The television camera pans over a devastated cityscape, pausing on piles of rubble that were once houses, apartments, and office buildings. Dazed and dusty survivors pick through the remnants of their lives seeking keepsakes, valuables, or simply food and water. People avoid even those buildings that seem intact; and at night the survivors sleep in the open, fearing the aftershocks that can cause the damaged homes to collapse around them. What are these aftershocks, and why are they to be feared?
An aftershock is a smaller earthquake that happens after a large earthquake at the same place. The movements happen when the crust of the earth adjusts after the main shock.
The terms “foreshock,” “main shock,” and “aftershock” are given to earthquakes depending on their order and strength. The main shock of an earthquake is the biggest movement. Small movements before the main shock are called foreshocks, and those after it are called aftershocks. A group of related earthquakes including foreshocks, main shock, and aftershocks, is called an earthquake cluster.
Major earthquake aftershocks can continue up to several years after the main shock, though both their size and frequency decrease in time. When the number and size of earthquake movements near a large earthquake can no longer be distinguished from the everyday tiny shifts in the earth’s crust, the period of aftershocks is considered over.
Aftershock strength and frequency follow simple rules. One is that the number of aftershocks decreases with time after the main shock. For instance, the number of aftershocks on day three Is about 1/3 the number of aftershocks on day one; and the number on the sixth day is about 1/6th. Rule two is that the main shock is about 1.1 to 1.2 times as powerful as its largest aftershock. This causes ground movement of about 1/11th to 1/12th of the movement of the main shock. In the case of the 2010 Haiti Earthquake, the main shock had a magnitude of approximately 7.1 and the first major aftershock six days later had a magnitude of approximately 6.1 – in the ballpark.
Both aftershock strength and the length of the aftershock period are related to the size of the main earthquake. Big quakes have powerful aftershocks and have aftershocks longer than smaller earthquakes. Some scientists believe that recent minor earthquakes in the U. S. Midwest are aftershocks of a major earthquake near New Madrid, Missouri, almost two centuries ago.
The greatest danger of earthquake aftershocks is that they’re unpredictable. After a disastrous earthquake, strong aftershocks add to destruction caused by the main shock. Buildings that were weakened by the main quake can collapse, and damaged infrastructure takes another pounding. Rescue and relief operations are slowed by additional damage and the increased terror of the populace. Unfortunately, though seismologists can estimate the probability that aftershocks will happen, they can’t predict either their size or timing.
In summary, aftershocks are less powerful earthquakes that happen after a large earthquake. The frequency and power of aftershocks decrease through time, and their biggest dangers are that they can’t be predicted and that they worsen the damage caused by the main shock.
Sources: British Geological Survey, United States Geological Survey, Science Daily, University of Washington
