Aftershocks Explained

Explainer · 5-minute read

After a big earthquake the ground rarely goes quiet straight away. Smaller quakes — aftershocks (called réplicas in Spanish and many other languages) — keep rattling the same region for days, weeks or even years. They're completely normal, and understanding them helps you stay safe in the hours after a major quake.

Mainshock, foreshocks and aftershocks

An earthquake sequence usually has three parts:

• Foreshocks: smaller quakes that sometimes precede the largest event. They can only be identified as foreshocks after the bigger quake happens, so they're not a reliable warning.
• Mainshock: the largest earthquake in the sequence.
• Aftershocks: the many smaller quakes that follow, on or near the same fault, as the crust adjusts to its new stress balance.

If an "aftershock" turns out to be larger than the original quake, scientists relabel it the mainshock and call the earlier one a foreshock.

Why aftershocks happen

When a fault slips, it doesn't relieve stress evenly — it actually raises stress on nearby patches of rock and neighbouring faults. Those stressed patches then slip in turn, producing aftershocks until the whole region settles. Larger mainshocks rupture bigger areas, so they trigger more, and larger, aftershocks over a wider zone.

How long do they last?

Aftershocks are most frequent and strongest right after the mainshock and then taper off. This decay follows a well-known pattern (Omori's law): the rate of aftershocks drops roughly in proportion to one over the time since the mainshock. In practice that means the first day has far more aftershocks than the second, and so on. A general rule of thumb (Båth's law) is that the largest aftershock is typically about one magnitude smaller than the mainshock — still large enough to topple buildings already weakened by the first quake.

What this means for you

• Expect them. After a significant quake, be ready to Drop, Cover and Hold On again at any moment.
• Be cautious with damaged buildings. Structures cracked by the mainshock can collapse in an aftershock — stay out until they're checked.
• Watch the tracker. Filter to your region and you'll often see a cluster of dots around a recent mainshock — that's the aftershock sequence playing out.

Can aftershocks be forecast?

Individual aftershocks can't be predicted, but their overall likelihood can be forecast statistically. Agencies such as USGS publish aftershock forecasts after major quakes — for example, the chance of one or more M5+ aftershocks in the next week. These are probabilities, not certainties, and they steadily decrease as the sequence dies down.

More: Earthquake safety & response · What is an earthquake · The magnitude scale.