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Aftershocks are earthquakes of smaller magnitude that follow a large quake. They happen because the newly-moved rock has to be settled. Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can catch people off guard if they are salvaging items from a damaged building.
There can also be foreshocks, which are smaller earthquakes that precede a much larger one. The 9.5-magnitude Great Chilean Earthquake had a 7.9-magnitude foreshock.
For large earthquakes (M=8) aftershocks may occur over hundreds of kilometres. Depending on the size, and depth of the earthquake, aftershocks may occur for many months after the mainshock. However, both the size, and the rate of aftershock activity dies off quickly with time.
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