Aftershocks? Swarm? What is the difference, and what do they mean?

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In Yellowstone, we often talk about earthquake swarms.  But especially in recent weeks, we’ve also discussed aftershock sequences.  What is the difference?  And what to these different types of seismic events mean?

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mike Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory; Jamie Farrell, assistant research professor with the University of Utah Seismograph Stations and Chief Seismologist of the Yellowstone Volcano Observatory; and Mike Stickney, Director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology.

Map of earthquakes greater than M1.5 that occurred between March 1 and May 12, 2020, in the Intermountain West.

Map of earthquakes greater than M1.5 that occurred between March 1 and May 12, 2020, in the Intermountain West. The main shock-aftershock sequences in central Idaho and near Salt Lake City are labeled. Orange dots are earthquakes that occurred within 24 hours, and yellow 1 week, of the map being generated (May 12, 2020)

(Credit: Jamie Farrell. Public domain.)

In recent weeks, YVO has received a number of questions about the sequences of earthquakes happening in Utah, and Idaho.  As we described in a previous edition of Caldera Chronicles, these smaller events are aftershocks of larger earthquakes—a M5.7 event on March 18 near Salt Lake City, and a M6.5 on March 31 in central Idaho—that were caused by tectonic stretching of the western USA.

But aren’t these clusters of earthquakes “swarms”?  And don’t “swarms” of earthquakes mean magma moving?

Not quite.  Aftershocks are smaller earthquakes that follow main shocks, and they are caused by adjustments of the fault that broke during the main shock.  They are the seismic equivalent of the fault “creaking” as it settles into a new relaxed state after the main earthquake. When an earthquake occurs, the state of stress around the earthquake dramatically changes.  The Earth wants to get back to some type of equilibrium, and that is the process that produces aftershocks.  If an event larger than the first strong quake occurs as part of the sequence, that then becomes the main shock and all prior events become foreshocks.

graphs showing the difference between earthquake types

Schematic diagram illustrating the difference between a main shock with aftershocks (top), a sequences of foreshocks, main shock, and aftershocks (middle), and a seismic swarm (bottom).  Adapted from manuscript by Farrell and others, 2009.

(Credit: Jamie Farrell; Stephan Husen; Robert B. Smith. Public domain.)

Aftershock sequences have been so well studied that the magnitudes and rates of earthquakes in an aftershock sequence can be forecast!  The largest aftershock event is usually about one magnitude level lower than the main shock.  So, for the central Idaho M6.5 main event, there could be at least one aftershock with a magnitude in the mid-5 range.  There would also be about 10 earthquakes in mid mid-four range, and 100 earthquakes in the mid-three range, and so forth.

In addition to the magnitudes of the aftershocks being smaller, the rate of aftershock occurrence decreases with time as well.  Most aftershocks happen within a few hours to days of the main shock, but the larger the main shock, the longer the aftershocks will last.  For the largest earthquakes, aftershock sequences can last for years to decades.  For the central Idaho M6.5, the aftershock sequence might last for about a year, while for the M5.7 near Salt Lake City, the aftershocks will continue for months.  These can be very unnerving to local residents, given that many of these aftershocks are large enough to be felt.

Swarms of earthquakes are a little bit different.  In a swarm, there is no main shock—no big earthquake that starts off a sequence.  Instead, the earthquakes occur at rates and with magnitudes that don’t obey any of the “rules” that aftershock sequences follow.  Swarms can last for hours, days, or months!  One of the largest and longest earthquakes swarms in Yellowstone was the 2017 Maple Creek sequence.  It lasted for about three months and included ~2400 located earthquakes, although many more occurred but were too small to be located.  Swarms can also slow down dramatically, stay at low levels, then intensify at a later date.  For example, the Maple Creek swarm seemingly quit in late 2017, but there was a renewed burst of activity in early 2018.

In Yellowstone, swarms are very common and account for about 40-50% of all the seismicity in the region.

Swarms are often associated with fluid movement.  Many Yellowstone swarms might be associated with water, especially lubricating the many faults in the region.  At volcanoes that are active and getting ready to erupt, swarms can be a sign of magma moving in the subsurface.  The 1980 eruption of Mount St. Helens, for example, was preceded by weeks of earthquake swarm activity.

Not all swarms are associated with fluids and volcanoes, however—tectonic forces can also trigger swarms!  In fact, such a swarm has been happening over the past two years in the Manhattan, MT, area.  Earthquakes there started in late September 2018 and continued into at least March 2020.  The swarm included two M4 events (M4.2 and M4.1) and numerous M3 earthquakes, many of which were felt.  A similar swarm of tectonic events occurred in NW Nevada during 2014-2018 and had 30 events in the M4 range (the largest was M4.7).  That was the most energetic and persistent earthquake sequence in Nevada’s recorded history!

These tectonic swarms are not necessarily caused by fluid migration, but rather extension of the western USA—the same force that caused the recent main shocks in Utah and Idaho.  Why the fault motion occurred as a swarm of relatively small events, versus a mainshock with aftershocks, is not known with certainty, but might be related to the properties of the fault.  Some faults might not slip suddenly, but rather slide in fits and starts due to lower friction or other properties.

Now you know the difference between an aftershock sequence and a swarm!  You also know that not all swarms are alike, with some caused by fluid movement and others caused by tectonic forces.  We hope this helps with understanding seismicity in the Yellowstone region, and indeed across the western USA.

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