‘Equal to 17 times that of Hiroshima explosion’

Scientists at the Indian Space Research Organisation (ISRO) have used a novel mathematical technique and analysed satellite images to estimate the strength of North Korea’s underground nuclear test of September 2017. Those tests are considered the most powerful thermonuclear devices to have been exploded by the country.

In the normal course, the detection and estimation of nuclear device explosions is based on the reading of earthquake monitoring sensors. However, North Korea’s relative isolation has meant that there were no accessible seismic stations near the test site at Mount Mantap, Punggye-ri, to accurately gauge the intensity of the explosion, and how deep into the earth the device was detonated. Such information is also important for determining the type of bomb, and consequently, the degree of know-how the detonating country possesses.

Test site

“We inferred the location of the source as 129.0764°E, 41.0324°N at a depth of about 540 m below Mt. Mantap. The explosive yield estimated (245-271 kt) is about 17 times that of the Hiroshima explosion,” the authors noted. “We inferred that the uncertainties in yield and source depth estimated using the Bayesian modelling of InSAR data were significantly less than that of seismic methods.”

The findings were published in the Geophysical Journal International. For the analysis, researchers K.M. Sreejith, Ritesh Agrawal and A.S. Rajawat used images of the location after the explosion, sourced from the ALOS-2, a Japanese satellite, and Sentinel 1B, a European radar imaging satellite. InSAR refers to the interferometric synthetic aperture radar and is a radar technique used to generate maps of how a place would look after an earthquake, or a detonation.

While other groups have also used InSAR based approaches to estimate impact from a detonation, the ISRO group claims to have used a mathematical technique called Bayesian inversion that can correct for errors associated with InSAR data.

These estimates, of a yield of 250 kiloton, are in line with an assessment this June by U.S. scientists, who said that the 2017 test was about 10 times more powerful than the tests first conducted by North Korea in 2016.

Sound waves

“North Korea detonated a nuclear device in 2017 equivalent to about 250 kilotons of TNT, creating an explosion 16 times the size of the bomb the United States detonated over Hiroshima, Japan, in 1945,” noted the American scientists’ paper published in the Journal of Geophysical Research. Their approach, however, relied on studying the signature of sound waves from an explosion when it travels through rock at the test site, and how it affected sensors around the world.

“...The Bayesian modelling of the InSAR data reduced the uncertainties in the yield and depth by 25-85% and 40-97%, [respectively]” the ISRO team underlined.