Editing Foreshock (section)

==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>