Earthquake swarm

February 2008 swarm near Mexicali.

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 may be of the order of days, weeks, or months.^[1] 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.

Examples

• Between February and April 2008 a swarm of 1,000 small magnitude quakes in the United States, referred to as the 2008 Reno earthquakes began in February and ended in November.^[2]
• During the 2011–12 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.^[3] The swarm was due to the movement of magma beneath the island, and on 9 October indications of a submarine volcanic eruption were detected.^[4]
• Over 500 quakes and aftershocks occurred during a two-week period in February 2008 near Mexicali, along the Cerro Prieto Fault.^[5]
• In 2013, the Santa Cruz Islands experienced a large earthquake swarm with many magnitude 5 and 6 earthquakes occurring in January and February - foreshocks to the 8.0 2013 Solomon Islands earthquake on February 6.
• In 2014, an area near the California/Oregon/Nevada borders experienced more than 800 small earthquakes over a period of around three months.^[6] More than 550 quakes were on magnitude 2.0 or larger.^[7]
• In 2017, the Philippine province of Batangas experienced an earthquake swarm with magnitudes between 5 and 6. The quake was felt in varying intensities in surrounding areas and as far as Manila’s financial district of Makati. The movement was felt in varying intensities in about 40 towns in Batangas, Laguna, Cavite and Quezon and in metropolitan Manila. Nearly 800 small aftershocks were reported but they were too weak to trigger a tsunami.^[8]
• In April 2017, the Salvadoran municipality of Antiguo Cuscatlán experienced a series of earthquakes, totalling 450, as of 16:00:00 local time (UTC -6), Wednesday 12 April 2017. Only 47 of these earthquakes were noticed by the population, whereas the rest were registered by monitoring equipment. The earthquake with the highest magnitude occurred at 17:55 local time (UTC -6) Tuesday 11 April 2017, measuring 5.1 in the Richter scale. The Department of the Environment and Natural Resources declared an alert situation in the entire country, which is to be lifted only after 24 hours without any earthquakes. The aftermath of this earthquake swarm is at least one death and minor material damages to buildings and houses.^[9]

See also

• 1951 East Rift Valley earthquakes
• 2009–17 Oklahoma earthquake swarms
• Blanco Fracture Zone
• Gutenberg–Richter law
• Guy-Greenbrier earthquake swarm
• Remotely triggered earthquakes
• Seismology

References