’s the job of another group, the international Comprehensive Nuclear Test-ban Treaty Organization, headquartered in Vienna, Austria, which was set up to monitor possible violations of the 1996 Comprehensive Nuclear Test Ban Treaty. Like USGS, the CTBTO monitors a worldwide network of seismographs. The organization officially confirmed Friday that the event its seismic monitors detected in North Korea was a man-made explosion.
The CTBTO also monitors networks of sensors in the ocean and atmosphere, as well as sensors that can detect nuclear particles and gases.
To confirm that the recent North Korean explosion was nuclear, it’s waiting for the next piece of key data — the presence of nuclear particles or gases.
“Radioactive noble gases will seep out [from an underground test site] even if you contain the particles,” says CTBTO spokesperson Annika Thunborg. “You can never contain it entirely.”
Those radioactive gases travel with the wind, so the CTBTO works with the World Meteorological Organization to forecast and track wind direction, to figure out which of the many monitoring stations in the region — in China, Japan, Mongolia and the Philippines, among others — would pick up the radioactive material first.
Right now, she says, it looks like the station in Japan is likely to see the first signs of this week’s test, assuming that it was in fact nuclear, perhaps as early as Monday.