|MadSci Network: Earth Sciences|
The focus of an earthquake is the actual location of the rupture within the body of the Earth. The epicenter is simply that point (or area) on the Earth's surface directly above the focus. Similarly, the focal depth is the vertical distance from the focus to the epicenter. http://www.geophys.washington.edu/seismosurfing.html has a variety of additional information, including links to sites of interest. Also, a quick search using Yahoo or a similar search engine (for Yahoo, doubleclick on the following sequential subjects 'science,' 'geology and geophysics,' 'earthquakes'). A fairly advanced text on geophysics in general is "Engineering Geology: an environmental approach" by RAHN (Prentice Hall, 2nd edition 1996)
Try the links in the MadSci Library for more information on Earth Sciences.