What the seismic data says about North Korea's bomb

Not even a chemical analysis of the air will be able to prove whether North Korea detonated a hydrogen bomb as opposed to classic atomic weapon. Thermonuclear weapons, popularly called hydrogen bombs, leave no known "sniffable" traces.

That leaves scientific experts with a single source of hard data:

Seismic waves.

Different seismological research organizations have come up with slightly differing figures about the strength of the quake following North Korea's test of what it calls a "miniatiurized" hydrogen bomb.

The German Research Center for Geosciences in Potsdam (GFZ) registered a tremor of 5.3 on the Richter scale. The Federal Institute for geosciences and Natural Resources (BGR) and the U.S. Geological Survey (USGS) registered a bit lower quake of a magnitude of 5.1.

Whatever the case, the strength of the explosion remains in a familiar spectrum: In 2013, a North Korean nuclear test resulted in a magnitude 5.1 tremor. In 2009, a similar test registered 4.9 (BGR) or 5.0 (GFZ) respectively.

"The fact that both blasts of 2013 and the current one are almost identical in magnitude certainly does not suggest a technological quantum leap into a whole new dimension," says Nicolai Gestermann says, a geophysicist at the national data center for the verification of the nuclear test ban treaty - an office of the BGR.

By comparison, underground tests conducted by the established nuclear powers before the nuclear test ban treaty took effect, usually reached levels of 6.9 to 7.0 on the Richter scale.

Certainly not an earthquake

What scientists are certain of, however, is that the 5.1 "earthquake" was triggered by a nuclear test.

Professor Torsten Dahm, head of the department for physics of earthquakes and volcanoes at the GFZ, identifies three indicators that make clear it was a manmade blast rather than a natural earthquake.

"The first indicator is the characteristic in which the waves are radiated," Dahm says. "That gives us a strong signal that is different from a tectonic earthquake."

Another indicator is the spectrum of frequencies, including the energy of the waves.

The third is the depth of the epicenter. "It is clear that a manmade explosion cannot take place as deep as ten kilometers (30,000 feet) or more in the earth's crust."

Explosions, by comparison, are usually very close to the surface.

While it's easier to determine the precise depth of the explosion the closer the seismometers are to the epicenter of the event, researchers can still interpret many things from far away - even if the country in question provides no data. They use something called "seismic antennas."

Seismic antennas are not actual antennas, but rather large networks of seismometers located in "quiet" parts of the world at huge distances from one another.

"With those, you can try to modulate deep phases from incoming waves. This will give you an idea of how deep in the ground the event took place."

With the latest North Korean nuclear test, the case is clear, anyhow. The epicenter is right in the middle of a well-known nuclear testing ground. And that area is far away from any seismic activity.

Not even a chemical analysis of the air will be able to prove whether North Korea detonated a hydrogen bomb as opposed to a more classical atomic bomb.

That leaves scientific experts with a single source of hard data: seismic waves.

Different seismological research organizations have come up with slightly differing figures about the strength of the quake following North Korea's test of what it calls a thermonuclear weapon.

The German Research Center for Geosciences in Potsdam (GFZ) registered a tremor of 5.3 on the Richter scale. The Federal Institute for geosciences and Natural Resources (BGR) and the U.S. Geological Survey (USGS) registered a bit lower quake of a magnitude of 5.1.

Whatever the case, the strength of the explosion remains in a familiar spectrum: In 2013, a North Korean nuclear test resulted in a magnitude 5.1 tremor. In 2009, a similar test registered 4.9 (BGR) or 5.0 (GFZ) respectively.

"The fact that both blasts of 2013 and the current one are almost identical in magnitude certainly does not suggest a technological quantum leap into a whole new dimension," says Nicolai Gestermann says, a geophysicist at the national data center for the verification of the nuclear test ban treaty - an office of the BGR.

By comparison, underground tests conducted by the established nuclear powers before the nuclear test ban treaty took effect, usually reached levels of 6.9 to 7.0 on the Richter scale.

Certainly not an earthquake

What scientists are certain of, however, is that the 5.1 "earthquake" was triggered by a nuclear test.

Professor Torsten Dahm, head of the department for physics of earthquakes and volcanoes at the GFZ, identifies three indicators that make clear it was a manmade blast rather than a natural earthquake.

"The first indicator is the characteristic in which the waves are radiated," Dahm says. "That gives us a strong signal that is different from a tectonic earthquake."

Another indicator is the spectrum of frequencies, including the energy of the waves.

The third is the depth of the epicenter. "It is clear that a manmade explosion cannot take place as deep as ten kilometers (30,000 feet) or more in the earth's crust."

Explosions, by comparison, are usually very close to the surface.

While it's easier to determine the precise depth of the explosion the closer the seismometers are to the epicenter of the event, researchers can still interpret many things from far away - even if the country in question provides no data. They use something called "seismic antennas."

Seismic antennas are not actual antennas, but rather large networks of seismometers located in "quiet" parts of the world at huge distances from one another.

"With those, you can try to modulate deep phases from incoming waves. This will give you an idea of how deep in the ground the event took place."

With the latest North Korean nuclear test, the case is clear, anyhow. The epicenter is right in the middle of a well-known nuclear testing ground. And that area is far away from any seismic activity.