NPR logo Earthquake Physics: By The Numbers


Earthquake Physics: By The Numbers

Japan's terrible earthquake once again makes it painfully clear how fragile our existence is in this planet. We seldom stop to think about this, but we are here as guests. Our impressive power, even our capacity for planetary destruction, pales when confronted with the real power of planetary dynamics. We may have learned to harness the power of energy, and are very good at describing regular, periodic natural phenomena, but our ability to predict sudden planetary changes is still in its infancy.

The February 27, 2010 magnitude 8.8 earthquake in Chile shortened the duration of a day by 1.26 microseconds (a microsecond is one millionth of a second). The 2004 Sumatran 9.1 earthquake shortened the day by 6.8 microseconds. It is estimated that the March 11 earthquake in Japan, with magnitude 9.0, shortened the length of the day by a bit more than the Chilean one, about 1.8 microseconds.

The change in day duration is due to a redistribution of Earth's mass. Just as an ice skater can speed up her spin by bringing her arms closer to her body, the shifting of Earth's mass will make it rotate faster. The closer the earthquake is to the equator, the more it will spin up the Earth. And if the Earth spins a bit faster, days are a bit shorter.

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Perhaps even more impressive, initial data indicates that the earthquake shifted the position of Japan's main island by 8 feet. It also shifted the position of Earth's figure axis (the axis long which Earth's mass distribution is balanced, like the balance axis in a see-saw) by about 6.5 inches (17 centimeters). The figure axis is not the same as the north-south axis, around which Earth rotates in space at about 1,000 mph. They are off by about 33 feet. Earthquakes can't shift the north-south axis; only external gravitational forces, such as that exerted by the sun, planets or moon on Earth, could.

To put us on our right place, it's possible to compare the energy released in an earthquake to that released by nuclear bombs. A magnitude 9.0 earthquake releases the equivalent of about 2 million Hiroshima bombs, or 31 billion tons of TNT. Of course, earthquakes aren't purposefully targeted at killing people or release radiation—nature doesn't have a plan for us, either way. Unless, of course, earthquakes or other natural disasters damage nuclear power plants, as is happening in Japan.

Hopefully, the tragedy of recent earthquakes will raise our awareness of our status in this planet and contribute to change our attitude towards it.