  Evidence for the triggering of earthquakes by tides has been largely lacking
for the continents but detectable in the oceans where the tides are larger. By
far the strongest tidal triggering signals are in volcanic areas of mid-ocean
ridges. These areas offer the most promise for the study of this process, but
even the most basic mechanism of tidal triggering at the ridges has been
elusive. The triggering occurs at low tides, but as the earthquakes are of the
normal faulting type, low tides should inhibit rather than encourage faulting.
Here, treating the most well documented case, Axial Volcano on the Juan de Fuca
ridge, we show that the axial magma chamber inflates or deflates in response to
tidal stresses and produces Coulomb stresses on normal faults opposite in sign
to those produced by the tidal stresses. If the bulk modulus of the magma
chamber is below a critical value, the magma chamber Coulomb stresses will
exceed the tidal ones and the phase of tidal triggering will be inverted. The
stress dependence of seismicity rate agrees with triggering theory with
unprecedented faithfulness, showing that there is no triggering threshold.
