Foreshocks, Mainshocks, and Aftershocks

Part of living with earthquakes is living with aftershocks. Earthquakes usually come in clusters called an earthquake sequence and although foreshocks may or may not occur, aftershocks are a certainty. In any earthquake sequence, the largest event is called a main-shock; anything before that is called a foreshock and anything after is an aftershock. Foreshocks and aftershocks can be large enough to be damaging -- particularly aftershocks that can additionally damage structures that are weakened by the mainshock. Following a large earthquake we must be seriously mindful of aftershocks to avoid preventable injuries and damage.

Foreshocks

A mainshock becomes a foreshock if a later event is larger. This happens about 6 percent of the time in Nevada. The chance of it happening dies off quickly with time just like aftershocks. The most likely time for a mainshock is within the first hour (a quarter of all mainshocks happen within an hour of their foreshock) and after three days this hazard is reduced back to normal levels.

Scientists have tried to find ways to tell if an earthquake is a foreshock before the main shock would appear . . . in other words, to use them to predict earthquakes. A valid earthquake prediction must specify the location, magnitude, and the beginning and end of the time window for an earthquake. Depending on the length of the time window, predictions can be "long" or "short term." Long-term predictions (for example, in the next 30 years) can be based on studies of the behavior of individual faults and the interactions of nearby faults. Even that is very complicated and results have significant uncertainty.

Earthquakes, which could be short-term precursors, are so far indistinguishable from earthquake sequences with smaller magnitude main shocks. Sometimes, however, advisories are issued to remind people that there is the chance that an ongoing earthquake sequence might turn out to be a foreshock sequence. Such reminders are justified because there are foreshocks before many large earthquakes in Nevada, although only a small fraction of earthquakes turn out to be foreshocks.

Aftershocks

Aftershocks usually occur near their mainshock. The stress on and around the mainshock's fault changes during the mainshock and produces most of the aftershocks. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well.

An earthquake large enough to cause damage will produce several felt aftershocks within the first hour. The rate of aftershocks dies off quickly -- the decrease is proportional to the inverse of time since the mainshock. This simply means that the second day after the mainshock has about one-half and the tenth day about one-tenth the number of aftershocks of the first day. Aftershocks can continue for weeks to decades.

Larger earthquakes have more and larger aftershocks. The difference in magnitude between the mainshock and the largest aftershock can be 0.1 to 3 or more, but averages about 1.2. There are many more small aftershocks than large ones. Aftershocks of all magnitudes die off at the same rate -- magnitude 5 after-shocks are one-tenth as common by day 10 as day one, as are magnitude 2 aftershocks. Large aftershocks can occur months or even years after the mainshock.