Exploratorium
Exploratorium: Earthquake Faultline Earthquake Activities
This outstanding website provides an excellent variety of hands-on earthquake activities to help you understand what causes earthquakes.
US Geological Survey
Central California Earthquake Hazards Project
Here is a review of the geology behind earthquakes and what is being done to predict the "Big One" on the San Andreas Fault.
Other
University of Delaware: Plate Tectonics
This site is a very simple overview of plate tectonics. It includes a map of all the major plates and a diagram of the layers of the earth.
The Tech Interactive
Tech Museum of Innovation: Earthquakes
As an overview of the online exhibit on earthquakes, this site provides brief information on faults and mentions the San Andreas Fault along with a picture.