Foreshocks and Aftershocks

It is quite obvious that earthquakes are not always random events. In other words, the occurrence of one earthquake often is related to another earthquake. There are patterns of earthquake occurrence that, if understood, can tell us something about the nature of earthquakes and the rocks wherein they occur.

Following almost any sizable earthquake, there is a train of many lesser earthquakes. Simply for the reasons that they occur after the big shock and appear to be related to it, these earthquakes are called aftershocks. Usually, the bigger the main earthquake, the more numerous and bigger are the aftershocks. Following a magnitude 7-plus, tsunami-generating earthquake in the Aleutian Islands in 1965, there were more than 750 substantial aftershocks within the first 24 hours. Sometimes these aftershock trains continue on for months. As time goes by, the frequency and the size of the aftershocks tend to decrease.

The characteristics of aftershocks indicate that they are continuation of the strain release processes that the main shock participated in, rather than being caused by elastic rebound. Hence, if the main earthquake is caused by a big sudden breakage of the rocks in a particular direction, the aftershocks are caused by lesser breakages in the same direction.

Some major earthquakes are preceded by earthquakes obviously related to the occurrence of the main shock, the so-called foreshocks. Laboratory experiments show that as the elastic limits of rocks are exceeded by a stress, the rocks tend to develop minor cracks before undergoing catastrophic failure. This incipient cracking may be the cause of foreshocks.

So far, seismologists are mostly unable to identify a particular earthquake as being a foreshock, a mainshock or an aftershock until the sequence is over and hindsight is applied. But some regions do display repeating sequences; there, it may be possible to use foreshock activity as a prediction tool.