Aftershocks: why the ground keeps shaking
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A big earthquake rarely ends with a single jolt. In the hours, days and weeks that follow comes a tail of smaller quakes — the aftershocks. They are not a sign that something is wrong; they are the underground finding a new balance.
What is an aftershock?
When the mainshock moves the rock along the fault, the stresses change across the whole area around the rupture. Neighbouring fractures must adapt to the new conditions, and every small adjustment is a new, smaller quake. Aftershocks therefore strike in the same area as the mainshock — typically within about one rupture-length's distance.
How strong, and for how long?
Two rules of thumb from seismology: the number of aftershocks decays quickly with time (the Omori law — day two typically brings about half as many as day one), and the largest aftershock is usually around one magnitude below the mainshock. After an M7 you can expect aftershocks around M6 — itself a serious quake. After very large quakes the tail can last months or years.
Why aftershocks are dangerous
Aftershocks are weaker than the mainshock, but they hit buildings that are already weakened. Walls that barely survived the mainshock can fall in an aftershock. Hence the advice after big quakes: do not enter damaged buildings — and know the basic safety rules.
Watch an aftershock sequence live
After any big quake you can follow the sequence on EarthquakeIndex's live map: search the area, pick 7 or 30 days, and watch the dots cluster around the mainshock. Drag across the seismogram at the bottom and you can see the quakes crowd right after the mainshock and slowly fade — Omori's law with the naked eye.