Large earthquakes are more likely to occur at times of full or new Moon, according to a study published online this week in Nature Geoscience.
Although it seems intuitive that the fault lines on Earth that are already close to failure could be pushed into slipping by the gravitational forces of the Sun and Moon, firm evidence for tidal triggering of earthquakes has been lacking.
Satoshi Ide and colleagues reconstruct the size or amplitude of tidal stresses - rather than just the timing of high tide or tidal phase - in the two weeks prior to large earthquakes (magnitude 5.5 or greater) that have occurred over the past two decades. Although they find no clear correlation between tidal stress and small earthquakes, they do find that some of the largest earthquakes: including 2004 Sumatra, Indonesia; 2010 Maule, Chile; and 2011 Tohoku-oki, Japan, occurred during times of high tidal stress amplitude. They also find that the fraction of large earthquakes compared to small earthquakes increases as the amplitude of tidal stress increases.
Precisely how large earthquakes initiate and evolve is not fully understood, but they may grow via a cascading process whereby a tiny fracture builds up into a large-scale rupture. If so, the authors’ results imply that the likelihood of a small fracture cascading into a large earthquake are greater during the Spring tide. Thus, knowledge of the tidal stress state in seismic regions could help in assessing the probability of an earthquake.