TOKYO — Have North Korea's nuclear tests become so big that they have altered the geological structure of the land? Some analysts now see signs that Mount Mantap, the 7,200-foot-high peak under which North Korea detonates its nuclear bombs, is suffering from "tired mountain syndrome."
The mountain visibly shifted during the last nuclear test, an enormous detonation that was recorded as a 6.3-magnitude earthquake in North Korea's northeast. Since then, the area, which is not known for natural seismic activity, has had three more quakes.
"What we are seeing from North Korea looks like some kind of stress in the ground," said Paul G. Richards, a seismologist at Columbia University's Lamont-Doherty Earth Observatory. "In that part of the world, there were stresses in the ground, but the explosions have shaken them up."
Chinese scientists already have warned that further nuclear tests could cause the mountain to collapse and release the radiation from the blast.
North Korea has conducted six nuclear tests since 2006, all of them in tunnels burrowed deep under Mount Mantap at a site known as the Punggye-ri Nuclear Test Facility. Intelligence analysts and experts alike use satellite imagery to keep close track of movement at the three entrances to the tunnels for signals that a test might be coming.
After the latest nuclear test, on Sept. 3, Kim Jong Un's regime claimed that it had set off a hydrogen bomb and that it had been a "perfect success."
The regime is known for brazen exaggeration, but analysts and many government officials said the size of the earthquake that the test generated suggested that North Korea had detonated a thermonuclear device at least 17 times the size of the U.S. bomb dropped on Hiroshima in 1945.
It registered as an artificial 6.3-magnitude earthquake, so big that it shook houses in northeastern China. Eight minutes later, there was a 4.1-magnitude earthquake that appeared to be a tunnel collapsing at the site.
Images captured by Airbus, a space technology company that makes Earth-observation satellites, showed the mountain literally moving during the test. An 85-acre area on the peak of Mount Mantap visibly subsided during the explosion, an indication of both the size of the blast and the weakness of the mountain.
Since that day, there have been three much smaller quakes at the site, in the 2- to 3-magnitude range, each of them prompting fears that North Korea had conducted another nuclear test that perhaps had gone wrong. But they all turned out to be natural.
That has analysts Frank V. Pabian and Jack Liu wondering if Mount Mantap is suffering from "tired mountain syndrome," a diagnosis previously applied to the Soviet Union's atomic test sites.
"The underground detonation of nuclear explosions considerably alters the properties of the rock mass," Vitaly V. Adushkin and William Leith wrote in a report on the Soviet tests for the United States Geological Survey in 2001. This leads to fracturing and rocks breaking, as well as changes along tectonic faults.
Earthquakes also occurred at the United States' nuclear test site in Nevada after detonations there.
"The experience we had from the Nevada test site and decades of monitoring the Soviet Union's major test sites in Kazakhstan showed that after a very large nuclear explosion, several other significant things can happen," said Richards, the seismologist. This included cavities collapsing hours or even months later, he said.
Pabian and Liu said that the North Korean test site also seemed to be suffering.
"Based on the severity of the initial blast, the post-test tremors, and the extent of observable surface disturbances, we have to assume that there must have been substantial damage to the existing tunnel network under Mount Mantap," they wrote in a report for the specialist North Korea website 38 North.
But the degradation of the mountain does not necessarily mean that it would be abandoned as a test site — just as the United States did not abandon the Nevada test site after earthquakes there, they said. Instead, the United States kept using the site until a nuclear test moratorium took effect in 1992.
For that reason, analysts will continue to keep a close eye on the Punggye-ri site to see if North Korea starts excavating there again — a sign of possible preparations for another test.
The previous tests took place through the north portal to the underground tunnels, but even if those tunnels had collapsed, North Korea's nuclear scientists might still use tunnel complexes linked to the south and west portals, said Pabian and Liu.
Chinese scientists have warned that another test under the mountain could lead to an environmental disaster. If the whole mountain caved in on itself, radiation could escape and drift across the region, said Wang Naiyan, former chairman of the China Nuclear Society and a senior researcher on China's nuclear weapons program.
"We call it 'taking the roof off.' If the mountain collapses and the hole is exposed, it will let out many bad things," Wang told the South China Morning Post last month.
The recent seismic events have triggered another environmental concern, at least on the Internet: that the nuclear tests might trigger the eruption of Mount Paektu, an active volcano straddling the border between North Korea and China more than 80 miles away. The mountain has not experienced a major eruption for centuries, and its last small rumble was in 1903.
But this scenario, experts say, is a stretch.
Volcanic eruptions happen when molten rock flows into the magma chamber under the surface, said Colin Wilson, professor of volcanology at Victoria University of Wellington in New Zealand.
If an earthquake occurs when the magma is hot and, as Wilson puts it, "ready to roll," it could trigger an eruption. But if the molten rock is not activated, then even a large earthquake won't cause a volcanic eruption.
He cited the Tohoku earthquake in 2011, which had a magnitude of 9 but did not cause any of Japan's many volcanoes to blow their top.
"There's no point in kicking a dead horse," Wilson said. "If the horse is up and ready, and you give it a slap on the bum, it will take off. But if it's dead, even if you slap it, it's not going anywhere."