BACKGROUND: The U.S. Geological Survey has devised the best tool yet for forecasting when and where earthquake aftershocks could occur: an online map, available to the public, that displays the probability of the ground shaking significantly over the next 24 hours for any 50-kilometer-square area of California after an earthquake.
HOW IT WORKS: The online map updates itself hourly. But it doesn't predict a primary earthquake. Instead, the map uses patterns in the initial aftershocks that follow big quakes to forecast when and where more will strike. It won't forecast the first of a series of quakes.
WHAT CAUSES EARTHQUAKES: Most earthquakes are caused by faulting: a sudden movement of rock along a rupture in the earth's surface. This surface is in constant slow motion, called plate tectonics, because deeper in the earth, hot rock continually flows. The plates cover the entire surface of the globe and can rub against each other in certain spots, sliding above or below each other. But if the motion isn't smooth, strain builds up until the "fault" ruptures, slipping to new position to relieve the strain. An earthquake is the resulting shaking that radiates out from the breaking rock.
WHAT ARE AFTERSHOCKS: Aftershocks are smaller quakes that occur after the main quake. They happen because the newly moved rock must re-settle in its new formation. Bigger earthquakes have more and larger aftershocks. Because they are so unpredictable, an aftershock can be as damaging as the initial quake, especially if building structures were weakened but not destroyed the first time around.
The U.S. Geological Survey, the Incorporated Research Institutions for Seismology, Inc., and the American Geophysical Union contributed to the information contained in the TV portion of this report.