- A 5.1-magnitude "seismic event" was reported near North Korea's Punggye-ri nuclear testing site late Tuesday evening.
- North Korea's government is claiming the event was a hydrogen bomb test. Hydrogen bombs are a more powerful type of nuclear weapon than the North has previously tested, one that North Korea first claimed to have developed in December.
- There is a real chance that this is a nuclear test: South Korean, Japanese, and Chinese authorities have said they believe the earthquake is manmade, and it is the same magnitude as a 2013 North Korean underground nuclear test.
- However, experts caution, we do not yet have conclusive evidence that the earthquake was, in fact, caused by a nuclear detonation. Nor do we yet know if it was a hydrogen bomb even if it was nuclear.
Is it a hydrogen bomb? And why would that matter?
According to top experts, it's very plausible this was a test. "I think it is *probably* a test," Jeffrey Lewis, the director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies , tweeted. "DPRK [Democratic People's Republic of Korea, the formal name of North Korea] event epicenter close to test site and on 1/2 hour." Generally, earthquakes don't just happen on exactly the half hour.
This isn't yet conclusive. It "may take a few hours to sort out the initial readings," Daryl Kimball, the director of the Arms Control Association, writes. "Let's wait and see," Lewis cautions. So we don't yet know for sure whether it was a test, let alone specifically a hydrogen bomb.
Hydrogen bombs differ from other nuclear weapons by harnessing energy created by fusing hydrogen atoms together rather than by tearing atoms apart (atomic fission). This makes them much more powerful. "Nuclear weapons based on fission typically have a yield of around 10 kilotons or so, while nuclear weapons employing fusion can have a yield measured in megatons. (A kiloton is 1,000 tons; a megaton is 1,000 kilotons.)," Bruce Bennett, a defense analyst at the RAND Corporation, explains in a piece for CNN published when North Korea first claimed to have developed a fusion bomb in December 2015.
It's also possible North Korea is using a sort of in-between weapon called a "boosted" nuclear device. This involves a very small amount of fusion to "boost" the explosive capability of a fission bomb. According to Bennett, these weapons generally have a yield around 50 kilotons.
Lewis thinks that if the seismic event was a test, it was much more likely to be a boosted device rather than a full fission (or "staged") bomb. "Maybe boosted. Definitely not a successful staged device," he tweeted.
This, according to Bennett, is more consistent with North Korea's technology level. "North Korea appears to have had a difficult time mastering even the basics of a fission weapon," he writes. "Because some fusion is involved in such a weapon, Kim may be claiming that he has achieved a hydrogen bomb when in practice he only has a boosted weapon." North Korea does have a long history of exaggerating its military prowess.
If this turns out to be a successful test, hydrogen or otherwise, it won't fundamentally change the status quo in the Korean peninsula. North Korea has had nuclear weapons since 2006, and last tested one in 2013. A hydrogen bomb would be a major technological step up for the North, but would be more a change in degree than in kind in military terms.
Regardless, it would represent a significant provocation on the North's part. Why North Korea would do this is very hard to say: The country is notoriously secretive, and so the reasons for its actions are often difficult to interpret. One reason could be extracting concessions out of its enemies: North Korea occasionally heightens military tensions with South Korea and then demands increases in aid from international actors in exchange for backing down.
Another could be domestic politics. North Korea is a country "where the leadership culture demands a powerful leader, one capable of achieving great accomplishments," Bennett writes. "So it is not surprising that [ruler Kim Jong Un] needs to periodically demonstrate his power. His claim that he has achieved a major advance in nuclear weaponry could be just such a demonstration, focused significantly on his internal audience."