How Scientists Track North Korea's Nuclear Program

North Korea's ongoing series of nuclear tests are interesting to me, a geoscientist, because of the seismic signature that they leave behind.  Here is some background information, showing from a seismic perspective, how North Korea's ability to produce ever more powerful nuclear weapons is improving.

During the early stages of the nuclear age between 1945 to 1957, nuclear weapons testing took place in the atmosphere as shown in this video:

This led to potential issues since the tests exposed civilian populations to excessive levels of radiation.

After the signing of the Limited Test Ban Treaty in 1963, most tests took place underground with the exceptions of China which continued atmosphere testing until 1980 and France which continued atmospheric testing until 1974.  The explosions from underground testing creates a subsurface cavity which often collapses, creating a rubble-filled chimney that may go to the surface depending on the depth of the test.  Here is a graphic showing the Nevada test site and the impact of a nuclear weapons test on the subsurface:

The only certain way to track these underground tests is through seismic monitoring.  During the nuclear test ban discussions held in 1958, it was decided that there was a need for a global network of accurately calibrated and timed instruments that could measure and monitor underground nuclear explosions.  Seismologists had, for some time, recognized the need for a global network of seismographs to produce the data needed for further global studies in seismology.  This led to the creation of the World-Wide Standardized Seismic Network or WWSSN.  Construction began in 1961 and was completed at the end of 1967 with the installation of 121 WWSSN systems plus one which was given to the USSR.   

Here is a map showing the WWSSN stations in 1978:

One of the major benefits of the WWSSN was its assistance in developing the science of global plate tectonics, a key aspect of the geoscience world today.

In 1996, the Comprehensive Test Ban Treaty (CTBT) was adopted by the United Nations.  Here is a map showing the signatory, ratifying and non-signatory states (in red):

Monitoring is obviously a very important part of the CTBT Organization.  Four types of monitoring are used including seismic, hydro acoustic, radionuclide and infrasound.  We'll focus on the seismic monitoring for the purposes of this posting.  Seismic monitoring takes place at 170 seismic stations (50 primary and 120 auxiliary) in 76 nations around the world.  The primary stations operate 24 hours a day, 365 days a year and data is relayed in real time to the International Data Centre in Vienna, Austria.  Here is a video which explains how the seismic monitoring system works:

 

Now, let's look at the seismic history of North Korea's nuclear tests according to the CTBTO, focussing on the test of September 3, 2017.  The "event" was detected at 41 primary and 90 auxiliary stations along with two hydroacoustic and one infrasound stations.  The event took place at 03:30 UTC on September 3, 2017 and is consistent with a man-made explosion, however, CTBTO states that the explosion can only be classified as nuclear once airborne radioactivity is detected, a process which can take up to55 days which was the case in the 2013 test.  The initial estimate of the event's magnitude was 5.8 which was later revised to 6.1.  Here is a graphic showing how this event's seismic signature compares to North Korea's other nuclear tests:

As you can see, there was significantly greater seismic activity after the September 3, 2017 test than there was in all previous nuclear tests.  Calculations now suggest that the yield of the most recent blast is approximately 250 kilotons (one quarter of a megaton), by far North Korea's largest yield as shown here:

By way of comparison, the Little Boy weapon that destroyed Hiroshima had a yield of 15 kilotons and the Fat Man weapon that destroyed Nagasaki had a yield of 22 kilotons.

The data from the latest North Korean nuclear test suggests that the hermit kingdom has now developed a weapon that would be capable of creating significant damage to its intended target.  With a weapon of this size, accuracy becomes somewhat less important since destruction will be widespread.