Explosion, collapse, earthquakes: North Korea’s 2017 nuclear test

The researchers used seismic data from the first DPRK nuclear test as the “master event” to calibrate their location analysis, since the epicenter of that small explosion could be visually located using satellite images of localized ground surface damage. The much larger September 2017 explosion produced surface damage over an area about nine square kilometers, however, in ground that was already disturbed by previous nuclear tests. “For example, after the sixth North Korean nuclear test, large displacements occurred on the west and south flank [of the test site] and debris flows were localized in pre-existing channels,” Zhao explained. “These spatially distributing phenomena made it difficult for us to directly determine the epicenter of the explosion.”

Zhao and colleagues used regional seismic data instead to calculate that the epicenter of the September 2017 explosion was at 41.3018°N and 129.0696°E. A seismic event that took place about eight minutes after the explosion occurred very close to the explosion epicenter—less than 200 meters away—and probably represents the seismic signature of the collapse of a cavity left by the underground explosion, the researchers suggested.

Two subsequent seismic events, one on 23 September and one on 12 October, were located about eight kilometers northeast of the nuclear test site. Zhao and colleagues said that the seismic signatures of these two events indicate that they are not explosions, but may have resulted from mechanisms such as landslide or ground collapse. They may also be very shallow natural earthquakes that were triggered by the explosion, they noted, a possibility that will require more research on the pre- and post-explosion stresses on the faults where the events occurred.

Careful analysis of data collected after the 3 September 2017 North Korean declared nuclear test explosion has allowed seismologists to distinguish the separate seismic signatures of the explosion, the collapse of the explosion cavity and even several small earthquakes that occurred after the collapse.

The data, compared with those collected from 20^th-century Nevada nuclear test sites, can help refine seismologists’ methods of identifying nuclear test explosions around the world, write William R. Walter and his colleagues at Lawrence Livermore National Laboratory in the SRL focus section.

The researchers used a method that compares the ratio between regional P- and S-wave amplitudes to distinguish the seismic signature of an explosion compared to an earthquake, at distances about 200 to 1500 kilometers away from the seismic wave source. (P-waves compress rock in the same direction as the seismic wave’s movement, while S-waves move rock perpendicular to the direction of the wave.) “In the P/S ratio discriminant we use to identify explosions, it is the lack of S-waves at high frequency that is distinctive of the explosions,” Walter explained.

Walter and colleagues showed that the ratio could separate the six North Korean declared nuclear tests from natural earthquakes in the region, and that the same method could be used to successfully distinguish between historic Nevada Test Site nuclear explosions and earthquakes in the western United States.

However, there was another unusual seismic event, occurring about eight and half minutes after the explosion, which also drew the attention of the seismologists. Models of seismic waveforms of the event led the scientists to conclude that the event may have been the ground collapsing around an underground cavity left by the explosion.

Although collapses similar to this were sometimes seen after Nevada Test Site explosions, “this is the first time, to my knowledge, that we have remotely observed seismic waves from a collapse with modern instrumentation at a foreign test site,” said Walter. “It is important to be able to determine this collapse was not another nuclear test.”

Several features of the post-explosion event’s waveforms mark it as a collapse rather than an explosion, the researchers say, including the relative lack of high frequency energy compared to explosion waveforms.

“Identifying the event as a collapse is another indicator the 3 September 2017 event was a nuclear test that generated a large vaporization cavity that collapsed eight and half minutes later,” said Walter. “But we want to continue to develop methods to identify collapses to distinguish them from both explosions and earthquakes.”

Researchers studying the September 2017 nuclear test data also noted two smaller seismic events occurring after the explosion, of magnitudes 2.6 and 3.4, that appear from the P- to S-wave ratios to be small earthquakes located four to eight kilometers north of the explosion site.

“We had not remotely observed any aftershocks from the prior DPRK declared nuclear tests, so the earthquakes following the explosion got people’s attention,” Walter said. “Again, we wanted to determine they were not additional smaller nuclear tests. Alternatively, we wanted to determine they were not associated with the collapse event.” Upon careful re-analysis of the continuous data the researchers found a number of additional small earthquakes, including some that occurred before the 3 September 2017 declared nuclear test.

Given the timing these earthquakes do not appear to be true “aftershocks” of the nuclear test, Walter and colleagues concluded, though they may be related and possibly induced by the explosion. “The fact that apparent tectonic earthquakes are occurring near the DPRK test site reveals information about the state of [seismic] stress in the region, which may help us better understand the explosion seismic signatures,” said Walter.

— Read more in Xi He et al., “High‐Precision Relocation and Event Discrimination for the 3 September 2017 Underground Nuclear Explosion and Subsequent Seismic Events at the North Korean Test Site,” Seismological Research Letters (2018) (doi: org/10.1785/0220180164)