[dblshot]

Wouldn't an explosion that size be picked up by seismic earthquake sensors?

[Strategerist]

Wouldn't an explosion that size be picked up by seismic earthquake sensors?

Yes. Since it wasn't, it means there wasn't an explosion.

[Tijeras_Slim]

Wouldn't an explosion that size be picked up by seismic earthquake sensors?

Yes. The 2000 explosion of the Russian Sub Kursk was detected. This blast is likely much larger and coupled into the ground better.

http://www.agu.org/news/press/pr_archives/2001/prrl0105.html