Can the Moon Cause Earthquakes?

John Roach
National Geographic News

May 23, 2005
Coast dwellers are accustomed to the daily rhythm of the tides, which are primarily lulled in and out by the gentle gravitational tug of the moon. Some scientists wonder whether the moon´s tugging may also influence earthquake activity.

"The same force that raises the ´tides´ in the ocean also raises tides in the [Earth´s]crust," said Geoff Chester, an astronomer and public affairs officer with the U.S. Naval Observatory in Washington, D.C.


Chester said the tides in the Earth´s crust are subtle—on the order of a few centimeters, as opposed to the several-meter ocean tides.

"We live on the crust, so we don´t really notice the deviation from what would be sort of the normal form of the geoid," he said. "So the effect is small but nonetheless there."

(The geoid is an imaginary outline that coincides with the mean sea level in the ocean and its extension through the continents.)

In theory, this slight deformation of the Earth´s crust could be sufficient to trigger an earthquake—like the proverbial straw that broke the camel´s back.

"Most earthquakes occur on preexisting tectonic lines, and the vast majority do occur as a result of geophysical processes, but there may be some correlation [between the moon] and earthquakes," Chester said.

For example, he said that in general there is a higher incidence of earthquake activity in the Northern Hemisphere when the moon is north of the Equator and an increase in earthquake activity in the Southern Hemisphere when the moon is south of the Equator.

The moon´s orbit is inclined in relation to the Earth, causing the moon´s position in the sky to nod north and south on an 18.6-year cycle.

Is the observed correlation between the moon´s position in its 18.6-year cycle (or any other lunar phase) and earthquake activity a coincidence or something more? That question, Chester said, is best answered by the U.S. Geological Survey.

"There´s no evidence to support that," said John Bellini, a geophysicist with the survey´s National Earthquake Information Center in Golden, Colorado. "There were some studies in the past that tried to link lunar effects to seismicity [the relative frequency and distribution of earthquakes] and there was nothing found."

Syzygy

James O. Berkland is a Glen Ellen, California-based geologist and editor of Syzygy—An Earthquake Newsletter. He believes the gravitational tugs of the moon, sun, and other planets can influence earthquake activity. Berkland said he has accurately predicted tremors based on factors such as syzygy.

"Syzygy" refers to the alignment of three celestial objects. Syzygy of the sun, Earth, and moon occur twice a month, at the full and new moons. At such times, gravitational forces are at a maximum, especially when the bodies are close together, Berkland said.

The Earth and moon are closest together—at perigee—once a month. The Earth and sun are closest together—at perihelion—once a year. Perihelion currently occurs in early January. Maximum gravitation force occurs when a syzygy and perigee occur on the same day as perihelion.

According to Berkland, seismometers left on the moon by Apollo astronauts show that moonquakes occur most frequently at perigee.

"So we know Earth´s gravity triggers moonquakes. I don´t think any scientist disputes that," Berkland said. "When I learned that, I went to my former [U.S. Geological Survey] colleagues in Menlo Park [California] and pointed out this really exists, so what´s so difficult about turning it around?"

According to Berkland, the U.S. Geological Survey said such a theory is ridiculous—the Earth is 82 times more massive than the moon. Though the Earth can trigger quakes on the moon, they said, the moon is too small to trigger any earthquakes.

But the moon is mostly solid and lacks a liquid core like the Earth, Berkland said. The Earth "is an active, living planet, and so it is not at all surprising that minor gravitational stresses can trigger earthquakes," he said.

Using syzygy and other factors—such as the number of cats and dogs listed in the lost and found in newspaper classified advertisements—Berkland said he accurately predicted several earthquakes, including the October 17, 1989 earthquake in San Francisco, California. Berkland said the number of cats and dogs reported missing goes up prior to an earthquake. The numbers went up significantly prior to the 1989 San Francisco quake, he said.

At least two major quakes may suppoort Berkland´s theory. The December 26, 2004, magnitude 9.1 in Sumatra, Indonesia, occurred on the day of a full moon. Likewise, the March 27, 1964, magnitude 9.2 earthquake in Alaska occurred on the day of maximum high tide.

According to Berkland, such correlations are more than coincidences. They demonstrate a true connection between the moon and earthquake activity, he said.

But Bellini, the U.S. Geological Survey geophysicist, said, "There is still no known observation of an effect related to the moon and seismicity."

In a follow-up email to National Geographic News, Bellini questioned the scientific validity of Berkland´s predictions. He said they appear to be "self-selected statistical analysis of historical seismicity rates and are so vague in time and location that they are certain to be correct."

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