Washington ? North Korea announced Monday that it had tested a nuclear weapon underground. Here are some questions and answers about the significance of that announcement.

How do we know that North Korea actually detonated a nuclear device?

We don’t. We know only that the North Korean government said it did and that a seismic event was detected in northeastern North Korea. The U.S. Geological Service measured the event at magnitude 4.2; the South Koreans said the magnitude was 3.6. By contrast, the 1994 Los Angeles earthquake was magnitude 6.7 – as much as 1,000 times stronger than the North Korean event.

How does the North Korean nuclear explosion – if that’s what it was – compare with traditional weapons?

The South Koreans estimated that the North Korean explosion was equivalent to 550 tons of TNT, and the French set it at 500 tons. Either way, it would be dwarfed by the bombs the United States dropped on Hiroshima and Nagasaki, Japan, which had the force of 15,000 and 20,000 tons of TNT. The Russians measured the North Korean blast at 5,000 to 15,000 tons.

Does North Korea have the nuclear fuel to make actual bombs?

It does. The Institute for Science and International Security in Washington, D.C., says it has enough plutonium for between four and 13 weapons, depending on their size.

Does the test mean that North Korea can deliver nuclear bombs to targets in the United States?

Probably not. North Korea is generally thought to lack the missiles it would need. Adm. William J. Fallon, the top U.S. military officer in the Pacific, says the failure of a July test launch of a Taepodong 2 long-range missile – it blew up within a minute of takeoff – showed the North Koreans’ deficiency.

Then why so much concern about the North Koreans’ test announcement?

The North Koreans could do a lot of mischief short of attacking the United States. For one thing, they could use the bomb as a deterrent to U.S. military action against them. They could threaten to respond to an American attack by striking Japan, South Korea or another nearby U.S. ally. That prospect could lead to an arms race between North Korea and U.S. allies in Asia.

What about the possibility that North Korea would give or sell its nuclear technology and materials to some of the world’s other bad guys?

That’s another problem. The North Koreans dropped out of the Nuclear Nonproliferation Treaty nearly four years ago, potentially setting the stage for nuclear dealings with the likes of Iran and Syria. “They are an active proliferator,” U.S. Defense Secretary Donald H. Rumsfeld said last week.

What is the United States doing to stop this?

It is monitoring every cargo ship and plane that leaves North Korea. And it is insisting that Pyongyang return to the six-party talks aimed at reining in the North Korean government. (The other four nations are China, Russia, South Korea and Japan.) Pyongyang insists on holding talks limited to the U.S. and North Korea.

How many countries now have nuclear weapons?

Besides the United States and – presumably – North Korea, the nations known to possess nuclear weapons are Britain, France, Russia, China, India and Pakistan. Israel is widely thought to have them but has never acknowledged it.