What is the Difference Between an Earthquake Swarm and Aftershocks

Both an earthquake swarm and aftershocks are a series of multiple earthquakes happening in a concentrated region.  So what is the difference?

What is an Earthquake Swarm?

An earthquake swarm is a series of small earthquake not following a mainshock.  A mainshock is the largest earthquake in a sequence of earthquakes that may have foreshocks and/or aftershocks.  A swarm is usually a sequence of earthquakes in comparable size that can occur over a number of days or even months.  Many swarms are associated with geothermal activity. Aftershocks are part of the “readjustment process” after the main slip on the fault (USGS). As with swarms, aftershocks can continues for days and even up to years after a main earthquake event although the magnitude lessens over time.


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Map showing a satellite image of the Aurora-Bodie Volcanic Field with December 28, 2016 Nevada earthquake swarm locations. Map: USGS, public domain

Map showing a satellite image of the Aurora-Bodie Volcanic Field with December 28, 2016 Nevada earthquake swarm locations. Map: USGS, public domain

What are Aftershocks?

Aftershocks are a series of earthquakes that occur after a mainshock has happened.  Aftershocks occur near the fault where the main earthquake occurred. This means there is one large earthquake and a series of smaller earthquakes than can occur before (foreshocks) and/or afterwards (aftershocks).

Map showing aftershocks from a 5.3 magnitude earthquake occurring on September 2, 2017, at 05:57 pm MDT beneath the Aspen Range in southeastern Idaho. Source: University of Utah

Map showing aftershocks from a 5.3 magnitude earthquake occurring on September 2, 2017, at 05:57 pm MDT beneath the Aspen Range in southeastern Idaho. Source: University of Utah

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