Earthquake Swarms
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In
seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
, 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 An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
(main shock) is followed by a series of
aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...
s: 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.


History and generalities

The
Ore Mountains The Ore Mountains (, or ; ) lie along the Czech–German border, separating the historical regions of Bohemia in the Czech Republic and Saxony in Germany. The highest peaks are the Klínovec in the Czech Republic (German: ''Keilberg'') at ab ...
(Erzgebirge), which form the border between the
Czech Republic The Czech Republic, also known as Czechia, and historically known as Bohemia, is a landlocked country in Central Europe. The country is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the south ...
and
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
, western
Bohemia Bohemia ( ; ; ) is the westernmost and largest historical region of the Czech Republic. In a narrow, geographic sense, it roughly encompasses the territories of present-day Czechia that fall within the Elbe River's drainage basin, but historic ...
and the
Vogtland Vogtland (; ) is a region spanning the German states of Bavaria, Saxony and Thuringia and north-western Bohemia in the Czech Republic. It overlaps with and is largely contained within Euroregio Egrensis. The name alludes to the former leadershi ...
region, have been known since the 16th century as being prone to frequent earthquake swarms, which typically last a few weeks to a few months. In 1899, Austrian geologist Josef Knett, while studying a swarm of about a hundred events felt in western Bohemia/Vogtland between January and February 1824, coined the noun ''Schwarmbeben'', i.e. "swarm
arth Arth is a village, a town, and a municipality in Schwyz District in the canton of Schwyz in Switzerland. The municipality consists of the villages Arth, Oberarth, and Goldau. The four settlements Rigi Kulm, Rigi First, Rigi Klösterli, and ...
uake". The term "swarm" comes from the fact that
hypocentre A hypocenter or hypocentre (), also called ground zero or surface zero, is the point on the Earth's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In seismology, the hypocenter of an earthquake is its p ...
s give the impression of agglutinating like a bee swarm when plotted onto a map, a cross-section or a 3D model. One of the best-documented swarms occurred near Matsushiro, a suburb of Nagano, to the north-west of
Tokyo Tokyo, officially the Tokyo Metropolis, is the capital of Japan, capital and List of cities in Japan, most populous city in Japan. With a population of over 14 million in the city proper in 2023, it is List of largest cities, one of the most ...
. 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. The phenomenon was clearly identified as linked to a
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
uplift, perhaps initiated by the
1964 Niigata earthquake The 1964 Niigata earthquake () struck at 13:01 local time (04:01 UTC) on 16 June with a magnitude of either 7.5 or 7.6. The epicenter was on the continental shelf off the northwest coast of Honshu, Japan, in Niigata Prefecture, about north of ...
, which occurred the previous year. Earthquake swarms are common in volcanic regions such as
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
,
Central Italy Central Italy ( or ) is one of the five official statistical regions of Italy used by the National Institute of Statistics (ISTAT), a first-level NUTS region with code ITI, and a European Parliament constituency. It has 11,704,312 inhabita ...
, the
Afar depression The Afar Triangle (also called the Afar Depression) is a geological depression caused by the Afar triple junction, which is part of the Great Rift Valley in East Africa. The region has disclosed fossil specimens of the very earliest hominins; ...
or
Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...
, where they occur before and during eruptions, but they are also observed in zones of
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
volcanism or of
hydrothermal circulation Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
, such as
Vogtland Vogtland (; ) is a region spanning the German states of Bavaria, Saxony and Thuringia and north-western Bohemia in the Czech Republic. It overlaps with and is largely contained within Euroregio Egrensis. The name alludes to the former leadershi ...
/western
Bohemia Bohemia ( ; ; ) is the westernmost and largest historical region of the Czech Republic. In a narrow, geographic sense, it roughly encompasses the territories of present-day Czechia that fall within the Elbe River's drainage basin, but historic ...
and the
Vosges The Vosges ( , ; ; Franconian and ) is a range of medium mountains in Eastern France, near its border with Germany. Together with the Palatine Forest to the north on the German side of the border, they form a single geomorphological unit and ...
massif, and less frequently far from
tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
boundaries in locations such as
Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...
,
Oklahoma Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...
or
Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
. In all cases, high-pressure fluid migration in the Earth's crust seems to be the trigger mechanism and the driving process that govern the evolution of the swarm in space and time. The
Hochstaufen The Hochstaufen is the easternmost mountain of the Chiemgau Alps, Germany. It is located in the north of Bad Reichenhall (Landkreis Berchtesgadener Land). The mountain belongs to the ''Staufen'' massif and is a popular destination for mountaineeri ...
earthquake swarm in
Bavaria Bavaria, officially the Free State of Bavaria, is a States of Germany, state in the southeast of Germany. With an area of , it is the list of German states by area, largest German state by land area, comprising approximately 1/5 of the total l ...
, with
foci Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film * ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel * ''Focus'' (2015 film), a 201 ...
, is one of the rare examples where an indisputable relationship between seismic activity and
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
could be established. Earthquake swarms raise public-safety issues: first, because the end of seismic activity cannot be predicted; second, because it is uncertain whether another earthquake with a
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
larger than those of previous shocks in the sequence will occur (the
2009 L'Aquila earthquake An earthquake occurred in the region of Abruzzo, in central Italy, at 03:32 Central European Time#Central European Summer Time, CEST (01:32 Coordinated Universal Time, UTC) on 6 April 2009. It was rated 5.8 or 5.9 on the Richter scale and 6.3 on ...
in Italy illustrates this, with an ''M''W 6.3 shock following a swarm activity with magnitudes between 1 and 3). Even though swarms usually generate moderate shocks, the persistence of felt earthquakes can be disruptive and cause distress to the population.


Examples

The following examples were chosen for peculiarities of certain swarms (for instance: large number of events, complex interaction with larger shocks, long period of time, ultra-shallow focal depth), or because of their geographical region, some swarms occurring in otherwise aseismic regions. It is not intended to be a list of all the swarms happening worldwide.


Asia


India

* Since 11 November 2018, an earthquake swarm has been observed in the region of
Dahanu Dahanu (Pronunciation: Help:IPA/Marathi, əɦaːɳuː is a coastal town and a municipal council in Palghar district of Maharashtra, Maharashtra state in Konkan division. It is located 110 km from Mumbai city and hosts Adani Power’s ...
,
Maharashtra Maharashtra () is a state in the western peninsular region of India occupying a substantial portion of the Deccan Plateau. It is bordered by the Arabian Sea to the west, the Indian states of Karnataka and Goa to the south, Telangana to th ...
, 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. * Bamhori village in Seoni district is also experiencing regular earthquakes since February 2000.


Philippines

* An earthquake swarm occurred from early April 2017 to mid August 2017 in the province of
Batangas Batangas, officially the Province of Batangas ( ), is a first class province of the Philippines located in the southwestern part of Luzon in the Calabarzon region. According to the 2020 census, it has a population of 2,908,494 people, making ...
. Four shocks in the 5.5–6.3 magnitude range ( 2017 Batangas earthquakes) caused damage in southern
Luzon Luzon ( , ) is the largest and most populous List of islands in the Philippines, island in the Philippines. Located in the northern portion of the List of islands of the Philippines, Philippine archipelago, it is the economic and political ce ...
; they occurred at the beginning of the swarm: ''M''s5.5 (4 April), ''M''s5.6 and ''M''s6.0 (8 April), and ''M''s6.3 (11 April). The origin of the 3 first major quakes seems established since they had practically the same epicentre; they occurred within the crust ( depth range). However, the strongest and latest quake does not seem related to the swarm: its epicentre is away, and its focal depth is moreover very different (, according to 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
Panay Panay is the sixth-largest and fourth-most populous island in the Philippines, with a total land area of and a total population of 4,542,926, as of 2020 census. Panay comprises 4.4 percent of the entire population of the country. The City of Il ...
ranging from magnitudes 2.5-4.5. Most of these quakes felt in Iloilo City. A previous swarm also hit Panay on November 5, 2018 (Including
Antique An antique () is an item perceived as having value because of its aesthetic or historical significance, and often defined as at least 100 years old (or some other limit), although the term is often used loosely to describe any object that i ...
,
Iloilo Iloilo ( ; ), officially the Province of Iloilo (; ; ; ), is a province in the Philippines located in the Western Visayas region. Its capital and largest city is Iloilo City, the regional center of Western Visayas and politically independen ...
and
Guimaras Guimaras (), officially the Province of Guimaras (; ), is an island province in the Philippines located in the Western Visayas region. The capital is Jordan, while the largest local government unit is the municipality of Buenavista. The provin ...
) ranging from magnitudes 4.0-4.8. The first earthquake (magnitude 4.7) at 7:45 A.M, occurred at
San Jose, Antique San Jose de Buenavista, officially the Municipality of San Jose de Buenavista (; ; ), is a municipality and capital of the province of Antique, Philippines. According to the 2020 census, it has a population of 65,140 people, making it the m ...
. Just a few minutes after, the second quake (magnitude 4.0) occurred at Sibunag, Guimaras. At 10:54 A.M, The third quake (magnitude 4.8) occurred at Guimbal, Iloilo. Intensity 4 was felt in
Iloilo City Iloilo City, officially the City of Iloilo (; ; ), is a Cities of the Philippines#Legal classification, highly urbanized city in the Western Visayas Regions of the Philippines, region of the Philippines, located on the southeastern coast of th ...
* A series of earthquakes that hit Mindanao in 2019 were classified as an earthquake swarm.
On October 16, a magnitude 6.3 ( Mwp) Struck Tulunan cotabato (Epicenter of earthquake). Intensity VII was felt in Tulunan,
M'lang M'lang, officially the Municipality of M'lang ( Hiligaynon: ''Banwa sang M'lang;'' Maguindanaon: ''Inged nu Melang'', Jawi: ايڠايد نو ملڠ), is a municipality in the province of Cotabato, Philippines. According to the 2020 census, i ...
, Makilala and
Kidapawan Kidapawan, officially the City of Kidapawan (; ; Maguindanao language, Maguindanaon: ''Kuta nu Kidapawan''; Obo language, Obo Monuvu: ''Ingod to Kidapawan''; ), is a Cities of the Philippines#Legal classification, component city and the capital ...
City,
Cotabato Cotabato, formerly and still commonly referred to as North Cotabato and officially the Province of Cotabato, is a landlocked Provinces of the Philippines, province in the Philippines located in the Soccsksargen Regions of the Philippines, regi ...
. Intensity VI was felt in
Digos Digos, officially the City of Digos (; ), is a component city and capital of the province of Davao del Sur, Philippines. According to the 2020 census, it has a population of 188,376 people. The city lies on the western shores of Davao Gul ...
City,
Davao del Sur Davao del Sur (; ), officially the Province of Davao del Sur (; ), is a Provinces of the Philippines, province in the Philippines located in the Davao Region in Mindanao. Its capital is Digos. Davao City is the largest city in terms of area an ...
;
Santo Niño, South Cotabato Santo Niño, officially the Municipality of Santo Niño (; ; ; , Jawi: ايڠد نو سنتو نيڽو), is a municipality in the province of South Cotabato, Philippines. According to the 2020 census, it has a population of 39,796 people, makin ...
and
Tacurong Tacurong, officially the City of Tacurong (; Maguindanaon: ''Kuta nu Takurung'', Jawi: كوت نو تكروڠ; ), is a component city in the province of Sultan Kudarat, Philippines. According to the 2020 census, it has a population of 109,3 ...
City,
Sultan Kudarat Sultan Kudarat, officially the Province of Sultan Kudarat (; Maguindanao language, Maguindanaon: ''Dairat nu Sultan Kudarat'', Jawi Alphabet, Jawi: دايرت نو سولتان كودرت; ; Ilocano language, Ilocano: ''Probinsia ti Sultan Ku ...
. On October 29, a magnitude 6.6 ( Mww) struck Tulunan again. At least ten deaths were reported and four hundred individuals were injured. Intensity VII was recorded in Tulunan and Makilala; Kidapawan City; Digos City, Bansalan and
Magsaysay, Davao del Sur Magsaysay, officially the Municipality of Magsaysay (; ), is a municipality in the province of Davao del Sur, Philippines. According to the 2020 census, it has a population of 56,263 people. As with much of the rural area of Davao del Sur, the p ...
and Malungon, Sarangani. Intensity VI was recorded in
Koronadal Koronadal, officially the City of Koronadal (; ; ; Maguindanao language, Maguindanaon: ''Kuta nu Kurunadal'', ) and also known as Marbel, is a Cities of the Philippines#Legal classification, component city and capital of the Provinces of the P ...
City and
Davao City Davao City, officially the City of Davao, is a City of the Philippines#Legal classification, highly urbanized city in the Davao Region, Philippines. The city has a total land area of , making it the List of Philippine cities and municipalities ...
. Just 2 days after on October 31, an earthquake struck Tulunan again (Magnitude 6.5 Mww) The death toll of two quakes was raised into 24 and 563 were injured. Eva's Hotel in Kidapawan City collapsed during the quake * On October 14-18 2021 an earthquake swarm occurred on
Camarines Sur Camarines Sur (; ), officially the Province of Camarines Sur (Central Bikol language, Bikol: ''Probinsya kan Habagatan na Camarines''; ), is a Provinces of the Philippines, province in the Philippines located in the Bicol Region on Luzon. Its ca ...
ranging from magnitudes 1.7-4.3 with the depth of 1–40 km. Some of these events were felt on
Camarines Norte Camarines Norte (; ), officially the Province of Camarines Norte (; ), is a Provinces of the Philippines, province in the Philippines located in the Bicol Region in Luzon. Its capital is Daet, Camarines Norte, Daet, the most populous town in the ...
and
Albay Albay (IPA: ), officially the Province of Albay (; ; Baybayin, ᜎᜎᜏᜒᜄᜈ᜔ ᜈᜅ᜔ ᜀᜎ᜔ᜊᜌ᜔), is a Provinces of the Philippines, province in the Bicol Region of the Philippines, mostly on the southeastern part of the isla ...
. PHIVOLCS recorded at least 27 earthquakes (10 were felt) in Camarines Sur.


Europe


Iceland

* A swarm of intense earthquakes in the
Reykjanes Peninsula Southern Peninsula (, ) is an administrative unit and part of Reykjanesskagi (pronounced ), or Reykjanes Peninsula, a region in southwest Iceland. It was named after Reykjanes, the southwestern tip of Reykjanesskagi. The region has a populatio ...
,
Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...
began on 24 October 2023, due to a magmatic intrusion underneath the area. The frequency and intensity of the earthquakes dramatically increased 10 November, with 20,000 tremors recorded by that time, the largest of which exceeded magnitude 5.2. An evacuation was ordered in the town of
Grindavík Grindavík () is a fishing town in the Southern Peninsula district of Iceland, not far from Þorbjörn, a tuya (a type of flat-topped, steep-sided volcano). It is one of the few towns with a harbour on this coast. Most of the inhabitants work ...
, which is located near the area of the seismic activity. Large-scale subsidence in and around the town is reported to have caused significant damage. An earthquake swarm began on the evening of 24 October due to the magmatic intrusion, with the intensity of the earthquakes decreasing by 30 October. Approximately 8,000 earthquakes were detected; most of these tremors occurred at a depth of . The
Icelandic Meteorological Office Icelandic Meteorological Office (IMO; ) is Iceland's national weather service and as such a government agency under the Ministry of Environment and Natural Resources. It is also active in volcano monitoring, esp. volcano seismology, and, toget ...
(IMO) reported that the swarm was focused around Svartsengi, north of Grindavík. About 700 earthquakes were recorded earlier in the month, the largest reaching magnitude 3.3.The largest of the earthquakes to date reached magnitude 5.1 on 10 November. By this time, over 22,000 earthquakes had been recorded since the beginning of the swarm in October. The IMO predicted that an eruption was likely, stating that "it will take several days (rather than hours) for magma to reach the surface." The greatest extent of the magma intrusion was inferred to be around the Sundhnúkur crater chain, approximately north of Grindavík. Instruments detected the presence of
sulphur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
in the atmosphere on 14 November, indicating that magma was now only a few hundred metres under the surface. Although the number of earthquakes decreased somewhat since 10 November, the IMO was still recording between 700 and 1,000 earthquakes daily by 14 November. Ground deformation sensors at Festarfjall fɛstarˌfjatl̥and Svartsengi recorded that the ground had moved apart by . Satellite measurements recorded the subsidence by about of a swathe of land measuring approximately five kilometers long and two kilometers wide (), running from the Sundhnúkur craters to the western side of Grindavík. The creation of this
graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
-like formation has enabled scientists to estimate the volume of the magmatic intrusion as approximately . It is estimated that the subsidence has been continuing at a rate of about four centimeters (1.6 inches) a day. A large crack opened up through the town, which old maps indicate is a reactivation of an existing fault. Scientists at the
University of Iceland The University of Iceland ( ) is a public research university in Reykjavík, Iceland, and the country's oldest and largest institution of higher education. Founded in 1911, it has grown steadily from a small civil servants' school to a modern co ...
believe that the fault was created by the last Sundhnúkur eruption over 2,000 years ago. Sensors emplaced in a borehole in Svartsengi detected the presence of
sulphur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
on 16 November, a classic signature of magma close to the surface. This has led the IMO to conclude that the area around the volcanic edifice of Hagafell, approximately north of Grindavík, is at the highest level of risk. A rapid uplift of the ground in the Svartsengi area was recorded from 18 to 21 November, likely indicating an upwelling of magma from a source five or more kilometers below the ground. A series of eruptions started on 18 December 2023 and was ongoing .


Czech Republic / Germany

* The western
Bohemia Bohemia ( ; ; ) is the westernmost and largest historical region of the Czech Republic. In a narrow, geographic sense, it roughly encompasses the territories of present-day Czechia that fall within the Elbe River's drainage basin, but historic ...
/
Vogtland Vogtland (; ) is a region spanning the German states of Bavaria, Saxony and Thuringia and north-western Bohemia in the Czech Republic. It overlaps with and is largely contained within Euroregio Egrensis. The name alludes to the former leadershi ...
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 Nový Kostel () is a municipality and village in Cheb District in the Karlovy Vary Region of the Czech Republic The Czech Republic, also known as Czechia, and historically known as Bohemia, is a landlocked country in Central Europe. The cou ...
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).


France

* In
Alpes-de-Haute-Provence Alpes-de-Haute-Provence (sometimes abbreviated as AHP; ; ; ), formerly until 1970 known as Basses-Alpes (, ), is a department in the Provence-Alpes-Côte d'Azur region of France, bordering Alpes-Maritimes and Italy to the east, Var to the sou ...
, the
Ubaye Valley The Ubaye Valley is an area in the Alpes de Haute-Provence ''département'', in the French Alps, having approximately 7,700 residents. Its residents are called ''Ubayens''. Its principal town is the ''sous-préfecture'' of Barcelonnette. 350px, A ...
is the most active seismic zone in the
French Alps The French Alps are the portions of the Alps mountain range that stand within France, located in the Auvergne-Rhône-Alpes and Provence-Alpes-Côte d'Azur regions. While some of the ranges of the French Alps are entirely in France, others, such a ...
. Earthquakes can follow there the classical scheme "mainshock + aftershocks" (for instance the 1959 ''M''5.5 earthquake, which caused heavy damage and two casualties). But seismic energy is principally released by swarms. This is particularly the case in the upper valley, between
Barcelonnette Barcelonnette (; , also ; obsolete ) is a Communes of France, commune of France and a Subprefectures in France, subprefecture in the Departments of France, department of Alpes-de-Haute-Provence, in the Provence-Alpes-Côte d'Azur region. It is l ...
and the French-Italian border. At the beginning of the 21st century, La Condamine-Châtelard experienced an exceptional swarm activity in an area where usually only a few low-magnitude events occur every year. A first swarm developed in 2003–2004 when more than 16,000 events were detected by the local monitoring network, but with magnitudes keeping to low values (2.7). On a map, the 2003–2004 swarm is long. After a period of almost complete inactivity, it was followed by a second swarm (2012–2014), slightly offset by a few kilometres, and with a length of . This second swarm was initiated by an ''M''4.3 earthquake in February 2012. Another ''M''4.8 earthquake in April 2014 reactivated the swarm in 2014–2015. These two major shocks, which caused damage in the nearby localities, were of course followed by their own short sequence of aftershocks, but such a 4-year activity for moderate magnitude shocks clearly characterizes a swarm. Most
foci Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film * ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel * ''Focus'' (2015 film), a 201 ...
were located in the depth range, within the crystalline basement.
Focal mechanism The focal mechanism of an earthquake describes the Fault (geology)#Slip.2C heave.2C throw, deformation in the Hypocenter, source region that generates the seismic waves. In the case of a Fault (geology), fault-related event, it refers to the ori ...
s involve normal faulting, but also
strike-slip faulting In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geology ...
. * In the lower
Rhône Valley The Rhône ( , ; Occitan: ''Ròse''; Arpitan: ''Rôno'') is a major river in France and Switzerland, rising in the Alps and flowing west and south through Lake Geneva and Southeastern France before discharging into the Mediterranean Sea ( Gu ...
, the
Tricastin The Tricastin () is a natural and historic region in the southern Rhône valley of southeastern France comprising the southwestern portion of the Drôme department and the northwestern portion of Vaucluse and centered on the modern town of Saint-P ...
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). Had their foci not been sited just under a hamlet in the vicinity of Clansayes, and very close to the surface ( 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. Most foci were located in an Upper-Cretaceous reef-
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
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 French Alps are the portions of the Alps mountain range that stand within France, located in the Auvergne-Rhône-Alpes and Provence-Alpes-Côte d'Azur regions. While some of the ranges of the French Alps are entirely in France, others, such a ...
, the
Maurienne Valley Maurienne (; ; ) is one of the provinces of Savoy, corresponding to the arrondissement of Saint-Jean-de-Maurienne in France. It is also the original name of the capital of the province, now Saint-Jean-de-Maurienne. Location The Maurienne valle ...
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. Some earthquakes of the sequence caused damage in the region close to
Saint-Jean-de-Maurienne Saint-Jean-de-Maurienne (; or ''Sant-Jian-de-Môrièna''; ) is a Subprefectures in France, subprefecture of the Savoie Departments of France, department, in the regions of France, region of Auvergne-Rhône-Alpes (formerly Rhône-Alpes), in south ...
, 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. 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.


Greece

Since January 26, 2025, intense seismic activity has been observed between
Amorgos Amorgos (, ; ) is the easternmost island of the Cyclades island group and the nearest island to the neighboring Dodecanese island group in Greece. Along with 16 neighbouring islets, the largest of which (by land area) is Nikouria Island, it compr ...
and
Santorini Santorini (, ), officially Thira (, ) or Thera, is a Greek island in the southern Aegean Sea, about southeast from the mainland. It is the largest island of a small, circular archipelago formed by the Santorini caldera. It is the southern ...
, centered on
Anydros Anydros () is an uninhabited Greek islet in the municipality of Santorini, which is a group of islands in the Cyclades. It is north of the island Anafi, and southwest of Amorgos. It is sometimes called . The island hosts a seismometer, part of ...
. Specifically, over 7,700 earthquakes of which around 100 exceeded magnitude 4 on the
Richter scale The Richter scale (), also called the Richter magnitude scale, Richter's magnitude scale, and the Gutenberg–Richter scale, is a measure of the strength of earthquakes, developed by Charles Richter in collaboration with Beno Gutenberg, and pr ...
have been observed, with the largest earthquake being 5.2R on the evening of February 5.


Central America


El Salvador

* In April 2017, the Salvadoran municipality of Antiguo Cuscatlán, a suburb of
San Salvador San Salvador () is the Capital city, capital and the largest city of El Salvador and its San Salvador Department, eponymous department. It is the country's largest agglomeration, serving as the country's political, cultural, educational and fin ...
, 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.


North America


United States

* Between February and November 2008,
Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...
experienced a swarm of 1,000 low-magnitude quakes generally referred to as the 2008 Reno earthquakes. 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 The Yellowstone Caldera, also known as the Yellowstone Plateau Volcanic Field, is a Quaternary caldera complex and volcanic plateau spanning parts of Wyoming, Idaho, and Montana. It is driven by the Yellowstone hotspot and is largely within Yel ...
, a
supervolcano A supervolcano is a volcano that has had an eruption with a volcanic explosivity index (VEI) of 8, the largest recorded value on the index. This means the volume of deposits for such an eruption is greater than . Supervolcanoes occur whe ...
in NW
Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...
, 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 The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...
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 Yellowstone Lake is the largest body of water in Wyoming and the largest in Yellowstone National Park. The lake is above sea level and covers with of shoreline. While the average depth of the lake is , its greatest depth is at least . Yellowst ...
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. * The Guy–Greenbrier earthquake swarm occurred in
central Arkansas Central Arkansas, also known as the Little Rock metro, designated by the United States Office of Management and Budget as the Little Rock-North Little Rock-Conway Metropolitan Statistical Area, is the most populous metro area in the U.S. state ...
beginning in August 2010.
Epicentre The epicenter (), epicentre, or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Determination The primary purpose of a s ...
s 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. * On 2 September 2017, an earthquake swarm appeared around
Soda Springs, Idaho Soda Springs is a city in Caribou County, Idaho, United States. Its population was 3,133 at the time of the 2020 census. The city has been the county seat of Caribou County since the county was organized in 1919. In the 1860s, Soda Springs ser ...
. 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 Idaho ( ) is a landlocked U.S. state, state in the Pacific Northwest and Mountain states, Mountain West subregions of the Western United States. It borders Montana and Wyoming to the east, Nevada and Utah to the south, and Washington (state), ...
had experienced an ''M''6.9 earthquake in 1983, experts warned residents that a stronger quake could follow (an unlikely but still possible scenario for them). * From early 2016 to late 2019, a swarm of earthquakes occurred near
Cahuilla The Cahuilla, also known as ʔívil̃uqaletem or Ivilyuqaletem, are a Native Americans in the United States, Native American people of the various tribes of the Cahuilla Nation, living in the inland areas of southern California. ...
in
Riverside County, California Riverside County is a County (United States), county located in the Southern California, southern portion of the U.S. state of California. As of the 2020 United States Census, 2020 census, its population was 2,418,185, making it the fourth-most ...
. 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 wide, long, with the earliest seismic swarm events localized down near its base at below the surface and the latest events migrating upwards to 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 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 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.


Atlantic Ocean

* In
El Hierro El Hierro (), nicknamed ''Isla del Meridiano'' (the "Meridian Island"), is the farthest south and west of the Canary Islands (an autonomous community of Spain), in the Atlantic Ocean off the coast of Africa, with a population of 11,659 (2023). ...
, the smallest and farthest south and west of the
Canary Islands The Canary Islands (; ) or Canaries are an archipelago in the Atlantic Ocean and the southernmost Autonomous communities of Spain, Autonomous Community of Spain. They are located in the northwest of Africa, with the closest point to the cont ...
, hundreds of small earthquakes were recorded from July 2011 until October 2011 during the
2011–12 El Hierro eruption The 2011–2012 El Hierro eruption occurred just off the island of El Hierro, the second smallest and farthest south and west of the Canary Islands (an autonomous community of Spain), in the Atlantic Ocean off the coast of Africa. The island is ...
. The accumulated energy released by the swarm increased dramatically on 28 September. The swarm was due to the movement of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
beneath the island, and on 9 October a submarine
volcanic eruption A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior h ...
was detected.


Indian Ocean

* An earthquake swarm began east of
Mayotte Mayotte ( ; , ; , ; , ), officially the Department of Mayotte (), is an Overseas France, overseas Overseas departments and regions of France, department and region and single territorial collectivity of France. It is one of the Overseas departm ...
on 10 May 2018. The strongest quake (''M''5.9), the largest-magnitude event ever recorded in the Comoro zone, struck on 15 May 2018. The swarm includes thousands of quakes, many of them felt by Maorais residents. Temporarily-installed ocean-bottom
seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...
s showed that the swarm active zone was sited east of Mayotte, deep into the oceanic
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
(in the depth range), a rather surprising result because the swarm was believed to be caused by the deflation of a
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
reservoir located east of Mayotte, at a depth of . (Accordingly, an oceanographic campaign discovered in May 2019 a new submarine volcano, and located east of Mayotte.) The swarm had been tapering off between August and November 2018 when the 11-November-2018 event occurred. This event had no detectable P nor S waves, but generated
surface waves In physics, a surface wave is a mechanical wave that propagates along the interface between differing media. A common example is gravity waves along the surface of liquids, such as ocean waves. Gravity waves can also occur within liquids, at ...
which could be observed worldwide by seismological observatories. Its origin is thought to be east of Mayotte. The swarm continued to be active all through 2019.


Pacific Ocean

* In January and February 2013, the
Santa Cruz Islands The Santa Cruz Islands form an archipelago in Temotu Province, Solomon Islands. They lie approximately to the southeast of the Solomon Islands (archipelago), Solomon Islands archipelago, just north of the archipelago of Vanuatu and are con ...
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 The 2013 Solomon Islands earthquake (also known as the Santa Cruz earthquake) struck Temotu Province within Solomon Islands on 6 February with a moment magnitude of 8.0 and a maximum Mercalli intensity of VIII (''Severe''). The epicentre ...
(6 February 2013).


See also

* 1951 East Rift Valley earthquakes * 2009– Oklahoma earthquake swarms * Blanco fracture zone *
Gutenberg–Richter law In seismology, the Gutenberg–Richter law (GR law) expresses the relationship between the Richter magnitude scale, magnitude and total number of earthquakes in any given region and time period of ''at least'' that magnitude. : \log_ N = a - b M ...
* Guy–Greenbrier earthquake swarm *
Remotely triggered earthquakes Remotely triggered earthquakes are a result of the effects of large earthquakes at considerable distance, outside of the immediate aftershock zone. The farther one gets from the initiating earthquake in both space and time, the more difficult it ...


References

{{reflist Swarm Swarm