Why do earthquakes happen in clusters?

There are two types of seismic clusters:

  • The clusters of foreshocks and aftershocks that span several months before and after a big earthquake.
  • The clusters of big seismic events (with a magnitude bigger than 7) spanning several years on a given plate boundary.

These seismic clusters result from the fact that a big earthquake puts further strain on the surrounding faults, which are likely to rupture soon after. Earthquakes therefore trigger one another. This is what happened on the Sunda Megathrust during the last decade (earthquakes of magnitude 9.3 in 2004, 8.6 in 2005 and 7.9 in 2007). Each patch of the fault seemingly ruptured because the previous earthquake had added stress on it.