United States fusion explosion at Eniwetok Atoll in the Pacific, 1952. (Photo by Three Lions/Getty Images) (Edit)

A sharp, shallow tremor has been recorded in North Korea — strong signs that the nation has conducted another underground nuclear test.

US seismologists said the 6.3 magnitude quake in the north-east of the country was a "possible explosion". … The tremor comes hours after North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.

A hydrogen bomb implies a fusion weapon, which is both considerably more complex and generally much larger than any weapon previously tested by North Korea. The strongest test conducted by North Korea so far has been reported at 30 kilotons — about twice the yield of the weapon which the United States used on Hiroshima. However, a typical fusion device has a yield several times higher. In the case of some designs, over 1,000 times higher. The first US test of a fusion weapon in 1951 had a yield of over 250 kilotons.

The last North Korean test, associated with the 30 kiloton weapon in September 2016, generated an earthquake with a magnitude of 5.3. This new test appears to have been considerably larger.

Initial reports from the US Geological Survey put the tremor at 5.6 magnitude with a depth of 10km (six miles) but this was later changed to 6.3 magnitude at 0km.

The scale for earthquakes is logarithmic. A 6.3 magnitude is actually 10x as strong as a 5.3 magnitude. If that was directly indicative of the energy released, the weapon involved would seem to have been much larger than previous tests. Anything beyond 60KT is very likely to represent a test of a fusion device.

There appears to have been a cave in related to the test, which could result in the release of radioactive materials.