Earthquakes rarely occur as isolated incidents, instead manifesting as a series of tremors clustered in time and space. Seismologists classify individual earthquakes within a cluster based on their size relative to one another. The term “mainshock” is central to this classification system, providing a framework for analyzing the sequence of rupture events along a fault zone. Understanding the mainshock allows scientists to map the distribution of stress release and the resulting adjustments of the Earth’s crust.
What Defines the Main Event
The mainshock is defined as the single largest earthquake within an entire seismic sequence, with its size determined by its magnitude. Seismologists rely on the Moment Magnitude (\(M_W\)) scale, which provides a more accurate measure of the total energy released by the event than older scales. This magnitude is calculated from the seismic moment (\(M_0\)), which accounts for the physical properties of the rupture, including the area of the fault that slipped, the average distance of that slip, and the rigidity of the rock.
The mainshock releases the vast majority of the total stored elastic strain energy of the entire sequence. In a typical sequence, the mainshock alone can account for nearly 97% of the total energy radiated as seismic waves. Because a whole number increase in magnitude represents a 32-fold increase in energy release, the largest event dominates the sequence’s energy budget.
Placing the Mainshock in the Earthquake Sequence
The mainshock temporally divides an earthquake cluster into two parts: the foreshock period and the aftershock period. Foreshocks are smaller earthquakes that precede the mainshock, occurring in the same localized area and sharing a spatial and temporal connection to the larger event. Not all mainshocks are preceded by these smaller tremors. When foreshocks do occur, they are considered part of a preparatory process leading to the primary rupture.
The mainshock is followed by a series of aftershocks, which are smaller earthquakes occurring in the same general region of the main rupture. Aftershocks represent the Earth’s crust adjusting to the stress changes caused by the primary fault slip. The frequency and magnitude of these aftershocks decrease systematically over time, a pattern known as Omori’s Law. Aftershocks typically occur within one to two fault lengths of the mainshock rupture and can continue for days, months, or even years.
Determining the Sequence After the Event
The classification of an earthquake as a mainshock is often a retrospective designation, meaning it cannot be definitively identified in real-time. An earthquake is only confirmed as a mainshock after scientists have recorded the subsequent seismic activity and confirmed that no larger event followed it within the defined spatial and temporal window. What is initially reported as a mainshock could potentially be reclassified as a foreshock if a larger tremor occurs later.
Sometimes, an earthquake sequence involves two events of very similar magnitude, which seismologists refer to as a “doublet.” Doublets consist of two mainshocks that occur close in time and space, often with magnitudes within 0.4 to 1.0 unit of each other. In these cases, both events are classified as mainshocks because the second event is too large to be considered a typical aftershock. The identification process is therefore statistical, relying on comparing the magnitudes of all events in the cluster to determine the largest one.