A nuclear test by North Korea will generate sound waves, seismic shock waves similar to an earthquake and, if the test site is not properly sealed, a spike in levels of radiation that will all be quickly detected by a global network of sensors, analysts say.
"If North Korea tests, it's highly likely we'll find it out. We have before," said Mark Hibbs, a Berlin-based senior associate in the Carnegie Endowment for International Peace's Nuclear Policy Program.
"After sensors register what looks like a test, we can anticipate there will be some surveillance aircraft flying around the neighborhood and following weather patterns to try to pick up nuclides," he added, referring to particles that would indicate an atomic explosion.
North Korea last week threatened to conduct a third nuclear test and launch more long-range rockets.
The belligerent rhetoric was in retaliation for a new U.N. resolution that reprimands Pyongyang for launching a rocket in December and imposes new sanctions.
North Korea has given no time line for carrying out a nuclear test.
"The most important clue will be a seismic-shock event like an earthquake that will be detected at many stations," said Robert Kelley, the Vienna, Austria-based former director of the International Atomic Energy Agency, the U.N.'s nuclear watchdog.
A very small test would be detected by several nearby stations, he said.
"If it is a larger test — like a 'typical' 10-kiloton bomb test that might be expected — the shock waves will be detected and measured by dozens of stations, which can triangulate right in on ground zero," Mr. Kelley explained.
The United States and North Korea's neighbors in Asia have "extensive seismic, infrasound, and radionuclide monitoring technologies that will detect any North Korean nuclear detonation," said Daryl Kimball, executive director of the Arms Control Association.
"Underground nuclear-test explosions also emit small quantities of certain types of radioactive gases that can be detected by mobile air-monitoring equipment deployed by the U.S. Air Force, and, depending on weather patterns, by certain ground stations controlled by Russia, Japan and South Korea, as well as the international radionuclide monitoring stations operated by the Comprehensive Test Ban Treaty Organization," he added.
In October 2006, a relatively low-yield nuclear explosion by North Korea was easily detected by the International Monitoring System, a worldwide network of technology that helps verify compliance with the Comprehensive Nuclear Test-Ban Treaty.
Telltale radioactive gases from this test were also detected at a monitoring station 4,600 miles away in Canada, said Mr. Kimball.
North Korea's second nuclear test, in May 2009, was confirmed by seismic shock waves. However, no radiation was detected. Analysts attribute the absence of radiation to the likelihood that the explosion took place very deep underground.
Recent satellite photographs of the Punggye-ri nuclear test site in northeastern North Korea show that the site is in a state of readiness and that a test could be conducted within weeks of an order being given, according to an analysis by 38 North, a program of the School of Advanced International Studies' U.S.-Korea Institute.
As North Korea appears set to test another nuclear device, the question remains: Does Pyongyang have enough highly enriched uranium to build a nuclear weapon? A third nuclear test could provide the answer.
"The answer to that question isn't trivial," said Mr. Hibbs.
Gary Milhollin, founder of the Wisconsin Project on Nuclear Arms Control, said North Korea "has had at least one enrichment plant running successfully for some time, and we have to assume that plant has made enough material for a test."
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