Courtesy Won-Young Kim, Lamont-Doherty Earth Observatory

The explosion took place near Kilju, in North Korea. Seismic waves were recorded by a station in Mudanjiang, China, 205 miles north. That is the closest station with data civilian researchers can get access to in real time.Courtesy Won-Young Kim, Lamont-Doherty Earth Observatory

North Korea's nuclear bombs are getting a lot bigger, say civilian seismologists who have been calculating the energy of the rogue state's bomb tests using the same methods applied to measuring earthquakes.

Based on data from the nearest non-classified seismological station in China, the Feb. 11 nuclear test was found to have had the energy equivalent of a 5.1-magnitude earthquake, which is about equal to an explosion of 7.3 kilotons of TNT, according to Won-Young Kim of Columbia University's Lamont-Doherty Earth Observatory (LDEO). That measurement jives with statements released by the South Korean Defense Ministry and Director of National Intelligence, James R. Clapper, Jr.

For comparison, that's less than half the explosive power of the bombs dropped on Hiroshima and Nakasaki in 1945. That said, the explosion is two or three times more powerful than North Korea's 2009 test and several times more powerful than that country's first test in 2006.

NEWS: North Korea Confirms Third Nuke Test

“Every one of these explosions is dangerous,” said LDEO seismologist Paul Richards. “These would really be an antisocial thing to drop on a modern city.”

Data from a seismic station in Mudanjiang, China, also was used by Kim and his colleagues last week to study North Korean tests in 2009 and 2006. There is also genuine earthquake data from that station, as well as artificial explosions of non-nuclear sorts, used to generate seismic waves to study the structure of a volcano in the same area, said Kim. All that comparative data has made it possible to glean quite a lot from the new signal, he explained.

This week’s test shows that the North Koreans are definitely learning the practical details of how to put material together, press the button, and get more out of it each time, he said.

ANALYSIS: 3 Reasons N. Korea Can’t Nuke the U.S. (Yet)

"They are very similar signals (in 2009 and 2013), so it must be the same tunnel in the mountain.” said Kim.

In fact, it looks like the latest bomb was detonated as little as 200 meters from the previous test, he said.

As for why non-government scientists are studying the test at all, when there are government scientists using more numerous and classified seismic stations in the area, it's a matter of wanting to shine a light.

NEWS: Google Maps North Korea

“We don't have any obligation or limitation,” said Kim. “Some government agencies sometimes cannot speak. We have been doing this to help get the Comprehensive Test Ban Treaty (CTBT) signed and enforced.”

The CTBT is designed stop the development of nuclear weapons by halting all tests of nuclear devices.

“If you can't do the testing, you can't develop the weapon,” said Kim. And already, even without the treaty, there is no hiding tests from civilian seismologists. “Nobody can cheat greater than a kiloton.”