Underground nuclear explosions generate seismic waves which propagate through the earth and are recorded by the numerous monitoring stations deployed around the globe. NORSAR is one of the seismic observatories which contribute data to the CTBTO‘s International Data Center in Vienna – the heart of the verification regime for compliance with the Comprehensive Nuclear-Test-Ban Treaty. On September 9, 2016, the DPRK or North Korea conducted its fifth declared nuclear test at its test-site in the mountainous north of the country. The explosion generated seismic waves indicating an event with a magnitude in excess of 5 – you can see the signal received on the NORSAR or NOA array near the top of the plot below:

We can compare the signals from the five declared DPRK nuclear tests recorded on this station to estimate the relative sizes of the different explosions as shown below:

By measuring tiny relative differences in the arrival times between the signals recorded on different stations, we can estimate the locations of the explosions relative to each other far more accurately than we can measure the absolute locations using seismic data alone. However, other technologies (such as commercial satellite imagery) provide constraints on the test-site given observation of human activity, infrastructure and topography. With very accurate relative location estimates, the terrain in the region may place significant constraints upon where the absolute locations can be. The figure below shows our current best estimates as to where the explosions took place in the mountains at the test-site, based upon high-precision evaluation of the seismic signals and considerations from satellite observations.

Estimating the relative event locations is rather more difficult than would at first appear; it seems that the seismic wavefield leaving the test-site is rather more complicated than the simplest models would indicate. Why? Because there are significant differences between the relative event location estimates using regional stations (within 1000 km of the test-site) and teleseismic stations (up to many thousands of km away). A paper in Geophysical Journal International earlier this year addresses these questions to find estimates for the relative event locations which are consistent both for near and distant seismic stations.

This super article by Alex Witze in Science News this month looks at several issues involved in the remote monitoring of underground nuclear tests.