Was it an earthquake? Or something gone awry? Or not so awry...
Earthquake.
Look at a worldwide seismic hazard map and Iran is a bright Red blob....
Not like you need to go fishing for causes of earthquakes other than an earthquake.
Also seismic quakes are VERY easily distinguishable from nuclear tests.
And this quake released about 5 megatons of energy. The Iranians would have to have a rather large HYDROGEN bomb to achieve that, which is rather implausible.
Cyborg's right. Seismologists can tell the difference between an earthquake and a nuclear boom-boom.