Petra, as pointed out by, I think, EQF, there was a paper in "Geology" in 2002 by Tolstoy and other(s) that showed a tidal influence on quakes on or near the Juan de Fuca spreading center (my details may be off because I have not looked at this in a while and don't feel like walking over to the shelf and re-reading this). So, tides may influence earthquakes under certain circumstances.
Chris