Pull of the Moon may trigger major quakes

The same gravitational influence that the Moon has on the oceans may also trigger earthquakes along Earth’s most fragile faults, scientists say.

Researchers at the University of Tokyo calculated the levels of ‘‘tidal stress’’ before major earthquakes of the past few decades and found that high levels of stress were often followed by major earthquakes, according to a study published in the journal Nature Geoscience this week.

‘‘This suggests that the probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels,’’ the scientists wrote.

‘‘We conclude that large earthquakes are more probable during periods of high tidal stress.’’

The magnitude 9.1 quake in Sumatra, Indonesia in December 2004, which caused a tsunami that killed 230,000 people, was likely precipitated by the influence of the Moon, the scientists say. So was the 2011 quake in Japan which left at least 15,000 dead and caused a meltdown at the Fukushima nuclear reactor.

Earth’s tides are caused by the gravitational pull of the Moon as it moves over the surface of the planet. When the satellite is directly above a certain spot, it pulls the water there towards itself, causing the ocean on that side of the planet (and the opposite side) to bulge.

This effect is especially evident when Earth, the Sun and the Moon are aligned, because the Sun is able to contribute its bulk to reinforce the Moon’s gravity, causing dramatic high tides.

‘‘The results are plausible,’’ University of Washington seismologist John Vidale said.

Vidale was not involved in the research; in fact, he has previously helped to debunk studies claiming a more tenuous connection between earthquakes and tides.

The connection still needs to be verified. Scientists point out that many large earthquakes of the past decade have happened at times when tidal stress wasn’t a factor.