'2010 N Korean seismic event was earthquake, not nuclear test'

A tiny seismic event that occurred in North Korea on May 12, 2010 may have been an earthquake rather than a small underground nuclear explosion, according to a new study.
The study led by seismologists from Columbia University in the US contradicts the findings of a 2015 report which concluded that the magnitude 1.5 seismic event was a small nuclear explosion.
The seismic signature of an explosion can be distinguished from that of a natural earthquake by looking at the ratio of two different types of seismic waves produced by the event, researchers said.
In the study, Paul G Richards and his colleagues suggest that that the May 12 event's signature is much more like that of an earthquake than an explosion.

More importantly, Richards noted, their analysis demonstrates how even very small seismic events can be monitored and analysed for signs of being a nuclear explosion.
The May 12 event produced seismic signals that are about three thousand times smaller than those coming from confirmed nuclear tests in North Korea conducted in 2013 and this year.

Seismological monitoring of underground nuclear explosions has been underway since the late 1950s, "but we can now monitor down to extraordinarily small sizes of seismic events with high confidence," Richards said.
"We'll never be able to do it perfectly, but we can do it down to such a low magnitude that for all practical purposes we can know whether a nuclear explosion has taken place in a nuclear weapon development programme," he added.

The first signs that a nuclear explosion might have taken place in North Korea in May 2010 came from stations located in South Korea, Japan and Russia that picked up traces of radioactive isotopes indicative of a nuclear explosion, researchers said.
By retracing the path of these unusual radionuclides through the atmosphere, researchers suggested that they were produced in a region of North Korea where earlier confirmed nuclear tests had taken place.
At first, scientists could find no signs of the sort of small seismic event that would indicate a low-yield nuclear explosion related to the radionuclide release.
However in 2015, a team of Chinese scientists identified a very small seismic event in the region and concluded that it came from a nuclear explosion.

"The event they found was so small that we did not detect it during our original study, but it all seemed to fit together with the radionuclide data," Richards said.
The researchers examined the seismic event discovered by the Chinese team, with the help of newly available data from a temporary network of seismic stations deployed in China that captured the May 2010 event.
However, when they compared these data to seismic signatures from confirmed nuclear explosions and earthquakes captured on another Chinese network, they "realised that the problem event turns out to look more like an earthquake".
The study was published in the journal Bulletin of the Seismological Society of America (BSSA).