To be honest, what does China have to gain? They are destroying the very “province” they seek to reunify? If they don’t claim responsibility, I don’t see how that brings them closer to claiming the island.

It would make sense if China were to use it against the US though. To stoke “America first” Republicans and make the US withdraw from geopolitical positions.

Consider the physics of it, a seismic event of a sufficient scale as to be a useful weapon would represent a colossal release of energy. Where would that energy come from?