Foreshock activity has been detected for about 40 % of all moderate to large earthquakes , and about 70 % for events of M > 7.0 . They occur from a matter of minutes to days or even longer before the main shock ; for example , the 2002 Sumatra earthquake is regarded as a foreshock of the 2004 Indian Ocean earthquake with a delay of more than two years between the two events
2002 Sumatra earthquake
2002 Sumatra earthquake
Foreshock