Tides DO affect quakes

...slightly. A paper last month in Science showed that significant subduction earthquakes are slightly more likely to occur when tides in the ocean and Earth affect stress in the crust. The effect was documented by two UCLA scientists, Elizabeth Cochrane and John Vidale, with data from Sachiko Tanake of the Japanese earthquake agency, and is strongest for larger earthquakes. A University of California news release has more detail if you can't get to the November 12 Science. This doesn't affect the worthless prediction methods you'll find at places like GeoForecaster and syzygyjob.com, though.