North Korea Says It Tested a Hydrogen Bomb Meant for Missiles – NYTimes.com

The blast was so powerful that the first tremor was followed by a second, weaker one minutes later, which the United States Geological Survey called a “collapse.” The second tremor was detected in China but not in South Korea; officials in the South said that would be consistent with a cave-in at the North’s underground test site.

https://mobile.nytimes.com/2017/09/03/world/asia/north-korea-tremor-possible-6th-nuclear-test.html