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]

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

Examples of earthquakes with foreshock events
Date (Foreshock) Magnitude (Foreshock) Flag and Country Region Date Depth Magnitude Intensity Name Deceased Tsunami
April 4, 1904 (23 minutes) 6.3 MW Bulgaria Blagoevgrad region April 4, 1904 15 km 7.0 MW X-XI Mercalli 1904 Krupnik earthquake >60 ✖
May 21, 1960 (1 day) 7.9 MW Chile Araucanía Region May 22, 1960 35 km 9.5 MW XII Mercalli 1960 Valdivia earthquake 1,655 ✔
November 2, 2002 (2 years) 7.3 MW Indonesia Sumatra December 26, 2004 30 km 9.1 MW IX Mercalli 2004 Indian Ocean earthquake and tsunami 230,000 ✔
October 20, 2006 (299 days) 6.4 MW[9] Peru Ica Region August 15, 2007 35 km 8.0 MW VIII Mercalli 2007 Peru earthquake 596 ✔
January 23, 2007 (3 months) 5.2 ML[10] Chile Aysén Region April 21, 2007 6 km 6.2 MW VII Mercalli 2007 Aysén Fjord earthquake 10 ✔
March 9, 2011 (2 days) 7.3 MW[11] Japan Miyagi Prefecture March 11, 2011 30 km 9.0 MW IX Mercalli and 7 Shindo 2011 Tōhoku earthquake and tsunami 15,891 ✔
March 16, 2014 (15 days) 6.7 MW[12] Chile Tarapacá Region April 1, 2014 20.1 km 8.2 MW VIII Mercalli 2014 Iquique earthquake 7 ✔
April 14, 2016 (2 days) 6.2 MW Japan Kumamoto Prefecture April 16, 2016 11 km 7.0 MW IX Mercalli 2016 Kumamoto earthquakes 41 ✖
April 22, 2017 (2 days) 4.8 MW Chile Valparaíso Region April 24, 2017 24.8 km 6.9 MW VII Mercalli 2017 Valparaiso earthquake 0 ✔
July 4, 2019 (1 day) 6.4 MW United States California July 5, 2019 10.7 km[13] 7.1 MW IX Mercalli 2019 Ridgecrest earthquakes 0 ✖
December 28, 2020 (1 day) 5.2 MW Croatia Central Croatia December 29, 2020 10 km 6.4 MW IX Mercalli 2020 Petrinja earthquake 7 ✖
  • 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. Retrieved 29 November 2010.
  7. Ludwin, R. (16 September 2004). "Earthquake Prediction". The Pacific Northwest Seismic Network. Retrieved 29 November 2010.
  8. 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.
  9. "El Sismo del 20 de Octubre de 2006" (PDF) (in Spanish). IGP.
  10. "Informe de sismo sensible" (in Spanish). GUC.
  11. "Magnitude 7.3 - NEAR THE EAST COAST OF HONSHU, JAPAN". USGS. Archived from the original on 2011-03-12.
  12. "Informe de sismo sensible" (in Spanish). GUC.
  13. "M 4.0 - 11km SW of Searles Valley, CA". United States Geological Survey. July 4, 2019. Retrieved July 9, 2019.
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