Can the Moon Cause Earthquakes?

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

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