In September 2017 North Korea tested its largest nuclear bomb yet. It was 10 times the blast strength of any of the five previous underground detonations (map). How do we know? A global network of more than 300 earthquake-monitoring stations stands sentry. After an explosion, seismometers pick up two types of shock waves within minutes and alert intelligence officers. Scientists learn even more afterward. Since the last blast, they have combined the seismic signals with satellite images and other data to pinpoint more details, recently published, such as the precise location and bomb size (chart). Should North Korea—or any other nation—try another bomb, the world will know.
This article was originally published with the title "Watching North Korea" in Scientific American 319, 5, 84 (November 2018)