Guide
Aftershocks and foreshocks: what to expect after an earthquake
Aftershocks are smaller earthquakes that follow a larger mainshock on or near the same fault, and they become less frequent over time. They can continue for weeks to years. A foreshock is simply an earthquake that turns out to be followed by a larger one, and it can only be identified after the fact.
Why aftershocks happen
A large earthquake changes the stress on the fault and the rock around it. Nearby parts of the crust then adjust through many smaller slips. The rate of aftershocks falls off roughly in proportion to the time since the mainshock, a pattern known as Omori’s law. Most come in the first hours and days, and some continue for years after a very large earthquake.
How big can they be?
The largest aftershock is typically about 1.2 magnitude units smaller than the mainshock (Båth’s law). It is still possible for an aftershock to be damaging, particularly to buildings already weakened by the mainshock. This is why guidance after a large earthquake is to stay out of damaged structures.
Foreshocks are only labelled afterwards
Most earthquakes are not followed by a bigger one. Scientists cannot tell in real time whether a small earthquake is a foreshock. USGS estimates that, for a given earthquake, there is roughly a 5% chance that a larger one follows within a few days nearby.
What to do
- Expect aftershocks and be ready to drop, cover and hold on again.
- Avoid damaged buildings, even for a quick return.
- Check earthquakes near you for the list of recent events and their magnitudes.