"Earthquake swarms are pervasive at volcanoes, but have seldom been studied systematically. Most swarms that are described in the literature are those that occurred in association with eruptions; indeed, earthquake swarms are the most reliable method of forecasting eruptions. For the purpose of this report, a swarm is defined as many earthquakes of the same size occurring in a small volume. Swarms are different in these two ways from a mainshock-aftershock sequence or a foreshock-mainshock-aftershock sequence (fig. 1). Swarms are especially common in volcanic areas. Because swarms are such a common and important phenomenon, we undertook a systematic and comprehensive study of swarms at volcanoes using modern commercially available database software. We term the result the Global Volcanic Earthquake Swarm Database (GVESD)."
Open-File Report 96-69
Alaska Volcano Observatory: http://www.avo.alaska.edu/