North Korea's provocative nuclear testing may not necessarily lead to war. But there may be another catastrophic risk: the mountain beneath which it has exploded six bombs may be on the point of collapse.

Hong Kong's South China Morning Postis quoting a Chinese scientist as saying if the peak crumbles, clouds of radioactive dust and gas will blanket the region.

At the heart of the fears is the Punggye-ri test site in the rogue-nation's northeast.
It's carved deep into the side of Mount Mantap.

This is a nondescript granite peak in the remote and heavily forested Hamgyong mountain range about 80 kilometres from Chongjin, the nearest big city.

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Geophysicist Wen Lianxing and his team at the University of Science and Technology of China in Hefei, Anhui province, has posted an analysis of data collected from more than 100 seismic monitoring sites across China. This has narrowed down the location of Pyongyang's nuclear tests with a margin of error of just 100m.

They've all been under the same mountain.

And the weekend's blast was by far the biggest of them all. It produced a shockwave similar to that of a magnitude 6.3 earthquake, indicating a bomb equivalent to some 120 kilotons of TNT explosive.

Eight minutes later, a second earthquake was detected.

Geophysicists around the world have been wracking their brains as to the implications of this second event.

So far the general consensus is it was likely part of the interior of the mountain collapsing in on a cavern created where the rock was vaporised by the blast of the hydrogen bomb.

Satellite imagery also shows the blast caused numerous landslides around the Punggye-ri test site, according to the Washington-based 38 North monitoring project.

Radiation sampling in China, South Korea and Japan have so far returned no abnormal readings. So the chances of a leak having already been generated are very low.

But Chinese nuclear weapons researcher and chair of the China Nuclear Society Wang Naiyan has told the Morning Post such an event indicates a potential major environmental disaster in the making.

Essentially, the mountain has been structurally weakened by the five blasts.

Even one more test could cause it to collapse in on itself, he warns.

"We call it 'taking the roof off'," Wang told the Morning Post. "If the mountain collapses and the hole is exposed, it will let out many bad things.

"A 100 kiloton bomb is a relatively large bomb. The North Korean government should stop the tests as they pose a huge threat not only to North Korea but to other countries, especially China."

Earthquake and Volcano of the Korea Meteorological Administration Director Lee Mi-seon briefs to the media about artificial earthquake in North Korea. Photo / AP
Earthquake and Volcano of the Korea Meteorological Administration Director Lee Mi-seon briefs to the media about artificial earthquake in North Korea. Photo / AP

Satellite photos taken just a day after the blast and released by 38 North reveal new gravel and scree fields shaken loose by the blasts at an elevation of about 2205m. 38 North analysts say these appear more numerous and widespread than those caused by previous detonations - which would be in keeping with the increased size of the bomb.

But they report no sign as yet of a collapse crater caused by the strong post-test tremor.
Chances are, North Korea will continue to test at the Punggye-ri test site as it has few other suitable locations.

Nuclear testing requires a mountain of solid rock with a peak, but it must also have only gently sloping sides to reduce the risk of landslides. Another factor is whether North Korea has drilled vertically beneath the mountain before planting its bombs, or tunnelled horizontally into its heart.

Things may not be so dire if North Korea had expended the extra time and effort necessary to go deep, Wang says. But if it had simply drilled into the side of the mountain, this increased the risk of "blowing the top off".

North Korea has given no indication it intends to end - or slow - its testing of hydrogen bomb warheads intended to be carried by its new intercontinental ranged ballistic missiles.

A top North Korean diplomat has warned that his country is ready to send "more gift packages" to the United States as world powers struggle for a response to Pyongyang's latest nuclear weapons test.

North Korea's ambassador to the United Nations Han Tae Song overnight declared the testing of its sixth and largest bomb to be a complete success.

"The recent self-defence measures by my country, DPRK, are a gift package addressed to none other than the US," Han told a disarmament conference.

"The US will receive more 'gift packages' ... as long as it relies on reckless provocations and futile attempts to put pressure on the DPRK."