Earthquake Swarms

Abstract

Introduction Earthquake swarms are a specific type of seismicity. They are generally defined as sequences of seismic events closely clustered in space and time, without a single outstanding earthquake (Mogi 1963). Globally, most seismic energy is released in ordinary earthquakes, represented by a single strong event succeeded by a series of smaller aftershocks (in some cases also preceded by foreshocks) with magnitudes of one or more magnitude units lower than that of the main event. These are so-called mainshock-aftershock sequences. In contrast, an earthquake swarm consists of numerous, mostly shallow, earthquakes, which are missing a single large event. They produce few dominant earthquakes of similar magnitudes so that smaller events are not associated with any identifiable mainshock. Earthquake swarms distinctively cluster in time and space and last from several hours to several months, with some exceptional cases lasting more than a year. Swarms originate in the crust from local stresses generated by volcanic or tectonic activity. In many cases, there is an overlapping signature of both volcanic and tectonic causes (Scholz 2002). The term “earthquake swarm” (“Schwarmbeben” in German) was very likely first used in the 1870s to describe this peculiar seismicity that occurred repeatedly in W-Bohemia/Vogtland (border of the Czech Republic, Saxony, and Bavaria).