Verification experts have a panoply of techniques to determine when and where underground detonations take place and how big they are.

But only one -- detection of nuclear particles or rare gases vented into the atmosphere from deep beneath the ground -- can "unambiguously" show a blast is nuclear in origin, says Vertic, an independent non-governmental organisation in London.

Confirmation of North Korea's second claimed nuclear test will depend on the weather for blowing such evidence towards monitoring stations, the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) said in Vienna on Monday.

"It would take at least two days for closest stations to be reached, and the farther we go, the more time it takes," said CTBTO chief Tibor Toth.

The monitors' frontline tool is the seismological sensor, of the kind that also listens out for earthquakes.

The sensitivity of these instruments, and their expanding network around the world, has vastly improved surveillance over the past two decades.

Even so, the technology has flaws and this was demonstrated by North Korea's first nuclear test on October 9, 2006.

Writing in the journal Nature Physics in 2007, Paul Richards and Won-Young Kim of the Lamont-Doherty Earth Observatory at New York's Columbia University reported how a seismic "event" was swiftly picked up by a station at Mudanjiang, northern China, 370 kilometres (230 miles) north of North Korea's test site.

Within five hours, other stations gave an excellent fix as to location and depth and estimated the "event" as a low 4.0 magnitude on the seismic scale.

But this was the easy part, said Richards and Kim: "Identifying the event as an explosion, not an earthquake -- and as a nuclear explosion -- was more difficult."

A signature from a typical 4.0-magnitude earthquake begins with a rumble of waves that amplifies into a thick squiggle on the seismogram, followed by another packet of waves that then tails away.

The North Korean signal, though, comprised two sharp, early bursts that swiftly faded to an angry buzz, and this showed it was man-made.

"When they are of comparable magnitude, an explosion has a more condensed signal than an earthquake," explained Bruno Seignier of France's Atomic Energy Commission (CEA).

"In an explosion, energy is released in an extremely violent fashion, signaled by a release in higher frequencies."

After determining that a bomb was indeed the cause, the scientists next had to estimate its yield and what kind of device made the blast.

On the first score, the estimates were at first wide-ranging, possibly confused by sensors' distance from the blast and the amplitude of signals carried through Earth's crust.

The 2006 blast was initially estimated by Russia at the equivalent of between 5,000 and 15,000 tonnes of TNT.

That would have potentially have put it in the same range as "Little Boy," the US atomic bomb that destroyed Hiroshima in August 1945, which was around 12.5 kilotonnes.

But other experts swiftly downgraded the test to a kilotonne or even less.

Monday's blast, measured at around 4.5 magnitude, was estimated by Russia at around 10 and 20 kilotonnes, but the CTBTO put it "in the low kilotonne range for this particular magnitude."

As for the type of bomb, seismograms may give little help at very low blast levels.

In fact, the seismic signature in 2006 "remarkably" mirrored that of a blast by two tonnes of conventional explosive, said Richards and Kim.

Confusion was such that experts speculated for several days that Pyongyang had simply detonated a huge pile of TNT to con the world into believing it had joined the nuclear club.

What weighed against this argument was that North Korea would have had to dig a big tunnel and truck in huge amounts of explosives, which would have been spotted by spy satellites.

The speculation was laid to rest several days later when radionuclide sensors proved the blast was nuclear.

But this evidence failed to still debate, continuing to this day, that the bomb may have been a dud -- a big device that fizzled out after failing to achieve a sustained nuclear reaction.

earlier related report
US sees 'progress' toward UN sanctions on NKorea
US ambassador to the UN Susan Rice on Monday reported progress in efforts by seven major powers to agree a draft resolution for tougher UN sanctions on North Korea over its nuclear defiance.

"I think we are making progress and I am hopeful that in due course we will be producing a very worthy and strong resolution," Rice told reporters after emerging from closed-door bargaining with envoys from Britain, China, France, Russia, Japan and South Korea.

The US envoy said the talks were continuing and a Western diplomat, speaking on condition of anonymity, said the seven were hoping that, after consultations with their respective capitals, they would be able to present a compromise text to the full 15-member Security Council on Tuesday.

Last week, the seven powers unveiled a tentative draft that would condemn "in the strongest terms" North Korea's test underground nuclear test last Monday which violated UN resolutions.

Diplomats said there was a consensus on broadening the sanctions against Pyongyang, but the text left out details of a key paragraph on possible, additional sanctions that would be slapped on the Stalinist state.

A Western diplomat, speaking on condition of anonymity, said that under consideration was extending the list of entities targeted for travel bans or financial sanctions.

In addition, a broader arms embargo, tougher inspections of cargo, a freeze on North Korean assets abroad and denial of access to the international banking and financial services were also being mulled, the diplomat said.

The North Korean foreign ministry has meanwhile warned that "any hostile acts by the UN Security Council will be tantamount to the demolition of the armistice," referring to the truce which ended fighting in the 1950-53 Korean War.

It did not elaborate on what it would do but reiterated that Monday's test was a "self-defense measure."

Earlier Monday, Washington warned North Korea not to fire a long-range missile, saying it would worsen tensions after the communist state's nuclear test.

State Department spokesman Robert Wood said that a launch would be a "clear violation" of a UN Security Council resolution approved after Pyongyang's first nuclear test in 2006.

US and South Korean defense officials say there are signs that North Korea is preparing to fire an intercontinental ballistic missile, in what would be its second such launch in as many months.

In an address to the UN General Assembly, UN Secretary General Ban Ki-moon voiced regret that Pyongyang was choosing "to go in a negative direction" that "runs counter to the ongoing efforts of the international community to curb nuclear proliferation and promote nuclear disarmament."

"I repeat my call for the country (North Korea) to refrain from taking any further provocative actions and to return to the process of dialogue," Ban added, referring to the six-party talks involving the two Koreas, China, Japan, Russia and the United States.

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