Have a look at this. http://www.geonet.org.nz/earthquake/quakes/recent_quakes.htm... Christchurch, New Zealand. Main quake, 7.1 on the scale (and with hindsight, not all that bad), was almost six months before a very shallow 6.3 aftershock which was very bad and had lots of deaths. So far there have been something like 8000 aftershocks. People from the area often seem shell-shocked.

An aftershock is, essentially, an earthquake caused either by the continuing event that caused the first quake or by the resettling of the earth in the area, and especially the latter can take a long time.