Second North Korean Nuclear Test Or Japan's 6.0 Earthquake?

As of the past hour, the Japanese monitoring what they believed to be a second North Korean nuclear missile/bomb test may instead be a 6.0 Richter scale earthquake.

What's interesting, however, is that North Korea said there test was entirely below ground. I'm no physicist or earth engineer. But I'm assuming a nuclear incident could trigger earthquakes.

For example, when during WWII the Manhattan Project was preparing for the desert nuclear tests, one scenario they anticipated was a shock going right to the core of the middle of this planet which would wipe out all life very quickly. Oddly enough, they went ahead anyway.