Reported December 2008

What Causes Earthquakes?: An earthquake is the result of a sudden release of stored energy in the earth's crust triggered by shifting tectonic plates. The earth's lithosphere is an elaborate network of interconnected plates that move constantly. Occasionally, they lock up at the boundaries, and this creates frictional stress. When that strain becomes too large, the rocks give way, break and slide along fault lines. This can give rise to a violent displacement of the earth's crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10 percent or so of the total energy is released in the seismic waves. The rest is converted into heat, used to crush and deform rock or released as friction.

How Do Scientists Rate Earthquakes?: An earthquake's magnitude describes how much the ground moves. The scale is logarithmic, which means when the magnitude increases by one the amount of ground motion increases by ten times. That is, a magnitude 3 quake leads to ten times as much ground motion as a magnitude 2 quake, and a magnitude 2 leads to ten times as much motion as a magnitude 1. This means a magnitude 3 is 100-times as violent as a magnitude 1, and -times less violent than a magnitude 5.

The magnitude scale also tells us just how much energy an earthquake released. For example, a magnitude 1 earthquake releases the same amount of energy as 30 pounds of TNT exploding. Although a magnitude 2 earthquake makes the ground move ten times as much as a magnitude 1, it releases 32-times as much energy -- or roughly as much as a ton of TNT. A magnitude 5 earthquake packs the punch of a moderate nuclear weapon, and a magnitude 12 quake would be enough to put a crack all the way through the center of Earth.

The American Geophysical Union and the Incorporated Research Institutions for Seismology contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Kristine Pankow
Salt Lake City, Utah 84112-0011
(801) 585-6484
pankow@seis.utah.edu

Peter Weiss
American Geophysical Union
Washington, DC 20009-1277
(800) 966-2481
http://www.agu.org
pweiss@agu.org

John Taber
Education and Outreach Program Manager
Incorporated Research Institutions for Seismology
Washington, DC 20005
(202) 682-2220
taber@iris.edu