Foreshock

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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.[1]

Occurrence

Foreshock activity has been detected for about 40% of all moderate to large earthquakes,

2004 Indian Ocean earthquake with a delay of more than two years between the two events.[4]

Some great earthquakes (M>8.0) show no foreshock activity at all, such as the M8.6 1950 India–China earthquake.[3]

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.[5]

Mechanics

The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to nucleation.[2] 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.[6] In practice, there are two main conflicting theories about foreshocks: earthquake triggering process (described in SOC models and ETAS-like models) and the loading process by aseismic slip (nucleation models). This debate about the prognostic value of foreshocks is well known as Foreshock Hypothesis.[7]

Earthquake prediction

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.[8] Earthquakes along oceanic transform faults do show repeatable foreshock behaviour, allowing the prediction of both the location and timing of such earthquakes.[9]

Examples of earthquakes with foreshock events

Foreshock Date
(Delay) 		Magnitude
(Foreshock) 		Location Date Depth Magnitude
(Mainshock) 		Intensity
(
MMI
) 		Name Type Comments
April 4, 1904 (23 minutes) 6.3 Mw
Blagoevgrad region, Bulgaria
 April 4, 1904 15 km 7.0 Mw  X-XI 1904 Kresna earthquakes
Normal
 [10]
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 9.2 Mw  IX 2004 Indian Ocean earthquake and tsunami Thrust
October 20, 2006 (10 months) 6.4 Mw 
Ica Region, Peru
 August 15, 2007 35 km 8.0 Mw  VIII 2007 Peru earthquake Thrust [11]
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
 [12]
March 9, 2011 (2 days)
 7.3 Mw  Miyagi Prefecture, Japan March 11, 2011 30 km 9.0 Mw  IX 2011 Tōhoku earthquake and tsunami Thrust [13]
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 [14]
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 [15]
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
  • Note: dates are in local time

References

  1. ISBN 978-0-8160-6302-4
    . Retrieved 29 November 2010.
  2. ^
    ISBN 978-0-309-06562-7
    . Retrieved 29 November 2010.
  3. ^
    ISBN 978-1-4020-8179-8
    . Retrieved 29 November 2010.
  4. doi:10.1785/0120050616. Archived from the original
    (PDF) on 23 July 2011. Retrieved 29 November 2010.
  5. ISBN 978-3-7643-6209-6
    . Retrieved 29 November 2010.
  6. doi:10.1785/0120030069. Archived from the original
    (PDF) on 3 July 2011. Retrieved 29 November 2010.
  7. PMID 24526224
    .
  8. ^ Ludwin, R. (16 September 2004). "Earthquake Prediction". The Pacific Northwest Seismic Network. Archived from the original on 16 June 2010. Retrieved 29 November 2010.
  9. S2CID 4337369
    . Retrieved 29 November 2010.
  10. doi:10.1046/j.0956-540x.2001.01589.x
    .
  11. ^ "El Sismo del 20 de Octubre de 2006" (PDF) (in Spanish). IGP.
  12. ^ "Informe de sismo sensible" (in Spanish). GUC.
  13. ^ "Magnitude 7.3 - NEAR THE EAST COAST OF HONSHU, JAPAN". USGS. Archived from the original on 2011-03-12.
  14. ^ "Informe de sismo sensible" (in Spanish). GUC.
  15. ^ "M 4.0 - 11km SW of Searles Valley, CA". United States Geological Survey. July 4, 2019. Retrieved July 9, 2019.
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