Editing Earthquake swarm (section)

== 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".<ref>{{cite journal|language=de|last=Knett|first=Josef|date=1899|title=Das Erzgebirgische Schwarmbeben zu Hartenberg vom 1. Jänner bis Feber 1824|journal=Sitzungsber. Deutsch. Naturwiss.-Med. Ver. Böhmen|volume=19|pages=167–191}}</ref> The term "swarm" comes from the fact that [[hypocentre]]s give the impression of agglutinating like a [[Swarming (honey bee)|bee swarm]] when plotted onto a map, a cross-section or a 3D model.{{citation needed|date=July 2022}}

One of the best-documented swarms occurred near [[Matsushiro, Nagano|Matsushiro]], a suburb of [[Nagano (city)|Nagano]], to the north-west of [[Tokyo]]. The [[Matsushiro earthquake swarm|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.<ref>{{cite web|title=Matsushiro earthquake swarm|url=http://www.data.jma.go.jp/svd/eqev/data/matsushiro/en/index.html|publisher=Japan Meteorological Agency|access-date=2017-10-31}}</ref> The phenomenon was clearly identified as linked to a [[magma]] uplift, perhaps initiated by the [[1964 Niigata earthquake]], which occurred the previous year.<ref>{{cite journal|last=Mogi|first=Kiyoo|date=1989|title=The mechanics of the occurrence of the Matsushiro earthquake swarm in central Japan and its relation to the 1964 Niigata earthquake|journal=Tectonophysics|volume=159|issue=1–2|pages=109–119|doi=10.1016/0040-1951(89)90173-X|bibcode=1989Tectp.159..109M}}</ref>

Earthquake swarms are common in [[Volcanism|volcanic regions]] such as [[Japan]], [[Central Italy]], the [[Afar triangle|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 [[plate tectonics|tectonic plate]] boundaries in locations such as [[Nevada]], [[Oklahoma]] or [[Scotland]]. In all cases, high-pressure fluid migration in the Earth's [[crust (geology)|crust]] seems to be the trigger mechanism and the driving process that govern the evolution of the swarm in space and time.<ref name=Thouvenot>{{cite journal|last1=Thouvenot|first1=François|last2=Jenatton|first2=Liliane|last3=Scafidi|first3=Davide|last4=Turino|first4=Chiara|last5=Potin|first5=Bertrand|last6=Ferretti|first6=Gabriele|date=2016|title=Encore Ubaye: Earthquake swarms, foreshocks, and aftershocks in the southern French Alps|journal=Bull. Seismol. Soc. Am.|volume=106|issue=5|pages=2244–2257|doi=10.1785/0120150249|bibcode=2016BuSSA.106.2244T}}</ref><ref>{{cite journal|last=Špičák|first=Aleš|date=2000|title=Earthquake swarms and accompanying phenomena in intraplate regions: a review|journal=Studia Geophysica et Geodaetica|volume=44|issue=2|pages=89–106|doi=10.1023/A:1022146422444|bibcode=2000StGG...44...89S |s2cid=126768561}}</ref>
The [[Hochstaufen]] earthquake swarm in [[Bavaria]], with {{Convert|2|km|4=-deep|abbr=on|adj=mid}} [[focus (earthquake)|foci]], is one of the rare examples where an indisputable relationship between seismic activity and [[precipitation]] could be established.<ref>{{cite journal|last1=Kraft|first1=Toni|last2=Wassermann|first2=Joachim|last3=Schmedes|first3=Eberhard|last4=Igel|first4=Heiner|date=2006|title=Meteorological triggering of earthquake swarms at Mt. Hochstaufen, SE-Germany|journal=Tectonophysics|volume=424|issue=3|pages=245–258|doi=10.1016/j.tecto.2006.03.044|bibcode=2006Tectp.424..245K}}</ref>

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 (earthquake)|magnitude]] larger than those of previous shocks in the sequence will occur (the [[2009 L'Aquila earthquake]] in Italy illustrates this, with an [[Moment magnitude|''M''<sub>W</sub>]]&nbsp;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.