How do aftershocks happen after a major earthquake?

When a major fault ruptures, it redistributes stress to nearby sections of the crust. Those neighboring areas may then fail in a series of smaller ruptures, producing aftershocks. Large earthquakes can have thousands of aftershocks, and the biggest ones sometimes cause additional damage to already weakened buildings. The frequency of aftershocks usually declines with time, but the exact pattern depends on the size and geometry of the main rupture. This is why emergency planners warn that the danger is not over after the first large quake.