Probably the same as terrorist threats in NYC, Hurricanes in the South, Tornadoes in the midwest and Neil Diamond in Vegas - I have no clue what that last one means.
But as you can tell buyers really don't buy with logic. No matter how hard we try to conivince ourselves, we buy homes/properties because of emotions - we feel successful, inteligent, daring, etc. Even when someone thinks they bought for price/value, they only use that to rationalize a purchase of something they like... so that being said, If people were scared of earthquakes they never would have rebuilt S.F. after the fire. Millions wouldn't live here, this place is special and beautiful, these things far outweigh any logic that may sway you about earthquakes. Plus buildings and safety have caught up to help deal with earhtquake dangers. Take your pick: quakes, terrorists, tornadoes, hurricanes, water shortages, winter storms, no jobs, floods, you name it every city has some reason why you should not live there, but people do.