A large seismic event frequently precedes a series of smaller tremors that can continue for days or weeks in the same area. This sequence is a normal part of how the Earth’s crust adjusts to a major fault rupture. Following the massive release of stored energy, the surrounding rock mass must settle into a new state of equilibrium, involving numerous subsequent, smaller fractures. Understanding this pattern helps seismologists forecast the general risk of continued ground movement.
Identifying the Aftershock
The smaller earthquake that occurs after a large one is specifically called an aftershock. This term is used to describe any tremor that follows the largest event in a sequence, which is formally identified as the mainshock. Aftershocks are themselves earthquakes, but they are characterized by their location within the same general fault system and their smaller magnitude compared to the main event.
Typically, the largest aftershock will be at least one full magnitude unit lower than the mainshock. For instance, a magnitude 7.0 mainshock would most likely be followed by aftershocks no larger than a magnitude 6.0. These subsequent events are a sign that the crust is undergoing a process of readjustment following the primary rupture. Most aftershocks are concentrated along the main fault plane or on nearby, secondary faults that were affected by the sudden shift.
How Stress Transfer Triggers Subsequent Quakes
The mechanical cause of aftershocks is the concept of stress transfer or stress redistribution within the Earth’s crust. Before an earthquake, tectonic forces build up immense pressure, or stress, along a fault line until the rock breaks, causing the mainshock. This rupture does not simply relieve all the built-up stress evenly across the entire area.
Instead, the sudden release of energy during the mainshock shifts the mechanical stress to adjacent segments of the fault or to nearby, intersecting fault lines. These newly stressed areas, which may have been close to failure already, are pushed past their breaking point by the transferred pressure. Even a small increase in stress, sometimes equivalent to less than a tenth of the atmospheric pressure at sea level, can be enough to trigger an additional rupture.
This mechanism of static stress change, particularly the Coulomb failure stress change, is the primary model used to explain the location of aftershocks. The mainshock effectively creates a “stress shadow” where pressure has been relieved, and a region of increased stress where the aftershocks are concentrated. Delayed triggering can also occur through time-dependent mechanisms like afterslip, where the fault continues to creep aseismically, slowly transferring stress over weeks to months.
The Timeline of Aftershock Activity
Aftershocks begin almost immediately after the mainshock, sometimes within seconds to minutes, and their activity follows a predictable pattern of decay over time. The frequency and magnitude of aftershocks are highest immediately following the main event, sometimes numbering hundreds or thousands of tremors. This rapid decline in activity is described by the empirical relationship known as Omori’s Law, which states that the rate of aftershocks decreases roughly with the inverse of the time since the mainshock.
While the rate decreases quickly, the entire aftershock sequence can last for a surprisingly long time, depending on the magnitude of the mainshock. For moderate earthquakes, the sequence may only last for a few weeks or months, but for very large events, aftershocks can continue for years or even decades. The sequence is considered over only when the rate of seismic activity returns to the long-term background level for that region.
Differentiating Foreshocks and Earthquake Swarms
While aftershocks follow the largest event, other seismic sequences occur that can sometimes be confused with them. A foreshock is a smaller earthquake that precedes the mainshock in the same area. An event can only be identified as a foreshock retroactively, after the larger mainshock has occurred.
Earthquake swarms represent a different type of seismic activity, as they are a series of tremors that lack a single, clear mainshock. In a swarm, the earthquakes are generally of similar size and do not follow the typical mainshock-aftershock sequence pattern. Swarms often relate to the movement of fluids, such as water or magma, beneath the Earth’s surface, which increases localized pressure and triggers repeated fault slips.