Editing Foreshock

{{short description|Earthquake that occurs before a mainshock but is of lower magnitude}}
{{Earthquakes}}

A '''foreshock''' is an [[earthquake]] that occurs before a larger seismic event{{snd}}the [[mainshock]]{{snd}}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.<ref name="Gates">{{cite book|last1=Gates|first1=A.|last2=Ritchie|first2=D.|title=Encyclopedia of Earthquakes and Volcanoes|url=https://books.google.com/books?id=b1sXfJCiCHQC&dq=foreshock+earthquake&pg=PA89|year=2006|publisher=Infobase Publishing|isbn=978-0-8160-6302-4|page=89|access-date=29 November 2010}}</ref>

==Occurrence==
Foreshock activity has been detected for about 40% of all moderate to large earthquakes,<ref name="NRS">{{cite book|last=National Research Council (U.S.). Committee on the Science of Earthquakes|title=Living on an Active Earth: Perspectives on Earthquake Science|chapter-url=https://archive.org/details/livingonactiveea0000unse/page/418|access-date=29 November 2010|year=2003|publisher=National Academies Press|location=Washington D.C.|isbn=978-0-309-06562-7|page=[https://archive.org/details/livingonactiveea0000unse/page/418 418]|chapter=5. Earthquake Physics and Fault-System Science}}</ref> and about 70% for events of M>7.0.<ref name="Kayal"/> 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.<ref name="Vallée">{{cite journal|last=Vallée|first=M.|year=2007|title=Rupture Properties of the Giant Sumatra Earthquake Imaged by Empirical Green's Function Analysis|journal=Bulletin of the Seismological Society of America|volume=97|issue=1A|pages=S103–S114|doi=10.1785/0120050616|url=http://mahabghodss.net/NewBooks/www/web/digital/nashrieh/bssa/2007/january%201%202007-%2097%20issue%201a/S103.pdf|access-date=29 November 2010|bibcode=2007BuSSA..97S.103V|url-status=dead|archive-url=https://web.archive.org/web/20110723201958/http://mahabghodss.net/NewBooks/www/web/digital/nashrieh/bssa/2007/january%201%202007-%2097%20issue%201a/S103.pdf|archive-date=23 July 2011}}</ref>

Some great earthquakes (M>8.0) show no foreshock activity at all, such as the M8.6 [[1950 Assam–Tibet earthquake|1950 India–China earthquake]].<ref name="Kayal">{{cite book|last=Kayal|first=J.R.|title=Microearthquake seismology and seismotectonics of South Asia|url=https://books.google.com/books?id=POLg0eGi3EAC&dq=foreshock+earthquake&pg=PA15|access-date=29 November 2010|year=2008|publisher=Springer|isbn=978-1-4020-8179-8|page=15}}</ref>

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 [[Power law|inverse power law]] type. This may either indicate that foreshocks [[Coulomb stress transfer|cause stress changes]] resulting in the mainshock or that the increase is related to a general increase in stress in the region.<ref name="Maeda">{{cite book|last=Maeda|first=K.|editor=Wyss M., Shimazaki K. & Ito A.|title=Seismicity patterns, their statistical significance and physical meaning|chapter-url=https://books.google.com/books?id=QIy6le4sCMAC&dq=foreshock&pg=PA381|access-date=29 November 2010|series=Reprint from Pageoph Topical Volumes|year=1999|publisher=Birkhäuser|isbn=978-3-7643-6209-6|pages=381–394|chapter=Time distribution of immediate foreshocks obtained by a stacking method}}</ref>

==Mechanics==
The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to [[Earthquake#Rupture dynamics|nucleation]].<ref name="NRS"/> 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.<ref name="Felzer">{{cite journal|last=Felzer|first=K.R.|author2=Abercrombie R.E.|author3=Ekström G.|year=2004|title=A Common Origin for Aftershocks, Foreshocks, and Multiplets|journal=Bulletin of the Seismological Society of America|volume=94|issue=1|pages=88–98|doi=10.1785/0120030069|url=http://128.197.153.21/rea/web_online/SSA03069_felzer_feb04.PDF|access-date=29 November 2010|bibcode=2004BuSSA..94...88F|archive-date=3 July 2011|archive-url=https://web.archive.org/web/20110703033726/http://128.197.153.21/rea/web_online/SSA03069_felzer_feb04.PDF|url-status=dead}}</ref> 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.<ref name= "Mignan"> {{cite journal|title=The debate on the prognostic value of earthquake foreshocks: A meta-analysis|last=Mignan|first=A.|date=14 February 2014|volume=4 |page=4099 |journal=Scientific Reports|doi=10.1038/srep04099 |pmid=24526224 |pmc=3924212 |bibcode= }}</ref>

==Earthquake prediction==
An increase in seismic activity in an area has been used as a method of [[Earthquake prediction|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.<ref name="Ludwin">{{cite web|url=http://www.ess.washington.edu/SEIS/PNSN/INFO_GENERAL/eq_prediction.html|title=Earthquake Prediction|last=Ludwin|first=R.|date=16 September 2004|publisher=The Pacific Northwest Seismic Network|access-date=29 November 2010|archive-date=16 June 2010|archive-url=https://web.archive.org/web/20100616210148/http://www.ess.washington.edu/SEIS/PNSN/INFO_GENERAL/eq_prediction.html|url-status=dead}}</ref> Earthquakes along oceanic [[transform fault]]s do show repeatable foreshock behaviour, allowing the prediction of both the location and timing of such earthquakes.<ref name="McGuire">{{cite journal|last=McGuire|first=J.J. |author2=Boettcher M.S. |author3=Jordan T.H.|year=2005|title=Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults|journal=Nature|volume=434|pages=457–461|doi=10.1038/nature03377|url=http://www.nature.com/nature/journal/v434/n7032/abs/nature03377.html|access-date=29 November 2010|pmid=15791246|issue=7032|bibcode = 2005Natur.434..457M |s2cid=4337369 |url-access=subscription}}</ref>

== Examples of earthquakes with foreshock events ==
* The strongest recorded mainshock that followed a foreshock is the [[1960 Valdivia earthquake]], which had a magnitude of 9.5 [[Moment magnitude scale|M<sub>W</sub>]].
{| class="wikitable sortable col1izq col2izq col3izq col4der col5der"
! Foreshock Date<br>(Delay)
! Magnitude<br>(Foreshock)
! Location
! Date
! Depth
! Magnitude<br>(Mainshock)
! Intensity<br>([[Modified Mercalli Intensity Scale|MMI]])
! Name
! Type
! Comments
|-
| April 4, 1904 (23 minutes)
| [[1904 Kresna earthquakes|6.3]] {{M|w|link=yes}}
| [[Blagoevgrad region]], [[Bulgaria]]
| April 4, 1904
| 15&nbsp;km
| 7.0 {{M|w}}
| X-XI
| [[1904 Kresna earthquakes]]
| [[Normal fault|Normal]]
| <ref name="Meyer_etal_2002">{{Cite journal |last1=Meyer |first1=B. |last2=Armijo |first2=R. |last3=Dimitroy |first3=D. |date=2002 |title=Active faulting in SW Bulgaria: possible surface rupture of the 1904 Struma earthquakes |journal=Geophysical Journal International |volume=148 |issue=2 |pages=246–255 |doi=10.1046/j.0956-540x.2001.01589.x|bibcode=2002GeoJI.148..246M |doi-access=free }}</ref>
|-
| May 21, 1960 (1 day)
| [[1960 Concepción earthquakes|7.9]] {{M|w}}
| [[Arauco Province (historical)|Arauco Province]], [[Chile]]
| May 22, 1960
| 35&nbsp;km
| 9.5 {{M|w}}
| XII
| [[1960 Valdivia earthquake]]
| [[Thrust fault|Thrust]]
| 
|-
| November 2, 2002 (2 years)
| [[2002 Sumatra earthquake|7.3]] {{M|w}}
| [[Sumatra]], [[Indonesia]]
| December 26, 2004
| 30&nbsp;km
| 9.2 {{M|w}}
| IX
| [[2004 Indian Ocean earthquake and tsunami]]
| Thrust
| 
|-
| October 20, 2006 (10 months)
| 6.4 {{M|w}}
| [[Ica Region]], [[Peru]]
| August 15, 2007
| 35&nbsp;km
| 8.0 {{M|w}}
| VIII
| [[2007 Peru earthquake]]
| Thrust
| <ref>{{cite web|url=http://www.igp.gob.pe/portal/images/documents/sismos/ise-2006/pisco_2006.pdf|title=El Sismo del 20 de Octubre de 2006|language= es|publisher=IGP}}</ref>
|-
| January 23, 2007 (3 months)
| 5.2 {{M|l|link=yes}}
| [[Aysén Region]], Chile
| April 21, 2007
| 6&nbsp;km
| 6.2 {{M|w}}
| VII
| [[2007 Aysén Fjord earthquake]]
| [[Strike-slip fault|Strike-slip]]
| <ref>{{cite web|url=http://www.sismologia.cl/events/sensibles/2007/01/23-2042-12L.S200701.html|title=Informe de sismo sensible|language=es|publisher=GUC}}</ref>
|-
| [[March 2011 Sanriku earthquake|March 9, 2011 (2 days)]]
| 7.3 {{M|w}}
| [[Miyagi Prefecture]], [[Japan]]
| March 11, 2011
| 30&nbsp;km
| 9.0 {{M|w}}
| IX
| [[2011 Tōhoku earthquake and tsunami]]
| Thrust
| <ref>{{cite web|url=https://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001r57/|title=Magnitude 7.3 - NEAR THE EAST COAST OF HONSHU, JAPAN|publisher=USGS|url-status=dead|archive-url=https://web.archive.org/web/20110312171744/http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001r57/|archive-date=2011-03-12}}</ref>
|-

|-
| March 16, 2014 (15 days)
| 6.7 {{M|w}}
| [[Tarapacá Region]], Chile
| April 1, 2014
| 20.1&nbsp;km
| 8.2 {{M|w}}
| VIII
| [[2014 Iquique earthquake]]
| Thrust
| <ref>{{cite web|url=http://www.sismologia.cl/events/sensibles/2014/03/16-2116-30L.S201403.html|title=Informe de sismo sensible|language= es|publisher=GUC}}</ref>
|-
| April 14, 2016 (2 days)
| 6.2 {{M|w}}
| [[Kumamoto Prefecture]], Japan
| April 16, 2016
| 11&nbsp;km
| 7.0 {{M|w}}
| IX
| [[2016 Kumamoto earthquakes]]
| Strike-slip
| 
|-
| April 22, 2017 (2 days)
| 4.8 {{M|w}}
| [[Valparaíso Region]], Chile
| April 24, 2017
| 24.8&nbsp;km
| 6.9 {{M|w}}
| VII
| [[2017 Valparaiso earthquake]]
| Thrust
| 
|-
|July 4, 2019 (1 day)
|6.4 {{M|w}}
|[[California]], [[United States]]
|July 5, 2019
|10.7&nbsp;km
|7.1 {{M|w}}
|IX
|[[2019 Ridgecrest earthquakes]]
| Strike-slip
|<ref>{{cite web|publisher=United States Geological Survey|date=July 4, 2019|access-date=July 9, 2019|title=M 4.0 - 11km SW of Searles Valley, CA|url=https://earthquake.usgs.gov/earthquakes/eventpage/ci38443095/executive}}</ref>
|-
| December 28, 2020 (1 day)
| 5.2 {{M|w}}
| [[Central Croatia]]
| December 29, 2020
| 10&nbsp;km
| 6.4 {{M|w}}
| IX
| [[2020 Petrinja earthquake]]
| Strike-slip
| 
|-
|March 5, 2021 (2 hours)
|7.4 {{M|w}}
|[[Kermadec Islands]], [[New Zealand]]
|March 5, 2021
|55.6 km
|8.1 {{M|w}}
|VIII
|[[2021 Kermadec Islands earthquake]]
| Thrust
|
|}
* <small>'''Note''': dates are in local time</small>

==References==
{{Reflist}}

[[Category:Seismology]]
[[Category:Types of earthquake]]