What is the difference between aftershocks and swarms?
Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks occur near the fault zone where the mainshock rupture occurred and are part of the "readjustment process” after the main slip on the fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock.
A swarm, on the other hand, is a sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months. They often recur at the same locations. Most swarms are associated with geothermal activity.
Learn more: Aftershocks? Swarm? What is the difference, and what do they mean?
Related Content
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?
Foreshocks, aftershocks - what's the difference?
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.
A closer look at the 2017 Maple Creek earthquake swarm
In June of 2017, an earthquake swarm began beneath the western edge of Yellowstone National Park, just east of Hebgen Lake. This swarm proved to be one of the more persistent swarms observed in Yellowstone, with the main episode lasting more than 3 months and producing thousands of recorded earthquakes.
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.
A Gassy Link to Past Earthquake Swarm
Over the past three years we've witnessed two large earthquake swarms at Yellowstone (2010 and 2009). Recent history tells us that these earthquake swarms are common, but we would like to know how frequently they have occurred in the more distant past. USGS researcher Bill Evans may have found an answer.
2004 Yellowstone Earthquake Swarm
In April 2004 there was an increase in earthquake activity at Yellowstone National Park that drew interest from scientists and the public.
Map of earthquake swarms in the San Ramon valley since 1970
Map of earthquake swarms in the San Ramon valley since 1970. The current 2018 sequence is shown by the red dots. The dot sizes scale with earthquake magnitude.
Foreshocks, mainshocks, and aftershocks - oh, my!
The graph shows the daily number of aftershocks (as of November 12) for the October 15, 2006 M6.7 Kiholo Bay earthquake. Note the exponential decay in activity.
Nevada Earthquake Swarm Dec. 28, 2017
Satellite image of the Aurora-Bodie Volcanic Field with December 28, 2016 Nevada earthquake swarm locations.
Oaxaca, Mexico earthquake aftershock sequence
Aftershock sequence from the M7.8 Oaxaca, Mexico earthquake on February 16, 2018. The blue circle is the mainshock and the gray and white smaller circles are the aftershocks through April 3, 2018.