Study: Radar imaging shows how the mountain collapsed after North Korea’s most recent nuclear test
As North Korea’s president pledges to “denuclearize” the Korean peninsula, an international team of scientists is publishing the most detailed view yet of the site of the country’s latest and largest underground nuclear test on Sept. 3, 2017.
The new picture of how the explosion altered the mountain above the detonation highlights the importance of using satellite radar imaging, called SAR (synthetic aperture radar), in addition to seismic recordings to more precisely monitor the location and yield of nuclear tests in North Korea and around the world.
The researchers—Teng Wang, Qibin Shi, Shengji Wei and Sylvain Barbot from Nanyang Technological University in Singapore, Douglas Dreger and Roland Bürgmann from the University of California, Berkeley, Mehdi Nikkhoo from the German Research Centre for Geosciences in Potsdam, Mahdi Motagh from the Leibniz Universität Hannover, and Qi-Fu Chen from the Chinese Academy of Sciences in Beijing—will report their results online this week in advance of publication in the journal Science.
That explosion took place under Mt. Mantap at the Punggye-ri nuclear test site in the country’s north, rocking the area like a 5.2-magnitude earthquake. Based on seismic recordings from global and regional networks, and before-and-after radar measurements of the ground surface from Germany’s TerraSAR-X and Japan’s ALOS-2 radar imaging satellites, the team showed that the underground nuclear blast pushed the surface of Mt. Mantap outward by as much as 11 feet (3.5 meters) and left the mountain about 20 inches (0.5 meters) shorter.
By modelling the event on a computer, they were able to pinpoint the location of the explosion, directly under the mile-high summit, and its depth, between a quarter and a third of a mile (400-600 meters) below the peak.
They also located more precisely another seismic event, or aftershock, that occurred 8.5 minutes after the nuclear explosion, putting it some 2,300 feet (700 meters) south of the bomb blast. This is about halfway between the site of the nuclear detonation and an access tunnel entrance and may have been caused by the collapse of part of the tunnel or of a cavity remaining from a previous nuclear explosion.
“This is the first time the complete three-dimensional surface displacements associated with an underground nuclear test were imaged and presented to the public,” said lead author Teng Wang of the Earth Observatory of Singapore at Nanyang Technological University.
From Science Magazine, the paper: http://science.sciencemag.org/content/early/2018/05/09/science.aar7230.full
The rise, collapse, and compaction of Mt. Mantap from the 3 September 2017 North Korean nuclear test
Surveillance of clandestine nuclear tests relies on a global seismic network, but the potential of spaceborne monitoring has been underexploited. Here, we determined the complete surface displacement field of up to 3.5 m of divergent horizontal motion with 0.5 m of subsidence associated with North Korea’s largest underground nuclear test using satellite radar imagery. Combining insight from geodetic and seismological remote sensing, we found that the aftermath of the initial explosive deformation involved subsidence associated with sub-surface collapse and aseismic compaction of the damaged rocks of the test site. The explosive yield from the nuclear detonation with seismic modeling for 450m depth was between 120-304 kt of TNT equivalent. Our results demonstrate the capability of spaceborne remote sensing to help characterize large underground nuclear tests.
Combining the available space-borne geodetic and seismic records provided new insights into the mechanics of deformation surrounding North Korea’s sixth underground nuclear test, revealing the explosion, collapse, and subsequent compaction sequence (Fig. 4). The modeling of the geodetic observations reduces the epicentral and depth uncertainties that otherwise hinder the analysis of seismic waveforms. The derived horizontal location of the first event is important to relatively relocate the second event, which likely indicates the collapse of the tunnel system of the test site. The inclusion of geodetic data also helps resolving the aseismic deformation processes that may follow nuclear tests. Finally, our findings demonstrate the capability of monitoring shallow underground nuclear tests using remote-sensing observations and seismic sensors.