"Nuclear explosions and quakes, even of the same magnitude, are INSTANTLY and easily distinguished on all seismographs."
Definitely NOT true.
The evaluation process cannot be done "instantly", is definitely not "easy", and definitely cannot be done on "all seismographs". In many cases, it is easy to classify as "definite earthquake", but classifying as "nuclear explosion" is much more difficult.
But at 55km deep, this isn't anything nuke related, right?