What determines that an earthquake is really an aftershock?
Aftershocks are earthquakes, and seismologists use the term, along with foreshocks and mainshocks, to describe the relationship between the intensity of various earthquakes. The mainshock of an earthquake sequence is generally the event with the largest magnitude. Aftershocks do not have a precise definition but are less intense than a mainshock and occur in the same area. Any subsequent earthquake in the same area, but with less magnitude, is considered an aftershock, until activity in that area returns to normal. Aftershocks are generally caused by the “readjustment” of Earth’s crust in response to a larger earthquake. Aftershocks occur even after very small earthquakes.