Swarms are made up of numerous earthquakes that occur locally over an extended period, without a clear sequence of high-intensity main quakes, preceded and succeeded by lower-intensity foreshocks and aftershocks.
Many earthquake swarms occur in regions with complex contiguous fracture systems. The theory is that they are related to the movement of fluid gases and liquids in the Earth’s crust.
An earthquake swarm can last for weeks, and the pace of subsidence is gradual, relative to aftershocks in normal earthquakes. Swarms are observed in volcanic environments, hydrothermal systems, and other active geothermal areas.
Swarms are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock. While the April 2015 Nepal earthquake demonstrated the foreshocks-main shock (7.8)- aftershocks trend, the seismic activity in Indonesia since July 29 2018 fits the typology of the swarm.