Incredible Luck
Clearly a significant release of energy from San Andreas slip-faulting, in the absolute middle of somewhere (i.e. halfway from LA to the Bay area - that part of the coast is still practically a wilderness and very difficult to access from inland). About the best possible quake you guys could suffer!
(Interesting note: These San Andreas quakes seem to be evenly spaced in time and space and usually around R6.5 in strength. You guys are due.)
-drl