The Punggye-ri Nuclear Test Site is North Korea's sole known site for testing atomics, and is located underground, beneath Mount Mantap. It has been the subject of much attention since the September 3 hydrogen bomb test that it hosted. That detonation was estimated to be 250 kilotons, 16-17 times more powerful than the atomic bomb dropped by the US on Hiroshima, Japan, in 1945.
The test also caused an estimated 6.1 magnitude earthquake and potentially caused a cave-in of the testing chamber. Aftershocks of the main earthquake followed, including a 4.6 magnitude quake just eight minutes after the test. Most recently, a 2.9 magnitude earthquake occurred on October 12, more than five weeks after the initial detonation.
The thermonuclear detonation was powerful enough to have potentially made the site geologically unstable and no longer suitable for testing. This is known as "Tired Mountain Syndrome," when nuclear blasts damage underground rock formations and make the site volatile.
Mainstream outlets replied to the reports of earthquakes with headlines such as "Has North Korea Nuked Itself Out of a Nuclear Test Site?" from CBS and "North Korea's Nuclear Test Site Could Be Unstable" from NBC News. However, experts have warned against such alarmism.
Punggye-ri has three tunnel complexes, meaning that at least two are still usable. "There is no valid reason to assume that the Punggye-ri test site is unable to contain additional underground nuclear tests," said analysts with 38 North, a website that focuses on North Korean affairs.
The US dealt with Tired Mountain Syndrome themselves as the Nevada Test Site, which saw over 800 underground nuclear tests over a 41 year period and remained usable. Many of them were followed by seismic activity as well, which according to 38 North "provides evidence that such post-test tremors are not unusual."
"For the time being," 38 North concluded, "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.
The DPRK has conducted six nuclear tests, three of them in the past two years. The September 2017 test utilized a hydrogen bomb 10 times as powerful as the one tested just a year prior. The rapid advance of the country's nuclear program has upset foreign powers such as the United States and the European Union, but Pyongyang insists the program is peaceful and for deterrence purposes only.
North Korean leader Kim Jong-un called the nuclear program a "treasured sword" and a "powerful deterrent" against the "protracted nuclear threats of the US imperialist" in a speech before the Central Committee of the Workers' Party earlier in October.