スマトラ島沖地震
Foreshock activity has been detected for about 40% of all moderate to large earthquakes, and about 70% for events of M&gt;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.