The gravitational pull of the moon and sun may serve as the last straw in triggering earthquakes, researchers have found.

Though the effects of the two heavenly bodies--known as the lunar and solar tidal forces, respectively--have only about 0.1 per cent of the accumulated-strain energy that triggers a powerful earthquake, such a force could act as the last push for a major tremor when seismic strain strong enough to cause a quake has accumulated, according to the researchers, including Sachiko Tanaka, a research fellow at the National Research Institute for Earth Science and Disaster Prevention.

The researchers' findings were announced in a US science journal.

Tanaka studied about 600 magnitude-5-and-above earthquakes that occurred in the 33 years until 2008 around the epicentre of a powerful earthquake that hit off Sumatra, Indonesia, in 2004--an area measuring 1,500km long and 500km wide.

According to the research, in the eight years prior to the 2004 earthquake, many quakes occurred when the lunar or solar tidal forces were at their most powerful, indicating that such forces could cause tremors before a powerful earthquake.

The research proposes that if relatively weak earthquakes often occur in one area when a tidal force is strong, this force could help trigger accumulated strain in the form of a powerful earthquake.

Tanaka said she hopes to conduct similar research around the epicenters of other possible major earthquakes, such as a major earthquake feared to strike the Tokai region.