This overlay is a US Geological Survey map depicting the major faults in the California and Nevada. Source: http://quake.usgs.gov/info/faultmaps/index.html
From that page, you can click the map and get a more detailed fault map of a particular section of the map with some of the smaller faults in higher resolution.

USGS is actively working on KML data that will give us GE users even better data than my simple overlays. Check out their realtime quake overlays:
Colored by age: http://earthquake.usgs.gov/eqcenter/recenteqsww/catalogs/eqs7day-age.kmz

Colored by depth: http://earthquake.usgs.gov/eqcenter/recenteqsww/catalogs/eqs7day-depth.kmz

Use the fault overlay with the quake overlay and you can see where the really active faults are.

The proximity to major faults is a consideration during paranormal investigations conducted by Beyond Investigation Magazine. http://www.beyondinvestigation.com