What is the relationship between worldwide earthquake distribution and tectonic boundaries?

earthquakes tend to occur along these boundaries.
see this map of plate tectonic boundaries and this one of earthquake hot spots.

the two sites i lifted these from are:
http://volcano.und.nodak.edu/vwdocs/volc_images/tectonic_plates.html
and
http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html

Thanks, methos5000.