Aftershocks are lesser earthquakes which occur after a major earthquake, along the same fault. They can be unsettling for people who experienced the initial earthquake, because they do not always fall into a recognizable pattern or system, jolting people and houses without warning. Because aftershocks, like earthquakes, cannot be predicted, if you experience an earthquake, you should be prepared for aftershocks, and take steps to prepare yourself including getting family members and pets out of the way of unsafe structures.

An earthquake is caused by stress under the Earth's crust. There are a number of different types of earthquakes, and they vary in intensity from very mild quakes which are barely felt to severe earthquakes which cause large amounts of damage and can trigger tsunamis and volcanic eruptions. The actual sensation of moving earth is caused by seismic waves, which are measured along a scale called the Richter Scale. The Richter Scale classifies intensity in magnitudes of 10: an earthquake which measures four on the Richter Scale, for example, is 10 times more intense than a three. Aftershocks follow almost every earthquake, as the Earth settles and adjusts itself, and they can sometimes be very severe.

Aftershocks happen along the same fault line that the initial earthquake occurred on, and they can go on for months. The longer the delay, the more difficult it can be to distinguish between an aftershock and an entirely new earthquake. Seismologists look at the epicenter of the quake, the seismic history of the area, and the type of fault to determine whether or not an earthquake is an aftershock. Generally, the longer the time period after a major quake, the less likely aftershocks are, although a major seismic event in a normally calm area can stimulate large aftershocks for months.

Generally, aftershocks are less strong than the initial quake, but they can still do a lot of damage, especially to structures. Structures weakened by the original quake may collapse from the aftershocks, which put immense amounts of stress on compromised support beams. For this reason, people are encouraged to evacuate buildings after an earthquake, in case aftershocks cause the building to collapse. After a major earthquake, buildings, elevated roadways, and bridges should be inspected to determine whether or not they are safe, and people should be wary of falling trees and power lines.