The March 19, 2011 full Moon coincides with the perigee, the closest distance as the Moon orbits Earth. Adopting terminology astronomers don't use, popular media call this phenomenon the supermoon.
Perigee at Full Moon and Earthquakes
Claims that tidal forces from the supermoon cause increased earthquake activity are circulating in popular media. Can close perigees during the full Moon cause earthquakes?
A plausible mechanism exists. Lunar tidal forces causing twice daily tides also stress Earth's rocky interior. Tidal forces are greater during the full and new Moon phases because tidal forces from the Sun add to the tidal forces from the Moon. Tidal forces are also greater during perigee because the Moon is closer to Earth. Are these effects significant?
What Are Tidal Forces?
Tidal forces are caused by the difference in gravitational force. The portion of Earth closest to the Moon experiences the strongest gravitational pull from the Moon. The Moon's gravity is weakest on the portion of Earth farthest from the Moon. This difference in gravitational force, the tidal force, causes two tidal bulges. The portions of Earth closest to and most distant from the Moon bulge, causing two high and low tides a day.
Mathematical Calculation of Tidal Forces
Gravitational forces are directly proportional to the mass and inversely proportional to the distance squared. While still directly proportional to the mass, tidal forces are inversely proportional to the cube (not square) of the distance. To those versed in calculus, this inverse cube effect comes from the derivative of the gravitational force law. (For details see Zeilik and Gregory, Introductory Astronomy & Astrophysics, Saunders, 1998.) These proportionalities allow calculating ratios of tidal forces.
The ratio of the Sun's to the Moon's tidal force on Earth is the ratio of the Sun's mass divided by its distance cubed to the Moon's mass divided by its distance cubed. Using the relevant distances and masses, I calculate that the tidal force from the Sun is about 45% of the tidal force from the Moon.
During the full and new Moon phases, the Earth, Sun, and Moon form a line. Hence the Sun's tidal forces add to the Moon's tidal forces. Tides are more extreme during full and new Moon phases. During the first and third quarter lunar phases, however, the Sun and Moon are 90 degrees apart as seen from Earth. Hence the Sun's tidal forces subtract from the Moons tidal forces. Therefore tidal forces on Earth vary significantly as the Moon cycles through its phases.
How much does the very close perigee affect the tidal force? Tables posted by the South African Astronomical Observatory list the Moon's distance as 367,577 kilometers on March 19, 2011. The average distance to the Moon during the 13 perigees listed for 2011 is about 362,000 kilometers. Cubing the ratio of these distances shows that the Moon's tidal force during the March 19 perigee would be only about 5% greater than on the average perigee if it were during a full Moon.
These numbers suggest the tidal effect of the March 19, 2011 perigee is too small to suddenly unleash rampant tectonic destruction.
Moon During Recent Earthquakes
Looking at the Moon's distance and phase during recent strong earthquakes listed by the USGS can further test the claims that a perigee during full Moon causes major earthquakes.
Most recently, Japan suffered a devastating 9.0 magnitude earthquake on March 11, 2011. That sounds suspiciously close to the March 19 perigee and full Moon, but remember that the Moon's complete cycle only takes a month. The Moon was at apogee, its farthest distance from Earth, on March 6, so on March 11 the Moon was slightly closer to apogee than to perigee. On March 11, the Moon was 1 day before first quarter, so the Sun's tidal force subtracted from the Moon's tidal force to decrease the total tidal effect. In addition, March 11 is before the March 19 supermoon; in cause-effect relationships the cause customarily precedes the effect.
Chile's 8.8 magnitude earthquake occurred on February 27, 2010. The Moon was at perigee, but not the closest perigee, and was within a day of being full.
Haiti's devastating earthquake occurred on January 12, 2010. This date initially sounds close to the last supermoon on January 30, 2010, when the Moon was only 15 kilometers farther from Earth than on March 19, 2011. Again the Moon's cycle only takes a month, so Haiti's earthquake was only 5 days before lunar apogee on January 17, 2010 and 2 days before the new Moon on January 14. The supermoon did not cause Haiti's earthquake.
On September 29, 2009 there was an 8.1 magnitude earthquake in the Samoan Islands. The Moon was at apogee and in the gibbous phase between first quarter and full. The supermoon effect clearly did not cause this strong earthquake.
These data show no correlation between the Moon's phase or distance and the occurrence of devastating earthquakes. Caveat: A sample of only four earthquakes is to small to be statistically valid. Hence these earthquake data are anecdotal evidence not a statistically valid argument. A valid statistical study is beyond the scope of this article.
How Tidal Forces Do Contribute to Earthquakes
Heat deep in Earth's interior drives convection currents in the mantle. Continental plates floating on the mantle drift to cause earthquake, volcanic, and other tectonic activity. Tidal forces acting on Earth's interior generate friction and therefore some, not all, of the heat energy driving tectonic activity. Showing this mechanism works, tidal forces heat Jupiter's moon, Io, and provide the energy to make Io the most volcanically active world in the solar system. This tidal heating is however a very long term effect, so the number of earthquakes does not change in response to rapid changes in tidal forces.
One nice thing about physics is that anyone with the necessary knowledge can test outlandish claims. Knowing physics allows one to pull the small grains of truth from big bunches of baloney.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
