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.
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.
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.
The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to nucleation. In one model of earthquake rupture, the process forms as a cascade, starting with a very small event that triggers a larger one, continuing until the main shock rupture is triggered. However, analysis of some foreshocks has shown that they tend to relieve stress around the fault. In this view, foreshocks and aftershocks are part of the same process. This is supported by an observed relationship between the rate of foreshocks and the rate of aftershocks for an event.
An increase in seismic activity in an area has been used as a method of predicting earthquakes, most notably in the case of the 1975 Haicheng earthquake in China, where an evacuation was triggered by an increase in activity. However, most earthquakes lack obvious foreshock patterns and this method has not proven useful, as most small earthquakes are not foreshocks, leading to probable false alarms. Earthquakes along oceanic transform faults do show repeatable foreshock behaviour, allowing the prediction of both the location and timing of such earthquakes.
|April 4, 1904 (23 minutes)||6.3 Mw||Blagoevgrad region, Bulgaria||April 4, 1904||15 km||7.0 Mw||X-XI||1904 Krupnik earthquake||Normal|||
|May 21, 1960 (1 day)||7.9 Mw||Arauco Province, Chile||May 22, 1960||35 km||9.5 Mw||XII||1960 Valdivia earthquake||Thrust|
|November 2, 2002 (2 years)||7.3 Mw||Sumatra, Indonesia||December 26, 2004||30 km||9.1 Mw||IX||2004 Indian Ocean earthquake and tsunami||Thrust|
|October 20, 2006 (299 days)||6.4 Mw||Ica Region, Peru||August 15, 2007||35 km||8.0 Mw||VIII||2007 Peru earthquake||Thrust|||
|January 23, 2007 (3 months)||5.2 ML||Aysén Region, Chile||April 21, 2007||6 km||6.2 Mw||VII||2007 Aysén Fjord earthquake||Strike-slip|||
|March 9, 2011 (2 days)||7.3 Mw||Miyagi Prefecture, Japan||March 11, 2011||30 km||9.0 Mw||IX||2011 T?hoku earthquake and tsunami||Thrust|||
|March 16, 2014 (15 days)||6.7 Mw||Tarapacá Region, Chile||April 1, 2014||20.1 km||8.2 Mw||VIII||2014 Iquique earthquake||Thrust|||
|April 14, 2016 (2 days)||6.2 Mw||Kumamoto Prefecture, Japan||April 16, 2016||11 km||7.0 Mw||IX||2016 Kumamoto earthquakes||Strike-slip|
|April 22, 2017 (2 days)||4.8 Mw||Valparaíso Region, Chile||April 24, 2017||24.8 km||6.9 Mw||VII||2017 Valparaiso earthquake||Thrust|
|July 4, 2019 (1 day)||6.4 Mw||California, United States||July 5, 2019||10.7 km||7.1 Mw||IX||2019 Ridgecrest earthquakes||Strike-slip|||
|December 28, 2020 (1 day)||5.2 Mw||Central Croatia||December 29, 2020||10 km||6.4 Mw||IX||2020 Petrinja earthquake||Strike-slip|
|March 5, 2021 (2 hours)||7.4 Mw||Kermadec Islands, New Zealand||March 5, 2021||55.6 km||8.1 Mw||VIII||2021 Kermadec Islands earthquake||Thrust|