[count-your-change]

A nuke explosion also has its own unique signal through the earth which is how an underground test shows up on sensors around the world. The signal of an earthquake is different and the difference is easily detected. Some what analogous to running different signals over a phone line.

[SolaGratiaSolaFideSolaScript]

You are correct. As a matter of fact if the cause of this earthquake was a nuclear explosion even an underground one then the U.S> and Russia who monitor each other because of the Test Ban treaty would know that there had been a nuclear explosion. International organizations that monitor nuclear activity would also know. The likelihood of this being a nuclear explosion is slim unless of course there is a conspiracy and everyone is in on it.