How Scientists Know the North Korea Blast Probably Wasn't an H-Bomb

National security experts were quick to express doubt about North Korea's claim that it tested its first hydrogen bomb on Wednesday. This chart explains why: The seismic effects of the explosion are too similar to previous explosions, in 2013, 2009, and 2006, which were all confirmed as underground nuclear tests. A true hydrogen bomb would have caused a larger seismic reading.

The chart shows seismic activity detected in the northeastern Chinese city of Mudanjiang early this morning compared to the previous explosions, according to scientists at Columbia University's Earth Institute. The monitor is still for nearly a minute into the reading, followed by two large pulses about eight seconds apart. The shockwaves gradually  dissipate after several minutes.

The magnitude of the seismic activity—5.1 on the Richter scale—was on a par with North Korea's previous tests. The explosive yield was probably, at a minimum, the equivalent of 3.5 kilotons of TNT, according to Columbia's Won-Young Kim. That's a smaller yield than the U.S. bombs that destroyed Hiroshima and Nagasaki in August 1945. 

An actual hydrogen bomb has a seismic signature similar to an atomic weapon's. But its explosive yield is in the much larger megaton range. It's more likely North Korea "turbo-charged" a normal atomic explosion by adding a small amount of tritium to the bomb's core rather than inventing a miniature hydrogen bomb from scratch.

The original version of this story misspelled the name of Columbia seismologist Won-Young Kim.