The Landers fault (red lines at left) and the Hector Mine fault and its aftershocks (red lines and circles at right). The Hector Mine earthquake was a magnitude 7.1, and it produced thousands of aftershocks. Image credit: Egill Hauksson, Caltech.
An aftershock is an earthquake that follows the largest shock of an earthquake sequence. Aftershocks are smaller than the main shock and within 1–2 fault lengths distance from the main shock fault. Aftershocks can continue over a period of weeks, months, or years. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they will continue.