North Korean nuclear tests trigger earthquakes and fears of radiation leaks

Tensions between the United States and North Korea took a dramatic turn for the worse on Labour Day weekend. That's when North Korea tested what they said was a hydrogen bomb - many times more powerful than the ones dropped on Hiroshima and Nagasaki.

That September 3rd underground nuclear test produced a seismic signal of 6.3 on the Richter scale. And since then, the area has experienced three separate earthquakes.

Many are starting to wonder if Mt. Mantap, where North Korea conducts their underground nuclear tests, is starting to show signs of something called "Tired Mountain Syndrome." And perhaps more worrying, an official from South Korea thinks one more test could trigger a radioactive leak.

Dr. William Leith is the Senior Science Advisor for Earthquake and Geologic Hazards with the United States Geological Survey in Reston, Virginia. In 2001, he and a colleague were the first to use the term "Tired Mountain Syndrome" when describing the results of the Soviet Union's underground nuclear tests.

This interview has been edited for length and clarity.     

Bob McDonald: So once they detonate what they call a hydrogen bomb, what does that do to the rock in the mountain?

Dr. William Leith: The nuclear device of that size is going to vaporize a very large amount of rock. It's going to create a cavity and in that cavity is going to remain the vaporized rock and gas - the nuclear explosion byproducts. Now the other thing that happens is that once that cavity cools or the pressure is released...then that cavity is prone to collapse soon after that pressure relief occurs.

BM: Well tell me about the earthquakes that have happened since their nuclear test on Labor Day weekend. What's happened?

WL: So there have been three small events since the September 3rd nuclear explosion. The second earthquake that occurred on September 3rd may very well have been the seismic signal from the collapse of the cavity created by the nuclear explosion.

These subsequent two smaller earthquakes that occurred after the day of the explosion and what might have been the cavity collapse, those may very well have been triggered small earthquakes, the release of what we call "tectonic stress."- Dr. William Leith, USGS

BM: OK. So that was the earthquake right after the blast. What about the other ones that followed?

WL: Right. So those may have been either a further cavity collapse or they may have been natural small earthquakes triggered by the explosion itself. And the way that those earthquakes are triggered is just because of the significant redistribution of stress in the area right around the explosion.

We also know that there is a history of natural earthquakes in Korea. That is a reflection of the fact that there's natural stress in the crust. So these subsequent two smaller earthquakes that occurred after the day of the explosion and what might have been the cavity collapse, those may very well have been triggered small earthquakes, the release of what we call "tectonic stress."

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BM: So once they've done one underground nuclear test what about trying to do others in the same area under the same mountain?

WL: They've had half a dozen nuclear tests in at least a couple of tunnels at the nuclear test site in North Korea. But there's a lot of mountain there. So what we'd expect to see is for the North Koreans to excavate either a new nuclear test tunnel or a new tunnel off the side of one of their existing tunnels. So you'd see new excavation and that'd be to keep the next test, if there is one, to keep it away from the previous explosions. And that's where we get to this question of a "Tired Mountain Syndrome."

When we published that in 2001 that was to confirm convey that the Soviet nuclear test site in Degelen Mountain that they had essentially run out of real estate. They had more than 200 nuclear explosions at the Degelen test site in more than 100 tunnels and they were out of fractured real estate. So the idea came about through my co-author that this was a tired mountain and it had been hit hard by a very large number of nuclear explosions and it didn't have the strength that it did previously.

BM: So what about the mountain in North Korea. How tired is it?

WL: I would say not very tired. And that's because they've only had as far as we know six underground nuclear explosions and there's a lot of mountain left there.

BM: The director of the Korean Seismological Institute in South Korea says that he thinks if North Korea conducts another test he's concerned about the risk of radioactive pollution. So how likely is it that radioactive material could reach populated zones in neighboring countries?

Highly pressurized hot gas is going to try to make its way out through fractures both the fractures that are created by the explosion itself and by the natural fractures in the rock.- Dr. William Leith

WL: That would happen if there was a significant vent of underground nuclear explosion an unintended vent of the radioactive products of an explosion. Highly pressurized hot gas is going to try to make its way out through fractures both the fractures that are created by the explosion itself and by the natural fractures in the rock.