What is an earthquake swarm?

Unlike a typical earthquake sequence with one large main shock followed by aftershocks, a swarm consists of many events of similar magnitude clustered in time and space. Swarms are often associated with volcanic activity, where moving magma or hot fluids fracture rock. They also occur along fault systems and in geothermal areas. The Yellowstone region regularly experiences swarms. Most individual events in a swarm are too small to be felt, but occasionally a larger event can occur. Scientists monitor swarms closely because they can sometimes, though not always, signal an impending volcanic eruption.