Testing those multi-megaton atomic bombs deep underground a good idea?

A 6.5-magnitude earthquake occurred six miles north-northwest of South Kona, Hawaii, at 7:08 a.m. (their local time) today.

Flash over to the North Korean alleged nuclear bomb test(s).
Luckily, it seems their test only yielded about 5 kilotons or just under a third of the explosive power used in the Hiroshima weapon.

It would be nice if the weapon tests and the world's earthquakes fail to coordinate.
If it does, however, what will happen when they test a multi-megaton weapon underground?