The suspicious tremor comes just hours before President Obama is to give the State of the Union address, and it marks the first diplomatic test in the region for new Secretary of State John Kerry.
Also, South Korea's new president, Park Geun-hye, is scheduled to be sworn in Feb. 25. One of North Korea's biggest holidays, Kim Jong-il's birthday, falls on Feb. 16.
Both of North Korea's previous tests used a plutonium-based method for making bomb fuel. The first was deemed a failure, the second only slightly less so. If the most recent test used HEU (highly enriched uranium), a far more difficult-to-detect method of producing bomb fuel, it would be a significant and worrisome step forward for North Korea's weapons program.
U.S. officials and experts say they believe North Korea has enough plutonium for four to 10 devices. But the country's small nuclear reactor in Yongbyon was dismantled in 2008. It is believed North Korea's plutonium stockpile is therefore limited.
Both HEU and plutonium can be used to fuel a miniaturized nuclear device, or warhead. The North Koreans are not believed to currently have the technology to produce a warhead small enough to travel on the end of a long-range rocket. Observers estimate such capability is still many years away.
Plutonium is produced in an above-ground nuclear facility that is relatively simple to monitor using satellite imagery, a drawback for a regime intent on keeping its nuclear program shrouded in secrecy. By comparison, a highly enriched uranium facility, or centrifuge plant, is easy to hide underground. But plutonium bomb fuel is widely considered to be better suited to the kind of warhead North Korea hopes to develop than HEU.
But many believe that A.Q. Khan, the father of Pakistan's atomic bomb who has admitted to selling nuclear secrets to rogue states, sold the North Koreans a Pakastani HEU bomb design that could travel on one of the country's existing rockets.
As Siegfried Hecker, a former director of the Los Alamos National Laboratory who toured the Yongbyon site in 2010, wrote in February on the Web site of Foreign Policy magazine, "If Khan provided both design and test-performance data, Pyongyang may have decided that HEU, albeit less effective than plutonium, was a quicker and more certain route to miniaturized nuclear devices."
Said retired Marine Col. Stephen Ganyard, a former deputy assistant secretary of state for plans, programs and operations in the State Department's Bureau of Political-Military Affairs: "A successful test gives the North Koreans credibility with their main clients, Iran, and ensures a continued flow of Iranian funds to support further development," s
ABC News' Joohee Cho contributed to this report.