Roger, I'm pretty sure that Tolstoy and others have found a tide effect on earthquakes on the spreading center between Pacific Plate and Juan de Fuca plate; this was published maybe 5 years ago. Being under deep water, the question is whether this was earth tides or ocean tides; I don't remember (I did not read the paper; I just read a summary of the paper). Perhaps magma chambers are more sensitive to tides.
Chris