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 Wisconsin Green Bay: Faults and Earthquakes
This site is primarily set up as an outline and is loaded with graphs, maps, and images. It covers a variety of earthquake-related topics, such as what causes earthquakes, fault lines and structures, seismology, a historical look at...