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Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey points out that an event may be on the order of days, weeks, or months. They are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock. Earthquake swarms are one of the events typically preceding eruptions of volcanoes.
In the United States there was the so-called "Mogul earthquake sequence" that began in February 2008 near Reno, Nevada and continued for several months, ending in November 2008. Between February and April the swarm produced more than 1,000 quakes of small magnitude, although the largest measured 4.7.
Another example was that affecting a Spanish island in the eastern Atlantic during the 2011-2012 El Hierro eruption. From July 2011 until October 2011, hundreds of small earthquakes were measured. The accumulated energy released by the swarm increased dramatically on 28 September. The swarm was due to the movement of magma beneath the island, and on 9 October indications of a submarine volcanic eruption were detected.
Between February 2011 and December 2011 more than 500 earthquakes were recorded during the 2011 Christchurch earthquakes, with 31 of them greater than magnitude 5.
In 2013, the Santa Cruz Islands had been experiencing a large earthquake swarm with many magnitude 5 and 6 earthquake occurring since January and continuing into February. This later turned out to be a series of foreshocks that led up to the 8.0 2013 Solomon Islands earthquake on February 6.
See also 
- 1951 East Rift Valley earthquakes
- Blanco Fracture Zone
- Earthquake storm
- Gutenberg–Richter law
- Guy-Greenbrier earthquake swarm
- Remotely triggered earthquakes
- USGS. "Earthquake Swarms at Yellowstone". United States Geological Survey. Retrieved 2008-08-27.
- Ashley Powers; Thomas H. Maugh II. "Swarm of earthquakes shakes Reno area". Los Angeles Times. Retrieved 2008-08-27.
- Bernardo Marin; R. Mendez (October 11, 2011). "La erupción volcánica submarina de El Hierro libera magma y gases en el océano". El País. Retrieved May 7, 2012.
- Sandra Dibble. "Ground stays still, but residents in quake area rattled". The San Diego Union-Tribune. Retrieved 2008-08-27.