North Korea says it has successfully tested a hydrogen bomb amid reports of a tremor near the main nuclear test site.State media announced the test after monitors detected a 5.1 magnitude quake close to the Punggye-ri site.The North is thought to have conducted three previous underground nuclear tests there since 2006.A hydrogen bomb uses fusion to create a blast far more powerful than that of a more basic atomic bomb.This test was apparently a hydrogen bomb, a step up in destructive power from the plutonium used in previous tests. It gives more explosive power for a lighter weight.After the test in 2013, there was widespread condemnation. The UN Security Council held an emergency meeting at which its members, including China, "strongly condemned" the test. Similar outrage is expected this time. Prime Minister Abe of Japan has said this fourth test was a "serious threat to the safety of his nation".On top of any fourth nuclear test, North Korea also appears to have tested a submarine-launched missile. The ability to launch missiles from submarines would change the whole calculation of military response because warning times of an attack on, for example, the West Coast of the United States would be much shorter.Before the test, North Korean state media said the country "deserved to hold nuclear weapons... to counter nuclear threats by the US". Experts believed before the fourth test that North Korea was still some years from being able to hit a target with a nuclear bomb delivered by a missile. But it is crystal clear that it is absolutely determined to be able to do so. It is also clear that it is improving its abilities rapidly.