Why you cannot identify an earthquake's true scale until the shaking finally stops

Nature · Published 3 October 2026 Swipe more like this →

In short: We often label earthquakes as they happen, but in seismology, the term mainshock is retrospective. You cannot definitively identify the largest event in a sequence until the entire series of tremors has concluded. What feels like the main event might simply be a precursor to something much larger.

In the field of seismology, a mainshock is defined as the most significant seismic event within a specific sequence. However, this designation is inherently backward-looking. Because a sequence often includes foreshocks—smaller tremors occurring before the main event in both time and space—it is impossible to confirm the mainshock while the ground is still moving. Only after the full sequence has played out can scientists accurately categorize the events.

Following the mainshock, the Earth’s crust undergoes a period of adjustment, leading to aftershocks. These are smaller earthquakes that occur in the same vicinity as the primary rupture. A single large earthquake can trigger hundreds or even thousands of these subsequent tremors. While their magnitude and frequency generally decline over time according to established physical laws, they remain a persistent feature of the seismic recovery process as the crust settles into a new equilibrium.

The classification becomes even more complex when dealing with doublet earthquakes. In these instances, the primary rupture does not occur in a single release of energy but unfolds in two or more distinct steps. These events are characterized by similar magnitudes and nearly identical seismic waveforms, which distinguishes them from standard aftershocks. Because these doublets involve multiple main shocks, they challenge the simple narrative of a single, identifiable peak event, highlighting the intricate and often unpredictable nature of crustal displacement.

Understanding these sequences is vital for assessing seismic risk, as the distinction between a foreshock and a mainshock is not merely academic. It dictates how we interpret the energy release of a tectonic event. By studying the patterns of these sequences, seismologists gain insight into how the crust redistributes stress, though the reality of these events often proves more layered and multi-staged than our initial observations might suggest during the heat of the moment.

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