South Korea’s weather agency, the Korea Metrological Administration, said it detected an “artificial earthquake” 30 miles north of Kilju, where North Korea’s main nuclear testing site is located. According to the Associated Press, the U.S Geological Survey confirmed seismic activity of magnitude 5.1.

All three of North Korea’s previous atomic detonations took place in this location, although the AP reports that South Korean government officials could not immediately confirm whether this cause of the seismic activity was indeed a nuclear blast (or, in fact, why they believed the earthquake was artificial).

On Wednesday, Reuters reports, a Japanese government spokesman, Chief Cabinet Secretary Yoshihide Suga, speculated, based on past experience, that the earthquake was a nuclear test.

Also from Reuters:

South Korea’s Yonhap News Agency cited an unnamed South Korean government official as saying Pyongyang appeared to have conducted an ejection test of a submarine-launched ballistic missile (SLBM) in December, following a reported failure of such a test in November.

A South Korean military official told Reuters that North Korea continued to developed submarine-launched missile capability but expects it will take a substantial period of time for it to be able to successfully deploy such a weapon.

North Korea’s third nuclear test was conducted in February 2013. North Korea said it would make a “special announcement” later on Wednesday.

Update – 10:39 pm

North Korea says it has successfully tested a hydrogen bomb, the AP reports. This is the regime’s fourth atomic test. From the Guardian:

North Korea claims the reason for the test is its “legal right” to defend the country against the United States.

The broadcast claimed that if the US does not threaten North Korean sovereignty, it will not need to use nuclear weapons.

The “self-defence” claim is one that Pyongyang has attempted to use before.

Update – 11:05 pm

Some reporters and analysts are skeptical of North Korea’s claim.

Photo via AP Images. Contact the author of this post: brendan.oconnor@gawker.com.