FACTBOX - Key facts about earthquake aftershocks

(Reuters) - A 5-magnitude aftershock rattled windows in Sichuan's provincial capital of Chengdu in the early hours of Tuesday, spooking residents who feared another quake after the devastating May 12 main quake.

A number of fresh aftershocks and forecasts of heavy rain are threatening to hinder efforts by China's military, government and private workers to ensure food and housing for millions made homeless by last week's 7.9 magnitude earthquake.

Here are some facts on aftershocks and their causes.

* Earthquakes tend to occur in clusters that strike the same area within a limited time period. The largest quake in a cluster is called the main shock, those before it are foreshocks, and those which follow are aftershocks.

* Aftershocks mostly occur on the same fault line as the main shock, and are thought to be triggered by the main shock. Sometimes the stress caused by the main shock is great enough to trigger aftershocks on nearby faultlines.

* Bigger earthquakes have more and larger aftershocks. Earthquakes large enough to cause damage typically produce several aftershocks that are large enough to be felt within the first hour. In any cluster, most quakes are aftershocks.

* Most aftershocks are too small to cause damage. But one or more that follow the large main shock can be powerful enough to cause additional damage and casualties and threaten the lives of rescuers.

* Aftershocks follow patterns, decreasing in number and magnitude over time. Scientists say the second day has about half the number of aftershocks of the first day. The 10th day has about 1/10 the number of the first day.

* The U.S. Geological Survey (USGS) began forecasting aftershocks in 1989. This information lets authorities decide when it is safe to demolish, repair, or allow people to use damaged structures.