Even if a country doesn’t announce an underground nuclear test, seismologists can distinguish a subterranean bomb blast or other explosion from an earthquake by examining the event’s seismic waveforms. A seismometer translates seismic waves into a graph called a seismogram, which researchers can then evaluate. Credit: hakandogu/Adobe Stock

Last week, North Korea tested what it claimed to be a hydrogen bomb, or as the North Korean government declared in its official statement, an “H-Bomb of justice.” However, it’s not likely that North Korea has actually developed a hydrogen bomb and successfully tested it on 6 January local time (the evening of 5 January on the U.S. East Coast), as announced. The U.S. Geological Survey recorded the subsequent seismic event as having a 5.1 magnitude, which is much lower than would be expected from such a powerful weapon.

But even if North Korea or anyone else conducting a clandestine nuclear test makes no announcement, seismologists can still figure out if an underground bomb test or an earthquake took place by analyzing how energy propagates from the seismic event in question.

Explosions Send Compression Waves in All Directions

P waves are the fastest-moving type of seismic waves. They alternately compress or dilate the material they move through. When an explosion, such as a nuclear test, occurs within the Earth, all of the force of the blast strikes the surrounding material.

When an earthquake strikes, seismologists use diagrams called focal mechanism plots to determine what type of faults moved. Each type generates a different pattern, with both black and white regions. Earthquake faults of different configurations, such as strike-slip or thrust faults, yield distinctive plot patterns when earthquakes occur along them. However, all regions of the diagram go dark if the jolt is caused by an explosion. Credit: Mikenorton, CC BY-SA 3.0, cropped from the original.

“As the bomb is detonating, it’s compressing the rock immediately adjacent to it, and that propagates out to the recording stations” as P waves, said Douglas Dreger, a seismologist at the University of California, Berkeley. The first wave to reach the seismometer generates an “up” signal. Seismologists use the term “up” because the ground actually moves up when the compression phase of a P wave arrives and the squeezed underground rock and soil juts upward at the surface.

Seismologists then plot the up signals from P wave compressions and down signals from dilations on black-and-white diagrams called focal mechanism plots. They divide these diagrams into four regions representing directions in which seismic waves travel from a shock. A focal mechanism plot would appear completely white before an earthquake and then shaded black in some spots once seismic detectors register an up signal in a region.

Each type of earthquake generates a different plot pattern in which there are some black and some white regions. By contrast, concussive signals propagating in all directions from an explosion would shade the entire plot black. Dreger’s focal mechanism plot for the North Korean nuclear test is entirely shaded.

Bigger Interior Waves Suggest an Explosion

The relative amplitudes of an event’s seismic waves that zip through the Earth’s interior, when compared to the amplitudes of its surface waves that radiate more slowly from the shock, can also indicate if an explosion or earthquake triggered the event. Explosions produce larger internally propagating waves than surface waves, whereas an earthquake doesn’t cause the same discrepancy.

Looking at only the waveforms from the North Korean test, Dreger said, “It’s very obviously an explosion.” So even if a less outspoken country tried to secretly test an atomic or hydrogen bomb, scientists could still uncover the truth.

Seismology Alone Can’t Determine What Type of Bomb Detonated

Once scientists know an underground explosion or bomb test occurred and they know the magnitude of the seismic event it caused, they can calculate how big of an explosive force caused that quake. This latest test, registering at 5.1, most likely falls below the magnitude a hydrogen bomb would produce, said Brian Stump, a seismologist at Southern Methodist University in Dallas, Texas.

But, Stump said, drawing a firm conclusion concerning what type of bomb was detonated requires radionuclide measurements that specially designed aircraft can make. Some news media outlets reported that U.S. Air Force planes will sample the air near the North Korean test area to determine what radioactive material, if any, leaked out of the underground blast site.

—Cody Sullivan, Writer Intern

Citation: Sullivan, C. (2016), Quake or bomb? Seismic waves speak truth, even if nations don’t, Eos, 97, doi:10.1029/2016EO043719. Published on 15 January 2016.

Text © 2016. The authors. CC BY-NC 3.0
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