Aftershocks are lesser earthquakes that occur after a major one, 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, like all earthquakes, they cannot be predicted, people who experience an earthquake should be prepared for smaller ones to follow and take steps to prepare themselves, including getting family members and pets out 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 that are barely felt to severe ones that 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 on the Richter Scale. The Richter Scale classifies intensity in magnitudes of 10: an earthquake that measures four, for example, is 10 times more intense than a three. Aftershocks follow almost every earthquake, as the ground 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 a new earthquake is connected to a past major one. Generally, the longer the time period after a major quake, the less likely these smaller quakes 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, which put immense amounts of stress on compromised support beams. For this reason, people are encouraged to evacuate buildings after an earthquake, in case later quakes cause the building to fall. 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.