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Foreshock
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.
==Occurrence==
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.
Some great earthquakes (M>8.0) show no foreshock activity at all, such as the M8.6 1950 India - China earthquake.
The increase in foreshock activity is difficult to quantify for individual earthquakes but becomes apparent when combining the results of many different events. From such combined observations, the increase before the mainshock is observed to be of inverse power law type. This may either indicate that foreshocks cause stress changes resulting in the mainshock or that the increase is related to a general increase in stress in the region.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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