North Korean leader Kim Jong Un points to a device with the characteristic two-hemisphere peanut shape of a fusion bomb in an image released by North Korean state media in 2017. Officials in Pyonyang said the object was a hydrogen bomb. Photo: Korean Central News Agency / AGU

North Korea detonated a nuclear device in 2017 equivalent to about 250 kilotons of TNT, a new study estimates, creating an explosion 16 times the size of the bomb the United States detonated over Hiroshima, Japan, in 1945.

That would make it powerful enough if detonated in the center of Washington to kill virtually everyone in a radius of 1.37 kilometers.

The new assessment of the 2017 explosion’s size is on the high end of previous estimate ranges. The 2017 test was an order of magnitude larger than the previous five underground tests at North Korea’s Punggye-ri test site, according to the new study by researchers from institutions including the University of California Santa Cruz and the Seismological Observatory of Costa Rico, published  in the Journal of Geophysical Research: Solid Earth.

The new study took into account the geology of the test site to estimate the size of the explosions from distant seismic recordings of the blasts.

The report comes during a week in which North Korea has warned that it’s losing patience with stalled peace and denuclearization negotiations. The US “should duly look back on the past year and cogitate about which will be a correct strategic choice before it is too late,” said a statement published by the official Korean Central News Agency. “The US would be well-advised to change its current method of calculation.”

Estimated explosive size of the six nuclear tests at the Punggye-ri test site on Mt Mantap, North Korea, in units equivalent to kilotons of TNT. Black lines mark the estimated yield of the bombs and blue boxes indicate the estimate range, factoring in uncertainty. Figure based on Table 8 of the new paper. Image: AGU

‘The scary thing’

“From 2006 to 2016 North Korea steadily increased the size of the events, from somewhere around 1 kiloton up to around 20 kilotons. The very early events looked like they didn’t work very well, because they were unusually small. And then in one year they jumped up to 250-ish kilotons,” Thorne Lay, a seismologist at the University of California Santa Cruz and an author of the new study said in remarks quoted in a news release from the research journal’s parent organization, AGU. “The scary thing is that this was such a big device.”

North Korea, AGU reported, has been testing nuclear devices underground since 2006. The 2017 nuclear test caused a magnitude 6.3 earthquake, according to the US Geological Survey, and produced sound waves recorded by seismometers around the world.

These reverberations are clues to the size of the blast, but determining the size of the explosion from a distance based on seismic recordings is not simple, said Lay, who has served on the Air Force Technical Application Center Seismic Review Panel tasked with advising on nuclear testing for over 25 years.

Unknown characteristics of the nuclear devices, the test sites and their geologic context create large uncertainties in estimates of explosion size. Previously published scientific estimates of the 2017 test’s size have ranged from 30 to 300 kilotons. The US intelligence community has estimated the size of the explosion at 140 kilotons according to a report in The Diplomat magazine.

US intelligence typically applies an uncertainty range of a factor of two to reported estimates. A reported yield of 100 kilotons, for example, would include the caveat that it could be as small as 50 kilotons or as large as 200 kilotons. The new research puts the magnitude of the 2017 test in a range of 148 to 328 kilotons.

Knowing the size of the tests relative to each other is itself useful information, demonstrating steady progress in North Korea’s nuclear program, said Steven Gibbons, a geophysicist with the program for Array Seismology and Test-Ban-Treaty Verification at NORSAR and a researcher unaffiliated with the new study.

“In 2006, when there was this little rumble, many people were quite dismissive that North Korea had the technology to do this properly, but I think from the progression we’ve seen with the increases of yield, it’s been a very well-executed weapons development program,” Gibbons said.

“When you got to 2017, there’s no question that this is an incredibly destructive weapon. Even at the lower end of this uncertain yield, it’s a horrific weapon.”

A 250-kiloton explosion could plausibly be produced by either a boosted fission bomb or a modest fusion device, Lay said. The fission bomb detonated at Hiroshima released a blast equivalent to 15 kilotons of TNT by splitting apart atomic nuclei.

Fusion devices, also known as thermonuclear or hydrogen bombs, derive their enormous explosive power from combining hydrogen nuclei to form helium and can yield as much as 50,000 kilotons.

Inherent uncertainties

The researchers had information about how sound travels in different types of rock from previous research. “There are differences, depending on the rock conditions under the test site, in how loud the sound is when you are listening far away,” Lay said.

“It would be different in granite than in say, sandstone. So we calibrated by setting tests off in our own diverse rock environments and recording right next to the source.”

Other factors like the depth of the explosion, geometry of the access tunnel, tectonic history of the region, and temperature and fluid content of the rock also influence the rate at which sound waves created by the explosion are dampened, Lay said.

Only a tiny amount of the energy generated by the blast is converted to sound and travels away from the test site as seismic waves.

Because of North Korea’s seclusion, details of the underground geology are not well known, AGU reported. The area experiences very few natural earthquakes that can reveal information about properties of the underlying rock.

Researchers used satellite images and other information to estimate the type of rock at the North Korea test site.

Google Earth image of the Democratic People’s Republic of Korea test site looking down from south‐southeast of Mount Mantap. Numbers mark the estimated locations of the six nuclear tests conducted from 2006 to 2017. From figure 1 of the new paper. Image: Google Earth / AGU

Rebooting cold war strategies

The researchers used the explosions from the nuclear tests North Korea conducted beginning in 2006 to calibrate models of how much explosive force transferred to the rock at the test site and how the sound waves traveled through the planet.

The echo of the explosion off the surface of the test site distorts the sound recorded far away. The distortion is affected by the depth and size of the explosion. If the echo were not present, the outgoing sound from the six test explosions would be similar.

The researchers used this idea to estimate the relative sizes of the bombs by finding a combination of depth and yield that compensated for the reflection of the sound from the surface.

The new study, AGU said, revived modeling strategies developed in the early 1980s to resolve suspicions that the Soviet Union had cheated on the Treaty on the Limitation of Underground Nuclear Weapon Tests by testing bombs larger than the 150-kiloton size threshold for testing they agreed to in 1974.

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