Aftershocks: what to expect
An aftershock is an earthquake that follows a larger one, close by, as the crust around the rupture adjusts. They are normal, they are expected, and they follow patterns reliable enough to describe statistically — but not reliably enough to tell you when the next one will be.
Three things the evidence supports
- They are most frequent immediately afterwards and decline over time. The rate falls off roughly with one over the time elapsed, so the first day brings far more than the second, and the second more than the tenth. This is an old and well-tested observation.
- The largest aftershock is typically about one magnitude smaller than the mainshock. Typically — not always, and not as a rule you can rely on for safety.
- The sequence can last a long time. A large earthquake produces aftershocks for months or years, at an ever-decreasing rate. A "swarm" that continues for weeks in western Nevada or northern California is often exactly this.
One thing that is not true
There is no rule that the first earthquake is the biggest. Any earthquake can be followed by a larger one, in which case seismologists rename the first a foreshock and the larger one becomes the mainshock. That renaming happens afterwards, because there is no way to tell at the time.
This is the core reason the advice after a felt earthquake is the same as the advice during one: be ready for more shaking, because nobody can rule out a larger event. Why prediction is not possible →
What a USGS aftershock forecast actually says
After significant earthquakes USGS issues an operational aftershock forecast. It is a genuine, useful statement — and it is a probability over a window, not a schedule. It reads like: the chance of one or more magnitude 5 or larger aftershocks in the next week is some percentage, and the likely number of magnitude 3 and above is a range.
QuakeIndex does not produce forecasts of its own. When USGS publishes one, we link it from the event page and quote it verbatim with attribution. That is the only forward-looking statement anywhere on this site, and it is not ours.
What QuakeIndex does show
On every event page: aftershocks so far — the earthquakes recorded within 50 km since that event, counted from our own database, with the time we last checked. That is an observation, and it answers the question people actually ask in the hours afterwards, which is "is it still going?"
On place pages: the 30-day activity strip, which makes an ongoing sequence visible at a glance — a run of marks day after day looks entirely different from an isolated event.
What to do during a sequence
The practical guidance does not change with the statistics: drop, cover and hold on each time you feel shaking; expect more; and take seriously that objects already loosened by the mainshock can fall in a smaller aftershock. If a building has visible structural damage, treat every aftershock as capable of making it worse, and do not rely on a website to judge whether a damaged building is safe.
Sources
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