like here, it could be the earth's crust still settling as a result of the melting ice at the end of the last glaciation; less weight on the crust makes it rebound to it's original state.
it's what's happening in my part of the world; with the last one, about 20 years ago) at around 6 (Richter scale).
just one day before that happened, i was telling my GF that it does not happen here. the next day she sees the vacuum cleaner walking past her and loud knocks on the door, and such.
when i was living in Iceland, i had the great pleasure of experiencing 2 strong earthquakes, one at 8.6 and another one at 9.4 - it was much better than what my sub can do and i enjoyed it a lot.
