April 7, 2010

A view of Concepción, Chile, on February 27, the day of the 8.8 magnitude earthquake, taken from the orbiting International Space Station. Photo: Courtesy of NASA

Ask The Professor

Why did the recent earthquake in Chile knock the Earth off its axis and shorten the average day?

Anne Gardulski, associate professor and chair of the Department of Geology in the School of Arts and Sciences, responds:

Geologic events as large as the recent Chilean earthquake can indeed affect the rate of Earth’s rotation. But the effect is miniscule, with a shortening of the day by only one or two millionths of a second. The great quakes—magnitude 8 and above—only occur about once a year on average. So it would take a million years to shorten the day by one or two seconds—not very significant for daily life! This effect has been going on for much of Earth’s 4.6 billion-year history. The reasons why this occurs are linked to how tectonic plate movement of rock changes the dynamics of rotation.

An earthquake is the sudden release of energy from somewhere on or in the Earth, and that energy moves through the Earth as seismic waves. Every year, there are about 500,000 earthquakes, but most are too small to be noticed. Earthquakes can be caused by movements between tectonic plates, along faults that separate two large blocks of rock or even by volcanic eruptions as molten rock moves upward in the Earth’s crust.

Along the west coast of South America, one of the oceanic plates of the Pacific is slowly moving to the east. It slides under the continent at an average rate of about three inches per year, and sinks downward at an angle into the mantle, the layer below the crust, in a process called subduction.

This movement is not smooth. It happens in fits and starts. The Chilean earthquake in February was an example of a significant vertical movement (about 30 feet) of the Pacific oceanic floor under South America. This caused the mass of the Earth to be redistributed ever so slightly. The redistribution is very small compared to the size of the Earth, but it is measurable. The Chilean earthquake and the Sumatran earthquake in 2004 involved just such vertical movements of rock, as the oceanic tectonic plates moved down into the Earth in subduction zones.

To understand why large earthquakes in subduction zones can affect rotation, consider the shape of the Earth. Our planet is not a perfect sphere—it is slightly flattened so that it bulges around the equator and may be ever so slightly wider in the southern hemisphere.

A line, called the figure axis, runs through the Earth, close to the position of the rotational axis. The mass of the Earth is symmetrically distributed around the figure axis, so when there is movement of a huge section of rock, the position of the figure axis moves a bit. It is not enough for us to perceive; it must be calculated.

That type of movement changes the distribution of mass slightly and affects the rate of spin of the Earth. The classic analogy is a figure skater: the skater spins faster as parts of the body’s mass (the arms) are pulled in closer to the body, closer to the axis of rotation. Similarly, when rock on the Earth slides vertically downward, closer to the axis, the rotation of the Earth will speed up very slightly, and thus the length of the day will be shortened by a few microseconds.

The Haitian earthquake in January did not have this effect. The fault motion was horizontal, parallel to the Earth’s surface, as tectonic plates abruptly slid past each other. Because there was no redistribution of mass vertically, it did not affect rotation.

Throughout geologic history, great earthquakes have caused changes in mass distribution on Earth, and therefore changes in rotation. The Earth’s rotation has been slowing down since the planet formed; the length of the day was about 22 hours about 600 million years ago. So speeding up the rotation by a microsecond every now and then just tends to counter that trend of lengthening the day.

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