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.[1]

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Occurrence

Foreshock activity has been detected for about 40% of all moderate to large earthquakes,[2] and about 70% for events of M>7.0.[3] 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.[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. Gates, A.; Ritchie, D. (2006). Encyclopedia of Earthquakes and Volcanoes. Infobase Publishing. p. 89. ISBN 978-0-8160-6302-4. Retrieved 29 November 2010.
  2. National Research Council (U.S.). Committee on the Science of Earthquakes (2003). "5. Earthquake Physics and Fault-System Science". Living on an Active Earth: Perspectives on Earthquake Science. Washington D.C.: National Academies Press. p. 418. ISBN 978-0-309-06562-7. Retrieved 29 November 2010.
  3. Kayal, J.R. (2008). Microearthquake seismology and seismotectonics of South Asia. Springer. p. 15. ISBN 978-1-4020-8179-8. Retrieved 29 November 2010.
  4. Vallée, M. (2007). "Rupture Properties of the Giant Sumatra Earthquake Imaged by Empirical Green's Function Analysis" (PDF). Bulletin of the Seismological Society of America. 97 (1A): S103–S114. Bibcode:2007BuSSA..97S.103V. doi:10.1785/0120050616. Archived from the original (PDF) on 23 July 2011. Retrieved 29 November 2010.
  5. Maeda, K. (1999). "Time distribution of immediate foreshocks obtained by a stacking method". In Wyss M., Shimazaki K. & Ito A. (ed.). Seismicity patterns, their statistical significance and physical meaning. Reprint from Pageoph Topical Volumes. Birkhäuser. pp. 381–394. ISBN 978-3-7643-6209-6. Retrieved 29 November 2010.
  6. Felzer, K.R.; Abercrombie R.E.; Ekström G. (2004). "A Common Origin for Aftershocks, Foreshocks, and Multiplets" (PDF). Bulletin of the Seismological Society of America. 94 (1): 88–98. Bibcode:2004BuSSA..94...88F. doi:10.1785/0120030069. Archived from the original (PDF) on 3 July 2011. Retrieved 29 November 2010.
  7. Mignan, A. (14 February 2014). "The debate on the prognostic value of earthquake foreshocks: A meta-analysis". Scientific Reports. 4: 4099. doi:10.1038/srep04099. PMC 3924212. 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. McGuire, J.J.; Boettcher M.S.; Jordan T.H. (2005). "Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults". Nature. 434 (7032): 457–461. Bibcode:2005Natur.434..457M. doi:10.1038/nature03377. PMID 15791246. S2CID 4337369. Retrieved 29 November 2010.
  10. Meyer, B.; Armijo, R.; Dimitroy, D. (2002). "Active faulting in SW Bulgaria: possible surface rupture of the 1904 Struma earthquakes". Geophysical Journal International. 148 (2): 246–255. 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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