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.