Aftershocks are the smaller, but sometimes just as damaging, earthquakes that occur after the initial shock. They can continue for days, or sometimes for years, but they normally get smaller and less intense with time.
In some cases, the aftershock is actually stronger than the first earthquake. The aftershock is then designated as the original earthquake, and the first quake, a foreshock.
Aftershocks are the result of the earth’s crust around the fault plane trying to adjust to new positions, caused by the original quake. These fault planes or lines are fractures in the earth’s rock, where one side moves in relation to the other. When the energy is released due to this movement along the fault, an earthquake occurs.
Most aftershocks occur along the original fault plane or in adjoining faults caused by the stress of the earthquake. The actual rupture length and the distance of the aftershock are usually equal. This helps determine how extensive the original rupture was.
In 1894, Fusakichi Omori developed a formula for determining how the strength of the aftershock decreases, and, how long it will take.
According to Omori, the rate of the aftershocks decreases quickly. So, the odds of an aftershock on the second day will be half that of the first. This is not always the case, however, and, aftershocks are more often random.
Bath’s Law states that the difference between the main shock, and the aftershock are always constant, regardless of how the main shock measures on the Moment magnitude scale, which measure earthquake intensity. The difference between the largest aftershock and the main earthquake is usually 0.1 to 3, but normally around 1.2.
Aftershocks are as unpredictable as earthquakes. They may go on for years, as those around the San Andreas, and the New Madrid fault areas.
Sequences of aftershocks are no longer counted when the seismic activity has returned to normal. However, in the case of the San Andreas, and the New Madrid fault areas, the activity continues.
The New Madrid earthquakes, named for New Madrid, Missouri, occurred in 1811 and 1812. They are the largest intraplate earthquake series ever to occur in the United States. There is still movement in the rupture, and small aftershocks have been felt for the last two hundred years. Movement along this fault line is about .0079 inches per year.
The San Andreas fault line moves about 1.5 inches a year, and, it has been experiencing aftershocks for ten years.
http://earthquake.usgs.gov/earthquakes/step/explain.php
