Earthquake swarm

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Earthquakes
Types Mainshock Foreshock Aftershock Blind thrust Doublet Interplate Intraplate Megathrust Remotely triggered Slow Submarine Supershear Tsunami Earthquake swarm
Causes Fault movement Volcanism Induced seismicity
Characteristics Epicenter Epicentral distance Hypocenter Shadow zone Seismic waves P wave S wave
Measurement Seismometer Seismic magnitude scales Seismic intensity scales
Prediction Coordinating Committee for Earthquake Prediction Forecasting
Other topics Shear wave splitting Adams–Williamson equation Flinn–Engdahl regions Earthquake engineering Seismite Seismology
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In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period. The time span used to define a swarm varies, but may be days, months, or years. Such an energy release is different from the situation when a major earthquake (main shock) is followed by a series of aftershocks: in earthquake swarms, no single earthquake in the sequence is obviously the main shock. In particular, a cluster of aftershocks occurring after a mainshock is not a swarm.^[2]

The

One of the best-documented swarms occurred near Matsushiro, a suburb of Nagano, to the north-west of Tokyo. The Matsushiro swarm lasted from 1965 to 1967 and generated about 1 million earthquakes. This swarm had the peculiarity of being sited just under a seismological observatory installed in 1947 in a decommissioned military tunnel. It began in August 1965 with three earthquakes too weak to be felt, but three months later, a hundred earthquakes could be felt daily. On 17 April 1966, the observatory counted 6,780 earthquakes, with 585 of them having a magnitude great enough to be felt, which means that an earthquake could be felt, on average, every two and a half minutes.^[4] The phenomenon was clearly identified as linked to a magma uplift, perhaps initiated by the 1964 Niigata earthquake, which occurred the previous year.^[5]

Earthquake swarms are common in

• Since 11 November 2018, an earthquake swarm has been observed in the region of Dahanu, Maharashtra, an otherwise aseismic area. Ten to twenty quakes are felt daily, with magnitudes usually smaller than 3.5 (maximum magnitude 4.1 in February 2019). Even with this low-level of magnitude, two shocks proved destructive and even lethal, probably because their foci were very shallow.^[9]
• Bamhori village in Seoni district is also experiencing regular earthquakes since February 2000.

• An earthquake swarm occurred from early April 2017 to mid August 2017 in the province of
  Phivolcs
  , the local seismic monitoring agency, a value which classifies this quake as an "intermediate-depth event"). This example shows how complex can be the interaction between a swarm and an independent earthquake, even though this last one is very likely to have been triggered by the swarm activity.
• October 15, 2020, an earthquake swarm occurred on the island of
  Guimbal, Iloilo. Intensity 4 was felt in Iloilo City^[12]
• A
  Malungon, Sarangani. Intensity VI was recorded in Koronadal City and Davao City.^[16] Just 2 days after on October 31, an earthquake struck Tulunan again (Magnitude 6.5M_ww) The death toll of two quakes was raised into 24 and 563 were injured.^[17] Eva's Hotel in Kidapawan City collapsed during the quake^[18]
• On October 14-18 2021 an earthquake swarm occurred on Camarines Sur ranging from magnitudes 1.7-4.3 with the depth of 1–40 km. Some of these events were felt on Camarines Norte and Albay. PHIVOLCS recorded at least 27 earthquakes (10 were felt) in Camarines Sur.^[19]

• A swarm of intense earthquakes in the
  A series of eruptions started on 18 December 2023 and was ongoing as of April 2025^[update].^[20]

• The western Bohemia/Vogtland region is the border area between the Czech Republic and Germany where earthquake swarms were first studied at the end of the 19th century. Swarm activity is recurrent there, sometimes with large maximum magnitudes, as for instance in 1908 (maximum magnitude 5.0), 1985–1986 (4.6), 2000 (3.2), or 2008 (3.8). This latter swarm occurred near Nový Kostel in October 2008 and lasted only 4 weeks, but up to 25,000 events were detected by WEBNET, the local monitoring network. The swarm is located on a steeply-dipping fault plane where an overall upward migration of activity was observed (first events at the bottom and last events at the top of the activated fault patch).^[21]

• In
  foci were located in the 4–11 km (2.5–6.8 mi) depth range, within the crystalline basement. Focal mechanisms involve normal faulting, but also strike-slip faulting.^[1][6]
• In the lower Rhône Valley, the Tricastin has been known from the 18th century as the seat of earthquake swarms which sometimes caused damage, as in 1772–1773 and 1933–1936, and which were characterized by barrage-like detonations—at least so reported by the inhabitants. No seismic activity had been documented in the region since 1936, when a very weak swarm appeared for a few months in 2002–2003 (maximal magnitude 1.7).^[22] Had their foci not been sited just under a hamlet in the vicinity of Clansayes, and very close to the surface (200 m (660 ft) deep), these shocks would have gone unnoticed. In such a scenario of "ultra-shallow" seismicity, even earthquakes of very low magnitude (1, or 0, or even negative magnitude) can be felt as explosions or water-hammer noises, more than as vibrations.^[23] Most foci were located in an Upper-Cretaceous reef-limestone slab which bursts out periodically in the course of centuries for still unknown reasons for a few months or a few years. A 200-m focal depth is believed to be a worldwide record value for tectonic events.
• In the French Alps, the Maurienne Valley is from time to time prone to earthquake swarms. During the 19th century, a protracted swarm lasted 5 years and a half, from December 1838 to June 1844.^[24] Some earthquakes of the sequence caused damage in the region close to Saint-Jean-de-Maurienne, but this long swarm with many felt events made things particularly difficult for the population. More recently, a swarm appeared in October 2015 near Montgellafrey, in the lower part of the valley.^[25] Its activity kept low until 17 October 2017, when more than 300 earthquakes occurred within two weeks, with a maximal magnitude of 3.7 being reached twice in late October 2017. The seismic activity lasted another full year, thus yielding a duration of more than 3 years for the full swarm.

Since January 26, 2025, intense seismic activity has been observed between Amorgos and Santorini, centered on Anydros. Specifically, over 7,700 earthquakes of which around 100 exceeded magnitude 4 on the Richter scale have been observed, with the largest earthquake being 5.2R on the evening of February 5.^[26]

• In April 2017, the Salvadoran municipality of Antiguo Cuscatlán, a suburb of San Salvador, experienced a sequence of close to 500 earthquakes within 2 days, with magnitudes in the 1.5–5.1 range. There was one casualty and minor damage due to the strongest quake. Local experts did not identify any anomalous activity at nearby volcanoes.^[27]

• Between February and November 2008, Nevada experienced a swarm of 1,000 low-magnitude quakes generally referred to as the 2008 Reno earthquakes.^[28] The peak activity was in April 2008, when 3 quakes with magnitudes larger than 4 occurred within 2 days. The largest one registered a moment magnitude of 4.9 and caused damage in the immediate area around the epicenter.
• The Yellowstone Caldera, a supervolcano in NW Wyoming, has experienced several strong earthquake swarms since the end of the 20th century. In 1985, more than 3,000 earthquakes were observed over a period of several months. More than 70 smaller swarms have been detected since. The United States Geological Survey states these swarms are likely caused by slips on pre-existing faults rather than by movements of magma or hydrothermal fluids. At the turn of the year 2008, more than 500 quakes were detected under the NW end of Yellowstone Lake over a seven-day span, with the largest registering a magnitude of 3.9. Another swarm started in January 2010, after the Haiti earthquake. With 1,620 small events in late January 2010, this swarm is the second-largest ever recorded in the Yellowstone Caldera. Interestingly, most of these swarms have "rapid-fire" characteristics: they seemingly appear out of nowhere and can churn out tens or hundreds of small to moderate quakes within a very short time frame. Such swarms usually occur within the caldera boundary, as was especially the case in 2018.^[29]

• The
  Epicentres show a linear distribution, with a clear overall shift in activity towards the southwest with time, and a magnitude of 4.7 was computed for the largest event. Analysis of the swarm has suggested a link with deep waste disposal drilling. It has led to a moratorium on such drilling.^[31]
• On 2 September 2017, an earthquake swarm appeared around Soda Springs, Idaho. Five quakes with magnitudes between 4.6 and 5.3 occurred within 9 days. Keeping the 2009 L'Aquila case in mind, and because Idaho had experienced an M6.9 earthquake in 1983, experts warned residents that a stronger quake could follow (an unlikely but still possible scenario for them).^[32]
• From early 2016 to late 2019, a swarm of earthquakes occurred near Cahuilla in Riverside County, California. More than 22,000 individual seismic events were recorded, ranging in magnitude from 0.7 to 4.4. The strongest one occurred in August 2018, south of Lake Riverside, just off Cahuilla Road (SR 371). By using computer algorithms and machine learning, researchers were able to infer the following detailed picture of the Cahuilla fault zone responsible for the earthquake swarm. The fault zone is no more than 50 m (160 ft) wide, 4 km (2.5 mi) long, with the earliest seismic swarm events localized down near its base at 9 km (5.6 mi) below the surface and the latest events migrating upwards to 5 km (3.1 mi) below the surface and spreading throughout the fault zone's length. Containing complex subterranean horizontal channels and prominent bents in its depth profile, the fault zone sits on top of a deeper natural underground reservoir of fluid under pressure with a connector at 8 km (5.0 mi) below the surface that was initially sealed off from the fault zone. When that seal broke open in early 2016, fluids were injected up into the fault zone's base and diffused slowly through the complex channels up to 5 km (3.1 mi) below the surface, which triggered the prolonged earthquake swarm that lasted until late 2019. This analysis provides detailed evidence that fault zone valving is a mechanism for seismogenesis in swarms.^[33][34]

• In El Hierro, the smallest and farthest south and west of the Canary Islands, hundreds of small earthquakes were recorded from July 2011 until October 2011 during the 2011–12 El Hierro eruption. 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 a submarine volcanic eruption was detected.^[35]

• An earthquake swarm began east of
  surface waves which could be observed worldwide by seismological observatories. Its origin is thought to be east of Mayotte.^[40]
  The swarm continued to be active all through 2019.

• In January and February 2013, the Santa Cruz Islands experienced a large earthquake swarm with many magnitude 5 and 6 earthquakes: more than 40 quakes with magnitude 4.5 or larger took place during the previous 7 days, including 7 events with magnitude larger than 6. The swarm degenerated into the M8.0 2013 Solomon Islands earthquake (6 February 2013).^[41]

• 1951 East Rift Valley earthquakes
• 2009–25 Oklahoma earthquake swarms
• Blanco fracture zone
• Gutenberg–Richter law
• Guy–Greenbrier earthquake swarm
• Remotely triggered earthquakes