All Eyes and Ears: Insights from a Seismo-Geodesy Study into an Underground Nuclear Test Site

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All Eyes and Ears: Insights from a Seismo-Geodesy Study into an Underground Nuclear Test Site

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Mount Mantap, the site of the 6th North Korean Nuclear Test (Source: Google Earth)

On 3 September 2017, the Democratic People’s Republic of Korea (North Korea) conducted its sixth underground nuclear test at the Punggye-ri test site. In collaboration with scientists from Germany, USA, and China, my colleagues from the Earth Observatory of Singapore (EOS) and I published our findings in Science on 11 May 2018, revealing the complex physical processes associated with the nuclear test.

We found that the top of the mountain experienced a rise, collapse, and compaction at different time scales after the explosion. The explosive yield from the nuclear detonation with seismic and geodetic modeling was between 120-304 kilotons of TNT, which is more than 10 times the power of the Hiroshima bombing (~15 kilotons).

Despite the explosion occurring in an inaccessible region, space geodesists can “see” the displacement via radar images acquired by satellites flying on the orbit 500 kilometres above the ground. By tracking the features in the satellite images acquired before and after the explosion, we were able to translate the microwave signals into surface displacements. The results showed as much as 3.5 metres (m) of divergent horizontal motion with 0.5 m of subsidence on the top of Mt. Mantap, which hosted this and four previous nuclear tests. This is the first time the complete three-dimension (3D) surface displacements associated with an underground nuclear test were imaged and presented to the public. From the modeling of the displacement data, we were able to locate the explosion precisely and determine the depth of the buried detonation. 

A summary deformation scenario for the North Korea Nuclear Test that took place on 3 September 2007. The unfolding of events includes a succession of explosive, collapse, and compaction processes, with different associated surface displacement patterns. The displacements measured from radar imagery are a combination of the three processes as shown in the subplot. (Source: Sylvain Barbot/Earth Observatory of Singapore)

In addition to “seeing”, seismometers deployed hundreds of kilometers away can “hear” the explosion and other seismic events. The analysis of these waveforms shows a predominant explosive source mechanism buried at a shallow depth, which is expected from an underground nuclear test. We had also located a smaller seismic event, that had occurred 8.5 minutes after the main explosion, with an implosive source mechanism probably caused by the collapse of the tunnel system at the site.
 
The source characteristics independently derived from space-borne geodetic and seismic records are highly consistent, revealing the explosion, collapse, and subsequent compaction sequence. Combining the depth constraints from geodesy and energy constraints from seismology allows us to derive the location, depth, magnitude, focal mechanism, and yield of any underground nuclear test without direct access to the test site. The integration of space-geodetic observations, deformation modeling, and detailed waveform analysis also sheds light on the nature of explosive sources buried at a shallow depth and the associated elastic and inelastic deformations. 
 
3D displacements associated with the North Korean Nuclear Test (NKNT 6) that took place on 3 September 2017

World peace benefits from the adherence to internationally-negotiated nuclear-test-ban treaties that strive to promote the non-proliferation of nuclear weapons. Surveillance of clandestine nuclear tests relies on a global seismic network, but the potential of spaceborne monitoring has been underexploited. This study demonstrates the capability of space borne remote sensing to help characterise large underground nuclear tests, if any, in the future.

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