Jovirunner
Not sure if the:
We were on a Monarch airbus and were really pitching/rolling quite heavily ...
was on the approach or on the landing rollout?
If the former, then winds are the likely, if the latter, the result of the winds requiring the pilots to straighten it up, can cause the machne to 'rock and roll' a little.
Of course, since it is possible that your landing coincided with the quake, it may well have been that. The timing would be very close - but always possible.