What is an aftershock?

There is often confusion surrounding the terminology of earthquakes. The largest earthquake in an earthquake sequence is called a mainshocks. Mainshocks are followed by 100s or even 1000s of smaller earthquakes, called aftershocks. Aftershocks are smaller earthquakes on patches of the rupture zone (areas of the fault that broke during the earthquake) and adjacent faults that occur as rocks along and nearby the rupture adjust to the new state of stress. Although there is some variation in what scientists “count” as an aftershock, a common definition is that an aftershock is any earthquake that occurs within one fault length of the fault where the mainshock occurred AND before the level of seismicity in the area returns to what is was before the mainshock happened.

Occasionally, about 5% of the time, an aftershock will be larger than the mainshock. When this happens we change to the terminology to reflect the sizes of the earthquakes and their timing. We rename the first earthquake and call it a “foreshock” and we rename the bigger aftershock and call it the “mainshock”, because it’s now the largest earthquake in the sequence.

Image of Omori and the logarithmic graphic showing the graph of aftershocks per day (y-axis) versus days after the mainshock (x-axis). Picture from IRIS

A scientist named Omori found that the decay of the aftershock rate was best analyzed using a logarithmic graph of aftershocks per day versus days after the mainshock. He showed that the rate of aftershocks decreased w/ the reciprocal of time since the mainshock. This is called Omori’s Law. Put another way, the most aftershocks will occur right after the mainshock, and the rate of aftershocks will decrease with time.

Although only a small portion of the energy of an earthquake sequence is released by aftershocks, these quakes can still cause damage, or make the damage from previous quakes worse. The *highest rate* of aftershocks occur during the critical first hours and days immediately following the major earthquake. This is exactly the time when emergency workers are locating and rescuing injured people from collapsed or damaged buildings. Because those buildings can shift or even collapse during aftershocks, rescuers are at great risk when their expertise and service is most important.

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