Earthquakes involve the shaking of the ground caused by the abrupt release of energy stored in the planet’s crust. This movement results from the fracturing and displacement of rock along a fault line. While a single, large tremor captures immediate attention, a major seismic event is rarely isolated. The initial, most powerful shock is followed by a long sequence of smaller, related tremors that can persist long after the main shaking has stopped.
Defining the Earthquake Sequence
The largest earthquake in any seismic sequence is designated the mainshock. Smaller tremors that follow the mainshock in the same general area are known as aftershocks. Seismologists classify an event as an aftershock based on three criteria: timing (occurring after the main event), location (within the fault zone), and magnitude.
An aftershock is always smaller than the mainshock. The magnitude difference between the mainshock and the largest subsequent tremor averages around 1.2 units (e.g., a magnitude 7.0 mainshock is followed by a largest aftershock of about magnitude 5.8). Completing the sequence are foreshocks, which are smaller tremors that precede the mainshock, though they are only identified in retrospect.
The Mechanism of Stress Redistribution
The reason for subsequent tremors lies in the redistribution of geological stress within the crust. When the mainshock occurs, the rupture along the fault relieves strain energy across the slipped segment, but this release does not dissipate all stress uniformly. Instead, the rupture shifts and concentrates pressure onto adjacent, intact segments or nearby subsidiary fault lines.
These newly stressed areas are pushed closer to their breaking point. Aftershocks are essentially the process of the crust adjusting to this new state of strain. They represent the failure of these smaller, stressed patches of rock that were not strong enough to withstand the load transferred to them by the mainshock. The result is a continuing series of smaller earthquakes until the accumulated stress is sufficiently dissipated.
Patterns of Frequency and Magnitude Decline
The occurrence of these smaller tremors follows a predictable temporal pattern. Both the frequency and the magnitude of the aftershocks decrease systematically over time. The rate of aftershock activity drops off rapidly following the mainshock, with the highest number of events concentrated within the first hours and days.
The number of aftershocks decreases roughly in proportion to the reciprocal of the time elapsed since the main event. This means that the rate will have fallen significantly by the tenth day compared to the first day. While the largest and most frequent aftershocks occur early on, the entire sequence can continue for weeks, months, or even years, depending on the magnitude of the mainshock.