A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake
One of a group of small to medium earthquakes which precede a large earthquake Perhaps 1/4 of large eqs have foreshocks which can be identified afterward At the time it can be difficult to know whether a cluster of earthquakes is predictive of a large eq The Chinese prediction of a disastrous earthquake at Haicheng is one case where foreshocks were successfully identified and used to predict an earthquake
A minor, barely detectable earthquake, generally preceding a full-scale earthquake with approximately the same focus Major quakes may follow a cluster of foreshocks by as little as a few seconds or as much as several weeks
Any earthquake which is followed, within a short time span, by a larger earthquake in the exact same location can be labelled a "foreshock" (In the case of an earthquake swarm, this terminology is not generally applied )
A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates at or near the focus of the larger earthquake