Earthquake swarm

In seismology, an earthquake swarm is a sequence of seismic events occurring in a local area within a relatively short period of time. The length of time used to define the swarm itself varies, but may be of the order of days, months, or even years. Such an energy release is different from what happens commonly when a major earthquake (mainshock) is followed by a series of aftershocks: in earthquake swarms, no single earthquake in the sequence is obviously the mainshock. 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 and Germany, western Bohemia and the Vogtland region have been known since the 16th century as frequently prone to 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 [earth]quake". The term "swarm" is explained by hypocentres giving the impression to agglutinate like a bee swarm when plotted onto a map, a cross-section, or still better onto a 3D model.
One of the best ever 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 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 accounted 6,780 earthquakes, with 585 of them having a magnitude large enough to be felt, which meant that one earthquake could be felt every 2 minutes 30 seconds on the average. The phenomenon was clearly identified as linked to a magma uplift, perhaps initiated by the 1964 Niigata earthquake which happened one year before.
Earthquake swarms are indeed common in volcanic regions (for instance 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 (for instance Vogtland/western Bohemia or the Vosges massif); or also—though less frequently—far from tectonic plate boundaries (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 2-km-deep foci, is one of the rare examples where an indisputable relationship between seismic activity and precipitation could be established.
Earthquake swarms raise issues from the point of view of public safety: 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 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.