Earthquake swarm

thumb |upright=1.35 |[[Noto earthquake swarm (2020–2024)]]

[[File:HistoCrise.jpg|thumb|upright=1.35|Chronology of the 2003–2004 Ubaye earthquake swarm

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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.

History and generalities

The Ore Mountains (Erzgebirge), which form the border between the Czech Republic and Germany, western Bohemia and the Vogtland 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 [earth]quake". The term "swarm" comes from the fact that hypocentres 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. 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 uplift, perhaps initiated by the 1964 Niigata earthquake, which occurred the previous year.

Earthquake swarms are common in volcanic regions such as Japan, Central Italy, the Afar depression or Iceland, where they occur before and during eruptions, but they are also observed in zones of Quaternary volcanism or of hydrothermal circulation, such as Vogtland/western Bohemia and the Vosges massif, and less frequently far from tectonic plate boundaries in locations such as Nevada, Oklahoma or Scotland. 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, with foci, is one of the rare examples where an indisputable relationship between seismic activity and precipitation 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 larger than those of previous shocks in the sequence will occur (the 2009 L'Aquila earthquake in Italy illustrates this, with an MW 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, 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.
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. Four shocks in the 5.5–6.3 magnitude range (2017 Batangas earthquakes) caused damage in southern Luzon; they occurred at the beginning of the swarm: Ms5.5 (4 April), Ms5.6 and Ms6.0 (8 April), and Ms6.3 (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 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, Iloilo and Guimaras) ranging from magnitudes 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, a magnitude 6.3 (Mwp) Struck Tulunan cotabato (Epicenter of earthquake). Intensity VII was felt in Tulunan, M'lang, Makilala and Kidapawan City, Cotabato. Intensity VI was felt in Digos City, Davao del Sur; Santo Niño, South Cotabato and Tacurong City, Sultan Kudarat. 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 and Malungon, Sarangani. Intensity VI was recorded in Koronadal City and Davao City. Just 2 days after on October 31, an earthquake struck Tulunan again (Magnitude 6.5Mww) 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 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.

Europe

Iceland

A swarm of intense earthquakes in the Reykjanes Peninsula, Iceland 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, 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 (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 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 [ˈfɛstarˌfjatl̥] and 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-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 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 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/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).

France

[[File:Carte150728.jpg|thumb|Ubaye earthquake swarms