An aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to known laws. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.
Most large earthquakes are followed by additional earthquakes, called aftershocks, which make up an aftershock sequence. While most aftershocks are smaller than the mainshock, they can still be damaging or deadly. A small fraction of earthquakes are followed by a larger earthquake, in which case the first earthquake is referred to as a foreshock. For example, the 2011 M9.1 Japan earthquake and tsunami was preceded by a M7.3 foreshock two days before. When the M7.3 earthquake first occurred, it was called the mainshock, and then when the M9.1 earthquake occurred, that larger earthquake became the mainshock.