The blast was so powerful that the first tremor was followed by a second, weaker one minutes later, which the United States Geological Survey called a “collapse.” The second tremor was detected in China but not in South Korea; officials in the South said that would be consistent with a cave-in at the North’s underground test site.
Just last week, North Korea fired a ballistic missile over Japan, sharply escalating tensions in the region. In July, Pyongyang launched an intercontinental ballistic missile capable of reaching the American mainland, and the North responded to Mr. Trump’s “fire and fury” rhetoric by threatening to fire missiles into waters around Guam, a United States territory that is home to military bases.
Japan’s foreign minister, Taro Kano, said Japan had requested an emergency meeting of the United Nations Security Council. President Moon Jae-in of South Korea and Prime Minister Shinzo Abe of Japan called emergency meetings of their national security council.
Earlier in the day, Mr. Abe and Mr. Trump had spoken by telephone and resolved to put more pressure on North Korea.
North Korea has conducted a series of nuclear and ballistic missile tests since 2006. Its previous nuclear tests have produced increasingly larger blasts. The last test, in September 2016, yielded one about as powerful as the bomb the United States dropped on Hiroshima in 1945.
In its fourth nuclear test, in January 2016, North Korea claimed to have used a hydrogen bomb. Other countries dismissed the claim for lack of evidence, but experts have said that the North may have tested a “boosted” atomic bomb, in which a small amount of thermonuclear fuel produced a slightly higher explosive yield but fell well short of a true hydrogen bomb.
Hours before the tremor was detected on Sunday, North Korea’s state news agency said the country had developed a hydrogen bomb that could be mounted on an intercontinental ballistic missile. The report offered no evidence for the claim, other than photos of Kim Jong-un, the country’s leader, inspecting what it said was the weapon.
Mr. Trump’s aides have concluded that his options in responding to a North Korean nuclear…