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.