Re: What is the relationship between the focus an earthquake and its epicenter

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)