Depth & aftershocks
Earthquakes happen from the surface down to about 700 km. Depth changes how strongly shaking reaches us, and tells us about the plates beneath.
Live: depth of … recent earthquakes (7 days)
Aftershocks per day after a mainshock (Omori’s law)
Shallow, intermediate, deep
Shallow earthquakes (0–70 km) are the most common and most damaging, because the energy has little distance to fade. Intermediate (70–300 km) and deep (300–700 km) quakes occur almost only inside cold slabs of subducting plate; they are felt over wide areas but rarely cause heavy damage.
Aftershocks
A large earthquake changes the stress on nearby faults, triggering aftershocks. Their rate falls roughly as 1 / time (Omori’s law): about half as many on day two as on day one, and a tenth by day ten. The largest aftershock is typically about 1.2 magnitudes smaller than the mainshock (Båth’s law) — still enough to damage weakened buildings.
Foreshocks
Sometimes an earthquake turns out to be the foreshock of a bigger one. That can only be known afterwards, which is why aftershock forecasts give probabilities, not predictions.