The Mechanism of Tidal Triggering of Earthquakes at Mid-Ocean Ridges

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.