Besides earthquake-monitoring organizations like the USGS, the Comprehensive Test-Ban Treaty Organization in Vienna, has a network of seismic, sonar and radiation instruments designed to pick up nuclear tests. It also has sensors that can detect gases that may leak into the atmosphere from the explosion.

But determining the sophistication of the nuclear device, and what kind of material -- plutonium or uranium -- was used, will be considerably more difficult, experts say.

At some point, North Korea is likely to announce that the explosion has taken place.

"Pyongyang will almost certainly claim that the test was successful and will tout its sophistication. It will be difficult to distinguish truth from propaganda, but experience shows there is often a nugget of truth in North Korea's claims," Stanford's Hecker says.

What stage will North Korea's nuclear weapons program be at following a new test?

With hard facts about the test so scarce, analysts are busy theorizing what exactly North Korea means when it says the test will be of a "higher level."

There is a widespread expectation that it will involve the use of highly enriched uranium, whereas the country's two previous tests are understood to have involved plutonium-based devices.

"A successful uranium test indicates that Pyongyang has advanced centrifuge technologies and related support systems," Notre Dame's Lopez said. "It means that North Korea, if left unchecked, can both produce and export such material."

In an article for the Bulletin of the Atomic Scientists last year, Hecker and another analyst, Frank Pabian, speculated that North Korea could test two devices at the same time, one using plutonium and the other uranium.

"Two detonations will yield much more technical information than one, and they will be no more damaging politically than if North Korea conducted a single test," they wrote.

Some observers have even suggested that Pyongyang could make an early attempt at testing a thermonuclear device, which uses nuclear fusion to create a more powerful explosion. But others say they don't believe the North has that ability within its grasp yet.

In any case, the test is expected to take North Korea closer to having a nuclear weapon it can direct at its enemies. But actually achieving that goal still remains a longer-term effort, according to Joseph Cirincione, president of the Ploughshares Fund.

"I still think we're years away from North Korea having a capability to deliver a nuclear warhead on a missile even to a country as close as Japan or South Korea," Cirincione said recently. "And they're even further away from having a long-range missile that could hit the United States."

What are the consequences likely to be?

The region is already braced for the test to take place, and countries like the United States, South Korea and Japan are already preparing their response.

John Kerry, the new U.S. Secretary of State, spoke to his counterparts in Tokyo and Seoul by phone on Sunday, and all of three of them agreed that the North must understand "that it will face significant consequences from the international community if it continues its provocative behavior," according to a summary of the calls from the U.S. State Department

A push for fresh condemnation and sanctions from the U.N. Security Council is likely, but whether or not the new measures have much bite depends on China.

In the event of a new nuclear test, Beijing is likely "reduce its assistance to North Korea," the the state-run Chinese newspaper Global Times said in an editorial last month.

But it added that "if the U.S., Japan and South Korea promote extreme U.N. sanctions on North Korea, China will resolutely stop them and force them to amend these draft resolutions."

Fundamentally, analysts say, a new test won't upend the geopolitical situation in Northeast Asia. But it will seriously harm the chances of any meaningful dialog between Pyongyang, Seoul and Washington in the near future.