There have now been three detected and reported earthquake-like events subsequent to the most recent, very large (~6.1 magnitude) underground nuclear test conducted by North Korea on September 3, 2017 at its Punggye-ri Nuclear Test Site. Recent media reporting has suggested that, as a result, the site may no longer be suitable for further underground nuclear testing. The three earthquakes were likely induced by the ~250 kiloton nuclear test; however, US nuclear test history at the Nevada Test Site provides evidence that such post-test tremors are not unusual. Furthermore, even in the face of what has been dubbed “Tired Mountain Syndrome,” abandonment of the site for nuclear testing should not be expected. Such historical precedent, combined with the presence of two other, as yet unused tunnel complexes within the test site, leads us to conclude that there is no valid reason to assume that the Punggye-ri test site is unable to contain additional underground nuclear tests.
Figure 1. An interferometric SAR (InSAR) derived graphic, obtained through the processing of radar imagery from the Japanese Advanced Land Observing Satellite 2 (ALOS-2) of the area encompassing the Punggye-ri Nuclear Test Site, overlain on Google Earth. The overall centralization of the radar-detected displacements encompassing all of Mt. Mantap provides additional empirical evidence that the sixth nuclear test occurred directly under the peak. Note that pixels with correlation less than 0.1 are transparent.
On September 3, 2017, a 4.6 magnitude earthquake-like event was detected at the Punggye-ri Nuclear Test Site only eight minutes after North Korea’s sixth and largest underground nuclear test had been conducted. Its seismic waveform, together with the nominal “five kilometers” depth, rule out a nuclear test explosion (the likely deepest depth of burial under Mt. Mantap is 800 meters, see Figure 1). It has generally been speculated that the first post-test earthquake-like event was somehow related to a structural collapse or cave-in, but this remains unverified at this time. Another similar event of 3.5 magnitude having “earthquake characteristics” was detected at essentially the same location on September 23. Most recently, on October 12, a 2.9 magnitude earthquake with similar waveform and depth (~5 kilometers) was detected. These three earthquakes were not caused by human intervention and hence were not related to new nuclear testing. Moreover, in terms of scale, all three earthquakes were smaller (two significantly so) in magnitude than North Korea’s second nuclear test in 2009, which had a magnitude of 4.7 and an estimated explosive yield of about 5 kilotons.
Media Speculation on Punggye-ri Nuclear Test Site’s Unsuitability
As a result of these nuclear test-induced earthquakes, new media reporting has appeared with headlines such as “North Korea’s Nuclear Test Site Could Be Unstable” and “Has N. Korea nuked itself out of a nuclear test site?” While these do make for eye-catching headlines, there was little substance in the articles to back them up beyond quoting the speculative fears of “civilian experts.” Nonetheless, based on the severity of the initial blast, the post-test tremors, and the extent of observable surface disturbances, we have to assume that there must have been substantial damage to the existing tunnel network under Mt. Mantap. If North Korea were to attempt to continue testing under this mountain (such as, in the area more to the eastern side), then we would expect to see new tunneling in the future near the North Portal, still under Mt. Mantap. A lack of new tunneling in this area would provide evidence that this mountain has been abandoned for future testing. However, complete abandonment of the test site as a whole remains unlikely.
For comparative purposes, we can look to the US Nevada Test Site’s experience with multiple underground tests. A United States Geological Survey (USGS) report on nuclear test-induced earthquakes conducted at this site concluded:
…increases in seismic activity in the Nevada region were common following underground nuclear explosions at the US Nevada Test Site with equivalent seismic magnitude of 5.0 or greater. Periods of time before and after 21 explosions were examined. Most of the increased activity appears to have originated within 20 kilometers of the test shot-point. The time history of this activity indicates that it was directly related to the nuclear explosions. Most of the seismic events (earthquakes) following underground tests were very small. The decay of post-shot activity and its relationships to the equivalent magnitude of the explosion provided confirmation of the determinative (causal) role of the nuclear event. For the most part, this activity was confined to the test site.
However, despite the numerous post-test earthquakes, the Nevada Test Site was not abandoned for nuclear test purposes until the nuclear test moratorium that took effect in October 1992.
At Punggye-ri, the presence of two other test tunnel complexes, accessible via the South and West Portals, provides sufficient means to continue testing at the site irrespective of test-induced effects to Mt. Mantap.
Nuclear tests previously conducted at the US Nevada Test Site (as well as at the former Soviet nuclear test sites) show that test-induced seismic events (small post-test earthquakes), associated with tests having magnitudes of 5.0 or more, are not unusual. Moreover, such activity did not lead to site abandonment prior to the general test moratorium in 1992. Because Mt. Mantap has been the location for the last five of six of North Korea’s declared underground nuclear tests (via the North Portal) and has undergone widespread observable surface disturbances resulting from the most recent test, it is not surprising that there were a number of post-test earthquakes. This may have caused some concern both inside and outside North Korea about “Tired Mountain Syndrome.” For the time being, however, given the presence of additional test portals, we see no reason that the Punggye-ri Nuclear Test Site as a whole has or will be abandoned for future underground nuclear testing.
“Tired Mountain Syndrome” derives from the underground detonation of (multiple) nuclear explosions that considerably alter the properties of the surrounding rock mass. Fracturing and rock breakage are extensive and markedly increased, and permeability is appreciably increased both in the rock mass itself and along isolated tectonic faults. The zone of inelastic deformations of structural fractures can extend hundreds of meters, and specifically for North Korea’s sixth test, could extend out to ~1.4 kilometers from the shot-point cavity. Source: Vitaly V. Adushkin and William Leith, The Containment of Soviet Underground Nuclear Explosions, USGS Report 01-312, p.35, September 2001, https://pubs.usgs.gov/of/2001/0312/report.pdf.
NORSAR put the latest event a little higher than USGS—3.2—but their conclusion was the same. https://earthquake.usgs.gov/earthquakes/eventpage/us1000aqsx#executive.
Gary Boucher, Alan Ryall, Austin E. Jones, “Earthquakes associated with underground nuclear explosions,” July 15, 1969. Journal of Geophysical Research, Volume 74, Issue 15, Version of Record online: 20 SEP 2012 http://onlinelibrary.wiley.com/doi/10.1029/JB074i015p03808/abstract.