what is an aftershock

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. Most aftershocks are located over the full area of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with the main shock. Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the main shock. Bigger earthquakes have more and larger aftershocks, and the sequences can last for years or even longer, especially when a large event occurs in a seismically quiet area. Aftershocks are not to be confused with swarms, which are sequences of small earthquakes associated with geothermal activity with no identifiable mainshock.