WASHINGTON • The subtle tug of the moon's gravity helps keep our planet steady on its axis and generates the monthly rise and fall of the ocean tides. But occasionally, that pull gets to be a bit too much.

According to a study published in Nature Geoscience this week, the same gravitational influence that the moon has on the oceans may also trigger earthquakes along Earth's most fragile faults.

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. "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 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 tremblor in Japan which left at least 15,000 dead and caused a meltdown at the Fukushima nuclear reactor.

Much as the moon tugs at the ocean, it also exerts subtle pressures on the Earth's crust. We don't notice it, but the ground beneath us is stressed by the tidal influence of the moon.

"The results are plausible," University of Washington seismologist John Vidale told Nature.

The scientist was not involved in the research, but he has in the past helped debunk studies claiming a more tenuous link between earthquakes and tides.

But the connection with the moon 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.

Whether the phase of the moon can truly help predict large earthquakes remains to be seen.

WASHINGTON POST