Bart – Regarding your comment, the following
excerpts are from an article by Kasahara, entitled Tides, Earthquakes, and Volcanoes, which appeared in Science 297:38-349, 2002
The elastic strain resulting from Earth tides is extremely small, on the order of 10−8, which seems too small to trigger earthquakes and volcanism (2). Nevertheless, the idea that tides may influence these geophysical events has been discussed since 1930, when an interesting earthquake sequence was observed during an earthquake swarm east of Ito on the Izu Peninsula, central Japan.
The effects of tides on submarine volcanism were not observed until the summer of 1994, when the U.S. Navy Sound Surveillance System (SOSUS) array identified intense earthquake activity around Axial Volcano on the Juan de Fuca Ridge (11). The ridge is located about 400 to 800 km west to southwest off the western coast of North America. The data showed a clear correlation between tidal change and earthquake activities on two occasions (1, 12).
The Ito swarm was thought to be related to volcanism, although magma was not identified at the time. Nasu et al. (3) observed that for several days, the hourly numbers of earthquakes were higher during low tide than during high tide. They suggested that the swarm was triggered by the ocean tide, but did not offer a convincing triggering mechanism.
But at least one other example of semidiurnal variation in earthquake swarm activity was detected near Ito in 1978 (4). Analysis of stress due to ocean loading effects suggested a strong influence of ocean tides.
By themselves the tidal forces are too small to generate earthquakes, but in the critical stage of faulting they can trigger volcanic earthquakes.