The motion of the ocean is rocking our world, or at least helping to give it a vigorous shake in some locations when the conditions are right, a team of seismologists says.

The idea that celestial bodies can cause earthquakes is one of the oldest theories in science. In 1687 Newton’s universal law of gravitation revealed ocean tides are caused by the attraction of the sun and moon. And in the 1700s scientists started to wonder if these same distant bodies might also affect geologic faults. This idea flourished in the 19th century. The eminent French seismologist Alexis Perrey spent decades searching for a link between earthquakes and the phases of the moon. Scientific American published an 1855 article on his work. Even Charles Darwin mused on the subject (page 259).

At the end of the 20th century the notion the heavens could have a hand in earthquakes seemed to have been discounted. Despite many attempts, researchers had repeatedly failed to find hard evidence tides and temblors were connected. But in the past 20 years some studies have suggested that this long-suspected phenomenon might actually be real. The most recent of these was published in February by researchers at the Aristotle University of Thessaloniki in Greece who analyzed records from more than 17,000 earthquakes that struck the south of that country between 1964 and 2012. Rather than occurring at random intervals, the quakes seemed to be related to oceanic tidal effects, the team found.

The results reveal that the number of earthquakes with magnitudes between 2.5 and 6.0 on the Richter scale was strongly correlated with two of the four gravitational factors that cause Earth’s tides—one due to the gravitational attraction of the sun, called S2, and the other caused by the combined attraction of the sun and moon, or K1. (These, along with two other effects of lunar gravity, O1 and M2, are largely responsible for ocean tides.)

The team found that earthquakes were around 15 percent more likely to strike at times of the day when the pull of the sun (S2) was strongest, compared with when it was at its weakest. For the combined influence of the sun and moon (K1), the opposite trend was observed; when at its weakest, earthquakes were around 16 percent more likely to strike than when K1 was at its strongest. Such a correlation does not prove that tides are triggering temblors in Greece. Nevertheless, the observational results are intriguing and could represent one of the largest tidal effects on earthquakes ever measured.

Whatever is going on beneath Greece, enough evidence has accumulated since the late 1990s so that some scientists have begun to accept the concept of tidal triggers for temblors. “Logically there must be a connection between tides and earthquakes,” says John Vidale, a seismologist at the University of Washington who wasn’t involved in the Greek study. “Tides stress faults and earthquakes [occur] when the stress is sufficient.” The contribution of this effect, however, is probably vanishingly small and only occurs on certain parts of the planet.

One of the places that tides have the strongest influence on seismicity is in deep ocean basins, says Elizabeth Cochran, a geophysicist with the U.S. Geological Survey who also was not involved in the study. "The largest impact of tides on earthquakes is in oceanic regions,” she says, “where the ocean tides can in some locations impart a large force on shallow faults.”

The gravitational pull of the sun and moon is far too weak to trigger an earthquake on its own. When seawater accumulates above submarine faults that are already close to rupture, however, the increased pressure can reduce friction on the fault and thereby hasten a quake’s onset. This idea suggests that as an undersea fault builds toward an earthquake, it may become more sensitive to the small nudges of tidal forces.

Some scientists think this sensitivity could someday be used to forecast dangerous earthquakes. In 2012 a seismologist discovered that in the decade leading up to the devastating Tohoku earthquake, which hit Japan in 2011, smaller quakes started to follow the pattern of the tides. As soon as the larger temblor struck, however, the correlation disappeared. The correlation could derive from the fact that the Tohoku event’s epicenter was located in the Pacific Ocean. The movement of overlying seawater might have magnified the minute tidal forces, enabling them to trigger small tremors as the fault became stressed in the years leading up to the larger quake. The Tohoku study offers promise that large submarine earthquakes could someday be forecast by studying the tides. This approach, however, is unlikely to prove useful for quakes which strike far inland, such as the devastating magnitude 7.8 event that struck Nepal April 25. Such tidal forces are weaker in the absence of water.

Tides may not aid in forecasting all earthquakes globally, although in some regions they could become an important tool for seismologists. Beyond centuries of scientific interest, the effect of the sun and moon on the Earth’s deep geology could prove more than scientifically interesting—it could save lives.