North Korea nuclear test

North Korea is claiming a second, successful, nuclear device test.
Coincident with a shallow medium magnitude earthquake in the test region.

Earthquake is magnitude 4.7 or so on the Richter scale (Swiss are estimating 5.1).
Very shallow, consistent with surface origin, 380 km north east of Pyongyang, close to the Chinese border.

The North Korean's previous nuclear test was a fizzle, but this one is several times stronger and shallow enough there doesn't seem to have been any attempt to hide it.

So, probably few kiloton equivalent yield, depending on the local geology and where exactly they set it off, if it was in a cavern, borehole or right at the surface (unlikely).
If the yield is confirmed to be in the few kT range, then the North Koreans have working nuclear bombs large enough to be a concern.
South Korea and Japan are likely to be seriously concerned by this.

Sateliite and air recon should confirm very soon, and isotope leaks to the atmosphere will tell everything about the device.

My guess: they finally got a Pu-239 implosion working, with material from their old magnox clone reactor. Be interesting to know if this is more of the material they extracted over the last few years and used up part of their stash, or it if it is new stuff.
And whether they are getting external funding to keep working on this.

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Arms Control Wonk is reporting ~4 kT based on the yield/magnitude relation. They point out that the small yield could be a result of the need to keep the weapon light enough to fly on a rocket.

yeah, if you take the USGS magnitude estimate and the default, "not tricks to mask or amplify the blast" conversion for shallow explosion, then it comes ot right at ~ 4 kT
If the Swiss estimate is right, then it is closer to 10-12 kT.
If they used a cavern or geological formation to muffle it, could be a bit higher also.
Saw a waveform on one of the seismographs, looked like a textbook explosion with sharp initial spike.

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