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 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 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 [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 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 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 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