At the surface, the energy released in an 7+ magnitude earthquake is equivalent to a
very large nuclear bomb, equal to the largest ever detonated. However, there is currently no known method to release all that energy in the form of a shockwave. In a nuclear explosion its released mainly as heat. Super-secret doomsday weapons are pure science fiction, with emphasis on fiction. An earthquake weapon would be dependent upon the discovery some way to first generate the waves (like the discovery of nuclear fission led to the first nuclear bombs). On top of that,
if it was a test of an earthquake weapon why not target it against an unpopulated pacific island instead of a poor nation with millions of people? It just makes no sense.
And the notion that it was a nuclear detonation, buried deep underground is equally preposterous. First, it would need to be several times larger than the largest nuke ever detonated because its underground. And second,
someone would have made a huge announcement. Most highly developed nations can detect earthquakes and nuclear explosions (even underground ones) and know the difference between the two. Using a similar line of thought, the Soviets indirectly proved that the Apollo moon landings were not a hoax ... they could intercept the radio transmissions coming from the moon and would have said something if things didn't match up.