Geophys. Res. Lett. doi:10.1029/2009GL041581 (2010)

The gravitational pull of the Sun and Moon raise minute tides in Earth's crust, which can alter the stress on faults. Seismologists have long debated whether these 'Earth tides' can trigger earthquakes. Now Sachiko Tanaka of the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, Japan, reports finding a link between Earth tides and the three huge earthquakes that occurred off the west coast of Sumatra in 2004, 2005 and 2007.

Tanaka found that during the decade before each megashock, faults in the region tended to produce smaller, more frequent earthquakes when stressed by tidal forces.

This correlation vanished after the mammoth quakes, suggesting that the relatively small tidal forces helped trigger quakes on fault segments that were near a critical threshold.