Gravitational pull 'has role in quakes'
The gravitational forces responsible for high tides may also play a role in triggering major earthquakes, a study suggests.
A Japanese research team found that large earthquakes are more likely to occur at times of a full or new Moon.
Tides arise from the effects of the gravitational interaction of the Moon and Sun on a rotating Earth.
This could put extra strain on geological faults that are already close to slipping, the team reports.
The researchers led by Satoshi Ide, from the University of Tokyo, have published their findings in the journal Nature Geoscience.
"Large earthquakes are more probable during periods of high tidal stress," they wrote in their paper.
When the tug of gravity is strongest, they added, "the probability of a tiny rock failure expanding to a gigantic rupture increases".
While the link might seem intuitive, firm evidence for the tidal triggering of earthquakes has been lacking.
The team reconstructed the size of tidal stresses - rather than just looking at the timing of tidal phases - in the two weeks prior to large quakes (of Magnitude 5.5 or greater).
There was no clear correlation found between tidal stress and small earthquakes. But they found that the fraction of large earthquakes increases in line with the size of tidal stress.
These massive quakes, such as the Magnitude 9.0 Great Tohoku event in Japan and the Magnitude 8.8 earthquake that hit Chile in 2010 tended to occur near the times of maximum tidal stress - during new and full Moons.
The findings "can be used to improve probabilistic earthquake forecasting, especially for extremely large earthquakes," Prof Ide and colleagues concluded.