What is the probability that an earthquake is a foreshock to a larger earthquake?
Worldwide the probability that an earthquake will be followed within 3 days by a large earthquake nearby is somewhere just over 6%. In California, that probability is about 6%. This means that there is about a 94% chance that any earthquake will NOT be a foreshock. In California, about half of the biggest earthquakes were preceded by foreshocks; the other half were not. At this time, we cannot tell whether or not an earthquake is a foreshock until something larger happens after it... so only in retrospect.
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United States Geological Survey (USGS) Natural Hazards Response
The primary goal of U.S. Geological Survey (USGS) Natural Hazards Response is to ensure that the disaster response community has access to timely, accurate, and relevant geospatial products, imagery, and services during and after an emergency event. To accomplish this goal, products and services provided by the National Geospatial Program (NGP)...
Lamb, Rynn M.; Jones, Brenda K.
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Filson, John R.; McCarthy, Jill; Ellsworth, William L.; Zoback, Mary Lou; Stauffer, Peter H.; Hendley, James W.
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Frankel, A.D.; Mueller, C.S.; Barnhard, T.P.; Leyendecker, E.V.; Wesson, R.L.; Harmsen, S.C.; Klein, F.W.; Perkins, D.M.; Dickman, N.C.; Hanson, S.L.; Hopper, M.G. Foreshocks and aftershocks of the Great 1857 California earthquake
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Meltzner, A.J.; Wald, D.J. Occurrence patterns of foreshocks to large earthquakes in the western United States
[No abstract available]
Abercrombie, R.E.; Mori, J.
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New Audiences, New Products for the National Seismic Hazard Maps
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USGS map displaying intensity of potential ground shaking from natural and human-induced earthquakes. There is a small chance (one percent) that ground shaking intensity will occur at this level or higher. There is a greater chance (99 percent) that ground shaking will be lower than what is displayed in these maps.
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Earthquake-damaged building
Building damanged by the February 2011 earthquake in Christchurch, New Zealand.
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Damage from the 1964 Alaskan Earthquake. Credit: USGS
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2014 National Seismic Hazard Map
2014 U.S. seismic hazard map showing the earthquake peak ground acceleration (PGA) that has a 2% chance of being exceeded in 50 years. Red indicates the highest hazard, and gray indicates the lowest hazard.