What Are Aftershocks, Foreshocks and Earthquake Clusters?

The calculations in this system are based on known behaviors of aftershocks. Scientists have shown that the rules governing aftershock behavior also apply to “aftershocks” that are larger than their main shock - i.e., the possibility that the first event was a foreshock. These rules include:

Aftershock Facts: In a cluster, the earthquake with the largest magnitude is called the main shock; anything before it is a foreshock and anything after it is an aftershock. A main shock will be redefined as a foreshock if a subsequent event has a larger magnitude. The rate of main shocks after foreshocks follows the same patterns as aftershocks after main shocks. Aftershock sequences follow predictable patterns as a group, although the individual earthquakes are random and unpredictable. This pattern tells us that aftershocks decay with increasing time, increasing distance, and increasing magnitude. It is this average pattern that this system uses to make real-time predictions about the probability of ground shaking.

Distance: Aftershocks usually occur geographically near the main shock. The stress on the main shock's fault changes drastically during the main shock and that fault produces most of the aftershocks. Sometimes the change in stress caused by the main shock is great enough to trigger aftershocks on other, nearby faults, and for a very large main shock sometimes even farther away. As a rule of thumb, we call earthquakes aftershocks if they are at a distance from the main shock's fault no greater than the length of that fault. The automatic system keeps track of where aftershocks have occurred, and when enough aftershocks have been recorded to pinpoint the more and less active locations, the system adjusts the probabilities on the map to reflect those local variations.

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Time: An earthquake large enough to cause damage will probably be followed by several felt aftershocks within the first hour. The rate of aftershocks decreases quickly - the decrease is proportional to the inverse of time since the main shock. This means the second day has about 1/2 the number of aftershocks of the first day and the tenth has about 1/10 the number of the first day. These patterns describe only the overall behavior of aftershocks; the actual times, numbers and locations of the aftershocks are random. We call an earthquake an aftershock as long as the rate at which earthquakes occur in that region is greater than the rate before the main shock. How long this lasts depends on the size of the main shock (bigger earthquakes have more aftershocks) and how active the region was before the main shock (if the region was seismically quiet before the main shock, the aftershocks continue above the previous rate for a longer time). Thus, an aftershock can occur weeks or decades after a main shock.

Magnitude: Bigger earthquakes have more and larger aftershocks. The bigger the main shock the bigger the largest aftershock will be, on average. The difference in magnitude between the main shock and largest aftershock ranges from 0.1 to 3 or more, but averages 1.2 (a M5.5 aftershock to a M6.7 main shock for example). There are more small aftershocks than large ones. Aftershocks of all magnitudes decrease at the same rate, but because the large aftershocks are already less frequent, the decay can be noticed more quickly. Large aftershocks can occur months or even years after the main shock.