Foreshock

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

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

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.{{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}}

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.{{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}} 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. {{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= }}

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.{{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}} Earthquakes along oceanic transform faults do show repeatable foreshock behaviour, allowing the prediction of both the location and timing of such earthquakes.{{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}}

• The strongest recorded mainshock that followed a foreshock is the 1960 Valdivia earthquake, which had a magnitude of 9.5 M_W.

• Note: dates are in local time

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Category:Seismology

Category:Types of earthquake