![HistoCrise](https://upload.wikimedia.org/wikipedia/commons/5/57/HistoCrise.jpg)
In
seismology
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
, 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 earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
(main shock) is followed by a series of
aftershock
In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousand ...
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
In the
Ore Mountains (Erzgebirge), which form the border between the
Czech Republic
The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...
and
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
, western
Bohemia
Bohemia ( ; cs, Čechy ; ; hsb, Čěska; szl, Czechy) is the westernmost and largest historical region of the Czech Republic. Bohemia can also refer to a wider area consisting of the historical Lands of the Bohemian Crown ruled by the Bohem ...
and the
Vogtland
Vogtland (; cz, Fojtsko) 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 Euregio Egrensis. The name alludes to the former ...
region, have been known since the 16th century as prone to frequent earthquake swarms, which typically last a few weeks to a few months. Austrian geologist Josef Knett, while studying in 1899 a swarm of about a hundred events felt in western Bohemia/Vogtland in January-February 1824, coined the noun ''Schwarmbeben'', ''i.e.'' "swarm
arthuake". The term "swarm" is explained by
hypocentre
In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake.
Earthquakes
An earthquake's hypocenter is the position where the strain energy s ...
s giving the impression to agglutinate like a
bee swarm
Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...
when plotted onto a map, a cross-section, or still better onto a 3D model.
One of the best documented swarms occurred near
Matsushiro
is the capital and largest city of Nagano Prefecture, located in the Nagano Basin (Zenkoji Daira) in the central Chūbu region of Japan. Nagano is categorized as a core city of Japan. Nagano City is the highest prefectural capital in Japan, with ...
, a suburb of
Nagano Nagano may refer to:
Places
* Nagano Prefecture, a prefecture in Japan
** Nagano (city), the capital city of the same prefecture
*** Nagano 1998, the 1998 Winter Olympics
*** Nagano Olympic Stadium, a baseball stadium in Nagano
*** Nagano Universi ...
, to the north-west of
Tokyo
Tokyo (; ja, 東京, , ), officially the Tokyo Metropolis ( ja, 東京都, label=none, ), is the capital and largest city of Japan. Formerly known as Edo, its metropolitan area () is the most populous in the world, with an estimated 37.468 ...
. The
Matsushiro swarm lasted from 1965 to 1967 and generated about 1 million earthquakes. This swarm had the peculiarity to be 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-one-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 is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
uplift, perhaps initiated by the
1964 Niigata earthquake
Events January
* January 1 – The Federation of Rhodesia and Nyasaland is dissolved.
* January 5 - In the first meeting between leaders of the Roman Catholic and Orthodox churches since the fifteenth century, Pope Paul VI and Patriarc ...
which happened one year before.
Earthquake swarms are indeed common in
volcanic regions (for instance
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
,
Central Italy, 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; tha ...
or
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
), 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). It follows the Neogene Period and spans from 2.58 million years ...
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 ...
(for instance
Vogtland
Vogtland (; cz, Fojtsko) 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 Euregio Egrensis. The name alludes to the former ...
/western
Bohemia
Bohemia ( ; cs, Čechy ; ; hsb, Čěska; szl, Czechy) is the westernmost and largest historical region of the Czech Republic. Bohemia can also refer to a wider area consisting of the historical Lands of the Bohemian Crown ruled by the Bohem ...
or the
Vosges
The Vosges ( , ; german: Vogesen ; Franconian and gsw, Vogese) are a range of low 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 ...
massif); or also—though less frequently—far from
tectonic plate
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
boundaries (
Nevada
Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
,
Oklahoma
Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
or
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
). 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 earthquake swarm in
Bavaria
Bavaria ( ; ), officially the Free State of Bavaria (german: Freistaat Bayern, link=no ), is a state in the south-east of Germany. With an area of , Bavaria is the largest German state by land area, comprising roughly a fifth of the total lan ...
, with 2-km-deep
foci
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
, 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 under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
could be established.
Earthquake swarms raise public-safety issues: first because the end of seismic activity cannot be ascertained; secondly because one can never be sure that another earthquake with a
magnitude larger than those of previous shocks in the sequence will not occur. (The
2009 L'Aquila earthquake
The 2009 L'Aquila earthquake occurred in the region of Abruzzo, in central Italy. The main shock occurred at 03:32 CEST (01:32 UTC) on 6 April 2009, and was rated 5.8 or 5.9 on the Richter magnitude scale and 6.3 on the moment magnitude scale; ...
in Italy is emblematic, 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 for a long time 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 (; , abbr. MH or Maha) is a states and union territories of India, state in the western India, western peninsular region of India occupying a substantial portion of the Deccan Plateau. Maharashtra is the List of states and union te ...
, 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 Philippine province of
Batangas
Batangas, officially the Province of Batangas ( tl, Lalawigan ng Batangas ), is a province in the Philippines located in the Calabarzon region on Luzon. Its capital is the city of Batangas, and is bordered by the provinces of Cavite and La ...
. Four shocks in the 5.5–6.3 magnitude range (
2017 Batangas earthquakes
The 2017 Batangas earthquakes were an earthquake swarm that occurred from early April to mid-August 2017, affecting the province of Batangas in the Philippines and other nearby areas.
The first major earthquake occurred on April 4 at 8:58pm Phi ...
) caused damage in southern
Luzon
Luzon (; ) is the largest and most populous island in the Philippines. Located in the northern portion of the Philippines archipelago, it is the economic and political center of the nation, being home to the country's capital city, Manila, as ...
; 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 swarm origin of the 3 first major quakes seems established since they had practically the same epicentre; they occurred within the crust (7–28-km depth range). However, the strongest and latest quake does not seem related to the swarm: its epicentre is 50 km away, and its
focal depth
In seismology, the depth of focus or focal depth refers to the depth at which an earthquake occurs. Earthquakes occurring at a depth of less than are classified as shallow-focus earthquakes, while those with a focal depth between and are commonl ...
is moreover very different (177 km, 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 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 Philippine Island of Panay ranging Magnitude 2.5-4.5. Most of these quakes felt in Iloilo City. The last earthquake swarm also hit Panay island on November 5, 2018 (Including
Antique
An antique ( la, antiquus; 'old', 'ancient') 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 ...
,
Iloilo
Iloilo (), officially the Province of Iloilo ( hil, Kapuoran sang Iloilo; krj, Kapuoran kang Iloilo; tl, Lalawigan ng Iloilo), is a province in the Philippines located in the Western Visayas region. Its capital is the City of Iloilo, the ...
and
Guimaras) ranging Magnitude 4.0-4.8. The First earthquake (Magnitude 4.7) at 7:45 A.M, occurred at San Jose Antique. 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
* A
series of earthquakes that hit Mindanao in 2019 were classified as an earthquake swarm.
On October 16 2019, a Magnitude 6.3 (
Mwp) Struck Tulunan cotabato (Epicenter of earthquake). PHIVOLCS Earthquake Intensity Scale, Intensity VII was felt in Tulunan, M'lang, Makilala and Kidapawan City, Cotabato. Intensity VI was felt in Digos City, Davao del Sur; Sto. Nino, South Cotabato; Tacurong City, Sultan Kudarat. On October 29 2019, a Magnitude 6.6 (
Mww) struck Tulunan Cotabato again. At least ten deaths were reported and four hundred individuals were injured.
Intensity VII was recorded in Tulunan and Makilala, Cotabato; Kidapawan City; Digos City, Bansalan, Magsaysay, Davao del Sur and Malungon, Sarangani. Intensity VI was recorded in Koronadal City and Davao City. Just 2 days after on October 31 2019, an earthquake struck Tulunan Cotabato again (Magnitude 6.5
Mww) The death toll of two quakes was raised into 24 and 563 were injured and 11 are still missing.
Eva's Hotel in Kidapawan City collapsed during the quake
* October 14-18 2021 an earthquake swarm occurred on Camarines Sur ranging Magnitude 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.
Europe
Czech Republic / Germany
* The western
Bohemia
Bohemia ( ; cs, Čechy ; ; hsb, Čěska; szl, Czechy) is the westernmost and largest historical region of the Czech Republic. Bohemia can also refer to a wider area consisting of the historical Lands of the Bohemian Crown ruled by the Bohem ...
/
Vogtland
Vogtland (; cz, Fojtsko) 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 Euregio Egrensis. The name alludes to the former ...
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 (german: Neukirchen) is a municipality and village in Cheb District in the Karlovy Vary Region of the Czech Republic
The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as ...
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
![Carte150728](https://upload.wikimedia.org/wikipedia/commons/4/4e/Carte150728.jpg)
* In
Alpes-de-Haute-Provence
Alpes-de-Haute-Provence or sometimes abbreviated as AHP (; oc, Aups d'Auta Provença; ) is a department in the Provence-Alpes-Côte d'Azur region of France, bordering Alpes-Maritimes and Italy to the east, Var to the south, Vaucluse to the west ...
, the
Ubaye Valley is the most active seismic zone in the
French Alps. 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 (; oc, Barciloneta de Provença, also ; obsolete it, Barcellonetta) is a commune of France and a subprefecture in the department of Alpes-de-Haute-Provence, in the Provence-Alpes-Côte d'Azur region. It is located in the southern ...
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 8-km 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 11 km. 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, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
were located in the 4–11-km depth range, within the crystalline basement.
Focal mechanism
The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is ...
s involve
normal faulting, but also
strike-slip faulting.
* 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). Had their foci not been sited just under a hamlet in the vicinity of
Clansayes, and very close to the surface (200 m 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 ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
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
Maurienne ( frp, Môrièna) 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 Mau ...
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, 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
Montgellafrey () is a former commune in the Savoie department in the Auvergne-Rhône-Alpes region in south-eastern France. On 1 January 2017, it was merged into the commune Saint-François-Longchamp.Antiguo Cuscatlán
Antiguo Cuscatlán ''(colloquially known as Antiguo)'' is a municipality in the La Libertad Department (El Salvador), La Libertad department of El Salvador, and its eastern tip lays in San Salvador Department part of the Metropolitan Area of San ...
, a suburb of
San Salvador
San Salvador (; ) is the capital and the largest city of El Salvador and its eponymous department. It is the country's political, cultural, educational and financial center. The Metropolitan Area of San Salvador, which comprises the capital i ...
, 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 U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
experienced a swarm of 1,000 low-magnitude quakes generally referred to as the
2008 Reno earthquakes
The Reno earthquakes of 2008, also known as the "Mogul-Somersett earthquake sequence", occurred in or near the western Reno, Nevada, suburbs of Mogul and Somersett. The earthquake swarm began in February 2008, but the first significant quake of th ...
. 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, sometimes referred to as the Yellowstone Supervolcano, is a volcanic caldera and supervolcano in Yellowstone National Park in the Western United States. The caldera and most of the park are located in the northwest corn ...
, 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 1,000 cubic kilometers (240 cubic ...
in NW
Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
, 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), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
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 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 . Yellowstone Lake is the largest fre ...
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.
![Arkansas_swarm](https://upload.wikimedia.org/wikipedia/commons/f/f8/Arkansas_swarm.png)
* The
Guy-Greenbrier earthquake swarm occurred in
central Arkansas beginning in August 2010.
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.
* 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
Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyom ...
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 American people of the various tribes of the Cahuilla Nation, living in the inland areas of southern California.[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, the 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, the smallest and farthest south and west of the
Canary Islands
The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
, 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 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 also t ...
. 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 is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
beneath the island, and on 9 October a submarine
volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...
was detected.
Indian Ocean
* An earthquake swarm began east of
Mayotte
Mayotte (; french: Mayotte, ; Shimaore: ''Maore'', ; Kibushi: ''Maori'', ), officially the Department of Mayotte (french: Département de Mayotte), is an overseas department and region and single territorial collectivity of France. It is loc ...
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
seismometers showed that the swarm active zone was sited 10 km 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 (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
(in the 20–50-km 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 is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
reservoir located 45 km east of Mayotte, at a depth of 28 km. (Accordingly, an oceanographic campaign discovered in May 2019 a new submarine volcano, 800-m high and located 50 km 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 th ...
which could be observed worldwide by seismological observatories. Its origin is thought to be east of Mayotte. The swarm has continued to be active all through 2019.
Pacific Ocean
* 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 ''M''8.0
2013 Solomon Islands earthquake
The 2013 Solomon Islands earthquake struck Temotu Province within Solomon Islands on 6 February with a Moment magnitude scale, moment magnitude of 8.0 and a maximum Mercalli intensity scale, Mercalli intensity of VIII (''Severe''). The epic ...
(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 magnitude and total number of earthquakes in any given region and time period of ''at least'' that magnitude.
: \!\,\log_ N = a - b M
or
: \!\,N = 10^
wher ...
*
Guy-Greenbrier earthquake swarm
*
Remotely triggered earthquakes
References
{{reflist
Earthquake swarms
Earthquakes
Types of earthquake