What is the term for a cluster of earthquakes with no single mainshock?

A cluster of earthquakes with no single dominant mainshock is called an earthquake swarm.

In a swarm, many events occur close together in time and space, but none clearly stands out as the main earthquake followed by smaller aftershocks. Swarms can last hours, days, or months. They are common in volcanic regions and geothermal areas, where moving fluids or magma can alter pressure on faults.

Not every swarm signals an eruption, and swarms also occur in tectonic settings. Scientists examine the event depths, locations, magnitudes, and changes through time to assess the underlying process. A swarm may include earthquakes too small for people to feel, though larger events can occur.

An aftershock sequence has a different pattern: a mainshock is followed by progressively less frequent smaller events in the same region. The distinction can become uncertain while an active sequence is unfolding, because scientists may not know whether a later event will become the largest.