Aftershock
A smaller quake that follows a mainshock as nearby faults adjust to the new stress.
An aftershock is a smaller earthquake that occurs near the rupture of a larger mainshock, as surrounding faults settle into the redistributed stress. Sequences can last days to years, with frequency dropping roughly in proportion to time, a pattern known as Omori's law.
The largest aftershock is typically about 1.2 magnitudes below the mainshock, so a magnitude 7 can produce a damaging magnitude 6 follow-up. Aftershocks are why a strong quake is not an all-clear: the hours and days after are a period of elevated risk, clearly visible as a busy cluster on a live feed.
Related terms
Magnitude
A number describing the energy an earthquake released at its source, on a logarithmic scale.
Fault
A fracture in the Earth's crust where blocks of rock move past each other, releasing quakes.
Mainshock
The largest quake in a sequence; smaller ones before it are foreshocks, after it aftershocks.