Patrick Tucker writes: Shortly after North Korea claimed it had tested a hydrogen bomb—a weapon potentially hundreds of times more powerful than the fission bombs the country had already set off—seismologists at the United States Geological Survey, or USGS, went to work trying to understand the event. Their early findings suggest that a nuclear-bomb test did occur but that it wasn’t a hydrogen bomb. So how do you tell the difference?

First, you try to rule out the possibility that North Korea was just trying to claim credit for an earthquake. Geologists and seismologists look at several factors to determine whether a seismic event is natural or manmade. One is the location: Is it on a known fault line, a place where there’s a lot of mining activity, etc.? Another factor is the seismological waveform itself—the waving lines that appear on the seismograph. An explosion forms wiggles that are different from the ones generated by an earthquake, according to USGS seismologist Paul Earle.

Lay a Slinky on the floor, grab one end, and move it back and forth to create a wave that propagates down its length. This is called shear wave propagation, the kind created by tectonic plates slipping beneath the surface of the earth. “That side-to-side motion, you’ll get less of it in an explosion,” said Earle. [Continue reading…]