A foreshock is an earthquake that occurs before a larger seismic event – the mainshock – and is related to it in both time and space.
The designation of an earthquake as foreshock, mainshock or aftershock is only possible after the full sequence of events has happened.
[1] Foreshock activity has been detected for about 40% of all moderate to large earthquakes,[2] and about 70% for events of M>7.0.
[3] The increase in foreshock activity is difficult to quantify for individual earthquakes but becomes apparent when combining the results of many different events.
[5] The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to nucleation.