An aftershock is a smaller earthquake that follows a larger one, occurring in the same area as the original quake. Aftershocks are part of the readjustment process along a fault after it has slipped during a major event, and they can continue for days, weeks, months, or even years. They happen within one to two fault lengths of the original rupture and are technically no different from any other earthquake, just classified by their relationship to the bigger quake that came first.
How Aftershocks Work
When a large earthquake (called the mainshock) occurs, the Earth’s crust doesn’t settle neatly back into place. The sudden movement along a fault changes the stress on surrounding rock, and that stress needs somewhere to go. Aftershocks are the result: smaller ruptures along or near the same fault as the crust works toward a new equilibrium.
Think of it like cracking a large sheet of ice. The initial break sends fractures radiating outward, and smaller pieces continue to snap and shift long after the first crack. Each aftershock releases a bit of the remaining built-up stress, though the process can be remarkably slow. Research from the USGS has found that both the shaking from seismic waves passing through rock and the permanent shift in stress from the mainshock contribute to triggering aftershocks. Studies on volcanic and tectonic events suggest that the dynamic shaking effect is actually the dominant trigger, producing both immediate and delayed aftershocks over weeks.
How Aftershocks Differ From Other Quakes
Seismologists classify earthquakes within a sequence based on size and timing. The mainshock is simply the largest earthquake in the sequence. Any smaller quakes that precede it (identified only in hindsight) are called foreshocks. Any smaller quakes that follow it in the same area are aftershocks. If a subsequent earthquake turns out to be larger than the original mainshock, the labels get reassigned: the new, bigger quake becomes the mainshock, and the original one gets reclassified as a foreshock.
This is worth knowing because there’s no way to tell in real time whether a given earthquake is “the big one” or a foreshock leading to something larger. The classification only becomes clear after the sequence plays out.
Aftershock sequences also differ from earthquake swarms. A swarm is a cluster of mostly small earthquakes with no clear mainshock, often associated with geothermal activity. Swarms tend to be short-lived and frequently recur at the same locations. Aftershock sequences, by contrast, have a clear dominant event and a predictable pattern of tapering off.
How Long Aftershocks Last
Aftershock frequency follows a remarkably consistent pattern first described by Japanese seismologist Fusakichi Omori in the 1890s. Known as Omori’s Law, it states that aftershock rates drop off rapidly at first, then taper slowly over time in a predictable curve. In the hours after a major quake, dozens of aftershocks may strike. Within days, the rate slows considerably. Within weeks or months, the area gradually returns to its normal level of background seismic activity.
For very large earthquakes, though, “gradually” can mean a very long time. Seismologists have argued that earthquakes still felt near New Madrid, Missouri, are aftershocks of a series of four massive quakes in 1811 and 1812, more than two centuries ago. In Japan, some earthquakes have been classified as aftershocks of the 1891 Nobi earthquake despite occurring a full century later. One sequence in Washington State’s Entiat region has been documented at 145 years, among the longest-lived aftershock sequences ever recorded.
How Strong Aftershocks Can Be
Aftershocks are smaller than the mainshock by definition, but “smaller” is relative. A pattern known as Båth’s Law holds that the largest aftershock in a sequence is typically about 1.2 magnitude units lower than the mainshock. That means a magnitude 7.0 earthquake commonly produces a largest aftershock around magnitude 5.8, which is strong enough to cause serious damage on its own.
The real danger lies in cumulative impact. Buildings and infrastructure weakened by the mainshock are far more vulnerable when aftershocks hit. Research on high-rise concrete structures has found that seismic risk under a mainshock-aftershock sequence is 2.0 to 3.5 times higher than from the mainshock alone. Christchurch, New Zealand, illustrated this dramatically: the city’s main urban area was almost undamaged by the 2010 Darfield earthquake, but a subsequent large aftershock caused nearly 16.2% of reinforced concrete buildings in the area to collapse. The aftershock extended recovery timelines and increased economic losses well beyond what the original quake had caused.
Most building codes are designed around a single seismic event, not the cumulative stress of repeated shaking. This gap between how buildings are designed and how earthquake sequences actually behave is one reason aftershocks can be so destructive.
Can Aftershocks Be Predicted?
Not individually, but their general pattern can be forecast with reasonable accuracy. Most aftershock forecasts still rely on statistical models first developed in the 1980s. These models use the mainshock’s magnitude and the rate of early aftershocks to estimate how many aftershocks of various sizes are likely over the coming days and weeks. The USGS issues these forecasts publicly after significant earthquakes, giving affected communities a sense of what to expect.
Researchers have explored physics-based models that map out how the mainshock shifted stress along the fault, then predict where aftershocks are most likely. These can sometimes match the accuracy of statistical models but haven’t yet surpassed them. Machine learning approaches are showing promise, particularly as improved earthquake detection systems produce more detailed catalogs of seismic events to train on.
Staying Safe During Aftershocks
If you’ve just experienced a significant earthquake, expect aftershocks and prepare accordingly. Wear sturdy shoes to protect against broken glass and debris. Check for injuries, apply direct pressure to any bleeding wounds, and avoid moving seriously injured people unless they’re in immediate danger. Cover injured people with blankets to prevent shock.
The next priority is checking your surroundings for hazards that aftershocks could worsen. Shut off your main gas valve if you smell gas or suspect broken pipes. If electrical wiring looks damaged, turn off power at the breaker box. Stay away from downed power lines and anything touching them. Open closets and cupboards carefully, since items may have shifted and could fall. Don’t use a damaged chimney, which could leak toxic gases or start a fire.
For food and water, avoid anything from open containers near broken glass. If the power is out, eat perishable and frozen foods first (freezer contents typically stay safe for a couple of days). Your water heater and ice cubes are backup water sources. Use outdoor grills or camp stoves for cooking if you suspect a gas leak, never an indoor stove. Avoid drinking pool or spa water, which may contain unsafe chemical concentrations.
Because aftershocks can continue for weeks or longer, keep emergency supplies accessible rather than packed away. The strongest aftershocks tend to come in the first hours and days, but damaging ones can arrive with little warning well into the sequence.