Foreshocks, aftershocks - what's the difference?
"Foreshock" and "aftershock" are relative terms.
Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.
Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes. (modified from Univ. of Washington)
Related Content
What is the difference between aftershocks and swarms?
Do earthquakes occur in Antarctica?
Where can I find earthquake educational materials?
Can we cause earthquakes? Is there any way to prevent earthquakes?
What is surface faulting or surface rupture in an earthquake?
At what depth do earthquakes occur? What is the significance of the depth?
Why are there so many earthquakes in the Geysers area in Northern California?
What is an earthquake and what causes them to happen?
Can the position of the moon or the planets affect seismicity? Are there more earthquakes in the morning/in the evening/at a certain time of the month?
Earthquake information products and tools from the Advanced National Seismic System (ANSS)
This Fact Sheet describes post-earthquake products and tools provided by the Advanced National Seismic System (ANSS) through the U.S. Geological Survey Earthquake Hazards Program. The focus is on products that provide situational awareness immediately after significant earthquakes.
Wald, Lisa A.
On the potential duration of the aftershock sequence of the 2018 Anchorage earthquake
Currently, an aftershock sequence is ongoing in Alaska after the magnitude 7.0 Anchorage earthquake of November 30, 2018. Using two scenarios, determined with observations as of December 14, 2018, this report estimates that it will take between 2.5 years and 3 decades before the rate of aftershocks decays to the rate of earthquakes that were...
Michael, Andrew J.
Natural Hazards Science at the U.S. Geological Survey
The mission of the USGS in natural hazards is to develop and apply hazard science to help protect the safety, security, and economic well-being of the Nation. The costs and consequences of natural hazards can be enormous, and each year more people and infrastructure are at risk. The USGS conducts hazard research and works closely with stakeholders...
Perry, Suzanne C.; Jones, Lucile M.; Holmes, Robert R.
Fundamental questions of earthquake statistics, source behavior, and the estimation of earthquake probabilities from possible foreshocks
Estimates of the probability that an ML 4.8 earthquake, which occurred near the southern end of the San Andreas fault on 24 March 2009, would be followed by an M 7 mainshock over the following three days vary from 0.0009 using a Gutenberg–Richter model of aftershock statistics (Reasenberg and Jones, 1989) to 0.04 using a statistical model of...
Michael, Andrew J. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide
Large earthquakes trigger very small earthquakes globally during passage of the seismic waves and during the following several hours to days1, 2, 3, 4, 5, 6, 7, 8, 9, 10, but so far remote aftershocks of moment magnitude M≥5.5 have not been identified11, with the lone exception of an M=6.9 quake remotely triggered by the surface waves from an M...
Pollitz, Fred F.; Stein, Ross S.; Sevilgen, Volkan; Burgmann, Roland
Earthquake hazards: a national threat
Earthquakes are one of the most costly natural hazards faced by the Nation, posing a significant risk to 75 million Americans in 39 States. The risks that earthquakes pose to society, including death, injury, and economic loss, can be greatly reduced by (1) better planning, construction, and mitigation practices before earthquakes happen, and (2)...
The USGS Earthquake Hazards Program - investing in a safer future
Filson, John R.; McCarthy, Jill; Ellsworth, William L.; Zoback, Mary Lou; Stauffer, Peter H.; Hendley, James W.
Foreshocks and aftershocks of the great 1857 California earthquake
Meltzner, A.J.; Wald, D.J.
The severity of an earthquake
The severity of an earthquake can be expressed in terms of both intensity and magnitude. However, the two terms are quite different, and they are often confused. Intensity is based on the observed effects of ground shaking on people, buildings, and natural features. It varies from place to place within the disturbed region depending on the...
This dynamic earth: the story of plate tectonics
In the early 1960s, the emergence of the theory of plate tectonics started a revolution in the earth sciences. Since then, scientists have verified and refined this theory, and now have a much better understanding of how our planet has been shaped by plate-tectonic processes. We now know that, directly or indirectly, plate tectonics...
Kious, W. Jacquelyne; Tilling, Robert I.
Magnitude 7.8 Earthquake in Nepal Aftershocks
A magnitude 7.8 earthquake struck Nepal on April 25, 2015 at 06:11:26 UTC. Visit the USGS event page to learn more about this earthquake.
Virginia Earthquake Aftershocks Identify Previously Unknown Fault Zone
RESTON, Va.-- Aftershocks from the 2011 Virginia earthquake have helped scientists identify the previously unknown fault zone on which the earthquake occurred.
Rare Great Earthquake in April Triggers Large Aftershocks All Over the Globe
MENLO PARK, Calif. — Large earthquakes can alter seismicity patterns across the globe in very different ways, according to two new studies by U.S. Geological Survey seismologists. Both studies shed light on more than a decade of debate on the origin and prevalence of remotely triggered earthquakes.
USGS Issues Assessment of Aftershock Hazards in Haiti
RESTON, Va.—The aftershock sequence of the magnitude-7 earthquake that struck near Port-au-Prince, Haiti, on Jan. 12, 2010, will continue for months, if not years. The frequency of events will diminish with time, but damaging earthquakes will remain a threat.
Minor Aftershock hits Seattle
A minor aftershock struck the Seattle area early Thursday morning, March 1, 2001. The aftershock, which struck at 1:10 a.m. local time, had a preliminary magnitude of 3.4 and was felt throughout the Seattle-Tacoma-Olympia area.
ENTER AT YOUR OWN RISK...But Check the Aftershocks Probabilities
When is a badly damaged, but stable building safe to enter after an earthquake? That is a question that safety-response and building-department officials have to answer in order to let occupants retrieve important possessions and business records, and to let contractors begin emergency repairs
MEXICO EARTHQUAKE AFTERSHOCK
A magnitude 5.5 aftershock to Monday’s (Oct. 9) earthquake in Mexico occurred on Thurs., Oct. 12, 1995, at 12:52 p.m. EDT (10:52 a.m. local time in Mexico), according to the U.S. Geological Survey.
aftershocks M4.0 and larger within 48 hours after 1/23/18 quake
60 aftershocks greater than or equal to M4.0 occurred in the first 48 hours after the mainshock. The blue dot is the location of the main Jan 23 earthquake. Yellow and orange dots are aftershock epicenters.
Map of Christ Church, NZ aftershocks
Map of sesmic activity near Christchurch, New Zealand, from 9/4/2010 to 4/11/2014
Aftershock Hazards in Haiti
The aftershock sequence of the magnitude 7 earthquake that struck Haiti on Jan. 12, 2010, will continue for months, if not years. The frequency of events will diminish with time, but damaging earthquakes will remain a threat.
Michael Blanpied, USGS Associate Earthquakes Hazards Program coordinator, discusses concerns and precautions for the future in Haiti and the
Damaged Buildings
Civilians on street between severely damaged buildings in Mianyang.