A foreshock is a smaller earthquake preceding a much larger earthquake. Many scientists hope to use foreshocks to predict upcoming earthquakes.

In particular, the East Pacific Rise transform faults show foreshock activity before the main seismic event. Reviews of data of past events and their foreshocks showed that they have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. (McGuire et al., 2005)

The 9.5-magnitude Great Chilean Earthquake had a 7.9-magnitude foreshock.

References

Seismology