It's possible, but the simpler explanation is that the have a H-bomb. New graph from @ctbto_alerts shows range up to 1MTpic.twitter.com/740PDHsc4X
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Here's an observation. The DPRK went straight to underground testing, something China struggled with well into the 1970s.
Explain that more Jeffrey-- India, Pak, S Afr, Israel all did or planned for underground tests for first devices.
I don't think instrumentation is quite the challenge today that it was for China in 1970s. An observation about technology diffusion. (1/2)
Also, shafts are different that tunnels. I should have specified tunnel shots. (2/2)
Good point-- all were vertical shafts in deserts. Mountain tunnel testing very different. Got it! Concur.
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