My thoughts exactly, a nuke attack.
However, when that day comes we will all have reason to both celebrate and relax.
1) This was too big to be a nuke unless it was the largest of thermonuclear weapons (of a size even the US no longer has in its arsenal).
2) It was 35 miles deep. Can't be a nuke.
3) Nuclear explosions and quakes, even of the same magnitude, are INSTANTLY and easily distinguished on all seismographs. NEIC wouldn't have listed it as an earthquake otherwise.