North Korea nuclear test could go undetected by Western agencies, experts say

North Korean leader Kim Jong Un visits the General Satellite Control and Command Center on the outskirts of Pyongyang, Dec. 12, 2012. Kim issued the order for a rocket launch on Dec. 12, 2012, and stressed that "satellite" launches need to be continued, the North's official Korean Central News Agency reported.

SEOUL, South Korea North Korea appears all set to detonate an atomic device, but confirming the explosion when it takes place will be virtually impossible for outsiders, specialists said Tuesday.

The best indication of a test will be seismic tremors and abnormal radiation in the air, but even that can be masked if North Korea wants to. In all likelihood the first word of the test will come from Pyongyang itself, just as it happened when the country conducted nuclear tests in 2006 and 2009.

Last week, North Korea warned a third nuclear test is planned to protest toughened international sanctions meant to punish it for firing a long-range rocket in December. The world sees the launch as a ballistic missile test, banned by the U.N., while Pyongyang says it only shot a satellite aboard the rocket into orbit as part of a peaceful space development program.

The U.S. and its allies pressed the North to scrap its nuclear test plans, saying it would only worsen the country's decades-old international isolation.

Japan's Prime Minister Shinzo Abe told a television talk show Tuesday that Japan and the international community would need to impose "quite severe measures" against North Korea if it conducts the test. He said North Korea would not gain anything from such a provocation.

The threats have placed scientists and experts in South Korea on high alert as any test is likely to aggravate the already high tensions on the divided Korean Peninsula.

South Korea's Defense Ministry said Tuesday it believes North Korea has nearly completed nuclear test plans, confirming satellite analysis last week by the U.S.-Korea Institute, a research group at the Johns Hopkins School of Advanced International Studies.

Its satellite images of the Punggye-ri site -- where the previous two nuclear tests were conducted -- reveal that the North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.

In the event of such an underground nuclear test, earthquake monitoring stations in South Korea can detect seismic tremors accompanied by low-frequency sound waves. While earthquakes trigger seismic waves they don't produce sound waves.

But this is at best a strong indication of the test and not an absolute confirmation.

An earthquake expert at the state-run Korea Meteorological Administration said his office aims to find out the magnitude of the tremor, the time it started and the exact location on the map within 10 minutes of the explosion. He spoke on condition of anonymity because he wasn't authorized to speak to media.

Experts also note that an artificial earthquake, such as created by a nuclear explosion, rarely triggers the same wave patterns as natural quakes.